Concerning Manuscript “Deficient Binocular Combination of Second-Order Stimuli in Amblyopia”

Visual functions in adult amblyopia subjects can be improved by perceptual learning,¹ but this kind of training-induced plasticity is still limited to some extent. More comprehensive and effective training specific to deficits of amblyopia is a major concern for researchers. We read with great interest the article by Jiawei Zhou et al. on second-order deficits in amblyopia.² We appreciate the authors' rigorous experimental methods and deliberate mathematical analysis, and we have some of our own thoughts about the authors' work. 

After measuring sensory imbalance in 14 patients using a dichoptic phase combination task, it was found that the sensory imbalance in binocular combination for second-order images was comparable to that for first-order images in 8 of the subjects but was more severe in the other 6 subjects with amblyopia. For patients A4, A8, A9, and S10 though S14, the balance points in first- and second-order binocular combination were not significantly different. And for patients A1 through A3 and patients A5 through A7, the balance point of the second-order perceived phase versus interocular modulation depth ratio (PvR) was less than that of the first-order PvR.² Here we want to point out that the diagnosis of anisometropic amblyopia for subject A9 may need to be reconsidered. There are not unified anisometropic diopters for the definition of anisometropic amblyopia. The Multi-Ethnic Pediatric Eye Disease Study (MEPEDS) has reported the prevalence of amblyopia in different places, and defined unilateral anisometropic amblyopia with spherical equivalent (SE) anisohyperopia ≥ 1.00 diopters (D), SE anisomyopia ≤ 3.00 D, or anisoastigmatism ≥ 1.50 D.³ Other articles, such as “Adaptation to Laterally Displacing Prisms in Anisometropic Amblyopia” by Sklar et al., have defined the anisometropic amblyopia as amblyopia in the presence of an interocular refractive error difference of ≥1 D in spherical or cylindrical power.⁴ Amblyopia Preferred Practice Pattern Guidelines (PPP), written by the Pediatric Ophthalmology/Strabismus Preferred Practice Pattern Panel members, does not give the anisometropic degree.⁵ However, for subject A9 with astigmatism, we did not find any evidence from existing literature to support the diagnosis of anisometropic amblyopia. The results may be biased by subject A9. 

Besides that, the study investigated the deficient binocular combination of second-order stimuli modulated by contrast. Differences of contrast sensitivity between anisometropic and strabismus amblyopia have been reported,⁶ so we doubt whether different kinds of amblyopia involve different responses to the second-order stimuli, especially when we see that all those exhibited additional second-order deficits were biased toward anisometropic amblyopia. We look forward to further studies with a larger number of patients to verify this tendency. It will be further helpful to instruct individualized visual training, and we are seeing the dawn of a valuable suggestion regarding therapeutic approach for adult amblyopia.