Visual SystemStephen D Van Hooser, Brandeis University, Waltham, Massachusetts, USA

Sacha B Nelson, Brandeis University, Waltham, Massachusetts, USA

Humans and many other animals obtain much of their information about the world

through their eyes. Patterns of light are transformed into nerve impulses in the retina and

visual information is processed by nerve cells in the primary visual cortex. In the human

brain, about one-half of the cerebral cortex is dedicated in some way to the processing of

visual information.

Anatomy of the Visual System

Visual processing begins in the eye, where light passesthrough the lens and is focused on to photoreceptors in theretina. Axons of retinal ganglion cells, the output cells ofthe retina, leave the eye in a bundle called the optic nerve.At the optic chiasm, some axons cross over to the oppositehemisphere, so that axons representing the right half ofvisual space travel to the left hemisphere and axons rep-resenting the left half of visual space travel to the righthemisphere. From the optic chiasm, the retinal ganglioncell axons project to visual brain structures such as thelateral geniculate nucleus (LGN) of the thalamus, the su-perior colliculus in the midbrain, and the suprachiasmaticnucleus. In primates, over 90% of these projections are tothe LGN, where the retinal ganglion axons segregate intolayers based on eye of origin and other properties. LGNrelay cells receive large synaptic contacts from these axons,and make projections to the primary visual cortex (V1),where LGN axons representing each eye ramify in an al-ternating fashion. The anatomy of the primate visual sys-tem is shown in Figure 1.

There is a secondmajor visual pathway to the neocortexfrom the retina via the superior colliculus. The superiorcolliculus projects to the pulvinar in the thalamus, which inturn projects to specialized regions of the visual cortex lo-cated beyond V1. In primates, the superior colliculus isknown to be involved in eye movements, but it receivesmany fewer ganglion cell axons than the LGN. In manyother mammals, such as carnivores and rodents, the supe-rior colliculus receives a larger percentage of retinal affer-ents than in primates, and it is likely that the superiorcolliculus plays a larger role in vision in these animals. Thisarticle will focus on the pathway from the retina to theLGN toV1, since it is much larger in primates and is betterstudied than the pathway via the superior colliculus.

Since brains are limited in size by developmental andenergy constraints, the visual system does not represent allparts of the visual field equally, but instead emphasizes thearea in the centre of the eye. The centre of the retina, calledthe fovea, contains the highest density of photoreceptors,and the fovea is represented by a disproportionately large

number of cells in the LGN and cortex; almost half of V1,for example, represents the fovea. Visual information isgathered through activemovements of the eyes to bring the

Article Contents

Advanced article

. Anatomy of the Visual System

. Concept of a Receptive Field

. Retina

. Focusing of Light on to the Retina

. First Stage of Information Processing:

Hyperpolarization of Photoreceptor Cells

. Receptive Fields of Retinal Ganglion Cells

. Relay of Signals from the Lateral GeniculateNucleus to

the Visual Cortex

. Orientation andDirectional Selectivity in Cortical Cells

. Double-opponent Colour Cells in the Visual Cortex

. Columnar Organization of the Visual Cortex

. Beyond the Primary Visual Cortex

. Summary

doi: 10.1038/npg.els.0000230

Retina

Lateral geniculatenucleus (LGN)

Primary visualcortex (V1)

Optic radiations

Optictract

Optic nerve

Opticchiasm

Lens

Cornea

Figure 1 Anatomy of the visual system. Light arrives at the eye and is

focused by the lens on to the retina, where photoreceptors transduce thelight into electrical signals that are processed by local retinal neurons.

Axons of retinal ganglion cells, the output cells of the retina, leave the retinain a bundle called the optic nerve. At the optic chiasm, some axons cross

over, so that axons representing the right half of visual space travel to theleft lateral geniculate nucleus (LGN) and axons representing the left half of

visual space travel to the right LGN (not shown). In the LGN, the axonssegregate into layers according to eye of origin and other properties. LGN

relay cell axons form a band called the optic radiations and project to theprimary visual cortex, where LGN axons representing each eye ramify in an

alternating fashion.

1ENCYCLOPEDIA OF LIFE SCIENCES © 2005, John Wiley & Sons, Ltd. www.els.net

most informative parts of a scene into focus on the centre ofthe retina. Humans make over 100 000 such eye move-ments, or saccades, in a single day, typically one or moreper second. By devoting large numbers of cells to a smallregion of visual space and moving the eyes to informativeplaces in an image, the mammalian visual system affordshigher resolution than would be possible in an animal withfixed eyes and equal brain size.

