Interocular Suppression Quantification in Amblyopia and Strabismus: Protocol for a Scoping Review of Measurement Principles, Outcome Metrics, and Methodological Characteristics

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Interocular suppression is a characteristic binocular sensory adaptation observed in amblyopia and strabismus, in which visual information from one eye is reduced, inhibited, or attenuated during binocular viewing. Traditionally, suppression has been considered a mechanism for avoiding visual confusion and diplopia arising from abnormal binocular interactions. However, increasing evidence suggests that suppression is not merely a compensatory phenomenon but is closely associated with binocular dysfunction and may contribute to visual deficits observed in amblyopia and strabismus. As contemporary approaches to amblyopia and strabismus management increasingly emphasize binocular visual function, suppression has emerged as an important target for both clinical assessment and therapeutic intervention. A wide range of approaches have been used to assess and quantify interocular suppression in both clinical and research settings. Traditional clinical methods, such as the Worth Four Dot test and neutral density filter testing, have been widely used to identify the presence and relative magnitude of suppression in routine practice. In addition to conventional clinical tests such as the Worth Four Dot test and neutral density filter testing, synoptophore-based approaches have recently been explored for the quantitative assessment of suppression and binocular sensory function.6 In parallel, psychophysical paradigms employing dichoptic stimuli have been developed to provide quantitative measures of binocular imbalance and suppression. More recently, electrophysiological and neuroimaging approaches have also been explored to investigate the neural correlates of suppression. Collectively, these methods differ substantially in their underlying measurement principles, assessment procedures, and intended applications. The growing diversity of suppression assessment approaches reflects the complex nature of interocular suppression itself. Different suppression quantification methods are designed to assess different measurable characteristics of interocular suppression, including suppression size (or extent), suppression depth (or strength), binocular imbalance, and the spatial distribution of suppression across the visual field. These methods encompass clinical, psychophysical, electrophysiological, and neuroimaging-based assessment approaches. Although they aim to quantify interocular suppression, they do so by manipulating or measuring different physical or physiological properties—such as visual angle, luminance, contrast, stimulus opacity, or neural response characteristics. Consequently, they generate different outcome metrics and units of quantification. Furthermore, evidence suggests that suppression may manifest differently across clinical populations, including anisometropic and strabismic amblyopia, highlighting the importance of understanding the specific suppression characteristic assessed by a given method. While some suppression quantification methods have been translated into clinical practice, many remain confined to research settings, and their methodological characteristics, measurement principles, outcome metrics, and units of quantification have not been comprehensively mapped. The present scoping review is conceptually informed by a previously published systematic review protocol that examined the association between suppression scotoma and visual acuity in clinical subgroups of amblyopia. During the conduct of that review, substantial methodological diversity in suppression quantification approaches became apparent, including variation in assessment methods, measurement principles, and reported outcome metrics. This observation highlighted the need for a broader examination of how suppression is measured across amblyopia and strabismus research. The previous review provided an important foundation by synthesizing the available suppression quantification methods and summarizing their clinical applicability. Its primary objective was to provide an overview of suppression quantification approaches, their broad methodological classifications, and the clinically feasible and commercially available tools for amblyopia. Additionally, that review was conducted within the context of amblyopia, whereas interocular suppression is also routinely assessed in strabismus as part of binocular vision evaluation, irrespective of whether amblyopia is present. However, the objective of the present review extends beyond identifying available methods to systematically examine what each method measures, how suppression is quantified, and how methodological differences influence the interpretation and clinical application of suppression measurements. Different suppression quantification methods are designed to assess distinct measurable characteristics of interocular suppression, including suppression size (or spatial extent), suppression depth, binocular imbalance, and the spatial distribution of suppression. These characteristics may be assessed using clinical, psychophysical, electrophysiological, or neuroimaging approaches. Because these methods are based on different measurement principles, they generate different outcome metrics, units of quantification, and methodological attributes. This methodological diversity complicates the clinical interpretation of suppression measurements and limits our ability to meaningfully apply suppression findings for understanding binocular dysfunction and monitoring therapeutic outcomes. Despite the growing number of suppression quantification methods, their methodological and measurement-related attributes have not been systematically mapped and synthesized across the literature. Building on the findings of our previous systematic review, the present scoping review will comprehensively map suppression quantification methods used in amblyopia and strabismus, with particular emphasis on the suppression characteristics assessed, underlying measurement principles, outcome metrics, units of quantification, and broader methodological characteristics of each approach. A comprehensive synthesis of suppression quantification approaches using the Joanna Briggs Institute methodology of Scoping Reviews may facilitate a better understanding of the methods currently available, the suppression characteristics they assess, the outcome metrics they generate, and the methodological characteristics that influence their implementation in clinical and research settings. Such information may help identify gaps in the existing literature, highlight areas requiring further investigation, and inform future research and clinical efforts aimed at improving suppression assessments in amblyopia and strabismus.

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