COGNITIVE SCIENCE 17 The Visual System: Color Vision Part 2 Jaime A. Pineda, Ph.D. Visible Spectrum. Color we perceive an object to be is determined by which wavelengths of light are reflected or absorbed by object
Jaime A. Pineda, Ph.D.
Based on the existence of three types of receptors that are maximally sensitive to different, but overlapping, ranges of wavelengths
Cannot read right digit deuteranopia
Cannot read left digit protanopia
excited (depolarized) when light is directed to cones in center of receptive field; inhibited when light hits the surround
inhibited when light is directed to the center of receptive field; excited when light is directed to center
Two major classes of ganglion cells within retina:
M & P cells – named for separate projections to magnocellular ( large cell) and parvocellular (small cell) layers of lateral geniculate nucleus
Account for 90% of all ganglion cells
More P than M cells
Ganglion cells of retina
large; simple antagonistic receptive fields, some off-center, some on-center but in both types the center and surround have similar, broad spectral sensitivities.
Concerned with gross features of a stimulus and its movement
color information carried almost exclusively by these cells. These are smaller, have smaller receptive fields; respond selectively to specific wavelengths
Primarily involved in analysis of fine detail of visual image
Parietal (Dorsal) and Temporal (Ventral) Processing Streams
Areas MT and V4 in the Macaque Brain
Dorsal (where) pathway shown in green and blueand Ventral (what) pathway shown in yellow and redserve different functions. (Courtesy of Leslie Ungerleider).
Magnocellular (dorsal) and parvocellular (ventral) pathways from the retina to the higher levels of the visual cortex are separate at the lower levels of the visual system. At higher levels they show increasing overlap.
Multiple Cortical Areas Devoted to Visual Functions
David Van Essen developed the technique of unfolding the cortex to better appreciate the many areas that contribute to vision.
Colored areas are devoted to visual function and brownareas are devoted to other functions.
Separation and Integration of Function
Areas of the monkey visual system (shown previously on unfolded cortex) are heavily interconnected.
Pathway from retina to the suprachiasmic nucleus (SCN) carries information about the light-dark cycle in the environment to the SCN. The size of the SCN is enlarged for viewing. Axons from the left eye are labeled in red and from the right eye in green. Both eyes project so diffusely to the two overlying SCN that they are outlined in yellow. (SCN photograph courtesy of Cynthia L. Jordan).