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

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.