The temporal organization of posture changes during the first year of independent walking
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Although the development of upright posture has received considerable attention, the quiet stance of infants in their first months of learning this fundamental behavior has not been well studied. The purpose of the present study was to characterize the time evolutionary properties, or temporal organization, of these infants’ unperturbed upright stance as well as to elucidate how somatosensory information influences that organization. Six healthy, full-term infants were tested monthly from walk onset until 9 months of independent walking experience while standing either independently or touching a static surface. The structure of sway was assessed through stabilogram-diffusion analysis using an exponential Ornstein-Uhlenbeck characterization. The results of this analysis revealed two new insights into postural development. First, the developmental changes in quiet stance involved a decreased rate at which sway decays to maximal variance, rather than an attenuation of the magnitude of that variance. Specifically, measures indexing amount of sway variance were significantly reduced when touching a static surface as compared with an independent stance condition, but revealed no change with increased walking experience. Further, a reduction in the average rate constant of decay indicated an increased influence of long time-scale sway corrections on the overall sway trajectory. Second, it was shown that, at early walk ages, the use of touch both reduced the amount of variance and shifted the rate constant of sway towards longer time-scale displacements. Taken in the context of previous research, these results support our conclusion that early postural development embodies the dual tasks of calibrating sensorimotor relations for estimation of self-motion as well as identification and tuning of control system properties.
KeywordsPosture Stabilogram Sensorimotor Development Human infant
This research was supported by National Science Foundation Grant #9905315 (J. Clark). Portions of this research were presented at the annual meetings of the North American Society for the Psychology of Sport and Physical Activity (NASPSPA) in Baltimore, MD, 2002 and Savannah, GA, 2003 and the International Conference on Infant Studies, Chicago, IL, 2004.
The authors wish to thank Tim Kiemel for discussions regarding mathematical implications and applications of SDA methods, Larry Douglass for guidance on the statistical analysis and all of our research assistants who helped collect and code the data included in this paper. Finally, we wish to express special gratitude to the parents and infants who gave willingly of their time and effort throughout the duration of this study.
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