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  • ELSEVIER Gait and Posture 6 (1997) 210-217

    Monaural and binaural galvanic vestibular stimulation in human dynamic balance function

    Alexandra SCverac Cauquil a**, Philippe Bousquet b, Marie-Claude Costes Salon , Philippe Dupui a, Paul Bessou a

    a Centre de Recherche Cerveau et Cognition UMR 5549, Facultl de Mkdecine, 133 Route de Narbonne, 31062 Toulouse cedex, France b Service de Neurochirurgie, Hapital de Rangueil, I Avenue Jean Pouilh& 31403 Toulouse cedex 4, France

    Accepted 13 November 1996


    Spontaneous dynamic balance reactions to galvanic binaural and monaural stimulation were investigated in lateral sway. Low-intensity currents were used to stimulate the vestibular apparatus of subjects, who were standing with their eyes closed on a rocking platform. Head and body base movements were measured in the lateral plane, simultaneously. In monaural as well as in binaural stimulation mode, the onset of the current induced a lateral biphasic stereotyped postural responses: firstly the pressure centre moved towards the cathode side then the whole body swayed towards the anode side. The cut-off of the current resulted in a similar pattern of movement but in the opposite direction. Two main features characterised the response obtained in monaural mode. First, the amplitude of the dynamic balance response was half the size of that recorded in binaural stimulation mode. Second, cathodal monaural stimulation on one side or anodal monaural stimulation on the opposite side elicited superimposable responses. The results demonstrate that in healthy subjects, postural response to binaural galvanic vestibular stimulation results from the linear sum of two equivalent stimulations: one from the cathodal excitation and the other from the anodal inhibition. The method could be used in clinical studies to detect vestibular asymmetries and dysfunction. 0 1997 Elsevier Science B.V.

    Keywords: Vestibular system; Galvanic stimulation; Dynamic balance; Human

    1. Introduction

    Galvanic vestibular stimulation (GVS), though known since the last century [l], has not been widely used either in experimental or in clinical vestibular investigations, this is probably because the mechanisms of action of the electric current on the vestibular ap- paratus long remained questionable. There is evidence that the current has a peripheral action [2] and could act by modulating the tonic firing rate of vestibular afferent. In squirrel monkey, Goldberg et al. [3] have shown that externally applied cathodal current en-

    *Corresponding author. Tel.: + 33 5621 72835; fax: + 33 5621 72809.

    0966-6362/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO966-6362(97)0001 1-8


    hances the vestibular nerve discharge whereas anodal current decreases it.

    Early observations [4] described in humans the re- sponses to GVS according to the polarity of the stimu- lus. A postural response was observed for lower intensities (head and body tilt, at 1 mA), ocular re- sponses required higher intensities (nystagmus, at 3 mA). The postural response in humans has been usually recorded using a force-platform [5,6], electromyo- graphic activity of leg muscles [7- lo], or accelerometers placed at different sites of the body [9] of subjects standing still, usually on a stable base. In these condi- tions, the bipolar binaural galvanic stimulation of the vestibular apparatus has been shown to induce stereo- typed body sway response, directed along the binaural

  • Fig. I. (A) The stabilometer used to elicit and assess spontaneous dynamic balance function: platform with a curved base in contact with the ground through a line, the pivot (p.). When the platfotm tilts, a lever (1.) fixed to an extremity of the platform slides on the ground and rotates around its point of fixation. The rotation is assessed by an optic coder (o.c.). (B) The ataxiameter used to measure head linear displacement in the pivot plane. A string tightened between the head band (h.b.) fixed around the head and a pulley transmits head displacements to an optic coder (cc.) fixed to the shaft of the pulley. (C) Position of the subject on the platform to measure lateral body sway.

    axis towards the ear bearing the anode [I 1,121. When the stimulus polarity is reversed, the GVS-produced balance reaction is inverted. Nevertheless, this does not indicate the part each side of the vestibular system is playing in the whole response.

    The clinical interest of GVS has already been sug- gested and shown (e.g. [1,13-171. However, as the oculomotor response was mainly chosen as the response parameter, relatively strong intensities of stimulation were used ( > 1 mA). Moreover, the stimulations were often applied binaurally, which may be not ideal to assess unilateral vestibular function. Indeed Dix et al clinical observations [13,14] pointed out the interest of monaural GVS.

