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

To characterize dynamic properties of combined saccade-vergence eye movements that occur as the point of visual fixation is shifted between objects lying in different directions and at different depths.

Using the scleral search-coil technique, eye movements were measured in 10 normal subjects as they made voluntary, disjunctive gaze shifts comprising a range of saccades and vergence movements.

By analyzing eye acceleration records, the authors identified small-amplitude (0.2-0.7 degrees), high-frequency (23-33 Hz), conjugate horizontal oscillations of the eyes during the vergence movement that followed the initial saccade. When the shift of the fixation point required a large vergence component (17 degrees , every subject showed these oscillations; they were present in approximately a third of responses. Approximately 5% of responses showed oscillations that had horizontal and vertical components. Oscillations were less prominent with shifts that had smaller vergence components and were absent after saccades made between targets located at optical infinity.

These findings suggest that a common mechanism gates both the saccadic and vergence components of disjunctive gaze shifts, a likely candidate being the pontine omnipause neurons. When a saccade is immediately followed 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 experimental strategy to induce saccadic oscillations.