the visual system: central processing (2.2) Flashcards
retinal-geniculate-striate pathway
includes about 90% of axons of retinal ganglion cells; retina -> LGN -> V1
optic chiasm
connection of the eyes
lateral geniculate nucleus
maintains segregation of inputs from the two eyes (keeps eyes separate from each other; no depth perception); visual part of thalamus; 6 layers; projects to subdivisions of later IV of neocortex
layers of the LGN (6)
top 4 layers are P (parvocellular) cells; bottom 2 layers are M (magnocellular) cells
P vs M cells
P cells: small; color and detail; still or slow-moving objects; input from cones; important for color vision (daytime)
M cells: large; no color; motion; input from rods; blurry, black and white vision (nighttime)
overrepresentation in the striate cortex
fovea
simple cells
can be constructed “simply” from convergence of center-surround inputs; similar to center-surround but borders are straight lines rather than circles; vertical = maximally happy, horizontal = not happy
ocular dominance columns
binocular cells in primary visual cortex may respond more robustly to stimulation of one eye than they do to the same stimulation of the other
depth perception
done in the ocular dominance columns beyond layer 4
hypercolumns
consist of pairs of ocular dominance columns, blobs and many orientation columns
blobs
where color vision is processed (inputs from only P cells); process color difference
color constancy
color perception is not altered by varying reflected wavelengths
retinex theory (Land)
color is determined by comparing the light reflected from adjacent surfaces (contrast)
dual-opponent color cells
sensitive to color contrast; found in cortical “blobs”; receive P inputs
dorsal vs ventral streams
dorsal: “where”; contrast of behavior (behavioral interactions with objects)
ventral: “what”; conscious perception [of objects]; face processing center; inferotemporal cortex
