FOUNDATIONS OF BEHAVIORAL NEUROSCIENCE Copyright © 2014 Pearson Education, Inc. All Rights Reserved

FOUNDATIONS OF BEHAVIORAL NEUROSCIENCE

Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Chapter 6Vision


VisionLearning Objectives

1.Describe the characteristics of light and color, outline the anatomy of the eye and its connections with the brain, and describe the process of transduction of visual information.

2.Describe the coding of visual information by photoreceptors and ganglion cells in the retina.

3.Describe the striate cortex and discuss how its neurons respond to orientation, movement, spatial frequency, retinal disparity, and color.

4.Describe the anatomy of the visual association cortex and discuss the location and functions of the two streams of visual analysis that take place there.

5.Discuss the perception of color and the analysis of form by neurons in the ventral stream.

6.Describe the role of the visual association cortex in the perception of objects, faces, body parts, and places.

7.Describe the role of the visual association cortex in the perception of movement.

8.Describe the role of the visual association cortex in the perception of spatial location.


The StimulusAnatomy of the Visual System

The Eyes

Photoreceptors

Connections Between Eye and Brain

Coding of Visual Information in the Retina

Coding of Light and Dark

Coding of Color

Analysis of Visual Information: Role of the Striate Cortex

Anatomy of the Striate Cortex

Orientation and Movement

Spatial Frequency

Retinal Disparity

Color

Modular Organization of the Striate Cortex


The StimulusAnalysis of Visual Information: Role of the Visual Association Cortex

Two Streams of Visual Analysis

Perception of Color

Perception of Form

Perception of Movement

Perception of Spatial Location


Prologuesensory receptor

A specialized neuron that detects a particular category of physical events.

sensory transductionThe process by which sensory stimuli are

transduced into slow, graded receptor potentials.

receptor potentialA slow, graded electrical potential produced by a receptor cell in response to a physical stimulus.


The Stimulussensory receptor

A specialized neuron that detects a particular category of physical events.

sensory transductionThe process by which sensory stimuli are

transduced into slow, graded receptor potentials.

receptor potentialA slow, graded electrical potential produced by a receptor cell in response to a physical stimulus.


The Stimulus


Our eyes detect the presence of light.

For humans light is a narrow band of the spectrum of electromagnetic radiation.

Electromagnetic radiation with a wavelength of between 380 and 760 nm (a nanometer, nm, is one-billionth of a meter) is visible to us. (See Figure 6.1.)

Other animals can detect different ranges of electromagnetic radiation.


The Stimulushue

One of the perceptual dimensions of color; the dominant wavelength.

brightnessOne of the perceptual dimensions of color; intensity.

saturationOne of the perceptual dimensions of color; purity.



Anatomy of the Visual System


The Eyes

The eyes are suspended in the orbits, bony pockets in the front of the skull.

They are held in place and moved by six extraocular muscles attached to the tough, white outer coat of the eye called the sclera.

Normally, we cannot look behind our eyeballs and see these muscles, because their attachments to the eyes are hidden by the conjunctiva.



The Eyes

These mucous membranes line the eyelid and fold back to attach to the eye (thus preventing a contact lens that has slipped off the cornea from “falling behind the eye”).

Figure 6.3 illustrates the anatomy of the eye. (See Figure 6.3.)



The Eyes

saccadic movement (suh kad ik)The rapid, jerky movement of the eyes used in

scanning a visual scene.

pursuit movementThe movement that the eyes make to maintain an image of a moving object on the fovea.




The Eyes

retinaThe neural tissue and photoreceptive cells located

on the inner surface of the posterior portion of the eye.

rodOne of the receptor cells of the retina; sensitive to

light of low intensity.

coneOne of the receptor cells of the retina; maximally

sensitive to one of three different wavelengths of light and hence encodes color vision.

Anatomy of the Visual SystemThe Eyes

photoreceptorOne of the receptor cells of the retina; transduces photic energy into electrical potentials.

fovea (foe vee a) The region of the retina that mediates the most

acute vision of birds and higher mammals. Color-sensitive cones constitute the only type of photoreceptor found in the fovea.




optic diskThe location of the exit point from the retina of the

fibers of the ganglion cells that form the optic nerve; responsible for the blind spot.




bipolar cellA bipolar neuron located in the middle layer of the retina, conveying information from the

photoreceptors to the ganglion cells.

ganglion cellA neuron located in the retina that receives visual information from bipolar cells; its axons give rise to

the optic nerve.

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horizontal cellA neuron in the retina that interconnects adjacent photoreceptors and the outer processes of the

bipolar cells.

amacrine cell (amm a krin)A neuron in the retina that interconnects adjacent

ganglion cells and the inner processes of the bipolar cells.



Anatomy of the Visual SystemPhotoreceptors

lamellaA layer of membrane containing photopigments;

found in rods and cones of the retina.

photopigmentA protein dye bonded to retinal, a substance

derived from vitamin A; responsible for transduction of visual information.


Anatomy of the Visual SystemPhotoreceptors

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

the photopigments.

retinal (rett i nahl) A chemical synthesized from vitamin A; joins with an

opsin to form a photopigment.

