EMG responses in the soleus muscles evoked by unipolar galvanic vestibular stimulation

Abstract

This study compared the effects of transmastoid galvanic stimulation with unilateral galvanic stimulation of vestibular afferents. We recorded the effects on soleus EMG occurring at short (SL) and medium (ML) latency, both in normal subjects and in patients with previous unilateral vestibular neurectomy. Unipolar cathodal and anodal stimulation on the same side produced opposite effects for both SL and ML responses. Responses to unilateral cathodal or anodal stimulation were smaller, but otherwise resembled those of transmastoid stimulation with the cathode or the anode placed on the same side, respectively. Unilateral cathodal stimulation resulted in a larger SL response, which occurred at shorter latency than unilateral anodal stimulation. With unipolar stimulation on the side of previous vestibular nerve section, typical SL and ML responses were absent. With stimulation of the intact side, the patients showed smaller SL responses than normal subjects with unilateral stimulation. The larger responses to unilateral cathodal compared to unilateral anodal stimulation are consistent with previous reports that cathodal stimulation produces an increase and anodal a decrease in vestibular nerve firing. The smaller SL responses in the patients may be a consequence of central nervous system reorganization following unilateral vestibular nerve section.

Introduction

Transmastoid direct current (`galvanic') stimulation has long been used as a means of vestibular activation to induce both ocular and postural movements. It is only relatively recently that characteristic EMG changes occurring within postural muscles have been defined (e.g. Nashner and Wolfson, 1974; Iles and Pisini, 1992; Britton et al., 1993; Fitzpatrick et al., 1994). These studies have shown that in standing subjects lower limb EMG excitability changes of opposing types occur at short (approximately 60 ms) and medium (approximately 100 ms) latencies (Britton et al., 1993; Fitzpatrick et al., 1994) following the onset of the stimulus. These effects are profoundly influenced by polarity of stimulation, head and body posture and postural task (Nashner and Wolfson, 1974; Tokita et al., 1989; Britton et al., 1993; Fitzpatrick et al., 1994). Primary vestibular afferents are characterized by a high level of tonic discharge, and studies of the effects of DC current in monkeys have shown that both excitatory and inhibitory effects may follow stimulation, depending on its polarity (Goldberg et al., 1984). Thus transmastoid galvanic stimulation, as used in human subjects, has the potential to produce its effects through simultaneously exciting one vestibular apparatus and inhibiting the other.

Clearly, the ability to stimulate each vestibular apparatus separately would be advantageous, both in furthering our understanding of basic physiology, and also for any clinical applications. Such lateralized stimulation montages have been used in the past in studies of induced eye movements (e.g. Skurczynski and Ernst, 1989) and posture (e.g. Coats and Stoltz, 1969; Watanabe et al., 1989; Cass et al., 1996).

The object of the present experiments was to investigate the lower limb EMG responses to lateralized vestibular stimulation and to compare these to those occurring with transmastoid stimulation. In addition, we have studied a series of patients with unilateral vestibular nerve section. The absence of EMG responses with unipolar stimulation on the side of the sectioned nerve would provide strong evidence confirming that the responses are vestibular dependent, and that the DC current acts at, or distal to, the level of the vestibular nerve fibres.