Spatial-frequency properties of letter identification in amblyopia

Amblyopia is characterized by a deficit in identifying small letters (acuity deficit) in the absence of identifiable ocular pathology. One explanation for this deficit is that the amblyopic visual system lacks appropriate channels tuned to high spatial frequencies for identifying small letters. The purpose of this study was to examine the spatial-frequency properties of letter identification in the amblyopic visual system. To do so, we measured contrast thresholds for identifying letters that were band-pass filtered to different bands of spatial frequencies, for letter sizes ranging from 2x to 19.2x larger than acuity letters. Letters were digitally filtered using a set of band-pass filters, with peak object spatial frequencies ranging from 0.88 to 10 c/letter. The bandwidth of the filters was 1 octave. For any given letter size, contrast sensitivity for identifying letters exhibits a spatial-tuning function. The shape of these tuning functions was found to be similar between amblyopic and non-amblyopic eyes, and across all letter sizes. The peak of these functions shifted progressively toward lower object spatial frequency when the letter size became smaller. When compared with the non-amblyopic eyes, the amblyopic eyes have a limited range of tuning functions sensitive to letters. However, when scaled with respect to acuity, the relationship between the peak frequency of the tuning functions and letter size becomes essentially identical in the amblyopic and non-amblyopic eyes. An ideal-observer analysis that takes into account spectral information about letter identity and the contrast-sensitivity function of the observer, but does not invoke narrow-band channels, also shows that the properties of the tuning functions for letter identification are similar between the amblyopic and non-amblyopic eyes. We conclude that the deficit in identifying small letters in amblyopes is not attributable to differences in the shape or selection of "channels", when compared with the normal visual system. Rather, it is a consequence of the difference in the resolution limit between the amblyopic and the non-amblyopic eyes.