Concept of a Receptive Field

In each brain structure described here, an individual cellresponds to images in a small part of the visual field andonly responds strongly to particular image patterns. Thepart of the visual field to which a cell responds is calledthe receptive field of the cell, and the relationship betweenimage patterns in the receptive field and the activity of thecell is referred to as the cell’s receptive field properties.Figure 2b shows an example of the receptive field propertiesof one neuron in the retina that is excited by light in thecentre of its receptive field but inhibited by light in thesurrounding part.

Retina

The retina is a sheet of neurons and specialized receptorcells located in the back of the eye. As shown in Figure 2a,the retina consists of six layers: the photoreceptor layer(PRL), the outer nuclear layer (ONL), the outer plexiformlayer (OPL), the inner nuclear layer (INL), the inner plexi-form layer (IPL), and the ganglion cell layer (GCL). Theorganization of the layers is peculiar in that the photo-receptors are located at the back of the retina so that lightpasses through all of the layers before reaching them. Thephotoreceptors of the retina transduce light into electricalsignals that are processed by the local neurons of the retina.The retinal ganglion cells are the only output cells of theretina, so all visual information available to the brain istransmitted by the axons of these cells.

Focusing of Light on to the Retina

Light enters the eye through a transparent portion of theexternal membrane of the eye (the cornea), passes throughthe lens and the vitreous space, and forms an image on theretina.Light is bent, or refracted, as it enters compartmentsthat possess different refractive indices. This refractionpermits the formation of a focused image on the retina. Thelens contributes only about 1/4 of the refractory power ofthe eye (the remainder is due to the cornea), but because theshape of the lens can be actively adjusted, it allows objectsat various distances to be brought into focus.

First Stage of Information Processing:Hyperpolarization of PhotoreceptorCells

The transduction of light into electrical activity occurs intwo types of photoreceptors: rods and cones. Both rodsand cones consist of an inner segment that contains the cellbody and nucleus, and an outer segment containing a stackof membranous disks specialized for phototransduction.Rods have an elongated outer segment, are specialized for

PRL

ONL

OPL

INL

IPLGCL

Ligh

t

Rod

Cone

Horizontal cell

Bipolar cell

Amacrine cell

Ganglion cell

To optic nerve(a)

Stimulus

Time+ –

ONOFF

(b)

Centre light Surround light

(c)H

yper

pol

ariz

ing/

dep

olar

izin

gIn

hibi

tion/

exci

tatio

n

Volta

ge

Stimulus on

Time

Invertingsynapse

Noninvertingsynapse

Figure 2 (a) The major cell types in the retina and their laminarorganization. PRL5photoreceptor layer; ONL5outer nuclear layer;

OPL5outer plexiform layer; INL5 inner nuclear layer; IPL5 innerplexiform layer; GCL5ganglion cell layer. (b) A centre–surround retinal

ganglion cell that responds to light in the centre of its receptive field and isinhibited by light in the surrounding region. The stimulus is shown on the

left, and action potentials in the cell relative to the onset and offset of thestimulus are shown on the right. Note that the cell responds most

vigorously to a light spot in the centre surrounded by a dark annulus(second from the top), but the cell responds much less vigorously when

stimulated by a large white spot (third from the top) because of theinhibitory surround. These properties are often denoted symbolically with

the notation at bottom, with ‘+’ indicating a preference for more lightrelative to backgroundand ‘2 ’ indicating less light. (c) Schematic diagram

of retinal circuitry that mediates the centre–surround cell depicted in (b).Light in the centrehyperpolarizes a cone,whichexcites abipolar cell,which

in turn excites the retinal ganglioncell.Horizontal cellsmediate the effect ofthe surround, providing inhibition to the bipolar cell in the centre when

there is light in the surround. This figure is adapted from Werblin andDowling (1969), who studied the salamander Necturus maculosus, and

similar circuitry has been found in other vertebrates.

