Visual acuity and spatial interaction zones: investigating the periphery in anisometropic amblyopia

Abstract

Anisometropic amblyopia is often simulated in healthy individuals by imposing blur and measuring foveal visual acuity. We examine (1) acuity and the shape of spatial interaction zones at the fovea and in the periphery, in normal participants and anisometropic amblyopes and (2) the effect of imposed blur on acuity at different eccentricities in normal participants to compare with the amblyopic periphery. Acuity was measured using a Method of Constant Stimuli and Sheridan-Gardiner letters in 6 normal and 6 amblyopic participants. Crowding was assessed by comparing performance for a letter flanked by other letters placed at horizontal, vertical and oblique orientations. Stimuli were presented foveally and at 2.5, 5 and 10deg in the lower visual field. In addition, foveal acuity of 4 different normal participants was blurred to match the mean amblyopic acuity using Gaussian (σGaussian 4-6 arcmin) and optical defocus. Letter acuity was then measured across the lower visual field. Anisometropic amblyopes exhibit asymmetric crowding regions, similar to normal, at all locations. At the fovea, crowding extent is ~1.2× bigger horizontally, whereas in the periphery it is ~1.6× bigger vertically. Crowding depth for both groups is consistently larger when target and flankers are arranged vertically, than horizontally (0.3-2.0×; p<0.05). Without blur, normal participants show larger acuity deterioration with increasing eccentricity (E2 of 2.1±0.2) than do anisometropic amblyopes (E2 of 7.7±2.3). With Gaussian blur, normal participant acuity across eccentricity is not significantly different from that found for the anisometropic amblyopes (p=0.91; E2 of 6.3±0.8). Optical defocus reveals similar E2 results. Anisometropic amblyopia demonstrates similar horizontal-vertical asymmetry of crowding regions to those found in normal vision, i.e., generally larger crowding for target arrangements radial to the fixation point. Foveal acuity in anisometropic amblyopia is worse due to increased intrinsic blur relative to normal vision; however their periphery appears to be functionally normal.

Meeting abstract presented at VSS 2014