Physiology of Behavior 10e Ch06 (1)

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Vision

outline The Stimulus

Anatomy of the Visual System

The EyesPhotoreceptors

Connections Between Eyeand Brain

Interim Summary

Coding of Visual Informationin the RetinaCoding of Light and Dark

Coding of Color

Interim Summary

Analysis of VisualInformation: Role of the

Striate CortexAnatomy of the Striate CortexOrientation and Movement

Spatial Frequency

Retinal Disparity

Color

Modular Organization ofthe Striate Cortex

Interim Summary

Analysis of VisualInformation: Role of the

Visual Association CortexTwo Streams of Visual Analysis

Perception of Color

Perception of Form

Perception of Movement

Perception of Spatial Location

Interim Summary

chapter

6

ISBN

0-558-46775-X

Physiology of Behavior, Tenth Edition, by Neil R. Carlson. Published by Allyn & Bacon. Copyright 2010 by Pearson Education, Inc.

Because of permissions issues, some material (e.g., photographs) has been removed from this chapter, though reference to itmay occur in the text. The omitted content was intentionally deleted and is not needed to meet the University's requirements forthis course.

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170 Chapter 6 Vision

A

s we saw in Chapter 3, the brain performs twomajor functions: It controls the movements ofthe muscles, producing useful behaviors, andit regulates the bodys internal environment.

To perform both these tasks, the brain must beinformed about what is happening both in the externalenvironment and within the body. Such information isreceived by the sensory systems. This chapter and thenext are devoted to a discussion of the ways in whichsensory organs detect changes in the environment andthe ways in which the brain interprets neural signalsfrom these organs.

We receive information about the environmentfrom sensory receptorsspecialized neurons thatdetect a variety of physical events. (Do not confuse

sensory receptors with receptors for neurotransmitters,neuromodulators, and hormones. Sensory receptors arespecialized neurons, and the other types of receptorsare specialized proteins that bind with certain mole-

cules.) Stimuli impinge on the receptors and, throughvarious processes, alter their membrane potentials. Thisprocess is known as sensory transduction because sensoryevents are transduced(transferred) into changes in thecells membrane potential. These electrical changes are

Dr. L., a young neuropsychologist, waspresenting the case of Mrs. R. to a group of medicalstudents doing a rotation in the neurology department at the

medical center. The chief of the department had shown them

Mrs. R.s CT scans, and now Dr. L. was addressing the students.

He told them that Mrs. R.s stroke had not impaired her abilityto talk or to move about, but it had affected her vision.

A nurse ushered Mrs. R. into the room and helped her

find a seat at the end of the table.

How are you, Mrs. R.? asked Dr. L.

Im fine. Ive been home for a month now, and I can do

just about everything that I did before I had my stroke.

Good. How is your vision?

Well, Im afraid thats still a problem.

What seems to give you the most trouble?

I just dont seem to be able to recognize things. When

Im working in my kitchen, I know what everything is as long

as no one moves anything. A few times my husband tried to

help me by putting things away, and I couldnt see them any

more. She laughed. Well, I could see them, but I just couldnt

say what they were.

Dr. L. took some objects out of a paper bag and placed

them on the table in front of her.

Can you tell me what these are? he asked. No, he

said, please dont touch them.

Mrs. R. stared intently at the objects. No, I cant rightly

say what they are.

Dr. L. pointed to one of them, a wristwatch. Tell me

what you see here, he said.Mrs. R. looked thoughtful, turning her head one way and

then the other. Well, I see something round, and it has two

things attached to it, one on the top and one on the bottom.

She continued to stare at it. There are some things inside the

circle, I think, but I cant make out what they are.

Pick it up.

She did so, made a wry face, and said, Oh. Its a wrist-

watch. At Dr. L.s request, she picked up the rest of the

objects, one by one, and identified each of them correctly.

Do you have trouble recognizing people, too? asked Dr. L.

Oh, yes! she sighed. While I was still in the hospital, my

husband and my son both came in to see me, and I couldnt tellwho was who until my husband said somethingthen I could

tell which direction his voice was coming from. Now Ive trained

myself to recognize my husband. I can usually see his glasses

and his bald head, but I have to work at it. And Ive been fooled

a few times. She laughed. One of our neighbors is bald and

wears glasses, too, and one day when he and his wife were vis-

iting us, I thought he was my husband, so I called him honey.

It was a little embarrassing at first, but everyone understood.

What does a face look like to you? asked Dr. L.

Well, I know that its a face, because I can usually see

the eyes, and its on top of a body. I can see a body pretty

well, by how it moves. She paused a moment. Oh, yes, I for-

got, sometimes I can recognize a person by how he moves.

You know, you can often recognize friends by the way they

walk, even when theyre far away. I can still do that. Thats

funny, isnt it? I cant see peoples faces very well, but I can

recognize the way they walk.

