Fig. 4. Stimulus expansion evokes saccades. (A) The time course of visual stimulation (stimulus half-angle, red trace), overlaid with the fly's orientation (black dots). The gray boxes during stimulus expansion and contraction represent the 500 ms time windows during which a saccade must occur to be considered `triggered' by the stimulus. The blue box is the time during which the spontaneous saccade rate was calculated. (B) Constant-velocity, accelerating and decelerating stimuli all elicit saccades (N=11 flies). (C) Stimuli expanding along only the horizontal, vertical or diagonal axes evoke saccades with the same probability as full-square expansion (N=9). (D) Stimuli approaching with different constant velocities have equal probability of triggering saccades. Stimuli with additional expanding edges (concentric squares) also evoke saccades at the same rate. Contraction of the low-velocity and concentric square stimuli inhibit the saccade behavior (N=15). ^*P<0.01; P<0.05, relative to the spontaneous rate. Total n=2933 saccades. The error bars represent the s.e.m.; the dotted line shows the spontaneous probability.