Proceedings of The Physiological Society

University College London December 2005 (2006) Proc Physiol Soc 1, C6

Oral Communications

Body sway responses evoked by audio frequency vibration of the mastoid process in standing humans

Welgampola, Miriam S; Day, Brian L;

1. Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, London, United Kingdom.

Audio frequency mastoid vibration is known to activate vestibular afferents in humans and experimental animals (1,2). However, balance responses evoked by mastoid vibration have not been reported thus far. In the present study we measured whole body responses evoked by mastoid vibration and endeavoured to determine their origin. Ten healthy subjects were studied while standing upright upon a force platform, with their feet together and eyes shut. 500 Hz bone-conducted tones of 127 dB FL (force level) and 2 s duration were delivered over the mastoid process using a B71 clinical bone vibrator. The 3-D movement of a marker attached over the C7 spinous process was measured at 200 Hz using a motion-capture system (CODA mpx 30). Ground reaction forces were measured using a force platform (Kistler). In a baseline study, right mastoid, left mastoid or null stimuli were delivered randomly to subjects facing forwards. Subjects were then studied with their heads rotated by 45 and 90 deg to the left and the right sides. Because low frequency vibration applied directly to neck muscles may produce body sway (3) we compared the response to mastoid stimulation with that evoked by stimulation directly above the external acoustic meatus or along the length of the sternocleidomastoid (SCM) muscle. Mastoid vibration evoked a force response that commenced at <200 ms and peaked at <450 ms. The medio-lateral (ML) components were oppositely directed for left and right sided stimulation while antero-posterior (AP) components had a similar direction. This resulted in oblique sway away from the stimulated ear and forwards. Peak force amplitudes (mean

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