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