The Transition to Reaching: Mapping Intention and Intrinsic Dynamics

TLDR

Directed reaching marks the emergence of a qualitatively new motor skill in infants. The study examined how intentional reaching develops from infants’ intrinsic movement dynamics and how initial reaches are progressively adapted to the task. Researchers observed four infants weekly during a standard reaching task, pinpointing the week of first arm‑extended reach and the two weeks before and after. Infants first reached between 12 and 22 weeks, employing varied strategies—large movers reduced force, quieter movers increased speed—while progressively modulating reach force and compliance; these adaptations arise from natural system dynamics and active exploration rather than pre‑programmed motor plans.

Abstract

The onset of directed reaching demarks the emergence of a qualitatively new skill. In this study we asked how intentional reaching arises from infants' ongoing, intrinsic movement dynamics, and how first reaches become successively adapted to the task. We observed 4 infants weekly in a standard reaching task and identified the week of first arm-extended reach, and the 2 weeks before and after onset. The infants first reached at ages ranging from 12 to 22 weeks, and they used different strategies to get the toy. 2 infants, whose spontaneous movements were large and vigorous, damped down their fast, forceful movements. The 2 quieter infants generated faster and more energetic movements to lift their arms. The infants modulated reaches in task-appropriate ways in the weeks following onset. Reaching emerges when infants can intentionally adjust the force and compliance of the arm, often using muscle coactivation. These results suggest that the infant central nervous system does not contain programs that detail hand trajectory, joint coordination, and muscle activation patterns. Rather, these patterns are the consequences of the natural dynamics of the system and the active exploration of the match between those dynamics and the task.