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LIGHT AND THE VISUAL APPARATUS

COLOR VISION

FORM VISION

THE PERCEPTION OF OBJECTS, COLOR, AND MOVEMENT

Vision and Visual PerceptionChapter 10

Light and the Visual Apparatus

Visible light—the adequate stimulus for vision—is a part of the electromagnetic spectrum. The electromagnetic spectrum includes a variety of

energy forms.

These range from gamma rays at one extreme of frequency to the radiations of alternating current circuits at the other.

The visible part of the spectrum accounts for only 1/70 of the range.

Light is described by its wavelength, the distance the oscillating energy travels before it reverses direction.

Visible light ranges from about 300 nm to 800 nm.

The Electromagnetic SpectrumFigure 10.1


The eye is a spherically shaped structure filled with a clear liquid. The outer covering, or sclera, is opaque except for the

cornea, which is transparent.

Behind the cornea is the lens.

Muscles stretch the lens flatter to focus the image of a distant object on the retina, or relax to focus the image of a near object.

The lens is partly covered by the iris, a circular muscle.

It controls the amount of light entering the eye by contracting reflexively in bright light and relaxing in dim light.

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The Human EyeFigure 10.2


The retina, at the back of the eye, contains two main types of light-sensitive photoreceptors.

These receptors contain photopigments, which break down in the presence of light.

Rods contain rhodopsin.

Rhodopsin is extremely sensitive to light.

Rods function well in dim light and poorly in bright light.

Rods distinguish only different levels of light.

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Cones contain iodopsin.

Iodopsin requires bright light to function.

Cones perform well in daylight and are nonfunctional in dim light.

Three types of iodopsin allow the cones to distinguish among different wavelengths.

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The receptors connect to bipolar cells, which in turn connect to ganglion cells.

Photoreceptors are most active in darkness.

Sodium and calcium channels are open.

The receptor is depolarized.

The receptor releases glutamate, which inhibits the bipolar cells.

When light strikes the photopigment

sodium and calcium channels close, reducing glutamate release;

bipolar cells release more transmitter, increasing the firing rate in the ganglion cells.

The Cells of the RetinaFigure 10.3


The area of the retina from which a ganglion cell (or any other cell in the visual system) receives its input is the cell’s receptive field.

Receptive fields of cones are small.

Few cones are attached to each ganglion cell; in the center of the fovea each cone has its own ganglion cell.

As a result, visual acuity—the ability to distinguish details—is great.

Cones are more numerous in the center, or fovea, and decrease toward the periphery.

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Receptive fields of rods are larger.

Many rods share each ganglion cell.

This enhances their already greater sensitivity to light.

But it reduces their acuity.

Rods are more numerous in the periphery and absent in the fovea.

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The axons of the ganglion cells join together and pass out of each eye to form the two optic nerves.

Where the nerve exits the eye there are no receptors, so it is referred to as the blind spot.

The two optic nerves run to a point just in front of the pituitary, where they join for a short distance at the optic chiasm.

Separating again, they travel to their first synapse in the lateral geniculate nuclei of the thalamus.

Information goes from the thalamus to each visual cortex.


At the optic chiasm axons from the nasal sides of the eyes cross to the other side and go to the occipital lobe in the opposite hemisphere.

Neurons from the outside half of the eyes (the temporal side) do not cross over, but go to the same side of the brain.

The visual field is the part of the environment that is being registered on the retina.

Information from the left visual field is detected by the right half of each retina, and transmitted to the right hemisphere.

An image in the right visual field will similarly be projected to the left hemisphere.

Projections from Retina to CortexFigure 10.4


The separation between your eyes produces retinal disparity.

More distant objects cast their image toward the nasal side of the retina.

Closer objects cast their image toward the temporal part of the retina.

Figure 10.5


In the visual cortex, different neurons fire depending on the image’s displacement on the two retinas. This provides information about the object’s distance.

The anterior parietal cortex combines this depth information with information about the object’s shape and location to provide three-dimensional location of objects.

Demonstrations of retinal disparity:

stereograms, such as Magic Eye

3-D movies such as Avatar (requiring glasses).

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Blindness often occurs due to deterioration of visual receptors, but the neural structures remain intact.

Hence, sight could be restored by replacing the receptors with an artificial retina.

The Argus Retinal Prosthesis System uses images from a small video camera mounted on a pair of glasses.

The signal is then transmitted wirelessly to a small chip implanted in the retina. The chip stimulates neurons to produce a crude image.

Another device delivers patterned stimulation to the tongue. Users learn to recognize doorways and elevator buttons and can pick up objects.


Repair rather than artificial replacement may be the best strategy.

Transplantation of fetal retinal tissue has resulted in quadrupled visual acuity.

