Brainstem mechanisms underlying control of large amplitude saccades

The primary aims of this study were to examine the dynamics of large amplitude saccades and to present a more accurate oculomotor model of the underlying brainstem mechanisms responsible for generating these movements. We examined large amplitude gaze movements under 2 conditions: 1) pure centrifugal from primary position up to the limits of the oculomotor range (50/spl deg/); and 2) combined centripetal & centrifugal movements across the midline. Initial eye position in the orbit markedly influences the neuronal activity in the pons required to generate both types of movements. Using a new analytical method for quantifying the differences in the accelerating and decelerating aspects of the saccades we demonstrate that the deceleration phase of eye increases dramatically with saccade amplitude, far exceeding the duration of the acceleration period. In addition, the deceleration phase differed amongst subjects and between centrifugal and combined movements. A new model of oculomotor control is proposed to account for these findings.