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E.E. Birch, V.L. N. Fu, A.M. Norcia; Fusional Suppression During Infancy . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2449.
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© ARVO (1962-2015); The Authors (2016-present)
Little is known about suppressive binocular interactions in newborn human infants. Infant monkeys show primarily suppressive binocular interactions up to 3 weeks of age (Endo et al., IOVS 41:2022). This finding, together with the finding that interocular suppression is the primary form of binocular interaction remaining in the V1 of monkeys reared with strabismus, has led to the hypothesis that abnormal persistence of neonatal suppression may contribute to the onset of infantile esotropia in humans (Sengpiel & Blakemore, Eye 10:250). Here we assess the development of fusional suppression during human infancy.
Fourteen infants provided fusional suppression data, 8 pre–stereoptic (6–10 w) and 6 post–stereoptic (3–11 m). Five normal adults served as controls. Fusional suppression was evaluated by recording the VEP to a dichoptic multi–bar vernier target composed of 5 static segments interspersed with 4 oscillating segments that aligned and misaligned at 2 Hz. Offset magnitude was swept in 7 steps from 10 min to 1 min during each 7 sec trial. The fellow eye viewed the same multi–bar target with either 0 min or 5 min standing disparity. Fusional suppression was defined by a decrease in amplitude in the presence of 5 min standing disparity compared with the 0 min condition.
For all infants, VEP amplitude in the 0 min condition was a monotonically increasing function of vernier offset magnitude. In post–stereoptic infants, the addition of 5 min standing disparity resulted in a decrease in VEP amplitude, consistent with fusional suppression. The amount of amplitude suppression (42%) was nearly as strong as found in adults (55%). Despite large amplitude responses in the 0 min condition (mean = 5.5 µV), pre–stereoptic infants unexpectedly had no VEP response to even the largest offset the presence of 5 min standing disparity.
By 3 months of age, fusional suppression is robust. The lack of a recordable VEP in pre–stereoptic infants under conditions that yield fusional suppression in post–stereoptic infants and adults may reflect very strong suppressive interactions which abolish the VEP response, similar to those reported in newborn monkeys. Alternatively, the absence of response may reflect the presence of a neonatal form of binocularity which ceases to function in the presence of the 5 min mismatch between monocular images.
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