Vision How does our body construct our conscious visual experience? How de we transform particles of light energy to another? We do this by receiving light.

Vision

• How does our body construct our conscious visual experience?

• How de we transform particles of light energy to another?

• We do this by receiving light energy and transforming it into neural messages that our brains process into what we consciously see.

The Stimulus Input- Light Energy

• What strikes are eyes IS NOT color, but rather electromagnetic energy that we perceive as color.

• The next slide shows the range from imperceptibility short waves of gamma rays to the narrow band we see as visible light, to the long waves of radio transmission and AC circuits.

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VisibleSpectrum

The Stimulus Input: Light Energy

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Wavelength (Hue)

Hue (color) is the dimension of color determined by the wavelength of the

light.

Wavelength is the distance from the

peak of one wave to the peak of the next.

The amount of energy in light waves or

intensity, determined by a wave’s

amplitude, or height, influences the

brightness of the light

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Intensity (Brightness)

Intensity Amount of energy in a

wave determined

by the amplitude. It is related to perceived

brightness.

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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)

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 redmakes 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

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Parts of the eye

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

2. Pupil- a small adjusting opening that allows light to pass through

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

4. Lens: Focuses the light rays on the retina.

5. Retina: Contains sensory receptors that process visual information and sends it to the brain.

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The LensLens: Transparent

structure behind the 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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Retina

Retina: The light-sensitive inner surface of the

eye, containing receptor rods and cones in

addition to 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 the optic nerve leaves the eye because there are no receptor cells located there. This 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

The neural signals produced in the rods and cones activate the neighboring BIPOLAR cells, which

then activate a network of GANGLION cells. The axons of ganglion cells converge to from the

OPTIC NERVE, which carries visual information to the BRAIN. Where the nerve leaves the eye, there

are no receptors; thus, the area is called the BLIND SPOT

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Test your Blind Spot

Use your textbook (E-BOOK). Close your left eye, and fixate your right eye on the

black dot. Move the page towards your eye and away from your eye. At some point the

car on the right will disappear due to a blind spot.

Fovea, Rods and Cones • Most cones are clustered around

the retina’s point of central focus called the FOVEA, whereas the rods are concentrated in more peripheral visions of the retina. Many cones have their own bipolar cells to communicate with the visual cortex.

• Cones- enable color• Rods- enable black and white

vision• Unlike cones, rods are sensitive.

Adapting to a darkened room will take the retina approximately 20 minutes.

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Visual Information Processing

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

connects to the visual cortex.

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Feature Detection

Hubel and Wiesel discovered that specific features called Feature Detectors.

Examples include - respond to specific features, such as edges, angles, and

movement.

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Feature Detection

• Feature detectors pass the information to higher level cells in the brain, which respond to specific visual scenes. Research has shown that in monkey brains such cells that specialize in responding to specific gazes, head angles, posture, or body movements. In may cortical areas, teams of cells supercell clusters respond to complex patters.

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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. Other brain damaged people may demonstrate blind sight by

responding to a stimulus that is not consciously perceived.

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Theories of Color Vision

Trichromatic theory: Based on behavioral experiments, Helmholtz suggested that the retina

should contain three receptors that are sensitive to red, blue and green colors. An object appears to be red in color because it rejects the long wavelengths of red and because of our mental construction of the color.

Blue Green Red

Medium LowMax

Standard stimulus

Comparison stimulus

Theories of Color Vision

• One out of every 50 people is color deficient; this is usually a male because the defect is genetically

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Subtraction of Colors

If three primary colors (pigments)

are mixed, subtraction of all

wavelengths occurs and the

color black is the result.

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Addition of Colors

If three primary colors (lights) are mixed, the wavelengths are added and the color

white is the result.

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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

Ishihara Test

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

Trichromatic theory.

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Opponent Colors

Gaze at the middle of the flag for about 30Seconds. When it disappears, stare at the dot and report

whether or not you see Britain's flag.

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Opponent Process Theory

Hering proposed that we process four primary colors combined in pairs of red-

green, blue-yellow, and black-white.

Cones

RetinalGanglion

Cells

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Color Constancy

Color of an object remains the same under different illuminations. However, when

context changes the color of an object may look different.

R. B

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