Dendritic Relationships between Cholinergic Amacrine Cells and Direction-Selective Retinal Ganglion Cells

Although the apparent simplicity of direction selectivity has fascinated retinal physiologists since the process was first characterized by Barlow & Levick (1965), its synaptic mechanism still eludes us. There is substantial cholinergic input to direction-selective (DS) retinal ganglion cells (Masland & Ames 1976, Ariel & Daw 1982) but the functions of cholinergic amacrine cells in complex visual processing are poorly understood. Direction selectivity is believed to arise from non-linear interactions between cholinergic and GABAergic inputs, either in the ganglion cell itself or at a presynaptic level. The recent finding that the cholinergic amacrines also contain γ-aminobutyric acid (GABA; Vaney & Young 1988) and its synthetic enzyme (GAD; Kosaka et al 1988, Brecha et al 1988), is compatible with a single type of interneuron mediating both the excitation and inhibition to DS ganglion cells (Dowling 1970).