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A sense is a
biological system A biological system is a complex network which connects several biologically relevant entities. Biological organization spans several scales and are determined based different structures depending on what the system is. Examples of biological syst ...
used by an organism for sensation, the process of gathering information about the world through the detection of
stimuli A stimulus is something that causes a physiological response. It may refer to: *Stimulation **Stimulus (physiology), something external that influences an activity **Stimulus (psychology), a concept in behaviorism and perception *Stimulus (economi ...
. (For example, in the human body, the
brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...
which is part of the
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...
receives signals from the senses which continuously receive information from the environment, interprets these signals, and causes the body to respond, either chemically or physically.) Although traditionally five human senses were identified as such (namely
sight Visual perception is the ability to interpret the surrounding Biophysical environment, environment through photopic vision (daytime vision), color vision, scotopic vision (night vision), and mesopic vision (twilight vision), using light in the ...
,
smell Smell may refer to; * Odor, airborne molecules perceived as a scent or aroma * Sense of smell, the scent also known scientifically as olfaction * "Smells" (''Bottom''), an episode of ''Bottom'' * The Smell, a music venue in Los Angeles, Californ ...
,
touch In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch (haptic perception), as well as temperature (thermoception), body position (proprioception), and pain. It is ...
,
taste The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception produced or stimulated when a substance in the mouth reacts chemically with taste receptor ...
, and
hearing Hearing, or auditory perception, is the ability to perceive sounds In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psycholog ...
), it is now recognized that there are many more. Senses used by non-human organisms are even greater in variety and number. During sensation, sense organs collect various stimuli (such as a sound or smell) for transduction, meaning transformation into a form that can be understood by the brain. Sensation and perception are fundamental to nearly every aspect of an organism's
cognition Cognition refers to "the mental action or process of acquiring knowledge and understanding through thought, experience, and the senses". It encompasses all aspects of intellectual functions and processes such as: perception, attention, thought, ...
,
behavior Behavior (American English) or behaviour (British English) is the range of actions and mannerisms made by individuals, organisms, systems or artificial entities in some environment. These systems can include other systems or organisms as wel ...
and
thought In their most common sense, the terms thought and thinking refer to conscious cognitive processes that can happen independently of sensory stimulation. Their most paradigmatic forms are judging, reasoning, concept formation, problem solving, a ...
. In organisms, a sensory organ consists of a group of interrelated
sensory cells Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cell ...
that respond to a specific type of physical stimulus. Via
cranial Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
and
spinal nerve A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. In the human body there are 31 pairs of spinal nerves, one on each side of the vertebral column. These are grouped into th ...
s (nerves of the Central and Peripheral nervous systems that relay sensory information to and from the brain and body), the different types of sensory receptor cells (such as
mechanoreceptors A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, a ...
, photoreceptors,
chemoreceptor A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance (endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorecept ...
s,
thermoreceptor A thermoreceptor is a non-specialised sense receptor, or more accurately the receptive portion of a sensory neuron, that codes absolute and relative changes in temperature, primarily within the innocuous range. In the mammalian peripheral nervous s ...
s) in sensory organs transduct sensory information from these organs towards the
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...
, finally arriving at the sensory cortices in the
brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...
, where sensory signals are processed and interpreted (perceived). Sensory systems, or senses, are often divided into external (exteroception) and internal (
interoception Interoception is contemporarily defined as the collection of senses perceiving the internal state of the body. This can be both conscious and unconscious. It encompasses the brain's process of integrating signals relayed from the body into specif ...
) sensory systems. Human external senses are based on the sensory organs of the
eyes Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conve ...
,
ears An ear is the organ that enables hearing and, in mammals, body balance using the vestibular system. In mammals, the ear is usually described as having three parts—the outer ear, the middle ear and the inner ear. The outer ear consists o ...
,
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...
,
nose A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for respiration alongside the mouth. Behind the nose are the olfactory mucosa and the sinuses. Behind the nasal cavity, air next passes th ...
, and
mouth In animal anatomy, the mouth, also known as the oral cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on ...
. Internal sensation detects stimuli from internal organs and tissues. Internal senses possessed by humans include the
vestibular system The vestibular system, in vertebrates, is a sensory system that creates the sense of balance and spatial orientation for the purpose of coordinating movement with balance. Together with the cochlea, a part of the auditory system, it constitutes ...
(sense of balance) sensed by the
inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
, as well as others such as
spatial orientation In geometry, the orientation, angular position, attitude, bearing, or direction of an object such as a line, plane or rigid body is part of the description of how it is placed in the space it occupies. More specifically, it refers to the imagina ...
,
proprioception Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by proprioceptors, mechanosensory neurons ...
(body position) and
nociception Nociception (also nocioception, from Latin ''nocere'' 'to harm or hurt') is the sensory nervous system's process of encoding noxious stimuli. It deals with a series of events and processes required for an organism to receive a painful stimulus, co ...
(pain). Further internal senses lead to signals such as
hunger In politics, humanitarian aid, and the social sciences, hunger is defined as a condition in which a person does not have the physical or financial capability to eat sufficient food to meet basic nutritional needs for a sustained period. In the ...
,
thirst Thirst is the craving for potable fluids, resulting in the basic instinct of animals to drink. It is an essential mechanism involved in fluid balance. It arises from a lack of fluids or an increase in the concentration of certain osmolites, suc ...
,
suffocation Asphyxia or asphyxiation is a condition of deficient supply of oxygen to the body which arises from abnormal breathing. Asphyxia causes generalized hypoxia, which affects primarily the tissues and organs. There are many circumstances that can i ...
, and
nausea Nausea is a diffuse sensation of unease and discomfort, sometimes perceived as an urge to vomit. While not painful, it can be a debilitating symptom if prolonged and has been described as placing discomfort on the chest, abdomen, or back of the ...
, or different involuntary behaviors, such as
vomiting Vomiting (also known as emesis and throwing up) is the involuntary, forceful expulsion of the contents of one's stomach through the mouth and sometimes the Human nose, nose. Vomiting can be the result of ailments like Food-poisoning, foo ...
. Some animals are able to detect
electrical Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...
and
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
s,
air moisture (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous phas ...
, or
polarized light Polarization (also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the ...
, while others sense and perceive through alternative systems, such as echolocation.
Sensory modalities Stimulus modality, also called sensory modality, is one aspect of a stimulus or what is perceived after a stimulus. For example, the temperature modality is registered after heat or cold stimulate a receptor. Some sensory modalities include: light, ...
or sub modalities are different ways sensory information is encoded or transduced.
Multimodality Multimodality is the application of multiple literacies within one medium. For example, understanding a televised weather forecast (medium) involves understanding spoken language, written language, weather specific language (such as temperature s ...
integrates different senses into one unified perceptual experience. For example, information from one sense has the potential to influence how information from another is perceived. Sensation and perception are studied by a variety of related fields, most notably
psychophysics Psychophysics quantitatively investigates the relationship between physical stimuli and the sensations and perceptions they produce. Psychophysics has been described as "the scientific study of the relation between stimulus and sensation" or, m ...
,
neurobiology Neuroscience is the scientific study of the nervous system (the brain, spinal cord, and peripheral nervous system), its functions and disorders. It is a multidisciplinary science that combines physiology, anatomy, molecular biology, development ...
,
cognitive psychology Cognitive psychology is the scientific study of mental processes such as attention, language use, memory, perception, problem solving, creativity, and reasoning. Cognitive psychology originated in the 1960s in a break from behaviorism, which ...
, and cognitive science.


Definitions


Sensory organs

Sensory organs are
organs In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a fu ...
that sense and transduce stimuli. Humans have various sensory organs (i.e. eyes, ears, skin, nose, and mouth) that correspond to a respective
visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (the a ...
(sense of vision),
auditory system The auditory system is the sensory system for the sense of hearing. It includes both the sensory organs (the ears) and the auditory parts of the sensory system. System overview The outer ear funnels sound vibrations to the eardrum, increasin ...
(sense of hearing),
somatosensory system In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch (haptic perception), as well as temperature (thermoception), body position (proprioception), and pain. It ...
(sense of touch),
olfactory system The olfactory system, or sense of smell, is the sensory system used for smelling (olfaction). Olfaction is one of the special senses, that have directly associated specific organs. Most mammals and reptiles have a main olfactory system and an ac ...
(sense of smell), and
gustatory system The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception produced or stimulated when a substance in the mouth reacts chemically with taste receptor ...
(sense of taste). Those systems, in turn, contribute to
vision Vision, Visions, or The Vision may refer to: Perception Optical perception * Visual perception, the sense of sight * Visual system, the physical mechanism of eyesight * Computer vision, a field dealing with how computers can be made to gain un ...
,
hearing Hearing, or auditory perception, is the ability to perceive sounds In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psycholog ...
,
touch In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch (haptic perception), as well as temperature (thermoception), body position (proprioception), and pain. It is ...
,
smell Smell may refer to; * Odor, airborne molecules perceived as a scent or aroma * Sense of smell, the scent also known scientifically as olfaction * "Smells" (''Bottom''), an episode of ''Bottom'' * The Smell, a music venue in Los Angeles, Californ ...
, and the ability to
taste The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception produced or stimulated when a substance in the mouth reacts chemically with taste receptor ...
. Internal sensation, or interoception, detects stimuli from internal organs and tissues. Many internal sensory and perceptual systems exist in humans, including the
vestibular system The vestibular system, in vertebrates, is a sensory system that creates the sense of balance and spatial orientation for the purpose of coordinating movement with balance. Together with the cochlea, a part of the auditory system, it constitutes ...
(sense of balance) sensed by the
inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
and providing the perception of
spatial orientation In geometry, the orientation, angular position, attitude, bearing, or direction of an object such as a line, plane or rigid body is part of the description of how it is placed in the space it occupies. More specifically, it refers to the imagina ...
;
proprioception Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by proprioceptors, mechanosensory neurons ...
(body position); and
nociception Nociception (also nocioception, from Latin ''nocere'' 'to harm or hurt') is the sensory nervous system's process of encoding noxious stimuli. It deals with a series of events and processes required for an organism to receive a painful stimulus, co ...
(pain). Further internal
chemoreception A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance (endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorecept ...
- and osmoreception-based sensory systems lead to various perceptions, such as
hunger In politics, humanitarian aid, and the social sciences, hunger is defined as a condition in which a person does not have the physical or financial capability to eat sufficient food to meet basic nutritional needs for a sustained period. In the ...
,
thirst Thirst is the craving for potable fluids, resulting in the basic instinct of animals to drink. It is an essential mechanism involved in fluid balance. It arises from a lack of fluids or an increase in the concentration of certain osmolites, suc ...
,
suffocation Asphyxia or asphyxiation is a condition of deficient supply of oxygen to the body which arises from abnormal breathing. Asphyxia causes generalized hypoxia, which affects primarily the tissues and organs. There are many circumstances that can i ...
, and
nausea Nausea is a diffuse sensation of unease and discomfort, sometimes perceived as an urge to vomit. While not painful, it can be a debilitating symptom if prolonged and has been described as placing discomfort on the chest, abdomen, or back of the ...
, or different involuntary behaviors, such as
vomiting Vomiting (also known as emesis and throwing up) is the involuntary, forceful expulsion of the contents of one's stomach through the mouth and sometimes the Human nose, nose. Vomiting can be the result of ailments like Food-poisoning, foo ...
. Nonhuman animals experience sensation and perception, with varying levels of similarity to and difference from humans and other animal species. For example, mammals in general have a stronger sense of smell than humans. Some animal species lack one or more human sensory system analogues and some have sensory systems that are not found in humans, while others process and interpret the same sensory information in very different ways. For example, some animals are able to detect
electrical field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
s and
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
s,
air moisture (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous phas ...
, or
polarized light Polarization (also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the ...
. Others sense and perceive through alternative systems such as echolocation. Recent theory suggests that plants and
artificial agents In artificial intelligence, an intelligent agent (IA) is anything which perceives its environment, takes actions autonomously in order to achieve goals, and may improve its performance with machine learning, learning or may use knowledge represe ...
such as
robot A robot is a machine—especially one programmable by a computer—capable of carrying out a complex series of actions automatically. A robot can be guided by an external control device, or the control may be embedded within. Robots may be c ...
s may be able to detect and interpret environmental information in an analogous manner to animals.


Sensory modalities

Sensory modality refers to the way that information is encoded, which is similar to the idea of transduction. The main sensory modalities can be described on the basis of how each is transduced. Listing all the different sensory modalities, which can number as many as 17, involves separating the major senses into more specific categories, or submodalities, of the larger sense. An individual sensory modality represents the sensation of a specific type of stimulus. For example, the general sensation and perception of touch, which is known as somatosensation, can be separated into light pressure, deep pressure, vibration, itch, pain, temperature, or hair movement, while the general sensation and perception of taste can be separated into submodalities of
sweet Sweetness is a basic taste most commonly perceived when eating foods rich in sugars. Sweet tastes are generally regarded as pleasurable. In addition to sugars like sucrose, many other chemical compounds are sweet, including aldehydes, ketones ...
, salty,
sour The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception produced or stimulated when a substance in the mouth reacts chemically with taste receptor ...
,
bitter Bitter may refer to: Common uses * Resentment, negative emotion or attitude, similar to being jaded, cynical or otherwise negatively affected by experience * Bitter (taste), one of the five basic tastes Books * '' Bitter (novel)'', a 2022 nove ...
, spicy, and
umami Umami ( from ja, 旨味 ), or savoriness, is one of the five basic tastes. It has been described as savory and is characteristic of broths and cooked meats. People taste umami through taste receptors that typically respond to glutamates and ...
, all of which are based on different chemicals binding to
sensory neuron Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cell ...
s.