Dr. L. made some movements with his hands. Can you

tell what Im pretending to do? he asked.

Yes, youre mixing somethinglike some cake batter.

He mimed the gestures of turning a key, writing, and

dealing out playing cards, and Mrs. R. recognized them with-

out any difficulty.Do you have any trouble reading? he asked.

Well, a little, but I dont do too badly.

Dr. L. handed her a magazine, and she began to read the

article aloudsomewhat hesitantly but accurately. Why is

it, she asked, that I can see the words all right but have so

much trouble with things and with peoples faces?

sensory receptor A specialized neuron that detects a particularcategory of physical events.

sensory transduction The process by which sensory stimuli aretransduced into slow, graded receptor potentials.











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The Stimulus 171

called receptor potentials. Most receptors lack axons; aportion of their somatic membrane forms synapses withthe dendrites of other neurons. Receptor potentialsaffect the release of neurotransmitters and hence modi-fy the pattern of firing in neurons with which these cellsform synapses. Ultimately, the information reaches the

brain.People often say that we have five senses: sight, hear-

ing, smell, taste, and touch. Actually, we have more thanfive, but even experts disagree about how the linesbetween the various categories should be drawn.Certainly, we should add the vestibular senses; as well asproviding us with auditory information, the inner earsupplies information about head orientation and move-ment. The sense of touch (or, more accurately,somatosensation) detects changes in pressure, warmth,cold, vibration, limb position, and events that damagetissue (that is, produce pain). Everyone agrees that wecan detect these stimuli; the issue is whether we shouldsay that they are detected by separate senses.

This chapter considers vision, the sensory modalitythat receives the most attention from psychologists,anatomists, and physiologists. One reason for this atten-tion derives from the fascinating complexity of the sen-sory organs of vision and the relatively large proportionof the brain that is devoted to the analysis of visual infor-mation. Approximately 20 percent of the cerebral cor-tex plays a direct role in the analysis of visual informa-tion (Wandell, Dumoulin, and Brewer, 2007). Anotherreason, I am sure, is that vision is so important to us asindividuals. A natural fascination with such a rich source

of information about the world leads to curiosity abouthow this sensory modality works. Chapter 7 deals withthe other sensory modalities: audition, the vestibularsenses, the somatosenses, gustation, and olfaction.

THE STIMULUSAs we all know, our eyes detect the presence of light. Forhumans, light is a narrow band of the spectrum of elec-tromagnetic radiation. Electromagnetic radiation with a

wavelength of between 380 and 760 nm (a nanometernm, is one-billionth of a meter) is visible to us. (SeeFigure 6.1.) Other animals can detect different ranges ofelectromagnetic radiation. For example, honeybees candetect differences in ultraviolet radiation reflected byflowers that appear white to us. The range of wave

lengths we call lightis not qualitatively different from therest of the electromagnetic spectrum; it is simply thepart of the continuum that we humans can see.

The perceived color of light is determined by threedimensions: hue, saturation, and brightness. Light travelsat a constant speed of approximately 300,000 kilometers(186,000 miles) per second. Thus, if the frequency ofoscillation of the wave varies, the distance between thepeaks of the waves will vary similarly but in inverse fashion. Slower oscillations lead to longer wavelengths, andfaster ones lead to shorter wavelengths. Wavelengthdetermines the first of the three perceptual dimensionsof light: hue. The visible spectrum displays the range ofhues that our eyes can detect.

Light can also vary in intensity, which correspondsto the second perceptual dimension of light: brightnessIf the intensity of the electromagnetic radiation isincreased, the apparent brightness increases, too. Thethird dimension, saturation, refers to the relative purityof the light that is being perceived. If all the radiation isof one wavelength, the perceived color is pure, or fullysaturated. Conversely, if the radiation contains all wavelengths, it produces no sensation of hueit appearswhite. Colors with intermediate amounts of saturationconsist of different mixtures of wavelengths. Figure 6.2

shows some color samples, all with the same hue buwith different levels of brightness and saturation. (SeeFigure 6.2.)

Wavelength in nanometers

400 500 600 700

The visible spectrum

Gammarays

X rays Ultravioletrays

RadarTelevision and radio

broadcast bandsAC circuits

Infraredrays

FIGURE 6.1 The Electromagnetic Spectrum .

receptor potential A slow, graded electrical potential produced bya receptor cell in response to a physical stimulus.

hue One of the perceptual dimensions of color; the dominantwavelength.

brightness One of the perceptual dimensions of color; intensity.

saturation One of the perceptual dimensions of color; purity.

