Segregation and integration of visual channels: layer-by-layer computation of ON-OFF signals by amacrine cell dendrites.

The visual system analyzes images through parallel channels, and our data suggest that the first set of parallel representations of the visual world is embodied in the inner plexiform layer (IPL) of the retina, in which light-evoked excitatory inputs of the ON and OFF bipolar cells to amacrine cells (ACs) are organized in a layer-by-layer manner. Approximately 30% of ACs have narrowly monostratified dendrites in 1 of the 10 strata of the IPL, and they receive segregated bipolar cell inputs: the light-evoked excitatory cation current, DeltaI(C), in strata 1, 2, and 4 is OFF (predominantly mediated by the OFF bipolar cells), the current in strata 3 and 7-10 is ON (predominantly mediated by ON bipolar cells), and the current in strata 5 and 6 is ON-OFF (mediated by both ON and OFF bipolar cells). The remaining 70% of ACs have broadly monostratified, multistratified, or diffuse dendrites, and they integrate bipolar cell signals through layer-by-layer summation: ACs with dendrites ramified in multiple strata exhibit DeltaI(C)s that are sums of DeltaI(C)s of individual strata. The light-evoked inhibitory chloride current, DeltaI(Cl), in strata 1, 2, and 4-6 is ON-OFF (mediated predominantly by ON-OFF ACs or ON ACs plus OFF ACs), and the DeltaI(Cl) in strata 3 and 7-10 is ON (mediated predominantly by ON ACs). This indicates that the amacrine-amacrine inhibitory synaptic circuitry in the IPL is asymmetrical in favor of the ON channels.