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detection of low intensity (scotopic) light, and are homo-geneous in their wavelength sensitivity. Cones have a ta-pering outer segment, are specialized for detection ofhigher intensity (photopic) light, and individually aremoresensitive to long (L-cones), medium (M-cones), or shorterwavelengths of light (S-cones). Comparisons of intensityacross different wavelengths are the basis of colour vision.In the human retina there are approximately 100 millionrods and 5 million cones. The most sensitive portion of theretina, the fovea, contains exclusively cones and the densityof cones falls off with increasing distance from the fovea.Rods are absent in the fovea but are present throughout therest of the retina. The proportion and distribution of rodsand cones varies widely across animal species.

In the dark, photoreceptors have a relatively depolarizedmembrane potential (� 2 40 mV) and continually releaseglutamate from their synaptic terminals. This depolariza-tion is caused by continual activation of mixed cationchannels located in the outer segments by cytoplasmic cy-clic GMP. Current flow through these channels is termedthe ‘dark current’. The dark current is opposed by a restingpotassium conductance that would otherwise hyperpolar-ize the photoreceptor to � 2 80 mV. Absorption of lightactivates the photopigment (rhodopsin in rod photorecep-tors) and initiates a biochemical cascade that leads to ac-tivation of a phosphodiesterase, rapidly reducingcytoplasmic cGMP levels and thereby closing the cGMP-sensitive cation channels. The effect of light is therefore tohyperpolarize the photoreceptor by shutting off the darkcurrent. This hyperpolarization shuts off release of gluta-mate from the photoreceptor terminal.

Receptive Fields of Retinal GanglionCells

Most mammalian retinal ganglion cells have an opposingcentre–surround (or concentric) receptive field organiza-tion, which means they are excited by one stimulus in thecentre of their receptive field and inhibited by anotherstimulus in the area surrounding this centre (Kuffler, 1953).An example of a centre–surround cell that responds vig-orously to a light spot surrounded by a dark annulus isshown in Figure 2b. Since neurons fire action potentials andcannot signal negatively, centre–surround cells usually ex-ist in two polarities. For example, most vertebrates possessa retinal ganglion cell type that responds to a light/darkcontrast between its receptive field centre and surround,and in addition possess another retinal ganglion cell typethat responds to a dark/light contrast between the centreand the surround.

The fact that most retinal ganglion cells, such as thelight/dark centre–surround cell described above, respondto a contrast between two regions rather than absolutebrightness allows the visual system touse the same circuitry

to operate at different light levels. If one reads a newspaperoutside on a bright sunny day, the absolute amount of lightreflected fromboth the black text and thewhite pagewill bemuch greater than if one reads the same newspaper in-doors. However, the contrast between the black text andthe white page conveys meaningful information in bothsettings.Each retinal ganglion cell type receives input from a dif-

ferent arrangement of the local retinal circuitry. The localcircuitry that produces the light/dark opponent retinalganglion cell described above is shown in Figure2c (WerblinandDowling, 1969). Photoreceptors synapse directly on tobipolar neurons, which in turn provide the primary inputto retinal ganglion cells. In the case of a light/dark oppo-nent cell, the bipolar cell is excited by the photoreceptors.Horizontal cells, which make inhibitory connections to bi-polar cells, provide surround inhibition to the bipolar cell,which then provides less excitation to the ganglion cellwhen there is light in the surround. Bipolar neurons thatrespond to a dark/light contrast between the centre andsurround hyperpolarize in response to light in their centreregions because they receive sign-conserving synapsesfrom their photoreceptor inputs. They are depolarized bythe release of glutamate during the dark and then hyper-polarize back towards their resting membrane potentialwhen glutamate release is shut off by light. Bipolar andganglion cells that respond to a light/dark contrast be-tween the centre and surround are called ON cells, whilethose that respond to a dark/light contrast between thecentre and surround are called OFF cells.Retinal ganglion cells are the only output cells of the

retina, so these cells must convey all information necessaryfor a variety of visual functions such as seeing in colour,seeing form, and detecting motion. Thus, it is not surpris-ing that there is a large diversity of retinal ganglion celltypes (DeMonasterio and Gouras, 1975), although theprecise number of these types and the exact qualities thatidentify each type are still a subject of debate. Ganglioncells are commonly grouped on the basis of their size andtheir axonal projections; cell types vary slightly from spe-cies to species, and here we describe cells found in themacaque monkey. Small ganglion cells projecting to theparvocellular layers of the LGN are called midget cells,while the larger ganglion cells projecting to the magnocel-lular layers of the LGN are called parasol cells. There is athird, less well studied class of cells that project to smallcells throughout the LGN called the koniocellular cells.The midget cells are primarily responsible for seeing de-

tail and colour. They comprise about 80% of all retinalganglion cells and have very small receptive fields. In hu-mans andOldWorldmonkeys, the vast majority ofmidgetcells are colour-opponent, being excited by red in the centreand inhibited by green in the surround or vice versa. It isimportant to note that this type of colour opponency doesnot correspond to a preference for a red/green contrastbetween the centre and surround, as such a neuron would

