THE RETINATHE RETINA

DR.DR.

AMER ISMAIL ABU IMARAAMER ISMAIL ABU IMARA

JORDANIAN BOARD OF OPHTHALMOLOGYJORDANIAN BOARD OF OPHTHALMOLOGY

INTERNATIONAL COUNCILOF OPHTHALMOLOGYINTERNATIONAL COUNCILOF OPHTHALMOLOGY

PALESTINIAN BOARD OF OPHTHALMOLOGYPALESTINIAN BOARD OF OPHTHALMOLOGY

Development of the retinaDevelopment of the retina The eye is externalized portion of the brain .The eye is externalized portion of the brain . Formation of the eye begins with lateral Formation of the eye begins with lateral

outpouchings of the forebrain during the third outpouchings of the forebrain during the third week of development .week of development .

The development of the optic cup ( optic The development of the optic cup ( optic vesicle ) reaches a stage where the outer layer vesicle ) reaches a stage where the outer layer of the optic vesicle becomes the retinal pigment of the optic vesicle becomes the retinal pigment epithelium , while the inner layer of the optic epithelium , while the inner layer of the optic vesicle becomes the multilayered neurosensory vesicle becomes the multilayered neurosensory retina . anterior extension of both layers become retina . anterior extension of both layers become the double layer ciliary epithelium . the double layer ciliary epithelium .

The ocular ventricle is the potential space The ocular ventricle is the potential space between the retinal pigment epithelium and the between the retinal pigment epithelium and the neurosensory retina.neurosensory retina.

PIGMENT EPITHELIUMPIGMENT EPITHELIUM Is homolog of the epithelium of the choroid Is homolog of the epithelium of the choroid

plexus of the brain .plexus of the brain .The retinal pigment epithelial cells acquire The retinal pigment epithelial cells acquire

during development tight junctions that during development tight junctions that form a barrier between the neurosensory form a barrier between the neurosensory retina and the choriocapillaries .retina and the choriocapillaries .

CELLULAR ORGANIZATION OF THE RETINACELLULAR ORGANIZATION OF THE RETINA The layers of cell nuclei are as follows :The layers of cell nuclei are as follows : the outer nuclear layer (ONL) , which contains the outer nuclear layer (ONL) , which contains

the cell bodies of the photoreceptors.the cell bodies of the photoreceptors. The inner nuclear layer (INL) , which contains The inner nuclear layer (INL) , which contains

the cell bodies of horizontal neurons , bipolar the cell bodies of horizontal neurons , bipolar neurons , amacrine neurons , displaced ganglion neurons , amacrine neurons , displaced ganglion cells and those of the glial cells of Muller .cells and those of the glial cells of Muller .

The ganglion cell layer , which contains the cell The ganglion cell layer , which contains the cell bodies of most of the ganglion cells , displaced bodies of most of the ganglion cells , displaced amacrine cells and those of the astroglial cells .amacrine cells and those of the astroglial cells .

Between the ONL and the INL is the outer Between the ONL and the INL is the outer plexiform layer (OPL)plexiform layer (OPL)

OPL = synapses of the OPL = synapses of the photoreceptors ,bipolar cells and photoreceptors ,bipolar cells and horizontal cells .horizontal cells .

Between the INL and the ganglion cell Between the INL and the ganglion cell layer is the inner plexiform layer ( IPL)layer is the inner plexiform layer ( IPL)

IPL= synapses of the bipolar cells , IPL= synapses of the bipolar cells , amacrine cells and ganglion cells .amacrine cells and ganglion cells .

The optic fibers consist of the axons of the The optic fibers consist of the axons of the ganglion cells and are unmyelinated while ganglion cells and are unmyelinated while within the retina . these fibers leave the within the retina . these fibers leave the retina at the optic disc going out of the retina at the optic disc going out of the globe posteriorly as the optic nerve . globe posteriorly as the optic nerve .

In the retina Muller cell processes fill in In the retina Muller cell processes fill in almost all volumes not occupied by nerve almost all volumes not occupied by nerve cells , relatively rare astroglia or blood cells , relatively rare astroglia or blood vessels .vessels .

There appear no physical barrier to There appear no physical barrier to diffusion of molecules of moderate size diffusion of molecules of moderate size from the vitreous through the retina into from the vitreous through the retina into the ocular ventricle .the ocular ventricle .

There is no hindrance to electrical current .There is no hindrance to electrical current .

BLOOD SUPPLY OF THE RETINA BLOOD SUPPLY OF THE RETINA Blood vessels coming from the optic nerve Blood vessels coming from the optic nerve

head supply the inner two thirds of the head supply the inner two thirds of the retina . the outer one third is supplied by retina . the outer one third is supplied by the choroid .the choroid .

The inner blood –retinal barrier is formed The inner blood –retinal barrier is formed by tight junctions between retinal blood by tight junctions between retinal blood vessels endothelial cells .vessels endothelial cells .

RETINAL NEUROANATOMY AND ITS RETINAL NEUROANATOMY AND ITS PHYSIOLOGIC SIGNIFICANCE PHYSIOLOGIC SIGNIFICANCE

The input to the retina is a time-varying two-The input to the retina is a time-varying two-dimensional display of an image in its focal dimensional display of an image in its focal plane .plane .

The image consists of patches of illumination The image consists of patches of illumination varying in shape, intensity and spectral content . varying in shape, intensity and spectral content . the information input is received by the the information input is received by the PHOTORECEPTORS . the output of the PHOTORECEPTORS . the output of the photoreceptors is processed by a variety of photoreceptors is processed by a variety of subsequent retinal neurons and finally by the subsequent retinal neurons and finally by the retinal ganglion cells whose axons leave the retinal ganglion cells whose axons leave the retina for higher brain centers .retina for higher brain centers .

The information leaving the retina via The information leaving the retina via axons of the ganglion cells represents a axons of the ganglion cells represents a small number of information processing small number of information processing streams parceling certain types of streams parceling certain types of information contained in the visual input to information contained in the visual input to axons with particular routing . the axons of axons with particular routing . the axons of the ganglion cells have several principal the ganglion cells have several principal as well as minor destinations, and the cells as well as minor destinations, and the cells are sometimes classified by their axonal are sometimes classified by their axonal targets .targets .

The retina also has outputs to the outermost The retina also has outputs to the outermost layers of the superior colliculus , where the layers of the superior colliculus , where the information directly or indirectly interacts with information directly or indirectly interacts with motor pathways influencing the extraocular motor pathways influencing the extraocular muscles ,visually concerned cerebellar pathways muscles ,visually concerned cerebellar pathways , such as those dealing with head and neck , such as those dealing with head and neck movements , and with vestibular and auditory movements , and with vestibular and auditory centers .centers .

Pretectal region indirectly receives retinal Pretectal region indirectly receives retinal information important for parasympathetic and information important for parasympathetic and sympathetic regulation of the pupil and ciliary sympathetic regulation of the pupil and ciliary muscle .muscle .

PHOTORECEPTORSPHOTORECEPTORS These cells are rods and cones . their cell These cells are rods and cones . their cell

bodies lie in the ONL.bodies lie in the ONL. They synapse at the OPL.They synapse at the OPL. An elongated part of the cell protrudes toward An elongated part of the cell protrudes toward

the RPE and this part is divided to outer the RPE and this part is divided to outer segment and inner segment which are linked by segment and inner segment which are linked by the ciliary stalk .the ciliary stalk .

The ellipsoid ( the apical portion of the inner The ellipsoid ( the apical portion of the inner segment ) is rich of mitochondria .segment ) is rich of mitochondria .

There are two types of photoreceptors : rods and There are two types of photoreceptors : rods and cones .cones .

Little is understood about how photoreceptor shape Little is understood about how photoreceptor shape affects function .affects function .

The outer segments of both rods and cones contain The outer segments of both rods and cones contain many double membrane discs or flattened saccules .many double membrane discs or flattened saccules .

The discs are isolated in rods , but in cones they The discs are isolated in rods , but in cones they connects to cell membrane .connects to cell membrane .

The discs are of great importance because the visual The discs are of great importance because the visual pigments , which capture the photons to begin the visual pigments , which capture the photons to begin the visual process , appear to be built into the discs .process , appear to be built into the discs .

The visual pigments are insoluble .The visual pigments are insoluble . They are intrinsic membrane proteins .They are intrinsic membrane proteins . They constitute > 50% of the protein of the outer They constitute > 50% of the protein of the outer

segment .segment .

Visual pigment = aldehyde of vit.A and various Visual pigment = aldehyde of vit.A and various proteins .proteins .

Outer segments are capable of regeneration .Outer segments are capable of regeneration . Destruction may occur on RD , vit.A def. Destruction may occur on RD , vit.A def. Surrounding the photoreceptor outer and inner Surrounding the photoreceptor outer and inner

segment a gel termed interphotoreceptor matrix segment a gel termed interphotoreceptor matrix (IPM) .(IPM) .

Both cone and rod discs shed and are Both cone and rod discs shed and are phagocytosed by RPE.phagocytosed by RPE.

Rods shed shortly after morning Rods shed shortly after morning Cones peak shedding at the end of the day.Cones peak shedding at the end of the day. Outer segment … production and destruction .Outer segment … production and destruction .

RECEPTOR OUTER SEGMENT AND RECEPTOR OUTER SEGMENT AND PIGMENT EPITHELIUM RELATIONS PIGMENT EPITHELIUM RELATIONS

RPE is implicated in the ocular transport of RPE is implicated in the ocular transport of vit.A and it’s derivatives .vit.A and it’s derivatives .

The regeneration of visual pigment is one The regeneration of visual pigment is one factor in dark adaptation after the factor in dark adaptation after the significant bleaching of such pigment .significant bleaching of such pigment .

The RPE contains melanosomes which The RPE contains melanosomes which contain melanin .contain melanin .

The melanosomes minimize the scattering The melanosomes minimize the scattering of light from one photoreceptor to another.of light from one photoreceptor to another.

Detachment of the retina consists of the physical Detachment of the retina consists of the physical separation of the retina from its close separation of the retina from its close approximation to the RPE .approximation to the RPE .

Parameters that contribute to attachment are :Parameters that contribute to attachment are : factors regulating the volume of fluid in the factors regulating the volume of fluid in the

ocular ventricle .ocular ventricle . acid mucopolysaccharides , known to be present acid mucopolysaccharides , known to be present

in the fluid of the ocular ventricle , which could in the fluid of the ocular ventricle , which could contribute to its viscosity or to the cohesion of contribute to its viscosity or to the cohesion of neighboring membranes .neighboring membranes .

a barb action of the elongated melanosomes in a barb action of the elongated melanosomes in the long microvilli from the RPE .the long microvilli from the RPE .

the RPE also has phagocytic function . the the RPE also has phagocytic function . the membrane of the outer segment heals membrane of the outer segment heals over.over.

The receptor axes are so tipped as to The receptor axes are so tipped as to orient them to the exit pupil of the eye orient them to the exit pupil of the eye rather than to the center of the ocular rather than to the center of the ocular sphere . this maximizes the ability of any sphere . this maximizes the ability of any one photoreceptor to capture light . one photoreceptor to capture light .

During the act of accommodation During the act of accommodation orientation of receptor outer segments is orientation of receptor outer segments is altered .altered .

It is now clear that after bleaching of It is now clear that after bleaching of photopigment , the 11-cis-retinaldehyde has photopigment , the 11-cis-retinaldehyde has been converted to all-trans-retinaldehyde . there been converted to all-trans-retinaldehyde . there is then a conversion to all-trans-retinol by a is then a conversion to all-trans-retinol by a dehydrogenase .dehydrogenase .

The RPE is the site where reoxidation of retinol The RPE is the site where reoxidation of retinol to retinal occurs , as well as reisomerization of to retinal occurs , as well as reisomerization of the all-trans-isomer to the 11-cis-isomer .the all-trans-isomer to the 11-cis-isomer .

Important carrier proteins are involved in moving Important carrier proteins are involved in moving these vitamin A derivatives between the these vitamin A derivatives between the photoreceptors and RPE in both directions .photoreceptors and RPE in both directions .

DISTRIBUTION OF PHOTORECEPTORS DISTRIBUTION OF PHOTORECEPTORS AND OTHER NEURONS WITHIN THE AND OTHER NEURONS WITHIN THE RETINA RETINA

How different types of photoreceptors are How different types of photoreceptors are distributed in retinas .distributed in retinas .

Regions biased for inspecting details are Regions biased for inspecting details are richer in cones by virtue of containing richer in cones by virtue of containing thinner cones and more of them per unit thinner cones and more of them per unit area than elsewhere and more ganglion area than elsewhere and more ganglion cells per unit area as well . such a region cells per unit area as well . such a region is termed central region .is termed central region .

Physiologically ,central regions tend to be Physiologically ,central regions tend to be free of major blood vessels and in certain free of major blood vessels and in certain retinas even capillaries .retinas even capillaries .

In the human the extent of the cone-rich In the human the extent of the cone-rich area is about 5.5mm in diameter, and it area is about 5.5mm in diameter, and it tends to be variably demarked by the tends to be variably demarked by the presence of yellow , nonphotolabile presence of yellow , nonphotolabile carotenoids in photoreceptor axons and carotenoids in photoreceptor axons and some inner retinal cells . the pigment is some inner retinal cells . the pigment is largely zeaxanthin . these pigments give largely zeaxanthin . these pigments give the region the name .. macula lutea .the region the name .. macula lutea .

The center of the cone-rich region The center of the cone-rich region contains a pit or fovea .contains a pit or fovea .

In the human the full depression occupies In the human the full depression occupies about 5 degrees of arc or about 1.5mm on about 5 degrees of arc or about 1.5mm on the retina .the retina .

In the center of the fovea there is the In the center of the fovea there is the foveola ( 54 minutes of arc = foveola ( 54 minutes of arc = 260micrometer ).260micrometer ).

Here only photoreceptor type present Here only photoreceptor type present ( cones ). ( cones ).

Cones in this region have the finest Cones in this region have the finest diameters of the retinal cones ( 1.5 diameters of the retinal cones ( 1.5 micrometer ) and this is the region of micrometer ) and this is the region of highest concentration of cones in the highest concentration of cones in the retina . retina .

Functionally the fovea is the position of the Functionally the fovea is the position of the retina to which , by turning the eye ball , a retina to which , by turning the eye ball , a person brings the image of what ever is of person brings the image of what ever is of greatest psychologic interest in the visual greatest psychologic interest in the visual field .field .

Anatomically , the retina in the central Anatomically , the retina in the central fovea consists entirely of the outer and fovea consists entirely of the outer and inner segments of the photoreceptors , the inner segments of the photoreceptors , the photoreceptor cell bodies , and the photoreceptor cell bodies , and the intervening glial cell processes .intervening glial cell processes .

The axons of the photoreceptors , the so-The axons of the photoreceptors , the so-called Henle fibers , are swept horizontally called Henle fibers , are swept horizontally and leave the foveal area . the terminals of and leave the foveal area . the terminals of foveal cones , the horizontal neurons and foveal cones , the horizontal neurons and bipolar neurons with which they interact , bipolar neurons with which they interact , and those amacrine cells and ganglion and those amacrine cells and ganglion cells that receive information from the cells that receive information from the foveal cones are centrifugally and laterally foveal cones are centrifugally and laterally displaced so that, in the foveolar region , displaced so that, in the foveolar region , all these elements are missing , and they all these elements are missing , and they are minimized elsewhere in the fovea .are minimized elsewhere in the fovea .

The foveola is surrounded by a parafoveal The foveola is surrounded by a parafoveal region , and this by a perifoveal region .region , and this by a perifoveal region .

They are 2.5mm and 5.5mm in diameter They are 2.5mm and 5.5mm in diameter respectively .respectively .

If one imagines a vertical line passing If one imagines a vertical line passing through the central fovea , thus separating through the central fovea , thus separating nasal retina from temporal retina , axons nasal retina from temporal retina , axons from ganglion cells of the temporal retina from ganglion cells of the temporal retina will project to the LGN and superior will project to the LGN and superior colliculus on the same side of the brain as colliculus on the same side of the brain as the eye , whereas ganglion cells from the the eye , whereas ganglion cells from the nasal half of the retina will cross in the nasal half of the retina will cross in the optic chiasm and terminate in the LGN and optic chiasm and terminate in the LGN and superior colliculus of the contralateral superior colliculus of the contralateral brain . brain .

The adult human retina has about 120 The adult human retina has about 120 million rods and about 6-7 million cones .million rods and about 6-7 million cones .

Cone density peaks in the fovea at about Cone density peaks in the fovea at about 199.000 cones / mm2 , and then falls off 199.000 cones / mm2 , and then falls off sharply in all directions , although there is sharply in all directions , although there is some concentration of cones along the some concentration of cones along the horizontal meridian , particularly in the horizontal meridian , particularly in the nasal retina .nasal retina .

The area for useful color vision in humans The area for useful color vision in humans has a diameter of 9mm centered on the has a diameter of 9mm centered on the fovea .fovea .

The rod-free center of the fovea may be The rod-free center of the fovea may be deficient in blue sensitive cones .deficient in blue sensitive cones .

The human rod density peaks in a The human rod density peaks in a somewhat elliptical ring .somewhat elliptical ring .

The highest rod concentration The highest rod concentration ( 160.000 /mm2 ) along this configuration ( 160.000 /mm2 ) along this configuration occurs in the superior retina .occurs in the superior retina .

It is important to realize that when light It is important to realize that when light levels are in the photopic range of cone levels are in the photopic range of cone function , their activity tends to command function , their activity tends to command all retinal output .all retinal output .

INL contains the somal regions of bipolar INL contains the somal regions of bipolar neurons and also contains those of neurons and also contains those of horizontal and amacrine neurons , horizontal and amacrine neurons , interplexiform neurons , rare displaced interplexiform neurons , rare displaced ganglion cells , and the somal regions of ganglion cells , and the somal regions of the glial cells of Muller .the glial cells of Muller .

In this layer it is difficult to know the exact In this layer it is difficult to know the exact distribution of cells across the retinal area. distribution of cells across the retinal area.

Situation for the distribution of ganglion Situation for the distribution of ganglion cells is somewhat better , because this cells is somewhat better , because this region belong only to ganglion cells and region belong only to ganglion cells and displaced amacrines .displaced amacrines .

However , there are several varieties in However , there are several varieties in ganglion cells in terms of size and ganglion cells in terms of size and distribution of processes distribution of processes

It is fair to state that the macular region in It is fair to state that the macular region in the human retina is rich in small ganglion the human retina is rich in small ganglion cells and that, by comparison to cells and that, by comparison to concentration of cones in this region , it concentration of cones in this region , it seems likely that there are enough small seems likely that there are enough small ganglion cells to permit the consideration ganglion cells to permit the consideration that each could receive information via that each could receive information via intermediate cells from a rather small intermediate cells from a rather small population of cones .population of cones .

A chain of information transmission in A chain of information transmission in which the ratio of receptors connected via which the ratio of receptors connected via intermediates to ganglion cells approaches intermediates to ganglion cells approaches 1:1 is what one might idealize for a region 1:1 is what one might idealize for a region of high detail discrimination .of high detail discrimination .

In other retinal regions there is a high ratio In other retinal regions there is a high ratio of rods to ganglion cells and , as of rods to ganglion cells and , as expected , a high sensitivity to detecting expected , a high sensitivity to detecting light but poor form discrimination .light but poor form discrimination .

SYNAPTIC CONNECTIONS OF THE SYNAPTIC CONNECTIONS OF THE RETINARETINA

Receptor terminals are spherules or Receptor terminals are spherules or pedicles .pedicles .

Spherules are small and round while Spherules are small and round while pedicles are large and have flat bases pedicles are large and have flat bases facing the rest of the OPL .facing the rest of the OPL .

