The spatial localization deficit in amblyopia

There have now been numerous reports of a spatial localization deficit in amblyopia but none so far have tackled (1) the relationship between the contrast sensitivity and spatial localization deficits and (2) whether the spatial localization deficit is best described in units of visual angle or in terms of the underlying filter size. These issues are germane because they lie at the very heart of our understanding of the underlying deficit in amblyopia. To answer these questions we use spatially bandpass stimuli so that we can readily compare detection and localization for the same stimuli at each of a number of spatial scales. For some amblyopes (all strabismics and a minority of anisometropes) the contrast sensitivity defect neither underlies nor covaries with the spatial localization deficit. In the majority of anisometropic amblyopes, the contrast sensitivity loss is a complete description. The spatial localization deficit in amblyopia is of two independent kinds; positional inaccuracy and positional distortion. The positional inaccuracy deficit which can occur in varying degrees in both strabismic and anisometropic amblyopia, affects all spatial scales equally and therefore is best thought of in terms of a constant fraction of the underlying filter size in the space-frequency plane. The positional distortion deficit which can also occur to varying degrees in both strabismic and anisometropic forms can not be easily understood within this metric at least for strabismics.