Contrast-coding in amblyopia. I. Differences in the neural basis of human amblyopia.

To shed light on the basis of normal contrast perception in general and its susceptibility during early visual development in particular the contrast-coding deficit of amblyopic eyes was investigated. This was accomplished by using two different but complementary paradigms, one involving equating the contrast sensations between the amblyopic and normal fellow eye and the other involving the assessment of incremental sensitivity at different contrast levels. Since human amblyopia is known to have three different forms, representatives of each were tested. These include strabismics, anisometropes and astigmats. The results of the contrast-matching approach suggest that (i) strabismic and anisometropic (including meridional) amblyopes show important differences in suprathreshold contrast matching (this difference, which occurs across different spatial frequencies, a t different luminances and for different field sizes, suggests a different neural basis for the two main forms of human amblyopia); (ii) all forms of amblyopia share one common feature, that of large threshold losses relative to the extent of the suprathreshold anomaly; (iii) the accelerating growth in subjective contrast above the raised threshold shown by the matching procedure is not evident in the assessment of incremental contrast sensitivity. From these results it is argued that, although contrast perception (in the low contrast range) is disturbed in amblyopia, amblyopia cannot be adequately understood in these terms alone. This raises important questions for our understanding of the physiological basis of contrast-coding in normal vision and its perceptual importance.