Rods end in spherules and cones in Rods end in spherules and cones in pedicles . pedicles .

Processes of horizontal cells and bipolar Processes of horizontal cells and bipolar neurons are deeply invaginated in rod neurons are deeply invaginated in rod spherules but only superficially spherules but only superficially invaginated into the bases of pedicles .invaginated into the bases of pedicles .

The receptor terminal is full of synaptic The receptor terminal is full of synaptic vesicles .vesicles .

There is some contacts between cones There is some contacts between cones and cones and cones and rods . these and cones and cones and rods . these contacts helps in spread of current contacts helps in spread of current between cells .between cells .

Horizontal cells occur in the outer portion Horizontal cells occur in the outer portion of the INL and are neurons whose of the INL and are neurons whose processes are disposed in a manner processes are disposed in a manner suggesting a role in the horizontal suggesting a role in the horizontal integration of retinal activity .integration of retinal activity .

An amacrine cell is a neuron with no An amacrine cell is a neuron with no morphologically definable axon . there morphologically definable axon . there soma lie in the inner aspect of the INL.soma lie in the inner aspect of the INL.

RETINAL SYNAPTIC MECHANISMS AND RETINAL SYNAPTIC MECHANISMS AND PUTATIVE CHEMICAL PUTATIVE CHEMICAL NEUROTRANSMITTERS NEUROTRANSMITTERS

The photoreceptors have terminals rich in The photoreceptors have terminals rich in synaptic vesicles and evidence strongly synaptic vesicles and evidence strongly indicates that the transmitter of the indicates that the transmitter of the photoreceptor is glutamate , an excitatory photoreceptor is glutamate , an excitatory ( depolarizing ) aminoacid . ( depolarizing ) aminoacid .

Interphotoreceptor contacts between cones , or Interphotoreceptor contacts between cones , or rods and cones , have frequently been noted rods and cones , have frequently been noted and appear to include gap junctions , indicating and appear to include gap junctions , indicating the possibility of electronic interactions between the possibility of electronic interactions between these cells .these cells .

The photoreceptors have terminals rich in The photoreceptors have terminals rich in synaptic vesicles and evidence strongly synaptic vesicles and evidence strongly indicates that the transmitter of the indicates that the transmitter of the photoreceptor is glutamate , an excitatory ( photoreceptor is glutamate , an excitatory ( depolarizing ) aminoacid . depolarizing ) aminoacid .

Interphotoreceptor contacts between Interphotoreceptor contacts between cones , or rods and cones , have cones , or rods and cones , have frequently been noted and appear to frequently been noted and appear to include gap junctions , indicating the include gap junctions , indicating the possibility of electronic interactions possibility of electronic interactions between these cells .between these cells .

The action of a neurotransmitter or The action of a neurotransmitter or neuromodulator , promoting excitation or neuromodulator , promoting excitation or inhibition , is both a parameter of the inhibition , is both a parameter of the nature of the agent and of the membrane nature of the agent and of the membrane mechanisms determining the response of mechanisms determining the response of a particular cell to the agent .a particular cell to the agent .

For example , the action of acetylcholine For example , the action of acetylcholine on skeletal muscle is excitatory , but its on skeletal muscle is excitatory , but its action on cardiac muscle is inhibitory .action on cardiac muscle is inhibitory .

Finally , when transmitter or Finally , when transmitter or neuromodulator are released it is neuromodulator are released it is obviously desirable to terminate their obviously desirable to terminate their presence by enzyme action or other presence by enzyme action or other mechanisms after they have carried out mechanisms after they have carried out their signaling function .their signaling function .

Thus the glial cells of Muller appear to Thus the glial cells of Muller appear to take up and metabolize glutamate .take up and metabolize glutamate .

ELECTRICAL ACTIVITY AND ELECTRICAL ACTIVITY AND INFORMATION PROCESSING BY INFORMATION PROCESSING BY RETINAL NEURONES . RETINAL NEURONES .

The electrical activity of individual cells The electrical activity of individual cells can be recorded by intracellular electrodes can be recorded by intracellular electrodes and sometimes by extracellular electrodes and sometimes by extracellular electrodes ( animals ) .( animals ) .

Each cell in the chain of nerve cells Each cell in the chain of nerve cells processing visual information has its own processing visual information has its own receptive field .receptive field .

There is a considerable overlapping of There is a considerable overlapping of receptive fields of cells near each other in receptive fields of cells near each other in the retina .the retina .

Any receptive field has sometimes distinct Any receptive field has sometimes distinct regions , such as ( center ) and regions , such as ( center ) and ( surround ). ( surround ).

When a small spot of light , at an intensity When a small spot of light , at an intensity above back ground , is first positioned on above back ground , is first positioned on the center and then on the surround , the center and then on the surround , opposite responses are often elicited . opposite responses are often elicited .

If the spot of light is expanded to stimulate If the spot of light is expanded to stimulate simultaneously both center and surround simultaneously both center and surround diminished or absent response will be diminished or absent response will be elicited .elicited .

Spot of darkness also has the same Spot of darkness also has the same response .response .

The spatial dimensions of receptive field The spatial dimensions of receptive field centers are one determinant of spatial centers are one determinant of spatial resolution – the smaller the center the resolution – the smaller the center the smaller the possible spatial resolution .smaller the possible spatial resolution .

Electrodes across the eye ( see ) a summation Electrodes across the eye ( see ) a summation of the various individual cell responses.of the various individual cell responses.

Retina with its population of rods and cones Retina with its population of rods and cones modifies the signals reaching ganglion cells as a modifies the signals reaching ganglion cells as a function of its adaptational state , that is to say , function of its adaptational state , that is to say , when it is dark adapted to a lower level of when it is dark adapted to a lower level of illumination or when it is light adapted to a more illumination or when it is light adapted to a more intense illumination .intense illumination .

Altering the adaptational level involves both Altering the adaptational level involves both photochemical and electrochemical changes in photochemical and electrochemical changes in the receptors and probably at subsequent retinal the receptors and probably at subsequent retinal processing levels .processing levels .

ALL EVIDENCE POINTS TO A ALL EVIDENCE POINTS TO A FUNCTIONAL ORGANISATION IN THE FUNCTIONAL ORGANISATION IN THE RETINA AND HIGHER VISUAL SYSTEM RETINA AND HIGHER VISUAL SYSTEM THAT IS RELATIVISTIC AND DIRECTED THAT IS RELATIVISTIC AND DIRECTED AT DISCERNING LOCAL CONTRASTS AT DISCERNING LOCAL CONTRASTS THAT ESTABLISH BORDERS BETWEEN THAT ESTABLISH BORDERS BETWEEN AREAL ELEMENTS IN THE COMPLEX AREAL ELEMENTS IN THE COMPLEX IMAGE OF THE VISUAL FIELD , IMAGE OF THE VISUAL FIELD , RATHER THAN MECHANISMS FOR RATHER THAN MECHANISMS FOR ASSAYING THE ABSOLUTE LEVELS OF ASSAYING THE ABSOLUTE LEVELS OF LIGHT IN LOCAL AREAS .LIGHT IN LOCAL AREAS .

A retinal locus receiving an image of an A retinal locus receiving an image of an area perceived as ( black ) at a high level area perceived as ( black ) at a high level of illumination may actually be receiving a of illumination may actually be receiving a greater absolute quantity of light than a greater absolute quantity of light than a retinal locus receiving an image of an area retinal locus receiving an image of an area perceived as ( white ) at a dim illumination perceived as ( white ) at a dim illumination if , in the former instance , the black area if , in the former instance , the black area is receiving is receiving relativelyrelatively much less light than much less light than its general surround and in the later its general surround and in the later instance , if the white area is receiving instance , if the white area is receiving relatively relatively much more light than its much more light than its surround .surround .

Moreover , the color perceived to be Moreover , the color perceived to be present in a patch will depend on the present in a patch will depend on the nature of the perceived color in its nature of the perceived color in its surround .surround .

NEURAL NETWORK OF VISUAL NEURAL NETWORK OF VISUAL APPARATUS ARE MORE KEYED TO APPARATUS ARE MORE KEYED TO DETECTING FLUCTUATIONS IN THE DETECTING FLUCTUATIONS IN THE RETINAL IMAGE CAUSED BY RETINAL IMAGE CAUSED BY CHANGES IN LOCAL RELATIVE CHANGES IN LOCAL RELATIVE INTENSITY THAN FOR DETECTING INTENSITY THAN FOR DETECTING STEADY DISPLAYS.STEADY DISPLAYS.

ONE SOURCE OF THIS FLUCTUATION ONE SOURCE OF THIS FLUCTUATION IS MOVEMENT OF THE IMAGE OF THE IS MOVEMENT OF THE IMAGE OF THE VISUAL FIELD ON THE RETINA .VISUAL FIELD ON THE RETINA .

THE LATTER FACT RAISES AN THE LATTER FACT RAISES AN IMPORTANT POINT REGARDING IMPORTANT POINT REGARDING MOVEMENTS OF THE EYE .MOVEMENTS OF THE EYE .

More over if by some means the image of the More over if by some means the image of the visual field is made to hold its position on the visual field is made to hold its position on the retina despite eye movements , the image fades retina despite eye movements , the image fades and is no longer seen by the observer .and is no longer seen by the observer .

This explains why the shadows of the blood This explains why the shadows of the blood vessels of the retina are not in constant view in vessels of the retina are not in constant view in superimposition on the field of vision , because superimposition on the field of vision , because by having a fixed relation to the retina and the by having a fixed relation to the retina and the pathway of light , they are adapted out of the pathway of light , they are adapted out of the perceived image .perceived image .

The important point therefore is that a normal The important point therefore is that a normal fine instability of the eye contributes to the fine instability of the eye contributes to the normal visual process . normal visual process .

Photoreceptors hyperpolarize when Photoreceptors hyperpolarize when exposed to flashes of light .exposed to flashes of light .

Single rod may be excited by a single Single rod may be excited by a single quantum of light .quantum of light .

For a single rod to be excited by a single For a single rod to be excited by a single photon represents an exquisite sensitivity . photon represents an exquisite sensitivity .

There is more synaptic activity in the dark .There is more synaptic activity in the dark . In the dark a current is flowing into the In the dark a current is flowing into the

outer segment from the rest of the outer segment from the rest of the photoreceptor photoreceptor

The effect of a flash of light is to diminish The effect of a flash of light is to diminish this dark current . this diminution is this dark current . this diminution is achieved largely by decreasing the achieved largely by decreasing the conductance for sodium ion across the conductance for sodium ion across the plasma membrane of the outer segment .plasma membrane of the outer segment .

Light causes a decrease rather than Light causes a decrease rather than increase in calcium activity in outer increase in calcium activity in outer segments. segments.

In the dark a low level of calcium ions In the dark a low level of calcium ions enters outer segments along with sodium enters outer segments along with sodium ions. ions.

This entry of calcium ions is countered by This entry of calcium ions is countered by the activity of a counterporting mechanism the activity of a counterporting mechanism that exchanges intracellular calcium and that exchanges intracellular calcium and potassium for external sodium .potassium for external sodium .

Since in the dark calcium enters the cell Since in the dark calcium enters the cell along with sodium via the light modulated along with sodium via the light modulated channels , and as the exchange channels , and as the exchange mechanism is not regulated by light , when mechanism is not regulated by light , when the light sensitive channel is closed , the the light sensitive channel is closed , the continuing operation of the exchange continuing operation of the exchange mechanism decrease intracellular mechanism decrease intracellular calcium .calcium .

The main agent for regulating the The main agent for regulating the channels for entry of sodium is cyclic channels for entry of sodium is cyclic guanosine monophosphate (cGMP) .guanosine monophosphate (cGMP) .

Photoreceptors are extraordinarily rich in Photoreceptors are extraordinarily rich in cGMP.cGMP.

Light causes losses of cGMP by a Light causes losses of cGMP by a complex mechanism called the cyclic GMP complex mechanism called the cyclic GMP cascade , wherein bleached rhodopsin cascade , wherein bleached rhodopsin activates a nucleotide-binding protein , activates a nucleotide-binding protein , and this intermediate activates a and this intermediate activates a cyclicGMP phosphodiesterase that cyclicGMP phosphodiesterase that hydrolyzes cyclic GMP to GMP .hydrolyzes cyclic GMP to GMP .

Studies have shown that cyclic GMP can Studies have shown that cyclic GMP can act on the inner face of the plasma act on the inner face of the plasma membrane of the rod outer segment to membrane of the rod outer segment to open conductances .open conductances .

This effect is suppressed by light .This effect is suppressed by light .

It is therefore clear that a channel involved It is therefore clear that a channel involved in the transduction in rods , and also in in the transduction in rods , and also in cones , is light sensitive , because cones , is light sensitive , because appropriate levels of cGMP keep it open in appropriate levels of cGMP keep it open in the dark , while the loss of cGMP through the dark , while the loss of cGMP through the activation of cGMP phosphodiesterase the activation of cGMP phosphodiesterase via the light initiated cGMP cascade allows via the light initiated cGMP cascade allows the channel to close .the channel to close .

To complete the story , cGMP is To complete the story , cGMP is regenerated from GTP by the activity of regenerated from GTP by the activity of guanylate cyclase .guanylate cyclase .

Chelating calcium rapidly and massively Chelating calcium rapidly and massively increased cyclic GMP levels in increased cyclic GMP levels in photoreceptors , suggesting that this photoreceptors , suggesting that this enzyme is inhibited by calcium .enzyme is inhibited by calcium .

The significance of light insensitive The significance of light insensitive mechanism exchanging calcium and mechanism exchanging calcium and potassium for sodium is thus seen as a potassium for sodium is thus seen as a way to promote the resynthesis of cGMP way to promote the resynthesis of cGMP by reducing the calcium level after light by reducing the calcium level after light has caused a loss of this cyclic has caused a loss of this cyclic nucleotide .nucleotide .

Both rods and cones hyperpolarize to light Both rods and cones hyperpolarize to light flashes , but rods recover more slowly flashes , but rods recover more slowly than do cones from bright flashes than do cones from bright flashes presented against a dark background , presented against a dark background , and rods have a lower absolute threshold and rods have a lower absolute threshold to flashes presented against a dark to flashes presented against a dark background .background .

Horizontal cells only sensitive to light Horizontal cells only sensitive to light intensity or luminosity are called ( L ) type , intensity or luminosity are called ( L ) type , while those whose polarity is color while those whose polarity is color sensitive are called ( C ) type.sensitive are called ( C ) type.

Horizontal cells are often coupled by gap Horizontal cells are often coupled by gap junctions , and their effects often feed junctions , and their effects often feed back onto cones , possibly to rods , and back onto cones , possibly to rods , and possibly feed forward onto bipolars .possibly feed forward onto bipolars .

Bipolar neurons respond to contrast , not Bipolar neurons respond to contrast , not simply light intensity , and these center simply light intensity , and these center versus surround phenomena are also versus surround phenomena are also evident at the ganglion cell level .evident at the ganglion cell level .

In the photopic range spatial detail is best In the photopic range spatial detail is best detected by brightness contrast rather detected by brightness contrast rather than color contrast . the best detectors of than color contrast . the best detectors of brightness contrast are red or green cones brightness contrast are red or green cones , the relatively sparce blue cones less so., the relatively sparce blue cones less so.

Brightness contrast is poorly detected by Brightness contrast is poorly detected by rods operating in the scoptopic range .rods operating in the scoptopic range .

The likely anatomic basis for the center of The likely anatomic basis for the center of this receptive field is the population of this receptive field is the population of photoreceptors with which these bipolars photoreceptors with which these bipolars are in direct synaptic contact , whereas the are in direct synaptic contact , whereas the surround may represent another surround may represent another population of photoreceptors by which the population of photoreceptors by which the bipolars are indirectly influenced via bipolars are indirectly influenced via horizontal neurons .horizontal neurons .

The visual system contains ( on ) and The visual system contains ( on ) and ( off ) pathways .( off ) pathways .

Neurons that depolarize in response to an Neurons that depolarize in response to an increase in the intensity of light above increase in the intensity of light above background in their receptive field are background in their receptive field are termed (on ) cells , where as those that termed (on ) cells , where as those that depolarize in response to the offset or depolarize in response to the offset or diminution of light compared to diminution of light compared to background are termed ( off ) cells .background are termed ( off ) cells .

Since the light diminishes the sodium Since the light diminishes the sodium current entering the photoreceptors , all current entering the photoreceptors , all photoreceptors hyperpolarize in response photoreceptors hyperpolarize in response to increasing light and depolarize to to increasing light and depolarize to diminishing light and are therefore ( off ) diminishing light and are therefore ( off ) cells .cells .

Bipolars that depolarize to light are ( on ) Bipolars that depolarize to light are ( on ) cells , whereas those that hyperpolarize to cells , whereas those that hyperpolarize to light are ( off ) cells .light are ( off ) cells .

As the latter mimic the polarization As the latter mimic the polarization responses of photoreceptors , the synapse responses of photoreceptors , the synapse between them is said to be ( sign between them is said to be ( sign conserving ) .conserving ) .

Conversely , the synapse between Conversely , the synapse between photoreceptors and those bipolars that photoreceptors and those bipolars that depolarize to light is termed ( sign depolarize to light is termed ( sign inverting ) .inverting ) .

The two bipolar classes may function in The two bipolar classes may function in detecting relatively bright centers in darker detecting relatively bright centers in darker surrounds , or the converse .surrounds , or the converse .

Particular amacrine cells probably serve in Particular amacrine cells probably serve in abstracting particular environmental abstracting particular environmental features to permit ganglion cells to features to permit ganglion cells to respond to such things as movements in respond to such things as movements in particular directions or to objects of particular directions or to objects of particular sizes, orientations , shapes or particular sizes, orientations , shapes or patterns .patterns .

Ganglion cells :Ganglion cells : ( on ) cells ( on ) cells ( off ) cells ( off ) cells ( on-off ) cells ( on-off ) cells It should be remembered that individual It should be remembered that individual

ganglion cells summate the effects of ganglion cells summate the effects of numerous impinging inputs of electronic or numerous impinging inputs of electronic or chemical synapses and the location of a chemical synapses and the location of a synapse on a cell is also of possible synapse on a cell is also of possible significance .significance .

Ganglion cells largely fall into one of two Ganglion cells largely fall into one of two major groups characterized by whether major groups characterized by whether their discharge pattern is sustained or their discharge pattern is sustained or phasic .phasic .

A ganglion cell exhibiting a sustained A ganglion cell exhibiting a sustained discharge is not responsive to the discharge is not responsive to the rearrangement of dark and bright elements rearrangement of dark and bright elements in its receptive field provided that the net in its receptive field provided that the net illumination is constant , whereas phasic illumination is constant , whereas phasic ganglion. cells would respond to each ganglion. cells would respond to each such change .such change .

Fast-conducting axons from the retina Fast-conducting axons from the retina tend to end in the magnocellular layers of tend to end in the magnocellular layers of the LGN , whereas axons of medium the LGN , whereas axons of medium conduction velocity project to the conduction velocity project to the parvocellular layers of the LGN .parvocellular layers of the LGN .

Receptive fields of the ganglion cells tend Receptive fields of the ganglion cells tend to increase with distance from the central to increase with distance from the central area or fovea .area or fovea .

It should be clear from the preceding It should be clear from the preceding discussion that the retina is organized to discussion that the retina is organized to permit the lateral interaction of nerve cell permit the lateral interaction of nerve cell networks and that this often can take the networks and that this often can take the form of lateral inhibition .form of lateral inhibition .

