An illusion is a distortion of the

sense 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 ...

s, which can reveal how 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 ...

normally organizes and interprets

sensory stimulation In physiology, a stimulus is a detectable change in the physical or chemical structure of an organism's internal or external environment. The ability of an organism or organ to detect external stimuli, so that an appropriate reaction can be m ...

. Although illusions distort the human perception of

reality Reality is the sum or aggregate of all that is real or existent within a system, as opposed to that which is only imaginary. The term is also used to refer to the ontological status of things, indicating their existence. In physical terms, r ...

, they are generally shared by most people. Illusions may occur with any of the human senses, but visual illusions (

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 ...

s) are the best-known and understood. The emphasis on visual illusions occurs because

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 ...

often dominates the other senses. For example, individuals watching a

ventriloquist Ventriloquism, or ventriloquy, is a performance act of stagecraft in which a person (a ventriloquist) creates the illusion that their voice is coming from elsewhere, usually a puppeteered prop known as a "dummy". The act of ventriloquism is v ...

will perceive the voice is coming from the dummy since they are able to see the dummy mouth the words. Some illusions are based on general assumptions the brain makes during

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 ...

. These assumptions are made using organizational principles (e.g.,

Gestalt Gestalt may refer to: Psychology * Gestalt psychology, a school of psychology * Gestalt therapy, a form of psychotherapy * Bender Visual-Motor Gestalt Test, an assessment of development disorders * Gestalt Practice, a practice of self-exploration ...

theory), an individual's capacity for

depth perception Depth perception is the ability to perceive distance to objects in the world using the visual system and visual perception. It is a major factor in perceiving the world in three dimensions. Depth perception happens primarily due to stereopsis an ...

and

motion perception Motion perception is the process of inferring the speed and direction of elements in a scene based on visual, vestibular and proprioceptive inputs. Although this process appears straightforward to most observers, it has proven to be a difficult pr ...

, and

perceptual constancy Subjective constancy or perceptual constancy is the perception of an object or quality as constant even though our sensation of the object changes. While the physical characteristics of an object may not change, in an attempt to deal with the extern ...

. Other illusions occur due to biological sensory structures within the

human body The human body is the structure of a Human, human being. It is composed of many different types of Cell (biology), cells that together create Tissue (biology), tissues and subsequently organ systems. They ensure homeostasis and the life, viabi ...

or conditions outside the body within one's physical environment. The term ''illusion'' refers to a specific form of sensory distortion. Unlike a

hallucination A hallucination is a perception in the absence of an external stimulus that has the qualities of a real perception. Hallucinations are vivid, substantial, and are perceived to be located in external objective space. Hallucination is a combinatio ...

, which is a distortion in the absence of a

stimulus 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 ...

, an illusion describes a misinterpretation of a true sensation. For example, hearing voices regardless of the environment would be a hallucination, whereas hearing voices in the sound of running water (or another auditory source) would be an illusion. So, it should not be wrong to consider that illusions are just "misinterpretations" on how our brain perceives something that exists (unlike a hallucination where a stimulus is absent).

Visual

visual 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 percept that arguably appears to differ from reality. Illusions come in a wide variety; thei ...

or optical illusion is characterized by visually perceived images that are deceptive or misleading. Therefore, the information gathered by the visual sense is processed to create a

percept 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 ...

that does not tally with information from other senses or physical measurements. The visual system, which includes the eyes (namely the retinas) and the central nervous system (namely the brain's visual cortex), constructs reality through both perceptual and cognitive neural pathways. Visual illusions are (at least in part) thought to be caused by excessive competing stimuli. Each stimulus follows a dedicated neural path in the early stages of visual processing, and intense/repetitive activity or interaction with active adjoining channels (perceptual neural circuits, usually at the same level) causes a

physiological Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...

imbalance that alters perception. During low-level visual processing, the retinal circuit arranges the information in the photoreceptors, by creating initial visual percepts from the patterns of light which fall on the retina. The Hermann

grid illusion A grid illusion is any kind of grid that deceives a person's vision. The two most common types of grid illusions are the Hermann grid illusion and the scintillating grid illusion. Hermann grid illusion The Hermann grid illusion is an optical illus ...

and

Mach bands Mach bands is an optical illusion named after the physicist Ernst Mach. It exaggerates the contrast between edges of the slightly differing shades of gray, as soon as they contact one another, by triggering edge-detection in the human visual s ...

are two illusions that are widely considered to be caused by a biological phenomenon named

lateral inhibition In neurobiology, lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition disables the spreading of action potentials from excited neurons to neighboring neurons in the lateral direction ...

, where the receptor signal in the retina's

receptive fields The receptive field, or sensory space, is a delimited medium where some Stimulus (physiology), physiological stimuli can evoke a Sensory neuron, sensory neuronal response in specific organisms. Complexity of the receptive field ranges from the un ...

