Adaptation aftereffects in single neurons of cat visual cortex: response timing is retarded by adapting

Extracellular single-unit recordings were made from simple cells in area 17 of anesthetized cats. Cells were tested with drifting gratings under control and adapted conditions. Response amplitude and phase were measured as a function of either contrast or temporal frequency. Adapting not only reduces amplitude, but also retards phase. Adaptation alters the responses of simple cells in a particular way: the onset of the response to each cycle of a sinusoidally modulate stimulus is delayed. Once cells start to respond during each cycle, however, they generally recover to control levels, and the offset of the response is unaffected by adapting. The timing aftereffects are independent of the amplitude aftereffects. Timing aftereffects are tuned around the adapting temporal frequency, with a bias toward lower temporal frequencies. Adaptation thus modifies cortical responses even more specifically then previously thought. Firing rates are depressed primarily at response onset, even after several stimulus cycles have occurred following the end of adapting. Because all cells appear to adapt in this way, the data offer an opportunity to theorize about cortical connectivity. One implication is that inhibition onto a simple cell arises from other simple cells with similar response properties that fire a half-cycle out of phase with the target cell.