RESUMO
A stimulus located outside the classic receptive field (CRF) of a striate cortical neuron can markedly influence its behavior. To study this phenomenon, we recorded from two cortical sites, recorded and peripheral, with separate electrodes in cats anesthetized with Propofol and nitrous oxide. The receptive fields of each site were discrete (2-7.3 deg between centers). A control orientation tuning (OT) curve was measured for a single recorded cell with a drifting grating. The OT curve was then remeasured while stimulating simultaneously the cell's CRF as well as the peripheral site with a stimulus optimized for that location. For 22/60 cells, the peripheral stimulus suppressed the peak response and/or shifted the center of mass of the OT curve. For 19 of these 22 cells, we then reversibly blocked stimulus-driven activity at the peripheral site by iontophoretic application of GABA (0.5 M). For 6/19 cells, the response returned to control levels, implying that for these cells the inhibitory influence arose from the blocked site. The responses of nine cells remained reduced during inactivation of the peripheral site, suggesting that influence was generated outside the region of local block in area 17. This is consistent with earlier findings suggesting that modulatory influences can originate from higher cortical areas. Three cells had mixed results, suggesting multiple origins of influence. The response of each cell returned to suppressed levels after dissipation of the GABA and returned to baseline values when the peripheral stimulus was removed. These findings support a cortical model in which a cell's response is modulated by an inhibitory network originating from beyond the receptive field that supplants convergence of excitatory lateral geniculate neurons. The existence of cells that exhibit no change in peripherally inhibited responses during the GABA application suggests that peripheral influences may arise from outside area 17, presumably from other cortical areas (e.g. area 18).
