Fig. 1.

Directional selectivity first emerges in the fly medulla, which is also densely innervated by octopamine neurons. (A) Schematic model of the early fly visual system, depicting the columnar neuron types that make up the ON motion circuit. Directional selectivity is synthesized in the dendrites of T4 cells from inputs conveyed by the nonselective neurons Mi9, Tm3, Mi1, and Mi4 (23). These T4 input neurons in turn receive major inputs from three types of lamina monopolar cells (L1, L3, L5). Only one example of each columnar cell type is shown here, but neurons of each type are present in every column of the visual system. The horizontal bars extending from each neuron’s axon indicate the location of fine branches and synaptic contacts. The excitatory synaptic inputs from Tm3 and Mi1 onto T4 are indicated with a downward arrowhead, while the inhibitory inputs from Mi9 and Mi4 onto T4 are indicated by an upturned arrowhead (signs are based on ref. 23; the relative spatial offsets between the cell types are based on ref. 27). The octopamine (OA) neurons project from the posterior slope of the central brain into the medulla (14); innervation layers of the octopamine cells are indicated with gray horizontal bars. (B) The expression pattern of octopamine neurons (magenta) and T4 cells (green) in the fly medulla (outlined in gray; genotype in Table S1). The octopamine neurons do not project into the lamina, and so octopamine modulation of the ON motion pathway is expected to occur downstream of the lamina.