Receptors

Sensory receptors are the cells or structures that detect sensations.
Stimuli A stimulus is something that causes a physiological response. It may refer to: *Stimulation **Stimulus (physiology), something external that influences an activity **Stimulus (psychology), a concept in behaviorism and perception *Stimulus (economi ...
in the environment activate specialized receptor cells in the
peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system of bilateral animals, with the other part being the central nervous system (CNS). The PNS consists of nerves and ganglia, which lie outside the brain ...
. During transduction, physical stimulus is converted into
action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...
by receptors and transmitted towards the
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...
for processing. Different types of stimuli are sensed by different types of
receptor cells Receptor may refer to: *Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a n ...
. Receptor cells can be classified into types on the basis of three different criteria:
cell type A cell type is a classification used to identify cells that share morphological or phenotypical features. A multicellular organism may contain cells of a number of widely differing and specialized cell types, such as muscle cells and skin cells, ...
, position, and function. Receptors can be classified structurally on the basis of cell type and their position in relation to stimuli they sense. Receptors can further be classified functionally on the basis of the transduction of stimuli, or how the mechanical stimulus, light, or chemical changed the cell
membrane potential Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological cell. That is, there is a difference in the energy required for electric charges ...
.


Structural receptor types


= Location

= One way to classify receptors is based on their location relative to the stimuli. An exteroceptor is a receptor that is located near a stimulus of the external environment, such as the somatosensory receptors that are located in the skin. An
interoceptor Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cell ...
is one that interprets stimuli from internal organs and tissues, such as the receptors that sense the increase in blood pressure in the
aorta The aorta ( ) is the main and largest artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries). The aorta distributes ...
or
carotid sinus In human anatomy, the carotid sinus is a dilated area at the base of the internal carotid artery just superior to the bifurcation of the internal carotid and external carotid at the level of the superior border of thyroid cartilage. The carotid ...
.


= Cell type

= The cells that interpret information about the environment can be either (1) a
neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...
that has a
free nerve ending A free nerve ending (FNE) or bare nerve ending, is an unspecialized, afferent nerve fiber sending its signal to a sensory neuron. ''Afferent'' in this case means bringing information from the body's periphery toward the brain. They function as cu ...
, with
dendrite Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the n ...
s embedded in tissue that would receive a sensation; (2) a neuron that has an encapsulated ending in which the sensory nerve endings are encapsulated in
connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
that enhances their sensitivity; or (3) a specialized
receptor cell Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cel ...
, which has distinct structural components that interpret a specific type of stimulus. The
pain Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, ...
and temperature receptors in the dermis of the skin are examples of neurons that have free nerve endings (1). Also located in the dermis of the skin are lamellated corpuscles, neurons with encapsulated nerve endings that respond to pressure and touch (2). The cells in the retina that respond to light stimuli are an example of a specialized receptor (3), a photoreceptor. A
transmembrane protein A transmembrane protein (TP) is a type of integral membrane protein that spans the entirety of the cell membrane. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequentl ...
receptor is a protein in the
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...
that mediates a physiological change in a neuron, most often through the opening of
ion channel Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of io ...
s or changes in the
cell signaling In biology, cell signaling (cell signalling in British English) or cell communication is the ability of a cell to receive, process, and transmit signals with its environment and with itself. Cell signaling is a fundamental property of all cellula ...
processes. Transmembrane receptors are activated by chemicals called
ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...
s. For example, a molecule in food can serve as a ligand for taste receptors. Other transmembrane proteins, which are not accurately called receptors, are sensitive to mechanical or thermal changes. Physical changes in these proteins increase ion flow across the membrane, and can generate an
action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...
or a
graded potential Graded potentials are changes in membrane potential that vary in size, as opposed to being all-or-none. They include diverse potentials such as receptor potentials, electrotonic potentials, subthreshold membrane potential oscillations, slow-wave ...
in the
sensory neuron Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cell ...
s.


Functional receptor types

A third classification of receptors is by how the receptor transduces stimuli into
membrane potential Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological cell. That is, there is a difference in the energy required for electric charges ...
changes. Stimuli are of three general types. Some stimuli are ions and
macromolecule A macromolecule is a very large molecule important to biophysical processes, such as a protein or nucleic acid. It is composed of thousands of covalently bonded atoms. Many macromolecules are polymers of smaller molecules called monomers. The ...
s that affect transmembrane receptor proteins when these chemicals diffuse across the cell membrane. Some stimuli are physical variations in the environment that affect receptor cell membrane potentials. Other stimuli include the electromagnetic radiation from visible light. For humans, the only electromagnetic energy that is perceived by our eyes is visible light. Some other organisms have receptors that humans lack, such as the heat sensors of snakes, the ultraviolet light sensors of bees, or magnetic receptors in migratory birds. Receptor cells can be further categorized on the basis of the type of stimuli they transduce. The different types of functional receptor cell types are
mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, ...
s, photoreceptors,
chemoreceptor A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance (endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorecept ...
s (
osmoreceptor An osmoreceptor is a sensory receptor primarily found in the hypothalamus of most homeothermic organisms that detects changes in osmotic pressure. Osmoreceptors can be found in several structures, including two of the circumventricular organs – t ...
),
thermoreceptor A thermoreceptor is a non-specialised sense receptor, or more accurately the receptive portion of a sensory neuron, that codes absolute and relative changes in temperature, primarily within the innocuous range. In the mammalian peripheral nervous s ...
s,
electroreceptor Electroreception and electrogenesis are the closely-related biological abilities to perceive electrical stimuli and to generate electric fields. Both are used to locate prey; stronger electric discharges are used in a few groups of fishes to stu ...
s (in certain mammals and fish), and
nociceptor A nociceptor ("pain receptor" from Latin ''nocere'' 'to harm or hurt') is a sensory neuron that responds to damaging or potentially damaging stimuli by sending "possible threat" signals to the spinal cord and the brain. The brain creates the sens ...
s. Physical stimuli, such as pressure and vibration, as well as the sensation of sound and body position (balance), are interpreted through a mechanoreceptor. Photoreceptors convert light (visible
electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic field, electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, inf ...
) into signals. Chemical stimuli can be interpreted by a chemoreceptor that interprets chemical stimuli, such as an object's taste or smell, while osmoreceptors respond to a chemical solute concentrations of body fluids. Nociception (pain) interprets the presence of tissue damage, from sensory information from mechano-, chemo-, and thermoreceptors. Another physical stimulus that has its own type of receptor is temperature, which is sensed through a
thermoreceptor A thermoreceptor is a non-specialised sense receptor, or more accurately the receptive portion of a sensory neuron, that codes absolute and relative changes in temperature, primarily within the innocuous range. In the mammalian peripheral nervous s ...
that is either sensitive to temperatures above (heat) or below (cold) normal body temperature.


Thresholds


Absolute threshold

Each
sense organ A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of Stimulus (physiology), stimuli. (For example, in the human body, the brain which is part of the cen ...
(eyes or nose, for instance) requires a minimal amount of stimulation in order to detect a stimulus. This minimum amount of stimulus is called the absolute threshold. The absolute threshold is defined as the minimum amount of stimulation necessary for the detection of a stimulus 50% of the time. Absolute threshold is measured by using a method called
signal detection Detection theory or signal detection theory is a means to measure the ability to differentiate between information-bearing patterns (called Stimulus (psychology), stimulus in living organisms, Signal (electronics), signal in machines) and random pa ...
. This process involves presenting stimuli of varying intensities to a subject in order to determine the level at which the subject can reliably detect stimulation in a given sense.


Differential threshold

Differential threshold or just noticeable difference (JDS) is the smallest detectable difference between two stimuli, or the smallest difference in stimuli that can be judged to be different from each other. Weber's Law is an empirical law that states that the difference threshold is a constant fraction of the comparison stimulus. According to Weber's Law, bigger stimuli require larger differences to be noticed. Magnitude estimation is a psychophysical method in which subjects assign perceived values of given stimuli. The relationship between stimulus intensity and perceptive intensity is described by
Steven's power law Stevens' power law is an empirical relationship in psychophysics between an increased intensity or strength in a physical stimulus and the perceived magnitude increase in the sensation created by the stimulus. It is often considered to supersede ...
.


Signal detection theory

Signal detection theory quantifies the experience of the subject to the presentation of a stimulus in the presence of
noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...
. There is internal noise and there is external noise when it comes to signal detection. The internal noise originates from static in the nervous system. For example, an individual with closed eyes in a dark room still sees something—a blotchy pattern of grey with intermittent brighter flashes—this is internal noise. External noise is the result of noise in the environment that can interfere with the detection of the stimulus of interest. Noise is only a problem if the magnitude of the noise is large enough to interfere with signal collection. The
nervous system In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes th ...
calculates a criterion, or an internal threshold, for the detection of a signal in the presence of noise. If a signal is judged to be above the criterion, thus the signal is differentiated from the noise, the signal is sensed and perceived. Errors in signal detection can potentially lead to
false positives and false negatives A false positive is an error in binary classification in which a test result incorrectly indicates the presence of a condition (such as a disease when the disease is not present), while a false negative is the opposite error, where the test result ...
. The sensory criterion might be shifted based on the importance of the detecting the signal. Shifting of the criterion may influence the likelihood of false positives and false negatives.


Private perceptive experience

Subjective visual and auditory experiences appear to be similar across humans subjects. The same cannot be said about taste. For example, there is a molecule called
propylthiouracil Propylthiouracil (PTU) is a medication used to treat hyperthyroidism. This includes hyperthyroidism due to Graves' disease and toxic multinodular goiter. In a thyrotoxic crisis it is generally more effective than methimazole. Otherwise it is t ...
(PROP) that some humans experience as bitter, some as almost tasteless, while others experience it as somewhere between tasteless and bitter. There is a genetic basis for this difference between perception given the same sensory stimulus. This subjective difference in taste perception has implications for individuals' food preferences, and consequently, health.


Sensory adaptation

When a stimulus is constant and unchanging, perceptual sensory adaptation occurs. During this process, the subject becomes less sensitive to the stimulus.


Fourier analysis

Biological auditory (hearing), vestibular and spatial, and visual systems (vision) appear to break down real-world complex stimuli into
sine wave A sine wave, sinusoidal wave, or just sinusoid is a curve, mathematical curve defined in terms of the ''sine'' trigonometric function, of which it is the graph of a function, graph. It is a type of continuous wave and also a Smoothness, smooth p ...
components, through the mathematical process called Fourier analysis. Many neurons have a strong preference for certain sine
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
components in contrast to others. The way that simpler sounds and images are
encoded In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communication ...
during sensation can provide insight into how perception of real-world objects happens.


Sensory neuroscience and the biology of perception

Perception occurs when
nerve A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...
s that lead from the
sensory organs A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of stimuli. (For example, in the human body, the brain which is part of the central nervous system rec ...
(e.g. eye) to the brain are stimulated, even if that stimulation is unrelated to the target signal of the sensory organ. For example, in the case of the eye, it does not matter whether light or something else stimulates the optic nerve, that stimulation will results in visual perception, even if there was no visual stimulus to begin with. (To prove this point to yourself (and if you are a human), close your eyes (preferably in a dark room) and press gently on the outside corner of one eye through the eyelid. You will see a visual spot toward the inside of your visual field, near your nose.)


Sensory nervous system

All stimuli received by the
receptors Receptor may refer to: *Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a n ...
are transduced to an
action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...
, which is carried along one or more afferent
neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...
s towards a specific area (
cortex Cortex or cortical may refer to: Biology * Cortex (anatomy), the outermost layer of an organ ** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain'' *** Motor cortex, the regions of the cerebral cortex i ...
) of the
brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...
. Just as different nerves are dedicated to sensory and motors tasks, different areas of the brain (cortices) are similarly dedicated to different sensory and
perceptual Perception () is the organization, identification, and interpretation of sense, sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous ...
tasks. More complex processing is accomplished across primary cortical regions that spread beyond the primary cortices. Every nerve, sensory or
motor An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power gen ...
, has its own signal transmission speed. For example, nerves in the frog's legs have a 90 ft/s (99 km/h) signal transmission speed, while sensory nerves in humans, transmit sensory information at speeds between 165 ft/s (181 km/h) and 330 ft/s (362 km/h).


Multimodal perception

Perceptual experience is often multimodal. Multimodality integrates different senses into one unified perceptual experience. Information from one sense has the potential to influence how information from another is perceived. Multimodal perception is qualitatively different from unimodal perception. There has been a growing body of evidence since the mid-1990s on the neural correlates of multimodal perception.Calvert, G.A., Hansen, P.C., Iversen, S.D. and Brammer, M.J., 2001. Detection of audio-visual integration sites in humans by application of electrophysiological criteria to the BOLD effect. Neuroimage, 14(2), pp. 427–438.


