Directional responses in retinal ganglion cells are generated in large part by direction-selective release of γ-aminobutyric acid from starburst amacrine cells onto direction-selective ganglion cells (DSGCs). The excitatory inputs to DSGCs are also widely reported to be direction-selective, however, recent evidence suggests that glutamate release from bipolar cells is not directional, and directional excitation seen in patch-clamp analyses may be an artifact resulting from incomplete voltage control. Here, we test this voltage-clamp-artifact hypothesis in recordings from 62 ON-OFF DSGCs in the rabbit retina. The strength of the directional excitatory signal varies considerably across the sample of cells, but is not correlated with the strength of directional inhibition, as required for a voltage-clamp artifact. These results implicate additional mechanisms in generating directional excitatory inputs to DSGCs.
Directional tuning of excitatory currents recorded in direction-selective ganglion cells (DSGCs) is thought to be an artifact produced by the strong directional tuning of inhibitory inputs leading to directionally-asymmetric voltage-clamp errors. If this hypothesis is correct, then the strength of excitatory and inhibitory directional tuning should be positively correlated. The graph shows the results of experiments in a large sample of DSGCs that failed to detect the expected positive correlation.