Influence of background illumination on fixation and visually guided saccades in the rhesus monkey.

The influence of background illumination on saccades towards small target LEDs was examined in three rhesus monkeys. In darkness, fixational saccades and those aimed at horizontal targets had a trajectory that was biased upward. This bias was not observed in the illuminated condition. For horizontal saccades, the magnitude of the vertical final errors depended on target eccentricity relative to starting eye position. Downward saccades undershot the location where eye position landed in the illuminated condition whereas upward saccades overshot less eccentric targets. Background illumination also influenced the latency of saccades. The change in accuracy that affects large saccades is interpreted as resulting from a change in the encoding of the desired displacement signal that feeds the local feedback loop controlling saccade trajectory.