Abstract
Psychophysics has identified visual mechanisms tuned for particular orientations and color directions. The goal of the present research is to explore the neural bases of these behavioral mechanisms. Two series of experiments measured neural activity using fMRI while subject viewed stimuli and performed tasks borrowed from psychophysical paradigms. A first study measured V1 responses to stimuli of different colors. Prolonged exposure to a particular color reduced both perceptual sensitivity and V1 response to that color, while sparing other colors. These selective adaptation effects were selective for orientation as well. Our results strongly suggest that large numbers of neurons in V1 are jointly tuned for orientation and color. The similarity of our fMRI and behavioral measures indicates that the activity of these neurons supports perceptual judgments of color appearance. A second study measured V1 and V2 responses to stimuli of different orientations. At para-foveal locations, cortex responded more strongly to principal (vertical and horizontal) than to oblique orientations. This difference was not present at more eccentric locations, matching behavioral measurements made using a contrast detection task. Training in detecting an oblique parafoveal stimulus increased fMRI response to that stimulus relative to others.
Behavioral performance improved in a similarly selective fashion. These results suggest that either the number of neurons tuned to oblique orientations increased, or the gain of such neurons rose. Thus perceptual learning may strengthen responses to simple patterns at very early stages of the visual system. Overall, our results show that the combination of neuroimaging techniques and psychophysics can identify properties of populations of neurons that support perception.