Improving vision in adult amblyopia by perceptual learning

Abstract

Practicing certain visual tasks leads, as a result of a process termed "perceptual learning," to a significant improvement in performance. Learning is specific for basic stimulus features such as local orientation, retinal location, and eye of presentation, suggesting modification of neuronal processes at the primary visual cortex in adults. It is not known, however, whether such low-level learning affects higher-level visual tasks such as recognition. By systematic low-level training of an adult visual system malfunctioning as a result of abnormal development (leading to amblyopia) of the primary visual cortex during the "critical period," we show here that induction of low-level changes might yield significant perceptual benefits that transfer to higher visual tasks. The training procedure resulted in a 2-fold improvement in contrast sensitivity and in letter-recognition tasks. These findings demonstrate that perceptual learning can improve basic representations within an adult visual system that did not develop during the critical period.

Figures

Fig. 1.

Visual stimuli used during training. (a) Single GS (see Methods). (b) Triplets consisting of a target GS and two flankers used in the lateral-masking experiments. Three different target-flanker separations were used. Target contrast is enhanced for demonstration. In all experiments reported here, the GS spatial SD was equal to the wavelength (σ = λ). The spatial frequency and orientation of flankers were always set to those of the target.

Fig. 2.

Lateral-masking curves. (a Left) Comparison exposing the absence of lateral facilitation in the data of the amblyopic patients. Error bars denote ±1 SE. (a Right) Masking curves for three amblyopic patients (S1-S3, selected to demonstrate variability), which show variable amounts of increased suppression, pointing to abnormal connectivity. Data from untrained subjects were obtained during the first lateral-masking session and were averaged across subjects: first group of amblyopic patients (open squares; n = 40; 40 of 44 completed these sessions); second group of amblyopic patients (open triangles; n = 19); nonamblyopic subjects (filled circles; n = 16). Spatial frequencies ranged from 3 to 12 cpd (mean ± SD, 5.9 ± 2.8; n = 40) for the first treatment group, from 1.5 to 6 cpd (2.7 ± 1.6; n = 19) for the second treatment group, and from 3 to 12 (7.7 ± 3.6 cpd; n = 16) for the control group. Past medical records and childhood photographs were obtained whenever possible. Of the 77 amblyopes, 58 had been treated by occlusion in the past (occlusion treatment had been initiated in 12 patients before the age of 3 years, in 21 patients between the ages of 3 and 5 years, in 21 patients between 5 and 9 years, in 3 patients aged ≥9 years, and 1 patient could not recall the age at which the occlusion treatment started). Fourteen subjects had received no treatment in the past, and in five patients information was not available. (b) The sum of the threshold elevations (2-6λ) was recorded for each patient from the first treatment session. The mean threshold elevation was 0.13 log units ± 0.03 (mean ± SE) for the first amblyopic group (n = 40 of 44), -0.12 log units ± 0.02 for the second amblyopic group (n = 19), and -0.12 ± 0.03 (n = 16) for the group with normal vision. (c) Contrast-detection thresholds for Gabor targets in the presence and absence of flankers for an amblyopic patient with astigmatism. Thresholds were tested with flankers at a distance of 3λ from the target. Data obtained before the first training session show strong lateral suppression around the orientation corresponding to the astigmatism axis (maximal blur), whereas after training the suppression disappeared and some facilitation was observed for all orientations. These experiments were carried out with the eyes optically corrected.

Fig. 3.

CSFs. (a) CSF for the first treatment group (n = 39, tested after 1 year) before and after training. Sensitivity improved by a factor of ≈2 across the range tested, reaching normal performance (gray shaded area) for all spatial frequencies tested except for the highest one (18 cpd). Tests carried out 12 months after the training was terminated showed complete retention and additional improvement on spatial frequencies of 12 and 18 cpd. CSFs were estimated by using a sine-wave contrast test with constant grating size (1.4°; see Methods) (26). (b) Reduced lateral inhibition after training. Data show threshold elevation for targets separated from flankers by a distance of 3λ (for which facilitation is near-maximal in normal-sighted subjects, σ = λ); values are means for 19 patients tested at 6 cpd before and after treatment. Threshold elevation is compared at four orientations to allow for possible anisotropy (astigmatism). The initial suppression, observed with all four orientations tested during the second training session, was removed by training (up to 12 sessions). The improvement is ≈0.15 log units. Error bars denote SEM of ±1.

Fig. 4.

Learning curves. (a) VA learning curves for three amblyopic patients and one control subject (RTA). Whereas the patients going through the training schedule showed a marked improvement in VA, the control subject, practicing with only highly discriminable (high-contrast) targets, showed no improvement. The actual chart, viewed at 3 m, has measured letters that are ≈7.3 times larger than the chart shown. (b) VA learning curves: group data (two treatment groups, n = 63; control group, n = 14). A relatively rapid improvement in VA during the first eight sessions is followed by a phase of slower learning. Learning seems to occur at the same rate for anisometropic amblyopic patients (n = 32) and strabismic amblyopic patients (n = 14). Testing of retention after 3 (n = 44), 6 (n = 41), 9 (n = 31), and 12 (n = 39) months disclosed only a slight decrement of performance.