The impact of object size and rigidity on infant reaching

Abstract

Although the changes in kinematics of infant reaching have been studied, few researchers have investigated the improvement of reaching regarding objects of distinct physical properties. The aim of this longitudinal study was to verify the impact of object size and rigidity on the development of reaching in 4–6-month-old infants. Four infants were observed with a motion capture system during trials with four objects of distinct sizes and rigidity. A total of 188 reaches were analyzed by using the 3D movement reconstruction. Our results showed that reaching frequency, mean velocity, and straightness index increased with age. The number of movement units decreased with age and increased for small objects. Rigidity was not shown to affect reaching trajectories. These findings suggest that infants are capable of perceiving the more relevant object properties, thus using their available motor capabilities to modify the essential variables so that they can reach the target more accurately.

Introduction

The emergence of reaching for objects is an important milestone in motor and cognitive development of infants during their first 6 months of life (Corbetta, 1998; Thelen, Corbetta, Spencer, Schneider, & Zernicke, 1993). From acquiring and improving this skill, infants learn to interact with the environment and consequently become capable of adjusting their movements so that they can reach the target more accurately.

Studies have shown that purposeful reaching emerge at around the third to fourth month of age in healthy infants, and once it is acquired, such skill is continuously improved (von Hofsten, 1979, von Hofsten, 1984, Thelen et al., 1996). This process involves many different changes in arm movements. For instance, infants begin to perform reaching movements followed by grasping (Newman, Atkinson, & Braddick, 2001; Savelsbergh & van Der Kamp, 1994; Thelen & Spencer, 1998), and their reaches become smoother, straighter and accurate (Fallang, Saugstad, & Hadders-Algra, 2000; von Hofsten, 1991; Konczak & Dichgans, 1997; Thelen et al., 1996).

To better understand the changes in reaching movements, it seems necessary to take account of the environmental factors, since they are essential to comprehend the complexity as well as the adaptability of infant organisms. According to Gibson (1986), hand movements toward the object are modulated by affordances, that is, action possibilities available in the environment to the organism. As motor skills develop, infants become capable of perceiving new affordances, which, in turn, make new actions possible (Adolph, Eppler, & Gibson, 1993; Gibson, 1995). This perception–action cycle is, therefore, essential to infants acquire and improve functional actions, such as reaching for objects (Barela, 2001, Gibson, 1986; Gibson & Pick, 2000; von Hofsten, 2004).

Some studies have investigated the improvement of reaching concerning the influences of object physical properties. Newell, Scully, McDonald, and Baillargeon (1989) and Newell, McDonald, and Baillargeon (1993) showed that 4–5-month-old infants performed one-handed reaching for small objects and two-handed reaching for large objects. Corbetta, Thelen, and Johnson (2000) verified that object size and rigidity affected adjustments of proximal (one-or-two handed reaching) as well as distal movements (grasping) in 8–9-month-old infants. Pryde et al. (1998) demonstrated that objects of different sizes influenced spatio-temporal variables of reaching in 9–10-year-olds, suggesting that small objects require more corrections at the end of the trajectory.

Although the literature presents data regarding the impact of object properties on infant reaching in different age groups, no research has investigated whether distinct physical properties can influence kinematic characteristics of reaching in young infants. The aim of the current study, therefore, was to verify how size and rigidity might affect spatio-temporal variables of reaching in 4–6-month-old infants and how this might change over age.

To meet the objectives of this study, two hypotheses were tested. First, given that infants can improve their abilities to act on affordances throughout their development, we predict that spatio-temporal variables of reaching will change between 4 and 6 months of age, when reaching trajectory will be straighter, faster, and with fewer corrections. Second, given that infants are capable of adjusting proximal and distal arm movements when reaching for objects of distinct physical properties, we predict that spatio-temporal variables will change according to size and rigidity. Moreover, reaching trajectory will be less straight, slower, and more corrections will be required for both small and rigid objects. This hypothesis is based on the assumption that reaching for small objects requires more accuracy, control and coordination of arms (Brandão, 1984; Pryde et al., 1998), and that young infants have difficulty grasping rigid objects (Corbetta et al., 2000). In summary, we believe that the infants will change spatio-temporal variables of their reaches as they perceive the object affordances and become able to coordinate their movements based upon perception–action coupling.

This study may provide further information about which factors might influence reaching, thus contributing towards a better understanding of the complexity and adaptability of infant movements concerning the manipulation of object physical properties. Moreover, our findings may be applied to the clinical practice of professionals who work in the field of infant development.