Walsh & Hoyt: Vergence System

Vergence eye movements permit stereopsis and prevent diplopia. By moving the eyes in opposite horizontal directions, the vergence system maintains the image of an object on the fovea of each eye, whether the object is located far away or nearby and whether it is moving toward or away from the viewer. Fixation of a near object induces not only convergence but also accommodation of the lenses and constriction of the pupils; these three responses constitute the near reaction. Two major types of vergence stimuli may be distinguished: retinal blur and retinal disparity. Retinal blur refers to a loss of image sharpness, while retinal disparity is the separation of images of a single object such that they fall on noncorresponding parts of the retina. Retinal blur elicits accommodative vergence and retinal disparity elicits fusional vergence. In most natural circumstances, both stimuli combine to produce appropriate vergence eye movements; however, accommodative and fusional vergence can be tested separately under experimental conditions. Voluntary control of convergence without visual stimuli, and, to a minor degree, of divergence, is variable among individuals.