Amblyopic and peripheral vernier acuity: a test-pedestal approach

This manuscript is concerned with three visual systems with degraded spatial vision: (i) anisometropic amblyopia; (ii) strabismic amblyopia; and (iii) the normal periphery. The question we ask here, is whether the poor positional acuity of each of these visual systems can be understood on the basis of reduced sensitivity to the local contrast information in the stimulus. To answer this question, we use a "test-pedestal" approach to position acuity. In the first experiment we measure our observers' thresholds for detecting both the pedestal stimuli (edges and lines) and the test or cue stimuli (lines and dipoles). This approach also provides an estimate of the size of the spatial pooling (integration) region for the local contrast cue. In experiments two and three, we measure line and edge vernier acuity as a function of contrast, and compare the losses to those found for the detection of the respective offset cues. The local contrast hypothesis predicts similar losses in vernier acuity and in "cue" detection in amblyopic or peripheral vision. Moreover, the precise form of the contrast response function can provide insights into the nature of the loss, and places constraints on the likely models for amblyopic or peripheral vision. Our results suggest that the loss in vernier acuity of our anisometropic amblyopes can be understood on the basis of the reduced local contrast sensitivity and by increased spatial pooling. In strabismic amblyopes and in the normal periphery, there appears to be an extra loss, which cannot be accounted for by either reduced local contrast sensitivity or by increased spatial pooling. Additional experiments and computational modeling suggest that the "extra" loss is not due to spatial undersampling or additive positional jitter, but rather results from positional noise at a "second" stage.