Philosophy

The philosophy of perception is concerned with the nature of
perceptual experience Experience refers to Consciousness, conscious events in general, more specifically to perceptions, or to the practical knowledge and familiarity that is produced by these conscious processes. Understood as a conscious event in the widest sense, ex ...
and the status of perceptual data, in particular how they relate to beliefs about, or knowledge of, the world. Historical inquiries into the underlying mechanisms of sensation and perception have led early researchers to subscribe to various philosophical interpretations of perception and the
mind The mind is the set of faculties responsible for all mental phenomena. Often the term is also identified with the phenomena themselves. These faculties include thought, imagination, memory, will, and sensation. They are responsible for various m ...
, including
panpsychism In the philosophy of mind, panpsychism () is the view that the mind or a mindlike aspect is a fundamental and ubiquitous feature of reality. It is also described as a theory that "the mind is a fundamental feature of the world which exists thro ...
,
dualism Dualism most commonly refers to: * Mind–body dualism, a philosophical view which holds that mental phenomena are, at least in certain respects, not physical phenomena, or that the mind and the body are distinct and separable from one another ** ...
, and
materialism Materialism is a form of philosophical monism which holds matter to be the fundamental substance in nature, and all things, including mental states and consciousness, are results of material interactions. According to philosophical materiali ...
. The majority of modern scientists who study sensation and perception take on a materialistic view of the mind.


Human sensation


General


Absolute threshold

Some examples of
human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
absolute threshold In neuroscience and psychophysics, an absolute threshold was originally defined as the lowest level of a stimulus – light, sound, touch, etc. – that an organism could detect. Under the influence of signal detection theory, absolute threshold ...
s for the nine to 21 external senses.


Multimodal perception

Human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
s respond more strongly to multimodal stimuli compared to the sum of each single modality together, an effect called the ''superadditive effect of multisensory integration''. Neurons that respond to both visual and auditory stimuli have been identified in the
superior temporal sulcus The superior temporal sulcus (STS) is the sulcus separating the superior temporal gyrus from the middle temporal gyrus in the temporal lobe of the brain. A sulcus (plural sulci) is a deep groove that curves into the largest part of the brain, t ...
. Additionally, multimodal "what" and "where" pathways have been proposed for auditory and tactile stimuli.


External

External receptors that respond to stimuli from outside the body are called exteroceptors. Human external sensation is based on the sensory organs of the
eyes Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conve ...
,
ears An ear is the organ that enables hearing and, in mammals, body balance using the vestibular system. In mammals, the ear is usually described as having three parts—the outer ear, the middle ear and the inner ear. The outer ear consists o ...
,
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...
,
vestibular system The vestibular system, in vertebrates, is a sensory system that creates the sense of balance and spatial orientation for the purpose of coordinating movement with balance. Together with the cochlea, a part of the auditory system, it constitutes ...
,
nose A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for respiration alongside the mouth. Behind the nose are the olfactory mucosa and the sinuses. Behind the nasal cavity, air next passes th ...
, and
mouth In animal anatomy, the mouth, also known as the oral cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on ...
, which contribute, respectively, to the sensory
perception Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system ...
s of
vision Vision, Visions, or The Vision may refer to: Perception Optical perception * Visual perception, the sense of sight * Visual system, the physical mechanism of eyesight * Computer vision, a field dealing with how computers can be made to gain un ...
,
hearing Hearing, or auditory perception, is the ability to perceive sounds In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psycholog ...
,
touch In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch (haptic perception), as well as temperature (thermoception), body position (proprioception), and pain. It is ...
,
spatial orientation In geometry, the orientation, angular position, attitude, bearing, or direction of an object such as a line, plane or rigid body is part of the description of how it is placed in the space it occupies. More specifically, it refers to the imagina ...
,
smell Smell may refer to; * Odor, airborne molecules perceived as a scent or aroma * Sense of smell, the scent also known scientifically as olfaction * "Smells" (''Bottom''), an episode of ''Bottom'' * The Smell, a music venue in Los Angeles, Californ ...
, and
taste The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception produced or stimulated when a substance in the mouth reacts chemically with taste receptor ...
. Smell and taste are both responsible for identifying molecules and thus both are types of
chemoreceptor A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance (endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorecept ...
s. Both olfaction (smell) and gustation (taste) require the transduction of chemical stimuli into electrical potentials.


Visual system (vision)

The visual system, or sense of sight, is based on the transduction of light stimuli received through the eyes and contributes to
visual perception Visual perception is the ability to interpret the surrounding environment through photopic vision (daytime vision), color vision, scotopic vision (night vision), and mesopic vision (twilight vision), using light in the visible spectrum reflecte ...
. The visual system detects
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
on photoreceptors in the
retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
of each eye that generates electrical
nerve impulses An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...
for the perception of varying colors and brightness. There are two types of photoreceptors: rods and
cones A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex. A cone is formed by a set of line segments, half-lines, or lines conn ...
. Rods are very sensitive to light but do not distinguish colors. Cones distinguish colors but are less sensitive to dim light. At the molecular level, visual stimuli cause changes in the photopigment molecule that lead to changes in membrane potential of the photoreceptor cell. A single unit of light is called a
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
, which is described in physics as a packet of energy with properties of both a particle and a wave. The
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...
of a photon is represented by its
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
, with each wavelength of visible light corresponding to a particular
color Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associ ...
. Visible light is
electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic field, electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, inf ...
with a wavelength between 380 and 720 nm. Wavelengths of electromagnetic radiation longer than 720 nm fall into the
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
range, whereas wavelengths shorter than 380 nm fall into the
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
range. Light with a wavelength of 380 nm is
blue Blue is one of the three primary colours in the RYB colour model (traditional colour theory), as well as in the RGB (additive) colour model. It lies between violet and cyan on the spectrum of visible light. The eye perceives blue when obs ...
whereas light with a wavelength of 720 nm is dark
red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a secondar ...
. All other colors fall between red and blue at various points along the wavelength scale. The three types of cone
opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most pro ...
s, being sensitive to different wavelengths of light, provide us with color vision. By comparing the activity of the three different cones, the brain can extract color information from visual stimuli. For example, a bright blue light that has a wavelength of approximately 450 nm would activate the "red" cones minimally, the "green" cones marginally, and the "blue" cones predominantly. The relative activation of the three different cones is calculated by the brain, which perceives the color as blue. However, cones cannot react to low-intensity light, and rods do not sense the color of light. Therefore, our low-light vision is—in essence—in
grayscale In digital photography, computer-generated imagery, and colorimetry, a grayscale image is one in which the value of each pixel is a single sample representing only an ''amount'' of light; that is, it carries only intensity information. Graysca ...
. In other words, in a dark room, everything appears as a shade of
gray Grey (more common in British English) or gray (more common in American English) is an intermediate color between black and white. It is a neutral or achromatic color, meaning literally that it is "without color", because it can be composed o ...
. If you think that you can see colors in the dark, it is most likely because your brain knows what color something is and is relying on that memory. There is some disagreement as to whether the visual system consists of one, two, or three submodalities. Neuroanatomists generally regard it as two submodalities, given that different receptors are responsible for the perception of color and brightness. Some argue that
stereopsis Stereopsis () is the component of depth perception retrieved through binocular vision. Stereopsis is not the only contributor to depth perception, but it is a major one. Binocular vision happens because each eye receives a different image becaus ...
, the perception of depth using both eyes, also constitutes a sense, but it is generally regarded as a cognitive (that is, post-sensory) function of the
visual cortex The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalamus and ...
of the brain where patterns and objects in
images An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensiona ...
are recognized and interpreted based on previously learned information. This is called
visual memory Visual memory describes the relationship between perceptual processing and the encoding, storage and retrieval of the resulting neural representations. Visual memory occurs over a broad time range spanning from eye movements to years in order ...
. The inability to see is called
blindness Visual impairment, also known as vision impairment, is a medical definition primarily measured based on an individual's better eye visual acuity; in the absence of treatment such as correctable eyewear, assistive devices, and medical treatment ...
. Blindness may result from damage to the eyeball, especially to the retina, damage to the optic nerve that connects each eye to the brain, and/or from
stroke A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop functionin ...
(
infarct Infarction is tissue death (necrosis) due to Ischemia, inadequate blood supply to the affected area. It may be caused by Thrombosis, artery blockages, rupture, mechanical compression, or vasoconstriction. The resulting lesion is referred to as a ...
s in the brain). Temporary or permanent blindness can be caused by poisons or medications. People who are blind from degradation or damage to the visual cortex, but still have functional eyes, are actually capable of some level of vision and reaction to visual stimuli but not a conscious perception; this is known as
blindsight Blindsight is the ability of people who are cortically blind to respond to visual stimuli that they do not consciously see due to lesions in the primary visual cortex, also known as the striate cortex or Brodmann Area 17. The term was coined by ...
. People with blindsight are usually not aware that they are reacting to visual sources, and instead just unconsciously adapt their behavior to the stimulus. On February 14, 2013, researchers developed a
neural implant Brain implants, often referred to as neural implants, are technological devices that connect directly to a biological subject's brain – usually placed on the surface of the brain, or attached to the brain's Cerebral cortex, cortex. A common purpo ...
that gives
rat Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus ''Rattus''. Other rat genera include ''Neotoma'' ( pack rats), ''Bandicota'' (bandicoot ...
s the ability to sense
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
light which for the first time provides
living creatures In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fungi; ...
with new abilities, instead of simply replacing or augmenting existing abilities.


= Visual perception in psychology

= According to Gestalt Psychology, people perceive the whole of something even if it is not there. The Gestalt's Law of Organization states that people have seven factors that help to group what is seen into patterns or groups: Common Fate, Similarity, Proximity, Closure, Symmetry, Continuity, and Past Experience. The Law of Common fate says that objects are led along the smoothest path. People follow the trend of motion as the lines/dots flow. The Law of Similarity refers to the grouping of images or objects that are similar to each other in some aspect. This could be due to shade, colour, size, shape, or other qualities you could distinguish. The Law of Proximity states that our minds like to group based on how close objects are to each other. We may see 42 objects in a group, but we can also perceive three groups of two lines with seven objects in each line. The Law of Closure is the idea that we as humans still see a full picture even if there are gaps within that picture. There could be gaps or parts missing from a section of a shape, but we would still perceive the shape as whole. The Law of Symmetry refers to a person's preference to see symmetry around a central point. An example would be when we use parentheses in writing. We tend to perceive all of the words in the parentheses as one section instead of individual words within the parentheses. The Law of Continuity tells us that objects are grouped together by their elements and then perceived as a whole. This usually happens when we see overlapping objects. We will see the overlapping objects with no interruptions. The Law of Past Experience refers to the tendency humans have to categorize objects according to past experiences under certain circumstances. If two objects are usually perceived together or within close proximity of each other the Law of Past Experience is usually seen.


Auditory system (hearing)

Hearing, or audition, is the transduction of
sound waves In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the ...
into a neural signal that is made possible by the structures of the
ear An ear is the organ that enables hearing and, in mammals, body balance using the vestibular system. In mammals, the ear is usually described as having three parts—the outer ear, the middle ear and the inner ear. The outer ear consists of ...
. The large, fleshy structure on the lateral aspect of the head is known as the auricle. At the end of the
auditory canal The ear canal (external acoustic meatus, external auditory meatus, EAM) is a pathway running from the outer ear to the middle ear. The adult human ear canal extends from the pinna to the eardrum and is about in length and in diameter. Struc ...
is the tympanic membrane, or
ear drum In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear from the middle ear. Its function is to transmit sound from the air ...
, which vibrates after it is struck by sound waves. The auricle, ear canal, and tympanic membrane are often referred to as the
external ear The outer ear, external ear, or auris externa is the external part of the ear, which consists of the auricle (also pinna) and the ear canal. It gathers sound energy and focuses it on the eardrum (tympanic membrane). Structure Auricle Th ...
. The
middle ear The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear). The mammalian middle ear contains three ossicles, which transfer the vibrations of the eardrum into waves in the ...
consists of a space spanned by three small bones called the
ossicles The ossicles (also called auditory ossicles) are three bones in either middle ear that are among the smallest bones in the human body. They serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea). The absence of the auditory ...
. The three ossicles are the
malleus The malleus, or hammer, is a hammer-shaped small bone or ossicle of the middle ear. It connects with the incus, and is attached to the inner surface of the eardrum. The word is Latin for 'hammer' or 'mallet'. It transmits the sound vibrations fro ...
,
incus The ''incus'' (plural incudes) or anvil is a bone in the middle ear. The anvil-shaped small bone is one of three ossicles in the middle ear. The ''incus'' receives vibrations from the ''malleus'', to which it is connected laterally, and transmit ...
, and
stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other animals which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the foot ...
, which are Latin names that roughly translate to hammer, anvil, and stirrup. The malleus is attached to the tympanic membrane and articulates with the incus. The incus, in turn, articulates with the stapes. The stapes is then attached to the
inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
, where the sound waves will be transduced into a neural signal. The middle ear is connected to the
pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struc ...
through the
Eustachian tube In anatomy, the Eustachian tube, also known as the auditory tube or pharyngotympanic tube, is a tube that links the nasopharynx to the middle ear, of which it is also a part. In adult humans, the Eustachian tube is approximately long and in d ...
, which helps equilibrate air pressure across the tympanic membrane. The tube is normally closed but will pop open when the muscles of the pharynx contract during
swallowing Swallowing, sometimes called deglutition in scientific contexts, is the process in the human or animal body that allows for a substance to pass from the mouth, to the pharynx, and into the esophagus, while shutting the epiglottis. Swallowing i ...
or
yawn A yawn is a reflex lasting 4-7 seconds, and is characterized by a long inspiratory phase with gradual mouth gaping, followed by a brief climax (or acme) with muscle stretching, and a rapid expiratory phase with muscle relaxation. For fish an ...
ing. Mechanoreceptors turn motion into electrical nerve pulses, which are located in the inner ear. Since sound is vibration, propagating through a medium such as air, the detection of these vibrations, that is the sense of the hearing, is a mechanical sense because these vibrations are mechanically conducted from the eardrum through a series of tiny bones to hair-like fibers in the
inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
, which detect mechanical motion of the fibers within a range of about 20 to 20,000 
hertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that on ...
, with substantial variation between individuals. Hearing at high frequencies declines with an increase in age. Inability to hear is called
deafness Deafness has varying definitions in cultural and medical contexts. In medical contexts, the meaning of deafness is hearing loss that precludes a person from understanding spoken language, an audiological condition. In this context it is written ...
or hearing impairment. Sound can also be detected as vibrations conducted through the body by tactition. Lower frequencies that can be heard are detected this way. Some deaf people are able to determine the direction and location of vibrations picked up through the feet. Studies pertaining to Audition started to increase in number towards the latter end of the nineteenth century. During this time, many laboratories in the United States began to create new models, diagrams, and instruments that all pertained to the ear. There is a branch of Cognitive Psychology dedicated strictly to Audition. They call it Auditory Cognitive Psychology. The main point is to understand why humans are able to use sound in thinking outside of actually saying it. Relating to Auditory Cognitive Psychology is Psychoacoustics. Psychoacoustics is more pointed to people interested in music. Haptics, a word used to refer to both taction and kinesthesia, has many parallels with psychoacoustics. Most research around these two are focused on the instrument, the listener, and the player of the instrument.


