Fig. 1. Responses to large-field motion stimuli presented in open-loop conditions. At the onset of image motion, the fly generates a bilateral change in wing stroke amplitude that is highly correlated with yaw torque. (A) Uniform rotation across the entire visual field elicits a turning response in the same direction as the stimulus. (B) Motion confined to the front half of the visual field elicits a larger response compared with the full-field stimulus. (C) Motion across the rear visual field elicits a turning response in the opposite direction. The sum of separate front and rear field responses (dotted red lines in A) closely approximates the full-field response. (D) A lateral expansion/contraction stimulus with motion in opposite directions in the front and rear visual fields elicits the largest turning response. The dotted red line shows the sum of the responses to individual stimuli indicated in B and C. Each trace represents mean ± S.D. (shaded area) (N=10). In all cases, contrast frequency changed from 0 s^–1 to 10 s^–1 according to the motion stimulus trace. The scale bars indicate 1 V for the wingbeat amplitude and 10^–8 N m for torque. Wingbeat amplitude signals were normalized (see Materials and methods). (E) Effect of rear field contrast frequency on turning response, measured from changes in wing stroke amplitude. Contrast frequency in the front field was held constant at 10 s^–1 while the value in the rear field varied from –10 s^–1 to 10 s^–1. Negative values indicate motion in the same direction as the front field. Data points represent the mean values of the response ± S.D. (N=10). (F) Turning response amplitude varies with the azimuth of the focus of expansion (N=5). From –100 deg. s^–1 to 100 deg. s^–1, the turning response varies sigmoidally with the location of the focus of expansion. The response attenuates as the focus of expansion moves into the animal's rear field of view.