To summarize the foregoing discussion , To summarize the foregoing discussion , one can consider the retina ( and higher one can consider the retina ( and higher visual system ) to have parallel visual system ) to have parallel depolarizing ( on- center ) and depolarizing ( on- center ) and hyperpolarizing ( off-center ) informational hyperpolarizing ( off-center ) informational pathways , with a neuron assigned to a pathways , with a neuron assigned to a pathway by the polarity of its response to pathway by the polarity of its response to the onset of light on its receptive field .the onset of light on its receptive field .

Opposite effects occur with light offset , Opposite effects occur with light offset , and cells that exhibit ( on-off ) behavior are and cells that exhibit ( on-off ) behavior are thought to connect to both information thought to connect to both information streams .streams .

ROLE OF GLIA ROLE OF GLIA The membrane potential of the Muller cell The membrane potential of the Muller cell

may reflect its behavior as a potassium may reflect its behavior as a potassium electrode .electrode .

Glia undoubtedly have phagocytic Glia undoubtedly have phagocytic functions in pathologic states and certainly functions in pathologic states and certainly functions that are still unknown .functions that are still unknown .

THE OPTIC NERVETHE OPTIC NERVE

The optic nerve can be considered to have The optic nerve can be considered to have four portions : four portions :

- the intraocular portion .- the intraocular portion .

- intraorbital portion - intraorbital portion

- intracanalicular portion- intracanalicular portion

- intracranial portion- intracranial portion

1 million axons that pass from each eye 1 million axons that pass from each eye into the optic nerve conduct partially into the optic nerve conduct partially processed visual information from the processed visual information from the retinal ganglion cells to the lateral retinal ganglion cells to the lateral geniculate body, superior colliculus , geniculate body, superior colliculus , hypothalamus , and certain midbrain hypothalamus , and certain midbrain centers .centers .

There are now considered to be two There are now considered to be two parallel pathways in the anterior visual parallel pathways in the anterior visual pathways carrying different types of visual pathways carrying different types of visual information .information .

The luminance pathway , also called M The luminance pathway , also called M ( for magnocellular ) , utilizes larger retinal ( for magnocellular ) , utilizes larger retinal ganglion cells with larger diameter axons ganglion cells with larger diameter axons that synapse in the magnocellular layers of that synapse in the magnocellular layers of the lateral geniculate body . the lateral geniculate body .

They are most sensitive to change in They are most sensitive to change in luminance at low light levels , subserve luminance at low light levels , subserve motion perception , and are relatively motion perception , and are relatively insensitive to color .insensitive to color .

Only a small proportion of all ganglion Only a small proportion of all ganglion cells are part of the M pathway .cells are part of the M pathway .

The color or P ( for parvocellular ) pathway The color or P ( for parvocellular ) pathway consists of smaller ganglion cells that consists of smaller ganglion cells that project to the parvocellular lateral project to the parvocellular lateral geniculate layers and preferentially carry geniculate layers and preferentially carry information on color and fine detail .information on color and fine detail .

This group includes the majority of retinal This group includes the majority of retinal ganglion cells , including the midget cells ganglion cells , including the midget cells of the foveal area .of the foveal area .

After passing through the lamina cribrosa to After passing through the lamina cribrosa to enter the intraorbital portion , the fibers gain a enter the intraorbital portion , the fibers gain a myelin sheath .myelin sheath .

From this point onward the impulse is carried by From this point onward the impulse is carried by saltatory conduction typical of white matter tracts saltatory conduction typical of white matter tracts and myelinated peripheral nerves .and myelinated peripheral nerves .

Depolarization occurs only at the nodes of Depolarization occurs only at the nodes of Ranvier , with the impulse jumping from node to Ranvier , with the impulse jumping from node to node .with this saltatory conduction , the impulse node .with this saltatory conduction , the impulse passes along the axon more rapidly than it passes along the axon more rapidly than it would by conduction of an action potential , such would by conduction of an action potential , such as in unmyelinated fiber .as in unmyelinated fiber .

Saltatory conduction also conserves Saltatory conduction also conserves metabolic energy , because only the metabolic energy , because only the exposed portion of the axon membrane at exposed portion of the axon membrane at the node needs to be repolarized , not the the node needs to be repolarized , not the entire length of the axon .entire length of the axon .

What will happen in demyelinating What will happen in demyelinating diseases ?? diseases ??

The number of fibers appear to be related The number of fibers appear to be related to the size of the optic disc as it is seen to the size of the optic disc as it is seen clinically . the larger the optic disc , the clinically . the larger the optic disc , the more the number of fibers .more the number of fibers .

There are more than twice the number of There are more than twice the number of axons in a fetal primate eye than in the axons in a fetal primate eye than in the adult eye .adult eye .

It is believed that ganglion cells die if their axons It is believed that ganglion cells die if their axons do not successfully synapse with appropriate do not successfully synapse with appropriate targets in the brain .targets in the brain .

After birth , this attrition of the optic nerve fibers After birth , this attrition of the optic nerve fibers slows dramatically .slows dramatically .

During a 75-year human life , the further loss of During a 75-year human life , the further loss of ganglion cells , presumably from aging , ganglion cells , presumably from aging , encompasses only 25% of the total .encompasses only 25% of the total .

Blacks have larger discs and hence larger cups .Blacks have larger discs and hence larger cups . The movement along axons seems to occur by The movement along axons seems to occur by

at least two different processes : rapid axonal at least two different processes : rapid axonal transport and slow axoplasmic flow . transport and slow axoplasmic flow .

Rapid axonal transport is bidirectional , Rapid axonal transport is bidirectional , orthograde from the ganglion cell body to the orthograde from the ganglion cell body to the axon terminal and retrograde from the terminal axon terminal and retrograde from the terminal to the ganglion cell body .to the ganglion cell body .

This active transport requires metabolic energy , This active transport requires metabolic energy , which is obtained from ATP produced locally which is obtained from ATP produced locally within each axon segment along the way .within each axon segment along the way .

200 – 400 mm / day 200 – 400 mm / day A variety of chemical messages , hormones and A variety of chemical messages , hormones and

foreign material such as toxins and viruses can foreign material such as toxins and viruses can be passengers in this system .be passengers in this system .

Slow axonal flow can be traced as the Slow axonal flow can be traced as the movement of soluble proteins synthesized movement of soluble proteins synthesized in the cell body toward the axon terminal in the cell body toward the axon terminal at a rate of only 1 to 3 mm / day .at a rate of only 1 to 3 mm / day .

THE GLIA THE GLIA The predominant glial element in the optic The predominant glial element in the optic

nerve head is the astrocyte .nerve head is the astrocyte .Their function to support the bundles of Their function to support the bundles of

nerve fibers as they turn to enter the optic nerve fibers as they turn to enter the optic nerve from the retina nerve from the retina

Astrocytes also provide a cohesiveness to Astrocytes also provide a cohesiveness to neural compartment .neural compartment .

Astroglia in the nerve head presumably Astroglia in the nerve head presumably also serve to moderate conditions for also serve to moderate conditions for neural function , for example , by neural function , for example , by absorbing excess extracellular potassium absorbing excess extracellular potassium ions released by depolarizing axons and ions released by depolarizing axons and by storing glycogen for use during by storing glycogen for use during transient oligemia .transient oligemia .

They function in the nerve head like the They function in the nerve head like the Muller cells of the retina .Muller cells of the retina .

The oligodendrocytes form and maintain The oligodendrocytes form and maintain the myelin sheaths , as they do elsewhere the myelin sheaths , as they do elsewhere in the CNS .in the CNS .

THE BLOOD VESSELSTHE BLOOD VESSELSThe optic nerve microvascular bed The optic nerve microvascular bed

resembles anatomically the retinal and resembles anatomically the retinal and CNS vessels .CNS vessels .

The optic nerve vessels share with those The optic nerve vessels share with those of the retina ( and of the CNS in of the retina ( and of the CNS in general )the physiologic properties of general )the physiologic properties of autoregulation and the presence of the autoregulation and the presence of the blood brain barrier .blood brain barrier .

Because of autoregulation , the rate of Because of autoregulation , the rate of blood flow in the optic nerve is not much blood flow in the optic nerve is not much affected by intraocular pressure ( IOP ) .affected by intraocular pressure ( IOP ) .

In the retina and optic nerve , as in the In the retina and optic nerve , as in the brain , the vascular tone is increased by brain , the vascular tone is increased by autoregulation when blood pressure autoregulation when blood pressure rises , increasing the resistance to flow , rises , increasing the resistance to flow , so that the flow level is not affected by the so that the flow level is not affected by the elevated blood pressure .elevated blood pressure .

Elevated IOP compresses the vein , Elevated IOP compresses the vein , increasing the total resistance to flow increasing the total resistance to flow through the arteriovenous circuit , which through the arteriovenous circuit , which would reduce the blood flow for a given of would reduce the blood flow for a given of arterial blood pressure .arterial blood pressure .

However , autoregulation compensates for However , autoregulation compensates for venous compression by reducing the vascular venous compression by reducing the vascular tone in other parts of the circuit as IOP rises , so tone in other parts of the circuit as IOP rises , so that blood flow is maintained despite the venous that blood flow is maintained despite the venous compression caused by intraocular pressure .compression caused by intraocular pressure .

Autoregulation seems to be accomplished in part Autoregulation seems to be accomplished in part as a response to the degree of arterial stretching as a response to the degree of arterial stretching and in part as metabolic autoregulation … as and in part as metabolic autoregulation … as carbon dioxide concentration , pH , or oxygen carbon dioxide concentration , pH , or oxygen level .level .

CO2 is a particularly powerful vasodilator . CO2 CO2 is a particularly powerful vasodilator . CO2 accumulates because of inadequate blood flow , accumulates because of inadequate blood flow , the vessels dilate and the blood flow increases .the vessels dilate and the blood flow increases .

PAPILLEDEMA ( OPTIC NERVE-HEAD PAPILLEDEMA ( OPTIC NERVE-HEAD SWELLING )SWELLING )

A variety of intracranial conditions may A variety of intracranial conditions may result in papilledema .result in papilledema .

Obstruction of cerebrospinal fluid exit , Obstruction of cerebrospinal fluid exit , which results in an elevated intracranial which results in an elevated intracranial ( hydrostatic pressure .( hydrostatic pressure .

Subarachnoid space extends through the Subarachnoid space extends through the optic canal around the optic nerve and the optic canal around the optic nerve and the hydrostatic pressure is transmittied there hydrostatic pressure is transmittied there around the optic nerve .around the optic nerve .

The central retinal vein , as it crosses the nerve The central retinal vein , as it crosses the nerve sheaths in the mid-orbit is subject to external sheaths in the mid-orbit is subject to external compression by the subarachnoid pressure .compression by the subarachnoid pressure .

When the subarachnoid pressure exceeds When the subarachnoid pressure exceeds venous pressure , compression of the vein venous pressure , compression of the vein increases resistance to flow at that point and increases resistance to flow at that point and elevates slightly the venous pressure upstream .elevates slightly the venous pressure upstream .

If spontaneous venous pulsations are present in If spontaneous venous pulsations are present in the optic disc , as happens in many normal the optic disc , as happens in many normal individuals , they disappear when the elevated individuals , they disappear when the elevated central retinal venous pressure exceeds the central retinal venous pressure exceeds the normal IOP .normal IOP .

It is now thought that the main It is now thought that the main pathophysiologic event is an impairment of pathophysiologic event is an impairment of slow axoplasmic flow . the tissue pressure slow axoplasmic flow . the tissue pressure of the axons within the eyes is equal to of the axons within the eyes is equal to IOP , whereas in the orbit the intra-axonal IOP , whereas in the orbit the intra-axonal tissue pressure is governed by the tissue pressure is governed by the subarachnoid pressure .subarachnoid pressure .

the lamina cribrosa is the partition that the lamina cribrosa is the partition that separates the two pressure separates the two pressure compartments , where axons are compartments , where axons are subjected to this pressure gradient .subjected to this pressure gradient .

under normal conditions the IOP is greater under normal conditions the IOP is greater than the subarachnoid pressure and the than the subarachnoid pressure and the gradient may be thought of as augmenting gradient may be thought of as augmenting the movement of axoplasm out of the eye .the movement of axoplasm out of the eye .

however , when intracranial pressure however , when intracranial pressure equals or exceeds IOP , the forces of slow equals or exceeds IOP , the forces of slow axoplasmic flow encounter a diminished or axoplasmic flow encounter a diminished or reverse pressure gradient . the axons in reverse pressure gradient . the axons in the optic nerve head become distended .the optic nerve head become distended .

although slow axoplasmic flow is although slow axoplasmic flow is impaired , the axons often continue to impaired , the axons often continue to function .function .

visual function may be quite normal in visual function may be quite normal in chronic papilledema for a long time , chronic papilledema for a long time , except that the physiologic blind spot is except that the physiologic blind spot is enlarged when the swollen disc displaces enlarged when the swollen disc displaces the inner retina next to the optic nerve the inner retina next to the optic nerve head .head .

Not all swellings of the optic nerve head Not all swellings of the optic nerve head are papilledema due to increased are papilledema due to increased subarachnoid pressure .subarachnoid pressure .

For example : anterior ischemic optic For example : anterior ischemic optic neuropathy in which the nerve head swells neuropathy in which the nerve head swells is most likely an infarction of the anterior is most likely an infarction of the anterior optic nerve .optic nerve .

Other swellings may occur as a result of Other swellings may occur as a result of neoplastic infiltration , acute glaucoma , neoplastic infiltration , acute glaucoma , and hereditary neuropathy .and hereditary neuropathy .

OPTIC ATROPHY OPTIC ATROPHY Nonglaucomatous optic atrophy is characterized Nonglaucomatous optic atrophy is characterized

by a loss of axons ( and their retinal ganglion by a loss of axons ( and their retinal ganglion cells ) in response to lethal insult to the cell .cells ) in response to lethal insult to the cell .

With loss of its substance , the optic disc flattens With loss of its substance , the optic disc flattens and turns pale .and turns pale .

Localized insults , which occur most often in the Localized insults , which occur most often in the anterior optic nerve and retina , injure bundles of anterior optic nerve and retina , injure bundles of axons and in the visual field produce scotomas axons and in the visual field produce scotomas and other visual field defects that are shaped and other visual field defects that are shaped like the course of nerve fiber bundles .like the course of nerve fiber bundles .

Lesions of the posterior optic nerve also Lesions of the posterior optic nerve also can rarely injure bundles of axons and can rarely injure bundles of axons and produce nerve fiber bundle defects . the produce nerve fiber bundle defects . the usual compressive lesion in this location usual compressive lesion in this location almost always has a diffuse effect or almost always has a diffuse effect or affects preferentially the small diameter affects preferentially the small diameter macular fibers , resulting in a central macular fibers , resulting in a central scotoma .scotoma .

Whichever the underlying etiology of optic Whichever the underlying etiology of optic atrophy , the integrity of the axon is atrophy , the integrity of the axon is interrupted anatomically or physiologically interrupted anatomically or physiologically at some point in its course .at some point in its course .

The proximal segment of the axon , being The proximal segment of the axon , being disconnected from its ganglion cell , disconnected from its ganglion cell , promptly degenerates . it is no longer promptly degenerates . it is no longer receives support through the orthograde receives support through the orthograde axonal transport mechanism .axonal transport mechanism .

There remain the ganglion cell and the There remain the ganglion cell and the attached fragment of axon that extends attached fragment of axon that extends from the ganglion cell to the cite of injury .from the ganglion cell to the cite of injury .

It has been shown that mammalian optic It has been shown that mammalian optic nerve fibers cannot only regenerate , but nerve fibers cannot only regenerate , but reestablish connections with their target reestablish connections with their target sites in the central nervous system if sites in the central nervous system if provided with a segment of peripheral provided with a segment of peripheral nerve tissue as a bridge within which to nerve tissue as a bridge within which to grow .grow .

When atrophy occurs , the astroglia When atrophy occurs , the astroglia migrate into the spaces vacated by the migrate into the spaces vacated by the degenerated axons . blood vessels are degenerated axons . blood vessels are reduced in number but remain in reduced in number but remain in proportion to the glial and neuronal tissue proportion to the glial and neuronal tissue that persists and requires nutrition .that persists and requires nutrition .

When optic atrophy occurs , the typically When optic atrophy occurs , the typically red color of the optic disc rim becomes red color of the optic disc rim becomes pale . pale .

COLOR VISION COLOR VISION Color is purely a sensory phenomenon Color is purely a sensory phenomenon

and not a physical attribute .and not a physical attribute .Human awareness of color arises out of Human awareness of color arises out of

subjective visual experiences in which subjective visual experiences in which given sensations are ascribed names .given sensations are ascribed names .

Agreement between individuals in color Agreement between individuals in color naming derives from a tacit acceptance naming derives from a tacit acceptance that given sensations can be reliably that given sensations can be reliably described with color names .described with color names .

The perception of color varies complexly The perception of color varies complexly as a function of multiple parameters , as a function of multiple parameters , including the spectral composition of light including the spectral composition of light coming from the object , the spectral coming from the object , the spectral composition of light emanating from composition of light emanating from surrounding objects , and the state of light surrounding objects , and the state of light adaptation in the subject just prior to adaptation in the subject just prior to viewing any given object .viewing any given object .

A remarkable and as yet not completely A remarkable and as yet not completely understood phenomenon that is understood phenomenon that is characteristic of color vision is that of color characteristic of color vision is that of color constancy .constancy .

Color constancy refers to the phenomenon Color constancy refers to the phenomenon in which the apparent color of an object in which the apparent color of an object does not seem to vary appreciably when does not seem to vary appreciably when the wavelengths and intensity of light the wavelengths and intensity of light illuminating the object are altered .illuminating the object are altered .

Color constancy appears to be related to a Color constancy appears to be related to a phenomenon in which :phenomenon in which :

colors acquire their appearance primarily colors acquire their appearance primarily by relative comparisons to other objects in by relative comparisons to other objects in their immediate vicinity and their immediate vicinity and

these comparisons change only minimally these comparisons change only minimally with broad changes in spectral mixtures of with broad changes in spectral mixtures of light falling on scenes .light falling on scenes .

COLOR AND VISIBLE SPECTRUMCOLOR AND VISIBLE SPECTRUMThe rainbow of hues visible in the solar The rainbow of hues visible in the solar

spectrum was first reported by Sir Issac spectrum was first reported by Sir Issac Newton , who correctly supposed that Newton , who correctly supposed that individual components of the spectral individual components of the spectral mixture were in some way related to mixture were in some way related to differential stimulation of photoreceptor differential stimulation of photoreceptor units in the eye , providing the basis for units in the eye , providing the basis for the physical stimulus evoking color the physical stimulus evoking color sensation .sensation .

If a prism is placed in the path of a narrow beam If a prism is placed in the path of a narrow beam of sunlight , the path of the light beam will be of sunlight , the path of the light beam will be refracted or bent toward the base of the prism .refracted or bent toward the base of the prism .

Short wavelengths are bent to a greater extent Short wavelengths are bent to a greater extent than are long wavelengths.than are long wavelengths.

The index of refraction for an optical medium The index of refraction for an optical medium differs according to wavelength .differs according to wavelength .

This variable extent of refraction spreads a This variable extent of refraction spreads a polychromatic white beam of light into its polychromatic white beam of light into its component wavelengths , a phenomenon component wavelengths , a phenomenon referred to as spectral dispersion .referred to as spectral dispersion .

The array of individual wavelengths thus The array of individual wavelengths thus exposed is referred to as the visible spectrum .exposed is referred to as the visible spectrum .

The sensations that these individual The sensations that these individual wavelengths evoke are called the spectral wavelengths evoke are called the spectral colors.colors.

Violet light at a wavelength of 430 nm, blue light Violet light at a wavelength of 430 nm, blue light of 460nm , green light of 520nm , yellow light of of 460nm , green light of 520nm , yellow light of approximately 575nm, orange light of 600nm , approximately 575nm, orange light of 600nm , and red light of 650nm. and red light of 650nm.

