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Vision Our most dominant sense. Our Essential Questions. What are the major parts of the eye? How does the eye translate light into neural impulses?. Vision. Purpose of the visual system transform light energy into an electro-chemical neural response
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Our Essential Questions • What are the major parts of the eye? • How does the eye translate light into neural impulses?
Vision Purpose of the visual system • transform light energy into an electro-chemical neural response • represent characteristics of objects in our environment such as size, color, shape, and location
Light: The Visual Stimulus • Wavelength of a light is the distance of one complete cycle of the wave. • Visible light : 400nm - 700nm. • Wavelength of light is related to its perceived color
The Structure of the Visual SystemSo how does this stimulus (light) transform into messages in our brain?
Cornea • The clear bulge on the front of the eyeball • Begins tofocus the light by bending it toward a central focal point • Protects the eye
Iris • Colored portion of the eye • Does color affect vision? • A ring of muscle tissue that regulates the size of the pupil • Allows more or less light to enter the eye
Pupil • Opening in the center of the eye • Controls the amount of light entering the eye • bright conditions - iris expands, pupil gets smaller • dark conditions - iris contracts, pupil gets larger
Lens • A transparent structure behind the pupil • Focuses the image on the back of the eye • Muscles change the thickness of the lens change how light is bent focuses the image • Glasses or contacts correct problems
Retina • At the back of the eyeball • Light-sensitive surface with cells that convert light energy to neural impulses • This is where the magic happens!
Fovea • The central focal point of the retina • The spot where vision is best (most detailed)
Receptor Cells • In sight they change light into neural impulses the brain can understand • Visual system has two types of receptor cells – rods and cones
Distribution of Rods and Cones • Cones—concentrated in center of eye (fovea) • approx. 6 million • Rods—concentrated in periphery • approx. 120 million • Blind spot—region with no rods or cones
Differences Between Rods and Cones • Cones • allow us to see in bright light • allow us to see fine spatial detail • allow us to see different colors • Rods • allow us to see in dim light • can not see fine spatial detail • can not see different colors
Receptive Fields and Rod vs. Cone Visual Acuity • Cones—in the fovea, one cone often synapse onto only a single ganglion cell • Rods—the axons of many rods synapse onto one ganglion cell • This allows rods to be more sensitive in dim light, but it also reduces visual acuity
Let’s Review • Cone Characteristics • Rod Characteristics
Rods • Located in the retina • Can only detect black and white • Respond to less light than do cones
Cones • Located in the retina • Can detect sharp images and color • Need more light than the rods • Many cones are clustered in the fovea
Let’s do an experiment now • What do you see in your peripheral vision (that’s the stuff on the side)?
The Experiment • A will look straight ahead • B will look A in the eyes – to make sure that A doesn’t cheat! • C will move various colored pieces of paper in A’s peripheral vision • A will guess the color • Note: if the person is consistently guessing correctly then they are cheating!
Write up the results… • Results – correct guess versus bad • Your conclusion • What do your results tell you about our vision? • How do the different kinds of receptor cells affect our vision?
Distribution of Rods and Cones • Cones—concentrated in center of eye (fovea) • approx. 6 million • Rods—concentrated in periphery • approx. 120 million • Blind spot—region with no rods or cones
Let’s Compare… Cones Rods allow us to see in dim light can not see fine spatial detail can not see different colors • allow us to see in bright light • allow us to see fine spatial detail • allow us to see different colors
Optic Nerve • The nerve that carries visual information from eye occipital lobes
Blind Spot 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
Color Vision • Differences in wavelength of light = color • Rods are color blind, but cones can see different colors • We have only one type of rod but three types of cones
Color Vision • Two theories of color vision: • Trichromatic Theory • Opponent-Process Theory
Trichromatic (3-Color) Theory • Cones are “tuned” to be sensitive to red, green and blue light • All the colors we see are a combination of these 3 colors • Similar to the design of a color TV
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” redgreen greenred blueyellow yellowblue black white white black
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