EXTRA-RECEPTIVE-FIELD MOTION FACILITATION IN ON-OFF DIRECTIONALLY SELECTIVE GANGLION-CELLS OF THE RABBIT RETINA

The excitatory receptive-field centers of On-Off directionally selecti ve (DS) ganglion cells of the rabbit retina correspond closely to the lateral extent of their dendritic arborizations. Some investigators ha ve hypothesized from this that theories for directional selectivity th at entail a lateral spread of excitation from outside the ganglion cel l dendritic tree, such as from starburst amacrine cells, are therefore untenable. We show here that significant motion facilitation is condu cted from well outside the classical excitatory receptive field center (and, therefore, dendritic arborization) of On-Off DS ganglion cells for preferred-direction, but not null-direction moving stimuli. These results are consistent with a role in directional selectivity for cell s with processes lying beyond the On-Off ganglion cell's excitatory re ceptive-field center. These results also highlight the fundamental dis tinction in retinal ganglion cell receptive-field organization between classical excitatory mechanisms and those that facilitate other excit ation without producing directly observable excitation by themselves.