from light and dark areas compete with one another.Kandel E.R., & Schwartz J.H., & Jessell T.M., & Siegelbaum S.A., & Hudspeth A.J., & Mack S(Eds.), (2014). ''Principles of Neural Science, Fifth Edition''. McGraw Hill. The assembly of visual elements into a collective percept, that distinguishes objects from backgrounds, takes part during intermediate-level visual processing. Many common visual illusions are a consequence of the percept constructed during this processing stage, as the elements first captured during low-level processing might easily be interpreted to form an image that differs from objective reality. An example is when two objects of the same size are placed on a certain background which conditions us to believe that one object might be larger than the other, and when the background is removed or replaced our perception immediately changes to the correct scenario (effectively concluding that both objects have equal dimensions). High-level visual processing consolidates information gathered from various sources to apply cognitive influences that create a conscious visual experience. Thus, allowing us to recognize the complex identity of different elements, and the disparate relations between them through cognitive processes. Visual illusions are also often a product of this processing stage, and it is during this stage that we might ultimately become conscious of any optical illusion. There are two crucial properties of our visual system related mostly to high-level visual processing, referred to as selectivity and invariance (which we have consistently attempted to replicate in image recognition computer algorithms). Selectivity refers to the identification of particular features that are relevant to recognize a specific element or object, while abstracting from other features that are not fundamental to performing the same recognition (e.g. when we see the shape of a house, certain contours that are essential for us to recognize it while other contours or image properties are not, such as color). On the other hand, invariance refers to the ability to be indifferent to small variations of a given feature, effectively identifying all those variations as simply being different versions of the same feature (e.g. we can recognize a given handwritten letter of the alphabet, written by different people with distinct styles of calligraphy). The whole process that constructs our visual experience is extremely complex (with multiple qualities that are unmatched by any computer or digital system). It is organized by many sequential and parallel sub-processes, each of which is essential in building our conscious image of the world. Our whole visual system seeks to simplify and categorize the unstructured low-level visual information, through both selectivity and invariance. Thus, while trying to organize an image by "filling in the gaps" through assumptions, we become vulnerable to misinterpretation.The Cutting Edge of Haptics
(MIT

Technology Review ''MIT Technology Review'' is a bimonthly magazine wholly owned by the Massachusetts Institute of Technology, and editorially independent of the university. It was founded in 1899 as ''The Technology Review'', and was re-launched without "The" in ...

article)

Auditory

An auditory illusion is an illusion of

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 ...

, the auditory equivalent of a visual illusion: the listener hears either sound which are not present in the stimulus, or "impossible" sounds. In short, audio illusions highlight areas where the human ear and brain, as organic, makeshift tools, differ from perfect audio receptors (for better or for worse). One example of an auditory illusion is a

Shepard tone A Shepard tone, named after Roger Shepard, is a sound consisting of a superposition of sine waves separated by octaves. When played with the bass pitch of the tone moving upward or downward, it is referred to as the ''Shepard scale''. This cre ...

Tactile

Examples of tactile illusions include

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 ...

, the

thermal grill illusion The thermal grill illusion is a sensory illusion originally demonstrated in 1896 by the Swedish physician Torsten Thunberg. The illusion is created by an interlaced grill of bars which are warm, e.g. , and cool, e.g. . When someone presses a hand ...

, the

cutaneous rabbit illusion The cutaneous rabbit illusion (also known as cutaneous saltation and sometimes the cutaneous rabbit effect or CRE) is a tactile illusion evoked by tapping two or more separate regions of the skin in rapid succession. The illusion is most readily ev ...

and a curious illusion that occurs when the crossed index and middle fingers are run along the bridge of the nose with one finger on each side, resulting in the perception of two separate noses. The brain areas activated during illusory tactile perception are similar to those activated during actual tactile stimulation. Tactile illusions can also be elicited through haptic technology. These "illusory" tactile objects can be used to create "virtual objects".The Cutting Edge of Haptics
(MIT

article)

Temporal

temporal illusion 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 a distortion in the perception of time, which occurs when the time interval between two or more events is very narrow (typically less than a second). In such cases, a person may momentarily perceive time as slowing down, stopping, speeding up, or running backward.

Intersensory

Illusions can occur with the other senses including those involved in food perception. Both sound and touch have been shown to modulate the perceived staleness and crispness of food products. It was also discovered that even if some portion of the taste receptor on the tongue became damaged that illusory taste could be produced by tactile stimulation. Evidence of

olfactory The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, it ...

(smell) illusions occurred when positive or negative verbal labels were given prior to olfactory stimulation. The McGurk effect shows that what we hear is influenced by what we see as we hear the person speaking; when the auditory component of one sound is paired with the visual component of another sound, leading to the perception of a third sound.

Disorders

Some illusions occur as a result of an illness or a disorder. While these types of illusions are not shared with everyone, they are typical of each condition. For example, people with migraines often report

fortification illusions Scintillating scotoma is a common visual aura that was first described by 19th-century physician Hubert Airy (1838–1903). Originating from the brain, it may precede a migraine headache, but can also occur acephalgically (without headache), a ...

Neuroscience

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 ...

is linked to specific brain activity and so can be elicited by brain stimulation. The (illusory) percepts that can be evoked range from simple

phosphene A phosphene is the phenomenon of seeing light without light entering the eye. The word ''phosphene'' comes from the Greek words ''phos'' (light) and ''phainein'' (to show). Phosphenes that are induced by movement or sound may be associated wit ...

s (detections of lights in the

visual field The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments". Or simply, visual field can be defined as the entire area that can be seen when an eye is fixed straight at a point ...

) to high-level percepts. In a single-case study on a patient undergoing presurgical evaluation for epilepsy treatment, electrical stimulation at the left temporo-parietal junction evoked the percept of a nearby (illusory) person who "closely 'shadowed' changes in the patient's body position and posture".

References

External links

Universal Veiling Techniques

by J.R. Block.
Optical illusions and visual phenomena
by Michael Bach
Auditory illusions
- touch illusions, forces and the geometry of objects, by Gabriel Robles-De-La-Torre.
Silencing awareness of visual change by motion
{{Authority control Perception Deception Articles containing video clips