Increasing selectivity of interlimb coordination during spontaneous movements in 2- to 4-month-old infants.

In the field of motor development, a question exists whether spontaneous activity in early infancy serves as a precursor to later-emerging goal-directed behaviors. To answer this question, it is necessary to investigate in detail the properties of spontaneous movements in individual infants. In the current study, we longitudinally examined the spontaneous movements of the end points of the limbs in 6 infants aged 2-4 months. Examinations were carried out every week by using a motion analysis system, and the number of recordings performed for each infant varied from 6 to 9 times. Our major finding was that there was an age-related increase in the velocity and position correlation between arms and between legs, whereas there was no significant change in the velocity, duration, and amplitude of movements of individual limbs. That is, the pattern of spontaneous movements changes from a general activity involving all the limbs to an activity involving more selective interlimb coordination from 2 to 4 months of age. These findings suggest that the dissociated movements of a selective combination of arms or legs during spontaneous movements may be a precursor to functionally dissociated movements during goal-directed behaviors.