Perceptual learning as a potential treatment for amblyopia: a mini-review

Abstract

Amblyopia is a developmental abnormality that results from physiological alterations in the visual cortex and impairs form vision. It is a consequence of abnormal binocular visual experience during the "sensitive period" early in life. While amblyopia can often be reversed when treated early, conventional treatment is generally not undertaken in older children and adults. A number of studies over the last twelve years or so suggest that Perceptual Learning (PL) may provide an important new method for treating amblyopia. The aim of this mini-review is to provide a critical review and "meta-analysis" of perceptual learning in adults and children with amblyopia, with a view to extracting principles that might make PL more effective and efficient. Specifically we evaluate: 1). What factors influence the outcome of perceptual learning? 2). Specificity and generalization - two sides of the coin. 3). Do the improvements last? 4). How does PL improve visual function? 5). Should PL be part of the treatment armamentarium? A review of the extant studies makes it clear that practicing a visual task results in a long-lasting improvement in performance in an amblyopic eye. The improvement is generally strongest for the trained eye, task, stimulus and orientation, but appears to have a broader spatial frequency bandwidth than in normal vision. Importantly, practicing on a variety of different tasks and stimuli seems to transfer to improved visual acuity. Perceptual learning operates via a reduction of internal neural noise and/or through more efficient use of the stimulus information by retuning the weighting of the information. The success of PL raises the question of whether it should become a standard part of the armamentarium for the clinical treatment of amblyopia, and suggests several important principles for effective perceptual learning in amblyopia.

Fig. 1

The effect of age on the Pre vs Post-training threshold Ratio (PPR), for the trained task (Top panel) and for transfer to Snellen acuity (Lower panel).Each symbol shows the mean data for a given study. A gray dotted line shows the mean improvement of previous PL studies. This format is repeated in Figs 2 and 3.

Fig. 2

The effect of task on the PPR (trained task – Top; Snellen acuity – Bottom). Format as in Fig. 1.

Fig. 3

The effect of duration on the PPR (trained task – Top; Snellen acuity – Bottom). Format as in Fig. 1. The dashed lines are power functions with exponents of −0.15 (top R = 0.34) and 0.11 (bottom R = 0.59). The dotted line in the lower panel is the dose-response curve for patching from Stewart et. al., 2004. The solid black circle is the patching outcome from Chen et al., 2008.

Fig. 4

The effect of severity of amblyopia (initial loss) on the outcome of perceptual learning. Top panel, number of hours required to reach plateau. Bottom panel, PPR following PL. After Li et al., 2008.

Fig. 5

The probability “success” of occlusion (defined as a visual acuity of 20/40 or better) vs the depth of visual loss before treatment (after Flynn et al., 1999). The three different symbols reflect 3 different age groups. The vertical gray line is drawn at an initial acuity of 20/60. Most amblyopes have acuities between 20/30 and 20/60.

Fig. 6

Acuity (MAR in minutes) vs weeks of treatment in 7 adult strabismic amblyopes (replotted from Kupfer, 1957).

Fig. 7

The change in acuity of the amblyopic (solid) fellow (open) eyes of amblyopic patients whose visual acuity spontaneously improved (positive values) in the wake of visual loss (negative numbers) due to macular degeneration in the fellow eye (replotted from El Mallah et al., 2000).