The Transition to Reaching: Mapping Intention and Intrinsic Dynamics

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.

The confluence of intrinsic and extrinsic constraints on 3- to 9-month-old infants’ catching behavior

“…The youngest infants displayed a strong preference for trying to intercept the moving ball with both hands. It seems reasonable to suggest that this predominance of bimanual catching reflects the infants' intrinsic constraints (Corbetta & Thelen, 1996;Thelen et al, 1993). At this age infants show their first reaches, which are often but not always two handed, even when they reach for relatively small balls.…”

Stability of coupled fluctuations in movement and visual attention in infants

“…Because infants' goal-directed actions emerge from spontaneous movements (Thelen et al, 1993), the early coupling of visual attention and spontaneous movement could facilitate motor learning as the infant explores the linkages between his or her looking patterns and body movements. For example, infants may learn to modulate their movements by selectively attending to objects or people.…”

“…For example, eye±head movements and, under some conditions, eye±hand movements are spatially and temporally linked in young infants (Ennouri & Bloch, 1996;van der Meer, van der Weel, & Lee, 1995;von Hofsten, 1984). Additionally, orienting in infants, often following the appearance of a new or unexpected visual stimulus, is characterized by an increase in visual inspection of the stimulus and a transient decrease in body movement (Sokolov, 1963;Stechler & Latz, 1966).…”

The Neuronal Group Selection Theory: a framework to explain variation in normal motor development

“…According to Dynamic Systems Theory, patterns of behaviour, such as the typical smooth and straight trajectory of a reaching movement of an adult, act as 'attractors'. This means that a specific type of behaviour, which results from the effects of (1) its multiple component parts, such as muscle strength, body weight, postural support, the infant's mood, and brain development, and (2) the effect of environmental condition and task requirements, spontaneously adopts a specific organization [21][22][23] . Due to the properties of dynamic pattern formation, continuously occurring changes of the above mentioned component parts or changes of the environment or task can induce discontinuous changes (transitions) in behaviour.…”