3.2 VISION

• 70% of your receptor cells are in your eyes• taste and touch need direct contact where as

sight and smell don’t• Sight can be experienced from farthest distance

VISION

Draw the diagram of the eye above and label each part (hint: use color to distinguish different parts of the eye)

THE VISUAL SYSTEM

• Cornea • Transparent protective

coating over the front of the eye

• Pupil• Small opening in the

iris through which light enters the eye

• Iris• Colored part of the

eye

THE VISUAL SYSTEM

• Lens• Focuses light onto the

retina, is transparent and changes shape to focus on things at different distances

• Retina• Lining of the eye

containing receptor cells that are sensitive to light

• Fovea• Center of the visual field,

images are in sharpest focus here

DIAGRAM

• Create a diagram showing the path of light from a subject through the different parts of the eye, ending at the optic nerve.

RECEPTOR CELLS IN THE EYE

• Cells in the retina that are sensitive to light• Light- small segment of electromagnetic

spectrum to which our eyes are sensitive.• Visual receptors are called rods and cones

RECEPTOR CELLS

• Rods• About 120 million

rods• Respond to light and

dark• Very sensitive to

light• Provide our night

vision• Dominant outside of

fovea

• Cones• About 8 million

cones• Respond to color as

well as light and dark

• Work best in bright light

• Found mainly in the fovea

Both get sparse as you move away from the fovea at the back of the eye.

RECEPTOR CELLS

• Bipolar cells• Receive input from

receptor cells(rods/cones)

• Only have one axon and dendrite

• In fovea, cones usually only connect to ONE bipolar cell

• Elsewhere, several rods/cones share a single cell

RECEPTOR CELLS

• Ganglion cells• Receive input from

bipolar cells• Axons of these join

to form optic nerve that carries messages to the brain

• Cells summarize and organize information for the brain

RECEPTOR CELLS

• Blind spot• Area where axons of

ganglion cells leave the eye

• No receptor cells• Even when light from

a small object is focused directly on the blind spot, the object will not be seen

• Pg. 101 test

RECEPTOR CELLS

• Visual acuity- ability to distinguish fine details visually• Pg. 101 x test

FROM EYE TO BRAIN

• Optic nerve• Made up of axons of

ganglion cells• carries neural

messages from each eye to brain

• Optic chiasm• Point where part of

each optic nerve crosses to the other side of the brain

ADAPTATION

• Dark adaptation• Increased sensitivity of rods and cones in darkness• Usually only see in grey scale, little color• Reason for many accidents at night

• Light adaptation• Decreased sensitivity of rods and cones in bright light• Squint to reduce amount of light striking your retina• Within a minute you usually fully adapt

• Afterimage• Sense experience that occurs after a visual stimulus has

been removed• Light experiment

FEATURE DETECTORS

• Feature detectors- specialized brain cells that only respond to particular elements in the visual field such as movement or lines of specific orientation.

• Cortical blindness- severe damage to visual cortex• Blightsight-they behave as if they can see forms, colors,

and motion– even though they cannot see• Ex. Patient might duck if an object is thrown at her, even

though she reports not having seen it; she might turn her head toward a bright flashing light although she can’t see it.

COLOR VISION

• Properties of color• Hue – refers to colors such as red and green

• Most people can identify about 150 hues

• Saturation – refers to the vividness of a hue• Brightness – the nearness of a color to white

THEORIES OF COLOR VISION

• Additive color mixing• Mixing of lights of different hues to create new hues• Each light adds additional wavelengths to the over all mix• Lights, T.V., computer monitors (RGB)

• Subtractive color mixing• Mixing pigments, e.g., paints, each of which absorbs

some wavelengths of light and reflects others

THEORIES OF COLOR VISION

• Trichromatic theory• Three different types of cones

• Red• Green• Blue-violet

• Experience of color is the result of mixing of the signals from these receptors

• Can account for some types of colorblindness

FORMS OF COLORBLINDNESS

• Approximately 10% of men and 1% of women have some form of colorblindness

• Dichromats• People who are blind to either

red-green or blue-yellow

• Monochromats• People who see no color at all,

only shades of light and dark• Tests

• http://www.youtube.com/watch?v=yEIM4jmK1F0

THEORIES OF COLOR VISION

• Trichromatic theory cannot explain all aspects of color vision• People with normal vision cannot see “reddish-green” or

“yellowish-blue”• Color afterimages

THEORIES OF COLOR VISION

• Opponent-process theory• Three pairs of color receptors

• Yellow-blue• Red-green• Black-white

• Members of each pair work in opposition• Can explain color afterimages

• Both theories of color vision are valid

COLOR VISION IN OTHER SPECIES

• Other species see colors differently than humans• Most other mammals are dichromats• Rodents tend to be monochromats, as are owls

who have only rods• Bees can see ultraviolet light