ISBN

0-558-46775-X











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ANATOMY OF THEVISUAL SYSTEM

For an individual to see, an image must be focused onthe retina, the inner lining of the eye. This image causeschanges in the electrical activity of millions of neuronsin the retina, which results in messages being sentthrough the optic nerves to the rest of the brain. (I saidthe rest because the retina is actually part of the brain; itand the optic nerve are in the centralnot peripheralnervous system.) This section describes the anatomy ofthe eyes, the photoreceptors in the retina that detect thepresence of light, and the connections between theretina and the brain.

The EyesThe eyes are suspended in the orbits, bony pockets in thefront of the skull. They are held in place and moved bysix extraocular muscles attached to the tough, whiteouter coat of the eye called the sclera. (See Figure 6.3.)Normally, we cannot look behind our eyeballs and seethese muscles, because their attachments to the eyes are

hidden by the conjunctiva. These mucous membranesline the eyelid and fold back to attach to the eye (thuspreventing a contact lens that has slipped off the corneafrom falling behind the eye). Figure 6.4 illustrates theanatomy of the eye. (See Figure 6.4.)

The eyes make three types of movements: vergencemovements, saccadic movements, and pursuit move-ments.Vergence movements are cooperative movementsthat keep both eyes fixed on the same targetor, moreprecisely, that keep the image of the target object oncorresponding parts of the two retinas. If you hold up a

finger in front of your face, look at it, and then bringyour finger closer to your face, your eyes will make ver-gence movements toward your nose. If you then look atan object on the other side of the room, your eyes willrotate outward, and you will see two separate blurryimages of your finger.

When you scan the scene in front of you, your gazedoes not roam slowly and steadily across its features.Instead, your eyes make jerkysaccadic movementsyou

shift your gaze abruptly from one point to another.(Saccadecomes from the French word for jerk.) Whenyou read a line in this book, your eyes stop several times,moving very quickly between each stop. You cannot con-sciously control the speed of movement between stops;during each saccade the eyes move as fast as they can.Only by performing apursuit movementsay, by look-ing at your finger while you move it aroundcan youmake your eyes move more slowly.

The white outer layer of most of the eye, the sclera,is opaque and does not permit entry of light. However,the cornea, the outer layer at the front of the eye, istransparent and admits light. The amount of light that

enters is regulated by the size of the pupil, which is an

FIGURE 6.2 Color Wavelength and .Saturation .

This figure shows examples of colors with the same

dominant wavelength (hue) but different levels of

saturation or brightness.

Decreasingsaturation

Increasingsaturation

Increasingbrightness

Decreasingbrightness

FIGURE 6.3 The Extraocular Muscles, .Which Move the Eyes .

Extraocularmuscles

vergence movementThe cooperative movement of the eyes,which ensures that the image of an object falls on identical portionsof both retinas.

saccadic movement(suh kadik) The rapid, jerky movement ofthe eyes used in scanning a visual scene.

pursuit movement The movement that the eyes make to maintainan image of a moving object on the fovea.











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Anatomy of the Visual System 173

opening in the iris, the pigmented ring of musclessituated behind the cornea. The lens, situated immedi-ately behind the iris, consists of a series of transparent,onionlike layers. Its shape can be altered by contractionof the ciliary muscles. These changes in shape permit theeye to focus images of near or distant objects on theretinaa process called accommodation.

After passing through the lens, light traverses themain part of the eye, which is filled with vitreous humor(glassy liquid), a clear, gelatinous substance. Afterpassing through the vitreous humor, light falls on theretina, the interior lining of the back of the eye. In theretina are located the receptor cells, the rods and cones(named for their shapes), collectively known asphotoreceptors.

The human retina contains approximately 120 mil-lion rods and 6 million cones. Although they are greatlyoutnumbered by rods, cones provide us with most of theinformation about our environment. In particular, theyare responsible for our daytime vision. They provide us with information about small features in the environ-

ment and thus are the source of vision of the highestsharpness, or acuity(from the Latin acus, needle). Thefovea, or central region of the retina, which mediatesour most acute vision, contains only cones. Cones arealso responsible for color visionour ability to discrim-inate light of different wavelengths. Although rods donot detect different colors and provide vision of pooracuity, they are more sensitive to light. In a very dimlylighted environment we use our rod vision; therefore, indim light we are color-blind and lack foveal vision. (SeeTable 6.1.)

Another feature of the retina is the optic disk where the axons conveying visual information gathetogether and leave the eye through the optic nerve. Theoptic disk produces a blind spotbecause no receptors arelocated there. We do not normally perceive our blindspots, but their presence can be demonstrated. If youhave not found yours, you may want to try the exercisedescribed in Figure 6.5.

