Fig. 8. Between saccades, asymmetries in visual motion cause deviation from straight flight, resulting in the fly turning away from the side experiencing the stronger visual motion signal. (A) Each inter-saccade flight segment was rotated and translated such that the initial trajectory, estimated by a regression through the first three points, was aligned downwards along the y-axis. Approach angle (defined in Fig. 3) was used to determine the side of the fly nearest to the wall of the arena, and the straight segments were separated and grouped accordingly. The overlaid plots demonstrate that the flies tend to deviate from straight flight by turning away from the nearest wall, particularly during flight within a textured background. (B) A plot of deviation angle against approach angle demonstrates that asymmetries in perceived visual motion cause flies to deviate from straight flight. Deviation angle is defined as the angle between the best-fitting straight line through the flight segment and the vertical axis. Linear fits yielded a slope of 0.26 (r2=0.11, P<0.001) with the textured background and a slope of 0.13 (r2=0.04, P<0.001) with the uniform background. The difference between the two slopes was significant (P<0.01, F-test). Uniform background data include 959 straight flight segments taken from 58 trajectories, textured data include 1231 straight segments from 36 trajectories.