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


Anatomy of the Visual SystemConnections Between Eye and Brain

dorsal lateral geniculate nucleus (LGN)A group of cell bodies within the lateral geniculate body

of the thalamus; receives inputs from the retina and projects to the primary visual cortex.

magnocellular layerOne of the inner two layers of neurons in the dorsal

lateral geniculate nucleus; transmits information necessary for the perception of form, movement, depth, and small differences in brightness to the primary visual cortex.



calcarine fissure (kal ka rine)A horizontal fissure on the inner surface of the

posterior cerebral cortex; the location of the primary visual cortex.

striate cortex (stry ate)The primary visual cortex.




optic chiasmA cross-shaped connection between the optic

nerves, located below the base of the brain, just anterior to the pituitary gland.



Coding of Visual Information in the BrainCoding of Light and dark

receptive fieldThat portion of the visual field in which the

presentation of visual stimuli will produce an alteration in the firing rate of a particular neuron.




Kuffler (1952, 1953), recording from ganglion cells in the retina of the cat, discovered that their receptive field consists of a roughly circular center, surrounded by a ring.

Stimulation of the center or surrounding fields had contrary effects: ON cells were excited by light falling in the central field (center) and were inhibited by light falling in the surrounding field (surround), whereas OFF cells responded in the opposite manner.



ON/OFF ganglion cells were briefly excited when light was turned on or off.

In primates these ON/OFF cells project to the superior colliculus, which is primarily involved in visual reflexes in response to moving or suddenly-appearing stimuli (Schiller and Malpeli, 1977), which suggests that they do not play a direct role in form perception. (See Figure 6.9.)



Coding of Visual Information in the BrainCoding of Color

So far, we have been examining the monochromatic properties of ganglion cells—that is, their responses to light and dark.

But, of course, objects in our environment selectively absorb some wavelengths of light and reflect others, which, to our eyes, gives them different colors.

The retinas of humans and many species of nonhuman primates contain three different types of cones, which provides them (and us) with the most elaborate form of color vision (Jacobs, 1996; Hunt et al., 1998).


Reserved

Coding of Visual Information in the BrainPhotoreceptors: Trichromatic Coding

Various theories of color vision have been proposed for many years—long before it was possible to disprove or validate them by physiological means.

In 1802, Thomas Young, a British physicist and physician, proposed that the eye detected different colors because it contained three types of receptors, each sensitive to a single hue.



His theory was referred to as the trichromatic (three-color) theory.

It was suggested by the fact that for a human observer any color can be reproduced by mixing various quantities of three colors judiciously selected from different points along the spectrum.



Three different types of photoreceptors (three different types of cones) are responsible for color vision.

Investigators have studied the absorption characteristics of individual photoreceptors, determining the amount of light of different wavelengths that is absorbed by the photopigments.



These characteristics are controlled by the particular opsin a photoreceptor contains; different opsins absorb particular wavelengths more readily.

The peak sensitivities of the three types of cones are approximately 420 nm (blue-violet), 530 nm (green), and 560 nm (yellow-green).

The peak sensitivity of the short-wavelength cone is actually 440 nm in the intact eye because the lens absorbs some short-wavelength light. For convenience the short-, medium-, and long-wavelength cones are traditionally called “blue,” “green,” and “red” cones, respectively.




protanopia (pro tan owe pee a)An inherited form of defective color vision in which red and green

hues are confused; “red” cones are filled with “green” cone opsin.

deuteranopia (dew ter an owe pee a)An inherited form of defective color vision in which red and green

hues are confused; “green” cones are filled with “red” cone opsin.



tritanopia (try tan owe pee a)An inherited form of defective color vision in which

hues with short wavelengths are confused; “blue” cones are either lacking or faulty.


Reserved

Coding of Visual Information in the BrainRetinal Ganglion Cells: Opponent-Process Coding

At the level of the retinal ganglion cell the three-color code gets translated into an opponent-color system.

Daw (1968) and Gouras (1968) found that these neurons respond specifically to pairs of primary colors: red versus green and yellow versus blue.



Coding of Visual Information in the BrainRetinal Ganglion Cells: Opponent-Process Coding

Thus, the retina contains two kinds of color-sensitive ganglion cells: red-green cells and yellow-blue cells.

Some color-sensitive ganglion cells respond in a center-surround fashion.

For example, a cell might be excited by red and inhibited by green in the center of their receptive field while showing the opposite response in the surrounding ring. (See Figure 6.11.)


Reserved

Analysis of Visual Information: Role of the Striate CortexAnatomy of the striate Cortex

The striate cortex consists of six principal layers (and several sublayers), arranged in bands parallel to the surface.

These layers contain the nuclei of cell bodies and dendritic trees that show up as bands of light or dark in sections of tissue that have been dyed with a cell-body stain. (See Figure 6.12.)



Analysis of Visual Information: Role of the Striate CortexOrientation of Movement

Most neurons in the striate cortex are sensitive to orientation.

That is, if a line or an edge (the border of a light and a dark region) is positioned in the cell’s receptive field and rotated around its center, the cell will respond best when the line is in a particular position—a particular orientation.

