SYNAPTIC INTEGRATION IN STRIATE CORTICAL SIMPLE CELLS

Simple cells in the visual cortex respond to the precise position of o riented contours (Hubel and Wiesel, 1962). This sensitivity reflects t he structure of the simple receptive field, which exhibits two sorts o f antagonism between on and off inputs. First, simple receptive fields are divided into adjacent on and off subregions; second, within each subregion, stimuli of the reverse contrast evoke responses of the oppo site sign: push-pull (Hubel and Wiesel, 1962; Palmer and Davis, 1981; Jones and Palmer; 1987; Ferster, 1988). We have made whole-cell patch recordings from cat area 17 during visual stimulation to examine the g eneration and integration of excitation (push) and suppression (pull) in the simple receptive field. The temporal structure of the push refl ected the pattern of thalamic inputs, as judged by comparing the intra cellular cortical responses to extracellular recordings made in the la teral geniculate nucleus. Two mechanisms have been advanced to account for the pull-withdrawal of thalamic drive and active, intracortical i nhibition (Hubel and Wiesel, 1962; Heggelund, 1986; Ferster, 1988). Ou r results suggest that intracortical inhibition is the dominant, and p erhaps sole, mechanism of suppression. The inhibitory influences opera ted within a wide dynamic range. When inhibition was strong, the membr ane conductance could be doubled or tripled. Furthermore, if a stimulu s confined to one subregion was enlarged so that it extended into the next, the sign of response often changed from depolarizing to hyperpol arizing. In other instances, the inhibition modulated neuronal output subtly, by elevating spike threshold or altering firing I-ate at a giv en membrane voltage.