Interocular acuity differences alter the size tuning function of stereopsis

Abstract

Purpose: Our recent data show that interocular differences in visual acuity results in spatial frequency specific losses of stereopsis. The Gabor targets adopted in those studies contain both Gaussian envelope cue and carrier spatial frequency cue. The purpose of this study was to examine how monocular blurring affects the envelope size tuning of stereoscopic perception.

Methods: The visual stimulus consisted of two horizontally separated square blocks, one presented to each eye. Each block contained a Gaussian target patch surrounded by four Gaussian reference patches. Binocular disparity was introduced by shifting the two Gaussian targets in opposite directions (controlled by 2 interleaved staircases), and a haploscope was used to enable binocular fusion. Gaussian standard derivation ranged from 1.75-28 arcmin. The visual task was to determine the stereoscopic depth of the Gaussian target (crossed disparity: in front of / uncrossed disparity: behind) relative to the four references. Four adult observers with corrected-to-normal vision were tested. Bangerter foils were used to reduce visual acuity in the dominant eye. Stereothresholds were measured for a range of acuity difference: from 1 to 8 letter-lines (D1-8) on a standard LogMAR letter chart.

Results: The stereoacuity versus stimulus size function is basically the inverse of a typical contrast sensitivity function, with the optimum size tuning at 2.3 arcmin. Increasing the interocular acuity difference degrades stereo thresholds more substantially for small targets, gradually shifting the optimum tuning to larger targets (D8: 7 arcmin). Interestingly, stereopsis for large targets was only modestly affected even with an acuity difference of as much as eight letter-lines (0.8 LogMAR).

Conclusions: The current study shows that Interocular acuity differences alter the size tuning of stereopsis. Our findings have important clinical implications for understanding both the sparing of coarse stereopsis and the deficits in fine stereopsis in anisometropic amblyopia.

Keywords: 434 binocular vision/stereopsis • 417 amblyopia • 676 refraction