Abstract
Introduction. Optometrists now utilize hand-held devices with enclosed stimulus displays to measure heterophorias. These instruments intend to accurately measure heterophorias at any simulated viewing distance; however, the proximity of the stimulus display necessitates minimization of the accommodative response when measuring heterophorias intended to reflect far viewing distances. We measured heterophorias with or without blur feedback to ask whether this minimization is best achieved via opening (OP) or clamping (CLA) the accommodative loop. Methods. The Eyelink 1000 Plus recorded the eye positions of seven observers under open loop fusional vergence conditions. Each observer wore a +3.00 ADD and viewed an enclosed stimulus display (33 cm) with red and blue dichroic filters. The stimulus, presented in primary gaze to the left eye, was blue and either opened (OP, a low spatial frequency DoG) or clamped (CLA, a high spatial frequency cross) the accommodative loop. Quaternions were used to estimate the horizontal distance heterophoria by first computing (kR, kL) and then comparing (p = kL * kR') the horizontal component of each quaternion across forty one-minute blocks. Results. The horizontal heterophoria across all blocks and participants in the OP and CLA conditions was convergent, with values (mean ± SEM) of -0.037 +/- 0.003 and -0.018 +/- 0.005, respectively. The lower limit of the 95% CI representing the difference (CLA – OP) between these two conditions fluctuated between values at or above zero throughout the course of the experiment. Conclusions. The presence of blur feedback promoted divergence; however, the convergent position of the eyes (~ 2° to 3°) in both conditions suggests a contribution from proximal vergence was present. Thus, the inclusion of monocular depth cues fostering a sense of farness is likely necessary to completely surmount the propiniquity of the stimulus display when measuring distance heterophorias in enclosed devices.