A topographic map is the ordered projection of a sensory surface, like the retina or the skin, or an effector system, like the musculature, to one or more structures 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 ...

. Topographic maps can be found in all

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 ...

s and in many

motor system The motor system is the set of central and peripheral structures in the nervous system that support motor functions, i.e. movement. Peripheral structures may include skeletal muscles and neural connections with muscle tissues. Central structur ...

Visual system

The

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 ...

refers to the part of the

that allows an organism to see. It interprets information from visible light to build a representation of the world. The ganglion cells of 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 ...

project in an orderly fashion to the

lateral geniculate nucleus In neuroanatomy, the lateral geniculate nucleus (LGN; also called the lateral geniculate body or lateral geniculate complex) is a structure in the thalamus and a key component of the mammalian visual pathway. It is a small, ovoid, ventral projec ...

of the

thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...

and from there to the primary

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 ...

(V1); adjacent spots on the retina are represented by adjacent neurons in the lateral geniculate nucleus and the primary visual cortex. The term for this pattern of projection is ''topography''. There are many types of topographic maps in the visual cortices, including retinotopic maps, occular dominance maps and orientation maps. Retinotopic maps are the easiest to understand in terms of topography. Retinotopic maps are those in which the image on the retina is maintained in the cortices (V1 and the LGN). In other words, if a specific region of the cortices was damaged that individual would then have a blind spot in the real world, they would not be able to see the bit of the world that corresponded to the retina damage. Orientation maps are also topographic. In these maps there are cells which have a preference to a certain orientation, the maximum firing rate of the cell will be achieved at that preference. As the orientation is moved away from the firing rate will drop. An orientation map is topographic because neighboring neural tissues have similar orientation preferences. The term ''retinotopic'' refers to the maintenance of the particular order of afferent connections from the retina along the afferent pathway via sub-cortical structures to V1 and other cortical visual areas. The primary visual cortex (V1, Brodmann's area 17) is the first cortical area to receive visual input. The stria of Gennari – a set of heavily myelinated, horizontally projecting axons within the termination zone of lateral geniculate nucleus (LGN) input to V1 – provides an anatomical marker particular to V1.

Development

According to the Chemoaffinity hypothesis, chemical labels are distributed in a graded fashion across the

and

tectum The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal ( alertness), and temperature regulation. The name comes from the Greek ''mesos'', " ...

. This allows each

retinal ganglion cell A retinal ganglion cell (RGC) is a type of neuron located near the inner surface (the ganglion cell layer) of the retina of the human eye, eye. It receives visual information from photoreceptor cell, photoreceptors via two intermediate neuron typ ...

to recognize its proper termination site. Experiments with artificially created compound eyes in ''

Xenopus ''Xenopus'' () (Gk., ξενος, ''xenos''=strange, πους, ''pous''=foot, commonly known as the clawed frog) is a genus of highly aquatic frogs native to sub-Saharan Africa. Twenty species are currently described within it. The two best-known ...

'' demonstrate that not only the ganglion cells but also their axons carry these specificities. Axons must be able to communicate with each other to ensure that ones with the same positional tags innervate the same area of the

superior colliculus In neuroanatomy, the superior colliculus () is a structure lying on the roof of the mammalian midbrain. In non-mammalian vertebrates, the homologous structure is known as the optic tectum, or optic lobe. The adjective form ''tectal'' is commonly ...

Categories of retinotopic maps

First-order representations are those in which adjacent points of the same hemifield always map to adjacent columns in the contralateral cortex. An example of this would be the map in

primary 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 ...

(V1). Second-order representations, also known as a field discontinuity map, are maps that are organized such that it appears that a discontinuity has been introduced in either the visual field or the retina. The maps in V2 and other

extrastriate cortex The extrastriate cortex is the region of the occipital cortex of the mammalian brain located next to the primary visual cortex. Primary visual cortex (V1) is also named striate cortex because of its striped appearance in the microscope. The extra ...

are second-order representations.

Auditory system

The

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 ...

is the sensory system for hearing in which the brain interprets information from the frequency of sound waves, yielding the perception of tones. Sound waves enter 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 ...

through the auditory canal. These waves arrive at the

eardrum 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 The outer ear, external ear, or auris externa is the extern ...

where the properties of the waves are transduced into vibrations. The vibrations travel through the bones of 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 ...

to the

cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...