Close examination of the retina shows that it con-sists of several layers of neuron cell bodies, their axonsand dendrites, and the photoreceptors. Figure 6.6 illustrates a cross section through the primate retina, whichis divided into three main layers: the photoreceptive

Optic nerve

Vitreous humor (upperhalf has been removed)

Blood vessels

Sclera

Layers of retinaConjunctiva (mergeswith inside ofeyelids)

Cornea

Iris

Lens

Pupil (openingin iris)

FIGURE 6.4 The Human Eye .

accommodation Changes in the thickness of the lens of the eye,accomplished by the ciliary muscles, that focus images of near ordistant objects on the retina.

retina The neural tissue and photoreceptive cells located on theinner surface of the posterior portion of the eye.

rod One of the receptor cells of the retina; sensitive to light of lowintensity.

cone One of the receptor cells of the retina; maximally sensitive toone of three different wavelengths of light and hence encodes colorvision.

photoreceptor One of the receptor cells of the retina; transducesphotic energy into electrical potentials.

fovea( foevee a) The region of the retina that mediates the mostacute vision of birds and higher mammals. Color-sensitive conesconstitute the only type of photoreceptor found in the fovea.

optic disk The location of the exit point from the retina of thefibers of the ganglion cells that form the optic nerve; responsiblefor the blind spot.

ISBN

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layer, the bipolar cell layer, and the ganglion cell layer.Note that the photoreceptors are at the backof the reti-na; light must pass through the overlying layers to get tothem. Fortunately, these layers are transparent. (SeeFigure 6.6.)

The photoreceptors form synapses with bipolarcells, neurons whose two arms connect the shallowestand deepest layers of the retina. In turn, these neuronsform synapses with the ganglion cells, neurons whoseaxons travel through the optic nerves (the second cra-nial nerves) and carry visual information into the rest ofthe brain. In addition, the retina contains horizontalcells and amacrine cells, both of which transmit informa-tion in a direction parallel to the surface of the retina andthus combine messages from adjacent photoreceptors.(See Figure 6.6.)

The primate retina contains approximately 55 dif-ferent types of neurons: one type of rod, three types ofcones, two types of horizontal cells, ten types of bipolarcells, 2429 types of amacrine cells, and 1015 types ofganglion cells (Masland, 2001).

PhotoreceptorsFigure 6.7 shows a drawing of two rods and a cone. Notethat each photoreceptor consists of an outer segment

connected by a cilium to the inner segment, which con-tains the nucleus. (See Figure 6.7.) The outer segmentcontains several hundred lamellae, or thin plates of

TABLE 6.1 Locations and Response Characteristics of Photoreceptors .

CONES RODS

Most prevalent in the central retina; found in the fovea Most prevalent in the peripheral retina; not found in the fovea

Sensitive to moderate to high levels of light Sensitive to low levels of light

Provide information about hue Provide only monochromatic information

Provide excellent acuity Provide poor acuity

+

+

+

Fovea

Optic disk(Blind spot)

FIGURE 6.5 A Test for the Blind Spot .

With your left eye closed, look at the plus sign with your right eye, and move the page

nearer to and farther from you. When the page is about 20 cm from your face, the green

circle disappears because its image falls on the blind spot of your right eye.

bipolar cell A bipolar neuron located in the middle layer of theretina, conveying information from the photoreceptors to theganglion cells.

ganglion cell A neuron located in the retina that receives visualinformation from bipolar cells; its axons give rise to the opticnerve.

horizontal cell A neuron in the retina that interconnects adjacentphotoreceptors and the outer processes of the bipolar cells.

amacrine cell (amm a krine) A neuron in the retina thatinterconnects adjacent ganglion cells and the inner processes of thebipolar cells.

lamellaA layer of membrane containing photopigments; found inrods and cones of the retina.











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membrane. (Lamella is the diminutive form of lamina,thin layer.)

Lets consider the nature of transduction of visualinformation. The first step in the chain of events thatleads to visual perception involves a special chemicalcalled a photopigment. Photopigments are special mol-ecules embedded in the membrane of the lamellae; a

single human rod contains approximately 10 million ofthem. The molecules consist of two parts: an opsin (aprotein) and retinal (a lipid). There are several forms of

opsin; for example, the photopigment of human rodsrhodopsin, consists ofrod opsinplus retinal. (Rhod-refersto the Greek rhodon, rose, not to rod. Before it isbleached by the action of light, rhodopsin has a pinkishhue.) Retinal is synthesized from vitamin A, whichexplains why carrots, which are rich in this vitamin, aresaid to be good for your eyesight.

When a molecule of rhodopsin is exposed to lightit breaks into its two constituents: rod opsin and retinalWhen that happens, the rod opsin changes from its rosycolor to a pale yellow; hence, we say that the lighbleaches the photopigment. The splitting of the photopigment produces the receptor potential: hyperpolarization of the membrane of the photoreceptor.

