Fig. 3. Visual input influences the direction but not the amplitude of a saccade. (A) Approach angle is defined as the angle that a continuation of the trajectory to the wall of the arena would make with the line perpendicular to the tangent at the intersection point. Approach angle is used as a rough measure of the asymmetry of visual motion experienced by the fly prior to the saccade. Positive approach angles indicate that the fly is closer to the arena wall on its left (L) side, and thus that the visual motion perceived on the left side is greater. Negative approach angles indicate that the perception of visual motion is stronger on the fly’s right (R) side. (B) Saccade angle plotted against approach angle for 1579 saccades from trajectories from 36 flies flying within a textured background. The two clusters around ±90° demonstrate that the fly does not alter the amplitude of the saccade on the basis of asymmetries in visual motions. Red lines show linear regressions for each cluster (r²<0.01, P>0.5 for the upper line both regressions, P>0.25 for the lower line). The histogram to the right of the scatterplot shows the distribution of saccade angles pooled over all measurements. (C) The probability of turning left or right depends on approach angle. To generate the probability distributions, saccade angles were binned according to approach angle. Each bin was 5° wide, and bin centers were separated by 5°.