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This Article Full Text Full Text (PDF) All Versions of this Article: 563/1/229    most recent jphysiol.2004.079525v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Cathers, I. Articles by Fitzpatrick, R. C Search for Related Content PubMed PubMed Citation Articles by Cathers, I. Articles by Fitzpatrick, R. C

¹ Prince of Wales Medical Research Institute and University of New South Wales, Sydney, Australia
² School of Biomedical Sciences, Faculty of Health Sciences, University of Sydney, Sydney, Australia
³ MRC Human Movement Group, Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, UK

We used galvanic vestibular stimulation (GVS) to identify human balance reflexes of the semicircular canals and otolith organs. The experiment used a model of vestibular signals arising from GVS modulation of the net signal from vestibular afferents. With the head upright, the model predicts that the GVS-evoked canal signal indicates lateral head rotation while the otolith signal indicates lateral tilt or acceleration. Both signify body sway transverse to the head. With the head bent forward, the model predicts that the canal signal indicates body spin about a vertical axis but the otolith signal still signifies lateral body motion. Thus, we compared electromyograms (EMG) in the leg muscles and body sway evoked by GVS when subjects stood with the head upright or bent forward. With the head upright, GVS evoked a large sway in the direction of the anodal electrode. This response was abolished with the head bent forward leaving only small, oppositely directed, transient responses at the start and end of the stimulus. With the head upright, GVS evoked short-latency (60–70 ms), followed by medium-latency (120 ms) EMG responses, of opposite polarity. Bending the head forward abolished the medium-latency but preserved the short-latency response. This is compatible with GVS evoking separate otolithic and canal reflexes, indicating that balance is controlled by independent canal and otolith reflexes, probably through different pathways. We propose that the short-latency reflex and small transient sway are driven by the otolith organs and the medium-latency response and the large sway are driven by the semicircular canals.

(Received 18 November 2004; accepted after revision 22 December 2004; first published online 23 December 2004)
Corresponding author R. C. Fitzpatrick: Prince of Wales Medical Research Institute, Easy Street, Randwick, NSW 2031 Australia. Email: r.fitzpatrick{at}unsw.edu.au

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