Interaction effects of galvanic vestibular stimulation and head position on the soleus H reflex in humans
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.
References (20)
- et al.
Modulation of the soleus H-reflex in prone human subjects using galvanic vestibular stimulation
Clin Neurophysiol
(2001) - et al.
Modifications and development of spinal reflexes in the alert baboon (papio papio) following an unilateral vestibular neurotomy
Brain Res
(1976) Convergence of macular vestibular and neck inputs on vestibulospinal and reticulospinal neurons projecting to the lumbosacral cord
Prog Brain Res
(1988)- et al.
Influence of head position and proprioceptive cues on short latency postural reflexes evoked by galvanic stimulation of the human labyrinth
Brain Res
(1974) The role of Renshaw cells in the dynamic control of posture during vestibulospinal reflexes
Prog Brain Res
(1988)- et al.
Influences of neck receptors on soleus motoneuron excitability in man
Exp Neurol
(1987) - et al.
EMG responses in the soleus muscles evoked by unipolar galvanic vestibular stimulation
Electroenceph clin Neurophysiol
(1997) - et al.
Postural electromyographic responses in the arm and leg following galvanic vestibular stimulation in man
Exp Brain Res
(1993) - et al.
Human body-segment tilts induced by galvanic stimulation: a vestibularly driven balance protection mechanism
J Physiol (Lond)
(1997) - et al.
Task-dependent reflex responses and movement illusions evoked by galvanic vestibular stimulation in standing humans
J Physiol (Lond)
(1994)
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