EFFECT OF ALTERED SENSORY CONDITIONS ON MULTIVARIATE DESCRIPTORS OF HUMAN POSTURAL SWAY

Multivariate descriptors of sway were used to test whether altered sen sory conditions result not only in changes in amount of sway but also in postural coordination. Eigenvalues and directions of eigenvectors o f the covariance of shnk and hip angles were used as a set of multivar iate descriptors. These quantities were measured in 14 healthy adult s ubjects performing the Sensory Organization test, which disrupts visua l and somatosensory information used for spatial orientation. Multivar iate analysis of variance and discriminant analysis showed that result ing sway changes were at least bivariate in character, with visual and somatosensory conditions producing distinct changes in postural coord ination. The most significant changes were found when somatosensory in formation was disrupted by sway-referencing of the support surface (P = 3.2 . 10(-10)). The resulting covariance measurements showed that su bjects not only swayed more but also used increased hip motion analogo us to the hip strategy. Disruption of vision, by either closing the ey es or sway-referencing the visual surround, also resulted in altered s way (P = 1.7 . 10(-10)), with proportionately more motion of the cente r of mass than with platform sway-referencing. As shown by discriminan t analysis, an optimal univariate measure could explain at most 90% of the behavior due to altered sensory conditions. The remaining 10%, wh ile smaller, are highly significant changes in posture control that de pend on sensory conditions. The results imply that normal postural coo rdination of the trunk and legs requires both somatosensory and visual information and that each sensory modality makes a unique contributio n to posture control. Descending postural commands are multivariate in nature, and the motion at each joint is affected uniquely by input fr om multiple sensors.