The amblyopic acuity deficit: Identification of letters distorted by spatial scrambling algorithms

The amblyopic deficit in letter acuity is often more severe than that predicted using periodic grating stimuli. One proposed explanation for this is spatial scrambling of the input received from the amblyopic eye. In this study, we generated letter stimuli simulating either: i) "cortical scrambling" at the output of oriented model "simple cells", or ii) "subcortical scrambling" of isotropic subunits forming the input to those simple cells. We used these stimuli to investigate whether amblyopic participants might be particularly vulnerable, or particularly resilient, to spatial scrambling. In the same experiments, we also investigated conventional "noise masking" with bandpass noise. We performed two letter identification experiments, equating the stimuli shown to each eye by normalising either: i) contrast, presenting stimuli at four times their contrast threshold; or ii) scale, presenting stimuli at twice the participant's threshold acuity for each eye. At the group level, we found participants using their amblyopic eyes were less efficient at performing letter identification in bandpass noise compared to fellow eyes or controls. We did not find a group-level difference when conducting equivalent comparisons with our two types of scrambling stimuli. Partitioning our participants by stereopsis ability revealed a deficit in identifying scrambled letters for participants with poor or absent stereopsis. By analysing the mistake responses, we found participants using amblyopic eyes showed distinct patterns of errors that correlated with the acuity deficit. These results demonstrate that our scrambled stimuli interrogate a component of amblyopic vision that is functionally distinct from that addressed by contrast noise masking.