Astigmatic blur alters the spatial frequency tuning characteristics of stereopsis : magnitude and orientation

Abstract

Purpose : It is well documented that uncorrected astigmatism results in reduced stereoacuity. However, previous studies have used tests that are broadband in their spatial frequency content. Our current study aimed to understand how astigmatism affects the spatial frequency tuning of stereopsis. 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, 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 (in front / behind) relative to the four references. Four young adult observers with normal visual acuity were tested. Plus cylindrical lenses were used to induce astigmatic blur in the two eyes. Appropriate plus lens powers were added to the distance correction in order to prevent accommodation for the viewing distances. Stereothresholds were measured for a range of against-the-rule astigmatism: from 0 to 2 D. Results : The stereoacuity versus spatial frequency function is the inverse of a typical contrast sensitivity function, with the optimum spatial frequency at 5-10 cyc/deg. Increasing the astigmatic blur elevated stereo thresholds selectively at high spatial frequencies, gradually shifting the optimum frequency to lower spatial frequencies. Stereopsis for low frequency targets was only mildly affected even with high astigmatism. We also obtained similar frequency-dependent patterns for oblique astigmatism, but interestingly, the deterioration effects on stereoacuity were not as much as against-the-rule astigmatism. As expected, with-the-rule astigmatism did not substantially affect stereoacuity. Conclusions : Here we show that astigmatic blur results in spatial frequency specific losses of stereopsis. These findings have clinical implications for understanding both the sparing of coarse stereopsis and the deficits in fine stereopsis in meridional amblyopia.