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D. F. Teel, S. R. Bharadwaj, T. R. Candy; Accommodative-Vergence and Vergence-Accommodation Cross-link Gains in Preschoolers. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2895.
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Accommodation and vergence interact through neural cross-links such that accommodation can drive vergence (AC/A ratio) and vergence can drive accommodation (CA/C ratio) to provide clear and single vision. Inappropriate cross-link gains are believed to lead to binocular vision anomalies. While the two systems interact from early infancy, adult-like interactions between them may be inappropriate during development. Relative to adults, reduced head size and typically hyperopic refractive error result in reduced vergence demands and elevated accommodative demands during infancy and early childhood. This study aims to make the first within-subject measures of the gains of both response AC/A and response CA/C ratios in a group of 2-4 year olds to understand strategies used to overcome these dissimilar demands; an age at which accommodative esotropia commonly appears.
Accommodation and vergence responses were measured in a group of 2-4 year olds (mean age=3.3 ± 0.5 years) and a group of adults (mean age=28.1 ± 5.4 years) using the PowerRefractor (25Hz). To measure the AC/A gain (MA/D), subjects watched a high contrast cartoon movie (1/f amplitude spectrum) monocularly (open-loop disparity driven vergence) while accommodation was stimulated using a -2D lens. To measure the CA/C gain (D/MA), subjects watched a blurred version of the same movie (filtered using a <0.2 cpd difference-of-gaussian) binocularly (open-loop blur driven accommodation) while their vergence was stimulated using a 2MA prism.
Response AC/A ratios were significantly higher (p=0.01) in adults (Mean + SD: 0.90 ± 0.15 MA/D) than in children (0.58 ± 0.21 MA/D) while the response CA/C ratios of adults (0.36 ± 0.2 D/MA) and preschoolers (0.45 ± 0.15 D/MA) were not significantly different (p=0.335).
The response AC/A ratio (in MA/D) would be higher in 2-4 year olds than in adults if the angular rotation of the eyes generated by the accommodative-vergence cross-link remained constant with age. Conversely, the response CA/C ratio (in D/MA) would be lower than in adults if the dioptric change in focus generated by the vergence-accommodation cross-link remained constant with age. These results do not follow these predictions, indicating a recalibration of both cross-link gains with age.
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