Vernier acuity, crowding and amblyopia.

When a vernier target is flanked by a pair of optimally positioned flanks, offset discrimination is strongly degraded. Spatial interference with vernier acuity was studied in each eye of observers with unilateral amblyopia associated with strabismus, anisometropia or both, and were compared to the functions obtained in the normal periphery (Levi et al., 1985). The results showed that: (1) For both strabismic and anisometropic amblyopes, as in normal central and peripheral vision, the extent of spatial interference was proportional to the unflanked vernier threshold. (2) For anisometropic amblyopes, grating and vernier acuity are affected similarly. (3) For strabismic amblyopes, like the normal periphery, vernier and grating acuity are decoupled, with vernier falling off faster than grating acuity. (4) The preferred eyes of strabismic but not anisometropic amblyopes have poorer vernier acuity than the normal controls. A conceptual framework for amblyopia based upon spatial filtering and spatial sampling is discussed.