One manifestation of contrast gain control in striate cortical cells is seen by measuring the response to continuous presentation of sinusoidal gratings with stepwise increasing, then decreasing contrast (Bonds, 1991). Firing rate is lowered after presentation of higher contrasts. Hysteresis, or stimulus history-dependent adaptation, is quantified by the difference in contrast required to produce the half-height firing rate between the increasing and decreasing slopes of the curve. Both intrinsic (cellular) and extrinsic (network) models for the mechanisms underlying this adaptation have been proposed, but the precise origins remain unclear. In cats paralyzed and anesthetized with Propofol and N2O, we investigated whether cortico-cortical projections were involved in history-dependent adaptation by blocking retinotopically-matched sites in Area 18 (A18) while measuring hysteresis in Area 17 (A17) cells. Control responses to systematically increasing/decreasing contrasts of a spatially optimized stimulus were measured from a single cell in A17 with a tungsten-in-glass electrode. Of 21 cells, all showed some level of hysteresis (mean = 0.40 ± 0.17 log units). The response was then remeasured while GABA (0.5M) was iontophoretically injected into a retinotopically-matched site in A18 via a multi-barrel injection and recording pipette. In 11 cells, the level of hysteresis decreased during the A18 blockade (mean = 58 ± 22%), suggesting that A18 was a significant source for adaptation in those cells. After terminating the GABA injection, all measurements returned to control levels. In 18/21 cells, orientation tuning responses were also measured pre-, during and post- GABA. Modulation of orientation tuning and hysteresis with A18 inactivation were not always paired (i.e., could occur independently), indicating that A18 can influence A17 responses through at least two different mechanisms.

Supported by EYR0103778, Training Grant EY07135