Fig. 1. Schematized experimental setup for measuring a fly's response to image expansion. During tethered flight, the fly's wingstroke amplitude and frequency are measured by optically tracking the shadows cast from an infra-red (IR) diode by each of the wings on an optical wing-beat analyzer (Dickinson and Lighton, 1995; Lehmann and Dickinson, 1997). During closed-loop experiments, the difference between the amplitude of each wing stroke controls the visual display, allowing the fly to orient actively toward the position of the 15°x15° square. At periodic intervals, the square symmetrically expands, eliciting a behavioral response.