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Zahra Hussain, Ben Webb, Andrew Astle, Paul McGraw; Perceptual learning alleviates crowding in amblyopia and the normal periphery. Journal of Vision 2011;11(11):425. doi: 10.1167/11.11.425.
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Foveal acuity in amblyopia is crowded: letter identification is impaired by the presence of flankers. Here, adult amblyopic participants were trained on a flanked letter-identification task to assess whether practice reduces crowding in the fovea, and whether a reduction in crowding improves letter acuity. Participants with normal vision trained on the same task in the lower peripheral field, where there is marked crowding. The stimulus configuration comprised four flanking letters surrounding a central letter; the task was to identify the central letter. Before training, both unflanked and flanked letter acuity thresholds were measured, yielding a “crowding ratio” (flanked/unflanked letter acuity) for three different target-flanker separations. Letter spacing was always a fixed proportion of letter size. During training (8–14 days; 450–600 trials/day), participants repeatedly performed the flanked letter-identification task at a fixed letter size, whilst target-flanker separation was varied to obtain a letter spacing threshold. After training, unflanked and flanked acuity measurements were repeated. Pre- and post-training crowding ratios of the trained groups were compared to those obtained from control groups who did not train on the task. On Day 1, we found significant crowding effects in the amblyopic fovea and in the normal lower peripheral field. For all participants, training reduced spacing thresholds across sessions. After training, we found modest improvements in unflanked letter acuity, but marked improvements in flanked letter acuity at all letter separations, both for amblyopic and normal participants. Amblyopic letter acuity improved, on average, by 0.15 logMAR. Normal participants' improvements were largely specific to the trained visual field. Crowding ratios were unchanged for the control groups. We conclude that perceptual learning alleviates crowding in the amblyopic fovea and normal peripheral visual field, and that reductions of crowding in amblyopia are accompanied by improvements in visual acuity.
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