Selective adaptation to color contrast in human primary visual cortex

How neural activity produces our experience of color is controversial, beca use key behavioral results remain at odds with existing physiological data. One important, unexplained property of perception is selective adaptation to color contrast. Prolonged viewing of colored patterns reduces the percei ved intensity of similarly colored patterns but leaves other patterns relat ively unaffected. We measured the neural basis of this effect using functio nal magnetic resonance imaging. Subjects viewed low-contrast test gratings that were either red-green (equal and opposite long- and middle-wavelength cone contrast, L-M) or light-dark (equal, same-sign, long- and middle-wavel ength cone contrast, L+M). The two types of test gratings generated approxi mately equal amounts of neural activity in primary visual cortex (V1) befor e adaptation. After exposure to high-contrast L-M stimuli, the L-M test gra ting generated less activity in V1 than the L+M grating. Similarly, after a daptation to a high-contrast L+M grating, the L+M test grating generated le ss activity than the L-M test grating. Behavioral measures of adaptation us ing the same stimuli showed a similar pattern of results. Our data suggest that primary visual cortex contains large populations of color-selective ne urons that can independently adjust their responsiveness after adaptation. The activity of these neural populations showed effects of adaptation that closely matched perceptual experience.