Compound grating discrimination in extrafoveal and amblyopic vision

Compound grating discrimination was measured in normal foveal and extrafove al vision as well as in central vision of amblyopic subjects. Two types of discrimination were examined: 0 degrees versus 180 degrees and 90 degrees v ersus 270 degrees phase shift of the second harmonic relative to the fundam ental. In common with several previous studies, we found that both 0/180 an d 90/270 discriminations are possible in extrafoveal vision. However, we sh ow that differences in foveal and extrafoveal sensitivity can be eliminated for both types of discrimination by scaling the stimulus size appropriatel y. The extent of spatial magnification necessary to equate foveal and extra foveal performance differed markedly, with 90/270 discriminations requiring much more magnification. In the amblyopic subjects, the magnitude of the 9 0/270 deficit was greater than the 0/180 deficit in all six amblyopes teste d. In common with previous investigations, we suggest that the visual syste m adopts a discrimination strategy, based upon differences in local feature s, between the patterns to be discriminated. One process registers position al relationships, while a second process registers local contrast differenc es.. In this context, the reduced ability of the normal periphery and ambly opic fovea to perform mirror-symmetric discriminations is explained in term s of losses in positional acuity.