    In the present work, we investigated the hypothesis that the body response to binaural GVS in humans results from the linear sum of two equivalent opposite actions, exerted on each vestibular apparatus: excita- tion on one side (cathodal stimulation) and inhibition on the other side (anodal stimulation). This could be of some interest in clinical assessment of unilateral vestibular function. For that, we compared the effect on balance of a bipolar monaural and a bipolar binau- ral stimulation of subjects facing straight ahead. Spon- taneous dynamic balance conditions were used to enhance the response since the effect of monaural GVS was expected half as large as the binaural one. Indeed, the responses to GVS are larger when the subjects stand on an unstable platform [lo]. In a previous study on motion sickness [ 1 S], standardised dynamic balance proved to be a sensitive and efficient means to detect the postural responses to GVS. The present study only deals with lateral sway since the subjects are facing straight ahead, anteroposterior body response to GVS only occurring when subjects rotate their head or trunk so as to align their binaural axis to the sagittal plane.

    Part of the present data have been presented in a brief form [19,20].

    2. Methods

    2.1. Gulvanic vestibular stimulatiw

    The stimulus was delivered by a DC stimulator driven by the microcomputer through two disposable electrodes (1 cm diameter) coated with conductive jelly and stuck on the skin. The stimulation device was a stimulator operating with 9-V batteries and driven by infrared command (thus unplugged in the mains power). The intensity of the current used was 0.4 mA, sufficient to induce a postural response [2X] but low enough to avoid any local cutaneous sensation and therefore awareness of the stimulation. The duration of the stimulation (7 s) was long enough to study and differentiate both phasic and tonic effects of the current application. Two different stimulation patterns were used, binaural and monaural. In the binaural mode the electrodes were situated over the mastoid bones, in the monaural mode one electrode was stuck on the fore- head and the other on the mastoid bone.

    Spontaneous dynamic balance conditions were ob- tained by asking the subject to stand on a stabilometer [22] derived from Freeman platforms used to develop co-ordination of the calf muscles [23]. The stabilometer consisted of a square platform (50 x 50 cm) supported by a segment of a cylinder (radius 55 cm, 6 cm height when horizontal) laying on the ground (Fig. 1). The fixed characteristics of the stabilometer produce stan-

  • 212 A. SPwrac Cuuquil er al. _I Gait and Posture 6 (1997) 210-217

    Table 1

    Cathode right Cathode left

    R s PI P2 R s Pl P2

    HMA (cm) 5.2 + 0.2 14.7 + 0.6 14.4 + 0.6 5.6 + 0.3 4.9 ) 0.2 14.3 + 0.5 13 +0.4 5.4 * 0.2 PMA (cm) 7.0 + 0.4 13.1 * 0.4 13.4 * 0.4 7.5 + 0.4 6.6 f 0.3 12.5 i. 0.3 12.5 + 0.3 6.9 + .0.3

    Means (n = 30) and standard errors of the maximum amplitude of the head (HMA) and pivot (PMA) displacements, in lateral dynamic balance, measured before (R), during (S) and after (Pl and P2) binaural galvanic vestibular stimulation.

    dardised conditions for subjects instability. The device reduces the area of support of a subject standing on it to a 50-cm line called the pivot. Because of the geome- try of the platform, at a given instant, the position of the pivot on the ground is the vertical projection of the centre of pressure of the subject on the platform. The measurement of the linear displacements of the pivot on the ground induced by a tilt of the platform was computed from the rotation of a lever, one extremity of which laid freely on the ground and slided with mini- mal friction according to the tilts of the platform. The lever rotation was assessed by an optical coder linked to a microcomputer fitted with a specific program (fre- quency sampling: 100 Hz; accuracy: 0.7 mm). The mechanical response time of the subject-stabilometer system (mean weight 75 kg) had been calculated by recording the pivot deviation elicited by a twitch of the triceps surae muscle obtained by stimulating the poste- rior tibia1 nerve (ptn) in both anteroposterior and lat- eral sway. We subtracted the delay of the EMG response to this stimulation (approximately 8 ms, [24]) from the response delay measured on averaged graphs (n = SO), giving a 178-ms ( + 8.13) mechanical response time. Therefore latencies could be calculated by sub- tracting the mechanical delay of the recording system from the response delay measured on the graphs.

    Head position, as well as support surface position, was recorded to analyse a global body response. An ataxiameter [25,26] was used to measure the head movements in the same plane as that of the pivot movements (Fig. 1B). A string was tightened between the head and a pulley by a small weight (20 g), the axis of the pull