. In the cochlea, the vibrations are transduced into electrical information through the firing of hair cells in the organ of Corti. The

organ of Corti The organ of Corti, or spiral organ, is the receptor organ for hearing and is located in the mammalian cochlea. This highly varied strip of epithelial cells allows for transduction of auditory signals into nerve impulses' action potential. Transd ...

projects in an orderly fashion to structures in the

brainstem 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 cont ...

(namely, the

cochlear nuclei The cochlear nuclear (CN) complex comprises two cranial nerve nuclei in the human brainstem, the ventral cochlear nucleus (VCN) and the dorsal cochlear nucleus (DCN). The ventral cochlear nucleus is unlayered whereas the dorsal cochlear nucleus ...

and the

inferior colliculus The inferior colliculus (IC) (Latin for ''lower hill'') is the principal midbrain nucleus of the auditory pathway and receives input from several peripheral brainstem nuclei in the auditory pathway, as well as inputs from the auditory cortex. The ...

), and from there to the medial geniculate nucleus of the

and the primary

auditory cortex The auditory cortex is the part of the temporal lobe that processes auditory information in humans and many other vertebrates. It is a part of the auditory system, performing basic and higher functions in hearing, such as possible relations to ...

. Adjacent sites on the

, which are themselves selective for the

sound 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 ...

frequency, are represented by adjacent neurons in the aforementioned CNS structures. This projection pattern has been termed

tonotopy In physiology, tonotopy (from Greek tono = frequency and topos = place) is the spatial arrangement of where sounds of different frequency are processed in the brain. Tones close to each other in terms of frequency are represented in topologically ...

. The tonotopic layout of sound information begins in the

where the

basilar membrane The basilar membrane is a stiff structural element within the cochlea of the inner ear which separates two liquid-filled tubes that run along the coil of the cochlea, the scala media and the scala tympani. The basilar membrane moves up and down in ...

vibrates at different positions along its length depending upon the frequency of the sound. Higher frequency sounds are at the base of the cochlea, if it were unrolled, and low frequency sounds are at the apex. This arrangement is also found in the

in the

temporal lobe The temporal lobe is one of the four Lobes of the brain, major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain. The temporal lobe ...

. In areas that are tonotopically organized, the frequency varies systematically from low to high along the surface of the cortex, but is relatively constant across cortical depth. The general image of topographic organization in

animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...

s is multiple tonotopic maps distributed over the surface of the cortex.

Somatosensory system

The

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 ...

comprises a diverse range of receptors and processing centers to produce the perception of touch, temperature,

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 ...

, 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 ...

. Receptors are located throughout the body including 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 ...

, epithelia, internal organs,

skeletal muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...

bone A bone is a Stiffness, rigid Organ (biology), organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red blood cell, red and white blood cells, store minerals, provid ...

s, and

joint A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw ...

s. The

cutaneous receptors 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 ...

of the skin project in an orderly fashion to the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...

, and from there, via different afferent pathways (

dorsal column-medial lemniscus tract Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal co ...

and

spinothalamic tract The spinothalamic tract is a part of the anterolateral system or the ventrolateral system, a sensory pathway to the thalamus. From the ventral posterolateral nucleus in the thalamus, sensory information is relayed upward to the somatosensory cor ...

), to the

ventral posterior nucleus The ventral posterior nucleus is the somato-sensory relay nucleus in thalamus of the brain. Input and output The ventral posterior nucleus receives neuronal input from the medial lemniscus, spinothalamic tracts, and trigeminothalamic tract. It p ...

of the

and the primary

somatosensory cortex 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 ...

. Again, adjacent areas on the skin are represented by adjacent neurons in all aforementioned structures. This projection pattern has been termed

somatotopy Somatotopy is the point-for-point correspondence of an area of the body to a specific point on the central nervous system. Typically, the area of the body corresponds to a point on the primary somatosensory cortex (postcentral gyrus). This cortex i ...

. One common diagram of the somatotopic map is the

cortical homunculus A cortical homunculus () is a distorted representation of the human body, based on a neurological "map" of the areas and proportions of the human brain dedicated to processing motor functions, or sensory functions, for different parts of the bo ...

. This illustration is a fairly accurate representation of how much cortical area represents each body part or region. It also maps what part of the cortex represents each region of the body.

