Fig. 2. Spontaneous behavior in the flight arena and methods used to quantify saccades. (A) Orientation data sampled every 10 ms (101 Hz) reveal periods of rapid turning (top). The small, slow (0.5 Hz) oscillations (arrow) were noticeable by eye, and do not represent tracking noise. The orientation data were low-pass filtered at 25 Hz before calculating the angular velocity (bottom). The saccade threshold (broken line) was set 4 s.d. away from 0, at about 350° s^-1. (B) Magnification of the region within the shaded box from A, containing one saccade. Saccade duration (green lines) was the time during which the fly's angular velocity exceeded one-quarter of its peak angular velocity during that saccade (broken line). Saccade amplitude (blue lines) was the difference between the median of the 5 points before and after the saccade.