Evidence for a phase transition in the early development of prehension.

A longitudinal study was conducted to examine the hypothesis that the development of prehension during the first 5 months of life is characterized by the presence of a discontinuous phase transition. Ten infants were observed weekly from 8 to 24 weeks of age. Video recordings were made of movements toward an attractive object which were classified according to two behavioral categories: reaching without grasping and reaching with grasping. The time evolution of the relative incidence of these behavioral categories was analyzed statistically. Evidence was found for a sudden jump from a (developmental) state in which reaching without grasping is predominant to a state in which reaching with grasping is predominant. Evidence was also found for bimodality, inaccessibility, and anomalous variance. In combination, these findings support the hypothesis that the investigated behavioral change constitutes a discontinuous phase transition. The behavioral change in question occurred at the moment in developmental time at which the attractor strength of reaching for objects as such relative to that of other behavioral activities appeared to be increased.