Motor system

Unlike the topographic maps of the senses, the

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 of the motor cortex are

efferent neurons Efferent nerve fibers refer to axonal projections that ''exit'' a particular region; as opposed to afferent projections that ''arrive'' at the region. These terms have a slightly different meaning in the context of the peripheral nervous syste ...

that exit the brain instead of bringing information to the brain through afferent connections. The

is responsible for initiating voluntary or planned movements (

reflex In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus. Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...

es are mediated at the

level, so movements that associated with a reflex are not initiated by the motor cortex). The activation from the motor cortex travels through

Betz cell Betz cells (also known as pyramidal cells of Betz) are giant pyramidal cells ( neurons) located within the fifth layer of the grey matter in the primary motor cortex. These neurons are the largest in the central nervous system, sometimes reaching ...

s down the

corticospinal tract The corticospinal tract is a white matter motor pathway starting at the cerebral cortex that terminates on lower motor neurons and interneurons in the spinal cord, controlling movements of the limbs and trunk. There are more than one million neur ...

through upper motor neurons, terminating at the anterior horn of the grey matter where lower motor neurons transmit the signal to peripheral motor neurons and, finally, the voluntary

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...

Movements not requiring use of topographic maps

There are several instances of movements that do not require the participation of topographic maps, or even the participation 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 ...

. One instance would be (as already stated) in some

es. Reflexes usually are mediated at the level of the spinal cord through

reflex arc A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons w ...

s. In humans, mono-, oligo-, and poly-synaptic reflex arcs, propriospinal

interneuron Interneurons (also called internuncial neurons, relay neurons, association neurons, connector neurons, intermediate neurons or local circuit neurons) are neurons that connect two brain regions, i.e. not direct motor neurons or sensory neurons. I ...

systems, and internuncial gray matter neurons collectively participate continuously to produce spinal cord reflex that activates muscle. The higher motor centers of

octopus An octopus ( : octopuses or octopodes, see below for variants) is a soft-bodied, eight- limbed mollusc of the order Octopoda (, ). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttle ...

es (large brained

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

s) are notable for organizing (unlike

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

s) highly skilled movements without the use of somatotopic maps of their bodies.

Olfactory system

The

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 ...

is the sensory system used for

olfaction 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 ...

, or the sense of smell. It detects volatile, airborne substances. Most sensory systems spatially segregate afferent input from primary sensory neurons to construct a topographic map that defines the location of a sensory stimulus within the environment, as well as the quality of the stimulus itself. Unlike other sensory systems, the topography in the olfactory system is not dependent on spatial properties of the stimuli. Relieved of the requirement to map the position of an olfactory stimulus in space, the olfactory system employs spatial segregation of sensory input to encode the quality of an

odorant An aroma compound, also known as an odorant, aroma, fragrance or flavoring, is a chemical compound that has a smell or odor. For an individual chemical or class of chemical compounds to impart a smell or fragrance, it must be sufficiently vol ...

. The topographic map revealed in the olfactory system differs in quality from the orderly representation inherent in the retinotopic, tonotopic, or somatotopic sensory maps. Olfactory neurons differ from one another by the nature of the receptor that they possess. However, the olfactory sensory neurons to which olfactory bulb neurons are connected are also distributed across the receptor sheet (the olfactory epithelium) depending on their chemical preferences. Locations on the olfactory epithelium and the olfactory bulb are correlated so, as with other sensory systems, the topographic map in the brain is linked to the structure of the peripheral organ. This principle is called rhinotopy. Individual olfactory sensory

neurons 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 ...

express only one of the thousand receptor

gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...

s, such that neurons are functionally distinct.

Cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...

s expressing a given receptor in the olfactory epithelium are randomly dispersed within 1 of 4 broad zones. Sensory neurons extend a single unbranched axon to the

olfactory bulb The olfactory bulb (Latin: ''bulbus olfactorius'') is a grey matter, neural structure of the vertebrate forebrain involved in olfaction, the sense of odor, smell. It sends olfactory information to be further processed in the amygdala, the orbitof ...

such that the projections from neurons expressing a specific receptor converge on 2 out of the 1800

glomeruli ''Glomerulus'' () is a common term used in anatomy to describe globular structures of entwined vessels, fibers, or neurons. ''Glomerulus'' is the diminutive of the Latin ''glomus'', meaning "ball of yarn". ''Glomerulus'' may refer to: * the filter ...

