Contrast-coding in amblyopia. II. On the physiological basis of contrast recruitment

The perceptual compensations that result from a disruption to early visual development (amblyopia) were investigated to highlight the adaptative capabilities of normal contrast-coding. In anisometropic and meridional ambylopia, contrast thresholds may be raised without sub­stantial impairment to the perception of high contrasts. This accelerating response linking the anomalous threshold function with normal suprathreshold behaviour is similar to that observed for loudness perception in cases of inner ear damage and termed recruitment. This phenomenon is not purely psychological adaptation to amblyopia because magnitude estimation of contrast change within the ‘recruiting range’ is not disturbed. Three possible physiological explanations for contrast recruit­ment are examined, namely (i) recruitment of different-frequency neu­rons, (ii) recruitment of different-threshold neurons and (iii) a gain change in the response–contrast function of individual neurons. Since contrast­-matching in the presence of band-limited noise designed to raise artificially the threshold of adjacent detectors does not alter the recruiting response, it is unlikely that ‘across-frequency’ recruiting represents an adequate explanation. Similarly, it is argued that since the incremental sensitivity to contrast change is not enhanced in the recruiting region it is also unlikely the amblyopic neurons have steeper response–contrast relations (gain) than normal. The remaining and more likely possibility, in the light of recent neurophysiological findings of separate low and high threshold systems in the primate, is that recruitment in amblyopia occurs across neurons with the same spatial but different threshold characteristics. This suggests that normal contrast perception is subserved by more than one contrast mechanism and that in amblyopia the more sensitive contrast channel (lower threshold) is selectively affected.