The effect of body orientation to gravity on early infant reaching.

The purpose of the study was to examine the effect of body orientation with respect to gravity on infant's reaching quantity and quality. Two groups, 12- to 19-week-old and 20- to 27-week-old infants, were seated in three positions: vertical (90 degrees from horizontal), recline (60 degrees), and supine (0 degrees). Nine balls on a black board were presented. Video recordings were used to measure quantity of reaching (number and duration of reaches) and quality of reaching (open or closed hand, starting position of the arm, and position of touched and grasped balls). On the quantity measure there was a significant age x body position interaction that indicated that the 12- to 19-week-olds showed reaching behavior in the vertical position equal to that of 20- to 27-week-old infants in all positions. A similar tendency, although not significant, was found for the quality measurements. These findings indicate that the development of reaching does not just reflect maturation of the central nervous system, but a changing interaction between organismic and environmental constraints.