Fig. 2.

The dark bar elicits a co-directional steering response, whereas the Fourier bar elicits a contra-directional orientation response. (A) Top: steering responses of 17 flies to clockwise (CW) and counter-clockwise (CCW) rotation of a Fourier bar (red) and a dark bar (black). Each trace is the mean response to multiple stimulus trials for a single fly. Bottom: population average responses to CW and CCW rotation of the Fourier bar (red) and the dark bar (black). Shaded regions indicate s.e.m., n=17. Arrows indicate the counter-directional turn toward the bar. Filled circles indicate the steering responses as the Fourier bar (red) and the dark bar (black) cross the visual midline. (B) Comparison of the ΔWBA value when the bar reaches the midline ‘zero crossing’ of the arena. Asterisks denote results of a paired t-test: P=1.20×10⁻⁷ for CW and P=6.25×10⁻⁷ for CCW. (C) Body angle recorded for a magnetically tethered fly in response to a bar revolving at 75 deg s⁻¹. Arrowheads indicate smooth body movements between saccades. (D) The angle between the fly's longitudinal body axis and the azimuthal position of the bar 300 ms segments before and after the bar crossed the fly's visual midline. n=26 flies for the Fourier bar and n=10 flies for the dark bar. Data are pooled from experiments in which the bars rotated at different speeds; thus, the gray line is merely indicative of the direction of bar movement, not its actual position. Mean body angle slope coefficients for the dark and Fourier bar are different from each other at the 99.9% confidence level for both CW and CCW rotation (linear regression, P<0.001, d.f.=29). For comparison, the right panel indicates single response trajectories from 17 flies in the rigid tether 1 s before and after the Fourier bar crosses the fly's visual midline.