Judith A. Hirsch, Jose-Manuel Alonso, and R. Clay Reid
Department of Neurobiology, Harvard Medical School
Striate cortical sensitivity to patterned stimuli has its roots in the spatial structure of the simple receptive field. Bright stimuli presented to a simple cell's on subregion evoke firing, whereas dark ones reduce activity (Hubel and Wiesel, 1962, J. Physiol 160,106). Signals of reverse contrast are often described as having a push-pull effect, (Palmer, 1987 J Neurophys. 58, 1187; Ferster, (1988) J Neurosci. 8, 1172). Two mechanisms have been proposed to account for the ``pull''. One process is passive- withdrawal of tonic excitation from the thalamus; the other is active-intracortical inhibition.
To study the synaptic basis of suppression in the simple receptive field we combined receptive field mapping with intracellular analyses of evoked responses. Our results show that visually-evoked hyperpolarizations are conveyed by active processes: they are accompanied by a large increase in the membrane conductance and reverse sign when the resting potential is made sufficiently negative. Furthermore, stimuli that recruit excitation and inhibition at once reveal the presence of a weak veto mechanism.