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S. R. Bharadwaj, T. Candy; Cues for the Control of Accommodation & Vergence During Postnatal Human Development. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4024. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Maintaining single & focused visual experience while an object moves in depth requires the coordination of vergence & accommodation responses. In adults, retinal blur and disparity are each capable of driving both accommodation and vergence responses. However, the relative importance of these cues in driving accommodation and vergence is largely unknown during development. Here we tested the importance of disparity by comparing the gain of accommodation & vergence in its presence (binocular viewing) & absence (monocular viewing).
Subjects (n = 159; age range: 1.9 months - 46 yrs) viewed a high contrast cartoon target either binocularly or monocularly (left eye) while the target moved at 0.5 D/s between 80 and 33 cms (stimulus size: 1.75 D or MA). Subjects’ accommodation, vergence & left and right eye positions were measured using the PowerRefractor (Multi Channel Systems). Cooperation with the task was ensured by only including responses where the left eye position & stimulus position were well-correlated (r>0.7).
The gains (response size/stimulus size) of accommodation & vergence were significantly reduced in monocular conditions compared with binocular conditions (p<0.001), even though the left eye tracked the target similarly in both conditions (p=0.2). The gain of monocular accommodation was lowest in infants (0.41) & it increased at the rate of 0.04yr-1 until an adult-like gain of 0.76 was reached by 10.9yrs. The gain of monocular vergence was also lowest in infants (0.33) & it increased at the rate of 0.01yr-1 until an adult-like gain of 0.57 was reached by 17.6yrs. The reduction in monocular accommodative gain was consistent across three forms of monocular occlusion (IR filter, opaque patch & +20D lens). The accommodation & vergence gains were poorly correlated with the subjects’ (n = 34; age range: 6.2 months - 16.4 years) cycloplegia refractive error (binocular accommodation: r = 0.17; monocular accommodation: r = -0.2; binocular vergence: r = -0.2; monocular vergence: r = -0.1).
Reduced vergence gain in monocular conditions is consistent with predictions based on neural coupling between accommodation & vergence. However, the reduced gain of accommodation during infancy & early childhood is a new observation that suggests the developing visual system is inefficient at generating accommodative responses in the absence of retinal disparity.
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