THE VISUAL SYSTEM
LIGHT
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Electromagnetic radiation that travels as a wave
Amplitude = brightness
Wavelength = color
Varies in purity (richness of colors)
THE EYE
• Light enters through cornea
• Cornea and lens form an upside down image
• Lens: transparent eye structure that focuses the light rays
falling on the retina
VISUAL DEFICIENCIES
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Nearsighted: close objects are clear; far objects are blurry
Caused by light from far objects falling short of retina
Farsighted: distant clear; close blurry
Caused by light from close objects falling behind retina and
eyeball being too short
EYE
• Iris: colored ring of muscle surrounding the pupil
• Pupil: opening in the center of iris that helps regulate the
amount of light passing into the rear chamber of the eye
RETINA: BRAINS ENVOY TO
THE EYE
• Retina: neural tissue lining the inside back surface of the eye;
it absorbs light, processes images, and sends visual info. to the
brain
• Optic disk: a hole in the retina where optic nerve fibers exit
the eye
RODS AND CONES
• Cones: specialized visual receptors that play a key role in
daylight vision and color vision
• Fovea: tiny spot in center of retina; contains only cones
• Rods: play key role in night vision and peripheral vision
DARK AND LIGHT
ADAPTATION
• Dark adaptation: process in which the eyes become more
sensitive to light in low illumination
• Light adaptation: eyes become more sensitive to light in high
illumination
INFO PROCESSING IN THE
BRAIN
• Light hits receptors; triggers neural signals
• Signals move to optic nerve
• Receptive field of visual cell: the retinal area that, when
stimulated affects the firing of that cell
VISUAL PATHWAYS TO THE
BRAIN
• Optic chiasm: pt at which the optic nerves from the inside half
of each eye cross over and then project to the opposite half of
the brain
• Optic fibers then diverge along 2 paths
• Main path projects into thalamus; retinal axons synapse in the
Lateral geniculate nucleus (LGN)
• Then to the occipital lobe to the primary visual cortex
• Main path divided into 2 paths: magnocellular and
parvocellular channels---engage in parallel processing
• 2nd path goes to Superior colliculus in the midbrain
INFO PROCESSING IN VISUAL
CORTEX
• Feature detectors: neurons that respond selectively to very
specific features of more complex stimuli
• Visual agnosia: inability to recognize objects
• Prosopagnosia: inability to recognize familiar faces