GABAERGIC INHIBITORY CONTROL OF THE TRANSIENT AND SUSTAINED COMPONENTS OF ORIENTATION SELECTIVITY IN A MODEL MICROCOLUMN IN LAYER-4 OF CAT VISUAL-CORTEX

Recently proposed models of orientation tuning in layer 4 of fat prima ry visual cortex (Somers, Nelson, & Sur, 1995; Douglas, Koch, Mahowald , Martin, & Suarez, 1995) rely on widespread inhibitory intracortical connections to suppress the nonoptimal component of a broadly tuned th alamic input, while local excitatory intracortical connections amplify the optimal component. However, new experimental data (Ferster, Chung , & Wheat,1996) and theoretical analyses (Ferster,1987; Krukowski, Pri ebe, & Miller, 1996) show that the temporally modulated component of t halamic input is well tuned and that the cortical circuitry must simpl y subtract an unmodulated DC component at nonoptimal orientations to o btain sharp tuning. In addition, within a single hypercolumn in layer 4, inhibitory and excitatory layer 4 neurons have approximately equal- sized axonal fields, making the most of their synapses within their ow n dendritic field (Kisvarday, Martin, Whitteridge, & Somogyi, 1985; Ma rtin & Whitteridge, 1984). We have constructed a model of a single mic rocolumn in which GABA(B) inhibition subtracts the DC and controls the sustained response, while GABA(A) inhibition controls the response to transient and suprathreshold inputs. The model fits experimental data based on stimulation with drifting sine-wave gratings as well as flas hed bars, explains a counterintuitive property of the GABA(B) K+ condu ctance, and at suboptimal orientations and submaximal contrasts produc es an exponential distribution of firing frequencies.