Abstract
The vergence system is thought to rotate the eyes through equal but opposite angles for gaze shifts in depth. A conjugate system also exists to rotate the eyes through equal, but the same angles. These systems are assumed to move the eyes appropriately for a given target motion, i.e., the eyes should rotate oppositely when a target moves only in depth, and conjugately when there is no depth component. It might be that the conjugate system is turned off when only vergence is required. Alternatively, vergence modulates a conjugate system that is always engaged. Observers pursued a physical target (small letter “E”) moving in depth on a motorized track on the midline. The target moved with a periodic profile between 33.3 cm (3.0 dpt) and 66.7 cm (1.5 dpt), with a peak velocity of 30 cm/s and acceleration of 50 cm/s2. Viewing was either binocular, or monocular with either eye viewing. Eye movements and accommodation were measured from both eyes with a PlusOptix photorefractor, and in separate sessions, eye movements only with an EyeLink 1000. We found typical vergence in binocular viewing. However, when one eye was covered, it surprisingly did not always rotate inward to follow the target in depth. Instead, the covered eye usually moved in the opposite direction, often nearly conjugate with the viewing eye despite the presence of other monocular depth cues. The covered movements were also frequently different between the eyes of a subject. In all cases accommodation was symmetric and unaffected by viewing condition. The results suggest that a conjugate drive remains active during pursuit of objects moving in depth, which can be overridden by binocular input to produce vergence.
