The Brainstem Switch for Gaze Shifts in Humans

Saccades are rapid eye movements that turn both eyes in the same direction. Brainstem omnipause neurons gate saccades. Most natural shifts of the fixation point are between targets lying in different directions and at different distances requiring a combination of saccades and vergence. We investigated whether the visual stimulus, the saccadic command or the vergence command turns off omnipause neurons during gaze shifts. Using the scleral search coil technique, eye movements were measured in seven normal subjects, as they made voluntary, disjunctive gaze shifts comprising saccades and vergence movements. Conjugate oscillations of small amplitude and high frequency were identified during the vergence movement that followed the initial saccade. These oscillations, which are an indication that the omnipause neurons are turned off, corresponded to the saccade onset, and not the vergence eye movement or the visual stimulus. These data were used to test and modify a model of the human saccade-vergence system.