Visual System

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be excited by red in the centre and also be excited by greenin the surround or vice versa; such ‘double opponent’ cellsare first seen in the primary visual cortex.Midget cells tendto respond to stimuli in a sustained manner, which meansthat they fire constantly to a constant stimulus.

Parasol cells, by contrast, primarily mediate seeing mo-tion and change. They make up about 10% of retinal gan-glion cells, and have larger receptive fields than midgetcells. There are few parasol cells in the fovea, but the ratioof parasol to midget cells increases with eccentricity. Par-asol cells are insensitive to colour and instead are lumi-nance-opponent.Unlikemidget cells, parasol cells respondtransiently to stimuli, which means they fire a few actionpotentials when a stimulus appears but do not fire con-stantly to a constant stimulus. Parasol cells can, however,respond to more rapid changes in a stimulus pattern thancan midget cells.

The function of the koniocellular cells is much less wellunderstood than that of the midget and parasol cells (Hen-dry and Reid, 2000). Koniocellular cells are a heterogene-ous class of neurons, some of which have large receptivefields (158), low firing rates, and slow-conducting axons.Only one specific type of receptive field properties, previ-ously thought to be associated with a class of midget neu-rons, has been conclusively identified as koniocellular.Cells of this type are colour-opponent neurons excited byblue in the centre and inhibited by red or green in the cen-tre. These cells have larger receptive field centre sizes thanthe midget cells and they lack a surround.

Relay of Signals from the LateralGeniculate Nucleus to the Visual Cortex

After leaving the retina, midget, parasol and koniocellularcell axons enter the optic chiasm and travel to the LGN,where they segregate into layers based on eye of origin andcell properties. The layers of theLGNare shown in Figure3.Layers 3–6 are the parvocellular layers, which receive inputfrommidget cells, and layers 1 and 2 are themagnocellularlayers, which receive input from parasol cells. Layers 1, 4,and 6 receive input from the contralateral eye, whereaslayers 2, 3, 5 receive input from the ipsilateral eye. Thekoniocellular axons project to layers K1–K6, which areintercalated among the parvocellular and magnocellularlayers, and koniocellular cells are also found diffuselythroughout the entire LGN. Each layer of the LGN is or-ganized topographically, which means there is a one-to-one relationship between receptive field locations in theretina and those in the LGN, and that adjacent cells alsohave adjacent receptive field locations.

Retinal ganglion cells make powerful synapses on toLGN cells, andmeasurements of LGN cell properties haveshown them to be similar to those of the retinal ganglioncells that drive them. For this reason, the LGN is often

referred to as a relay station between the retina and visualcortex, andLGNcells that receive retinal input and projectto cortex are called relay cells. Axons from these relay cellsform a band called the optic radiations and travel to theprimary visual cortex (see Figure 1). The LGN also receivesmassive direct and indirect connections from primary vis-ual cortex, that can modulate signals from the retina invarious ways. However, the functional role of these ‘feed-back’ connections and theirmodulation of the retinal inputare incompletely understood.Upon arriving in the primary visual cortex (V1), axons

from each of the three LGN neuron classes make synapticcontacts in different cortical layers. V1 is a six-layeredstructure, and a simplified wiring diagram of its connec-tions is shown in Figure 6. The majority of input from theparvocellular cells arrives at a subdivision of layer 4 calledlayer 4B, while the magnocellular cells largely project toanother layer called 4A.Cells in layers 4Band 4Aprimarilyexhibit receptive field properties similar to the parvocellu-lar and magnocellular neurons that provide their input.The koniocellular cells project to small bands of cellsspanning layers 1–3 called ‘blobs’ that will be discussedfurther below. Within the cortex, cells in layer 4 project tolayers 2 and 3 throughout the cortex, and layers 2 and 3cells in turn project to layers 5 and 6 in V1 and also projectto adjacent cortical areas. Cells in layer 5 project to adja-cent cortical areas andalso to subcortical structures such as

5

6

4

3

2

1

K6

K5

K4

K3

K2

K1

Figure 3 The lateral geniculate nucleus of the rhesus monkey. Midgetcells of the retina innervate layers 3–6, and the parasol cells innervate layers1 and 2. Koniocellular cells innervate K1–K6 but are also diffusely present

throughout the entire LGN.

