Figure 4. Sublinear summation of local motion signals by T4/T5 neurons.

A: Stimulus screen, over time (left) and space-time plots (right) depicting the local visual stimulus (as in Figure 3A). B: Average projection of GCaMP6f signals in T4 and T5 axons. Three ROIs are demarked by yellow circles. C-Q: GCaMP6f responses to dark edges moving 27°/s in the T4/T5 axon terminals shown in B. C,F,I,L,O: Space-time plots of the stimulus. D,G,J,M,P: GCaMP6F signals, plotted over time, in each of the three ROIs in B. E,H,K,N,Q: Responses of T4/T5 axon terminals to moving dark edges, with baseline fluorescence subtracted. The dashed lines in M and P are the expected responses, based on the sum of the responses to single edges, whereas the solid lines are the measured responses. The arrowheads in E, N and Q point to the T4/T5 axon terminals stimulated by edge 1 (E) and suppressed by edge 2 (N) but not edge 3 (Q). R-U: Sublinear responses to pairs of moving edges in T4/T5 axons. Responses are shown for dark (R-S) and light edges (T-U) moving at either 27°/s (Q,S) or 54°/s (R,T). The nonlinearity of the responses is captured using the same metric as in Figure 3. N indicates the number of ROIs; the total number of flies for R, S, T, and U was 12, 5, 5, and 5, respectively. Asterisks indicate p < 0.01 (*), p < 10⁻⁵ (**), and p < 10⁻¹⁰ (**), Independent Sample T-tests. See also Figure S5.