Wavelengths falling between these values Wavelengths falling between these values produce color sensations that are often given produce color sensations that are often given compound names , such as blue-green or compound names , such as blue-green or yellow – green .yellow – green .

Remember , though, that such stimuli remain Remember , though, that such stimuli remain monochromatic , and the sensations they evoke monochromatic , and the sensations they evoke depend on much more than just wavelength and depend on much more than just wavelength and intensity .intensity .

The color a given stimulus evokes The color a given stimulus evokes depends critically on the context within depends critically on the context within which it is seen, a phenomenon called which it is seen, a phenomenon called simultaneous color contrast .simultaneous color contrast .

As will be shown later , color is neural As will be shown later , color is neural encoded in the afferent visual system by encoded in the afferent visual system by cells whose receptive fields are tuned to cells whose receptive fields are tuned to the detection of simultaneous color the detection of simultaneous color contrasts .contrasts .

THE TRICHROMATIC THEORY OF THE TRICHROMATIC THEORY OF HUMAN COLOR VISION HUMAN COLOR VISION

Two major theories to explain the Two major theories to explain the properties of human color vision .properties of human color vision .

These two principal theories are now These two principal theories are now referred to as the theory of trichromacy referred to as the theory of trichromacy ( or the Young-Helmholtz-Maxwell theory ) ( or the Young-Helmholtz-Maxwell theory ) and the opponent process theory .and the opponent process theory .

Over the past several decades , it has become Over the past several decades , it has become apparent that human and nonhuman primate apparent that human and nonhuman primate color vision is indeed mediated by an essentially color vision is indeed mediated by an essentially trichromatic process at the receptor level , but is trichromatic process at the receptor level , but is encoded for neural transmission in a physiologic encoded for neural transmission in a physiologic paradigm of the color opponent process .paradigm of the color opponent process .

Studies of individual photoreceptors , allowed Studies of individual photoreceptors , allowed the identification of three mutually exclusive the identification of three mutually exclusive classes of cones in the primate retina having classes of cones in the primate retina having differing , but overlapping , spectral sensitivities .differing , but overlapping , spectral sensitivities .

One class of photoreceptors has a spectral One class of photoreceptors has a spectral sensitivity that peaks at approximately 440nm to sensitivity that peaks at approximately 440nm to 450nm . these receptors , which are more 450nm . these receptors , which are more sensitive to the short wavelength end of the sensitive to the short wavelength end of the spectrum , are sometimes referred to as short spectrum , are sometimes referred to as short wavelength sensitive receptors , or blue cones .wavelength sensitive receptors , or blue cones .

A second class of middle wavelength sensitive A second class of middle wavelength sensitive receptors has a spectral sensitivity that peaks at receptors has a spectral sensitivity that peaks at between 535and 550 nm , sometimes referred to between 535and 550 nm , sometimes referred to as green cones .as green cones .

The third class has a spectral sensitivity peaking The third class has a spectral sensitivity peaking at between 570 and 590 nm . these are referred at between 570 and 590 nm . these are referred to as long wavelength sensitive photoreceptors to as long wavelength sensitive photoreceptors or red cones .or red cones .

The overlapping of the spectral The overlapping of the spectral sensitivities of these three classes of sensitivities of these three classes of cones means that no individual class of cones means that no individual class of cones can be stimulated in isolation by cones can be stimulated in isolation by any one wavelength .any one wavelength .

THE OPPONENT COLOR THEORY THE OPPONENT COLOR THEORY Certain select pairs of colors such as red versus Certain select pairs of colors such as red versus

green or yellow versus blue were found to be green or yellow versus blue were found to be mutually exclusive .mutually exclusive .

Mixing lights of such colors did not yield Mixing lights of such colors did not yield composite sensations .composite sensations .

For instance , red light and green light mixed For instance , red light and green light mixed together produced an appearance of yellow , together produced an appearance of yellow , while mixing blue light with yellow light produced while mixing blue light with yellow light produced an appearance of white .an appearance of white .

Thus some colors seems to be mutually Thus some colors seems to be mutually exclusive or opponents of one another . exclusive or opponents of one another .

COLOR MIXING , METAMETRIC COLOR MIXING , METAMETRIC MATCHES , AND COMPLEMENTARY MATCHES , AND COMPLEMENTARY WAVELENGTHS . WAVELENGTHS .

Helmholtz found that any colored light one Helmholtz found that any colored light one wished to use as a reference could be wished to use as a reference could be matched by a suitable mixture of three matched by a suitable mixture of three strategically chosen lights mixed together . strategically chosen lights mixed together .

While Helmholtz established the While Helmholtz established the qualitative nature of this relationship , it qualitative nature of this relationship , it was later put on a quantitative basis by was later put on a quantitative basis by Maxwell .Maxwell .

Mixtures of light that produce identical-Mixtures of light that produce identical-appearing colors are called metameric appearing colors are called metameric matches .matches .

Mixtures that are physically identical to Mixtures that are physically identical to one another ( have identical spectral one another ( have identical spectral compositions ) are said to be isomeric compositions ) are said to be isomeric matches .matches .

Normal observers can always produce Normal observers can always produce metameric matches , but only if at least metameric matches , but only if at least three spectral lights are given.three spectral lights are given.

While such metameric matches can be While such metameric matches can be physically very different from one another ( physically very different from one another ( in terms of their spectral distributions ) , in terms of their spectral distributions ) , they nonetheless appear to be identical in they nonetheless appear to be identical in color and brightness .color and brightness .

This appearance of sameness is linked to This appearance of sameness is linked to the relative extent to which each of the the relative extent to which each of the three retinal cone classes are excited .three retinal cone classes are excited .

Light mixtures that cause the same Light mixtures that cause the same proportional stimulation of the three proportional stimulation of the three receptors will result in the same receptors will result in the same sensation .sensation .

In those special cases of metameric In those special cases of metameric matches of light seen as “ white “ , it is matches of light seen as “ white “ , it is often possible to achieve a match by the often possible to achieve a match by the proper mixing of only two appropriately proper mixing of only two appropriately chosen spectral lights .chosen spectral lights .

Such pairs of monochromatic light sources Such pairs of monochromatic light sources are said to have complementary are said to have complementary wavelengths .wavelengths .

NEURAL ENCODING OF COLOR NEURAL ENCODING OF COLOR The receptive fields of neurons are classified as The receptive fields of neurons are classified as

being color coded , if some aspect of the cell being color coded , if some aspect of the cell responses are found to be specific for some responses are found to be specific for some color attribute .color attribute .

For instance , a cell may respond more For instance , a cell may respond more vigorously to stimulation by light of one vigorously to stimulation by light of one wavelength than of another , or the nature of the wavelength than of another , or the nature of the response may differ as a function of response may differ as a function of wavelength .wavelength .

Two broadest categories of such cells in the Two broadest categories of such cells in the anterior visual system are opponent color cells anterior visual system are opponent color cells and double opponent cells .and double opponent cells .

OPPONENT COLOR CELLS OPPONENT COLOR CELLS Tracing afferent visual pathways beyond Tracing afferent visual pathways beyond

the photoreceptors , the first cells found to the photoreceptors , the first cells found to have specific color-related properties are have specific color-related properties are the opponent color cells .the opponent color cells .

Such cells have differing polarity of Such cells have differing polarity of responses for differing portions of the responses for differing portions of the visible spectrum .visible spectrum .

An example of an opponent color cell is as An example of an opponent color cell is as follows :follows :

Stimulation by yellow light increases the tonic Stimulation by yellow light increases the tonic firing rate of such a cell , whereas stimulation by firing rate of such a cell , whereas stimulation by blue light inhibits or eliminates its rate of firing .blue light inhibits or eliminates its rate of firing .

Stimulation by similar-sized spots of white light Stimulation by similar-sized spots of white light produces no response .produces no response .

The latter phenomenon may be explained by The latter phenomenon may be explained by presuming that simultaneous white stimulation of presuming that simultaneous white stimulation of both excitatory and inhibitory components of the both excitatory and inhibitory components of the cell’s receptive field will result in a net zero cell’s receptive field will result in a net zero response in firing rate .response in firing rate .

Opponency of such cells can be Opponency of such cells can be characteristically divided into two large characteristically divided into two large groups : those having blue-yellow groups : those having blue-yellow opponency , and those having red-green opponency , and those having red-green opponency .opponency .

Red-green color opponent cells are tuned Red-green color opponent cells are tuned to detection of varying levels of stimulation to detection of varying levels of stimulation of middle and long wavelength sensitive of middle and long wavelength sensitive cones , and are best suited to the cones , and are best suited to the detection of red-green color contrasting detection of red-green color contrasting borders .borders .

It is believed that there are also cells that It is believed that there are also cells that sum the input of red and green cones to sum the input of red and green cones to produce a yellow signal .produce a yellow signal .

Blue – yellow opponent cells then detect Blue – yellow opponent cells then detect levels of stimulation of blue cones as levels of stimulation of blue cones as compared to the summed effect of compared to the summed effect of stimulating both red and green cones .stimulating both red and green cones .

DOUBLE OPPONENT CELLSDOUBLE OPPONENT CELLSCells that have opponent receptive field Cells that have opponent receptive field

properties for both color and space are properties for both color and space are said to be double opponent .said to be double opponent .

Double opponent cells are optimally Double opponent cells are optimally organized for the detection of organized for the detection of simultaneous color contrast .simultaneous color contrast .

The phenomenon of simultaneous color The phenomenon of simultaneous color contrast can be demonstrated by viewing contrast can be demonstrated by viewing restricted areas surrounded by contrasting restricted areas surrounded by contrasting color regions .color regions .

For instance , a small field of gray For instance , a small field of gray surrounded by a field of red will appear to surrounded by a field of red will appear to contain a greenish cast .contain a greenish cast .

Conversely , the same gray spot viewed in Conversely , the same gray spot viewed in any region surrounded by green will any region surrounded by green will acquire a reddish appearance .acquire a reddish appearance .

The general rule of simultaneous color The general rule of simultaneous color contrast is that the color of a restricted contrast is that the color of a restricted region will tend toward the complementary region will tend toward the complementary color of its surround .color of its surround .

Simultaneous color contrast is closely Simultaneous color contrast is closely related to the phenomenon of color related to the phenomenon of color constancy referred to previously .constancy referred to previously .

The color that a given light will appear to The color that a given light will appear to have depends critically upon closely have depends critically upon closely adjacent areas with which it is compared adjacent areas with which it is compared and contrasted .and contrasted .

A localized area illuminated by A localized area illuminated by monochromatic light of 585nm wavelength monochromatic light of 585nm wavelength can take on a variety of seemingly can take on a variety of seemingly different colors , depending entirely on the different colors , depending entirely on the areas surrounding it .areas surrounding it .

For instance , a spot of 585nm will acquire a For instance , a spot of 585nm will acquire a green color when embedded in a surround of green color when embedded in a surround of 650nm but will appear to be red when 650nm but will appear to be red when surrounded by a field of 540 nm .surrounded by a field of 540 nm .

When surrounded by a field of identical When surrounded by a field of identical wavelength but 0.7 log unit brighter , it will wavelength but 0.7 log unit brighter , it will acquire a gray appearance , whereas if the acquire a gray appearance , whereas if the surround of the same wavelength is 2 log units surround of the same wavelength is 2 log units brighter , the spot will appear to be black .brighter , the spot will appear to be black .

Again , if the spot of 585nm light is surrounded Again , if the spot of 585nm light is surrounded by a field of somewhat shorter wavelength , for by a field of somewhat shorter wavelength , for instance 570nm , but 1 log unit brighter , the instance 570nm , but 1 log unit brighter , the spot will appear to be brown .spot will appear to be brown .

RETINAL DISTRIBUTION OF COLOR-RETINAL DISTRIBUTION OF COLOR-SPECIFIC NEURONS SPECIFIC NEURONS

The density of cones in the retina falls sharply The density of cones in the retina falls sharply outside the fovea , but cones of all three outside the fovea , but cones of all three varieties are present , though in much smaller varieties are present , though in much smaller numbers , all the way to the ora serrata .numbers , all the way to the ora serrata .

The center of the fovea is unique both in having The center of the fovea is unique both in having the highest spatial density of cones and in the highest spatial density of cones and in having a pure mosaic of red and green cones , having a pure mosaic of red and green cones , with blue cones being eliminated from the with blue cones being eliminated from the photoreceptor population within the central 1/8 photoreceptor population within the central 1/8 degree of the visual field .degree of the visual field .

PECULIARITIES OF THE BLUE CONE PECULIARITIES OF THE BLUE CONE SYSTEM SYSTEM

Both visual acuity and contrast sensitivity Both visual acuity and contrast sensitivity are poorer in blue light than in red or green are poorer in blue light than in red or green light .light .

This phenomenon is due not only to the This phenomenon is due not only to the absence of short wavelength sensitive absence of short wavelength sensitive cones in the foveal center but also to the cones in the foveal center but also to the relative scarcity of blue cones , which are relative scarcity of blue cones , which are much fewer in number in all retinal areas much fewer in number in all retinal areas than are middle wavelength sensitive or than are middle wavelength sensitive or long wavelength sensitive receptors .long wavelength sensitive receptors .

COLOR ENCODING IN THE CEREBRAL COLOR ENCODING IN THE CEREBRAL CORTEX CORTEX

The axons of color opponent and double The axons of color opponent and double opponent cells , arising from somas in the opponent cells , arising from somas in the LGN , synapse with cells located in LGN , synapse with cells located in several layers of the striate cortex .several layers of the striate cortex .

The first clue to color organization in the The first clue to color organization in the primary visual cortex was the finding of primary visual cortex was the finding of groups of cells that stain strongly for the groups of cells that stain strongly for the presence of cytochrome oxidase .presence of cytochrome oxidase .

These groups of cells are referred to as “ These groups of cells are referred to as “ blobs “ .blobs “ .

The receptive fields of striate cortical cells The receptive fields of striate cortical cells located between the blobs are located between the blobs are characteristically tuned to specific spatial characteristically tuned to specific spatial orientations , whereas cells within the orientations , whereas cells within the blobs have no apparent orientation blobs have no apparent orientation selectivity .selectivity .

Blob cells also have relatively simple , Blob cells also have relatively simple , concentric , center-surround receptive field concentric , center-surround receptive field properties , and strongly associated with properties , and strongly associated with color differentiation .color differentiation .

It farther appears that the cells of It farther appears that the cells of individual blob columns are devoted to one individual blob columns are devoted to one or the other major types of color or the other major types of color opponency : red versus green or blue opponency : red versus green or blue versus yellow .versus yellow .

Blobs devoted to red/green opponency Blobs devoted to red/green opponency outnumber those of blue yellow outnumber those of blue yellow opponency by a ratio of about 3 to 1 .opponency by a ratio of about 3 to 1 .

In peristriate cortex ( area 18 or V2 ) rather In peristriate cortex ( area 18 or V2 ) rather than blobs , the pattern of cytochrome than blobs , the pattern of cytochrome oxidase staining is one of parallel stripes oxidase staining is one of parallel stripes of alternating widths , thick and thin .of alternating widths , thick and thin .

Cells within the thin stripe regions of Cells within the thin stripe regions of area 18 are not orientation selective , area 18 are not orientation selective , show a high frequency of color-opponency show a high frequency of color-opponency , and probably receive their input directly , and probably receive their input directly from blob cells of area 17 .from blob cells of area 17 .

Cells in the thick stripes and in the pale Cells in the thick stripes and in the pale interstripe regions are orientation selective and interstripe regions are orientation selective and are frequently sensitive to binocular disparity are frequently sensitive to binocular disparity ( probably concerned with stereoacuity .( probably concerned with stereoacuity .

The thin stripes of area 18 appear to receive The thin stripes of area 18 appear to receive most of their input from the blobs of area most of their input from the blobs of area 17 ,while the pale stripes receive the majority of 17 ,while the pale stripes receive the majority of their input from the interblob regions of area 17 .their input from the interblob regions of area 17 .

Thus an anatomic and functional segregation of Thus an anatomic and functional segregation of form and color discrimination is maintained form and color discrimination is maintained through the striate and into the peristriate visual through the striate and into the peristriate visual cortex .cortex .

CONGENITAL DYSCHROMATOPSIAS CONGENITAL DYSCHROMATOPSIAS Two broad groups are represented , those Two broad groups are represented , those

in which reds and greens are confused in which reds and greens are confused with one another and those in which blues with one another and those in which blues and yellows are confused .and yellows are confused .

Congenital defects in color vision are Congenital defects in color vision are further subdivided into anomalous further subdivided into anomalous trichromacy , dichromacy , and trichromacy , dichromacy , and monochromacy .monochromacy .

Anomalous trichromats are those who still Anomalous trichromats are those who still require three primaries in order to match require three primaries in order to match the full gamut of color but who do not the full gamut of color but who do not accept matches made by those with accept matches made by those with normal color vision . normal color vision .

Dichromats need only two primaries to Dichromats need only two primaries to match any colored light within their match any colored light within their spectral range of vision , and will accept all spectral range of vision , and will accept all matches made by normals.matches made by normals.

Monochromacy is a term used somewhat Monochromacy is a term used somewhat confusedly , having been applied to two confusedly , having been applied to two different entities .different entities .

Rod monochromats are those with a Rod monochromats are those with a complete congenital absence of cone complete congenital absence of cone function , while blue cone monochromats function , while blue cone monochromats have no red or green cone function but have no red or green cone function but appear to have retinal pigments for both appear to have retinal pigments for both rods and blue cones .rods and blue cones .

One or more of the normal cone pigments One or more of the normal cone pigments are altered or missing altogether in are altered or missing altogether in individuals affected by congenital individuals affected by congenital dyschromatopsia .dyschromatopsia .

Subjects with complete dichromacy have Subjects with complete dichromacy have only two types of cones , each having only two types of cones , each having normal spectral sensitivity characteristics , normal spectral sensitivity characteristics , with the third type being absent .with the third type being absent .

Protanopes are dichromats having normal Protanopes are dichromats having normal green and blue cones , but an absence of green and blue cones , but an absence of cones containing long wavelength cones containing long wavelength sensitive pigment .sensitive pigment .

Conversely , deuteranopes have normal Conversely , deuteranopes have normal red and blue cones , but an absence of red and blue cones , but an absence of cones containing the middle wavelength cones containing the middle wavelength sensitive pigment .sensitive pigment .

While these subjects appear to have a While these subjects appear to have a numerical component of cones, those numerical component of cones, those cones that should have contained a given cones that should have contained a given variety of pigment ,either red or green , variety of pigment ,either red or green , have been genetically determined to have been genetically determined to contain the complementary variety .contain the complementary variety .

Anomalous trichromats have three classes Anomalous trichromats have three classes of cones , containing three different of cones , containing three different pigments , but the pigment in one of the pigments , but the pigment in one of the three has an abnormal spectral absorption three has an abnormal spectral absorption ..

Protanomalous individuals , for instance , Protanomalous individuals , for instance , lack a normal red-sensitive pigment , but lack a normal red-sensitive pigment , but instead have cones containing pigment instead have cones containing pigment with a spectral absorption more nearly like with a spectral absorption more nearly like that of the normal middle wavelength that of the normal middle wavelength sensitive variety .sensitive variety .

As a consequence , the spectral As a consequence , the spectral absorption curves of their red and green absorption curves of their red and green cones are more nearly alike .cones are more nearly alike .