Visual System

4

the superior colliculus. Finally, cells in layer 6 project backto the LGN. (Note that we have followed Casagrande andKaas (1994) in using Hassler’s labelling of V1 layers.)

Orientation and Directional Selectivityin Cortical Cells

With the exception of cells in the input layers 4A and 4B,luminance-sensitive neurons in V1 have very different re-ceptive field properties from cells in the LGN. These cellsrespond best to edges or bars at a particular orientation(see Figure 4a), and these orientated edges are an importantfeature for the nervous system because they frequently de-fine the boundaries of objects. V1 has two types of orien-tation-sensitive neurons, simple cells and complex cells(Hubel and Wiesel, 1962). Receptive fields of simple cellshave separate regions that respond to light increments orlight decrements, so simple cells respond to bars or edges atone particular position in space with a maintained re-sponse, as shown in Figure 4b. Complex cells, by contrast,

respond to the presence of a bar located anywhere withintheir receptive fields, and thus do not have specific regionsthat canbe stimulated by spots of light (see Figure4c).Manycomplex cells respondmost vigorously to bars or edges thatare moving, and some simple and complex cells only re-spond tomovement in one particular direction, as shown inFigure 4d.The synaptic connections and cellular mechanisms that

underlie orientation selectivity are still a subject of debate.The investigators who first characterized simple and com-plex cells, David Hubel and Torsten Wiesel, proposed atheory describing how simple and complex receptive fieldproperties could arise from input from LGN cells (or V1layer 4A/4B cells) and other cortical cells. They suggestedthat simple cell responses could arise from feed-forwardinput from centre–surround cells with co-linear receptivefield centres of like signs as shown in Figure 5a. Such anarrangement would produce a cell with excitatory and in-hibitory regions and orientation selectivity. The complexcell properties of orientation selectivity but indifference toprecise positioning could arise from input from multipleadjacent simple cells with similar orientation preferences,as shown in Figure 5b.The simplest and strongest evidence for Hubel and

Wiesel’s idea comes from anatomical and physiologicalstudies of cat primary visual cortex. In the cat, unlike in themonkey, cells in the input layer 4 of V1 show orientationselectivity. Almost all layer 4 cells in the cat are simple cells,while many cells in layers 2 and 3 and 5 and 6 are complexcells, consistent with the notion that simple cells can beproduced directly with synaptic input from centre–sur-round neurons, but that complex cells require an addi-tional layer of intervening neurons. In addition,simultaneous recordings of connected neurons in cat

BarON

OFF

(a)

BarON

OFF

(b)

BarON

OFF

(c) (d)

++ –

++ –

++ –

++ –

Figure 4 (a)Many neurons in the primary visual cortex respond to bars oredges at a particular orientation. The stimulus is shown at the left, and

action potentials in the cell relative to the onset and offset of the stimulusare shown at the right. The neuron in (a) responds to a bar rotated 458clockwise fromvertical, but respondspoorly tobarswithotherorientations.(b) Simple cells have separate regions of their receptive fields that respond

to light increments and light decrements and thus respond to bars atspecific locations. One example of such a receptive field pattern is shown.

(c) In contrast to simple cells, complex cells respond to a properly orientedbar anywhere in their receptive fields. (d) Many cells in V1 respond to

moving oriented bars. The arrows in the stimulus (left) indicate direction ofbarmovement.Most cells in V1 are orientation-selective and not direction-

selective, responding to movement in both directions (upper), but somecells are direction-selective and only respond to bars moving in a particular

direction (lower).

+–+–+–+–

+–+ +– +– +– +–

(a) (b)

––––

++++

Figure 5 Hubel and Wiesel’s model for the formation of simple and

complex receptive field properties. (a) A simple cell (at right) that respondsto a dark, oriented bar on a light background could receive input from

several adjacent centre–surround cells that respond to light decrements intheir receptive field centres. (b) A complex cell (at right) could obtain its

indifference to bar position by receiving input from several adjacent simplecells (at left) sharing one orientation preference. The complex cell’s

receptive field properties cannot be represented in the same form as theschematics for the LGNcells and simple cell in (a) and the simple cells in (b).

