Synaptic physiology of direction selectivity in the retina

Detection of the direction of image movement is accomplished first in the retina by an elegant neuronal circuit, which integrates multiple levels of spatially asymmetric synaptic interactions among subsets of bipolar, amacrine and ganglion cells. Central to these interactions is the asymmetric GABAergic inhibition exerted by the starburst amacrine cell (SAC), a cholinergic and GABAergic interneuron with a radially symmetric dendritic tree. SACs make reciprocal GABAergic synapses on each other to create a direct inhibitory receptive field surround, which suppresses the response of each SAC to centripetal image movement. Each radially projecting branch of a SAC responds to image movement with a centrifugal bias and, through directionally asymmetric synaptic connections with the dendrites of direction-selective ganglion cells (DSGCs), exerts a spatially offset inhibition that vetoes the response of DSGCs to image movement in a specific (null) direction. Recent physiological studies have greatly advanced our understanding of the mechanism of direction selectivity and also revealed a new level of complexity that remains to be understood.