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.A mblyopia is characterized by several functional abnormalities in spatial vision (for reviews see refs. 1-4), including reductions in visual acuity (VA), contrast-sensitivity function (CSF), and vernier acuity as well as spatial distortion (5), abnormal spatial interactions (6, 7), and impaired contour detection (8,9). In addition, amblyopic individuals suffer from binocular abnormalities such as impaired stereoacuity and abnormal binocular summation. The visual deficiencies are thought to be irreversible after the first decade of life (10-12), by which time the developmental maturation window has been terminated. The loss of vision is thought to result from abnormal operation of the neuronal network within the primary visual cortex, particularly of orientation-selective neurons and their interactions (13). The perceptual learning procedure described in this study was designed to train this network by efficiently stimulating these neuronal populations and effectively promoting their spatial interactions.Spatial interactions in human vision can be probed by contrast detection of a localized target in the presence of flankers (Fig. 1). These experiments show that the contrast threshold of a foveal Gabor signal (GS) is reduced in the presence of cooriented and coaligned (collinear) high-contrast GS flankers (14-18). The excitatory interaction is range-dependent and is maximal for target-flanker separation of approximately three times the GS wavelength. Smaller separations can raise the target threshold, depending on flanker contrast and phase (19). Singleunit recordings suggest that the underlying mechanisms reside within the primary visual cortex (20,21). Neuronal responses in the visual cortex are tuned for location, orientation, and spatial frequency. Recent evidence from studies in the cat and the monkey (20,(22)(23)(24)(25) shows that neuronal responses in the primary visual cortex are modulated by remote image parts, with both excitatory and inhibitory effects observ...

Recent studies have shown that perceptual learning has the potential to treat amblyopia. In this study we tested whether a recent perceptual learning technique that improved visual functions in adults can be applied to improve the vision of children after the conventional treatment of patching has failed. A prospective clinical pilot study was carried out in children who were non-compliant with patching or in whom patching had failed despite good compliance. Each child underwent a complete eye examination before and after treatment. The treatment was based on a perceptual learning technique that was similar to the adult study [Polat, U., Ma-Naim, T., Belkin, M., & Sagi, D. (2004). Improving vision in adult amblyopia by perceptual learning. Proceedings of the National Academy of Sciences of the United States of America, 101(17), 6692-6697]. Between blocks, children played a computer game to engage and maintain their attention in order to increase compliance. Each child received two treatment sessions a week, with a total of not more than 40 sessions. Each session lasted for about 1h and included a total practice time of about 30min. The age of the children (n=5) was between 7 and 8years (mean 7.3years). For the whole group, the average improvement in visual acuity was 1.5 Snellen lines or 2.12 ETDRS lines. The training improved the contrast sensitivity, which reached the normal range after treatment. Thus, the perceptual learning technique can be successfully used to treat children with amblyopia even after the conventional treatment of patching fails.