Influence of the intensity of galvanic vestibular stimulation and cutaneous stimulation on the soleus H-reflex in healthy individuals.

Modulation of the soleus H-reflex following galvanic vestibular stimulation (GVS) has been used to evaluate vestibulospinal tract function. It is not known whether this modulation is because of vestibular stimulation and/or cutaneous stimulation, and the suitable stimulating intensity of GVS for the modulation is not established. We investigated the influence of GVS intensity and cutaneous stimulation on the soleus H-reflex in healthy adults. We applied cathodal GVS (at 1, 2, and 3 mA) or 3-mA cutaneous stimulation as a conditioning stimulation in a random order to 17 individuals in the prone position with the head facing forward, and we examined the changes in the right soleus H-reflex amplitude. We maintained the interval between the conditioning stimulation's onset and the tibial nerve stimulation evoking the soleus H-reflex constant at 100 ms. The amplitude of all conditioned H-reflexes was significantly larger than that of the unconditioned H-reflexes. The greatest facilitation of the H-reflex occurred when 3-mA GVS was applied. The degree of facilitation of the H-reflex by 3-mA GVS was significantly larger than that produced by the 3-mA cutaneous stimulation. These findings indicate that (a) the facilitation of the soleus motor neuron pool excitability by GVS is derived from both vestibular stimulation and cutaneous stimulation, and (b) the intensity of GVS affects the degree of facilitation. When this technique is used to examine vestibulospinal tract function, no less than 3 mA GVS may be appropriate as the conditioning stimulation.