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Vision

Module 13

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Vision

Vision The Stimulus Input: Light

Energy

The Eye

Visual Information Processing

Color Vision

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TransductionConversion of one form of

energy to another.How is this important when studying sensation?

Stimulus energies to neural impulses.

For example:

Light energy to vision.

Chemical energy to smell and taste.

Sound waves to sound.

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Vision

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VisibleSpectrum

The Stimulus Input: Light Energy

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What makes up a light wave?

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Light Characteristics

1. Wavelength (hue/color)2. Intensity (brightness)3. Saturation (purity)

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Wavelength (Hue)

Hue (color): dimension of

color determined by wavelength of

light.

Wavelength the distance from

the peak of one wave to the peak

of the next.

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Wavelength (Hue)

Different wavelengths of light resultin different colors.

400 nm 700 nmLong wavelengthsShort wavelengths

Violet Indigo Blue Green Yellow Orange Red

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Intensity (Brightness)

Intensity Amount of energy in a

wave determined

by amplitude; related to perceived

brightness.

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Intensity (Brightness)

Blue color with varying levels of intensity.As intensity increases or decreases, blue color

looks more “washed out” or “darkened.”

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Purity (Saturation)

Monochromatic light added to green and redmake them less saturated.

Saturated

Saturated

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Color Solid

Represents all three

characteristics of light stimulus on this model.

http://www.visionconnection.org

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The Eye

• Cornea• Pupil/Iris• Lens: accommodation • Retina/rods and cones (fovea)• Bipolar cells -> ganglion cells• Optic nerves• Where they cross is the optic chiasm• Thalamus• Occipital lobe

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16Fig. 5.4 The human eye, a simplified view. Table of ContentsTable of Contents ExitExit

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Fig. 5.6 The iris and diaphragm.

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Parts of the eye

1. Cornea: Transparent tissue where light enters the eye.

2. Iris: Muscle that expands and contracts to change the size of opening (pupil) for light.

3. Lens: Focuses the light rays on the retina. Accommodation

4. Retina: Contains sensory receptors that process visual information and send it to the brain.

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Light Control (cont.)

• Visual Acuity: Sharpness of visual perception• Fovea: Area of the retina containing only cones• Peripheral Vision: Vision at edges of visual field;

side vision – Many superstar athletes have excellent peripheral

vision

• Tunnel Vision: Loss of peripheral vision

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Light Control

• Cones: Visual receptors for colors and bright light (daylight) Fine detail vision

• Rods: Visual receptors for dim light; only produce black and white

• Blind Spot: Area of the retina lacking visual receptors

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The LensLens: Transparent structure behind

pupil that changes shape to focus

images on the retina.

Accommodation: The process by which the eye’s lens changes shape to help focus

near or far objects on the retina.

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The Lens

Nearsightedness: A condition in which nearby objects are seen more clearly

than distant objects.

Farsightedness: A condition in which faraway objects are seen more clearly than near objects.

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Nearsighted Vision

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Farsighted Vision

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Retina

Retina: The light-sensitive inner

surface of the eye, containing

receptor rods and cones plus layers of other neurons (bipolar, ganglion cells) that process visual information.

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Optic Nerve, Blind Spot & Fovea

http://www.bergen.org

Optic nerve: Carries neural impulses from the eye to the brain. Blind Spot: Point where optic nerve leaves the eye, because there are no receptor cells located here, it creates a blind spot. Fovea: Central point in the retina, around which the eye’s cones cluster.

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Photoreceptors

E.R. Lewis, Y.Y. Zeevi, F.S Werblin, 1969

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Bipolar & Ganglion Cells

Bipolar cells receive messages from photoreceptors and transmit them to ganglion cells, which form the optic nerve.

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Visual Information Processing

Optic nerves connect to the thalamus in the middle of the brain, and the thalamus to the

visual cortex.

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Ganglion & Thalamic Cells

Retinal ganglion cells and thalamic neurons break down visual stimuli into small components and have receptive

fields withcenter-surround organization.

Action Potentials

ON-center OFF-Surround

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Feature Detection

Nerve cells in the visual cortex respond to specific features, like edges, angle, and

movement.