Somatosensory system (touch)

Somatosensation is considered a general sense, as opposed to the special senses discussed in this section. Somatosensation is the group of sensory modalities that are associated with touch and interoception. The modalities of somatosensation include
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
,
vibration Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic function, periodic, such as the motion of a pendulum ...
, light touch,
tickle Tickling is the act of touching a part of a body in a way that causes involuntary twitching movements or laughter. The word evolved from the Middle English ''tikelen'', perhaps frequentative of ''ticken'', to touch lightly. In 1897, psychol ...
,
itch Itch (also known as pruritus) is a sensation that causes the desire or reflex to scratch. Itch has resisted many attempts to be classified as any one type of sensory experience. Itch has many similarities to pain, and while both are unpleasant ...
,
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
,
pain Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, ...
,
kinesthesia Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by proprioceptors, mechanosensory neurons ...
.
Somatosensation In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch (haptic perception), as well as temperature (thermoception), body position (proprioception), and pain. It is ...
, also called
tactition In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch (haptic perception), as well as temperature (thermoception), body position (proprioception), and pain. It is ...
(adjectival form: tactile) is a perception resulting from activation of neural
receptors Receptor may refer to: *Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a n ...
, generally in the
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...
including
hair follicle The hair follicle is an organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction between h ...
s, but also in the
tongue The tongue is a muscular organ (anatomy), organ in the mouth of a typical tetrapod. It manipulates food for mastication and swallowing as part of the digestive system, digestive process, and is the primary organ of taste. The tongue's upper surfa ...
,
throat In vertebrate anatomy, the throat is the front part of the neck, internally positioned in front of the vertebrae. It contains the pharynx and larynx. An important section of it is the epiglottis, separating the esophagus from the trachea (windpipe ...
, and
mucosa A mucous membrane or mucosa is a membrane that lines various cavities in the body of an organism and covers the surface of internal organs. It consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It is ...
. A variety of
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
receptors respond to variations in pressure (firm, brushing, sustained, etc.). The touch sense of
itch Itch (also known as pruritus) is a sensation that causes the desire or reflex to scratch. Itch has resisted many attempts to be classified as any one type of sensory experience. Itch has many similarities to pain, and while both are unpleasant ...
ing caused by insect bites or allergies involves special itch-specific neurons in the skin and spinal cord. The loss or impairment of the ability to feel anything touched is called tactile
anesthesia Anesthesia is a state of controlled, temporary loss of sensation or awareness that is induced for medical or veterinary purposes. It may include some or all of analgesia (relief from or prevention of pain), paralysis (muscle relaxation), ...
.
Paresthesia Paresthesia is an abnormal sensation of the skin (tingling, pricking, chilling, burning, numbness) with no apparent physical cause. Paresthesia may be transient or chronic, and may have any of dozens of possible underlying causes. Paresthesias ar ...
is a sensation of tingling, pricking, or
numbness Hypoesthesia or numbness is a common side effect of various medical conditions that manifests as a reduced sense of touch or sensation, or a partial loss of sensitivity to sensory stimuli. In everyday speech this is generally referred to as num ...
of the skin that may result from nerve damage and may be permanent or temporary. Two types of somatosensory signals that are transduced by
free nerve ending A free nerve ending (FNE) or bare nerve ending, is an unspecialized, afferent nerve fiber sending its signal to a sensory neuron. ''Afferent'' in this case means bringing information from the body's periphery toward the brain. They function as cu ...
s are pain and temperature. These two modalities use
thermoreceptor A thermoreceptor is a non-specialised sense receptor, or more accurately the receptive portion of a sensory neuron, that codes absolute and relative changes in temperature, primarily within the innocuous range. In the mammalian peripheral nervous s ...
s and
nociceptor A nociceptor ("pain receptor" from Latin ''nocere'' 'to harm or hurt') is a sensory neuron that responds to damaging or potentially damaging stimuli by sending "possible threat" signals to the spinal cord and the brain. The brain creates the sens ...
s to transduce temperature and pain stimuli, respectively. Temperature receptors are stimulated when local temperatures differ from
body temperature Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...
. Some thermoreceptors are sensitive to just cold and others to just heat. Nociception is the sensation of potentially damaging stimuli. Mechanical, chemical, or thermal stimuli beyond a set threshold will elicit painful sensations. Stressed or damaged tissues release chemicals that activate receptor proteins in the nociceptors. For example, the sensation of heat associated with spicy foods involves
capsaicin Capsaicin (8-methyl-''N''-vanillyl-6-nonenamide) ( or ) is an active component of chili peppers, which are plants belonging to the genus ''Capsicum''. It is a chemical irritant for mammals, including humans, and produces a sensation of burning ...
, the active molecule in hot peppers. Low frequency vibrations are sensed by mechanoreceptors called
Merkel cell Merkel cells, also known as Merkel-Ranvier cells or tactile epithelial cells, are oval-shaped mechanoreceptors essential for light touch sensation and found in the skin of vertebrates. They are abundant in highly sensitive skin like that of the f ...
s, also known as type I cutaneous mechanoreceptors. Merkel cells are located in the
stratum basale The ''stratum basale'' (basal layer, sometimes referred to as ''stratum germinativum'') is the deepest layer of the five layers of the epidermis, the external covering of skin in mammals. The ''stratum basale'' is a single layer of columnar or ...
of the
epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rele ...
. Deep pressure and vibration is transduced by lamellated (
Pacinian Pacinian corpuscle or lamellar corpuscle or Vater-Pacini corpuscle; is one of the four major types of mechanoreceptors (specialized nerve ending with adventitious tissue for mechanical sensation) found in mammalian skin. This type of mechanorecept ...
) corpuscles, which are receptors with encapsulated endings found deep in the dermis, or subcutaneous tissue. Light touch is transduced by the encapsulated endings known as tactile (
Meissner Meissner, Meißner or Meisner may refer to: Geography Meissner is the name of the following geographic features: * the Meißner (range), an important mountain range in Hesse, Germany * Hoher Meißner, the highest peak of the Meißner range * Mei ...
) corpuscles. Follicles are also wrapped in a
plexus In neuroanatomy, a plexus (from the Latin term for "braid") is a branching network of vessels or nerves. The vessels may be blood vessels (veins, capillaries) or lymphatic vessels. The nerves are typically axons outside the central nervous syste ...
of nerve endings known as the hair follicle plexus. These nerve endings detect the movement of hair at the surface of the skin, such as when an insect may be walking along the
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...
. Stretching of the skin is transduced by stretch receptors known as
bulbous corpuscle The Bulbous corpuscle or Ruffini ending or Ruffini corpuscle is a slowly adapting mechanoreceptor located in the cutaneous tissue between the dermal papillae and the hypodermis. It is named after Angelo Ruffini Angelo Ruffini (Pretare of Arquata ...
s. Bulbous corpuscles are also known as Ruffini corpuscles, or type II cutaneous mechanoreceptors. The heat receptors are sensitive to infrared radiation and can occur in specialized organs, for instance in
pit vipers The Crotalinae, commonly known as pit vipers,Mehrtens JM (1987). ''Living Snakes of the World in Color''. New York: Sterling Publishers. 480 pp. . crotaline snakes (from grc, κρόταλον ''krotalon'' castanet), or pit adders, are a subfa ...
. The thermoceptors in the skin are quite different from the
homeostatic In biology, homeostasis (British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and i ...
thermoceptors in the brain (
hypothalamus The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...
), which provide feedback on internal body temperature.


Gustatory system (taste)

The gustatory system or the sense of taste is the
sensory system The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons (including the sensory receptor cells), neural pathways, and parts of the brain involved i ...
that is partially responsible for the
perception Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system ...
of taste (flavor). A few recognized submodalities exist within taste:
sweet Sweetness is a basic taste most commonly perceived when eating foods rich in sugars. Sweet tastes are generally regarded as pleasurable. In addition to sugars like sucrose, many other chemical compounds are sweet, including aldehydes, ketones ...
, salty,
sour The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception produced or stimulated when a substance in the mouth reacts chemically with taste receptor ...
,
bitter Bitter may refer to: Common uses * Resentment, negative emotion or attitude, similar to being jaded, cynical or otherwise negatively affected by experience * Bitter (taste), one of the five basic tastes Books * '' Bitter (novel)'', a 2022 nove ...
, and
umami Umami ( from ja, 旨味 ), or savoriness, is one of the five basic tastes. It has been described as savory and is characteristic of broths and cooked meats. People taste umami through taste receptors that typically respond to glutamates and ...
. Very recent research has suggested that there may also be a sixth taste submodality for fats, or lipids. The sense of taste is often confused with the perception of flavor, which is the results of the
multimodal integration Multisensory integration, also known as multimodal integration, is the study of how information from the different sensory modalities (such as sight, sound, touch, smell, self-motion, and taste) may be integrated by the nervous system. A coherent ...
of gustatory (taste) and olfactory (smell) sensations. Within the structure of the
lingual papillae Lingual papillae (singular papilla) are small structures on the upper surface of the tongue that give it its characteristic rough texture. The four types of papillae on the human tongue have different structures and are accordingly classified as c ...
are
taste bud Taste buds contain the taste receptor cells, which are also known as gustatory cells. The taste receptors are located around the small structures known as lingual papillae, papillae found on the upper surface of the tongue, soft palate, upper e ...
s that contain specialized gustatory receptor cells for the transduction of taste stimuli. These receptor cells are sensitive to the chemicals contained within foods that are ingested, and they release
neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell. Neuro ...
s based on the amount of the chemical in the food. Neurotransmitters from the gustatory cells can activate
sensory neuron Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cell ...
s in the
facial A facial is a family of skin care treatments for the face, including steam, exfoliation (physical and chemical), extraction, creams, lotions, facial masks, peels, and massage. They are normally performed in beauty salons, but are also a com ...
,
glossopharyngeal The glossopharyngeal nerve (), also known as the ninth cranial nerve, cranial nerve IX, or simply CN IX, is a cranial nerve that exits the brainstem from the sides of the upper Medulla oblongata, medulla, just anterior (closer to the nose) to t ...
, and
vagus The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and right v ...
cranial nerves Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...
. Salty and sour taste submodalities are triggered by the
cations An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
and , respectively. The other taste modalities result from food molecules binding to a
G protein–coupled receptor G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily-related p ...
. A G protein signal transduction system ultimately leads to
depolarization In biology, depolarization or hypopolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolarization is esse ...
of the gustatory cell. The sweet taste is the sensitivity of gustatory cells to the presence of
glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...
(or
sugar substitute A sugar substitute is a food additive that provides a sweetness like that of sugar while containing significantly less food energy than sugar-based sweeteners, making it a zero-calorie () or low-calorie sweetener. Artificial sweeteners may be d ...
s) dissolved in the
saliva Saliva (commonly referred to as spit) is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which DNA can be ...
. Bitter taste is similar to sweet in that food molecules bind to G protein–coupled receptors. The taste known as umami is often referred to as the savory taste. Like sweet and bitter, it is based on the activation of G protein–coupled receptors by a specific molecule. Once the gustatory cells are activated by the taste molecules, they release
neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell. Neuro ...
s onto the
dendrite Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the n ...
s of sensory neurons. These neurons are part of the facial and glossopharyngeal cranial nerves, as well as a component within the vagus nerve dedicated to the
gag reflex The pharyngeal reflex or gag reflex is a reflex muscular contraction of the back of the throat, evoked by touching the roof of the mouth, the back of the tongue, the area around the tonsils, the uvula, and the back of the throat. It, along with othe ...
. The facial nerve connects to taste buds in the anterior third of the tongue. The glossopharyngeal nerve connects to taste buds in the posterior two thirds of the tongue. The vagus nerve connects to taste buds in the extreme posterior of the tongue, verging on the
pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struc ...
, which are more sensitive to
noxious stimuli A noxious stimulus is a stimulus strong enough to threaten the body’s integrity (i.e. cause damage to tissue). Noxious stimulation induces peripheral afferents responsible for transducing pain (including A-delta and C- nerve fibers, as well as f ...
such as bitterness. Flavor depends on odor, texture, and temperature as well as on taste. Humans receive tastes through sensory organs called taste buds, or gustatory calyculi, concentrated on the upper surface of the tongue. Other tastes such as calcium and
free fatty acids In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, fr ...
may also be basic tastes but have yet to receive widespread acceptance. The inability to taste is called
ageusia Ageusia (from negative prefix ''a-'' and Ancient Greek ''γεῦσις'' geûsis 'taste') is the loss of taste functions of the tongue, particularly the inability to detect sweetness, sourness, bitterness, saltiness, and umami (meaning 'pleasant/ ...
. There is a rare phenomenon when it comes to the Gustatory sense. It is called Lexical-Gustatory Synesthesia. Lexical-Gustatory Synesthesia is when people can "taste" words. They have reported having flavor sensations they aren't actually eating. When they read words, hear words, or even imagine words. They have reported not only simple flavors, but textures, complex flavors, and temperatures as well.


