Contrast detection and orientation discrimination thresholds associated with meridional amblyopia.

Humans who have astigmatism resulting in meridional amblyopia exhibit deficits in performing visual tasks at or near detection thresholds. However, there is mounting evidence supporting the idea that performance at threshold may not reliably predict visual capabilities at supra-threshold levels of stimulation. In this study the threshold and supra-threshold performance of six meridional amblyopes were compared. A difference in the pattern of oblique effects was observed between contrast detection thresholds and supra-threshold orientation discriminations. This suggests there exists an independence between populations of neurons subserving contrast detection and the discrimination of visual stimuli. Meridional amblyopia may primarily involve a degradation in those mechanisms subserving visual contrast detection. Populations of cells subserving supra-threshold abilities such as orientation discrimination remain relatively unaffected in humans exhibiting meridional amblyopia.