Fig. 10. Large-field, unidirectional visual motion does not trigger saccades. (A) Large-field rotation calculation. A vector field representing the output of the local motion at a single instant of time is shown. The horizontal components of the visual motion perceived through the local motion detectors were spatially summed over each of the two frontal quadrants (together comprising 180° of azimuth) of the fly’s field of view (regions 2 and 3 in Fig. 9). For each saccade, the side away from which the fly turns is termed ipsilateral and the side towards which the fly turns is termed contralateral. Front-to-back rotation is plotted as positive for each side. Red arrows indicate the direction of large-field summation. (B) Event-triggered averages suggest that large-field horizontal motion does not trigger saccades. Each individual trace (gray lines) represents the time course of large-field horizontal motion (HIps and HCont) before and after each saccade. The y-axis of this and subsequent figures is a dimensionless quantity that represents the amplitude of the spatially summed output of the motion detectors without any sort of normalization (see Appendix for details). Each trace is aligned at the initiation of the saccade, referred to as time zero. The mean value is shown by the red lines; blue lines represent ±S.D. The gray regions of each plot indicate time after the initiation of each saccade. Large-field front-to-back rotation on both sides precedes each saccade during flight within a textured background (top traces), but is absent during flight within a uniform background (bottom traces). The time resolution of each of these traces was 6.67 ms. Individual traces were taken from 123 saccades from three flies within a textured background and 99 saccades from three flies within a uniform background. (C) Large-field vertical motion, calculated by spatially summing the output of the vertical elementary motion detectors. Upward motion is denoted as positive. (D) Event-triggered averages indicate that saccades are not triggered by vertical motion. Large-field downward motion (VIps and VCont) precedes saccades during flight within a textured background but not within a uniform background. The scaling of the y-axis is identical to that in B.
