Corresponding and disparate retinal points in normal and anomalous correspondence

Fusional (disparity) vergence requires that the ocular images fall on noncorresponding (disparate) retinal points. Study of the distribution of corresponding retinal points is accomplished by analyzing the locus of object points in space that are seen either singly or in the same direction for each eye. This locus, a horopter, is easily determined for persons with binocular fixation but requires a special tangent plane apparatus (which is described) for persons with strabismus. The identical visual direction horopter for the intermittent exotrope under conditions of bifixation passes through the fixation point and curves sharply toward the eyes, a phenomenon that may produce disparity conflicts between the central and peripheral fields. For squinters with normal anomalous correspondence (where the two foveas correspond), the horopter passes through the intersection of the visual axes, for squinters with anomalous correspondence, it does not; instead, it courses through the space between the visual axes, which indicates correspondence between opposite occipital hemispheres. Additionally, these horopters exhibit a localized central convexity toward the eyes (a notch) that implies correspondence being less anomalous centrally than peripherally. In validating the presence of this horopter notch, a three-light test was developed. Irregular fusional vergence movements of squinters may be attributed in part to their irregular horopters.