Direction-selective ganglion cells (DSGCs) in the retina respond strongly w
hen stimulated by image motion in a preferred direction but are only weakly
excited by image motion in the opposite null direction. Such coding repres
ents an early manifestation of complex information processing in the visual
system, but the cellular locus and the synaptic mechanisms have yet to be
elucidated. We recorded the synaptic activity of DSGCs using strategies to
observe the asymmetric inhibitory inputs that underlie the generation of di
rection selectivity. The critical nonlinear interactions between the excita
tory and inhibitory inputs took place postsynaptically within the dendrites
of the DSGCs.