Interocular acuity differences alter the spatial frequency tuning of stereopsis

Abstract

Purpose: It is well known that interocular acuity differences result in reduced stereo acuity. However, previous studies have used tests that are broadband in their spatial frequency content. The purpose of the present study was to investigate the effects of interocular differences in acuity on the spatial frequency tuning of stereoscopic depth perception.

Methods: The visual stimulus consisted of two horizontally separated square blocks, one presented to each eye. Each block contained a Gabor target patch surrounded by four Gabor reference patches. Binocular disparity was introduced by shifting the two Gabor targets in opposite directions (controlled by 2 interleaved staircases), and a haploscope was used to enable binocular fusion. Stimulus spatial frequency ranged from 1-20 cyc/deg. The visual task was to determine the stereoscopic depth of the Gabor target (crossed disparity: in front of / uncrossed disparity: behind) relative to the four references. Five 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 on a standard LogMAR letter chart.

Results: The stereoacuity versus spatial frequency function is basically the inverse of a typical contrast sensitivity function, with the optimum spatial frequency at 5- 10 cyc/deg. Increasing the interocular acuity difference degrades stereo thresholds selectively at high spatial frequencies, gradually shifting the optimum frequency to lower spatial frequencies. Interestingly, stereopsis for low frequency targets was only mildly 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 result in spatial frequency specific losses of stereopsis. These findings have important clinical implications for understanding both the sparing of coarse stereopsis and the deficits in fine stereopsis in anisometropic amblyopia.

Keywords: 495 depth • 754 visual acuity • 584 low vision