Stereopsis and amblyopia: A mini-review

Abstract

Amblyopia is a neuro-developmental disorder of the visual cortex that arises from abnormal visual experience early in life. Amblyopia is clinically important because it is a major cause of vision loss in infants and young children. Amblyopia is also of basic interest because it reflects the neural impairment that occurs when normal visual development is disrupted. Amblyopia provides an ideal model for understanding when and how brain plasticity may be harnessed for recovery of function. Over the past two decades there has been a rekindling of interest in developing more effective methods for treating amblyopia, and for extending the treatment beyond the critical period, as exemplified by new clinical trials and new basic research studies. The focus of this review is on stereopsis and its potential for recovery. Impaired stereoscopic depth perception is the most common deficit associated with amblyopia under ordinary (binocular) viewing conditions (Webber & Wood, 2005). Our review of the extant literature suggests that this impairment may have a substantial impact on visuomotor tasks, difficulties in playing sports in children and locomoting safely in older adults. Furthermore, impaired stereopsis may also limit career options for amblyopes. Finally, stereopsis is more impacted in strabismic than in anisometropic amblyopia. Our review of the various approaches to treating amblyopia (patching, perceptual learning, videogames) suggests that there are several promising new approaches to recovering stereopsis in both anisometropic and strabismic amblyopes. However, recovery of stereoacuity may require more active treatment in strabismic than in anisometropic amblyopia. Individuals with strabismic amblyopia have a very low probability of improvement with monocular training; however they fare better with dichoptic training than with monocular training, and even better with direct stereo training.

Keywords: Amblyopia; Anisometropia; Perceptual learning; Stereopsis; Strabismus; Videogames.

Copyright © 2015 Elsevier Ltd. All rights reserved.

Figures

Fig. 1

Stereoacuity vs. visual acuity. The dotted lines show the upper and lower limits of the test. The data for strabismic anisometropes (gray squares) have been slightly displaced for clarity. Data replotted from Levi et al., 2011. The blue regression line suggests that worse visual acuity goes hand in hand with worse stereoacuity in anisometropic amblyopes; however this relationship does not hold in strabismic amblyopes or strabismic anisometropes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Stereoacuity and crowding, as quantified by the spacing:acuity ratio (S/A). Data points above the upper horizontal dotted line at stereoacuity = 6.67 min “fail” the test. The vertical dashed line, S/A = 1.84, divides amblyopic patients into two groups with large and small spacing:acuity ratio, or in other words high crowding or low crowding. High levels of crowding appear systematically associated with loss of stereo-acuity. Indeed, all but one amblyopic patient with a small S/A ratio pass the test, and all but one with a large S/A ratio fail. Data replotted from Song et al. (2014).

Fig. 3

The cumulative probability of stereo-acuity being 40 arc s or worse. Cumulative probabilities for positive and negative values of vector blur anisometropia were computed separately, beginning at 0. Data replotted from Levi et al. (2011).

Fig. 4

The effect of patching on stereopsis recovery in children. The percentage of patients with very poor or no stereopsis (800 arc s or >) decreases, and there is a modest increase in the percentage of patients with good stereo acuity (100 arc s or

Fig. 5

The percentage of anisometropic and strabismic amblyopes achieving at least a two-level improvement in stereoacuity and a stereoacuity of 160” or better with all methods of treatment (based on the studies in Table 1). The numbers above each bar show the number of subjects achieving this improvement/the number of participants in that category.

Fig. 6

The effect of extensive monocular PL on stereoacuity in adult amblyopes. Both anisometropic (blue) and strabismic (red) amblyopes, including several who were “stereoblind” (i.e., unable to see a disparity of 500 arc s, the largest disparity tested – data in the turquoise rectangle) initially show improved stereo sensitivity after PL (replotted from Zhang et al., 2014). These subjects were arbitrarily assigned a threshold value of 600 arc s., which was used in the calculation of % improvement. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 7

The percentage of anisometropic amblyopes (A) and strabismic amblyopes (B) showing improved stereopsis with various methods of treatment. The selected criterion for stereopsis improvement is achieving at least a two-level improvement in stereopsis and a stereoacuity of 16000 or better (data plotted based on the studies in Table 1).

Fig. 8

Experimental training summary. The percentage of anisometropic (left 3 bars) and strabismic (right three bars) achieving the criterion improvement in stereopsis based on monocular training (combining PL & VGP – black bars), dichoptic training (combining PL & VGP – gray bars) and direct stereopsis training (blue bars). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 9

Post vs. pre-training stereo thresholds. This figure replots data from several of our studies, involving 94 subjects and multiple training approaches. Blue symbols – anisometropic amblyopes; red symbols – strabismic amblyopes. The diagonal gray line indicates no improvement. Symbols below the line show improved performance following training. Data below the dashed horizontal lines indicate a post-training stereothreshold of 140 arc s or better. Data within the turquoise rectangle indicate no measurable pre-training stereopsis. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)