In the vertebrate retina, photoreceptors provideinput to both bipolar cells and horizontal cells. Figure6.8 shows the neural circuitry from a photoreceptor to aganglion cell. The circuitry is much simplified and omitsthe horizontal cells and amacrine cells. The first twotypes of cells in the circuitphotoreceptors and bipolar

Photoreceptor Layer Bipolar Cell Layer Ganglion Cell Layer

PhotoreceptorsBipolar

cell

Ganglioncell

Amacrinecell

Horizontalcell

LightBackof Eye

Cone

Rod

FIGURE 6.6 Details of Retinal Circuitry .

(Adapted from Dowling, J. E., and Boycott, B. B. Proceedings of the Royal Society of London, B, 1966, 166, 80111.)

Lamellae

Lamellae

Cone innersegment

Rod innersegment

Nucleus

MitochondriaConnectingcilium

Back of retina Light

FIGURE 6.7 Photoreceptors .

photopigmentA protein dye bonded to retinal, a substancederived from vitamin A; responsible for transduction of visualinformation.

opsin (opp sin) A class of protein that, together with retinal,constitutes the photopigments.

retinal (retti nahl) A chemical synthesized from vitamin A; joinswith an opsin to form a photopigment.

rhodopsin (roh dopp sin) A particular opsin found in rods.

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cellsdo not produce action potentials. Instead, theirrelease of the neurotransmitter (glutamate) is regulatedby the value of their membrane potential; depolariza-tions increase the release, and hyperpolarizationsdecrease it. The contents of the circles indicate what would be seen on an oscilloscope screen recordingchanges in the cells membrane potentials in responseto a spot of light shining on the photoreceptor.

The hyperpolarizing effect of light on the mem-brane of a photoreceptor is shown in the left circle. Inthe dark, photoreceptors constantly release their neuro-transmitter. When light strikes molecules of the pho-topigment, the hyperpolarization that ensues reducesthe amount of neurotransmitter released by the pho-toreceptor. Because the neurotransmitter normallyhyperpolarizes the dendrites of the bipolar cell by bind-ing with inhibitory metabotropic alutamate receptors, areductionin its release causes the membrane of the bipo-lar cell to depolarize. Thus, light hyperpolarizes the pho-

toreceptor and depolarizes the bipolar cell. (See Figure6.8.) The depolarization of the bipolar cell causes it torelease more neurotransmitter, which depolarizes themembrane of the ganglion cell and raises this cells rateof firing. Thus, light shining on the photoreceptorexcites the ganglion cell and increases the rate of firingof its axon.

The circuit shown in Figure 6.8 illustrates a ganglioncell whose firing rate increases in response to light. As wewill see, other ganglion cells decreasetheir firing rate inresponse to light. These neurons are connected to bipo-lar cells that form different types of synapses with thephotoreceptors. The functions of these two types of cir-cuits are discussed in a later section, Coding of VisualInformation in the Retina. If you would like to knowmore about the neural circuitry of the retina, you shouldconsult the book by Rodieck (1998).

Connections BetweenEye and BrainThe axons of the retinal ganglion cells bring informa-tion to the rest of the brain. They ascend through theoptic nerves and reach the dorsal lateral geniculatenucleus (LGN) of the thalamus. This nucleus receives itsname from its resemblance to a bent knee (genuis Latin

for knee). It contains six layers of neurons, each ofwhich receives input from only one eye. The neurons inthe two inner layers contain cell bodies that are largerthan those in the outer four layers. For this reason theinner two layers are called the magnocellular layers, andthe outer four layers are called theparvocellular layers(parvo-refers to the small size of the cells). A third set ofneurons in the koniocellular sublayers are found ventralto each of the magnocellular and parvocellular layers.(Konisis the Greek word for dust.) As we will see later,these three sets of layers belong to different systems,which are responsible for the analysis of different typesof visual information. They receive input from different

types of retinal ganglion cells. (See Figure 6.9.)The neurons in the LGN send their axons through a

pathway known as the optic radiationsto the primary visualcortexthe region surrounding the calcarine fissure

FIGURE 6.8 Neural Circuitry in the Retina .

Light striking a photoreceptor produces a hyperpolarization,

so the photoreceptor releases less neurotransmitter. Because

the neurotransmitter normally hyperpolarizes the membraneof the bipolar cell, the reduction causes a depolarization.

This depolarization causes the bipolar cell to release more

neurotransmitter, which excites the ganglion cell.

(Adapted from Dowling, J. E., in The Neurosciences: Fourth Study Program,edited by F. O. Schmitt and F. G. Worden. Cambridge, Mass.: MIT Press, 1979.)

