Fig. 1. Stimulus parameters. (A) Target parameters. As seen at the eye, a target is described by its angular width _x and angular speed . For a target of given absolute width b, these parameters correspond to a particular distance s and flight speed u. (B) Lines of constant target flight speed u (solid) and distance s (broken), in the plane of target angular speed and angular width _x. (C) Target contrast C falls with distance because more distant targets appear smaller and suffer more optical blur. The effect is greater in females than in males. (D) Stimulus parameters. The light stimuli delivered to male (solid line) and female (broken line) photoreceptors, simulating a 3° target moving at 100 deg. s^-1. Owing to superior optics, the male stimulus has a higher contrast C and a shorter width _c at half maximum amplitude.

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