Optogenetics—genetic replacement of photoreceptors with plant rhodopsins—restored some visual function in mice.

Replacement of the RPE65 gene enables recipients to recognize faces and read large-print books.

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Color Vision

According to the Young-Helmholtz trichromatictheory, three color processes account for all the colors we are able to distinguish.

The primary colors in this theory are red, green, and blue.

The theory was based on the fact that observers cannot resolve these colors into separate components.

TV and computer screen are an application of trichromatic color mixing.

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Color Vision

Opponent process theory attempts to explain color vision in terms of opposing neural processes.

The photopigment in the red/green receptor is broken down by red light and regenerates in the presence of green light.

The chemical in the yellow/blue receptor is broken down in the presence of yellow light and regenerates in the presence of blue light.

This theory explained the uniqueness of yellow, as well as complementary colors, colors that cancel each other out to produce a neutral gray or white.

The theory did not have a solid physiological basis, but the opponent concept would become important.

The Color CircleFigure 10.7

Colors across from each other cancel out to a neutral

gray; this is an example of opponent process.

Negative Color AftereffectFigure 10.8

Negative color aftereffects also

demonstrate opponent processes.

Color Vision

Leo Hurvich and Dorothea Jameson combined trichromatic theory and the opponent process theory:

There are three kinds of color receptors

red-sensitive,

green-sensitive,

and blue-sensitive

which combine in opponent-process fashion at the ganglion cells to produce four color processes.

Hurvich and Jameson proposed the following arrangements:

Combine Color Processing TheoryFigure 10.10

Equal stimulation of “red” and “green” cones activates the yellow-blue ganglion cell and produces a sensation of yellow.

Medium-wavelength light excites the “green” cones and inhibits the red-green cell,signaling green to the brain.

Short-wavelength light excites “blue” cones andinhibits the yellow-blue ganglion cell, leading to a sensation of blue.

Long-wavelength light excites “red” cones and the red-green ganglion cell,giving a sensation of red.

Color Vision

Evidence for the Hurvich-Jameson Theory

Studies of light absorption in the retina demonstrated three peaks, conforming to red, blue, and green sensitivity.

(Note that sensitivities overlap, so the system must compare activity in all receptors to determine the color of a stimulus.)

Color-opponent ganglion cells were discovered in monkeys.

Receptive Fields of Color-Opponent CellsFigure 10.12

Some of these had retinal fields made up of color-complementary concentric circles of cones.

Color-opponent circular fields sharpen wavelength discrimination and enhance color contrast.

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Color Vision

The study of color blindness has helped researchers understand the processes of color vision.

People who lack cones are completely color blind.

Their vision is limited to that provided by rods,

they have poor visual acuity,

and they are very light sensitive.

This is a rare, inherited condition.

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Color Vision

Typically, a person is only partially color blind.

People with red-green color blindness see both red and green light, but are unable to distinguish between them.

Others cannot perceive blue, so their world appears in variations of green and red.

Both types of people have normal visual acuity.

This indicates there is no lack of cones.

Probably one photopigment is missing and replaced by the other.

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A Test for Color BlindnessFigure 10.13

Form Vision

Form vision is the detection of an object’s boundaries and features (such as texture).

The visual cortex contains a retinotopic map. This means that adjacent retinal receptors activate adjacent cells in the visual cortex.

However, this does not explain form vision.

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Form Vision

Edge detection involves lateral inhibition.

More specifically:

Each neuron’s activity inhibits the activity of its neighbors, and in turn its activity is inhibited by them.

This sharpens the contrast between darker and lighter boundaries.

• Contrast Enhancement and Edge Detection

Hubel and Wiesel’s TheoryFigure 10.20

Simple cells are located in the visual cortex.

They receive input from ganglion cells whose receptive fields are arranged in a line on the retina.

They respond to a line or an edge that is at a specific orientation and at a specific place on the retina.

• Hubel and Wiesel’s Theory

Hubel & Wiesel’s Complex CellsFigure 10.21

Complex cells receive input from simple cells that have receptive fields oriented in the same way, in adjacent areas of the cortex.

They continue to respond when a line or edge moves to a different location.

This theory can account for the detection of boundaries, but it is questionable whether edge detection cells can handle surface details.

Form Vision

Spatial Frequency Theory

According to this theory:

Visual cortical cells do a Fourier frequency analysis of the luminosity variations in a scene.

Different visual cortical cells have a variety of sensitivities, not just those required to detect edges.

As a result, visual cortical cells can detect not just edges but gradations of change.

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High vs. Low Frequencies in VisionFigure 10.23

Images with only high frequency transitions are not very meaningful. Those with more gradual transitions (low frequencies) are more recognizable.

