Early control of reaching: Effects of experience and body orientation

Abstract

Although research suggests that experience may be a better indicator of the acquisition of certain abilities by infants than age, little work addresses this issue in the development of reaching movements in particular. This longitudinal study fills this gap by verifying the effect of practice time on more- and less-skilled reachers of the same age group in different body orientations. Less- (n = 6) and more-skilled (n = 4) reachers were evaluated at the acquisition of reaching and again after 1 month of spontaneous practice. Kinematic analyses examined 3D wrist motion during reaching trials in supine (0°), reclined (45°) and seated (70°) orientations. The results indicated that skill level was a relevant factor in dealing with gravitational constraints imposed by different body orientations. Time of practice affected the way less- and more-skilled reachers explored and adapted the kinematic parameters of reaching to constraints imposed by body orientations. These findings suggest that not only age, but also experience and body orientations are important constraints that should be taken into account when examining infant reaching development.

Introduction

Infants acquire and develop important motor abilities during the first year of life. The emergence of reaching, in particular, has been studied extensively during the last thirty years (Konczak & Dichgans, 1997; Thelen, Corbetta, & Spencer, 1996; Von Hofsten, 1979, Von Hofsten, 1982, Von Hofsten, 1991). A reaching movement requires a certain level of interaction between infant and environment, and represents an improved perception of the world and recognition of some gamut of possible actions (Rosengren, Savelsbergh, & Van der Kamp, 2003; Savelsbergh & Van der Kamp, 1993; Von Hofsten, 1991). From a constraint-led perspective, the development of action is generally brought about by changes within and between the constraints imposed upon the organism–environment system (Newell, 1986). Particular constraints may act, at certain developmental time, as rate-limiting factors in the emergence and mastering of new motor actions. In other words, one can consider constraints as those factors that somehow set boundaries for the control and coordination of an action and changes therein. The interaction of these constraints does not prescribe the pattern or the performance of an action, but it guides its development by making the occurrence of certain patterns more or less probable. A change in the confluence of constraints, therefore, can eventuate in a change in action, with no predominant constraints. It is the interaction between intrinsic and extrinsic constraints that makes some but not other actions possible at a particular time and place (Rosengren et al., 2003). The present study examines how the interaction between intrinsic and extrinsic constraints affects the early coordination of reaching.

Newell's constraint-led perspective was supported by studies that attempted to understand the development of reaching by taking into account intrinsic constraints, like the level of postural control (Fallang, Saugstad, & Hadders-Algra, 2000; Rochat & Goubet, 1995), as well as extrinsic constraints, such as investigating the use of objects of different sizes (Van Hof, Van der Kamp, & Savelsbergh, 2006) and rigidity (Rocha, Silva, & Tudella, 2006), objects in motion (Van Hof, Van der Kamp, Caljouw, & Savelsbergh, 2005) and influence of gravity by imposing different body orientations (Rochat, 1992; Savelsbergh & Van der Kamp, 1994; Out, Van Soest, Savelsbergh, & Hopkins, 1998).

In the experiment conducted by Savelsbergh and Van der Kamp (1994), reaching movements were analyzed in 12–27-week-old infants positioned in an infant chair that could be adjusted to three body orientations: seated, reclined, and supine. Reaching and grasping were observed to be affected by age and body orientation. Specifically in the seated orientation, the 12–19-week-old infants showed patterns of reaching and grasping that were comparable to those of 20–27-week-old infants, regardless of body orientation. However, when the younger infants were in supine, they not only performed fewer reaching and grasping movements, but also seemed to have poorer control and coordination. The observation that only the young infants’ movements were affected by body orientation suggests that biomechanical constraints (e.g. muscle power in order to overcome the gravity force) may act as a rate-limiting factor during the development of reaching and grasping. Carvalho, Tudella and Savelsbergh (2007), when conducting a longitudinal study to verify the effect of seated and supine positions on spatio-temporal parameters of reaching at 4, 5 and 6 months, verified that frequency and straightness of reaching increased over age. Significant differences in reaching between the body orientations were observed at 4 months, that is, the frequency increased and the duration and deceleration time decreased in the seated position. There were no significant differences between the body orientations at 5 and 6 months. These findings suggest that young infants adapt their behavior to both intrinsic (i.e. age) and extrinsic (i.e. body orientation) constraints. In summary, this previous research linked adaptive response with infant's age, in which the latter is intercorrelated with developmental factors such as changes in body dimensions, duration of reaching experience, and reaching proficiency (Adolph, 1997). In this regard, it is important to note that Adolph's study (1997) on the locomotion over different slopes showed that the duration of crawling experience is a better predictor of adaptive response than age. Not only experience but also the infants’ level of skill may affect adaptiveness. Generally, more-skilled performers are better able to direct their attention to relevant properties, just like an expert rock climber may notice handholds and footholds that a novice would not (Gibson, 1969). Thus, infants’ reaching skill may also mediate adaptive responses. The literature provides scant data about effects of practice duration and skill on adaptive reaching. Previous work showed that older, probably more experienced and skilled reachers, behaved more adaptively than younger, probably less experienced and skilled reachers. The present study explores the possibility to investigate these factors separately in order to find whether practice time and infant's reaching skill are more important constraints than age.

There are many factors that can determine the level of reaching behavior in young infants, namely the number of opportunities to practice the movement, improvement in controlling arm movements, perception of the appropriate action for specific situations and inclination to reach (Lobo, Galloway, & Savelsbergh, 2004; Wimmers, Savelsbergh, Beek, & Hopkins, 1998; Van Hof et al., 2005). However, the question is: how do we determine that infants of the same age are more or less skilled in reaching? The works of Savelsbergh and Van der Kamp (1994) and Carvalho et al. (2007) provide us with a suggestion. Reaching movements were less controlled and coordinated in the infants that showed fewer reaches in the supine position. In contrast, better controlled reaching movements were observed in infants that showed more reaches in supine position. In other words, if an infant reaches more (i.e. a higher amount of reaches) in the supine body orientation, s/he is more-skilled in reaching. To classify infants as either less- or more-skilled reachers, we first identified the frequency of reaching in supine, reclined and seated orientations. Particularly the amount of reaching in supine was picked out as an indicator for the level of reaching skill.

The aim of the current exploratory study is to examine the effect of practice time on reaching movements for more- and less-skilled reachers in different body orientations. The experimental design adopted was longitudinal because it is well known that reaching does not emerge at the same age for every infant (Carvalho et al., 2007; Rocha et al., 2006; Thelen, Corbetta, Spencer, Schneider, & Zernicke (1993); Wimmers et al., 1998). The current experiment tracked infants’ reaching performance in different body orientations from their first reaching attempts to experienced reaching achieved several weeks later. Thus, the design controlled practice duration, but infants’ ages varied within test sessions. Anyway, all infants grew older across sessions. In order to determine the skill level at the acquisition of reaching, this behavior was evaluated not at a particular age, but at the ‘onset’ of reaching, i.e. at the first lab visit after the infants acquired the behavior.