Describe how colors are detected and processed in the magnocellular and parvocellular systems, in complete and partial color blindness, and in color constancy.

Question

What will be an ideal response?

Answer 1

The parvocellular ganglion cells are mostly located in the fovea and have small, circular, color-opponent receptive fields. These cells receive input from the cone receptors and thus process information about color. The magnocellular ganglion cells have large, circular, brightness-opponent receptive fields, and they process information about brightness contrast, giving us a perception of depth.
Individuals who are completely color-blind lack cone receptors. Thus there is no mechanism to respond to the different wavelengths of light. Individuals who are partially color-blind lack functioning in at least one of the three types of cone receptors. Based on which cone receptor is lacking, the individual will be blind to associated wavelengths of light or have difficulty distinguishing among certain colors. For example, those who are red-green color-blind cannot tell the difference between the two colors. This is due to either a deficit in the red cone receptors (protanopia) or a deficit in the green cone receptors (deuteranopia). An individual with a deficit in the blue cone receptors (tritanopia) is unable to perceive the color blue. Cells in the cortex (V4 in particular) give us the ability to recognize the color of an object as constant despite changes in illumination. This is called color constancy. The cells in area V4 have large, circular receptive fields that are also color opposed. This allows the cells to "subtract out" the color of any general illumination occurring in the visual field and thus keep the perception of the color of an object constant.