Model-Based Understanding of Clinical Vergence Testing

Abstract

Both symmetric and asymmetric disparity vergence have been investigated extensively over the past 50 years to gain a more complete understanding of the sensory, motor, and perceptual processes subserving and controlling binocular fusion, which is the primary goal of the vergence system (Schor and Ciuffreda, 1983; Ciuffreda and Tannen, 1995). Pure symmetric vergence can be elicited when a target is displaced in depth precisely along one’s egocentric midline. In this case, the retinal disparity is equally distributed between the two eyes (Fig. 17.1A). The response consists of a saccade-free, relatively slow and smooth, symmetric disparity vergence tracking movement in depth. On the other hand, asymmetric vergence can be elicited when a target is displaced in depth anywhere except precisely along the egocentric midline, as is true under most naturalistic conditions. In this case, the initial retinal disparity is unequally distributed between the two eyes (Fig. 17.1B). However, now the response consists of a rapid saccade and a relatively slower disparity vergence movement, with the conjugate saccade functioning to shift the eyes laterally such that the retinal disparity is once again symmetrically distributed between the two eyes for symmetric disparity vergence to correct the residual bifixation error within foveal Panum’s fusional areas (PFA). Furthermore, there is the special case of line-of-sight asymmetric vergence, in which the target is moved along the line-of-sight of one eye (Fig. 17.1C). Here too both a saccade and a disparity vergence movement are executed, as described earlier for the more general case of asymmetric vergence (Fig. 17.1B).