Elsevier

Clinical Neurophysiology

Volume 113, Issue 11, November 2002, Pages 1709-1714
Clinical Neurophysiology

Interaction effects of galvanic vestibular stimulation and head position on the soleus H reflex in humans

https://doi.org/10.1016/S1388-2457(02)00238-9Get rights and content

Abstract

Objectives: The objective of this study is to measure the influence of galvanic vestibular stimulation and head-on-body alignment on the soleus H-reflex in prone human subjects.

Methods: We studied changes in the amplitude of the right soleus H-reflex to monopolar monaural galvanic stimulation in 10 healthy prone lying subjects. Trials were randomly administered according to head position (left or right) and stimulus polarity (anode or cathode). We also investigated the influence of the conditioning stimuli by examining the amplitude of the H-reflex based on head position without galvanic stimulation. A one-way and two-way repeated measures analysis of variance were used to compare the mean amplitudes of the test and conditioned H-reflexes.

Results: The greatest facilitation was observed when the head was turned to the left and coupled with cathodal stimulation. The largest inhibitory effect was found when the head was turned to the right and paired with anodal stimulation. However, when head left was paired with anodal stimulation or head right was coupled with cathodal stimulation, only minimal changes in amplitude were observed.

Conclusions: The results demonstrate that there is an interaction between head position and galvanic stimulus polarity when measuring the excitability of the soleus motoneuron pool in prone lying subjects.

Introduction

Galvanic vestibular stimulation (GVS) has been widely used as a non-painful technique for activating the human vestibular system (Watson and Colebatch, 1997, Day et al., 1997). Typically, a low-intensity stimulus, delivered to an electrode placed over the mastoid process, can modulate the firing frequency of the irregularly firing afferents of the vestibular nerve (Goldberg et al., 1984). An anodal or positive stimulus tends to decrease the firing frequency whereas a cathodal or negative stimulus increases the level of activity of the irregular afferents (Minor and Goldberg, 1991). This change in the discharge properties of the vestibular nerve causes an upright individual to sway toward the anodal stimulus and away from the cathodal stimulus (Lund and Broberg, 1983).

During quiet stance, GVS produces changes in the electrical activity (electromyogram, EMG) of postural muscles, such as the soleus (Nashner and Wolfson, 1974, Iles and Pisini, 1992). The galvanically induced EMG response has both a short and medium latency component that are opposite in polarity (Britton et al., 1993, Watson and Colebatch, 1997). The magnitude of both responses appears to be dependent on the overall stability or steadiness of the individual (Welgampola and Colebatch, 2001). In fact, when subjects were either braced or seated, transient EMG responses in the soleus muscle were absent (Storper and Honrubia, 1992, Britton et al., 1993). This has led some to speculate that the GVS-evoked muscle responses observed in the soleus muscle are task dependent (Fitzpatrick et al., 1994).

Recently, it was reported that the amplitude of the soleus H-reflex could be influenced by GVS in prone human subjects (Kennedy and Inglis, 2001). This was the first study to show a galvanically induced vestibulospinal response in a quiescent muscle that was not posturally engaged. Interestingly, the timing of the effect coincided with the medium latency response observed during quiet stance (Britton et al., 1993, Watson and Colebatch, 1997). That is, the greatest change in H-reflex amplitude occurred when the galvanic stimulus was delivered 100 ms prior to the H-reflex stimulus, regardless of stimulus polarity (Kennedy and Inglis, 2001). However, the amplitude of the soleus H-reflex was modulated in the prone position with GVS only as the subjects faced forward.

While there is evidence to suggest that the position of the head can influence the GVS response in standing subjects (Nashner and Wolfson, 1974, Iles and Pisini, 1992), it is also known that rotation of the head can independently modulate the excitability of the motoneuron pool (Hayes and Sullivan, 1976, Traccis et al., 1987). Interestingly, vestibular and neck afferents activate the same neurons within the vestibular nuclei (Wilson, 1991). Consequently, these sensory afferents produce opposite responses, which cancel one another out at the level of the motoneuron pool (Kasper et al., 1988). However, it is not clear how GVS, an artificial vestibular input, may interact with a natural stimulus, such as head-on-body alignment when subjects are lying in a prone position.

Therefore, the purpose of the present study is to examine the interaction between head position and galvanic stimulus polarity. We wished to establish under what, if any, conditions could head position and artificial vestibular stimuli produce an optimal response at the level of the soleus motoneuron pool. The amplitude of the soleus H-reflex was measured in prone human subjects during periods of galvanic stimulation. In each condition, the head was either rotated towards the left or right shoulder. This allowed us to assess the impact of GVS and head-on-body alignment on the amplitude of the soleus H-reflex. The results suggest that although GVS can influence the excitability of the soleus motoneuron pool in prone human subjects, the overall effect is dependent upon the position of the head.

Section snippets

Methods

This experiment involved 10 healthy volunteers between the ages of 24 and 40 years (mean age=31.2 years). Written, informed consent was obtained and the clinical research ethics board at the University of British Columbia approved the following procedures. Subjects lay prone on a padded bed with both legs extended. The right foot was elevated and secured to a brace to limit ankle movement. The ankle angle was held constant at approximately 90°, measured as the internal angle between the lower

Results

Changes in the amplitude of the soleus H-reflex in response to the conditioning stimuli are illustrated in Fig. 1 for a representative subject. Superimposing the test and conditioned reflexes shows that the overall effect of GVS on the H-reflex in a prone individual is modulated by head-on-body alignment. This effect was monitored in 1200 H-reflexes from all 10 subjects. Between trials, the absolute amplitudes of both the M-waves and test H-reflexes did not exceed 5% of the total amplitude.

Discussion

This study shows that while it is possible to elicit a vestibulospinal response with GVS in prone lying individuals (Kennedy and Inglis, 2001), this response is highly dependent upon the position of the head relative to the trunk. For instance, anodal stimulation decreased the amplitude of the H-reflex when the head was positioned to the right. Conversely, cathodal stimulation significantly increased the amplitude of the conditioned reflex when the head was turned to the face the left shoulder.

Acknowledgements

This research was supported by an operating and equipment grant from the Natural Sciences and Engineering Research Council of Canada (J.T.I.) and an NSERC Studentship Scholarship supported P.M.K.

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