From spontaneous to instrumental behavior: kinematic analysis of movement changes during very early learning.

3-month-old infants who can activate a mobile with their spontaneous leg kicks increase their kick rate over baseline. Kinematic analyses of the temporal and topographic characteristics of reinforced leg kicks were compared with those of infants who saw noncontingent mobile movement. In both experimental and control groups, the flexion and extension movement phases of the kick remained remarkably invariant in duration. Pauses between movements varied inversely with rate. Kick amplitude increased with increasing rate except in the extinction phase, where in some infants, kicks became less frequent but more forceful. Movement duration was unaffected by reinforcement condition. By varying amplitude and frequency domains, infants adjusted movement by neuromuscular mechanisms similar to those of mature humans. The experimental imposition of a voluntary component on spontaneous kicking did not alter the modifiability of the movement control parameters.