Neural plasticity in adults with amblyopia

Purpose: To determine the nature and limits of cortical plasticity in human adults with naturally occurring amblyopia. Methods: Our key measure was a psychophysical estimate of Vernier acuity, since amblyopes have marked deficits in Vernier acuity which are highly correlated with their loss of Snellen acuity. Our experiment consisted of three phases: (1) pre-training measures of Vernier and detection thresholds in each eye for several line orientations. (2) a training phase where each observer repetitively trained on the Vernier task at a specific orientation until they had completed 4000-5000 trials. (3) post-training measures (identical to 1). Since we were interested in perceptual learning, as opposed to simply learning the psychophysical technique, or learning a strategy for making psychophysical observations with an amblyopic eye, five of the seven observers had previous experience in making Vernier and detection judgments (at a different orientation) using our signal-detection methodology. Results: All observers showed significant improvement in Vernier acuity in the trained orientation. Averaged across observers, the improvement was on the order of 50% (the Figure shows Vernier thresholds for each session normalized by the pre-training threshold, and averaged across observers). Learning was strongest at the trained orientation; did not transfer to an untrained task (detection), but did transfer partially to the untrained eye, primarily at the trained orientation. Interestingly, in the two observers tested, the improvement in Vernier acuity was accompanied by an improvement in Snellen acuity. Conclusions: Some adults with naturally occurring amblyopia retain a significant degree of neural plasticity. Their perceptual learning appears to reflect alterations which are, at least in part, in early neural processes that are orientation specific and are localized beyond the site of convergence of the two eyes. (Graph Presented).