Ganglioncell

To brainLight

Bipolar cell

Recordingof actionpotentials

Depolarizingmembranepotential

Hyperpolarizingmembranepotential

Stimulus

Photoreceptor

dorsal lateral geniculate nucleus (LGN) A group of cell bodieswithin the lateral geniculate body of the thalamus; receives inputsfrom the retina and projects to the primary visual cortex.

magnocellular layer One of the inner two layers of neurons in thedorsal lateral geniculate nucleus; transmits information necessaryfor the perception of form, movement, depth, and small differencesin brightness to the primary visual cortex.

parvocellular layer One of the four outer layers of neurons in thedorsal lateral geniculate nucleus; transmits information necessaryfor perception of color and fine details to the primary visual cortex.

koniocellular sublayer (koh nee oh sellyew lur) One of thesublayers of neurons in the dorsal lateral geniculate nucleus foundventral to each of the magnocellular and parvocellular layers;transmits information from short-wavelength (blue) cones to theprimary visual cortex.

calcarine fissure (kalka rine) A horizontal fissure on the innersurface of the posterior cerebral cortex; the location of the primaryvisual cortex.











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and the left hemisphere receives information from theright. It is not correct to say that each hemispherereceives visual information solely from the contralateraleye. (See Figure 6.11.)

Besides the primary retino-geniculo-cortical pathway, fibers from the retina take several other pathwaysFor example, one pathway to the hypothalamus synchronizes an animals activity cycles to the 24-hourrhythms of day and night. (We will study this system inChapter 9.) Other pathways, especially those that traveto the optic tectum and the pretectal nuclei, coordinate

eye movements, control the muscles of the iris (and thusthe size of the pupil) and the ciliary muscles (which control the lens), and help to direct our attention to suddenmovements in the periphery of our visual field.

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6

12

Lateralgeniculatenucleus

FIGURE 6.9 Lateral Geniculate Nucleus .

The photomicrograph shows a section through the right lateral geniculate nucleus of a

rhesus monkey (cresyl violet stain) Layers 1, 4, and 6 receive input from the contralateral

(left) eye, and layers 2, 3, and 5 receive input from the ipsilateral (right) eye. Layers 1 and 2

are the magnocellular layers; layers 36 are the parvocellular layers. The koniocellular

sublayers are found ventral to each of the parvocellular and magnocellular layers. The

receptive fields of all six principal layers are in almost perfect registration; cells located

along the line of the unlabeled arrow have receptive fields centered on the same point.

(Photomicrograph from Hubel, D. H., Wiesel, T. N., and Le Vay, S. Philosophical Transactions of the Royal Societyof London, B, 1977,278, 131163. Reprinted with permission.)

(calcarine means spur-shaped), a horizontal fissurelocated in the medial and posterior occipital lobe. Theprimary visual cortex is often called the striate cortexbecause it contains a dark-staining layer (striation) ofcells. (See Figure 6.10.)

Figure 6.11 shows a diagrammatical view of a hori-zontal section of the human brain. The optic nerves jointogether at the base of the brain to form the X-shapedoptic chiasm (khiasmais the Greek for cross). There,axons from ganglion cells serving the inner halves of theretina (the nasal sides) cross through the chiasm and

ascend to the LGN on the opposite side of the brain.The axons from the outer halves of the retina (the tem-poral sides) remain on the same side of the brain. (SeeFigure 6.11.) The lens inverts the image of the world pro- jected on the retina (and similarly reverses left andright). Therefore, because the axons from the nasalhalves of the retinas cross to the other side of the brain,each hemisphere receives information from the con-tralateral half (opposite side) of the visual scene. That is,if a person looks straight ahead, the right hemispherereceives information from the left half of the visual field,

striate cortex(stryate) The primary visual cortex.

optic chiasm A cross-shaped connection between the optic nerves,located below the base of the brain, just anterior to the pituitarygland.

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2 mm

FIGURE 6.10 Striate Cortex.

This photomicrograph is a cross section through the striate

cortex of a rhesus macaque monkey. The ends of the striate

cortex are shown by arrows.

(From Hubel, D. H., and Wiesel, T. N. Proceedings of the Royal Society ofLondon, B, 1977, 198, 159. Reprinted with permission.)

Visual fieldof right eye

Visual fieldof left eye

Region of overlapof two visual fields

Optic nerve

Lateralgeniculate

nucleus

Information fromright half ofvisual field (yellow)

Primaryvisualcortex

Information fromleft half ofvisual field (green)

Optic chiasm

FIGURE 6.11 The Primary .Visual Pathway .

InterimSummaryThe Stimulus and Anatomy of the

Visual System

Light consists of electromagnetic radiation, similar to radio

waves but of a different frequency and wavelength. Color can

vary in three perceptual dimensions: hue, brightness, and

saturation, which correspond to the physical dimensions of

wavelength, intensity, and purity.

The photoreceptors in the retinathe rods and the

conesdetect light. Muscles move the eyes so that images of

particular parts of the environment fall on the retina.

Accommodation is accomplished by the ciliary muscles, which

change the shape of the lens. Photoreceptors communicate

through synapses with bipolar cells, which communicate

through synapses with ganglion cells. In addition, horizontal

cells and amacrine cells combine messages from adjacent pho-

toreceptors.

