Using visual noise to characterize amblyopic letter identification

Amblyopia is a much-studied but poorly understood developmental visual disorder that reduces acuity, profoundly reducing contrast sensitivity for small targets. Here we use visual noise to probe the letter identification process and characterize its impairment by amblyopia. We apply five levels of analysis - threshold, threshold in noise, equivalent noise, optical MTF, and noise modeling - to obtain a two-factor model of the amblyopic deficit: substantially reduced efficiency for small letters and negligibly increased cortical noise. Cortical noise, expressed as an equivalent input noise, varies among amblyopes but is roughly 1.4x normal, as though only 1/1.4 the normal number of cortical spikes are devoted to the amblyopic eye. This raises threshold contrast for large letters by a factor of radical1.4 = 1.2x, a negligible effect. All 16 amblyopic observers showed near-normal efficiency for large letters (> 4x acuity) and greatly reduced efficiency for small letters: 1/4 normal at 2x acuity and approaching 1/16 normal at acuity. Finding that the acuity loss represents a loss of efficiency rules out all models of amblyopia except those that predict the same sensitivity loss on blank and noisy backgrounds. One such model is the last-channel hypothesis, which supposes that the highest-spatial-frequency channels are missing, leaving the remaining highest-frequency channel struggling to identify the smallest letters. However, this hypothesis is rejected by critical band masking of letter identification, which shows that the channels used by the amblyopic eye have normal tuning for even the smallest letters. Finally, based on these results, we introduce a new "Dual Acuity" chart that promises to be a quick diagnostic test for amblyopia.