The Physiology and Neurology of Vergence Eye Movements: An Update

Chi D Luu, Julie F Green

Human eye movements have been classified into three groups: reflex, voluntary and fusional (Bielschowsky, 1956). The reflex and voluntary eye movements are described as conjugate or versional movement which have the characteristics of both eyes moving the same direction of gaze. Reflex eye movements are controlled by the vestibulo-ocular and optokinetic systems. These two systems have very short latencies and exist to stabilised the eyes for compensatory head movements. Voluntary eye movements are controlled by the saccadic and smooth pursuit systems. The saccadic system has high level velocity eye movements to re-fixate targets from different gaze positions, whereas the smooth pursuit system involves low level eye movements which provide the advantage of tracking slowly moving targets.
The Vergence system is the controller for fusional eye movements. Unlike reflex and voluntary eye movements, innervation from the Vergence centre will initiate disjugate movements of the eyes, that is, both eyes moving in opposite directions. This type of movement is important in maintaining the state of binocular fixation at various viewing distances. This paper aims to give a summary of the physiology of vergence eye movements and to highlight recent findings concerning the neurology of the vergence system.