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