Changes in interlimb coupling, and their role in the development of bimanual coordination, were studied longitudinally in 6- to 12-month-old infants (N = 6). Infants were observed while they were reaching for simple objects of 2 different sizes. Their use of a uni- versus bimanual strategy for reaching as well as the coupling of their bimanual movements were compared; progress in bimanual coordination of complementary movements was evaluated on 3 different bimanual tasks. The bimanual tasks involved an asymmetrical cooperation between the 2 hands. Although spatiotemporal coupling of bimanual reaching movements did not decrease during the age period studied, infants around 7 months of age used their 2 hands infrequently for reaching. Occurrences of bimanual reaching were particularly low at the session preceding the first bimanual success at a bimanual task. This suggests that the temporal coincidence between greater independence of the 2 hands and progress in bimanual coordination of complementary movements acts in 2 directions: Infants may be more at ease when using their 2 hands in differentiated patterns as the hands move less in synchrony, but, in turn, they may be less likely to move their hands in synchrony as they anticipate mirror manipulations of the object less. The frequency of bimanual reaching increased toward the end of the 1st year. This might have been caused by an increase in the repertoire of bimanual asymmetrical object manipulations and by the fact that the development of bimanual coordination allows infants to manipulate objects with complementary movements even after a bimanual approach toward the object.

The organisation of reaching movements was studied longitudinally (every two weeks) in infants aged from 20 to 32 weeks, in order to detect possible differences between the two hands. An object was presented on a table, at a reachable distance, and four trials were retained. Analysis of left and right hand movements was performed, frame-by-frame, on video images obtained from two cameras. Three body points were considered: the elbow, the wrist, and the major finger for drawing the profile of the movement. Three parameters were considered: movement time (MT), straightness, and number of changes in direction. Data show that the right hand MT is always shorter than the left hand MT. Moreover, the right hand is also continuously more directly oriented towards the object and makes fewer corrective changes than the left hand. These data show that a different organisation of movement of the left and the right hand exists early in infancy.

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