Figure 3. HS dendrites exhibit sublinear summation of local motion signals.

A: Stimulus screen, over time (left) and space-time plots (right) depicting the local visual stimulus. A single moving dark edge expanded in the PD, covering 13.5° of visual space over either 0.5 or 0.25 seconds (stimulus speed = 27°/sec or 54°/sec) before disappearing. We presented dark edges alone (“single edge”) or in pairs (“edge pair”) offset by either 13.5° or 27°. B: Average projection of GCaMP6f signals in an HSN dendritic branch. C-Q: GCaMP6f responses to moving dark edges in the HS dendrite shown in part B. C: XT plot showing the timing and position of a single moving dark edge, edge 1. D: GCaMP6f signals, plotted over time, in either the whole dendritic branch (yellow line in B) or smaller regions of interest (ROI i-iii, white dashed lines in B). E: Response of the dendritic branch to a single moving edge at position 1, with the baseline fluorescence subtracted. F-Q are the same as C-E, except with different stimuli: a single moving edge at position 2 (F-H), a single moving edge at position 3 (I-K), edges 1 and 2 at the same time (L-N), and edges 1 and 3 at the same time (O-Q). In M and P, the dashed lines are the expected responses, based on the sum of the responses to single edges, whereas the solid lines are the measured responses. Arrowheads in E, N and Q point to the distal portion of the dendrite stimulated by edge 1 (E). The response to edge 1 is strongly suppressed by the addition of edge 2 (N), but not edge 3 (Q). R-U: Sublinear responses to pairs of moving edges in HS dendrites. A metric capturing the nonlinearity of the response ((M-E)/E; zero indicates linear summation and negative and positive values indicate sublinear and supralinear summation, respectively) is plotted as a function of distance between the edges. Positive distances indicate offset in the preferred direction, whereas negative distances indicate offset in the opposite, non-preferred direction, as shown in A. Responses are shown for dark (R-S) and light edges (T-U) moving at either 27°/s (R,T) or 54°/s (S,U). Box and whisker plots show the median (black line), interquartile range (boxes), 1.5 times the interquartile range (whiskers). Dot plots showing each data point are overlaid on the box plots. N indicates the number of ROIs; the total number of flies for R, S, T, and U was 13, 5, 7, and 3, respectively. Asterisks indicate p < 0.01 (*), p < 10⁻⁵ (**), and p < 10⁻¹⁰ (**), Independent Sample T-tests. See also Figures S2–4.