Comparison of Two Dimension Spatial Interaction Zones between Normal Vision and Anisometropic Amblyopia

Visual acuity of anisometropic amblyopes is often simulated in visually normal individuals by imposing blur on their foveal vision. However, their peripheral visual acuity and whether the same method can simulate their peripheral vision are unknown. We examined: acuity and shape of spatial interaction zones at the fovea and in the periphery, in normal participants and anisometropic amblyopes; and the effect of imposed blur on acuity at different eccentricities in normal participants to compare with the amblyopic periphery. Acuity was measured with Method of Constant Stimuli using 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 10 degrees in the lower visual field. In addition, foveal acuity of 4 normal participants was blurred to match the mean amblyopic acuity using Gaussian and optical defocus. Acuity was then re-measured across the lower visual field. Normal participants showed larger acuity deterioration with increasing eccentricity (E-2 of 2.25 +/- 0.21) than did anisometropic amblyopes (E-2 of 6.02 +/- 1.45). Both groups exhibited asymmetric crowding regions at all locations. Acuity was worse with optical blur compared to with Gaussian blur (significant blur type and eccentricity interaction, F(2.13,6.38)=4.93, p=0.049) but neither was significantly different from the acuity of amblyopes. Anisometropic amblyopia demonstrated similar asymmetric crowding regions to those found in normal vision, i.e. generally larger crowding for arrangements radial to the fixation point. Foveal acuity in anisometropic amblyopia was worse due to increased intrinsic blur relative to normal vision; however, their periphery appeared to be functionally normal.