Raffles Xingqi Zhu¹ (), Robert F. Hess¹, Alex S. Baldwin¹; ¹McGill University

Behavioural research in amblyopia has shown evidence of a “spatial scrambling”. These effects would be consistent with a topographical disorganization affecting neural projections in the visual system. We investigated where in the processing hierarchy this scrambling might occur, and how it might contribute to the letter acuity deficit that is used clinically to characterize the severity of amblyopia. We constructed our stimuli using a physiologically-inspired algorithm. We consider two possible sites for scrambling to occur, either affecting: i) the outputs of log-Gabor filters (which mimic the oriented simple cell receptive field in V1), or ii) the inputs from isotropic subunits (i.e. LGN afferents) that form those oriented receptive fields. We refer to these image space manipulations as cortical and subcortical scrambling (CS and SCS), respectively. These distortions differ from typical additive noise such as bandpass noise, which we also tested for the purpose of comparison. The experiment involved a monocular task where the participant identified one of four possible noisy letters (bandpass and centered at 3 cycles/letter). Their sizes were rendered at half of the eye’s acuity limit, obtained through a separate monocular letter acuity experiment. We hypothesized that amblyopic participants should exhibit different patterns of mistakes (obtained from confusion matrices) based on viewing condition as a result of spatial scrambling for amblyopic eye viewing. In total 18 amblyopic (13 strabismic, 5 anisometropic) participated in this study. We found moderate correlations between the ratio of interocular acuity difference and our metric of the mistake pattern divergence in the two eyes when stimuli were affected by scrambling noise (SCS: r=0.65, P=0.0038; CS: r=0.47, P=0.047) but not for letters shown in bandpass noise (P=0.35). These results suggest neural scrambling, as early as the monocular inputs to the cortex, may play an important role in limiting letter acuity in amblyopia.