Spectral-sensitivity curves were derived from the a−, b−, and d−waves of rhesus monkey ERGs after injection of bicuculline, strychnine, or no drug. Without drug injection, the a− and d−wave curves were well-fit by an additive model of weighted photoreceptor absorption spectra, while the b−wave curve requires inhibitory terms to produce an adequate fit. Bicuculline, but not strychnine, reduced the weight assigned to the inhibitory terms in a dose-dependent fashion, to the point that no inhibition was evident. The results suggest that GABAergic synapses are required for the expression of red/green color opponency in primate bipolar cells.
