Fig. 4.

Schematic diagrams interpreting the present results. a, A two-stage model for the generation of direction selectivity. The first stage creates motion sensitivity by symmetric facilitation (red lines). The interaction is shown (conventionally) as multiplicative, but a nondirectional facilitation could be additive without fundamentally changing the overall concept. The second stage creates direction selectivity by asymmetric inhibition (green lines). Both stages require delay components (ε). The two stages could occur in sequence or in parallel and could in principle be mediated by the starburst cell plus another amacrine or by the starburst cell alone. b, If the starburst cells mediate both the facilitation and the inhibition, their excitatory synapses (red dots) would need to contact the DS cell nonselectively to account for symmetrical facilitation (present results), but their inhibitory, GABAergic synapses (green dots) would contact them asymmetrically (only on the left sides of the starburst cells). Excitatory and inhibitory synapses are shown in a restricted set of dendritic crossings because the outputs of the starburst cells are restricted to the distal third of their dendrites (Famiglietti, 1991). This diagram shows only four starburst cells, a small subset of the total that cover each DS cell (Tauchi and Masland, 1984); in actuality, red and green dots would cover the entire dendritic arbor of the DS cell uniformly (Jeon et al., 2002).