Olfactory system (smell)

Like the sense of taste, the sense of smell, or the olfactiory system, is also responsive to chemical stimuli. Unlike taste, there are hundreds of
olfactory receptor Olfactory receptors (ORs), also known as odorant receptors, are chemoreceptors expressed in the cell membranes of olfactory receptor neurons and are responsible for the detection of odorants (for example, compounds that have an odor) which give ri ...
s (388 functional ones according to one 2003 study), each binding to a particular molecular feature. Odor molecules possess a variety of features and, thus, excite specific receptors more or less strongly. This combination of excitatory signals from different receptors makes up what humans perceive as the molecule's smell. The olfactory receptor neurons are located in a small region within the superior nasal cavity. This region is referred to as the
olfactory epithelium The olfactory epithelium is a specialized epithelial tissue inside the nasal cavity that is involved in smell. In humans, it measures 9 cm2 and lies on the roof of the nasal cavity about 7 cm above and behind the nostrils. The olfactory ...
and contains bipolar sensory neurons. Each olfactory sensory neuron has
dendrite Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the n ...
s that extend from the
apical surface The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (th ...
of the
epithelium Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...
into the
mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is ...
lining the cavity. As airborne molecules are inhaled through the
nose A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for respiration alongside the mouth. Behind the nose are the olfactory mucosa and the sinuses. Behind the nasal cavity, air next passes th ...
, they pass over the olfactory epithelial region and dissolve into the mucus. These odorant molecules bind to proteins that keep them dissolved in the mucus and help transport them to the olfactory dendrites. The odorant–protein complex binds to a receptor protein within the cell membrane of an olfactory dendrite. These receptors are G protein–coupled, and will produce a graded
membrane potential Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological cell. That is, there is a difference in the energy required for electric charges ...
in the
olfactory neurons An olfactory receptor neuron (ORN), also called an olfactory sensory neuron (OSN), is a sensory neuron within the olfactory system. Structure Humans have between 10 and 20 million olfactory receptor neurons (ORNs). In vertebrates, ORNs are b ...
. In the
brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...
, olfaction is processed by the
olfactory cortex The olfactory system, or sense of smell, is the sensory system used for smelling (olfaction). Olfaction is one of the special senses, that have directly associated specific organs. Most mammals and reptiles have a main olfactory system and an ac ...
. Olfactory receptor neurons in the nose differ from most other neurons in that they die and regenerate on a regular basis. The inability to smell is called
anosmia Anosmia, also known as smell blindness, is the loss of the ability to detect one or more smells. Anosmia may be temporary or permanent. It differs from hyposmia, which is a decreased sensitivity to some or all smells. Anosmia can be due to a num ...
. Some neurons in the nose are specialized to detect
pheromone A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...
s. Loss of the sense of smell can result in food tasting bland. A person with an impaired sense of smell may require additional
spice A spice is a seed, fruit, root, bark, or other plant substance primarily used for flavoring or coloring food. Spices are distinguished from herbs, which are the leaves, flowers, or stems of plants used for flavoring or as a garnish. Spices a ...
and
seasoning Seasoning is the process of supplementing food via herbs, spices, salts, and/or sugar, intended to enhance a particular flavour. General meaning Seasonings include herbs and spices, which are themselves frequently referred to as "seasonings". Ho ...
levels for food to be tasted. Anosmia may also be related to some presentations of mild depression, because the loss of enjoyment of food may lead to a general sense of despair. The ability of olfactory neurons to replace themselves decreases with age, leading to age-related anosmia. This explains why some elderly people salt their food more than younger people do. Causes of Olfactory dysfunction can be caused by age, exposure to toxic chemicals, viral infections, epilepsy, some sort of neurodegenerative disease, head trauma, or as a result of another disorder. As studies in olfaction have continued, there has been a positive correlation to its dysfunction or degeneration and early signs of Alzheimers and sporadic Parkinson's disease. Many patients don't notice the decline in smell before being tested. In Parkinson's Disease and Alzheimers, an olfactory deficit is present in 85 to 90% of the early onset cases. here is evidence that the decline of this sense can precede the Alzheimers or Parkinson's Disease by a couple years. Although the deficit is present in these two diseases, as well as others, it is important to make note that the severity or magnitude vary with every disease. This has brought to light some suggestions that olfactory testing could be used in some cases to aid in differentiating many of the neurodegenerative diseases. Those who were born without a sense of smell or have a damaged sense of smell usually complain about 1, or more, of 3 things. Our olfactory sense is also used as a warning against bad food. If the sense of smell is damaged or not there, it can lead to a person contracting food poisoning more often. Not having a sense of smell can also lead to damaged relationships or insecurities within the relationships because of the inability for the person to not smell body odor. Lastly, smell influences how food and drink taste. When the olfactory sense is damaged, the satisfaction from eating and drinking is not as prominent.


Internal


Vestibular system (balance)

The vestibular sense, or sense of balance (equilibrium), is the sense that contributes to the perception of balance (equilibrium), spatial orientation, direction, or acceleration (
equilibrioception The sense of balance or equilibrioception is the perception of balance and spatial orientation. It helps prevent humans and nonhuman animals from falling over when standing or moving. Equilibrioception is the result of a number of sensory systems ...
). Along with audition, the
inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
is responsible for encoding information about equilibrium. A similar
mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, ...
—a hair cell with
stereocilia Stereocilia (or stereovilli or villi) are non-motile apical cell modifications. They are distinct from cilia and microvilli, but are closely related to microvilli. They form single "finger-like" projections that may be branched, with normal cel ...
—senses head position, head movement, and whether our bodies are in motion. These cells are located within the
vestibule Vestibule or Vestibulum can have the following meanings, each primarily based upon a common origin, from early 17th century French, derived from Latin ''vestibulum, -i n.'' "entrance court". Anatomy In general, vestibule is a small space or cavity ...
of the inner ear. Head position is sensed by the utricle and
saccule The saccule is a bed of sensory cells in the inner ear. It translates head movements into neural impulses for the brain to interpret. The saccule detects linear accelerations and head tilts in the vertical plane. When the head moves verticall ...
, whereas head movement is sensed by the
semicircular canals The semicircular canals or semicircular ducts are three semicircular, interconnected tubes located in the innermost part of each ear, the inner ear. The three canals are the horizontal, superior and posterior semicircular canals. Structure The ...
. The neural signals generated in the
vestibular ganglion The vestibular ganglion (also called Scarpa's ganglion) is the ganglion of the vestibular nerve. It is located inside the internal auditory meatus. The ganglion contains the cell bodies of bipolar neurons whose peripheral processes form synapti ...
are transmitted through the
vestibulocochlear nerve The vestibulocochlear nerve or auditory vestibular nerve, also known as the eighth cranial nerve, cranial nerve VIII, or simply CN VIII, is a cranial nerve that transmits sound and equilibrium (balance) information from the inner ear to the bra ...
to the
brain stem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is conti ...
and
cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebel ...
. The semicircular canals are three ring-like extensions of the vestibule. One is oriented in the horizontal plane, whereas the other two are oriented in the vertical plane. The
anterior Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
and posterior vertical canals are oriented at approximately 45 degrees relative to the
sagittal plane The sagittal plane (; also known as the longitudinal plane) is an anatomical plane that divides the body into right and left sections. It is perpendicular to the transverse and coronal planes. The plane may be in the center of the body and divid ...
. The base of each semicircular canal, where it meets with the vestibule, connects to an enlarged region known as the
ampulla An ampulla (; ) was, in Ancient Rome, a small round vessel, usually made of glass and with two handles, used for sacred purposes. The word is used of these in archaeology, and of later flasks, often handle-less and much flatter, for holy water or ...
. The ampulla contains the hair cells that respond to rotational movement, such as turning the head while saying "no". The stereocilia of these hair cells extend into the cupula, a membrane that attaches to the top of the ampulla. As the head rotates in a plane parallel to the semicircular canal, the fluid lags, deflecting the cupula in the direction opposite to the head movement. The semicircular canals contain several ampullae, with some oriented horizontally and others oriented vertically. By comparing the relative movements of both the horizontal and vertical ampullae, the vestibular system can detect the direction of most head movements within three-dimensional ( 3D) space. The
vestibular nerve The vestibular nerve is one of the two branches of the vestibulocochlear nerve (the cochlear nerve being the other). In humans the vestibular nerve transmits sensory information transmitted by vestibular hair cells located in the two otolith organ ...
conducts information from sensory receptors in three
ampulla An ampulla (; ) was, in Ancient Rome, a small round vessel, usually made of glass and with two handles, used for sacred purposes. The word is used of these in archaeology, and of later flasks, often handle-less and much flatter, for holy water or ...
that sense motion of fluid in three
semicircular canal In mathematics (and more specifically geometry), a semicircle is a one-dimensional locus of points that forms half of a circle. The full arc of a semicircle always measures 180° (equivalently, radians, or a half-turn). It has only one line of ...
s caused by three-dimensional rotation of the head. The vestibular nerve also conducts information from the utricle and the
saccule The saccule is a bed of sensory cells in the inner ear. It translates head movements into neural impulses for the brain to interpret. The saccule detects linear accelerations and head tilts in the vertical plane. When the head moves verticall ...
, which contain hair-like sensory receptors that bend under the weight of
otolith An otolith ( grc-gre, ὠτο-, ' ear + , ', a stone), also called statoconium or otoconium or statolith, is a calcium carbonate structure in the saccule or utricle of the inner ear, specifically in the vestibular system of vertebrates. The sa ...
s (which are small crystals of calcium carbonate) that provide the inertia needed to detect head rotation, linear acceleration, and the direction of gravitational force.


Proprioception

Proprioception Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by proprioceptors, mechanosensory neurons ...
, the kinesthetic sense, provides the
parietal cortex The parietal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The parietal lobe is positioned above the temporal lobe and behind the frontal lobe and central sulcus. The parietal lobe integrates sensory informa ...
of the brain with information on the movement and relative positions of the parts of the body. Neurologists test this sense by telling patients to close their eyes and touch their own nose with the tip of a finger. Assuming proper proprioceptive function, at no time will the person lose awareness of where the hand actually is, even though it is not being detected by any of the other senses. Proprioception and touch are related in subtle ways, and their impairment results in surprising and deep deficits in perception and action.


Pain

Nociception Nociception (also nocioception, from Latin ''nocere'' 'to harm or hurt') is the sensory nervous system's process of encoding noxious stimuli. It deals with a series of events and processes required for an organism to receive a painful stimulus, co ...
(physiological
pain Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, ...
) signals nerve-damage or damage to tissue. The three types of pain receptors are cutaneous (skin), somatic (joints and bones), and visceral (body organs). It was previously believed that pain was simply the overloading of pressure receptors, but research in the first half of the 20th century indicated that pain is a distinct phenomenon that intertwines with all of the other senses, including touch. Pain was once considered an entirely subjective experience, but recent studies show that pain is registered in the
anterior cingulate gyrus In the human brain, the anterior cingulate cortex (ACC) is the frontal part of the cingulate cortex that resembles a "collar" surrounding the frontal part of the corpus callosum. It consists of Brodmann areas 24, 32, and 33. It is involved i ...
of the brain. The main function of pain is to attract our
attention Attention is the behavioral and cognitive process of selectively concentrating on a discrete aspect of information, whether considered subjective or objective, while ignoring other perceivable information. William James (1890) wrote that "Atte ...
to dangers and motivate us to avoid them. For example, humans avoid touching a sharp needle, or hot object, or extending an arm beyond a safe limit because it is dangerous, and thus hurts. Without pain, people could do many dangerous things without being aware of the dangers.


