The computation of directional selectivity in the retina occurs presynaptic to the ganglion cell.

Directional selectivity is a response that is greater for a visual stimulus moving in one (PREF) direction than for the opposite (NULL) direction, and its computation in the vertebrate retina is a classical issue in functional neurophysiology. To date, most quantitative experimental studies have relied on extracellular responses for identifying properties of the directionally selective circuit. Here I describe an intracellular analysis using whole-cell patch recordings of the synaptic events underlying the spike response in directionally selective ganglion cells of the turtle retina. These quantitative measurements allowed me to distinguish among various explicit classes of circuit models that can, in principle, account for ganglion cell directional selectivity. I found that ganglion cell directional selectivity is due to an excitatory input that itself is directionally selective, and that the crucial shunting inhibition implicated in this computation must act on cells presynaptic to the ganglion cell.