Development of role-differentiated bimanual manipulation during the infant's first year.

Role-differentiated bimanual manipulation (RDBM) is a complementary movement of both hands that requires differentiation between actions of the hands. Previous research showed that RDBM can be observed in infants as early as 7 months. However, RDBM could be considered a skill only when its frequency, duration, and use is appropriate for the type of manual task, and there is some evidence of intentionality in use. Twenty-four normally developing infants were studied longitudinally at 7, 9, 11, and 13 months to assess the frequency and duration of five clearly different types of RDBM with three "single-part" and three "two-part" toys as they emerge during development. Also, the sequences of actions that lead to RDBM were examined for evidence of "intentionality." The results show that although the each type of RDBM appears early in infancy, RDBM only begins to exhibit the characters of a skill by 13 months. Moreover, the type of toy influences not only the likelihood of eliciting role differentiation, but also the type of RDBM behavior and the organization of the sequence of actions that lead to RDBM. Some useful criteria for defining an infant sensorimotor skill are provided in discussion.