. The pattern of convergence is absolute and invariant in all individuals in a species. The bulb therefore provides a spatial map that identifies which of the numerous receptors have been activated within the sensory epithelium so that the quality of an olfactory stimulus is encoded by specific combination of glomeruli activated by a given odorant (combinatorial code). While refinement of the bulbar topographic code relies on activity, the development occurs partly without apparent contribution from activity-dependent processes. Mice lacking the olfactory cyclic nucleotide-gated ion channel fail to exhibit odor-evoked electrophysiological responses in the sensory epithelium, but the pattern of convergence of like axons in the bulb is unaltered in these mutant mice, arguing strongly that olfactory experience is not necessary for the establishment or refinement of the topographic map. These findings, however, do not exclude a role for activity-dependent processes in the maintenance or potential plasticity of the map after it is established. For example, neurons that do not express functional odorant receptors have been observed to have a significantly shorter half-life. Activity-dependent processes may therefore be essential for the survival of the olfactory neurons postnatally, and in that manner, may alter the input to individual glomeruli, altering the sensitivity to individual odors.

Gustatory system

The

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 ...

is the sensory system for

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 ...

. Like olfaction, taste requires a process of

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 ...

. The receptors for taste are

taste buds 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 papillae found on the upper surface of the tongue, soft palate, upper esophagus, the che ...

on 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 ...

. The tongue contains taste receptors, as well as mechanoreceptors. Afferents from taste receptors and mechanoreceptors of the tongue access different ascending systems in the brainstem. However, it is uncertain how these two sources of information are processed in cortex. The primary gustatory cortex (G) is located near the somatotopic region for the tongue (S1), in the

insular cortex The insular cortex (also insula and insular lobe) is a portion of the cerebral cortex folded deep within the lateral sulcus (the fissure separating the temporal lobe from the parietal and frontal lobes) within each hemisphere of the mammalian bra ...

deep in the

lateral fissure In neuroanatomy, the lateral sulcus (also called Sylvian fissure, after Franciscus Sylvius, or lateral fissure) is one of the most prominent features of the human brain. The lateral sulcus is a deep fissure in each hemisphere that separates the ...

with the secondary taste areas in the opercula. The peripheral taste system likely maintains a specific relationship between taste bud cells selectively responsive to one taste quality and the

ganglion cells {{stack, A ganglion cell is a cell found in a ganglion. Examples of ganglion cells include: * Retinal ganglion cell (RGC) found in the ganglion cell layer of the retina * Cells that reside in the adrenal medulla, where they are involved in th ...

signaling that particular quality. This explains the response specificity of some individual taste nerve, particularly because sweet, amino acid, and bitter receptors are expressed in distinct populations of taste cells. Although anatomical evidence for such an exclusive relationship is lacking at the level of single receptor and ganglion cells, the relationship between single buds and their innervating ganglion cells is tractable neuroanatomically. In taste, attempts to identify a spatial representation of taste receptors or taste qualities have revealed only an indistinct functional topography in the brain. Nevertheless, taste ganglion cells must distribute peripheral fibers to particular receptor cell types and disseminate impulses centrally in a structurally organized manner.

Benefits

For

, it is metabolically costly to make long connections, and the trade-off of connection distance is time. Thus, it is good design to group neurons together that are to be highly interconnected. Multiple topographic maps is a feature that is advantageous because it allows maps of different sizes that would accommodate varying levels of acuity and details in signals. A more detailed map has more neurons that would take up more area than a more global map, which would require fewer connections.

Techniques

A variety of techniques have been used to establish the topographic maps in the brain. The existence of topographical maps was shown early by electrical stimulation of the cortex, tracing patterns of epileptic seizures, stimulation sequences, and impairments due to lesions. Details in the maps came later through

microelectrode A microelectrode is an electrode used in electrophysiology either for recording neural signals or for the electrical stimulation of nervous tissue (they were first developed by Ida Hyde in 1921). Pulled glass pipettes with tip diameters of 0. ...

stimulation and recording techniques became commonly used in demonstrating somatotopic maps and later in the auditory and visual systems, both cortically and in subcortical structures such as the colliculi and geniculate nuclei of the

. Single-cell recording,

Transcranial magnetic stimulation Transcranial magnetic stimulation (TMS) is a noninvasive form of brain stimulation in which a changing magnetic field is used to induce an electric current at a specific area of the brain through electromagnetic induction. An electric pulse gener ...

(TMS), electrical stimulation of the cortex, and

Functional magnetic resonance imaging Functional magnetic resonance imaging or functional MRI (fMRI) measures brain activity by detecting changes associated with blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area o ...

(fMRI) are some of the techniques used to study maps in the brain. Many of the existing topographic maps have been further studied or refined using fMRI. For example, Hubel and Wiesel originally studied the retinotopic maps in the primary visual cortex using single-cell recording. Recently, however, imaging of the retinotopic map in the cortex and in sub-cortical areas, such as the lateral geniculate nucleus, have been improved using the fMRI technique.