Other internal sensations and perceptions

An internal sensation and perception also known as interoception is "any sense that is normally stimulated from within the body". These involve numerous sensory receptors in internal organs. Interoception is thought to be atypical in clinical conditions such as
alexithymia Alexithymia is a personality trait characterized by the inability to identify and describe emotions experienced by oneself. The core characteristic of alexithymia is marked dysfunction in emotional awareness, social attachment, and interpersonal ...
. Some examples of specific receptors are: *
Hunger In politics, humanitarian aid, and the social sciences, hunger is defined as a condition in which a person does not have the physical or financial capability to eat sufficient food to meet basic nutritional needs for a sustained period. In the ...
is governed by a set of brain structures (e.g., the
hypothalamus The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...
) that are responsible for
energy homeostasis In biology, energy homeostasis, or the homeostatic control of energy balance, is a biological process that involves the coordinated homeostatic regulation of food intake (energy inflow) and energy expenditure (energy outflow). The human brain, parti ...
. *
Pulmonary stretch receptors Pulmonary stretch receptors are mechanoreceptors found in the lungs. When the lung expands, the receptors initiate the Hering-Breuer reflex, which reduces the respiratory rate. This signal is transmitted by vagus nerve. Increased firing from the ...
are found in the lungs and control the
respiratory rate The respiratory rate is the rate at which breathing occurs; it is set and controlled by the respiratory center of the brain. A person's respiratory rate is usually measured in breaths per minute. Measurement The respiratory rate in humans is mea ...
. *
Peripheral chemoreceptors Peripheral chemoreceptors (of the carotid and aortic bodies) are so named because they are sensory extensions of the peripheral nervous system into blood vessels where they detect changes in chemical concentrations. As transducers of patterns of va ...
in the brain monitor the carbon dioxide and oxygen levels in the brain to give a perception of
suffocation Asphyxia or asphyxiation is a condition of deficient supply of oxygen to the body which arises from abnormal breathing. Asphyxia causes generalized hypoxia, which affects primarily the tissues and organs. There are many circumstances that can i ...
if carbon dioxide levels get too high. * The
chemoreceptor trigger zone The chemoreceptor trigger zone (CTZ) is an area of the medulla oblongata that receives inputs from blood-borne drugs or hormones, and communicates with other structures in the vomiting center to initiate vomiting. The CTZ is located within the area ...
is an area of the
medulla Medulla or Medullary may refer to: Science * Medulla oblongata, a part of the brain stem * Renal medulla, a part of the kidney * Adrenal medulla, a part of the adrenal gland * Medulla of ovary, a stroma in the center of the ovary * Medulla of th ...
in the brain that receives inputs from
blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the c ...
-borne
drug A drug is any chemical substance that causes a change in an organism's physiology or psychology when consumed. Drugs are typically distinguished from food and substances that provide nutritional support. Consumption of drugs can be via insuffla ...
s or
hormone A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required ...
s, and communicates with the
vomiting center The area postrema, a paired structure in the medulla oblongata of the brainstem, is a circumventricular organ having permeable capillaries and sensory neurons that enable its dual role to detect circulating chemical messengers in the blood and ...
. * Chemoreceptors in the circulatory system also measure salt levels and prompt thirst if they get too high; they can also respond to high
blood sugar Glycaemia, also known as blood sugar level, blood sugar concentration, or blood glucose level is the measure of glucose concentrated in the blood of humans or other animals. Approximately 4 grams of glucose, a simple sugar, is present in the blo ...
levels in diabetics. *
Cutaneous receptor A cutaneous receptor is the type of sensory receptor found in the skin ( the dermis or epidermis). They are a part of the somatosensory system. Cutaneous receptors include mechanoreceptors (pressure or distortion), nociceptors (pain), and th ...
s in the skin not only respond to touch, pressure, temperature and vibration, but also respond to vasodilation in the skin such as
blushing Blushing is the reddening of a person's face due to psychological reasons. It is normally involuntary and triggered by emotional stress associated with passion, embarrassment, shyness, fear, anger, or romantic stimulation. Severe blushing is ...
. * Stretch receptors in the
gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organ (biology), organs of the digestive syste ...
sense gas distension that may result in colic pain. * Stimulation of sensory receptors in the
esophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to the ...
result in sensations felt in the throat when
swallowing Swallowing, sometimes called deglutition in scientific contexts, is the process in the human or animal body that allows for a substance to pass from the mouth, to the pharynx, and into the esophagus, while shutting the epiglottis. Swallowing i ...
,
vomiting Vomiting (also known as emesis and throwing up) is the involuntary, forceful expulsion of the contents of one's stomach through the mouth and sometimes the Human nose, nose. Vomiting can be the result of ailments like Food-poisoning, foo ...
, or during
acid reflux Gastroesophageal reflux disease (GERD) or gastro-oesophageal reflux disease (GORD) is one of the upper gastrointestinal chronic diseases where stomach content persistently and regularly flows up into the esophagus, resulting in symptoms and/ ...
. * Sensory receptors in
pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struc ...
mucosa, similar to touch receptors in the skin, sense foreign objects such as mucous and food that may result in a
gag reflex The pharyngeal reflex or gag reflex is a reflex muscular contraction of the back of the throat, evoked by touching the roof of the mouth, the back of the tongue, the area around the tonsils, the uvula, and the back of the throat. It, along with othe ...
and corresponding gagging sensation. * Stimulation of sensory receptors in the
urinary bladder The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine enters ...
and
rectum The rectum is the final straight portion of the large intestine in humans and some other mammals, and the Gastrointestinal tract, gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the s ...
may result in perceptions of fullness. * Stimulation of stretch sensors that sense dilation of various blood vessels may result in pain, for example headache caused by vasodilation of brain arteries. * Cardioception refers to the perception of the activity of the heart. *
Opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most pro ...
s and
direct DNA damage Direct may refer to: Mathematics * Directed set, in order theory * Direct limit of (pre), sheaves * Direct sum of modules, a construction in abstract algebra which combines several vector spaces Computing * Direct access (disambiguation), a ...
in
melanocyte Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea), the inner ear, vaginal epithelium, meninges, bones, and heart. ...
s and
keratinocyte Keratinocytes are the primary type of Cell (biology), cell found in the epidermis (skin), epidermis, the outermost layer of the skin. In humans, they constitute 90% of epidermal skin cells. Basal cells in the stratum basale, basal layer (''str ...
s can sense
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
radiation, which plays a role in pigmentation and
sunburn Sunburn is a form of radiation burn that affects living tissue, such as skin, that results from an overexposure to ultraviolet (UV) radiation, usually from the Sun. Common symptoms in humans and animals include: red or reddish skin that is ho ...
. *
Baroreceptor Baroreceptors (or archaically, pressoreceptors) are sensors located in the carotid sinus (at the bifurcation of external and internal carotids) and in the aortic arch. They sense the blood pressure and relay the information to the brain, so that a ...
s relay blood pressure information to the brain and maintain proper homeostatic blood pressure. The
perception of time The study of time perception or chronoception is a field within psychology, cognitive linguistics and neuroscience that refers to the subjective experience, or sense, of time, which is measured by someone's own perception of the duration of the ind ...
is also sometimes called a sense, though not tied to a specific receptor.


Nonhuman animal sensation and perception


Human analogues

Other living organisms have receptors to sense the world around them, including many of the senses listed above for humans. However, the mechanisms and capabilities vary widely.


Smell

An example of smell in non-mammals is that of
sharks Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorp ...
, which combine their keen sense of smell with timing to determine the direction of a smell. They follow the nostril that first detected the smell.
Insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...
s have olfactory receptors on their antennae. Although it is unknown to the degree and magnitude which non-human mammals can smell better than humans, humans are known to have far fewer olfactory receptors than
mice A mouse ( : mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus' ...
, and humans have also accumulated more
genetic mutations In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mit ...
in their olfactory receptors than other primates.


Vomeronasal organ

Many animals (
salamander Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All ten ...
s,
reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s,
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s) have a
vomeronasal organ The vomeronasal organ (VNO), or Jacobson's organ, is the paired auxiliary olfactory (smell) sense organ located in the soft tissue of the nasal septum, in the nasal cavity just above the roof of the mouth (the hard palate) in various tetrapods. ...
that is connected with the mouth cavity. In mammals it is mainly used to detect
pheromones A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...
of marked territory, trails, and sexual state. Reptiles like
snake Snakes are elongated, Limbless vertebrate, limbless, carnivore, carnivorous reptiles of the suborder Serpentes . Like all other Squamata, squamates, snakes are ectothermic, amniote vertebrates covered in overlapping Scale (zoology), scales. Ma ...
s and
monitor lizard Monitor lizards are lizards in the genus ''Varanus,'' the only extant genus in the family Varanidae. They are native to Africa, Asia, and Oceania, and one species is also found in the Americas as an invasive species. About 80 species are recogn ...
s make extensive use of it as a smelling organ by transferring scent molecules to the vomeronasal organ with the tips of the forked tongue. In reptiles the vomeronasal organ is commonly referred to as Jacobson's organ. In mammals, it is often associated with a special behavior called
flehmen The flehmen response (; from German ''flehmen'', to bare the upper teeth, and Upper Saxon German ''flemmen'', to look spiteful), also called the flehmen position, flehmen reaction, flehmen grimace, flehming, or flehmening, is a behavior in which ...
characterized by uplifting of the lips. The organ is vestigial in humans, because associated neurons have not been found that give any sensory input in humans.


Taste

Flies Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advanced ...
and
butterflies Butterflies are insects in the macrolepidopteran clade Rhopalocera from the Order (biology), order Lepidoptera, which also includes moths. Adult butterflies have large, often brightly coloured wings, and conspicuous, fluttering flight. The ...
have taste organs on their feet, allowing them to taste anything they land on.
Catfish Catfish (or catfishes; order Siluriformes or Nematognathi) are a diverse group of ray-finned fish. Named for their prominent barbels, which resemble a cat's whiskers, catfish range in size and behavior from the three largest species alive, ...
have taste organs across their entire bodies, and can taste anything they touch, including chemicals in the water.


Vision

Cat The cat (''Felis catus'') is a domestic species of small carnivorous mammal. It is the only domesticated species in the family Felidae and is commonly referred to as the domestic cat or house cat to distinguish it from the wild members of ...
s have the ability to see in low light, which is due to muscles surrounding their irides–which contract and expand their pupils–as well as to the
tapetum lucidum The ''tapetum lucidum'' ( ; ; ) is a layer of tissue in the eye of many vertebrates and some other animals. Lying immediately behind the retina, it is a retroreflector. It reflects visible light back through the retina, increasing the light a ...
, a reflective membrane that optimizes the image.
Pit vipers The Crotalinae, commonly known as pit vipers,Mehrtens JM (1987). ''Living Snakes of the World in Color''. New York: Sterling Publishers. 480 pp. . crotaline snakes (from grc, κρόταλον ''krotalon'' castanet), or pit adders, are a subfa ...
,
pythons The Pythonidae, commonly known as pythons, are a family of nonvenomous snakes found in Africa, Asia, and Australia. Among its members are some of the largest snakes in the world. Ten genera and 42 species are currently recognized. Distribution ...
and some boas have organs that allow them to detect
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
light, such that these snakes are able to sense the body heat of their prey. The
common vampire bat The common vampire bat (''Desmodus rotundus'') is a small, leaf-nosed bat native to Latin America. It is one of three extant species of vampire bat, the other two being the hairy-legged and the white-winged vampire bats. The common vampire bat ...
may also have an infrared sensor on its nose. It has been found that
birds Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
and some other animals are tetrachromats and have the ability to see in the
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
down to 300 nanometers.
Bees Bees are winged insects closely related to wasps and ants, known for their roles in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyletic lineage within the superfamil ...
and
dragonflies A dragonfly is a flying insect belonging to the infraorder Anisoptera below the order Odonata. About 3,000 extant species of true dragonfly are known. Most are tropical, with fewer species in temperate regions. Loss of wetland habitat threa ...
are also able to see in the ultraviolet.
Mantis shrimp Mantis shrimp, or stomatopods, are carnivorous marine crustaceans of the order Stomatopoda (). Stomatopods branched off from other members of the class Malacostraca around 340 million years ago. Mantis shrimp typically grow to around in lengt ...
s can perceive both
polarized light Polarization (also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the ...
and
multispectral Multispectral imaging captures image data within specific wavelength ranges across the electromagnetic spectrum. The wavelengths may be separated by filters or detected with the use of instruments that are sensitive to particular wavelengths, ...
images and have twelve distinct kinds of color receptors, unlike humans which have three kinds and most mammals which have two kinds.
Cephalopod A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...
s have the ability to change color using
chromatophore Chromatophores are cells that produce color, of which many types are Biological pigment, pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and ...
s in their skin. Researchers believe that
opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most pro ...
s in the skin can sense different wavelengths of light and help the creatures choose a coloration that camouflages them, in addition to light input from the eyes. Other researchers hypothesize that
cephalopod eye Cephalopods, as active marine predators, possess sensory organs specialized for use in aquatic conditions.Budelmann BU. "Cephalopod sense organs, nerves and the brain: Adaptations for high performance and life style." Marine and Freshwater Behavi ...
s in species which only have a single
photoreceptor protein Photoreceptor proteins are light-sensitive proteins involved in the sensing and response to light in a variety of organisms. Some examples are rhodopsin in the photoreceptor cells of the vertebrate retina, phytochrome in plants, and bacteriorhodo ...
may use
chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the wave ...
to turn monochromatic vision into color vision, explaining pupils shaped like the letter U, the letter W, or a
dumbbell The dumbbell, a type of free weight, is a piece of equipment used in weight training. It can be used individually or in pairs, with one in each hand. History The forerunner of the dumbbell, halteres, were used in ancient Greece as lifting w ...
, as well as explaining the need for colorful mating displays. Some cephalopods can distinguish the polarization of light.


