Figure 5. Flies use body saccades to track a bar on a counter-rotating background, quickly switching to smooth movement between saccades, and saccades dynamics, but not saccade-triggering error angle, are influenced by ground speed.

A) The compound stimulus is depicted by a space-time diagram in shaded gray as in Figure 1D. Fly heading (blue) and error angle (red) between the fly and the bar. The fly switches between saccadic bar fixation and smooth movement. Note that the fly’s body angle lags behind the bar. B) Expanded view of the trajectory if a single saccade indicates rapid transitions between a bar pursuit saccade and smooth tracking of the ground. Median delay = 25 ms. C) Flies track the ground between saccades. Body angle (blue) is baseline-subtracted to show the time course of body angle between saccades during a bout of bar fixation. The yellow line denotes the best-fit line of the body angle data. n = 1,361 saccades from 10 animals. D) Top: space-time graph of task shown in A with bar and background revolving in opposite directions. Bottom: space-time graph in the fly’s reference frame showing rapid switching between saccades and smooth movement. The bar is highlighted in pink for visual comparison. E) Ground velocity has subtle but significant effect on saccade duration, amplitude, and peak angular velocity. F) Flies generate an initial torque followed by a counter torque. The black line shows the mean torque. Shaded areas are ± 1 STD. G) Initial peak torques and counter-torques are tuned to the ground speed. H) The error scales with ground speed and marginally for ISI whereas integrated error does not change. n = 1,361 saccades from 10 animals. See also Figure S5 and Movie S3.