Is tonic vergence protective against strabismus during development?

Abstract

Young children have increased accommodative demand due to their typically hyperopic refractive error, while they have reduced vergence demand relative to adults, due to their decreased interpupillary distance. The neural coupling of accommodation and vergence implies that young children may experience an apparent cue conflict for stable accommodative and vergence responses (Bharadwaj & Candy, 2009). Some children develop strabismus, potentially resulting from this conflict (Parks, 1958). How most children are able to avoid strabismus is poorly understood. The purpose of this study was to determine whether tonic adaptation of vergence might protect young children from this conflict by reducing the impact of the neural coupling. Participants were 3- to 10-year-old typically-developing children and adults, who watched a video at a 33cm viewing distance. They initially viewed it monocularly to determine their baseline heterophoria (with a near IR filter and MCS PowerRefractor, 25Hz). They then watched the video binocularly for 5 or 60 sec, after which their alignment with the filter was measured again to determine the time constant over which they returned to their baseline heterphoria (Schor & Horner, 1989). The mean phoria at the task distance in adults was 4.4 pd (±2.9) exophoria and in children was 3.5 pd (±3.9) exophoria (p = 0.60). For these tasks the mean time constants only differed by 1.5sec (p = 0.40) (after 5sec viewing) and 1.8sec (p = 0.54) (after 60sec viewing) between the adults and children. Thus the balance of tonic adaptation and phasic activity appear similar for children and adults for these tasks. Given that tonic vergence is active in adults (Schor, 1979), the fact that the data from children were comparable to adults' indicates that tonic vergence could play a role in achieving alignment during development.