RETINAL DIRECTION SELECTIVITY AFTER TARGETED LASER-ABLATION OF STARBURST AMACRINE CELLS

Directionally selective retinal ganglion cells respond strongly when a stimulus moves in their preferred direction, but respond little or no t at all when it moves in the opposite direction(1,2). This selectivit y represents a classic paradigm of computation by neural microcircuits , but its cellular mechanism remains obscure. The directionally select ive ganglion cells receive many synapses from a type of amacrine cell termed 'starburst' because of its regularly spaced, evenly radiating d endrites(3,4). Starburst amacrine cells have a synaptic asymmetry that has been proposed as the source of the directional response in the ga nglion cells(5,6). Here we report experiments that make this unlikely, and offer an alternative concept of the function of starburst cells. We labelled starburst cells in living retinas, then killed them by tar geted laser ablation while recording from individual directionally sel ective ganglion cells. Ablating starburst cells revealed no asymmetric contribution to the ganglion cell response. Instead of being directio n discriminators, the starburst cells appear to potentiate generically the responses of ganglion cells to moving stimuli. The origin of dire ction selectivity probably lies with another type of amacrine cell.