Suppressive and facilitatory spatial interactions in amblyopic vision.

Amblyopic vision is characterized by reduced spatial resolution, and inhibitory spatial interactions ("crowding") that extend over long distances. The present paper had three goals: (1) To ask whether the extensive crowding in amblyopic vision is a consequence of a shift in the spatial scale of analysis. To test this we measured the extent of crowding for targets that were limited in their spatial frequency content, over a large range of target sizes and spatial frequencies. (2) To ask whether crowding in amblyopic vision can be explained on the basis of contrast masking by remote flanks. To test this hypothesis we measured and compared crowding in a direction-identification experiment with masking by remote flanks in a detection experiment. In each of the experiments our targets and flanks were comprised of Gabor features, thus allowing us to control the feature contrast, spatial frequency and orientation. (3) To examine the relationship between the suppressive and facilitatory interactions in amblyopic contrast detection and "crowding". Our results show that unlike the normal fovea [Levi, Klein, & Hariharan, Journal of Vision 2 (2002a) 140] crowding in amblyopia is neither scale invariant, nor is it attributable to simple contrast masking. Rather, our results suggest that suppressive spatial interactions in amblyopic vision extend over larger distances than in normal foveal vision, similar to peripheral vision of non-amblyopic observers [Levi, Hariharan, & Klein, Journal of Vision 2 (2002b) 167], for targets of the same size. Observers can easily detect the features that comprise our targets (Gabor patches) under conditions where crowding is strong. Thus, our speculation is that crowding occurs because the target and flanks are combined or pooled at a second stage that is coarse in the amblyopic visual system, following the stage of feature extraction. In amblyopic vision, this pooling takes place over a large spatial distance.