Human body‐segment tilts induced by galvanic stimulation: a vestibularly driven balance protection mechanism.

Abstract

1. We have studied the effects of changes in posture on the motor response to galvanic vestibular stimulation (GVS). The purpose of the experiments was to investigate whether the function of the GVS-evoked response is to stabilize the body or the head in space. Subjects faced forwards with eyes closed standing with various stance widths and sitting. In all cases the GVS-evoked response consisted of a sway of the body towards the anodal ear. 2. In the first set of experiments the response was measured from changes in (i) electromyographic activity of hip and ankle muscles, (ii) the lateral ground reaction force, and (iii) lateral motion of the body at the level of the neck (C7). For all measurements the response became smaller as the feet were placed further apart. 3. In the second set of experiments we measured the GVS-evoked tilts of the head, torso and pelvis. The basic response consisted of a tilt in space (anodal ear down) of all three segments. The head tilted more than the trunk and the trunk tilted more than the pelvis producing a leaning and bending of the body towards the anodal ear. This change in posture was sustained for the duration of the stimulus. 4. The tilt of all three segments was reduced by increasing the stance width. This was due to a reduction in evoked tilt of the pelvis, the bending of the upper body remaining relatively unchanged. Changing from a standing to a sitting posture produced additional reductions in tilt by reducing the degree of upper body bending. 5. The results indicate that the response is organized to stabilize the body rather than the head in space. We suggest that GVS produces a vestibular input akin to that experienced on an inclined support surface and that the function of the response is to counter any threat to balance by keeping the centre of mass of the body within safe limits.Passing a small current across the mastoid processes stimulates the vestibular system and in standing subjects evokes a sway of the body. This 'galvanic vestibular stimulation' (GVS) acts by modulating the spontaneous firing of vestibular afferents, increasing their firing frequency on

Short-duration galvanic vestibular stimulation evokes prolonged balance responses

“…Previous authors observed biphasic CoP responses using longduration GVS pulses (Ͼ1-s stimuli), with the larger, second response directed toward the anode (8,24). The polarity and timing of the first and second CoP components observed by these authors correspond to the characteristics of the second and third CoP components observed here.…”

“…Such stimuli have been thought to evoke motor responses with minimal or no balance response, but it remains to be determined whether such brief GVS pulses specifically evoke vestibulomotor responses or if they evoke a concommittant balance response. The primary aim of the present study was to quantify the muscle and balance responses associated with brief 20-ms GVS pulses.Muscle and whole body responses elicited by GVS during quiet standing have been mostly described, regardless of the body and CoP position (8,38,39). One notable exception, however, is a study by Marsden et al (23), who showed that asymmetric stance alters vestibular-evoked postural responses.…”

“…Muscle and whole body responses elicited by GVS during quiet standing have been mostly described, regardless of the body and CoP position (8,38,39). One notable exception, however, is a study by Marsden et al (23), who showed that asymmetric stance alters vestibular-evoked postural responses.…”

“…GVS alters the firing rates of all vestibular afferents with a preference toward the irregular afferents (12), such that the anode decreases and the cathode increases afferent firing rates (13). In standing humans, binaural, bipolar GVS applied for a long duration (1-2 s) elicits a well-defined biphasic muscle response (4,9) and a multiphasic center of pressure (CoP) response often identified by lateral sway and tilt toward the side of the anode (8,21,28). Short-duration GVS (20 -50 ms) have also been used to evoke vestibulomotor responses in lower and upper limb muscles engaged in balance (4,40).…”

The medium latency muscle response to a vestibular perturbation is increased after depression of the cerebellar vermis

“…There is literature to suggest that the SL response may not be consistent or reliable when the vestibular stimulus is below 2 mA (Ali, Rowen, & Iles, 2003; Fitzpatrick et al., 1994). Additionally, the quality of the reflexes may have been compromised with head orientation, as the reflexes are obtainable [as evidenced by the results of our study and other experiments (Dakin, Son, Inglis, & Blouin, 2007; Day et al., 1997; Forbes et al., 2016; Nashner & Wolfson, 1974)], yet not as prominent in the soleus muscle with the head facing forward (Fitzpatrick et al., 1994). Most studies examining vestibular reflexes have subjects stand with their head facing over the shoulder (Britton et al., 1993; Fitzpatrick et al., 1994; Lund & Broberg, 1983; Nashner & Wolfson, 1974; Welgampola & Colebatch, 2001), however, we opted to test the reflexes with head forward as we have shown in a previous experiment that depressing the cerebellar vermis with cTBS can result in a modulation in the postural sway direction, such that it is no longer intra‐aural with the head facing over the shoulder (Lam et al., 2016).…”