Spatial localization in normal and amblyopic vision.

Spatial localization was investigated for each eye of amblyopic observers using a bisection paradigm. The stimuli were comprised of a grating composed of bright lines, and a test line. The test line was either placed above the grating (bisection-no overlap) or within the row of lines comprising the grating (bisection-with overlap) and thresholds for each bisection task were measured as a function of the fundamental spatial frequency of the grating. Vernier thresholds were also measured. For the nonamblyopic eyes at low spatial frequencies, bisection thresholds were a constant fraction ("Weber" fraction) of the space to be bisected, while at high spatial frequencies thresholds were approximately a constant retinal distance (a hyperacuity). However the spatial localization of an amblyopic eye depends upon both the type of amblyopia, and the stimulus configuration. Specifically, for anisometropic amblyopia, spatial localization (bisection-no overlap) and vernier, when scaled to the resolution losses, were normal. However, spatial adjacency (bisection with overlap), while enhancing the spatial localization of nonamblyopic eyes at high spatial frequencies, markedly elevated thresholds in the amblyopic eyes of anisometropic amblyopes. Strabismic amblyopes on the other hand show disturbances in both spatial localization tasks which can not be accounted for on the basis of reduced resolution. Their results are characterized by an absence of a constant Weber fraction at low spatial frequencies and "crowding" effects at high spatial frequencies. For strabismic amblyopes, the optimal localization thresholds were similar to the Snellen threshold, while for anisometropic amblyopes, the optimal localization thresholds were several times better than the Snellen threshold.