Vernier and Contrast Identification in Amblyopic Subjects

Abstract

Abstract: : Purpose: We studied the depth of binocular suppression in amblyopes for vernier stimuli. Others have shown that monocular vernier identification thresholds are degraded more than the resolution acuity or contrast discrimination thresholds in amblyopic subjects (Levi & Klein 1982; Levi, Klein et al 1994). Holopigian et al. studied the depth of binocular suppression in amblyopes by comparing contrast increment detection threshold under monocular viewing and binocular suppression. We studied binocular suppression in vernier identification acuity. Methods: We measured vernier and contrast identification performance in the affected eyes of two amblyopes, monocularly and during binocular suppression. The difference between those thresholds was a measure of the depth of binocular suppression. Binocular dissociation was brought about using red-green anaglyphs. The non-amblyopic fixating eye was occluded for monocular viewing; in the binocular suppression condition, the fixating eye viewed a high-contrast grating. Our 3 normal control subjects viewed the suppression stimulus at full luminance, and the test stimuli via a 0.6 ND filter. The stimuli were dark, gray, and light 0.25deg horizontal stripes. Vernier stimuli contained an abrupt phase shift, and contrast stimuli contained a sharp transition in contrast. Pairs of vernier and contrast discrimination stimuli were adjusted to have equal Fourier energy spectra. The observer judged which half of the stimulus had higher contrast or was displaced upward, respectively. Results: 1. (1) Normal subjects made few errors in identifying the contrast stimuli, and the suppressing stimulus had little effect on performance. 2. (2) The refractive amblyope made more errors than normal subjects in identifying the vernier or contrast stimuli, but there was no significant suppression of either vernier or contrast performance. 3. (3) The strabismic/refractive amblyopic subject showed considerable suppression of both vernier and contrast performance, but the suppression was similar for the two tasks. Particularly, there was no selective suppression of vernier identification performance. Conclusions: Vernier identification was impaired in amblyopia, especially under conditions of binocular suppression. However, contrast identification was also impaired, and the depth of suppression was no greater for vernier identification than for contrast identification.

Keywords: amblyopia • strabismus • visual acuity