The oculomotor "twitch"--a transient response to target motion.

The oculomotor "twitch" is a small, transient eye movement that occurs in response to onset of target motion during both passive and active regard of the target (Wyatt and Pola 1985 a, 1987). Among the properties described here, primarily tested with subjects remaining passive, the twitch (i) is elicited extrafoveally, but maximally at and near the fovea, (ii) often persists as a distinct early transient response even when target motion is onset of a simple ramp, and (iii) remains approximately constant over a wide range of target motions, varying only with direction. We suggest that the twitch reflects the working of a mechanism analyzing the direction in which a target starts to move.