Frequency-dependent modulation of muscle sympathetic nerve activity by sinusoidal galvanic vestibular stimulation in human subjects.

We have previously demonstrated that selective modulation of vestibular inputs, via sinusoidal galvanic vestibular stimulation (GVS) delivered at 0.5-0.8 Hz, can cause partial entrainment of muscle sympathetic nerve activity (MSNA). Given that we had seen interaction between the dynamic vestibular input and the normal cardiac-locked MSNA rhythm, we tested the hypothesis that frequencies of GVS remote from the cardiac frequency would cause a greater modulation of MSNA than those around the cardiac frequency. Bipolar binaural sinusoidal GVS (+/-2 mA, 200 cycles) was applied to the mastoid processes in 11 seated subjects at frequencies of 0.2, 0.5, 0.8, 1.1, 1.4, 1.7 and 2.0 Hz. In all subjects, the stimulation evoked robust vestibular illusions of "rocking in a boat" or "swinging from side to side." Cross-correlation analysis revealed a cyclic modulation of MSNA at all frequencies, with the modulation index being similar between 1.1 Hz (78.5 +/- 3.7%) and 2.0 Hz (77.0 +/- 4.3%). However, vestibular modulation of MSNA was significantly stronger at 0.2 Hz (93.1 +/- 1.7%) and significantly weaker at 0.8 Hz (67.2 +/- 1.8%). The former suggests that low-frequency changes in vestibular input, such as those associated with postural changes, preferentially modulate MSNA; the latter suggests that vestibular inputs compete with the stronger baroreceptor inputs operating at the cardiac rhythm (approximately 0.8 Hz), with vestibular modulation of MSNA being greater when this competition with the baroreceptors is reduced.