When light strikes a molecule of photopigment in a

photoreceptor, the retinal molecule detaches from the

opsin molecule. This detachment hyperpolarizes themembrane of the photoreceptor. As a result, the rate of

firing of the ganglion cell changes, signaling the detection

of light.

Visual information from the retina reaches the striate

cortex surrounding the calcarine fissure after being relayed

through the magnocellular, parvocellular, and koniocellular

layers of the LGN. Several other regions of the brain, includ-

ing the hypothalamus and the tectum, also receive visual

information. These regions help to regulate activity during

the daynight cycle, coordinate eye and head movements,

control attention to visual stimuli, and regulate the size of

the pupils.

Thought Question

People who try to see faint, distant lights at night are often

advised to look just to the side of the location where they

expect to see the lights. Can you explain the reason for this

advice?











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Coding of Visual Information in the Retina 179

CODING OF VISUALINFORMATION IN THE RETINA

This section describes the way in which cells of the retinaencode information they receive from the photoreceptors.

Coding of Light and DarkOne of the most important methods for studying thephysiology of the visual system is the use of microelec-trodes to record the electrical activity of single neurons.As we saw in the previous section, some ganglion cellsbecome excited when light falls on the photoreceptorsthat communicate with them. The receptive field of aneuron in the visual system is the part of the visual fieldthat an individual neuron seesthat is, the place in which a visual stimulus must be located to produce aresponse in that neuron. Obviously, the location of the

receptive field of a particular neuron depends on thelocation of the photoreceptors that provide it with visualinformation. If a neuron receives information from pho-toreceptors located in the fovea, its receptive field willbe at the fixation pointthe point at which the eye islooking. If the neuron receives information from pho-toreceptors located in the periphery of the retina, itsreceptive field will be located off to one side.

At the periphery of the retina many individualreceptors converge on a single ganglion cell, bringinginformation from a relatively large area of the retinaand hence a relatively large area of the visual field.However, the fovea contains approximately equal num-bers of ganglion cells and cones. These receptor-to-axonrelationships explain the fact that our foveal (central)vision is very acute but our peripheral vision is much lessprecise. (See Figure 6.12.)

Over seventy years ago, Hartline (1938) discoveredthat the frog retina contained three types of ganglioncells. ON cells responded with an excitatory burst whenthe retina was illuminated, OFF cells responded whenthe light was turned off, and ON/OFF cells respondedbriefly when the light went on and again when it wentoff. Kuffler (1952, 1953), recording from ganglion cellsin the retina of the cat, discovered that their receptive

field consists of a roughly circular center, surrounded bya ring. Stimulation of the center or surrounding fieldshad contrary effects: ON cells were excited by lightfalling in the central field (center) and were inhibited bylight falling in the surrounding field (surround), where-as OFF cells responded in the opposite manner.ON/OFF ganglion cells were briefly excited when light was turned on or off. In primates most of theseON/OFF cells project primarily to the superior collicu-lus, which is primarily involved in visual reflexes(Schiller and Malpeli, 1977); thus, these cells do not

appear to play a direct role in form perception. (SeeFigure 6.13.)

Figure 6.13 also illustrates a rebound effect thaoccurs when the light is turned off again. Neurons

whose firing is inhibited while the light is on will show abrief burst of excitation when it is turned off. In contrast, neurons whose firing is increased will show a briefperiod of inhibition when the light is turned off. (SeeFigure 6.13.)

The two major categories of ganglion cells (ON andOFF) and the organization of their receptive fields intocontrasting center and surround provide useful information to the rest of the visual system. Let us considerthese two types of ganglion cells first. As Schiller (1992)notes, ganglion cells normally fire at a relatively low rateThen, when the level of illumination in the center oftheir receptive field increases or decreases (for example, when an object moves or the eye makes a saccade)they signal the change. In particular, ON cells signalincreases and OFF cells signal decreases, but both signalthem by an increased rate of firing. Such a system is par-ticularly efficient. Theoretically, a single type of ganglion

FIGURE 6.12 Central Versus.Peripheral Acuity .

Ganglion cells in the fovea receive input from a smaller

number of photoreceptors than those in the periphery and

hence provide more acute visual information.

Receptive field in centerof retina (fovea)

Photoreceptors Bipolarcells

Ganglioncells

Receptive field inperiphery of retina

receptive field That portion of the visual field in which thepresentation of visual stimuli will produce an alteration in thefiring rate of a particular neuron.

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cell could fire at an intermediate rate and signalchanges in the level of illumination by increases ordecreases in rate of firing. However, in this case the aver-age rate of firing of the one million axons in each opticnerve would have to be much higher.

