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T R. Candy, Erin Babinsky, Tawna Roberts, Eric Seemiller, Vivian Wong; The Relationship Between Accommodation and Vergence in Binocular Viewing During Early Infancy. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1784.
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Classical models of accommodation and vergence incorporate a number of components either in the context of the Maddox/Heath classifications (blur-driven, disparity-driven, tonic, coupled, proximal and voluntary) or Controls Theory models (phasic, tonic, coupling and plant). In either vocabulary, these components may combine with different weights to generate relatively coupled or independent responses when all cues are available. Previous studies have reached inconsistent conclusions regarding the relative maturation of accommodation and vergence, but have implied that the final motor responses are not tightly coupled in early infancy. This study asked whether this relative independence only becomes apparent across individuals or exists within an individual.
Accommodation and vergence responses were recorded simultaneously from 74 3-4 month-old infants using an eccentric photorefractor (Multi Channel Systems, 25Hz). The infants were presented with an animated movie (a broadband spatial amplitude spectrum) moving repeatedly in a ramp fashion between dioptric distances of 1.1 and 2.85D. The subjects viewed the movie in full-cue binocular viewing conditions. The correlation between the accommodation and vergence responses to this stimulus was determined and the responses were simulated using Hung and Semmlow’s static model (1980) and Schor’s dynamic model (1992).
66 infants gave usable data and 37 of them had a correlation between their accommodation and vergence data of greater than 0.6. The vergence responses were more repeatable within individuals than accommodation and relative drift of the accommodation response was not uncommon at levels (on the order of 1-2 D) beyond the depth of focus predicted by their expected acuity. While the average responses could be simulated well using the models, the deterministic structure of the models prevented simulation of the drift.
While the accommodation and vergence systems demonstrate coupled open-loop responses during early infancy, the effective coupling in full-cue conditions is not tight in many 3-4 month-olds. While this may lead to inaccurate responses at times, it could also permit young infants to maintain both focused and aligned visual experience during early development and reduce the potential for cue conflict and strabismus.
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