Mammalian retinas contain about 20 types of
ganglion cells that respond to different aspects of the visual scene, including the direction of motion of objects in the visual field. The rabbit retina has long been thought to contain two distinct types of directionally selective (DS)
ganglion cell: a bistratified ON-OFF DS
ganglion cell that responds to onset and termination of light, and an ON DS
ganglion cell, which stratifies only in the ON layer and responds only to light onset. This division is challenged by targeted recordings from rabbit retina, which indicate that ON DS
ganglion cells occur in two discriminably different types. One of these is strongly tracer-coupled to amacrine cells; the other is never tracer-coupled. These two types also differ in branching pattern, stratification depth, relative latency, and transience of spiking. The sustained, uncoupled ON DS cell ramifies completely within the lower
cholinergic band and responds to
nicotine with continuous firing. In contrast, the transient, coupled ON DS
ganglion cell stratifies above the
cholinergic band and is not positioned to receive major input from
cholinergic amacrine cells, consistent with its modest response to the
cholinergic agonist nicotine. Much data have accrued that directional responses in the mammalian retina originate via
gamma-aminobutyric acid (
GABA) release from the dendrites of starburst amacrine cells (Euler et al., 2002). If there is an ON DS
ganglion cell that does not stratify in the starburst band, this suggests that its
GABA-dependent directional signals may be generated by a mechanism independent of starburst amacrine cells.