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S. T. L. Chung, R. W. Li, D. M. Levi, B. V. Nia; Improving Acuity And Reducing Crowding in Strabismic Amblyopia Through Perceptual Learning. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3819.
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© ARVO (1962-2015); The Authors (2016-present)
Learning to identify crowded letters ("uncrowd") reduces the crowding zone in normal peripheral vision (Chung, 2007). The first goal of this study was to determine if strabismic amblyopes, who generally exhibit excessive crowding, could benefit from learning to uncrowd. The second goal was to compare the effectiveness of learning to uncrowd with learning to identify near-acuity low-contrast single letters, which might be effective in improving acuities in strabismic amblyopes.
Observers with strabismic amblyopia were randomized into two training groups -- trained to identify crowded letters (center-to-center letter separation = 0.8 times x-height) or trained to identify near-acuity low-contrast single letters. A pre-test consisted of measurements of (1) high-contrast single letter acuity, (2) spatial extent of crowding as determined by measuring identification-accuracy of the middle target letters of trigrams for a range of letter separations, (3) visual-span profile as obtained by measuring letter-identification accuracy as a function of letter positions within 7 letter slots left and right of fixation and (4) contrast thresholds for identifying single and crowded letters with and without external white luminance noise (rms = 0.2). Training consisted of 10 sessions of repeated measurements (1,000 trials per day for a total of 10,000 trials) on the respective task. The post-test, identical to the pre-test, followed the last training session. All testings were conducted using only the amblyopic eyes.
All observers improved after training. Post-pretest comparisons reveal that following training, (1) observers in the single-letter training group showed improvements in acuity; (2) all observers demonstrated a reduced spatial extent of crowding; (3) there was practically no change in the visual-span profiles for all observers and (4) there was a reduction in thresholds for identifying letters with and without external noise.
Learning to uncrowd and learning to identify near-acuity low-contrast single letters were both effective in reducing the spatial extent of crowding for strabismic amblyopes. However, the single-letter training appears to be more effective in improving acuity. The mechanisms underlying the improvements (inferred from the measurements using external noise) appear to be similar for both training tasks.
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