The Visual System: The Structure of the Visual System Module 9: Sensation

Sight!

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

What if your retina is damaged?

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 White –Black/White Only one side is “on” at a timeOnly one side is “on” at a time

Opponent Process Theory ON”“OFF” red green green red blue yellow yellow blue white black white white 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