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Shape Detection

Specific combinations of temporal lobe activity occur as people look at shoes,

faces, chairs and houses.

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Perception in Brain

Our perceptions are a combination of sensory (bottom-up) and cognitive (top-

down) processes.

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Visual Information Processing

Processing of several aspects of the stimulus simultaneously is called parallel processing. The

brain divides a visual scene into subdivisions such as color, depth, form and movement etc.

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Parallel Processing• The processing of several aspects of a

problem simultaneously.

Color Motion Form Depth

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How do we see in color?

What color is this dragon?

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Color

• The dragon is anything but red.

• The dragon rejects the long wavelengths of light that to us are red- so red is reflected of and we see it.

• Also, light has no real color.

• It is our mind that perceives the color.

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Color Vision

• Trichromatic Theory: Color vision theory that states we have three cone types: red, green, blue– Other colors produced by a combination of these– Black and white produced by rods

• Opponent Process Theory: Color vision theory based on three “systems”: red or green, blue or yellow, black or white– Exciting one color in a pair (red) blocks the excitation in

the other member of the pair (green)– Afterimage: Visual sensation that remains after

stimulus is removed (seeing flashbulb after the picture has been taken)

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Young-Helmholtz Trichromatic (three color) Theory

• Realized that any color can be created by combining the light waves of three primary color-

•So they guessed that we have 3 different types of receptor cells in our eyes. Together they can pick any combination of our 7 million color variations.

•Most colorblind people simply lack cone receptor cells for one or more of these primary colors.

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Subtraction of Colors

If three primary colors (pigments)

are mixed it results in subtraction of all wavelengths

and the result is a black color.

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Addition of Colors

If three primary colors (lights) are mixed the wavelengths are added and they result

in white color.

Fritz Goro, L

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PhotoreceptorsRed

ConesGreenCones

Longwave

Mediumwave

Shortwave

MacNichol, Wald and Brown (1967)

measured directly the absorption

spectra of visual pigments of single cones

obtained from the retinas of humans.

BlueCones

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Color Blindness

• Inability to perceive colors; lacks cones or has malfunctioning cones– Total color blindness is rare

• Color Weakness: Inability to distinguish some colors– Red-green is most common; much more common

among men than women– Recessive, sex-linked trait on X chromosome

• Ishihara Test: Test for color blindness and color weakness

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Color Blindness

Genetic disorder in which people are blind to green or red colors supports

Trichromatic theory.

Ishihara Test

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Fig. 5.15 Color blindness and color weakness. (a) Photograph illustrates normal color vision. (b) Photograph is printed in blue and yellow and gives an impression of what a red-green color-blind person sees. (c) Photograph simulates total color blindness. If you are totally colorblind, all three photos will look nearly identical.

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Dark Adaptation

• Increased retinal sensitivity to light after entering the dark; similar to going from daylight into a dark movie theater

• Rhodopsin: Light-sensitive pigment in the rods; involved with night vision

• Night Blindness: Blindness under low-light conditions; hazardous for driving at night

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Opponent Process Theory

Hering, proposed that we process four primary colors opposed in pairs of red-green, blue-yellow, and black-white.

Cones

RetinalGanglion

Cells

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Fig. 5.11 Negative afterimages. Stare at the dot near the middle of the flag for at least 30 seconds. Then look immediately at a plain sheet of white paper or a white wall. You will see the American flag in its normal colors. Reduced sensitivity to yellow, green, and black in the visual system, caused by prolonged staring, results in the appearance of complementary colors. Project the afterimage of the flag on other colored surfaces to get additional effects.

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Fig. 5.12 Firing rates of blue, green, and red cones in response to different colors. The taller the colored bar, the higher the firing rates for that type of cone. As you can see, color sensations are coded by activity in all three types of cones in the normal eye. (Adapted from Goldstein, 1999.)

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Opponent-Process Theory• We cannot see certain colors together in

combination (red-green, blue-yellow, and white-black). These are antagonist/ opponent colors.

Tube and marble example.

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Color Constancy

Color of an object remains the same under different illuminations. However, when context changes color of an object may look different.

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