COLOR PROCESSING IN HUMAN CORTEX

The goal of this project is to understand the nature of color opponent processing in human visual areas. Color perception results from a sequence of neural transformations of a signal originally encoded by the photoreceptors. The first stages of this sequence are fairly well understood, but the neural basis for the second perceptually identified stage of processing, opponent color mechanisms, remains unclear. A long history of perceptual experiments indicate that a second stage encodes color as three combinations of cone signals, encoding the relative local amounts of red and green, blue and yellow, and light and dark. While parvocellular neurons in the lateral geniculate nucleus (LGN) receive opponent inputs from cones, these cells differ from perceptual mechanisms in important ways. LGN neurons appear to jointly encode red-green and light-dark, they respond well to high temporal frequencies, and they over-represent red-green relative to blue-yellow. The work proposed here attempts to localize cortical areas whose response properties more closely match perceptual mechanisms. We will measure the responsiveness of human visual areas to stimuli of many colors. Functional MRI (fMRI) data will be acquired while subjects view large colored patterns. Responses to each stimulus will be averaged within the LGN and visual areas VI-V4. Color tuning is a measure of responsiveness as a function of color. A series of experiments will measure the color tuning of visual areas under conditions that reveal identifying features of perceptual mechanisms. First, the linearity of color tuning will be tested. Next, color tuning will be measured at a variety of temporal and spatial frequencies. Then it will be measured in the presence of adapting stimuli. Finally, color tuning will be measured in the visual areas of dichromatic observers. All of these conditions effect perceptual opponent color mechanisms in distinctive ways. Visual areas whose response properties resemble perceptual mechanisms are likely to be important stages in the neural pathways that give rise to color perception. Differences in the response properties of areas will help reveal the sequence of neural transformations that results in perceptual color opponency.