Several studies have shown that ON cells and OFFcells do, indeed, signal different kinds of information.Schiller, Sandell, and Maunsell (1986) injected monkeys with APB (2-amino-4-phosphonobutyrate), a drug thatselectively blocks synaptic transmission in ON bipolarcells. They found that the animals had difficulty detect-ing spots that were made brighter than the backgroundbut had no difficulty detecting spots that were slightlydarker than the background. In addition, Dolan andSchiller (1989) found that an injection of APB com-

pletely blocked vision in very dim light, which is normallymediated by rods. Thus, rod bipolar cells must all be ofthe ON type. (If you think about it, that arrangementmakes sense; in very dim light we are more likely to seebrighter objects against a dark background than darkobjects against a light background.)

The second characteristic of the receptive fields ofganglion cellstheir center-surround organizationenhances our ability to detect the outlines of objectseven when the contrast between the object and the back-ground is low. Figure 6.14 illustrates this phenomenon.

This figure shows six gray squares arranged in order ofbrightness. The right side of each square looks lighterthan the left side, which makes the borders betweenthe squares stand out. But these exaggerated bordersdo not exist in the illustration; they are added by ourvisual system because of the center-surround organiza-tion of the receptive fields of the retinal ganglion cells.(See Figure 6.14.)

Figure 6.15 explains how this phenomenon works.We see the centers and surrounds of the receptive fieldsof several ganglion cells. (In reality these receptive fieldswould be overlapping, but the simplified arrangement iseasier to understand. This example also includes onlyON cellsagain, for the sake of simplicity.) The image

0 0

ON Cell

ON area

ON areaOFF area

OFF area

OFF Cell

Surround

Center

1. Spot of light incenter

2. Spot of light insurround

Light Light

Receptivefield

Time Time

0.5 0.51.0 1.0(s) (s)

Actionpotentials

(a) (b)

FIGURE 6.13 ON and OFF Ganglion Cells .

The figure shows responses of ON and OFF ganglion cells to stimuli presented in the

center or the surround of the receptive field.

(Adapted from Kuffler, S. W. Cold Spring Harbor Symposium for Quantitative Biology, 1952, 17, 281292.)

FIGURE 6.14 Enhancement of Contrast .

Although each gray square is of uniform darkness, the right

edge of each square looks somewhat lighter, and the left

edge looks somewhat darker. This effect appears to be

caused by the opponent center-surround arrangement of

the receptive fields of the retinal ganglion cells.











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Coding of Visual Information in the Retina 181

of the transition between lighter and darker regions fallsacross some of these receptive fields. The cells whosecenters are located in the brighter region but whose sur-rounds are located at least partially in the darker regionwill have the highest rate of firing. (See Figure 6.15.)

Coding of ColorSo far, we have been examining the monochromaticproperties of ganglion cellsthat is, their responses tolight and dark. But, of course, objects in our environ-ment selectively absorb some wavelengths of light andreflect others, which, to our eyes, gives them differentcolors. The retinas of humans, Old World monkeys, onespecies of New World monkey, and apes contain threedifferent types of cones, which provides them (and us)

with the most elaborate form of color vision ( Jacobs,1996; Hunt et al., 1998). Although monochromatic(black-and-white) vision is perfectly adequate for mostpurposes, color vision gave our primate ancestors theability to distinguish ripe fruit from unripe fruit andmade it more difficult for other animals to hide them-selves by means of camouflage (Mollon, 1989). In fact,the photopigments of primates with three types ofcones seem well suited for distinguishing red and yel-low fruits against a background of green foliage (Reganet al., 2001).

Color MixingVarious theories of color vision have been proposed formany yearslong before it was possible to disprove orvalidate them by physiological means. In 1802, ThomaYoung, a British physicist and physician, proposed thathe eye detected different colors because it contained

three types of receptors, each sensitive to a single hueHis theory was referred to as the trichromatic (threecolor) theory. It was suggested by the fact that for ahuman observer any color can be reproduced by mixing various quantities of three colors judiciously selectefrom different points along the spectrum.

I must emphasize thatcolor mixingis different frompigment mixing. If we combine yellow and blue pigments(as when we mix paints), the resulting mixture is greenColor mixing refers to the addition of two or more lighsources. If we shine a beam of red light and a beam ofbluish green light together on a white screen, we will seeyellow light. If we mix yellow and blue light, we get whitelight. When white appears on a color television screenor computer monitor, it actually consists of tiny dots ofred, blue, and green light. (See Figure 6.16.)

Another fact of color perception suggested to German physiologist, Ewald Hering (1905/1965), thathue might be represented in the visual system as opponencolors; red versus green and yellow versus blue. Peopleinterested in color perception have long regarded yellowblue, red, and green as primary colorscolors that seemunique and do not appear to be blends of other colors(Black and white are primary, too, but we perceive themas colorless.) All othe

Physiology of Behavior 10e Ch06 (1)

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