Lack of otolith involvement in balance responses evoked by mastoid electrical stimulation

TY - JOUR

T1 - Lack of otolith involvement in balance responses evoked by mastoid electrical stimulation

AU - Mian, Omar S

AU - Dakin, Christopher J

AU - Blouin, Jean-Sébastien

AU - Fitzpatrick, Richard C

AU - Day, Brian L

PY - 2010/11/15

Y1 - 2010/11/15

N2 - Passing current through mastoid electrodes (conventionally termed galvanic vestibular stimulation; GVS) evokes a balance response containing a short- and a medium-latency response. The origins of these two responses are debated. Here we test the hypotheses that they originate from net signals evoked by stimulation of otolith and semi-circular canal afferents, respectively. Based on anatomy and function, we predicted the directions of the stimulus-evoked net head rotation vector from the canals and the linear acceleration net vector from the otoliths. We tested these predictions in healthy adults by obtaining responses with the head in strategic postures to alter the relevance of the signals to the balance system. Cross-covariance between a stochastic waveform of stimulating current and motor output was used to assess the balance responses. Consistent with the canal hypothesis, with the head pitched down the medium-latency EMG response was abolished while the short-latency EMG response was maintained. The results, however, did not support the otolith hypothesis. The direction of the linear acceleration signal from the otoliths was predicted to change substantially when using monaural stimuli compared to binaural stimuli. In contrast, short-latency response direction measured from ground-reaction forces was not altered. It was always directed along the inter-aural axis irrespective of whether the stimulus was applied binaurally or monaurally, whether the head was turned in yaw through 90 deg, whether the head was pitched down through 90 deg, or combinations of these manipulations. We conclude that a net canal signal evoked by GVS contributes to the medium-latency response whilst a net otolith signal does not make a significant contribution to either the short- or medium-latency responses.

AB - Passing current through mastoid electrodes (conventionally termed galvanic vestibular stimulation; GVS) evokes a balance response containing a short- and a medium-latency response. The origins of these two responses are debated. Here we test the hypotheses that they originate from net signals evoked by stimulation of otolith and semi-circular canal afferents, respectively. Based on anatomy and function, we predicted the directions of the stimulus-evoked net head rotation vector from the canals and the linear acceleration net vector from the otoliths. We tested these predictions in healthy adults by obtaining responses with the head in strategic postures to alter the relevance of the signals to the balance system. Cross-covariance between a stochastic waveform of stimulating current and motor output was used to assess the balance responses. Consistent with the canal hypothesis, with the head pitched down the medium-latency EMG response was abolished while the short-latency EMG response was maintained. The results, however, did not support the otolith hypothesis. The direction of the linear acceleration signal from the otoliths was predicted to change substantially when using monaural stimuli compared to binaural stimuli. In contrast, short-latency response direction measured from ground-reaction forces was not altered. It was always directed along the inter-aural axis irrespective of whether the stimulus was applied binaurally or monaurally, whether the head was turned in yaw through 90 deg, whether the head was pitched down through 90 deg, or combinations of these manipulations. We conclude that a net canal signal evoked by GVS contributes to the medium-latency response whilst a net otolith signal does not make a significant contribution to either the short- or medium-latency responses.

KW - Adolescent

KW - Adult

KW - Electric Stimulation

KW - Electromyography

KW - Head Movements

KW - Humans

KW - Mastoid

KW - Muscle, Skeletal

KW - Orientation

KW - Otolithic Membrane

KW - Postural Balance

KW - Posture

KW - Reaction Time

KW - Young Adult

U2 - 10.1113/jphysiol.2010.195222

DO - 10.1113/jphysiol.2010.195222

M3 - Article

C2 - 20855437

SN - 1469-7793

VL - 588

SP - 4441

EP - 4451

JO - Journal of Physiology

JF - Journal of Physiology

IS - Pt 22

ER -