Algebraic Summation of Centre and Surround Inputs to Retinal Ganglion Cells of the Cat

Abstract

Two mutually antagonistic concentric regions, the centre and the surround, make up the receptive field of most cat retinal ganglion cells. It is believed that each region corresponds to a functional unit or response mechanism, the two overlapping spatially but each having its own receptor representation^1–3. In response to square-wave stimuli of such geometry and luminance that both mechanisms are stimulated, the ganglion cell receives excitatory stimuli from one mechanism, and inhibitory ones from the other at the onset of light. At the offset of light the mechanisms reverse their roles. The mutual antagonism of the two mechanisms is manifested, but according to rules that are not well established. In spite of the overlap in distribution of centre and surround receptors, it has been possible to apply stimuli, the geometry and luminance of which are such that either a pure central or a pure surround response, without cross-contamination, is elicited^4–6. (Response is here defined as a change in spike rate elicited by a change in stimulus luminance.) This, for the first time, makes it possible to explore directly the mode of antagonism between the two mechanisms. We present here results obtained from three on-centre cells which demonstrate unequivocally that the response of the ganglion cell to a combination of central and surround inputs is simply the algebraic sum of the pure responses to these same inputs delivered separately. The results strongly suggest that, up to the ganglion cell, there is indeed no interaction between the two mechanisms. The interpretation of experiments on retinal ganglion cells of the centre-surround type is thus greatly simplified.