The Visual System The Structure of the Visual

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 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

• The point at which the optic nerve travels through the retina to exit the eye Blind Spot • There are no rods and cones at this point, so there is a small blind spot in vision.

Parts of the Eye – Blind Spot

Module 9: Sensation The Visual System: Color Vision

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?

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/White • Only one side is “on” at a time

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

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

Afterimage Effect

Opponent-Process Theory • If one sensor is stimulated, the other is inhibited • If one sensor is overstimulated, and fatigues, the paired sensor will be activated, causing an afterimage