Conjugate ocular oscillations during shifts of the direction and depth of visual fixation

PURPOSE. To characterize dynamic properties of combined saccade-vergence ey e movements that occur as the point of visual fixation is shifted between o bjects lying in different directions and at different depths. METHODS. Using the scleral search-coil technique, eye movements were measur ed in 10 normal subjects as they made voluntary, disjunctive gaze shifts co mprising a range of saccades and vergence movements. RESULTS. By analyzing eye acceleration records, the authors identified smal l-amplitude (0.2-0.7 degrees), high-frequency (23-33 Hz), conjugate horizon tal oscillations of the eyes during the vergence movement that followed the initial saccade. When the shift of the fixation point required a lai ge ve rgence component (17 degrees), every subject showed these oscillations; the y were present in approximately a third of responses. Approximately 5% of r esponses showed oscillations that had horizontal and vertical components. O scillations were less prominent with shifts that had smaller vergence compo nents and were absent after saccades made between targets located at optica l infinity. CONCLUSIONS. These findings suggest that a common mechanism gates both the saccadic and vergence components of disjunctive gaze shifts, a likely candi date being the pontine omnipause neurons. When a saccade is immediately fol lowed by a prolonged vergence movement, the omnipause neurons remain silent , leading to small-amplitude saccadic oscillations. Shifts in the point of visual fixation that require a large vergence movement may be a useful expe rimental strategy to induce saccadic oscillations.