THE MOLECULAR BIOLOGY OF THE THE MOLECULAR BIOLOGY OF THE CONGENITAL DYSCHROMATOPSIAS CONGENITAL DYSCHROMATOPSIAS

Congenital dyschromatopsias of the most Congenital dyschromatopsias of the most common variety are caused by alterations in the common variety are caused by alterations in the genes encoding the red- and green-sensitive genes encoding the red- and green-sensitive photopigments .photopigments .

Color vision defects are produced by deletions of Color vision defects are produced by deletions of red or green pigment genes or by formation of red or green pigment genes or by formation of hybrid genes comprised of portions of both red hybrid genes comprised of portions of both red and green pigment genes , resulting from an and green pigment genes , resulting from an unequal crossing over between genes that are unequal crossing over between genes that are located in tandem within the X chromosome.located in tandem within the X chromosome.

Differences in severity of the color vision defects Differences in severity of the color vision defects are related to variations in the cross over sites .are related to variations in the cross over sites .

Even among color-normal trichromats , a certain Even among color-normal trichromats , a certain degree of polymorphism has been found .degree of polymorphism has been found .

Careful studies have shown that normal Careful studies have shown that normal trichromats can be further subdivided by the trichromats can be further subdivided by the patterns of the color matches that they make patterns of the color matches that they make with the Rayleigh anomaloscope .with the Rayleigh anomaloscope .

Thus even among color-normals there are Thus even among color-normals there are detectable ,discrete variations in capacities for detectable ,discrete variations in capacities for discriminating between middle and long discriminating between middle and long wavelength light .wavelength light .

It is believed that most normal humans It is believed that most normal humans have , in fact , more than three different have , in fact , more than three different cone pigment types represented on the X cone pigment types represented on the X chromosome .chromosome .

The inherited dyschromatopsias are :The inherited dyschromatopsias are :1- binocular 1- binocular 2- symmetrical2- symmetrical3- and do not change over time .3- and do not change over time .

HUE DISCRIMINATION TESTS FOR HUE DISCRIMINATION TESTS FOR CHARACTERIZING ABNORMAL COLOR CHARACTERIZING ABNORMAL COLOR VISION .VISION .

The Fransworth-Munsell 100 hue Test can The Fransworth-Munsell 100 hue Test can be used to estimate both the nature and be used to estimate both the nature and extent of defective color vision .extent of defective color vision .

The tests consists of a series of 85 colored The tests consists of a series of 85 colored caps .caps .

The 85 caps are divided into four The 85 caps are divided into four approximately equal-sized groups that are approximately equal-sized groups that are stored in separate boxes .stored in separate boxes .

In the course of testing , a subject is asked to In the course of testing , a subject is asked to arrange the caps in a linear sequence between arrange the caps in a linear sequence between pairs of fixed reference caps that are located at pairs of fixed reference caps that are located at either end of each box .either end of each box .

Confusion between similar hues in patients with Confusion between similar hues in patients with congenital color defects result in characteristic congenital color defects result in characteristic patterns in Fransworth-Munsell polar plots .patterns in Fransworth-Munsell polar plots .

Transpositional errors are usually confined to Transpositional errors are usually confined to restricted zones within the color circle that are restricted zones within the color circle that are located at directly opposite locations from one located at directly opposite locations from one another .another .

ACQUIRED DYSCHROMATOPSIASACQUIRED DYSCHROMATOPSIAS Acquired color vision defects , so called Acquired color vision defects , so called

dyschromatopsias , are different from congenital dyschromatopsias , are different from congenital color vision deficits in several respects .color vision deficits in several respects .

Most importantly , acquired defects in color Most importantly , acquired defects in color vision are noticeable to the observer , whereas vision are noticeable to the observer , whereas congenital defects usually are not .congenital defects usually are not .

Additionally , acquired defects may be Additionally , acquired defects may be monocular or markedly asymmetric and may monocular or markedly asymmetric and may even vary from one part of the visual field to even vary from one part of the visual field to another .another .

Acquired defects are commonly Acquired defects are commonly associated with reduction in visual acuity , associated with reduction in visual acuity , changes in dark adaptations , and/or changes in dark adaptations , and/or flicker discrimination .flicker discrimination .

Acquired deficits are caused by a variety Acquired deficits are caused by a variety of diseases that damage the retina , the of diseases that damage the retina , the optic nerve , or the visual cortex .optic nerve , or the visual cortex .

Toxic , vascular , inflammatory , neoplastic Toxic , vascular , inflammatory , neoplastic , demyelinating , and degenerative , demyelinating , and degenerative diseases are all well-recognized causes of diseases are all well-recognized causes of acquired dyschromatopsias .acquired dyschromatopsias .

CLASSIFICATIONS OF ACQUIRED CLASSIFICATIONS OF ACQUIRED DYSCHROMATOPSIAS DYSCHROMATOPSIAS

Three major types of acquired Three major types of acquired dyschromatopsias called types I , II , and dyschromatopsias called types I , II , and III are included in this classification .III are included in this classification .

The first two varieties are associated with The first two varieties are associated with a major axis of hue discrimination in the a major axis of hue discrimination in the red – green region of the Fransworth-red – green region of the Fransworth-munsell diagram , much like the patterns munsell diagram , much like the patterns found for the protan and deutan varieties found for the protan and deutan varieties of congenital dyschromatopsias .of congenital dyschromatopsias .

Type I is protanlike , and is manifested as Type I is protanlike , and is manifested as an acquired loss of discrimination between an acquired loss of discrimination between reds and greens with little or no loss of reds and greens with little or no loss of blue- yellow discrimination .blue- yellow discrimination .

This variety of dyschromatopsia is also This variety of dyschromatopsia is also associated with moderate to severe associated with moderate to severe reductions in VA .reductions in VA .

The type II dyschromatopsia is said to be The type II dyschromatopsia is said to be deutanlike , and involves mild to severe deutanlike , and involves mild to severe confusion of reds and greens with a confusion of reds and greens with a simultaneous but milder loss of simultaneous but milder loss of discrimination between blues and yellows .discrimination between blues and yellows .

Again type II is usually associated with moderate Again type II is usually associated with moderate to severe reductions in VA.to severe reductions in VA.

The third type of acquired color vision defect in The third type of acquired color vision defect in the Verriest classification , type III , is said to be the Verriest classification , type III , is said to be tritan like , and is manifested by mild to tritan like , and is manifested by mild to moderate confusions of blue and yellow hues moderate confusions of blue and yellow hues with a lesser or even absent impairment of red –with a lesser or even absent impairment of red –green discrimination . in this third type of green discrimination . in this third type of dyschromatopsia VA may be normal or only dyschromatopsia VA may be normal or only mildly reduced .mildly reduced .

HOW ACQUIRED DISEASES PRODUCE HOW ACQUIRED DISEASES PRODUCE THEIR VARIOUS PATTERNS OF COLOR THEIR VARIOUS PATTERNS OF COLOR DIFICITS .DIFICITS .

Because of the small blind spot for blue Because of the small blind spot for blue perception located at the center of the perception located at the center of the visual field , human observers making visual field , human observers making color judgments between the various caps color judgments between the various caps of the Fransworth-Munsell test must of the Fransworth-Munsell test must depend on comparisons between the more depend on comparisons between the more peripheral portions of the centrally viewed peripheral portions of the centrally viewed test objects in oreder to distinguish hues in test objects in oreder to distinguish hues in the blue-yellow dimension of color space .the blue-yellow dimension of color space .

Diseases of the retina and optic nerve Diseases of the retina and optic nerve produce characteristic patterns of damage produce characteristic patterns of damage in the central and peripheral portions of in the central and peripheral portions of the visual field .the visual field .

For instance , the most common form of For instance , the most common form of glaucoma notoriously damages the glaucoma notoriously damages the extracentral portions of the visual field , as extracentral portions of the visual field , as evidenced by sparing of VA until the latest evidenced by sparing of VA until the latest stages of the disease .stages of the disease .

Apparently selective damage to blue – yellow Apparently selective damage to blue – yellow discrimination with relative preservation of red-discrimination with relative preservation of red-green discrimination and visual acuity ( type III green discrimination and visual acuity ( type III dyschromatopsia in Verriest classification ) is dyschromatopsia in Verriest classification ) is common in chronic glaucoma .common in chronic glaucoma .

This should be expected if damage to the This should be expected if damage to the extrafoveal visual field ( outside the central half extrafoveal visual field ( outside the central half degree ) exceeds that at the foveal center .degree ) exceeds that at the foveal center .

In this situation the higher degree of red-green In this situation the higher degree of red-green discrimination and visual acuity found in the discrimination and visual acuity found in the foveal cone mosaic will be relatively preserved , foveal cone mosaic will be relatively preserved , but the perifoveal blue cone contribution to color but the perifoveal blue cone contribution to color discrimination will have been diminished .discrimination will have been diminished .

THE CENTRAL VISUAL PATHWAYTHE CENTRAL VISUAL PATHWAYThe ganglion cells are the only cells in the The ganglion cells are the only cells in the

retina that project from the eye to the retina that project from the eye to the brain.brain.

Their axons terminate in a thalamic relay Their axons terminate in a thalamic relay nucleous called the leateral genicualte nucleous called the leateral genicualte body . postsynaptic neurons of lateral body . postsynaptic neurons of lateral geniculate body receiving retinal input geniculate body receiving retinal input project in turn to the primary visual cortex .project in turn to the primary visual cortex .

The retino-geniculo-cortical pathway The retino-geniculo-cortical pathway provides the neural substrate for visual provides the neural substrate for visual perception .perception .

RETINAL GANGLION CELL TARGETS RETINAL GANGLION CELL TARGETS Although the lateral geniculate body is the Although the lateral geniculate body is the

main target of ganglion cells , at least nine main target of ganglion cells , at least nine other nuclei within the brain also receive other nuclei within the brain also receive retinal input .retinal input .

The superior colliculus contains a The superior colliculus contains a complete retinotopic map of the complete retinotopic map of the contralateral field of vision .contralateral field of vision .

Application of an electrical pulse to any Application of an electrical pulse to any point on this retinotopic map evokes a point on this retinotopic map evokes a saccade of appropriate direction and saccade of appropriate direction and amplitude to shift fixation to the receptive amplitude to shift fixation to the receptive field location of neurons at the stimulation field location of neurons at the stimulation site .site .

Findings suggest that the superior Findings suggest that the superior colliculus is important for visual orienting colliculus is important for visual orienting and foveation but is not essential for and foveation but is not essential for analysis of sensory information leading to analysis of sensory information leading to visual perception .visual perception .

The pupillary light reflex is governed by a retinal The pupillary light reflex is governed by a retinal projection that exits the optic tract before the projection that exits the optic tract before the lateral geniculate body to terminate bilaterally in lateral geniculate body to terminate bilaterally in a scattered , ill-defined cellular complex within a scattered , ill-defined cellular complex within the midbrain referred to as “ pretectal nuclei “ .the midbrain referred to as “ pretectal nuclei “ .

Neurons of pretectal nuclei send projections to Neurons of pretectal nuclei send projections to the epsilateral and contralateral Edinger-the epsilateral and contralateral Edinger-Westphal subdivisions of the oculomotor nuclei .Westphal subdivisions of the oculomotor nuclei .

Edinger-Westphal neurons provide Edinger-Westphal neurons provide parasympathetic input via an interneuron in the parasympathetic input via an interneuron in the ciliary ganglion to control the sphincter pupillae ciliary ganglion to control the sphincter pupillae of the iris .of the iris .

THE RETINO-GENICULO-CORTICAL THE RETINO-GENICULO-CORTICAL PATHWAY PATHWAY

RETINA TO LATERAL GENICULATE RETINA TO LATERAL GENICULATE BODY BODY

The superb visual acuity of humans is The superb visual acuity of humans is achieved at the fovea by thrusting aside all achieved at the fovea by thrusting aside all retinal elements except the photoreceptors retinal elements except the photoreceptors , to minimize absorption and scattering of , to minimize absorption and scattering of light .light .

This unique primate specialization requires This unique primate specialization requires fibers from ganglion cells in temporal retina to fibers from ganglion cells in temporal retina to follow a circuitous route to the optic disc to avoid follow a circuitous route to the optic disc to avoid passing over the fovea .passing over the fovea .

The horizontal raphe in the temporal nerve fiber The horizontal raphe in the temporal nerve fiber layer results in a complex , discontinuous layer results in a complex , discontinuous arrangement of ganglion cell axons at the optic arrangement of ganglion cell axons at the optic disc .disc .

Cells in the temporal retina just above and below Cells in the temporal retina just above and below the horizontal meridian send their fibers via a the horizontal meridian send their fibers via a roundabout route to enter the superior and roundabout route to enter the superior and inferior poles of the optic disc respectively .inferior poles of the optic disc respectively .

Although their cell bodies are situated Although their cell bodies are situated close together in the retina , their fibers close together in the retina , their fibers are widely separated in the optic disc by are widely separated in the optic disc by other fibers that directly enter the nasal other fibers that directly enter the nasal and temporal sides of the disc .and temporal sides of the disc .

Retinotopic organization is further Retinotopic organization is further complicated by intermingling between complicated by intermingling between peripheral and central axons as they peripheral and central axons as they approach the optic disc .approach the optic disc .

After leaving the eye at the optic disc , the After leaving the eye at the optic disc , the ganglion cell fibers become invested with ganglion cell fibers become invested with myelin to form the optic nerve .myelin to form the optic nerve .

The retinotopic organization of ganglion The retinotopic organization of ganglion cell fibers is generally preserved within the cell fibers is generally preserved within the optic nerve .optic nerve .

Near the eye the ganglion cell fibers are Near the eye the ganglion cell fibers are precisely arrayed in a manner that precisely arrayed in a manner that duplicates their arrangement within the duplicates their arrangement within the optic nerve head .optic nerve head .

Moving proximally toward the optic chiasm Moving proximally toward the optic chiasm the fibers gradually scatter in position until the fibers gradually scatter in position until the topography in the optic nerve becomes the topography in the optic nerve becomes quite imprecise .quite imprecise .

At least a third of the optic nerve is At least a third of the optic nerve is comprised of macular fibers . near the comprised of macular fibers . near the globe the macular fibers are clustered into globe the macular fibers are clustered into the central and temporal sectors of the the central and temporal sectors of the optic nerve , but more proximally they optic nerve , but more proximally they intermingle with other fibers to distribute intermingle with other fibers to distribute throughout all sectors of the optic nerve .throughout all sectors of the optic nerve .

At the optic chiasm the fibers originating At the optic chiasm the fibers originating from ganglion cells located nasal to the from ganglion cells located nasal to the fovea cross into the contralateral optic fovea cross into the contralateral optic tract .tract .

Wilbrand observed that some crossing Wilbrand observed that some crossing fibers loop briefly into the opposite nerve fibers loop briefly into the opposite nerve before entering the optic tract ( Wilbrand’s before entering the optic tract ( Wilbrand’s knee ).knee ).

At the chiasm the partial decussation of At the chiasm the partial decussation of optic nerve fibers merges input from the optic nerve fibers merges input from the two hemiretinas subserving the two hemiretinas subserving the contralateral field of vision .contralateral field of vision .

THE LATERAL GENICULATE BODYTHE LATERAL GENICULATE BODY The lateral geniculate body is the principal The lateral geniculate body is the principal

thalamic nucleous linking the retina and the thalamic nucleous linking the retina and the striate cortex . the majority of retinal ganglion striate cortex . the majority of retinal ganglion cell fibers terminate in the lateral geniculate cell fibers terminate in the lateral geniculate body .body .

The nucleous consists of six principal cellular The nucleous consists of six principal cellular laminae separated by thin cell-free zones .laminae separated by thin cell-free zones .

Laminae 1,4,6 receive axons from the Laminae 1,4,6 receive axons from the contralateral nasal retina and laminae 2,3,5 contralateral nasal retina and laminae 2,3,5 receive axons from the epsilateral temporal receive axons from the epsilateral temporal retina .retina .

Each lamina of the LGB contains a precise Each lamina of the LGB contains a precise retinotopic map of the contralateral retinotopic map of the contralateral hemifield of vision .hemifield of vision .

Central vision is thought to be Central vision is thought to be represented, in the caudal , 6-layered represented, in the caudal , 6-layered portion of the human LGB .portion of the human LGB .

Rostrally , the LGB is reduced to only 4 Rostrally , the LGB is reduced to only 4 laminae by fusion of each pair of dorsal laminae by fusion of each pair of dorsal laminae .laminae .

The periphery of the visual field is The periphery of the visual field is represented in this 4-layered region of the represented in this 4-layered region of the nucleous .nucleous .

THE OPTIC RADIATION THE OPTIC RADIATION Neurons of the LGB complete the relay of Neurons of the LGB complete the relay of

retinal input to the primary visual cortex by retinal input to the primary visual cortex by projecting to the epsilateral occipital lobe .projecting to the epsilateral occipital lobe .

Their axons form a sheet of white matter Their axons form a sheet of white matter called the optic radiation .called the optic radiation .

THE PRIMARY VISUAL CORTEX THE PRIMARY VISUAL CORTEX The upper and lower visual quadrants are The upper and lower visual quadrants are

represented in the lower and upper represented in the lower and upper calcarine banks respectively , separated calcarine banks respectively , separated by the horizontal meridian along the base by the horizontal meridian along the base of the calcarine fissure .of the calcarine fissure .

The fovea is represented at the occipital The fovea is represented at the occipital pole .pole .

Most of primary visual cortex is actually Most of primary visual cortex is actually buried within the depth of the calcarine buried within the depth of the calcarine fissure .fissure .

The primary visual cortex contains a topographic The primary visual cortex contains a topographic but highly distorted representation of the but highly distorted representation of the contralateral hemifield of vision .contralateral hemifield of vision .

The most striking feature of the visual field map The most striking feature of the visual field map is the enormous fraction of visual cortex is the enormous fraction of visual cortex assigned to the representation of central vision .assigned to the representation of central vision .

Quantitative measurements in macaque monkey Quantitative measurements in macaque monkey reveal that between 55 and 60 % of the surface reveal that between 55 and 60 % of the surface area of primary visual cortex is devoted to the area of primary visual cortex is devoted to the representation of the central 10˚ of vision .representation of the central 10˚ of vision .

The linear cortical ( magnification factor ) – The linear cortical ( magnification factor ) – the millimeters of cortex representing one the millimeters of cortex representing one degree of visual field – has a ratio of more degree of visual field – has a ratio of more than 40:1 between the fovea ( 0˚ than 40:1 between the fovea ( 0˚ eccentricity ) and the periphery ( 60˚ eccentricity ) and the periphery ( 60˚ eccentricity ) .eccentricity ) .

The representation of central vision is The representation of central vision is highly magnified compared with peripheral highly magnified compared with peripheral vision , so that the cortical area devoted to vision , so that the cortical area devoted to the central 1˚ of visual field roughly equals the central 1˚ of visual field roughly equals the cortical area allotted to the entire the cortical area allotted to the entire monocular temporal crescent .monocular temporal crescent .

The relatively magnified representation of the The relatively magnified representation of the macula in primary visual cortex furnishes an macula in primary visual cortex furnishes an important clue to how the cerebral cortex important clue to how the cerebral cortex analyzes sensory information .analyzes sensory information .

The linear magnification factor of the retina is The linear magnification factor of the retina is equal to about 250 micrometers of tissue per equal to about 250 micrometers of tissue per degree for all points in the visual field .degree for all points in the visual field .

The linear magnification factor of the retina must The linear magnification factor of the retina must remain nearly constant , because the eye is remain nearly constant , because the eye is engaged in processing an optical image of the engaged in processing an optical image of the visual environment .visual environment .

The steep gradient in visual acuity , from 20/20 The steep gradient in visual acuity , from 20/20 centrally to 20/400 peripherally , is achieved by centrally to 20/400 peripherally , is achieved by variation in the density of cells in the ganglion variation in the density of cells in the ganglion cell layer .cell layer .

