V1 responses can be suppressed by stimuli placed either within the classical receptive field (CRF) or in the surround. We have previously shown that surround suppression occurs roughly 20 ms later than CRF excitation (Bair et al, ARVO 1999). We have now used a similar paradigm to explore the timing of cross-orientation inhibition within the CRF. Our stimulus randomly assumed one of four possible states, based on the presence or absence of two drifting sinusoidal gratings: a stimulus at the cell's preferred orientation and a mask at an orthogonal orientation. Each state lasted for one full drift cycle, typically 80 or 160 ms. The rapid succession of these states allowed us to measure the time course of responses to the appearance of the preferred stimulus or the mask alone, to the appearance of the mask while the preferred stimulus was present, and to the simultaneous appearance of both. The latency of excitation from the preferred stimulus and the latency of suppression from an orthogonal mask were roughly equal when both stimuli were confined to the CRF. In contrast, the latency of suppression elicited by changing the orientation of a suppressive surround stimulus was about 20 ms longer. Our results suggest that suppression within the classical receptive field has the same dynamics as excitation, and may therefore arise within very local circuits near the neuron under study. Suppression from the surround acts more slowly, and may reflect the action of long range connections within V1 or of feedback connections from higher cortical areas.

Supported by the Howard Hughes Medical Institute (HHMI).