Spatial orientation

Many invertebrates have a
statocyst The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, ctenophorans, echinoderms, cephalopods, and crustaceans. A similar structure is also found in ''Xenoturbella''. The statocyst cons ...
, which is a sensor for acceleration and orientation that works very differently from the mammalian's semi-circular canals.


Not human analogues

In addition, some animals have senses that humans do not, including the following:


Magnetoception

Magnetoception Magnetoreception is a sense which allows an organism to detect the Earth's magnetic field. Animals with this sense include some arthropods, molluscs, and vertebrates (fish, amphibians, reptiles, birds, and mammals, though not humans). The sen ...
(or magnetoreception) is the ability to detect the direction one is facing based on the Earth's
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
. Directional awareness is most commonly observed in
bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
s, which rely on their magnetic sense to navigate during migration. It has also been observed in insects such as
bee Bees are winged insects closely related to wasps and ants, known for their roles in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyly, monophyletic lineage within the ...
s. Cattle make use of magnetoception to align themselves in a north–south direction.
Magnetotactic bacteria Magnetotactic bacteria (or MTB) are a polyphyletic group of bacteria that orient themselves along the magnetic field lines of Earth's magnetic field. Discovered in 1963 by Salvatore Bellini and rediscovered in 1975 by Richard Blakemore, this alig ...
build miniature magnets inside themselves and use them to determine their orientation relative to the Earth's magnetic field. There has been some recent (tentative) research suggesting that the
Rhodopsin Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene and a G-protein-coupled receptor (GPCR). It is the opsin of the rod cells in the retina and a light-sensitive receptor protein that triggers visual phototransduction ...
in the human eye, which responds particularly well to blue light, can facilitate magnetoception in humans.


Echolocation

Certain animals, including
bat Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most bi ...
s and
cetacea Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel them ...
ns, have the ability to determine orientation to other objects through interpretation of reflected sound (like
sonar Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigation, navigate, measure distances (ranging), communicate with or detect o ...
). They most often use this to navigate through poor lighting conditions or to identify and track prey. There is currently an uncertainty whether this is simply an extremely developed post-sensory interpretation of auditory perceptions or it actually constitutes a separate sense. Resolution of the issue will require brain scans of animals while they actually perform echolocation, a task that has proven difficult in practice. Blind people report they are able to navigate and in some cases identify an object by interpreting reflected sounds (especially their own footsteps), a phenomenon known as
human echolocation Human echolocation is the ability of humans to detect objects in their environment by sensing echoes from those objects, by actively creating sounds: for example, by tapping their canes, lightly stomping their foot, snapping their fingers, or makin ...
.


Electroreception

Electroreception Electroreception and electrogenesis are the closely-related biological abilities to perceive electrical stimuli and to generate electric fields. Both are used to locate prey; stronger electric discharges are used in a few groups of fishes to stu ...
(or electroception) is the ability to detect
electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
s. Several species of fish,
shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimo ...
s, and rays have the capacity to sense changes in electric fields in their immediate vicinity. For cartilaginous fish this occurs through a specialized organ called the
Ampullae of Lorenzini Ampullae of Lorenzini (singular ''Ampulla'') are electroreceptors, sense organs able to detect electric fields. They form a network of mucus-filled pores in the skin of cartilaginous fish (sharks, rays, and chimaeras) and of basal actinopterygia ...
. Some fish passively sense changing nearby electric fields; some generate their own weak electric fields, and sense the pattern of field potentials over their body surface; and some use these electric field generating and sensing capacities for social
communication Communication (from la, communicare, meaning "to share" or "to be in relation with") is usually defined as the transmission of information. The term may also refer to the message communicated through such transmissions or the field of inquir ...
. The mechanisms by which electroceptive fish construct a spatial representation from very small differences in field potentials involve comparisons of spike latencies from different parts of the fish's body. The only orders of mammals that are known to demonstrate electroception are the
dolphin A dolphin is an aquatic mammal within the infraorder Cetacea. Dolphin species belong to the families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontoporiidae (the ...
and
monotreme Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials (Metatheria). Monotremes are typified by structural differences in their brain ...
orders. Among these mammals, the
platypus The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal Endemic (ecology), endemic to Eastern states of Australia, eastern Australia, including Tasmania. The platypu ...
has the most acute sense of electroception. A dolphin can detect electric fields in water using electroreceptors in vibrissal crypts arrayed in pairs on its snout and which evolved from whisker motion sensors. These electroreceptors can detect electric fields as weak as 4.6 microvolts per centimeter, such as those generated by contracting muscles and pumping gills of potential prey. This permits the dolphin to locate prey from the seafloor where sediment limits visibility and echolocation. Spiders have been shown to detect electric fields to determine a suitable time to extend web for 'ballooning'.
Body modification Body modification (or body alteration) is the deliberate altering of the human anatomy or human physical appearance. In its broadest definition it includes skin tattooing, socially acceptable decoration (''e.g.'', common ear piercing in many s ...
enthusiasts have experimented with magnetic implants to attempt to replicate this sense. However, in general humans (and it is presumed other mammals) can detect electric fields only indirectly by detecting the effect they have on hairs. An electrically charged balloon, for instance, will exert a force on human arm hairs, which can be felt through tactition and identified as coming from a static charge (and not from wind or the like). This is not electroreception, as it is a post-sensory cognitive action.


Hygroreception

Hygroreception Hygroreception is the ability to detect changes in the moisture and humidity content of an environment. It is a sense that is not present in humans. Some insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class ...
is the ability to detect changes in the moisture content of the environment.


Infrared sensing

The ability to sense
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
thermal radiation evolved independently in various families of
snakes Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more joi ...
. Essentially, it allows these reptiles to "see" radiant heat at
wavelengths In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
between 5 and 30
μm The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...
to a degree of accuracy such that a blind
rattlesnake Rattlesnakes are venomous snakes that form the genera ''Crotalus'' and ''Sistrurus'' of the subfamily Crotalinae (the pit vipers). All rattlesnakes are vipers. Rattlesnakes are predators that live in a wide array of habitats, hunting small anim ...
can target vulnerable body parts of the prey at which it strikes. It was previously thought that the organs evolved primarily as prey detectors, but it is now believed that it may also be used in thermoregulatory decision making. The facial pit underwent
parallel evolution Parallel evolution is the similar development of a trait in distinct species that are not closely related, but share a similar original trait in response to similar evolutionary pressure.Zhang, J. and Kumar, S. 1997Detection of convergent and paral ...
in
pitvipers The Crotalinae, commonly known as pit vipers,Mehrtens JM (1987). ''Living Snakes of the World in Color''. New York: Sterling Publishers. 480 pp. . crotaline snakes (from grc, κρόταλον ''krotalon'' castanet), or pit adders, are a subfa ...
and some boas and
pythons The Pythonidae, commonly known as pythons, are a family of nonvenomous snakes found in Africa, Asia, and Australia. Among its members are some of the largest snakes in the world. Ten genera and 42 species are currently recognized. Distribution ...
, having evolved once in pitvipers and multiple times in boas and pythons. The
electrophysiology Electrophysiology (from Greek , ''ēlektron'', "amber" etymology of "electron"">Electron#Etymology">etymology of "electron" , ''physis'', "nature, origin"; and , '' -logia'') is the branch of physiology that studies the electrical properties of b ...
of the structure is similar between the two lineages, but they differ in gross structural
anatomy Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having its ...
. Most superficially, pitvipers possess one large pit organ on either side of the head, between the eye and the nostril (
Loreal pit The loreal pit is the deep depression, or fossa, in the loreal area on either side of the head in pit vipers (crotaline snakes). The area is located behind the nostril and in front of the eye, but below the line that runs between the centers o ...
), while boas and pythons have three or more comparatively smaller pits lining the upper and sometimes the lower lip, in or between the scales. Those of the pitvipers are the more advanced, having a suspended sensory membrane as opposed to a simple pit structure. Within the family
Viperidae The Viperidae (vipers) are a family of snakes found in most parts of the world, except for Antarctica, Australia, Hawaii, Madagascar, and various other isolated islands. They are venomous and have long (relative to non-vipers), hinged fangs tha ...
, the pit organ is seen only in the subfamily Crotalinae: the pitvipers. The organ is used extensively to detect and target
endotherm An endotherm (from Greek ἔνδον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions inste ...
ic prey such as rodents and birds, and it was previously assumed that the organ evolved specifically for that purpose. However, recent evidence shows that the pit organ may also be used for thermoregulation. According to Krochmal et al., pitvipers can use their pits for thermoregulatory decision-making while true vipers (vipers who do not contain heat-sensing pits) cannot. In spite of its detection of IR light, the pits' IR detection mechanism is not similar to photoreceptors – while photoreceptors detect light via photochemical reactions, the protein in the pits of snakes is in fact a temperature-sensitive ion channel. It senses infrared signals through a mechanism involving warming of the pit organ, rather than a chemical reaction to light. This is consistent with the thin pit membrane, which allows incoming IR radiation to quickly and precisely warm a given ion channel and trigger a nerve impulse, as well as vascularize the pit membrane in order to rapidly cool the ion channel back to its original "resting" or "inactive" temperature.


Other

Pressure detection uses the organ of Weber, a system consisting of three appendages of vertebrae transferring changes in shape of the
gas bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth w ...
to the middle ear. It can be used to regulate the buoyancy of the fish. Fish like the
weather fish Misgurnus is a genus of true loaches found in Europe and Asia. The origin of the name ''Misgurnus'' comes from the Greek word (to hate) and the Turkish (loud), a name given to them due to their habit of becoming very active during barometric ...
and other loaches are also known to respond to low pressure areas but they lack a swim bladder. Current detection is a detection system of water currents, consisting mostly of
vortices In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...
, found in the
lateral line The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...
of fish and aquatic forms of amphibians. The lateral line is also sensitive to low-frequency vibrations. The mechanoreceptors are
hair cell Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...
s, the same mechanoreceptors for vestibular sense and hearing. It is used primarily for navigation, hunting, and schooling. The receptors of the electrical sense are modified hair cells of the lateral line system.
Polarized light Polarization (also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the ...
direction/detection is used by
bees Bees are winged insects closely related to wasps and ants, known for their roles in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyletic lineage within the superfamil ...
to orient themselves, especially on cloudy days.
Cuttlefish Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control of ...
, some
beetles Beetles are insects that form the order Coleoptera (), in the superorder Endopterygota. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 400,000 describ ...
, and
mantis shrimp Mantis shrimp, or stomatopods, are carnivorous marine crustaceans of the order Stomatopoda (). Stomatopods branched off from other members of the class Malacostraca around 340 million years ago. Mantis shrimp typically grow to around in lengt ...
can also perceive the polarization of light. Most sighted humans can in fact learn to roughly detect large areas of polarization by an effect called
Haidinger's brush Haidinger's brush, more commonly known as Haidinger's brushes is an image produced by the eye, an entoptic phenomenon, first described by Austrian physicist Wilhelm Karl von Haidinger in 1844. Haidinger saw it when he looked through various mine ...
; however, this is considered an
entoptic phenomenon Entoptic phenomena () are visual effects whose source is within the human eye itself. (Occasionally, these are called entopic phenomena, which is probably a typographical mistake.) In Helmholtz's words: "Under suitable conditions light falling o ...
rather than a separate sense.
Slit sensilla The slit sensilla, also known as the slit sense organ, is a small mechanoreceptory organ or group of organs in the exoskeleton of arachnids which detects physical deformation or strain due to forces experienced by the animal. The organ appears in t ...
e of spiders detect mechanical strain in the exoskeleton, providing information on force and vibrations.


Plant sensation

By using a variety of sense receptors, plants sense light, temperature, humidity, chemical substances, chemical gradients, reorientation, magnetic fields, infections, tissue damage and mechanical pressure. The absence of a nervous system notwithstanding, plants interpret and respond to these stimuli by a variety of hormonal and cell-to-cell communication pathways that result in movement, morphological changes and physiological state alterations at the organism level, that is, result in plant behavior. Such physiological and cognitive functions are generally not believed to give rise to mental phenomena or qualia, however, as these are typically considered the product of nervous system activity. The emergence of mental phenomena from the activity of systems functionally or computationally analogous to that of nervous systems is, however, a hypothetical possibility explored by some schools of thought in the philosophy of mind field, such as functionalism and
computationalism In philosophy of mind, the computational theory of mind (CTM), also known as computationalism, is a family of views that hold that the human mind is an information processing system and that cognition and consciousness together are a form of comp ...
. However, plants can perceive the world around them, and might be able to emit airborne sounds similar to "screaming" when stressed. Those noises could not be detectable by human ears, but organisms with a
hearing range Hearing range describes the range of frequencies that can be heard by humans or other animals, though it can also refer to the range of levels. The human range is commonly given as 20 to 20,000 Hz, although there is considerable variati ...
that can hear ultrasonic frequencies—like mice, bats or perhaps other plants—could hear the plants' cries from as far as away.