It responds to a properly oriented bar at any position in its receptive field.

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LGN and V1 by Reid and Alonso show that an LGN neu-ron is much more likely to contact a V1 simple cell with anoverlapping receptive field if the LGN neuron’s receptivefield centre has the same sign as the overlapping region inthe simple cell’s receptive field than if the two signs do notagree (Reid and Alonso, 1995).

Another idea for generation of orientation selectivityposits that this selectivity arises from an interaction ofsynaptic input from centre–surround cells and recurrentsynaptic connections with other V1 neurons (Sompolinskyand Shapley, 1997). In this view, orientation-selective cellsreceive centre–surround input that has a small orientationbias (not as strong as pictured in Figure 5a), and receivestrong excitatory input from nearby V1 neurons with sim-ilar orientation preferences and inhibitory input fromnearby cells with many different orientation preferences.When an oriented edge is observed, many V1 cells with anorientation preference close to that of the stimulus fireweakly initially, but over time the recurrent input fromother V1 cells to V1 neurons with the proper orientationpreference is amplified. This model is consistent with theexperimental observations that orientation selectivity issharpened over time and that orientation selectivity in oneregion of V1 can be disrupted by inactivation of cells in aV1 region hundreds of micrometres away.

Double-opponent Colour Cells in theVisual Cortex

Humans are able to perceive relationships between coloursover a wide range of lighting conditions, whichmeans theymust be able to detect colour contrasts. In the LGN, themajority of neurons are colour-selective, being excited byone colour in one region of their receptive fields and in-hibited by another colour in another region, but they donot respond to colour contrasts. The cortex contains a classof cells, called ‘double-opponent cells’, that performs thisfunction (Livingstone and Hubel, 1984).

Double-opponent cells show two types of colour op-ponency with a centre–surround organization. They areexcited by one colour in the centre of their receptive fieldand inhibited by another colour, and they are excited bythis second colour in the surround region and inhibited bythe first colour. For example, a double-opponent cellmightbe excited by red and inhibited by green in its centre, and beexcited by green and inhibited by red in the surround (de-noted r+g2 /r2 g+). These cells seem to only exist infour types: r+g2 /r2 g+, r2 g+/r+g2 , b+y2 /y+b2 , b2 y+/y2 b+, where b is blue and y is red andgreen together.

Double opponent cells are found in regions of layers 2and 3 that show increased staining for cytochromeoxidase,a mitochondrial enzyme shown to exist more densely incells with generally higher activity since such cells require

more energy and thus have more mitochondria. These re-gions, called ‘blobs’ for their appearance in tangential cor-tical sections stained for cytochrome oxidase are depictedin the wiring diagram in Figure 6. The blobs receive inputfrom the koniocellular layers of the LGN, so it is possiblethat double-opponent cells get their receptive field prop-erties only from input from koniocellular neurons; alter-natively, they might receive input from the neurons incortical layer 4B that receive input from the parvocellularLGN layers.

Columnar Organization of the VisualCortex

In addition to the laminar organization described above,V1 has a considerable degree of horizontal organization atmany scales. Like the LGN, V1 has a topographic repre-sentation of visual space, so that each position on the V1sheet of cells corresponds to a particular point in visualspace, and adjacent points on the sheet correspond to ad-jacent points in visual space (see Figure 7a).Within this topographic organization is a segregation of

the input from the two eyes. Axons from the LGN relaycellsmediating each eye ramify in layer 4 and in the blobs inan alternating fashion, as shown in Figure 6 and Figure 7b.Each of these ocular dominance bands is about 450mm

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3

1

2

4A

4BV1

To superiorcolliculus

LGN

To MT, V2

To V2

Magnocellular – ipsilateralKoniocellular – contralateralKoniocellular – ipsilateral

Contralateral Ipsilateral Contralateral

Parvocellular – contralateralParvocellular – ipsilateralMagnocellular – contralateral

Figure 6 Selected synaptic connections in the primary visual cortex. Ineach ocular dominance band, parvocellular LGN neurons from one eye

project to cortical layer 4B,magnocellular LGNneurons project to layer 4A,and koniocellular LGN neurons project to the cytochrome oxidase blobs in

layers 1–3. Neurons in layer 4 make connections with neurons in layers 2and 3, both in the blobs and between the blobs, and these cells in turn

project to layers 5 and 6. Cells in layers 2, 3 and 5 make connections withadjacent cortical areas, and cells in layer 5 also make connections with

subcortical structures such as the superior colliculus. Finally, cells in layer 6project back to the lateral geniculate nucleus. Adapted from Casagrande

and Kaas (1994).

