Motion sensitivity and spatial undersampling in amblyopia

The nature of the visual deficit in human amblyopia has been keenly sought over the last decade. Some confusion has arisen as to whether the motion-sensitive mechanisms known to exist in normal vision are selectively affected in humans with amblyopia. To address this issue we compare contrast thresholds for detection and direction discrimination of drifting sine-wave gratings in a group of humans with amblyopia. The results suggest that over the vast majority of the spatio-temporal range, direction of motion can be discriminated at detection threshold. Over a narrow part of the visible range there is a differential loss of sensitivity for direction discrimination over that of simple detection. However such an effect also occurs for normal vision under spatially scaled conditions and it seems likely that it is mediated by non-motion sensitive mechanisms. We show that one possible cause of this loss of direction discrimination, namely spatial undersampling within the central region of the amblyopic visual field, is not a viable explanation.