Anodal vestibular stimulation does not suppress vestibular reflexes in human subjects

Anodal current applied to the vestibular apparatus has previously been found to inhibit discharge from irregular vestibular afferents in squirrel monkeys. We wished to investigate whether anodal currents applied over the mastoid processes of human subjects would significantly reduce ongoing vestibular activity and thereby the size of galvanic-evoked vestibulospinal reflexes, measured by soleus electromyogram. Nine subjects were tested, of whom six subjects (five females, one male) provided data for the final analysis. Tonic anodal current was applied over one mastoid at 0 (baseline), 2, 4, 6 and 8 mA. The cathode was placed at C7. Superimposed on each intensity of tonic current were separate, short anodal currents of 4 mA, duration 20 ms, presented as 128 stimuli to the same side, and used to test vestibular responsiveness. These trials were then repeated with the anode overlying the contralateral mastoid. Short latency (SL) and medium latency (ML) reflexes were measured from the right soleus muscle. All six subjects used in the final analysis had readily identifiable reflexes to all stimuli. One-way ANOVA revealed no significant difference in the magnitude of the SL (P=0.99) or ML (P=0.96) components of the vestibulospinal reflexes across the group. Despite surface anodal currents of up to 8 mA, there was no consistent effect on the size of galvanic-evoked vestibulospinal reflexes. As 8 mA is close to the maximum intensity tolerated by volunteer subjects, our results indicate that anodal current applied over the mastoids is unlikely to be a useful means of suppressing vestibular function in human subjects.