[Laterality of upper extremity movements in infancy: observations at 4 and 9 months of age].

This study investigated the process involved in the lateralization of movements during infancy by observing upper extremity movements in a laboratory setting. Reaching for flying rings, balls, mini toy cars, and small round cookies were observed and recorded by videotape at 4 and 9 months of age. The subjects were 202 infants who participated in Japan Children's Study, a cohort study on the development of sociability. Infants reached for objects significantly more frequently at 9 months (98%) than at 4 months (40%) (p<0.001). Though the lateral preference in reaching for balls at 4 months was ambiguous, reaching for toy cars was performed more frequently with the right hand at 9 months (50%) than with the left one (19%) (p<0.01). Lateralization of the upper extremity movements is thought to appear by 9 months.