Characterizing the mechanisms of improvement for position discrimination in adult amblyopia.

Abstract

Abstract: : Purpose: Following practice, adults with amblyopia can improve their ability to judge position. Our goal was to study the neural mechanisms underlying this perceptual learning in position discrimination in adult amblyopia. Methods: To address this question we use an equivalent noise approach. Specifically, we measure positional acuity in noise in order to parse any changes during learning to two important factors: a decrease in equivalent input noise and/or an increase in the efficiency with which the stimulus information is used. The test stimulus was comprised of two horizontal segments with a 34 minute gap between the two segments. Each segment consisted of eight discrete Gabor patches with a vertical misalignment between the two segments. Positional noise was produced by distributing each Gabor patch around the mean line position according to a Gaussian probability function. We used a three–spatial–alternative forced–choice interleaved staircase paradigm to measure position discrimination thresholds in seven adult amblyopes. Observers were tested and trained (at least 10000 trials) at five noise levels (including zero). Trial–by–trial feedback was provided. Results: We found that 6 of the 7 observers showed systematic and significant improvements in positional acuity which were both eye and orientation specific. For those six observers, the improvements in thresholds was about 30% and 20% for zero and the highest noise level respectively. Asymptotic performance was reached in about six to seven sessions. There are important individual differences in learning mechanisms: three observers showed increased efficiency (mean 58%) with no change in equivalent input noise across sessions; two observers showed reduced equivalent input noise (mean 35%) with no change in efficiency, and one observer showed both a decrease in equivalent input noise (43%) and an increase in efficiency (24%). Interestingly all seven amblyopic observers showed substantial improvement (mean 33%) in single letter acuity. Three observers were also tested on a counting task and all three improved (52%) after practicing positional discrimination. Conclusions: Our results reveal the mechanisms underlying perceptual learning in amblyopic vision, and may provide a basis for developing more effective and efficient strategies for the treatment of amblyopia.

Keywords: amblyopia • learning • visual acuity