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Fixation disparities (FD) were measured as a function of forced vergence using binocular scleral search coils and simultaneously with nonius lines. The slope of the objective FD curve was significantly greater than the subjective FD curve for three of five subjects. This indicates an alteration in retinal correspondence of up to one degree, that shifts Panum's area to avoid the diplopia normally present with large disparities. This process allows for fusion in the presence of large objective fixation disparities which would normally cause diplopia. The shift in correspondence enhances the range of forced vergence, since the larger objective FDs serve as more effective stimuli to fusional vergence. The remaining subjects who lacked this effect had "flat" FD curves indicative of high vergence adaptation.

Adaptation to the simple visual displacement of prisms was compared to that for refractive lenses, which have a varied prismatic effect. Subjects were made myopic using contact lenses, then corrected using spectacle lenses. The effect on the perceived direction of a randomly located target was assessed from pointing behavior. Prism adaptation showed a negative directional aftereffect but lacked intermanual transfer. Lens adaptation lacked a negative aftereffect but exhibited intermanual transfer. The results suggest that lens adaptation involves a recalibration of extraretinal eye movement information and multiple sets of lens adaptation can be retained for short periods.

The accuracy of directional localization with refractive lenses was determined by open-loop pointing for four myopic habitual contact lens wearers. The target was a single point of light at random locations (dark), or was embedded in a horizontal array of lights (light). In the dark, the regression slope of indicated vs actual target position was significantly less for spectacles than for contact lenses, as predicted from the prismatic effects of the spectacles. In the light, slopes for spectacles and contacts were not significantly different. These results suggest that spectacle wearers do not utilize eye position information in making direction judgments in the light. Copyright © 1996 Elsevier Science Ltd.

The spatial characteristics of the changes in retinal correspondence produced by forced convergence were studied. The vertical extent of lateral shifts in binocular correspondence were quantified by comparing the convergence of the eyes measured with binocular search coils to the convergence of the eyes as determined using nonius lines having vertical separations (gaps) between the nonius lines of 0.5-4.8 degrees. Lateral shifts in binocular correspondence only occurred for nonius gaps < 3-4 degrees. The effects of horizontal retinal eccentricity on lateral shifts in correspondence were determined by measuring the nonius horopter of the subject under forced convergence, using 11 nonius line eccentricities between 4.5 degrees left and right. The nonius horopter was shifted toward the fusion target maximally near the fixation point. There was no shift beyond 3 degrees of eccentricity. We conclude that the nonius horopter is 'dimpled' vertically and horizontally, facilitating local fusion by shifting the line horopter and the region of single binocular vision toward the point of regard over a region of 3 degrees around the fixation point.