In central retina the ganglion cells are stacked 6 In central retina the ganglion cells are stacked 6 to 8 cells deep , declining to a broken monolayer to 8 cells deep , declining to a broken monolayer in peripheral retina .in peripheral retina .

Free of any optical constrains , the cerebral Free of any optical constrains , the cerebral cortex handles the richer flow of visual cortex handles the richer flow of visual information emanating from the central retina in information emanating from the central retina in a different fashion .a different fashion .

The cortical mantle maintains uniform thickness The cortical mantle maintains uniform thickness throughout the primary visual cortex but throughout the primary visual cortex but allocates more tissue for the analysis of central allocates more tissue for the analysis of central vision .vision .

In the visual cortex the magnification factor In the visual cortex the magnification factor , rather than the cell density , varies with , rather than the cell density , varies with eccentricity in the visual field eccentricity in the visual field representation .representation .

From the fovea to the periphery of the From the fovea to the periphery of the visual field a roughly parallel relationship visual field a roughly parallel relationship exists between cortical magnification exists between cortical magnification factor , ganglion cell density and visual factor , ganglion cell density and visual acuity . acuity .

VISUAL FIELD EXAMINATIONVISUAL FIELD EXAMINATIONVision may be impaired by damage to the Vision may be impaired by damage to the

afferent visual pathway anywhere from the afferent visual pathway anywhere from the retina to the occipital lobe .retina to the occipital lobe .

The preservation of topographic order The preservation of topographic order within the retino-geniculo-cortical pathway within the retino-geniculo-cortical pathway usually allows accurate localization of usually allows accurate localization of lesions causing a disturbance in vision by lesions causing a disturbance in vision by careful examination of the visual fields .careful examination of the visual fields .

PRECHIASMAL LESIONS PRECHIASMAL LESIONS The crux of visual field analysis is to The crux of visual field analysis is to

decide whether a lesion is located decide whether a lesion is located before ,at, or behind the optic chiasm .before ,at, or behind the optic chiasm .

A visual field deficit confined to one eye A visual field deficit confined to one eye must be due to a lesion anterior to the must be due to a lesion anterior to the chiasm involving either the optic nerve or chiasm involving either the optic nerve or the retina .the retina .

A visual field deficit in both eyes can result A visual field deficit in both eyes can result from either bilateral prechiasmal lesions or from either bilateral prechiasmal lesions or from a single lesion at or behind the from a single lesion at or behind the chiasm .chiasm .

Certain patterns of visual field loss are Certain patterns of visual field loss are characteristic of diseases that afflict the characteristic of diseases that afflict the optic nerve .optic nerve .

Glaucoma selectively injures axons that Glaucoma selectively injures axons that enter the superotemporal and enter the superotemporal and inferotemporal poles of the optic disc .inferotemporal poles of the optic disc .

This pattern of nerve fiber loss produces This pattern of nerve fiber loss produces arching , fan-shaped field defects that arching , fan-shaped field defects that emanate from the blind spot and curve emanate from the blind spot and curve around fixation to terminate flat against the around fixation to terminate flat against the nasal horizontal meridian .nasal horizontal meridian .

This type of field defect, known as Bjerrum This type of field defect, known as Bjerrum scotoma , mirrors the arcuate course of scotoma , mirrors the arcuate course of fibers in the temporal retinal nerve fiber fibers in the temporal retinal nerve fiber layer .layer .

When the papillomacular bundle is When the papillomacular bundle is damaged , the patient develops a visual damaged , the patient develops a visual field defect that encompasses the blind field defect that encompasses the blind spot and the macula .spot and the macula .

This field defect is called a cecocentral This field defect is called a cecocentral scotoma .scotoma .

The cecocentral scotoma is typical of the The cecocentral scotoma is typical of the optic neuropathy caused by toxins like optic neuropathy caused by toxins like ethanol , tobacco , methanol and ethanol , tobacco , methanol and ethambutol .ethambutol .

Inadequate blood supply to the optic disc Inadequate blood supply to the optic disc results in ischemic optic neuropathy .results in ischemic optic neuropathy .

This condition is frequently accompanied This condition is frequently accompanied by an altitudinal pattern of visual field by an altitudinal pattern of visual field loss .loss .

CHIASMAL LESIONSCHIASMAL LESIONSThe hallmark of chiasmal lesions is The hallmark of chiasmal lesions is

bitemporal hemianopia .bitemporal hemianopia .For reasons that remain quite unclear , For reasons that remain quite unclear ,

crossed fibers are more vulnerable than crossed fibers are more vulnerable than uncrossed fibers to compression of the uncrossed fibers to compression of the optic chiasm by mass lesions .optic chiasm by mass lesions .

The most common culprit is a tumor The most common culprit is a tumor arising from the pituitary gland within the arising from the pituitary gland within the sella turcica .sella turcica .

Lesions situated at the junction of the optic Lesions situated at the junction of the optic nerve with the optic chiasm can produce nerve with the optic chiasm can produce an anterior chiasmal syndrome consisting an anterior chiasmal syndrome consisting of blindness in one eye and temporal of blindness in one eye and temporal hemianopia in the other eye .hemianopia in the other eye .

POSTCHIASMAL LESIONS POSTCHIASMAL LESIONS

Any lesion behind the chiasm will produce Any lesion behind the chiasm will produce a homonymous hemianopia , namely a a homonymous hemianopia , namely a visual field defect involving matching visual field defect involving matching portions of the overlapping temporal portions of the overlapping temporal hemifield of the contralateral eye and the hemifield of the contralateral eye and the nasal hemifield of the epsialteral eye .nasal hemifield of the epsialteral eye .

It is important to realize that visual acuity It is important to realize that visual acuity will be entirely normal if the postchiasmal will be entirely normal if the postchiasmal pathway in the other hemisphere is intact .pathway in the other hemisphere is intact .

Input from only half the fovea is sufficient Input from only half the fovea is sufficient for 20/20 Snellen visual acuity .for 20/20 Snellen visual acuity .

A decrement in visual acuity should never A decrement in visual acuity should never be attributed to a unilateral postchiasmal be attributed to a unilateral postchiasmal lesion .lesion .

As a general rule : the more congruent a As a general rule : the more congruent a visual field defect,, the more posterior the visual field defect,, the more posterior the lesion in the visual pathway .lesion in the visual pathway .

Usually a lesion of the optic tract produces a Usually a lesion of the optic tract produces a homonymous hemianopia with an afferent pupil homonymous hemianopia with an afferent pupil defect in the contralateral eye .defect in the contralateral eye .

The afferent pupil defect in the contralateral eye The afferent pupil defect in the contralateral eye occurs because ganglion cells in the nasal occurs because ganglion cells in the nasal hemiretina outnumber those in the temporal hemiretina outnumber those in the temporal hemiretina .hemiretina .

Consequently , a lesion of the optic tract Consequently , a lesion of the optic tract damages more ganglion cell fibers driving the damages more ganglion cell fibers driving the pupil reflex of the contralateral eye the pupil reflex of the contralateral eye the epsilateral eye . this results in an afferent pupil epsilateral eye . this results in an afferent pupil defect in the contralateral eye .defect in the contralateral eye .

Small lesions of the superior colliculus Small lesions of the superior colliculus have been reported to produce an afferent have been reported to produce an afferent pupil defect in the contralateral eye with no pupil defect in the contralateral eye with no visual field defect in either eye .visual field defect in either eye .

The lesion causes selective injury to the The lesion causes selective injury to the asymmetric pupil fiber input to the asymmetric pupil fiber input to the pretectum from the two hemiretinae .pretectum from the two hemiretinae .

No field defect occurs because retino-No field defect occurs because retino-geniculate fibers are entirely spared .geniculate fibers are entirely spared .

An afferent pupil defect will develop after a An afferent pupil defect will develop after a lesion of the optic tract , but should not develop lesion of the optic tract , but should not develop after a lesion of the lateral geniculate body , after a lesion of the lateral geniculate body , because axons governing the pupil reflex exit for because axons governing the pupil reflex exit for the midbrain well before the lateral geniculate the midbrain well before the lateral geniculate body .body .

Lesions involving the optic radiations or the Lesions involving the optic radiations or the visual cortex do not result in homonymous visual cortex do not result in homonymous hemiretinal atrophy , because the primary hemiretinal atrophy , because the primary projection from the retina to the lateral projection from the retina to the lateral geniculate body remains completely intact .geniculate body remains completely intact .

Injury to just a portion of the optic Injury to just a portion of the optic radiations occurs quite frequently and radiations occurs quite frequently and usually produces a partial homonymous usually produces a partial homonymous hemianopia that appears roughly hemianopia that appears roughly quadrantic .quadrantic .

For example , tumors of the temporal lobe For example , tumors of the temporal lobe may selectively injure Meyer’s loop to may selectively injure Meyer’s loop to cause a homonymous inferior cause a homonymous inferior quadrantanopia .quadrantanopia .

Partial homonymous hemianopia also Partial homonymous hemianopia also occurs from lesions that damage only a occurs from lesions that damage only a portion of the primary visual cortex .portion of the primary visual cortex .

The conspicuous feature of an incomplete The conspicuous feature of an incomplete cortical hemianopia is the extreme degree cortical hemianopia is the extreme degree of congruity . this congruity results of congruity . this congruity results because axons from right eye and left eye because axons from right eye and left eye laminae of the LGB terminate side by side laminae of the LGB terminate side by side in a finely dovetailed pattern of the ocular in a finely dovetailed pattern of the ocular dominance columns in visual cortex .dominance columns in visual cortex .

MACULAR SPARING MACULAR SPARING The extreme cortical magnification of the The extreme cortical magnification of the

macula is the key to understanding the macula is the key to understanding the problem of macular sparing .problem of macular sparing .

In most individuals the vascular supply to In most individuals the vascular supply to primary visual cortex is provided by the primary visual cortex is provided by the posterior cerebral artery .after infarction to posterior cerebral artery .after infarction to the territory of the posterior cerebral artery the territory of the posterior cerebral artery , a complete , macula-splitting , a complete , macula-splitting homonymous hemianopia ensues .homonymous hemianopia ensues .

However , in some patients the occipital However , in some patients the occipital pole straddles the vascular territories of pole straddles the vascular territories of the posterior cerebral artery and the the posterior cerebral artery and the middle cerebral artery . in these patients middle cerebral artery . in these patients the occipital pole survives after posterior the occipital pole survives after posterior cerebral artery occlusion , due to perfusion cerebral artery occlusion , due to perfusion by the middle cerebral artery .by the middle cerebral artery .

Because the representation of central Because the representation of central vision is so magnified , the preservation of vision is so magnified , the preservation of posterior visual cortex spares tissue posterior visual cortex spares tissue devoted exclusively to macular vision .devoted exclusively to macular vision .

If only the occipital tip becomes infracted , If only the occipital tip becomes infracted , the converse is produced : a homonymous the converse is produced : a homonymous hemimacular field defect with peripheral hemimacular field defect with peripheral sparing .sparing .

Complete bilateral injury or infarction of Complete bilateral injury or infarction of the occipital lobes results in total blindness the occipital lobes results in total blindness . lesions of both optic nerves , tracts , or . lesions of both optic nerves , tracts , or the chiasm can also cause total the chiasm can also cause total blindness .blindness .

These two situations can be differentiated These two situations can be differentiated by examination of the pupils .by examination of the pupils .

Pupillary responses to light will be absent Pupillary responses to light will be absent in patients with total blindness of in patients with total blindness of infrageniculate origin .infrageniculate origin .

STRUCTURE AND FUNCTION OF THE STRUCTURE AND FUNCTION OF THE LATERAL GENICULATE BODYLATERAL GENICULATE BODY

In the nervous system afferent information In the nervous system afferent information from every sensory system except from every sensory system except olfaction passes through the thalamus olfaction passes through the thalamus before reaching the cerebral cortex .before reaching the cerebral cortex .

RECEPTIVE FIELD ORGANIZATION RECEPTIVE FIELD ORGANIZATION Cells in the visual system discharge action Cells in the visual system discharge action

potentials spontaneously even in the potentials spontaneously even in the absence of stimulation . For every cell this absence of stimulation . For every cell this spontaneous activity can be influenced by spontaneous activity can be influenced by stimulation with light in some region of the stimulation with light in some region of the visual field .visual field .

This special zone is called “ receptive field This special zone is called “ receptive field ” of the cell .” of the cell .

Neurons in the lateral geniculate body Neurons in the lateral geniculate body share with retinal ganglion cells the same share with retinal ganglion cells the same basic center – surround arrangement of basic center – surround arrangement of their receptive fields .their receptive fields .

On- center cells respond with a burst of On- center cells respond with a burst of spikes when a small spot of light spikes when a small spot of light stimulates the field center .stimulates the field center .

The maximal response is obtained by choosing The maximal response is obtained by choosing a spot size equal to the diameter of the receptive a spot size equal to the diameter of the receptive field center .field center .

If the spot is larger than the field center the cell’s If the spot is larger than the field center the cell’s response is attenuated , indicating antagonism response is attenuated , indicating antagonism between the center and the surround subfields .between the center and the surround subfields .

A light annulus suppresses spontaneous activity A light annulus suppresses spontaneous activity and produces a brisk “ off ” response.and produces a brisk “ off ” response.

The inputs of geniculate cells are wired together The inputs of geniculate cells are wired together to generate the more elaborate receptive fields to generate the more elaborate receptive fields of cortical cells . Cells in visual cortex are of cortical cells . Cells in visual cortex are virtually unresponsive to stimulation with diffuse virtually unresponsive to stimulation with diffuse light .light .

Information about absolute light intensity is Information about absolute light intensity is generally not important for the visual system , generally not important for the visual system , except perhaps for the small subclass of retinal except perhaps for the small subclass of retinal ganglion cells that drives the pupil light reflex .ganglion cells that drives the pupil light reflex .

Information about spatial discontinuities in Information about spatial discontinuities in patterns of light energy is more useful for image patterns of light energy is more useful for image analysis .analysis .

Cells with center- surround receptive field Cells with center- surround receptive field organization are ideally suited for detecting such organization are ideally suited for detecting such contrasts .contrasts .

Their best responses are elicited by contours Their best responses are elicited by contours illuminating just a portion of their receptive field .illuminating just a portion of their receptive field .

SYNAPTIC INPUTSSYNAPTIC INPUTS Any given optic tract fiber arborizes exclusively Any given optic tract fiber arborizes exclusively

within a single geniculate lamina. Each axon within a single geniculate lamina. Each axon terminal plexus makes about a 100 synaptic terminal plexus makes about a 100 synaptic contacts over an area 50 to 100 micrometers contacts over an area 50 to 100 micrometers wide .wide .

Each optic tract fiber may synapse with as few Each optic tract fiber may synapse with as few as 4 to 6 geniculate cells and each geniculate as 4 to 6 geniculate cells and each geniculate cell receives input from even fewer tract fibers . cell receives input from even fewer tract fibers .

A single spike from a retinal ganglion cell fiber is A single spike from a retinal ganglion cell fiber is sufficient to elicit a single spike from a geniculate sufficient to elicit a single spike from a geniculate cell .cell .

On-center ganglion cells trigger only on-On-center ganglion cells trigger only on-center geniculate cells and off-center center geniculate cells and off-center ganglion cells drive only off-center ganglion cells drive only off-center geniculate cells .geniculate cells .

Some geniculate neurons derive their Some geniculate neurons derive their excitatory input from only a single ganglion excitatory input from only a single ganglion cell. For the majority of geniculate cells the cell. For the majority of geniculate cells the excitatory input is provided by 2 or 3 excitatory input is provided by 2 or 3 ganglion cells .ganglion cells .

The lateral geniculate body contains about The lateral geniculate body contains about 1800,000 neurons .1800,000 neurons .

Approximately 90% of the retinal ganglion Approximately 90% of the retinal ganglion cells terminate in the lateral geniculate cells terminate in the lateral geniculate body , yielding a ratio of ganglion cell body , yielding a ratio of ganglion cell fibers to geniculate neurons of 1:2 .fibers to geniculate neurons of 1:2 .

This ratio is consistent with data This ratio is consistent with data suggesting that each geniculate cell suggesting that each geniculate cell receives input from 2 to 3 optic tract fibers receives input from 2 to 3 optic tract fibers and that optic tract fiber contacts 4 to 6 and that optic tract fiber contacts 4 to 6 geniculate cells . these average synaptic geniculate cells . these average synaptic ratios probably vary with eccentricity and ratios probably vary with eccentricity and may differ slightly depending upon the may differ slightly depending upon the geniculate lamina in question .geniculate lamina in question .

MAGNO VERSUS PARVOMAGNO VERSUS PARVOA striking difference is apparent in the A striking difference is apparent in the

morphology of neurons in the dorsal morphology of neurons in the dorsal laminae and the ventral laminae of the laminae and the ventral laminae of the primate lateral geniculate body.primate lateral geniculate body.

The two ventral laminae contain loosely The two ventral laminae contain loosely packed cells with giant somas that exceed packed cells with giant somas that exceed 30 micrometers in diameter .30 micrometers in diameter .

They are commonly referred to as the They are commonly referred to as the magnocellualr laminae .magnocellualr laminae .

The four dorsal laminae are comprised of The four dorsal laminae are comprised of much smaller neurons and hence are much smaller neurons and hence are known as the parvocellular laminae .known as the parvocellular laminae .

FUNCTIONAL SPECIFICITY OF FUNCTIONAL SPECIFICITY OF GENICULATE LAMINAE GENICULATE LAMINAE

In the primate lateral geniculate body the In the primate lateral geniculate body the parvocellular laminae receive input from parvocellular laminae receive input from the midget retinal ganglion cells , and the the midget retinal ganglion cells , and the magnocellular laminae receive input from magnocellular laminae receive input from the parasol cells .the parasol cells .

This pattern of innervations implies that the This pattern of innervations implies that the color-opponent and broad – band retinal color-opponent and broad – band retinal channels remain segregated at the level of the channels remain segregated at the level of the lateral geniculate body .lateral geniculate body .

In the parvocellular laminae the majority of cells In the parvocellular laminae the majority of cells have color-selective responses .have color-selective responses .

Wiesel and Hubel described three principal Wiesel and Hubel described three principal types of parvocellular units . the most common types of parvocellular units . the most common cell ( type 1 ) has a standard center-surround cell ( type 1 ) has a standard center-surround receptive field arrangement .receptive field arrangement .

The center and surround have different spectral The center and surround have different spectral sensitivities because they are fed by different sensitivities because they are fed by different cone systems.cone systems.

Parvo cells and mango cells differ in other Parvo cells and mango cells differ in other important receptive field parameters important receptive field parameters besides their color responses .besides their color responses .

At any given eccentricity the receptive At any given eccentricity the receptive fields of mango cells are several times fields of mango cells are several times larger than the fields of parvo cells .larger than the fields of parvo cells .

Mango axons conduct action potentials to Mango axons conduct action potentials to striate cortex more rapidly than parvo striate cortex more rapidly than parvo axons .axons .

Mango cells have higher contrast Mango cells have higher contrast sensitivity than parvo cells .sensitivity than parvo cells .

To visual stimulation mango cells give rapid , To visual stimulation mango cells give rapid , phasic responses whereas parvo cells give phasic responses whereas parvo cells give slow , tonic responses .slow , tonic responses .

In parvocellular geniculate the on-center cells In parvocellular geniculate the on-center cells and off-center cells are segregated into separate and off-center cells are segregated into separate laminae .laminae .

Laminae 5 and 6 receive input mostly from on-Laminae 5 and 6 receive input mostly from on-center retinal ganglion cells and consequently center retinal ganglion cells and consequently contain mostly on-center cells .contain mostly on-center cells .

