To investigate the developmental emergence of the ability to change their behavior depending on environmental conditions, we studied spontaneous limb movements and subsequent changes in amount and pattern of movement while playing with a mobile toy in infants 90-129 days old. We calculated two independent indices to represent amount and pattern of movements. While younger infants only increased the amount of movement, older infants first changed their movement pattern toward the arm-dominant pattern and then increased the amount of movement. Although the diversity of spontaneous movements did not differ with age, only the older infants showed the two-stage process. These results suggest that there is a drastic transition in the changeability of spontaneous movements toward movements suitable for the specific environmental condition.
The authors are with the Graduate School of Education, University of Tokyo, Japan.