A longitudinal study of prism adaptation in infants from six to nine months of age.

This study replicated a previous demonstration of prism-induced aftereffects in 5-9-month-old infants. Aftereffects were considered to be changes in direction of reaching for visual targets following prism exposure. The present study employed a longitudinal design. In the experimental group, infants were exposed to 25-diopter lateral displacing prisms once each month from 5 to 9 months. Since many of the subjects were unable to complete testing at 5 months, data for that age level were not included in the main analyses. In a control group, infants were tested each month but were exposed to prisms only at 9 months of age. The magnitude of aftereffects for control subjects at 9 months was not significantly greater than aftereffects obtained for experimentals at 6 months, so there was little evidence of changes in adaptability due to age or repeated exposure to the prisms. The finding of an increase in the frequency of reaching with the left hand during exposure to leftward-deviating prisms was replicated. Such a shift in hand preference probably represents an alternative form of adaptation which for methodological reasons has not been observed in adults. The aftereffect and frequency results were complementary and provide a clear demonstration of adaptation to prismatic displacement between 6 and 9 months.