Does microstimulation evoke fixed-vector saccades by generating their vector or by specifying their goal?

Summary

Electrical stimulation was performed at several sites of the monkey's forebrain producing fixed-vector saccades when the eyes were steady. When the same stimulation was applied during or immediately after a spontaneous eye movement, the saccade trajectory was considerably modified: the eyes were driven, from wherever they were deviated by the spontaneous movement, to the point where the fixed-vector saccade would have brought them if the spontaneous movement had not occurred. This finding implies first, that saccades evoked from these sites are directed toward a goal and, second, that the goal is defined with respect to an eye position sampled long in advance (before the spontaneous eye movement). This is consistent with the hypothesis that the electrical stimulation evoked the retinotopic representation of a target whose spatial coordinates were then computed at further stages to produce a saccade. Using the present paradigm, it may be possible to distinguish brain sites processing retinal error (at the visual stage) from those processing motor error (at the motor stage).