THE TIME-COURSE OF DIRECTION-SELECTIVE ADAPTATION IN SIMPLE AND COMPLEX CELLS IN CAT STRIATE CORTEX

1. Responses of single cortical neurons in area 17 of anesthetized cat s were recorded in response to prolonged stimulation with a patch of d rifting square-wave grating. 2. During adaptation in the preferred dir ection, all neurons showed some reduction in response to motion in the stimulated direction and most showed some reduction in the opposite, nonstimulated direction. 3. For complex cells, the time course of resp onse decrement in both the stimulated and nonstimulated directions was exponential, with an average time constant of 5 s. Response recovery was also exponential but significantly slower, with time constants of 8 and 13 s in the stimulated and nonstimulated directions, respectivel y. 4. For simple cells the dynamics of the adaptation effect depended on the direction of testing. In the nonstimulated direction the time c ourse of the change in sensitivity was similar to that of complex cell s. In the stimulated direction during both the adaptation and recovery periods, simple cells showed an initial rapid exponential change on t he order of a few seconds that was followed by a more gradual exponent ial change. 5. During prolonged stimulation in the nonpreferred direct ion, there was less overall change in sensitivity. For some neurons th e change in sensitivity during adaptation and recovery was exponential , with a short time constant for both simple and complex cells and for stimulated and nonstimulated directions. Other neurons showed no chan ge in sensitivity in either direction and a few neurons showed facilit ation during the adaptation period. 6. There appears to be a rapid gen eral or nonspecific process, which may be related to contrast gain con trol, underlying motion adaptation in striate cortical neurons. An add itional slow, direction-selective process is revealed when simple but not complex cells are stimulated in the preferred direction. We sugges t that this latter type of adaptation is a key feature underlying the perceptual motion aftereffect.