Fig. 7. Delay and steady-state balance of the expansion avoidance reflex depend upon the contrast frequency of large-field image motion. Flies had closed-loop control over the yaw position of the poles of expansion/contraction, while a pattern of stripes drifted at constant velocity. We periodically challenged the fly's closed loop responses to image expansion/contraction by reversing the drift direction, therefore exchanging the position of the two poles. (A) At each direction reversal, flies rapidly turn away from the pole of expansion to fixate the pole of contraction frontally. In this figure, drift direction is indicated by the polarity of the stimulus waveform. Dashed lines indicate pole positions along the y-axis. (B) Either doubling the drift velocity or (C) halving the functional wavelength of the pattern on one half of the arena resulted in a 20° shift in fixation towards the side of the arena showing the slower drift speed. (D) Increasing the drift velocity results in shorter delay to the onset of steady-state responses (N=13; ANOVA, F=14.7, P<0.01).