Position and Velocity Coupling of Postural Sway to Somatosensory Drive.
John Jeka, Kelvin Oie, Gregor Sch”ner, Tjeerd Dijkstra & Elaine Henson.
Department of Kinesiology, University of Maryland, College Park, MD 20742,
Centre de Recherche en Neurosciences Cognitives, Marseille, France, Department
of Psychology, Ohio State University, Columbus, OH 43210.
APStracts 4:368N, 1997.
ABSTRACT
Light touch contact of a fingertip to a stationary surface provides
orientation information that enhances control of upright stance. Slight
changes in contact force at the fingertip lead to sensory cues about the
direction of body sway, allowing attenuation of sway. In the present study,
the coupling of postural sway to a moving contact surface was investigated in
detail. Head, center of mass and center of pressure displacement were measured
as the contact surface moved rhythmically at 0.1, 0.2, 0.4, 0.6 and 0.8 Hz.
Stimulus amplitude decreased with frequency to maintain peak velocity constant
across frequency. Head and body sway were highly coherent with contact surface
motion at all frequencies except 0.8 Hz, where a dropoff in coherence was
observed. Mean frequency of head and body sway matched the driving frequency
up to 0.4 Hz. At higher frequencies, non 1:1 coupling was evident. The phase
of body sway relative to the touch plate averaged 20-30_ at 0.1 Hz drive and
decreased approximately linearly to -130_ at 0.8 Hz drive. System gain was
approximately 1 across frequency. The large phase lags observed cannot be
accounted for with velocity coupling alone, but indicate that body sway was
also coupled to the position of the touch plate. Fitting of a linear second
order model to the data suggest that postural control parameters are not
fixed, but adapt to the moving frame of reference. Moreover, coupling to both
position and velocity suggest that a spatial reference frame is defined by the
somatosensory system.
Received 28 April 1997; accepted in final form 2 December 1997.
APS Manuscript Number J331-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 12 December 1997