Some neurons respond best to a vertical line, some to a horizontal line, and some to a line oriented somewhere in between.




simple cellAn orientation-sensitive neuron in the striate cortex whose receptive field is organized in an opponent fashion.

complex cellA neuron in the visual cortex that responds to the presence of a line segment with a particular

orientation located within its receptive field, especially when the line moves perpendicularly to its orientation.



hypercomplex cellA neuron in the visual cortex that responds to the presence of a line segment with a particular

orientation that ends at a particular point within the cell’s receptive field.



Analysis of Visual Information: Role of the Striate CortexSpatial Frequency

sine-wave gratingA series of straight parallel bands varying

continuously in brightness according to a sine-wave function, along a line perpendicular to their lengths.

spatial frequencyThe relative width of the bands in a sine-wave

grating, measured in cycles per degree of visual angle.




Analysis of Visual Information: Role of the Striate CortexColor

cytochrome oxidase (CO) blobThe central region of a module of the primary visual cortex, revealed by a stain for cytochrome oxidase; contains wavelength-sensitive neurons; part of the parvocellular system.



Analysis of Visual Information: Role of the Striate CortexModular Organization of the Striate Cortex

Most investigators believe that the brain is organized in modules, which probably range in size from a hundred thousand to a few million neurons.

Each module receives information from other modules, performs some calculations, and then passes the results to other modules.

In recent years investigators have been learning the characteristics of the modules that are found in the visual cortex.




Analysis of Visual Information: Role of the Visual Association Cortex

Two Streams of Analysis

extrastriate cortexA region of visual association cortex; receives fibers

from the striate cortex and from the superior colliculi and projects to the inferior temporal cortex.






dorsal streamA system of interconnected regions of visual cortex involved in the perception of spatial location,

beginning with the striate cortex and ending with the posterior parietal cortex.

ventral streamA system of interconnected regions of visual cortex involved in the perception of form, beginning with

the striate cortex and ending with the inferior temporal cortex.




inferior temporal cortexThe highest level of the ventral stream of the visual association cortex; involved in perception of

objects, including people’s bodies and faces.

posterior parietal cortexThe highest level of the dorsal stream of the visual association cortex; involved in perception of

movement and spatial location.





Perception of Color

As we saw earlier, neurons within the CO blobs in the striate cortex respond to colors.

Like the ganglion cells in the retina (and the parvocellular and koniocellular neurons in the LGN), these neurons respond in opponent fashion.

This information is analyzed by the regions of the visual association cortex that constitute the ventral stream.



Studies with Laboratory Animals

color constancyThe relatively constant appearance of the colors of objects viewed under varying lighting conditions.



Studies with Humans

cerebral achromatopsia (ay krohm a top see a)Inability to discriminate among different hues;

caused by damage to area V8 of the visual association cortex.




Perception of Form

The analysis of visual information that leads to the perception of form begins with neurons in the striate cortex that are sensitive to orientation and spatial frequency.

These neurons send information to area V2 that is then relayed to the subregions of the visual association cortex that constitute the ventral stream.




In primates the recognition of visual patterns and identification of particular objects take place in the inferior temporal cortex, located on the ventral part of the temporal lobe.

This region of the visual association cortex is located at the end of the ventral stream.




It is here that analyses of form and color are put together and perceptions of three-dimensional objects and backgrounds are achieved.

Damage to this region causes severe deficits in visual discrimination (Mishkin, 1966; Gross, 1973; Dean, 1976).



Studies with Humans

visual agnosia (ag no zha)Deficits in visual form perception in the absence of

blindness; caused by brain damage.

prosopagnosia (prah soh pag no zha)Failure to recognize particular people by the sight of

their faces.

fusiform face area (FFA)A region of the visual association cortex located in the inferior temporal; involved in perception of faces.




Studies with Humans

extrastriate body area (EBA)A region of the visual association cortex located in

the lateral occipitotemporal cortex; involved in perception of the human body and body parts other than faces.




Studies with Humans

parahippocampal place area (PPA)A region of the medial temporal cortex; involved in perception of particular places (“scenes”).





We need to know not only what things are, but also where they are and if they are moving, where they are going.

Without the ability to perceive the direction and velocity of movement of objects, we would have no way to predict where they will be.

We would be unable to catch them (or avoid letting them catch us).




Studies with Humans

Functional imaging studies suggest that a motion-sensitive area (usually called MT/MST) is found within the inferior temporal sulcus of the human brain (Dukelow et al., 2001).

However, a more recent study suggests that this region is located in the lateral occipital cortex, between the lateral and inferior occipital sulci (Annese, Gazzaniga, and Toga, 2004).





Studies with Humans

akinetopsiaInability to perceive movement, caused by damage

to area V5 (also called MST) of the visual association cortex.




Studies with Humans

intraparietal sulcus (IPS)The end of the dorsal stream of the visual

association cortex; involved in perception of location, visual attention, and control of eye and hand movements.





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FOUNDATIONS OF BEHAVIORAL NEUROSCIENCE Copyright © 2014 Pearson Education, Inc. All Rights Reserved