Artificial sensation and perception

Machine perception is the capability of a
computer system A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These progr ...
to interpret
data In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted ...
in a manner that is similar to the way humans use their senses to relate to the world around them. Computers take in and respond to their environment through attached hardware. Until recently, input was limited to a keyboard, joystick or a mouse, but advances in technology, both in hardware and software, have allowed computers to take in
sensory input Sensory may refer to: Biology * Sensory ecology, how organisms obtain information about their environment * Sensory neuron, nerve cell responsible for transmitting information about external stimuli * Sensory perception, the process of acquiri ...
in a way similar to humans.


Culture

In the time of
William Shakespeare William Shakespeare ( 26 April 1564 – 23 April 1616) was an English playwright, poet and actor. He is widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's nation ...
, there were commonly reckoned to be five wits or five senses. At that time, the words "sense" and "wit" were synonyms, so the senses were known as the five outward wits. This traditional concept of five senses is common today. The traditional five senses are enumerated as the "five material faculties" (') in Hindu literature. They appear in allegorical representation as early as in the
Katha Upanishad The ''Katha Upanishad'' (Sanskrit: कठोपनिषद् or कठ उपनिषद्) (') is one of the ''mukhya'' (primary) Upanishads, embedded in the last eight short sections of the ' school of the Krishna Yajurveda.Paul Deussen. ...
(roughly 6th century BC), as five horses drawing the "
chariot A chariot is a type of cart driven by a charioteer, usually using horses to provide rapid motive power. The oldest known chariots have been found in burials of the Sintashta culture in modern-day Chelyabinsk Oblast, Russia, dated to c. 2000&nbs ...
" of the body, guided by the mind as "chariot driver". Depictions of the five traditional senses as
allegory As a literary device or artistic form, an allegory is a narrative or visual representation in which a character, place, or event can be interpreted to represent a hidden meaning with moral or political significance. Authors have used allegory th ...
became a popular subject for seventeenth-century artists, especially among
Dutch Dutch commonly refers to: * Something of, from, or related to the Netherlands * Dutch people () * Dutch language () Dutch may also refer to: Places * Dutch, West Virginia, a community in the United States * Pennsylvania Dutch Country People E ...
and
Flemish Baroque painter Flemish Baroque painting refers to the art produced in the Southern Netherlands during Spanish control in the 16th and 17th centuries. The period roughly begins when the Dutch Republic was split from the Habsburg Spain regions to the south with ...
s. A typical example is
Gérard de Lairesse Gerard or Gérard (de) Lairesse (11 September 1641 – June 1711) was a Dutch Golden Age painter and art theorist. His broad range of skills included music, poetry, and theatre. De Lairesse was influenced by the Perugian Cesare Ripa and Fr ...
's ''Allegory of the Five Senses'' (1668), in which each of the figures in the main group alludes to a sense: Sight is the reclining boy with a
convex mirror A curved mirror is a mirror with a curved reflecting surface. The surface may be either ''convex'' (bulging outward) or ''concave'' (recessed inward). Most curved mirrors have surfaces that are shaped like part of a sphere, but other shapes are ...
, hearing is the
cupid In classical mythology, Cupid (Latin Cupīdō , meaning "passionate desire") is the god of desire, lust, erotic love, attraction and affection. He is often portrayed as the son of the love goddess Venus (mythology), Venus and the god of war Mar ...
-like boy with a
triangle A triangle is a polygon with three Edge (geometry), edges and three Vertex (geometry), vertices. It is one of the basic shapes in geometry. A triangle with vertices ''A'', ''B'', and ''C'' is denoted \triangle ABC. In Euclidean geometry, an ...
, smell is represented by the girl with flowers, taste is represented by the woman with the fruit, and touch is represented by the woman holding the bird. In
Buddhist philosophy Buddhist philosophy refers to the philosophical investigations and systems of inquiry that developed among various schools of Buddhism in India following the parinirvana of The Buddha and later spread throughout Asia. The Buddhist path combi ...
, Ayatana or "sense-base" includes the mind as a sense organ, in addition to the traditional five. This addition to the commonly acknowledged senses may arise from the psychological orientation involved in Buddhist thought and practice. The mind considered by itself is seen as the principal gateway to a different spectrum of phenomena that differ from the physical sense data. This way of viewing the human sense system indicates the importance of internal sources of sensation and perception that complements our experience of the external world. File:1668_Gérard_de_Lairesse_-_Allegory_of_the_Five_Senses.jpg, Lairesse's ''Allegory of the Five Senses'' File:Peacock_served_in_full_plumage_(detail_of_BRUEGHEL_Taste,_Hearing_and_Touch).jpg, Detail of ''The Senses of Hearing, Touch and Taste'',
Jan Brueghel the Elder Jan Brueghel (also Bruegel or Breughel) the Elder (, ; ; 1568 – 13 January 1625) was a Flemish painter and draughtsman. He was the son of the eminent Flemish Renaissance painter Pieter Bruegel the Elder. A close friend and frequent collaborato ...
, 1618 File:Pietro_Paolini_-_Allegory_of_the_Five_Senses_-_Walters_372768.jpg, In this painting by
Pietro Paolini Pietro Paolini, called il Lucchese (3 June 1603 – 12 April 1681) was an Italian painter of the Baroque period. Working in Rome, Venice and finally his native Lucca, he was a follower of Caravaggio to whose work he responded in a very personal ...
, each individual represents one of the five senses.


See also

* Aesthesis *
Apperception Apperception (from the Latin ''ad-'', "to, toward" and ''percipere'', "to perceive, gain, secure, learn, or feel") is any of several aspects of perception and consciousness in such fields as psychology, philosophy and epistemology. Meaning in philo ...
*
Attention Attention is the behavioral and cognitive process of selectively concentrating on a discrete aspect of information, whether considered subjective or objective, while ignoring other perceivable information. William James (1890) wrote that "Atte ...
*
Āyatana ''Āyatana'' (Pāli; Sanskrit: आयतन) is a Buddhist term that has been translated as "sense base", "sense-media" or "sense sphere". In Buddhism, there are six ''internal'' sense bases (Pali: ''ajjhattikāni āyatanāni''; also known as ...
(sense bases in Theravada Buddhism) *
Chemesthesis Chemesthesis is the chemical sensitivity of the skin and mucous membranes. Chemesthetic sensations arise when chemical compounds activate receptors associated with other senses that mediate pain, touch, and thermal perception. These chemical-induc ...
*
Empiricism In philosophy, empiricism is an epistemological theory that holds that knowledge or justification comes only or primarily from sensory experience. It is one of several views within epistemology, along with rationalism and skepticism. Empir ...
*
Extrasensory perception Extrasensory perception or ESP, also called sixth sense, is a claimed paranormal ability pertaining to reception of information not gained through the recognized physical senses, but sensed with the mind. The term was adopted by Duke Universi ...
*
Entoptic phenomenon Entoptic phenomena () are visual effects whose source is within the human eye itself. (Occasionally, these are called entopic phenomena, which is probably a typographical mistake.) In Helmholtz's words: "Under suitable conditions light falling o ...
* Increased sensitivity: **
Hyperacusis Hyperacusis is the increased sensitivity to sound and a low tolerance for environmental noise. Definitions of hyperacusis can vary significantly; it can refer to normal noises being perceived as: loud, annoying, painful, fear-inducing, or a combina ...
**
Hyperesthesia Hyperesthesia is a condition that involves an abnormal increase in sensitivity to stimuli of the sense. Stimuli of the senses can include sound that one hears, foods that one tastes, textures that one feels, and so forth. Increased touch sensitivi ...
**
Supertaster A supertaster is a person whose sense of taste is of far greater intensity than the average person. Some studies also show that an increased sensitivity to bitter tastes may be a cause of selective eating. History The term originated with ...
*
Illusion An illusion is a distortion of the senses, which can reveal how the mind normally organizes and interprets sensory stimulation. Although illusions distort the human perception of reality, they are generally shared by most people. Illusions may o ...
s **
Auditory illusion Auditory illusions are false perceptions of a real sound or outside stimulus. These false perceptions are the equivalent of an optical illusion: the listener hears either sounds which are not present in the stimulus, or sounds that should not be p ...
**
Optical illusion Within visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual perception, percept that arguably appears to differ from reality. Illusions come in a wide v ...
**
Touch illusion A tactile illusion is an illusion that affects the sense of touch. Some tactile illusions require active touch (e.g., movement of the fingers or hands), whereas others can be evoked passively (e.g., with external stimuli that press against the skin ...
*
Intuition Intuition is the ability to acquire knowledge without recourse to conscious reasoning. Different fields use the word "intuition" in very different ways, including but not limited to: direct access to unconscious knowledge; unconscious cognition; ...
*
Multisensory integration Multisensory integration, also known as multimodal integration, is the study of how information from the different sensory modalities (such as sight, sound, touch, smell, self-motion, and taste) may be integrated by the nervous system. A coherent r ...
*
Phantom limb A phantom limb is the sensation that an amputated or missing limb is still attached. Approximately 80 to 100% of individuals with an amputation experience sensations in their amputated limb. However, only a small percentage will experience painf ...
*
Remote sensing Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Earth ...
* Saḷāyatana and Ayatana (the six senses as a concept in
Buddhism Buddhism ( , ), also known as Buddha Dharma and Dharmavinaya (), is an Indian religion or philosophical tradition based on teachings attributed to the Buddha. It originated in northern India as a -movement in the 5th century BCE, and gra ...
) *
Sensation and perception psychology A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of stimuli. (For example, in the human body, the brain which is part of the central nervous system rec ...
*
Sense of direction Sense of direction is the ability to know one's location and perform wayfinding. It is related to cognitive maps, spatial awareness, and spatial cognition. Sense of direction can be impaired by brain damage, such as in the case of topographical di ...
*
Sensitivity (human) Sensory processing is the process that organizes sensation from one's own body and the environment, thus making it possible to use the body effectively within the environment. Specifically, it deals with how the brain processes multiple sensory mod ...
*
Sensorium A sensorium (/sɛnˈsɔːrɪəm/) (plural: sensoria) is the apparatus of an organism's perception considered as a whole, the "seat of sensation" where it experiences, perceives and interprets the environments within which it lives. The term origi ...
*
Sensory processing disorder Sensory processing disorder (SPD, formerly known as sensory integration dysfunction) is a condition in which multisensory input is not adequately processed in order to provide appropriate responses to the demands of the environment. Sensory proces ...
*
Sensus divinitatis ''Sensus divinitatis'' (Latin for "sense of divinity"), also referred to as ''sensus deitatis'' ("sense of deity") or ''semen religionis'' ("seed of religion"), is a term first used by French Protestant reformer John Calvin to describe a postula ...
*
Synesthesia Synesthesia (American English) or synaesthesia (British English) is a perceptual phenomenon in which stimulation of one sensory or cognitive pathway leads to involuntary experiences in a second sensory or cognitive pathway. People who re ...
(
Ideasthesia Ideasthesia (alternative spelling ideaesthesia) is a neuropsychological phenomenon in which activations of concepts (inducers) evoke perception-like sensory experiences (concurrents). The name comes from the Ancient Greek () and (), meaning 'se ...
)


References


External links

* The 2004
Nobel Prize The Nobel Prizes ( ; sv, Nobelpriset ; no, Nobelprisen ) are five separate prizes that, according to Alfred Nobel's will of 1895, are awarded to "those who, during the preceding year, have conferred the greatest benefit to humankind." Alfr ...
in
Physiology or Medicine The Nobel Prize in Physiology or Medicine is awarded yearly by the Nobel Assembly at the Karolinska Institute, Nobel Assembly at the Karolinska Institutet, Karolinska Institute for outstanding discoveries in physiology or medicine. The Nobel Pr ...

announced
4 October 2004) was won by
Richard Axel Richard Axel (born July 2, 1946) is an American molecular biologist and university professor in the Department of Neuroscience at Columbia University and investigator at the Howard Hughes Medical Institute. His work on the olfactory system won hi ...
and
Linda Buck Linda Brown Buck (born January 29, 1947) is an American biologist best known for her work on the olfactory system. She was awarded the 2004 Nobel Prize in Physiology or Medicine, along with Richard Axel, for their work on olfactory receptors. She ...
for their work explaining olfaction, published first in a joint paper in 1991 that described the very large family of about one thousand genes for odorant receptors and how the receptors link to the brain.
Answers to several questions related to senses and human feeling from curious kids

The Physiology of the Senses tutorial
12 animated chapters on vision, hearing, touch, balance and memory. {{Authority control Perception Sensory systems