The Visual System: The Structure of the

Visual System

Cornea• The clear bulge on the front of the eyeball• Begins to focus the light by bending it

toward a central focal point• Protects the eye

Parts of the Eye – Cornea

Iris• A ring of muscle tissue that

forms the colored portion of the eye; creates a hole in the center of the iris (pupil)

• Regulates the size of the pupil by changing its size--allowing more or less light to enter the eye

Parts of the Eye - Iris

Pupil

• The adjustable opening in the center of the eye that controls the amount of light entering the eye (surrounded by the iris)

• In bright conditions the iris expands, making the pupil smaller.

• In dark conditions the iris contracts, making the pupil larger.

Parts of the Eye - Pupil

Lens• A transparent structure behind the pupil;

focuses the image on the back of the eye (retina)

• Muscles that change the thickness of the lens change how the light is bent thereby focusing the image

• Glasses or contacts correct problems in the lens’ ability to focus.

Parts of the Eye - Lens

Nearsighted - Myopia

Farsighted - Hyperopia

Retina• Light-sensitive surface with cells that

convert light energy to nerve impulses• At the back of the eyeball

Parts of the Eye - Retina

Receptor Cells

• These cells are present in every sensory system to change (transduce) some other form of energy into neural impulses.

• In sight they change light into neural impulses the brain can understand.

• Visual system has two types of receptor cells – rods and cones

• Visual receptor cells located in the retina

• Can only detect black and white

• Respond to less light than do cones

Rods

• Visual receptor cells located in the retina

• Can detect sharp images and color

• Need more light than the rods

• Many cones are clustered in the fovea.

Cones

Fovea

• The central focal point of the retina• The spot where vision is best (most

detailed)

Parts of the Eye - Fovea

Visual Processing in the Retina

Optic Nerve

• The nerve that carries visual information from the eye to the occipital lobes of the brain

Parts of the Eye – Optic Nerve

Blind Spot

• The point at which the optic nerve travels through the retina to exit the eye

• There are no rods and cones at this point, so there is a small blind spot in vision.

Parts of the Eye – Blind Spot

The Visual System: Color Vision

Module 9: Sensation

Color Vision

• There are two theories of color vision:–Trichromatic Theory–Opponent-Process Theory

Trichromatic (three-color) Theory

• Theory of color vision that says cones are “tuned” to be sensitive to red, green and blue light

• All the colors we see are a combination of these three colors.

• Similar to the design of a color TV

How do we see color?• Trichromatic (three color) Theory–three different retinal color receptors•Red green blue

Can you see what is in the middle?

Color Deficient Vision

• People who lack one of the three types of cones

• Usually the red or green receptors are missing

• Usually referred to as color blindness• In inherited and found more in males

Red-Green Color Blindness

• Opponent-Process Theory- Vision from opposing pairs of color receptors- only one “side” ON at a time

Opponent-Process Theory

• Sensory receptors in the retina come in pairs:–Red/Green–Yellow/Blue–Black/WhiteWhite

• Only one side is “on” at a timeOnly one side is “on” at a time

Opponent Process Theory

ON” “OFF”red greengreen red blue yellow yellow blue black whitewhitewhite white black

Afterimage Effect

Opponent-Process Theory• If one sensor is stimulated, the

other is inhibited• If one sensor is over-

stimulated, and fatigues, the paired sensor will be activated, causing an afterimage