Separate adaptive mechanisms for the control of reactive and volitional saccadic eye movements.

Adaptive reduction of the gain of the saccadic system was induced by means of two basically different paradigms. In the first approach the subjects had to follow a step-wise moving target. During each follow-up saccade the target was systematically displaced by 25% of the initial step, into the opposite direction of the saccade. In the second approach the subjects scanned a display of six small items. During each scanning saccade the whole display was displaced by 25% into the opposite direction of the saccade. Both conditions lead to fast and consistent saccadic gain reductions. However, adaptation with the stepping target did not transfer to the saccades in the scanning situation, nor to delayed saccades in an overlap paradigm, nor to memory-guided saccades. Conversely, when saccades were adapted in the scanning situation, induced gain changes transferred to overlap and memory-guided saccades, but not to saccades following steps of a single target. The results suggest that two separate and largely independent mechanisms are involved in the generation of reactive, stimulus-triggered and volitional, internally generated saccades, respectively. Both types of responses can be selectively adapted.