The spatial extent of excitatory and inhibitory zones in the receptive field of superficial layer hypercomplex cells

Abstract

The extent of inhibitory and excitatory components in the receptive field of superficial layer hypercomplex cells in the cat striate cortex and the relations of the components to the length preference exhibited by these cells were studied. Maximal responses were produced by an optimal length stimulus moving through a restricted region of the receptive field. The length of this receptive field region was less than the total length of the excitatory zone as mapped with a very short slit. Slits of similar length to the excitatory zone produced a smaller response than an optimal length slit. An increase of slit length so that it passed over receptive field regions on either side of the excitatory zone resulted in an elimination of the response. When background discharge levels were increased by the iontophoretic application of D,L-homocysteic acid, slits of this length produced a suppression of the resting discharge as they passed over the receptive field. They did not modify the resting discharge level when it was induced by the iontophoretic application of the GABA antagonist bicuculline. Apparently long slits activate a powerful post-synaptic inhibitory input to the cell. Maximal inhibitory effects were only observed if the testing slit passed over the receptive field center, i.e., slits with a gap positioned midway along their length, so as to exclude the optimal excitatory response region, surprisingly tended to produce excitatory effects rather than the expected inhibitory effects. It appears that simultaneous stimulation of the receptive field center is a precondition for the inhibitory effect of stimulation of regions on either side of the excitatory zone to be activated. The interneurons mediating the inhibitory input to the superficial layer hypercomplex cells are apparently driven by cells in adjacent hypercolumns with receptive fields spatially displaced to either side of the excitatory zone and by cells in the same column, optimal inhibitory effects only being achieved when both sets of input to the interneuron are activated.