Support for the theory: Researchers have found cortical cells that respond to light-dark “gratings” containing a specific combination of frequencies.

Perception of Objects, Color, & Movement

Modular processing refers to the segregation of the various brain functions into separate locations.

Hierarchical processing means that lower levels of the nervous system analyze their information and pass the results on to the next higher level for further analysis.

Some neuroscientists argue that visual function is not modular but distributed, meaning that it occurs across a relatively wide area of the brain.

Another view is that, like language, vision is a mix of modular and distributed processing.


Visual information travels from the retina to the brain in the parvocellular and magnocellularsystems.

Parvocellular ganglion cells are located mostly in the fovea.

They have circular receptive fields that are small and color opponent

As a result, the system is specialized for

discrimination of fine detail

and discrimination of color.

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Magnocellular ganglion cells are located mostly in the periphery.

They have larger circular receptive fields

that are brightness opponent

and respond only briefly to stimulation.

As a result, the magnocellular system is specialized for brightness contrast and for movement.

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Like the auditory system, the visual system is divided into a ventral stream and a dorsal stream.

The ventral stream

is dominated by the parvocellular system;

passes from V1 through V2 to V4, which is mostly concerned with color perception.

projects then to the inferior temporal cortex, providing object recognition;

is the visual “what” processor.

People with damage in the ventral stream can see objects, reach for them, and walk around them, but they can’t identify the objects.

The Ventral StreamFigure 10.25


The dorsal stream

is dominated by the magnocellular system;

projects to V5/MT and MST, which are concerned with movement;

projects then to the posterior parietal area, which provides location of objects in space.

The dorsal stream is the visual “where” processor.

People with damage in the dorsal stream can identify objects, but they have trouble orienting their gaze toward objects, reaching accurately, and shaping their hands to grasp an object using visual cues.

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The Dorsal and Ventral StreamsFigure 10.25


Both systems proceed to the prefrontal cortex.

The prefrontal cortex manages information in memory while it is being used.

For example, it integrates information about the body and about objects while planning movements.

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Damage to the inferior temporal cortex causes object and face agnosias.

Object agnosia is the impaired ability to recognize objects.

Prosopagnosia is the inability to visually recognize familiar faces.

Emotional recognition may remain intact.

Congenital prosopagnosia may be fairly common (though milder).

The fusiform face area is especially important for face recognition.


The inferior temporal area contains cells specific for geometric figures, houses, faces, hands, etc.

These cells likely receive input from cells with narrower sensitivities.

These capabilities may be “hardwired,” but learning plays a role.

Examples: subject’s experience with “greebles” and activity of the visual word form area, which responds to written words as a whole.

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Color agnosia is the loss of the ability to perceive colors, due to brain damage.

Cells in V1 are wavelength coded, whereas cells in v4 are color coded.

V4 provides color constancy, the ability to recognize the so-called natural color of an object in spite of the illuminating wavelength.

If it were not for color constancy, objects would seem to change colors as the sun shifts its position through the day or as we go indoors into artificial light.

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Movement agnosia is the inability to perceive movement.

Information about movement is integrated in area MT.

Patient LM had difficulty:

distinguishing between moving and stationary objects except in the periphery;

making visually guided eye and finger movements;

detecting movement of people if more than two people were present;

detecting radial movement (which indicates objects are moving closer or farther away).


Neglect and the Role of Attention

The posterior parietal cortex combines input from the visual, auditory, and somatosensory areas to help the individual locate objects in space and to orient the body in the environment.

Damage impairs abilities such as reaching for objects.

It also produces neglect, in which the patient ignores visual, touch, and auditory stimulation on the side opposite the injury.

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Neglect is not due to any defect in visual processing, but rather to a deficit in attention.

Symptoms of neglect can be seen in drawings, which lack detail on the left side.

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Neglect usually results from injury in the right hemisphere.

Figure 10.29


How the brain combines information from different areas into a whole is known as the binding problem.

Suggested areas where this might occur:

In a part of the superior temporal gyrus that receives input from both dorsal and ventral streams.

In the part of the parietal cortex where damage causes neglect.

In frontal areas where both streams converge.

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Some consider synesthesia to be an example of overbinding.

Stimulation in one sense triggers an experience in another or a concept evokes an unrelated sensory experience.

Activity increases in V4 during color synesthesia.

Grapheme/color synesthetes have more connections among areas involved in processing and integrating visual information.

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Vision and Visual Perception Chapter 10 - Weeblyacnusc.weebly.com/uploads/2/5/5/8/25588039/ch_10-vision.pdf · 2018-10-17 · Vision and Visual Perception Chapter 10. Light and the