Perceptual distortions in human amblyopia

Amblyopia is a developmental disorder of spatial vision, largely characterized by reduced monocular visual acuity and contrast sensitivity. These deficits are accompanied by supra-threshold perceptual distortions that have received much less empirical attention. Here, we examine these distortions in adult amblyopia using a novel, binocular space bisection task. Using red-green filters, we independently presented a probe and target stimulus to fellow and amblyopic eyes, respectively. The probe was randomly positioned at one of 32 positions (4 eccentricities x 8 axes); observers had to maintain central fixation (cross) and move the target from fixation to a position diametrically opposite the probe. The resulting maps of spatial distortions in 18 strabismic and 6 anisometropic amblyopic observers revealed errors in local binocular alignments (radial and tangential), which can be visualized as compressions, spatial offsets and expansions at all sampled locations. We identified distinct patterns of distortions across clinical subtypes of amblyopia. Maps from observers with anisometropic amblyopia were similar to normals, but showed increased positional variance. Global spatial offsets in the majority of strabismic maps corresponded with the angle of squint, whereas others revealed a pattern consistent with harmonious anomalous retinal correspondence. A small number of strabismic subjects showed unique compressions that were not easily explained by their clinical presentation. The magnitudes of error across eccentricities, when expressed in cortical units, yield an index of compromised cortical representation that differentiates between clinical groups. Overall, the task yields a rich quantitative characterization of perceptual anomalies in amblyopia, with potential for diagnostic applications in other visually impaired populations.