Development of reaching behavior from 9 to 36 months

概要

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The purpose of this study was to observe reaching from its primitive to its more complete form, using kinematic analysis. Subjects were 55 healthy full term infants, who were divided into age groups of 9, 12, 18, 24, and 36 months old. Test stimuli were transparent cubes of various sizes, cylinders, and small familiar objects. They were presented on a glass table through an aperture in a facing wall so that they lay in the subject's median plane. The infant's behavior was recorded via one television camera placed underneath the table. The screen image of reaching behavior on the video recorder was analyzed by an X-Y tracker. The trajectory and speed of the reaching were calculated. Findings were as follows: Both straightness and smoothness of hand movements increased with age. A significant change in smooth reaching was observed between 9 and 12 months old and thereafter. However, straightness deteriorated at 18 months for the right hand, and at 24 months for the left hand. Comparing these five groups with respect to velocity, acceleration and deceleration, the speed of movements increased significantly after 12 months, achieveing levels similar to those of adults. These findings suggest that significant developmental changes occur in reaching behavior between 9 and 12 months and again at 18 months.