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D. F. Teel, S. Bharadwaj, T. R. Candy; Accommodation and Vergence Coupling During Visual Development. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1837.
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Neural coupling between accommodation and vergence is present in early infancy (accommodative vergence [AC] & vergence accommodation [CA]). Infants have narrow IPDs and are typically hyperopic, requiring smaller vergence responses [in degrees or prism diopters] and larger accommodative responses than adults to binocularly fixate and focus. The relative demands also change with growth of eye (decreasing hyperopia) and head (widening IPD). Therefore, adult-like couplings may not be optimal during development. This study was designed to determine whether the couplings are optimized to avoid over-convergence or under-accommodation during development.
Measurements were taken from infants (3- 6 mos; mean age = 3.8 ± 0.7 mos), children (2-4 year olds; mean age=3.0 ± 0.5 years) and adults (mean age=28.1 ± 5.5 years) using the PowerRefractor (25Hz). To measure the response AC/Ar (Ar = accommodative response), subjects watched a high contrast cartoon movie (1/f amplitude spectrum) monocularly (feedback for disparity-driven vergence is open-loop) while accommodation was stimulated using a -2D lens. To measure the response CA/Cr (Cr = vergence response), subjects watched a blurred version of the same movie (filtered with <0.2 cpd difference-of-gaussian) binocularly (feedback for blur-driven accommodation is open-loop) while their vergence was stimulated using a 2MA Base-out prism (prism power adjusted for subject’s IPD).
AC/Ar ratios were lower in infants and children compared with adults while the CA/Cr ratios decreased with increasing age group. The AC/Ar and CA/Cr ratios were not significantly different across groups in units of meter angles (m-1) (p= 0.35 & p = 0.23, respectively) while in units of prism diopters (angular unit of eye rotation), the ratios were found to be significantly different (p<0.001 and p=0.002, respectively). Within-subjects comparison of ratios showed lower AC/Ar ratios to be correlated with higher CA/Cr ratios (r = -0.44, p=0.0004).
AC/Ar is lower while CA/Cr is higher (in units of prism diopters) for the younger groups. These data support the hypothesis that the accommodation and vergence couplings may help prevent over-convergence or under-accommodation for hyperopic accommodative demands and low vergence demands experienced during development in typically developing children.
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