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wide.All of the cells in layer 4 strictly respond to input fromone of the two eyes, but in layers 2, 3, 5 and 6, the cell inputismixed.Neurons in layers 2, 3, 5 and 6 that lie in the centreof an ocular dominance band show a strong preference forthat eye, but cells close to the ocular dominance band bor-ders show relatively mixed input.

Finally, woven into the topographic map and oculardominance bands of V1 is a semi-regular arrangementof neurons according to orientation preference. If onedrives a recording electrode into the cortex obliquely andrecords the orientation preferences of many neurons, onesees that nearby neurons tend to have the same orientationand that the orientation preference of cells changes slowlyas one moves tangentially through the cortex. These‘orientation maps’ have been imaged optically, and theshapes of areas containing neurons that share orientationpreferences loosely resemble the leaves of a pinwheel(see Figure 7c).

Beyond the Primary Visual Cortex

A complete discussion of other visual cortical areas is be-yond the scope of this article, but it is important to notethat neurons in theprimary visual cortexmake connectionswith cells in other visual cortical areas, and many of theseareas respond to even more specific stimuli than does V1.For example, while direction-selective cells in V1 respondto motion of local image features within their small recep-tive fields, direction-selective cells inmiddle temporal (MT)cortex respond to motion of a complete object or texture.Some cells in the inferior temporal (IT) cortex respond tostimuli as specific as faces. The segregation of cell prop-erties for sensitivity to motion or colour and form in theretina and LGN seems to be somewhat maintained in pro-jections to the second visual cortex (V2) and higher corticalareas likeMTand IT.V1 cells receiving indirect input fromthe LGN parvocellular cells largely project to visual areasmediating form; cells receiving indirect input from the ma-gnocellular cells largely project to areas mediating the per-ception of motion; and cells in the blobs largely project toareas mediating perception of colour. These and interven-ing visual areas send many feedback connections tothe visual cortex, and, as with the feedback connectionsto the LGN, the role of these connections is not wellunderstood.

Summary

In the human visual system, signals travel from the retina tothe lateral geniculate nucleus to the primary visual cortex. Inthe retina, photoreceptors transduce light into electrical sig-nals that are processed by the local neurons of the retina,which in turn provide input to the retinal ganglion cells, theoutput neurons of the retina.There aremany typesof retinalganglion cells, including those sensitive to colour and formandmotion and change, and retinal ganglion cells generallyhave receptive fields with a centre–surround organization.The lateral geniculate nucleus acts as a relay station betweenthe retina and primary visual cortex. In the cortex, the lu-minance-sensitive simple and complex cells respond to ori-ented bars or edges. Simple cells respond to properlyoriented bars of light at particular locations, while complexcells respond to properly oriented bars at any location with-in their receptive fields. Double-opponent cells in the pri-mary visual cortex allow the visual system to sense colourcontrasts. The primary visual cortex has a complex hori-zontal organization with overlapping maps of visual topog-raphy, ocular dominance and orientation tuning.

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Figure7 Horizontal organizationof the visual cortex. (a) The topographicprojection of visual space in the right visual hemifield on to an idealized,

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Further Reading

Burns ME and Baylor DA (2001) Activation, deactivation, and adap-

tation in vertebrate photoreceptor cells. Annual Review of Neurosci-

ence 24: 779–805.

Dowling JE (1987) The Retina: An Approachable Part of the Brain.

Cambridge, MA: Harvard University Press.

Ferster D and Miller KD (2000) Neural mechanisms of orientation se-

lectivity in the visual cortex. Annual Reviews of Neuroscience 23: 441–

471.

Hassler R (1966) Comparative anatomy of the central visual systems in

day- and night-active primates. In: Hassler R and Stephen H (eds)

Evolution of the Forebrain, pp. 419–434. Stuttgart: Thieme.

Leventhal AG (1991) The Neural Basis of Visual Function. London:

Macmillan Press Ltd.

McIlwain JT (1996)An Introduction to the Biology of Vision. Cambridge:

Cambridge University Press.

Wandell BA (1995) Foundations of Vision. SunderlandMA: Sinauer As-

sociates.

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