Laminae 3 and 4 receive input largely from off-Laminae 3 and 4 receive input largely from off-center ganglion cells and are more richly center ganglion cells and are more richly populated with off-center cells.populated with off-center cells.

This pattern of retinal innervation suggests This pattern of retinal innervation suggests that one major function of the primate that one major function of the primate lateral geniculate body isto sort retinal on-lateral geniculate body isto sort retinal on-off channels into different laminae .off channels into different laminae .

However , on-center and off-center cells However , on-center and off-center cells are intermingled through out the are intermingled through out the magnocellular geniculate laminae with no magnocellular geniculate laminae with no loss of specificity in their inputs from on-loss of specificity in their inputs from on-center and off-center parasol retinal center and off-center parasol retinal ganglion cells.ganglion cells.

A single excitatory postsynaptic potential from a A single excitatory postsynaptic potential from a ganglion cell is usually sufficient to evoke a ganglion cell is usually sufficient to evoke a discharge from a geniculate neuron .discharge from a geniculate neuron .

There is a little divergence or convergence in the There is a little divergence or convergence in the transmission of information through the lateral transmission of information through the lateral geniculate body . In all these respects the LGB geniculate body . In all these respects the LGB appears to behave as a relay nucleus .appears to behave as a relay nucleus .

The LGB receives a massive projection from The LGB receives a massive projection from neurons in layer VI of the visual cortex. This neurons in layer VI of the visual cortex. This reciprocal corticogeniculate projection might be reciprocal corticogeniculate projection might be expected to influence profoundly the receptive expected to influence profoundly the receptive fields of geniculate cells .fields of geniculate cells .

It offers an anatomic substrate for potential It offers an anatomic substrate for potential modulation of retinal inputs at the modulation of retinal inputs at the geniculate level before transfer to visual geniculate level before transfer to visual cortex .cortex .

However , reversible inactivation of the However , reversible inactivation of the cortico geniculate input by cooling striate cortico geniculate input by cooling striate cortex produces only slight effects upon cortex produces only slight effects upon the response properties of cells in the the response properties of cells in the lateral geniculate body. lateral geniculate body.

This surprising result leaves us without a This surprising result leaves us without a clear understanding of the role of the clear understanding of the role of the lateral geniculate body . lateral geniculate body .

THE PRIMARY VISUAL CORTEXTHE PRIMARY VISUAL CORTEXThe primary visual cortex is often called “ The primary visual cortex is often called “

striate cortex” referring to the prominent striate cortex” referring to the prominent stria found by Gennari .stria found by Gennari .

Later Brodmann parceled the cerebral Later Brodmann parceled the cerebral cortex into 47 different regions based upon cortex into 47 different regions based upon subtle distinctions in cortical histology .subtle distinctions in cortical histology .

He assigned the arbitrary label of “ area 17 He assigned the arbitrary label of “ area 17 ” to the primary visual cortex .” to the primary visual cortex .

In recent years other visual areas have In recent years other visual areas have been discovered in extrastriate cortex been discovered in extrastriate cortex surrounding the primary visual cortex .surrounding the primary visual cortex .

The primary visual has received the The primary visual has received the prosaic designation of V1 ( visual area prosaic designation of V1 ( visual area 1 )and adjacent extrastriate visual areas 1 )and adjacent extrastriate visual areas are named V2,V3,V4, and so on.are named V2,V3,V4, and so on.

Primary visual cortex , striate cortex , area Primary visual cortex , striate cortex , area 17 and V1 are all synonymous for the 17 and V1 are all synonymous for the same piece of tissue .same piece of tissue .

OCULAR DOMINANCE COLUMNS OCULAR DOMINANCE COLUMNS In a tissue section of visual cortex stained In a tissue section of visual cortex stained

for Nissl substance the most striking for Nissl substance the most striking feature is the horizontal lamination of cell feature is the horizontal lamination of cell bodies .bodies .

There are six fundamental layers in the There are six fundamental layers in the primate cortex .primate cortex .

In striate cortex some layers contain In striate cortex some layers contain multiple sublayers .multiple sublayers .

Magno cells and parvo cells project to Magno cells and parvo cells project to separate layers of striate cortex in the separate layers of striate cortex in the macaque monkey .macaque monkey .

Magnocellular axons terminate in layer Magnocellular axons terminate in layer IVCα of striate cortex with minor branches IVCα of striate cortex with minor branches entering the deeper portion of layer VI .entering the deeper portion of layer VI .

Parvocellular axons innervate layer IVCβ Parvocellular axons innervate layer IVCβ and layer IVA , with small small additional and layer IVA , with small small additional inputs to layer I and the upper portion of inputs to layer I and the upper portion of layer VI .layer VI .

Layer IVA is absent in humans , indicating Layer IVA is absent in humans , indicating that important differences exist among that important differences exist among similar primate species in the similar primate species in the cytoarchitecture of striate cortex .cytoarchitecture of striate cortex .

Axon terminals from right eye and left eye Axon terminals from right eye and left eye geniculate laminae are not randomly geniculate laminae are not randomly distributed in layer IVC of striate cortex , distributed in layer IVC of striate cortex , but rather , they are segregated into a but rather , they are segregated into a system of alternating parallel stripes called system of alternating parallel stripes called ocular dominance columns .ocular dominance columns .

In humans the ocular dominance columns have In humans the ocular dominance columns have been revealed in striate cortex by examining the been revealed in striate cortex by examining the distribution of cytochromoxidase .distribution of cytochromoxidase .

Ocular dominance columns are absent in the Ocular dominance columns are absent in the representation of the temporal monocular representation of the temporal monocular crescent.crescent.

Ocular dominance columns are also lacking in Ocular dominance columns are also lacking in the cortical representation of the blind spot .the cortical representation of the blind spot .

At the level of the LGB no binocular interaction At the level of the LGB no binocular interaction occurs , because retinal afferents terminate in occurs , because retinal afferents terminate in purely monocular laminae .purely monocular laminae .

Convergence of afferents representing the Convergence of afferents representing the right eye and the left eye occurs only in right eye and the left eye occurs only in striate cortex .striate cortex .

Binocular integration in the cortex is Binocular integration in the cortex is delayed beyond the initial tier of synaptic delayed beyond the initial tier of synaptic input by the segregation of geniculate input by the segregation of geniculate afferents into ocular dominance columns .afferents into ocular dominance columns .

Microelectrode recordings in macaque Microelectrode recordings in macaque monkey confirm that neurons in layer IVC monkey confirm that neurons in layer IVC are strictly monocular .are strictly monocular .

Cells that respond to stimulation from Cells that respond to stimulation from either eye are found only outside layer IV . either eye are found only outside layer IV . such cells owe their binocularity to such cells owe their binocularity to convergence of inputs from monocular convergence of inputs from monocular cells in layer IV .cells in layer IV .

The functional significance of ocular The functional significance of ocular dominance columns in humans and dominance columns in humans and macaques is uncertain macaques is uncertain

PHYSIOLOGY OF STRIATE CORTEX PHYSIOLOGY OF STRIATE CORTEX Hubel and Wiesel were the first scientists Hubel and Wiesel were the first scientists

to provide a coherent description of the to provide a coherent description of the receptive field properties of cells in striate receptive field properties of cells in striate cortex .cortex .

SIMPLE CELLSSIMPLE CELLSThe receptive field of simple cells can be The receptive field of simple cells can be

mapped into excitatory and inhibitory mapped into excitatory and inhibitory subdivisions with a small spot of light .subdivisions with a small spot of light .

They exhibit summation within their They exhibit summation within their separate excitatory and inhibitory subfields separate excitatory and inhibitory subfields and antagonism when both regions are and antagonism when both regions are stimulated simultaneously .stimulated simultaneously .

In these respects simple cells are similar In these respects simple cells are similar to the center-surround cells of the LGB.to the center-surround cells of the LGB.

The critical distinction between geniculate The critical distinction between geniculate cells and cortical simple cells lies with the cells and cortical simple cells lies with the spatial arrangement of their excitatory and spatial arrangement of their excitatory and inhibitory domains .inhibitory domains .

For cortical simple cells these domains are For cortical simple cells these domains are not arrayed concentrically , , but organized not arrayed concentrically , , but organized into parallel , flanking subfields separated into parallel , flanking subfields separated by straight boundaries .by straight boundaries .

The geometry of subfields varies The geometry of subfields varies considerably among simple cells . In the considerably among simple cells . In the most common layout a narrow elongated most common layout a narrow elongated region , either excitatory or inhibitory , is region , either excitatory or inhibitory , is sandwiched between two symmetric sandwiched between two symmetric subregions of the opposite type .subregions of the opposite type .

Some cells have subfields of unequal area Some cells have subfields of unequal area and other cells have only two antagonistic and other cells have only two antagonistic subfields .subfields .

For all simple cells the best stationary For all simple cells the best stationary stimulus is a slit or bar of light exactly the stimulus is a slit or bar of light exactly the right dimensions to activate only an right dimensions to activate only an excitatory ( on-response ) or inhibitory excitatory ( on-response ) or inhibitory ( off- response ) subfield.( off- response ) subfield.

Diffuse light evokes a meager response , Diffuse light evokes a meager response , because excitatory areas and inhibitory because excitatory areas and inhibitory areas cancel each other .areas cancel each other .

Correct orientation of the light slit is crucial Correct orientation of the light slit is crucial to obtain the maximum response .to obtain the maximum response .

If the stimulating light bar is not parallel to If the stimulating light bar is not parallel to the axis of the receptive field , it will the axis of the receptive field , it will stimulate part of the inhibitory subfield and stimulate part of the inhibitory subfield and fail to stimulate the entire excitatory fail to stimulate the entire excitatory subfield .subfield .

Orientation selectivity is thus a cardinal Orientation selectivity is thus a cardinal feature of cortical simple cells . feature of cortical simple cells .

For the sake of illustration all the cells in For the sake of illustration all the cells in figure are depicted with a preferred figure are depicted with a preferred orientation of 45˚ .orientation of 45˚ .

In fact , all orientations are represented In fact , all orientations are represented equally in the visual cortex .equally in the visual cortex .

Simple cells also respond briskly to Simple cells also respond briskly to moving bars , slits , or edges and moving bars , slits , or edges and sometimes the discharge pattern can be sometimes the discharge pattern can be predicted from the arrangement of the predicted from the arrangement of the excitatory and inhibitory subfields .simple excitatory and inhibitory subfields .simple cells usually fire a burst of spikes just as a cells usually fire a burst of spikes just as a moving light slit enters an excitatory region moving light slit enters an excitatory region ..

The most vigorous discharge is provoked The most vigorous discharge is provoked by simultaneously leaving an inhibitory by simultaneously leaving an inhibitory zone and entering an excitatory zone .zone and entering an excitatory zone .

Cells with symmetric subfield Cells with symmetric subfield arrangements generally give an equal arrangements generally give an equal response to movement in either direction .response to movement in either direction .

Cells with asymmetric subfields often give Cells with asymmetric subfields often give unequal responses to movement in unequal responses to movement in opposite directions .opposite directions .

The optimal speed of stimulus movement The optimal speed of stimulus movement can also vary among simple cells .can also vary among simple cells .

COMPLEX CELLSCOMPLEX CELLSThe receptive fields of complex cells The receptive fields of complex cells

cannot be mapped with stationary stimuli cannot be mapped with stationary stimuli into excitatory and inhibitory subregions .into excitatory and inhibitory subregions .

They give inconsistent on-off responses They give inconsistent on-off responses when tested with stationary slits or spots when tested with stationary slits or spots of light .of light .

However , when a light slit is swept across However , when a light slit is swept across the receptive field , it elicits a sustained the receptive field , it elicits a sustained barrage of impulses .barrage of impulses .

A complex cell may respond to movement A complex cell may respond to movement of the light stimulus anywhere within the of the light stimulus anywhere within the receptive field , provided the stimulus is receptive field , provided the stimulus is oriented correctly .oriented correctly .

By contrast , a simple cell fires only short By contrast , a simple cell fires only short bursts at the moment when the light slit bursts at the moment when the light slit crosses an interface between antagonistic crosses an interface between antagonistic subregions .subregions .

END-STOPPED CELLS END-STOPPED CELLS Ordinary complex cells show summation by Ordinary complex cells show summation by

responding more robustly as the length of a light responding more robustly as the length of a light stimulus is increased . the maximum response stimulus is increased . the maximum response occurs when a slit or bar equals the full length of occurs when a slit or bar equals the full length of the cell’s receptive field.the cell’s receptive field.

Extending the stimulus beyond the length of the Extending the stimulus beyond the length of the receptive field augments the response no receptive field augments the response no further .further .

A special subtype of complex cell behaves in a A special subtype of complex cell behaves in a different fashion : the cell’s response declines different fashion : the cell’s response declines sharply as the stimulus exceeds the length of the sharply as the stimulus exceeds the length of the activating portion of the receptive field .activating portion of the receptive field .

RECEPTIVE FIELD HIERARCHYRECEPTIVE FIELD HIERARCHY Receptive fields undergo a remarkable transformation in Receptive fields undergo a remarkable transformation in

the progression from the LGB to striate cortex .the progression from the LGB to striate cortex . Cells in cortex respond best to suitably oriented bars or Cells in cortex respond best to suitably oriented bars or

edges , rather than circular spots , and their responses edges , rather than circular spots , and their responses depend critically upon the speed and direction of stimuli .depend critically upon the speed and direction of stimuli .

How do geniculate cells generate the receptive fields of How do geniculate cells generate the receptive fields of cortical cells ??cortical cells ??

Simple cells are concentrated in layer IV of striate Simple cells are concentrated in layer IV of striate cortex , the same cortical layer that receives the bulk of cortex , the same cortical layer that receives the bulk of the projection from the LGB . simple cells are also the projection from the LGB . simple cells are also sprinkled throughout layer VI , another layer innervated sprinkled throughout layer VI , another layer innervated by geniculate neurons .by geniculate neurons .

This finding is unlikely to be a coincidence : it This finding is unlikely to be a coincidence : it suggests that simple cells receive their input suggests that simple cells receive their input directly from geniculate cells .directly from geniculate cells .

The axons of simple cells ramify widely to The axons of simple cells ramify widely to synapse upon cells located in other cortical synapse upon cells located in other cortical layers .layers .

Complex cells are common in all cortical layers Complex cells are common in all cortical layers except layer IV . the logical inference is that except layer IV . the logical inference is that simple cells in layers IV and VI feed their input to simple cells in layers IV and VI feed their input to complex cells situated outside layer IV.complex cells situated outside layer IV.

Hubel and Wiesel suggested that simple cell Hubel and Wiesel suggested that simple cell receptive fields are constructed from geniculate receptive fields are constructed from geniculate cell receptive fields .cell receptive fields .

For example , the simple field in the figure :For example , the simple field in the figure :

Might be generated by excitatory input Might be generated by excitatory input from a row of geniculate cells with on-from a row of geniculate cells with on-centers lined up as shown in the figure centers lined up as shown in the figure

In such a scheme a light stimulus falling within In such a scheme a light stimulus falling within the narrow rectangular zone containing the the narrow rectangular zone containing the geniculate on-centers will elicit a net response , geniculate on-centers will elicit a net response , despite partial stimulation of antagonistic field despite partial stimulation of antagonistic field surrounds .surrounds .

The inhibitory subfields of the simple cell might The inhibitory subfields of the simple cell might drive from the off-surrounds of the geniculate drive from the off-surrounds of the geniculate cells , or perhaps from off-center geniculate cells cells , or perhaps from off-center geniculate cells placed in rows on either side of the on-center placed in rows on either side of the on-center units .units .

The receptive fields of complex cells are The receptive fields of complex cells are probably built from simple cells that share the probably built from simple cells that share the same orientation tuning .same orientation tuning .

In one possible arrangement the receptive In one possible arrangement the receptive fields of simple cells are concatenated fields of simple cells are concatenated within the larger receptive field of a single within the larger receptive field of a single complex cell.complex cell.

A moving light slit will activate in A moving light slit will activate in succession each simple receptive field , succession each simple receptive field , eventually exciting a discharge from the eventually exciting a discharge from the complex cell .complex cell .

A stationary on-off stimulus elicits a feeble A stationary on-off stimulus elicits a feeble response because activation of only one response because activation of only one simple cell is not enough to drive the simple cell is not enough to drive the complex cell .complex cell .

The role of simple cells and complex cells The role of simple cells and complex cells in visual perception is unclear , although in visual perception is unclear , although their receptive field properties are well their receptive field properties are well defined .defined .

Simple cells and complex cells respond Simple cells and complex cells respond best to oriented contours , suggesting that best to oriented contours , suggesting that they process information about borders or they process information about borders or edges . edges .

MICROCIRCUIT OF PRIMATE STRIATE MICROCIRCUIT OF PRIMATE STRIATE CORTEXCORTEX

A cortical module is comprised of a few million A cortical module is comprised of a few million cells. Each cortical module receives input from cells. Each cortical module receives input from only a few thousands geniculate fibers .only a few thousands geniculate fibers .

The ratio of cortical cells to geniculate afferents ( The ratio of cortical cells to geniculate afferents ( 106 / 103 ) indicates that after relatively direct 106 / 103 ) indicates that after relatively direct transmission through the lateral geniculate transmission through the lateral geniculate body , the retinal signal activates a cortical unit body , the retinal signal activates a cortical unit containing approximately a thousandfold more containing approximately a thousandfold more processing elements .processing elements .

About 15% of the stellate cells in layer About 15% of the stellate cells in layer IVCβ are inhibitory interneurons .IVCβ are inhibitory interneurons .

Both mango and parvo geniculate fibers Both mango and parvo geniculate fibers send collaterals to pyramidal neurons in send collaterals to pyramidal neurons in layer VI . These pyramidal neurons also layer VI . These pyramidal neurons also receive geniculate input via apical receive geniculate input via apical dendrites branching in layer IVC .dendrites branching in layer IVC .

Layer VI pyramidal cells send axon Layer VI pyramidal cells send axon collaterals back to layer IVC , setting up a collaterals back to layer IVC , setting up a reverberating intracortical circuit of reverberating intracortical circuit of unknown function .unknown function .

The pyramidal cells in layer VI are also The pyramidal cells in layer VI are also believed to give rise to the major believed to give rise to the major reciprocal projection to the lateral reciprocal projection to the lateral geniculate body .geniculate body .

In summary , striate cortex receives In summary , striate cortex receives segregated input from two distinct segregated input from two distinct geniculate cell channels , mango and geniculate cell channels , mango and parvo .parvo .

The parvo input goes via layer IVCβ to The parvo input goes via layer IVCβ to layers II and III to supply cells within blobs layers II and III to supply cells within blobs and between blobs . Blob cells and and between blobs . Blob cells and interblob cells project to separate targets interblob cells project to separate targets in the next visual area , V2 .in the next visual area , V2 .

The mango input proceeds independently The mango input proceeds independently from layer IVCα via layer IVB to V2.from layer IVCα via layer IVB to V2.

EXTRASTRIATE VISUAL CORTEX EXTRASTRIATE VISUAL CORTEX According to the classical view , the striate According to the classical view , the striate

cortex performs a basic analysis of cortex performs a basic analysis of geniculate input and then transmits some geniculate input and then transmits some critical essence to higher peristriate critical essence to higher peristriate cortical areas for further interpretation .cortical areas for further interpretation .

Visual perception is thought to be Visual perception is thought to be enshrined in two visual association areas enshrined in two visual association areas surrounding striate cortex , which surrounding striate cortex , which Brodmann called area 18 and area 19 .Brodmann called area 18 and area 19 .

