Perceptual learning reduces crowding in amblyopia and in the normal periphery

Abstract

Amblyopia is a developmental visual disorder of cortical origin, characterized by crowding and poor acuity in central vision of the affected eye. Crowding refers to the adverse effects of surrounding items on object identification, common only in normal peripheral but not central vision. We trained a group of adult human amblyopes on a crowded letter identification task to assess whether the crowding problem can be ameliorated. Letter size was fixed well above the acuity limit, and letter spacing was varied to obtain spacing thresholds for central target identification. Normally sighted observers practiced the same task in their lower peripheral visual field. Independent measures of acuity were taken in flanked and unflanked conditions before and after training to measure crowding ratios at three fixed letter separations. Practice improved the letter spacing thresholds of both groups on the training task, and crowding ratios were reduced after posttest. The reductions in crowding in amblyopes were associated with improvements in standard measures of visual acuity. Thus, perceptual learning reduced the deleterious effects of crowding in amblyopia and in the normal periphery. The results support the effectiveness of plasticity-based approaches for improving vision in adult amblyopes and suggest experience-dependent effects on the cortical substrates of crowding.

Figure 1.

A, Stimuli used in the letter identification task. Unflanked letters (top) and flanked letters (bottom). B, Crowding in the amblyopic fovea: acuity thresholds for flanked letters at the closest separation (1.1× letter size) against acuity thresholds for unflanked letters. Ten subjects were tested. C, Crowding in the normal periphery (4° upper and lower). Ten subjects were tested.

Figure 2.

Improvement of two amblyopes (fovea; left) and two normal controls (periphery; right) on the spacing task. Open symbols denote letter size (in degrees) in that bin. Filled gray symbols denote the spacing threshold in that bin. As performance improves, spacing thresholds approach letter size (i.e., abutting). Filled black symbols denote the average spacing threshold for successive bins within a session, at a given letter size. Vertical dashed lines separate the sessions. Sessions were performed on consecutive days.

Figure 3.

Perceptual learning effects on crowding in the amblyopic fovea. A, Pretraining and posttraining crowding ratios (flanked letter acuity/unflanked letter acuity), in trained amblyopes for the amblyopic eye at three separations (1.1×, 1.2×, 1.4× letter size), and the fellow eye at the closest letter separation (1.1× letter size). Fellow eye was not trained. B, Crowding ratios of a subset of the amblyopic observers on two separate occasions with no intervening training. Error bars denote SEM. C, Change in unflanked letter acuity versus flanked letter acuity after training (black circles, amblyopic eye; white circles, fellow eye; diamonds, untrained amblyopic eye). Dashed gray lines indicate a ratio of 1 (i.e., no change). D, Change in logMAR acuity after training. Each symbol represents an observer. Average improvement is shown on the right.

Figure 4.

Perceptual learning effects on crowding in the normal periphery. A, Pretraining and post-training crowding ratios (flanked letter acuity/unflanked letter acuity), for the trained group at three separations (1.1×,1.2×, 1.4× letter size). B, Pretraining and posttraining crowding ratios for the control group at three separations. C, Change in unflanked letter acuity versus flanked letter acuity after training for the trained group (black inverted triangles) and the control group (white inverted triangles). Dashed gray lines indicate a ratio of 1 (i.e., no change). Points above the diagonal indicate greater change in flanked than in unflanked letter acuity. Error bars denote SEM.

Figure 5.

Absolute letter spacings in the flanked acuity tasks. Spacing was a fixed proportion of letter size; therefore, absolute spacings were calculated by multiplying each observer's acuity with the spacing in each condition (1.1×, 1.2×, 1.4× letter size). Error bars denote SEM.