Postural control has been suggested as an important factor for early motor development, however, little is known about how infants acquire the ability to control their upright posture in changing environments and differing tasks. This dissertation addresses these issues in the first two years of life when infants learn to sit, stand, and walk. Three specific aims will be addressed: 1) to characterize the development of unperturbed infant upright postural sway; 2) to establish the influence of static somatosensory information on infant postural sway; and 3) to characterize the dynamic relationship between the infant's posture and sensory information. Three studies were conducted.

The first study longitudinally examined infants' quiet stance and the influence of static touch in the 9 months following walk onset. With increasing walking experience, infants' upright postural sway developed toward lower frequency and slower velocity without changing the amount of sway. Additional touch information attenuated postural sway and decreased the sway velocity without affecting the frequency characteristics. We concluded that early postural development may involve increasing the use of sensory information to tune sensorimotor relationships that enhance estimating self-motion in the environment. The second study longitudinally characterized infants' unperturbed sitting postural sway and the influence of static touch. A temporary disruption of infant sitting posture was observed around walk onset. Light touch contact attenuated sitting postural sway only at this transition when infants' posture became unstable. These results suggest a sensorimotor re-calibration process in infant postural control to accommodate the newly emerging bipedal behavior of independent walking. The third study systematically examined the adaptive visual-postural dynamics, specifically the frequency- and amplitude-dependent features, in a cross-sectional sample of infants as they develop from sitting to standing and walking. The results revealed that infants as young as sitting onset were able to control their sitting posture responding to an oscillating visual stimulus as well as to re-weight the visual information as the stimulus amplitude changes. However, newly sitting infants, compared to experienced walkers, were more responsive but variable when the stimulus amplitude was small. We conclude from these three studies that infant postural development involves a complementary process between improving postural control of self-motion and an increasing sensitivity to environmental motion