Recent studies in monkeys using Recent studies in monkeys using physiologic recordings and anatomic physiologic recordings and anatomic tracers have revealed that areas 18 and tracers have revealed that areas 18 and 19 together contain at least five distinct 19 together contain at least five distinct cortical areas devoted to visual processing cortical areas devoted to visual processing : V2, V3 , V3A, V4 and V5 .: V2, V3 , V3A, V4 and V5 .

Other visual areas within and beyond Other visual areas within and beyond areas 18 and 19 remain to be completely areas 18 and 19 remain to be completely mapped and characterized .mapped and characterized .

So far 25 cortical areas predominantly or So far 25 cortical areas predominantly or exclusively engaged in vision have been exclusively engaged in vision have been identified in the macaque monkey .identified in the macaque monkey .

The striate cortex constitute the largest The striate cortex constitute the largest single cortical area , averaging 1200 mm2 single cortical area , averaging 1200 mm2 or about 12% of the neocortex .or about 12% of the neocortex .

V2 , just slightly smaller than V1 , is the V2 , just slightly smaller than V1 , is the second largest cortical area .second largest cortical area .

Together , V1 and V2 account for about Together , V1 and V2 account for about 20% of the entire surface area of the 20% of the entire surface area of the neocortex .neocortex .

After completion of initial processing in After completion of initial processing in striate cortex , visual information is striate cortex , visual information is transmitted to areas V2,V3,V4 and V5 .transmitted to areas V2,V3,V4 and V5 .

Projections unite corresponding retinotopic Projections unite corresponding retinotopic points in the visual field representation in points in the visual field representation in each of these cortical areas .each of these cortical areas .

V2 and V3V2 and V3V1,V2, and V3 are arranged to permit an V1,V2, and V3 are arranged to permit an

orderly topographic representation of the orderly topographic representation of the visual hemifield in each area while contact visual hemifield in each area while contact is maximized between adjacent cortical is maximized between adjacent cortical areas areas

V2 and V3 have ventral and dorsal halves V2 and V3 have ventral and dorsal halves that wrap around V1 , as a result the that wrap around V1 , as a result the superior and inferior visual field quadrants superior and inferior visual field quadrants are mapped in a retinotopic fashion in V2 are mapped in a retinotopic fashion in V2 and V3 .and V3 .

The cytochrome oxidase stain reveals an The cytochrome oxidase stain reveals an array of coarse parallel stripes in V2 array of coarse parallel stripes in V2

The stripes in V2 appear alternately thin and The stripes in V2 appear alternately thin and thick , and extend along the full width of V2 from thick , and extend along the full width of V2 from the V1-V2 border to the V2-V3 border .the V1-V2 border to the V2-V3 border .

Thin stripes receive input from V1 cells located Thin stripes receive input from V1 cells located within cytochrome oxidase blobs in layer II, III .within cytochrome oxidase blobs in layer II, III .

The thick stripes receive input from V1 cells The thick stripes receive input from V1 cells scattered through out layer IVB .scattered through out layer IVB .

The pale interstripe zones that separate the thin The pale interstripe zones that separate the thin and thick stripes of intense cytochrome oxidase and thick stripes of intense cytochrome oxidase activity get their input from V1 cells located activity get their input from V1 cells located between the blobs in layers II ,III .between the blobs in layers II ,III .

V4V4 This region has come to be known as the color This region has come to be known as the color

area in extrastriate cortex .area in extrastriate cortex .

V5 ( middle temporal area )V5 ( middle temporal area ) Neurons in V5 were exquisitely sensitive to Neurons in V5 were exquisitely sensitive to

stimulus motion .stimulus motion . Some units responded well to alight spot , bar , Some units responded well to alight spot , bar ,

or slit moved briskly in a preferred direction and or slit moved briskly in a preferred direction and gave no response to an opposite , null direction .gave no response to an opposite , null direction .

Directional selectivity of cells is such a singular Directional selectivity of cells is such a singular feature of V5 that it has been called the ( motion feature of V5 that it has been called the ( motion area ) in extrastriate cortex .area ) in extrastriate cortex .

VISUAL DEPRIVATIONVISUAL DEPRIVATIONAmblyopia can be defined as a condition Amblyopia can be defined as a condition

caused by abnormal visual experience caused by abnormal visual experience during childhood resulting in unilateral or during childhood resulting in unilateral or bilateral decrease in acuity that cannot be bilateral decrease in acuity that cannot be explained by a disorder of the eye itself .explained by a disorder of the eye itself .

Without an ocular explanation for the low Without an ocular explanation for the low acuity , ophthalmologists have speculated acuity , ophthalmologists have speculated that amblyopia is caused by anomalous that amblyopia is caused by anomalous wiring of the eye’s central connections in wiring of the eye’s central connections in the brain .the brain .

INTRAUTERINE DEVELOPMENT INTRAUTERINE DEVELOPMENT In the human retina most of the ganglion In the human retina most of the ganglion

cells are generated between the eighth cells are generated between the eighth and fifteenth weeks of gestation .and fifteenth weeks of gestation .

The ganglion cell population reaches a The ganglion cell population reaches a plateau of 2.2 to 2.5 million by week18 and plateau of 2.2 to 2.5 million by week18 and remains at that level until the thirtieth week remains at that level until the thirtieth week of gestation .of gestation .

After week 30 , the ganglion cell After week 30 , the ganglion cell population falls drastically during a period population falls drastically during a period of rapid cell death that lasts for about 6 to of rapid cell death that lasts for about 6 to 8 weeks .8 weeks .

Thereafter cell death continues at a low Thereafter cell death continues at a low rate through birth and into the first few rate through birth and into the first few postnatal months .postnatal months .

The ganglion cell population is reduced to The ganglion cell population is reduced to a final count of about 1 million . a final count of about 1 million .

The loss of more than a million The loss of more than a million supernumerary optic axons may serve to supernumerary optic axons may serve to refine the topography and specificity of the refine the topography and specificity of the retinogeniculate projection by eliminating retinogeniculate projection by eliminating inappropriate connections .inappropriate connections .

By week 10 the first retinal ganglion cell By week 10 the first retinal ganglion cell fibers begin to invade the primordial fibers begin to invade the primordial human lateral geniculate nucleus .human lateral geniculate nucleus .

In the macaque , Rakic has shown that In the macaque , Rakic has shown that initially the inputs from each eye initially the inputs from each eye intermingle to occupy the entire lateral intermingle to occupy the entire lateral geniculate body .geniculate body .

The segregation of ocular inputs occurs on The segregation of ocular inputs occurs on a parallel timetable with the development a parallel timetable with the development of lamination .of lamination .

Retinal afferents prune back their axon Retinal afferents prune back their axon terminals so that synaptic connections are terminals so that synaptic connections are preserved only within appropriate preserved only within appropriate geniculate laminae .geniculate laminae .

In the human fetus the geniculate laminae In the human fetus the geniculate laminae emerge between weeks 22 and 25 .emerge between weeks 22 and 25 .

In the macaque monkey the cells destined In the macaque monkey the cells destined to comprise striate cortex are born to comprise striate cortex are born between days E43 and E102 . This period between days E43 and E102 . This period corresponds to weeks 10 to 25 in the corresponds to weeks 10 to 25 in the human fetus .human fetus .

In macaques the geniculate afferents In macaques the geniculate afferents begin to innervate striate cortex by E110 , begin to innervate striate cortex by E110 , a time equivalent to gestational week 26 in a time equivalent to gestational week 26 in humans .humans .

Injection of anatomic tracers reveals that Injection of anatomic tracers reveals that initially the geniculate afferents initially the geniculate afferents representing each eye overlap extensively representing each eye overlap extensively in layer IVC .in layer IVC .

The segregation of inputs into ocular The segregation of inputs into ocular dominance columns transpires during the dominance columns transpires during the last few weeks of pregnancy and is almost last few weeks of pregnancy and is almost complete at birth .complete at birth .

The maturation of the ocular dominance The maturation of the ocular dominance columns requires thousands of left eye columns requires thousands of left eye and right eye geniculate afferents to and right eye geniculate afferents to gradually disentangle their overlapping gradually disentangle their overlapping axon terminals in striate cortex .axon terminals in striate cortex .

NEWBORN FUNCTIONNEWBORN FUNCTIONAny parent can testify that the visually Any parent can testify that the visually

guided behavior of human newborns is guided behavior of human newborns is quite primitive .quite primitive .

This observation implies that visual acuity This observation implies that visual acuity is still rather poor at birth . is still rather poor at birth .

A number of methods are available to test A number of methods are available to test vision quantitatively in babies .vision quantitatively in babies .

These techniques rely either upon visual These techniques rely either upon visual evoked potentials , optokinetic evoked potentials , optokinetic nystagmus , or preferential looking by an nystagmus , or preferential looking by an infant toward a patterned visual stimulus .infant toward a patterned visual stimulus .

Each technique exploits a different Each technique exploits a different approach to measure acuity , but all three approach to measure acuity , but all three techniques agree fairly well that visual techniques agree fairly well that visual acuity is only 20/400 at birth .acuity is only 20/400 at birth .

Visual acuity quickly improves to a level of Visual acuity quickly improves to a level of 20/20 within the first few years of life .20/20 within the first few years of life .

This rapid refinement in visual acuity is This rapid refinement in visual acuity is paralleled by maturation of mechanisms paralleled by maturation of mechanisms that control accommodation , stereopsis , that control accommodation , stereopsis , smooth pursuit , and saccadic eye smooth pursuit , and saccadic eye movements. movements.

The human macula is immature at birth .The human macula is immature at birth .The fovea is still covered by multiple cell The fovea is still covered by multiple cell

layers and only sparsely packed with layers and only sparsely packed with cones.cones.

During the first year of life the During the first year of life the photoreceptors redistribute within the photoreceptors redistribute within the retina and peak foveal cone density retina and peak foveal cone density increases by fivefold to achieve the increases by fivefold to achieve the concentration found in adult retina .concentration found in adult retina .

In newborns the white matter of the visual In newborns the white matter of the visual pathway is only scantily clad with myelin . pathway is only scantily clad with myelin .

For the first two years after birth the myelin For the first two years after birth the myelin sheaths enlarge rapidly .sheaths enlarge rapidly .

Myelination continues at a slower rate Myelination continues at a slower rate through out the first decade of life .through out the first decade of life .

At birth , neurons of the lateral geniculate At birth , neurons of the lateral geniculate body are only 60% of their average adult body are only 60% of their average adult size .size .

Their volume gradually increases until the Their volume gradually increases until the age of 2 years .age of 2 years .

In striate cortex refinement of synaptic In striate cortex refinement of synaptic connections continues for many years connections continues for many years after birth .after birth .

THE ROLE OF ACTIVITY THE ROLE OF ACTIVITY The visual system begins to form in utero The visual system begins to form in utero

before visual experience can exert any before visual experience can exert any possible influence .possible influence .

The continued development of the central The continued development of the central visual pathways after birth suggests a visual pathways after birth suggests a potential for postnatal activity to shape the potential for postnatal activity to shape the maturing visual system .maturing visual system .

Apparently the basic elements of the Apparently the basic elements of the cortical module are generated before cortical module are generated before birth , according to instructions that are birth , according to instructions that are innately programmed .innately programmed .

Surprisingly , physiologic activity in the Surprisingly , physiologic activity in the fetus plays a vital role in the development fetus plays a vital role in the development of normal anatomic connections in the of normal anatomic connections in the visual system .visual system .

In utero , mammalian retinal ganglion cells In utero , mammalian retinal ganglion cells discharge spontaneous action potentials discharge spontaneous action potentials in the absence of any visual stimulation .in the absence of any visual stimulation .

Abolishing these action potentials with Abolishing these action potentials with tetrodotoxin , a sodium channel blocker , tetrodotoxin , a sodium channel blocker , prevents the normal prenatal segregation prevents the normal prenatal segregation of the retinogeniculate axons into of the retinogeniculate axons into appropriate geniculate laminae .appropriate geniculate laminae .

Intraocular administration of tetrodotoxin Intraocular administration of tetrodotoxin also blocks the formation of ocular also blocks the formation of ocular dominance columns in striate cortex .dominance columns in striate cortex .

These experiments indicate that although These experiments indicate that although the functional architecture of the visual the functional architecture of the visual system is ordained by genetics , the system is ordained by genetics , the specificity and refinement of connections specificity and refinement of connections are molded by physiologic activity are molded by physiologic activity occurring in the fetus .occurring in the fetus .

EYELID SUTURE EYELID SUTURE If a newborn monkey is reared in the dark If a newborn monkey is reared in the dark

or with both eyes sutured closed , cells in or with both eyes sutured closed , cells in striate cortex eventually develop bizarre striate cortex eventually develop bizarre receptive field properties .receptive field properties .

The cells lose sharp orientation tuning and The cells lose sharp orientation tuning and normal binocular responses .normal binocular responses .

Some cells become oblivious to visual Some cells become oblivious to visual stimulation and can be detected only by stimulation and can be detected only by virtue of their erratic spontaneous activity .virtue of their erratic spontaneous activity .

The remaining units give sluggish and The remaining units give sluggish and unpredictable responses to visual unpredictable responses to visual stimulation .stimulation .

After a long period of deprivation , if the After a long period of deprivation , if the monkey is introduced to a normal visual monkey is introduced to a normal visual environment ( or the eyelids are environment ( or the eyelids are reopened ) , the animal is left profoundly reopened ) , the animal is left profoundly blind with minimal potential for recovery .blind with minimal potential for recovery .

Cells in striate cortex do not recover Cells in striate cortex do not recover normal response properties .normal response properties .

These laboratory observations These laboratory observations demonstrate that patterned visual demonstrate that patterned visual stimulation is required for a critical period stimulation is required for a critical period after birth to preserve and promote normal after birth to preserve and promote normal visual function .visual function .

In ophthalmology an analogy can be In ophthalmology an analogy can be drawn with the newborn baby suffering drawn with the newborn baby suffering dense , bilateral , congenital lens opacities dense , bilateral , congenital lens opacities . In this clinical situation the cataracts must . In this clinical situation the cataracts must be removed soon after birth to avoid be removed soon after birth to avoid permanent visual loss from bilateral permanent visual loss from bilateral amblyopia.amblyopia.

Cataract extraction delayed beyond the Cataract extraction delayed beyond the critical period will not allow the child to critical period will not allow the child to enjoy normal visual function .enjoy normal visual function .

If a monkey is visually deprived as an If a monkey is visually deprived as an adult by suturing closed both eyelids , adult by suturing closed both eyelids , there is no effect upon the properties of there is no effect upon the properties of cells in striate cortex .cells in striate cortex .

In adult patients , form deprivation induced In adult patients , form deprivation induced by slowly advancing cataracts does not by slowly advancing cataracts does not impair visual function in a permanent impair visual function in a permanent manner .manner .

After successful removal of the cataracts , After successful removal of the cataracts , the patient experiences full restoration of the patient experiences full restoration of sight .sight .

The occluded eye usually develops axial The occluded eye usually develops axial myopia .In the lateral geniculate body the myopia .In the lateral geniculate body the cells in the deprived laminae become cells in the deprived laminae become slightly shrunken compared with the cells slightly shrunken compared with the cells in normal laminae .in normal laminae .

Although cells within deprived laminae are Although cells within deprived laminae are shrunken , they have normal center-shrunken , they have normal center-surround receptive fields and respond surround receptive fields and respond briskly to visual stimulation . These briskly to visual stimulation . These findings imply that a defect at the level of findings imply that a defect at the level of the lateral geniculate body is unlikely to the lateral geniculate body is unlikely to account for amblyopia .account for amblyopia .

Monocular visual deprivation produces a Monocular visual deprivation produces a radical alteration in the ocular dominance radical alteration in the ocular dominance columns in striate cortex .columns in striate cortex .

The ocular dominance columns of the The ocular dominance columns of the closed eye appear severely narrowed closed eye appear severely narrowed when labeled with radioactive tracer . when labeled with radioactive tracer .

THE CRITICAL PERIODTHE CRITICAL PERIODThe conclusion from performing eyelid The conclusion from performing eyelid

closure in different animals at various ages closure in different animals at various ages is that macaque monkeys are vulnerable is that macaque monkeys are vulnerable to the effects of eyelid suture for only a to the effects of eyelid suture for only a few months after birth .few months after birth .

This period is defined as the critical This period is defined as the critical period .period .

In the macaque monkey the closure of one In the macaque monkey the closure of one eye any time during the critical period eye any time during the critical period even for just a week , can result in the even for just a week , can result in the shrinkage of ocular dominance columns shrinkage of ocular dominance columns and the loss of the deprived eye’s ability to and the loss of the deprived eye’s ability to drive cells in striate cortex .drive cells in striate cortex .

The critical period corresponds to a time The critical period corresponds to a time when the wiring of the striate cortex is still when the wiring of the striate cortex is still malleable and hence vulnerable to the malleable and hence vulnerable to the effects of visual deprivation .effects of visual deprivation .

During the critical period the deleterious During the critical period the deleterious effects of eyelid closure can also be effects of eyelid closure can also be corrected by reverse eyelid suture .corrected by reverse eyelid suture .

Reexapansion of deprived eye columns Reexapansion of deprived eye columns does not occur if reverse suture is carried does not occur if reverse suture is carried out beyond the critical period .out beyond the critical period .

It may explain in part why patching in a It may explain in part why patching in a child to improve vision in an amblyopic eye child to improve vision in an amblyopic eye is fruitless if instigated after the end of the is fruitless if instigated after the end of the critical period .critical period .

CLINICAL IMPLICATIONS CLINICAL IMPLICATIONS The most common etiologies are unilateral The most common etiologies are unilateral

ptosis and cataract .ptosis and cataract . In humans , patching of the normal eye is In humans , patching of the normal eye is

the mainstay of treatment for amblyopia . the mainstay of treatment for amblyopia . Recent clinical experience suggests that Recent clinical experience suggests that good visual function can be achieved in good visual function can be achieved in children with congenital monocular children with congenital monocular cataract . cataract .

Effective therapy requires early surgical Effective therapy requires early surgical removal of the offending cataract , removal of the offending cataract , appropriate refractive correction , and appropriate refractive correction , and vigorous patching of the normal eye .vigorous patching of the normal eye .

The critical period in humans has been The critical period in humans has been defined by documenting the visual defined by documenting the visual outcome in children after surgical removal outcome in children after surgical removal of congenital cataracts performed at of congenital cataracts performed at different ages .different ages .

These studies indicate that the human These studies indicate that the human critical period extends for at least several critical period extends for at least several years after birth .years after birth .

The duration of the critical period may also The duration of the critical period may also vary according to the etiology of the vary according to the etiology of the amblyopia .amblyopia .

A minority of cases of amblyopia are A minority of cases of amblyopia are caused by media opacity . Other common caused by media opacity . Other common etiologies in children include strabismus . , etiologies in children include strabismus . , anisometropia , nystagmus , and extreme anisometropia , nystagmus , and extreme refractive error .refractive error .

Raising an animal with alternate daily Raising an animal with alternate daily occlusion of one eye using a translucent occlusion of one eye using a translucent contact lens leads to selective loss of contact lens leads to selective loss of stereopsis with normal acuity in each eye stereopsis with normal acuity in each eye and this can be considered a special form and this can be considered a special form of amblyopia due to a breakdown of of amblyopia due to a breakdown of binocular connections in striate cortex .binocular connections in striate cortex .

After cutting one extraocular muscle , After cutting one extraocular muscle , some monkeys do not alternate fixation some monkeys do not alternate fixation but instead fixate constantly with the same but instead fixate constantly with the same eye . The deviating eye invariably eye . The deviating eye invariably develops amblyopia .develops amblyopia .

Few cells in striate cortex can be driven by Few cells in striate cortex can be driven by stimulation of the amblyopic eye .stimulation of the amblyopic eye .