The Visual System
Major Structural Features of the Eye
cornea
anterior chamber (filled with aqueous humor)
pupil & iris
vitreous chamber (filled with vitreous humor)
Retina
layers
ganglion cell layer (outermost)
bipolar layer (middle)
photoreceptor layer (innermost)
types of photoreceptors
rods
120 million
light sensitive (not colour)
periphery of retina
low activation threshold
cones
6 million
colour sensitive
found in fovea
photopigment: rhodopsin
responsible for scotopic vision
ability to see in dim light
3 types of photopigments
green
red
blue
responsible for phototropic vision
ability to see in bright light
Receptive Fields
attributes of a stimulus that will alter the firing rate of sensory cell
indicate the manner in which sensory info converges from level to level
Colour Vision Theories
Trichromatic Theory
Opponent Theory
3 different receptors in the eye
each sensitive to a single hue
any colour accounted for if mix 3 lights in various portions
3 primary colours
yellow
blue
red
Visual Association Cortex
visual info transmitted to extrastriate cortex via
dorsal stream
ventral stream
'what' an object is
equal mix of magnocellular & parvocellular input
projects to V2, V3, V4, V5 & inferior temporal cortex
'where' an object is
mostly magnocellular input
projects to posterior parietal association cortex
analysis of visual info
achromatopsia
inability to discriminate among different hues
caused by damage to visual assoc. cortex
analysis of form
agnosia
visual agnosia
aperceptive visual agnosia
prosopagnosia
associative visual agnosia
perception of spatial location
Balint's Syndrome
bilateral damage to parieto-occipital region
optic ataxia
ocular apraxia
difficulty in visual scanning
difficulty reaching for objects under visual guidance
simultanagnosia
difficulty perceiving more than 1 object at a time
scotoma
produces an area where vision is poor or missing
caused by stroke or traumatic brain injury