Journal of Vestibular Research 14 (2004) 307–319 307 IOS Press Postural responses exhibit multisensory dependencies with discordant visual and support surface motion Emily A. Keshner a,b,∗ , Robert V. Kenyon c and Jessica Langston a a Sensory Motor Performance Program, Rehabilitation Institute of Chicago, Chicago IL, USA b Department of Physical Medicine and Rehabilitation, Feinberg School of Medicine, Northwestern University, Chicago IL, USA c Department of Computer Science, University of Illinois at Chicago, Chicago IL, USA Received 6 June 2003 Accepted 10 December 2003 Abstract. The purpose of this study was to identify how the postural system weights coincident yet discordant disturbances of the visual and proprioceptive/vestibular systems. Eleven healthy subjects (25–38 yrs) received either fore-aft translations of an immersive, wide field-of-view visual environment (0.1 Hz, ± 3.7 m/sec), or anterior-posterior translations of the support surface (0.25 Hz, ± 15 cm/sec), or both concurrently. Kinematics of the head, trunk, and shank were collected with an Optotrak system and angular motion of each segment plotted across time. With only support surface translation, segmental responses were small (1 ◦ –2 ◦ ) and mostly opposed the direction of sled translation. When only the visual scene was moving, segmental responses increased as the trial progressed. When the inputs were presented coincidentally, response amplitudes were large even at the onset of the trial. Mean RMS values across subjects were significantly greater with combined stimuli than for either stimulus presented alone and areas under the power curve across subjects were significantly increased at the frequency of the visual input when both inputs were presented. Thus, intra-modality dependencies were observed, such that responses to the visual inputs significantly increased and responses to the somatosensory signals reflected the stimulus amplitude only when the two inputs were combined. We believe it unlikely that the role of any single pathway contributing to postural control can be accurately characterized in a static environment if the function of that pathway is context dependent. Keywords: Virtual reality, vestibular, postural control, motion analysis, sensory re-weighting 1. Introduction There is significant evidence that dynamic visual in- puts induce postural instability through measures of in- creased sway during quiet stance [26,46,47]. Studies with visual field motion have demonstrated large cen- ter of pressure changes [9,10,31], with the most robust ∗ Corresponding author: Dr. Emily Keshner, Sensory Motor Per- formance Program, Rehabilitation Institute of Chicago, Room 1406, 345 East Superior Street, Chicago, IL 606011, USA. Tel.: +1 312 238 2228; Fax: +1 312 238 2208; E-mail: eak@northwestern.edu. postural changes in the roll and pitch planes [14,46] and at frequencies below 0.2 Hz [4,21,31] Motion of the visual field affects more than center of pressure val- ues, however. Velocity and frequency of visual field stimuli have been correlated with individual segmen- tal velocities [10,13,24,27,28,35], muscle electromyo- graphic amplitudes [12], and the direction of gaze [17] in quietly standing subjects. We might hypothesize that when all sensory systems are functioning and subjects are faced with coincident disturbances in the visual field and at the base of sup- port, they are able to disregard the information from the environment (visual inputs) and attend only to their in- ISSN 0957-4271/04/$17.00  2004 – IOS Press and the authors. All rights reserved