Summation of rod signals within the receptive field centre of cat retinal ganglion cells.

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Abstract
The processing of rod signals within the receptive field center of cat retinal ganglion cells was investigated in 2 spot summation experiments using the analytical methods of response and sensitivity summation. The rod system was isolated by presenting test stimuli of short wave-length light against a completely dark background or a dim background of long wave-length light. Stimulus-response curves were obtained for 2 small, square-wave modulated test spots applied at points in the receptive field center of equal sensitivity. The test spots were presented singly or simultaneously. In the absence of surround antagonism the flux required to evoke a weak criterion response was the same whether the spots were presented singly or together. The flux required to evoke larger responses was typically half as great when the 2 spots were delivered together than when either was presented alone. Over a moderate response range, the magnitude of the response to the 2 test spots presented together equalled the algebraic sum of the 2 responses to the test spots presented alone. For responses of large magnitude the algebraic sum was larger. Permitting the surround to contribute substantially to the cell''s response changed the outcome of the 2 spot summation experiment. The data are consistent with a 3 stage model of signal processing within the receptive field center: an early compressive power law transformation (within each sub-area) of illuminance into a neural signal is followed by linear summation of sub-area signals and a 2nd compressive transformation.