Spatial scale shifts in amblyopia.

We used a masking paradigm to uncover the properties of the mechanisms engaged by the amblyopic visual system for vernier acuity and line detection. Line vernier and line detection thresholds were measured in the presence of one-dimensional noise masks varying in orientation, spatial frequency content or contrast. Our results reveal that in both normal and amblyopic eyes, there is a bimodal orientation tuning function for vernier acuity, i.e. vernier acuity is most strongly masked by mask orientations approx. +/- 10 deg on either side of the target lines. In contrast, in both normal and amblyopic eyes, line detection is most strongly masked when the mask and line target have the same orientation. In the normal fovea, the spatial frequency tuning is bandpass, with a peak spatial frequency of about 10 c/deg. In the amblyopic eyes, the spatial tuning is similar in specificity; however the peak is shifted to lower spatial frequencies, suggesting a shift in the scale of spatial processing of line stimuli. For all of the amblyopic eyes, the increased line detection thresholds are approximately proportional to the shift in spatial scale. In anisometropic amblyopes, the (unmasked) vernier threshold is elevated in proportion to the shift in spatial scale; however in some amblyopes with constant strabismus the shift in spatial scale is not sufficient to account for the degraded vernier acuity. The "extra" increase in vernier thresholds associated with strabismus may be a consequence of a high degree of positional uncertainty which adds noise at a stage following the combination of filter responses.