Increasing task precision demands reveals that the reach and grasp remain subject to different perception-action constraints in 12-month-old human infants

Abstract

The reach and grasp follow different developmental trajectories, but are often considered to have achieved nearly adult-like precision and integration by 12 months of age. This study used frame-by-frame video analysis to investigate whether increasing precision demands, by placing small reaching targets on a narrow pedestal rather than on a flat table, would influence the reach and grasp movements of 12-month-old infants in a complementary or differential fashion. The results reveal that placing the target atop a pedestal impaired the infants’s ability to direct an appropriate digit towards the small target, but did not produce a corresponding decrease in the frequency with which they used an index-thumb pincer grip to grasp the target. This was due to the fact that, although infants were more likely to contact the target with a suboptimal part of the hand in the pedestal condition, a greater proportion of these suboptimal contacts ultimately transitioned to a successful index-thumb pincer grip. Thus, increasing task precision demands impaired reach accuracy, but facilitated index-thumb grip formation, in 12-month-old infants. The differential response of the reach and grasp to the increased precision demands of the pedestal condition suggests that the two movements are not fully integrated and, when precision demands are great, remain sensitive to different perception-action constraints in 12-month-old infants.