The effects of galvanic vestibular stimulation (GVS) on the firing probability of single motor units (MUs) after combined motor cortex and ulnar nerve stimulation.
A–H show the peristimulus time histograms (PSTHs) of a single MU in the biceps brachii (BB) muscle after separate ulnar nerve stimulation (NERVE) at 1.0 × motor threshold (MT) of the first dorsal interosseous muscle (A, E), separate transcranial magnetic stimulation (TMS) (B, F) over the contralateral primary motor cortex at 1.25 × active motor threshold of the BB, and combined stimulation (COMB) (C, G) in the control (A–D) and anodal GVS conditions (E–H). Each PSTH was obtained after 50 stimuli. The counts in these PSTHs were subtracted by the mean counts during a 50 ms prestimulus period. D and H show differential PSTHs after subtraction of the summed PSTHs after separate stimuli from the PSTHs of the COMB. The number of counts in each bin was normalized by the number of triggers. The vertical dashed line represents the onset of the excitatory peak in the PSTH after separate TMS. The superimposed waveforms in the upper right corner of each PSTH show the MU action potentials (n = 50) obtained from each stimulus trial. I–L indicate the peak counts of the MU firings in the differential PSTHs in the control and anodal GVS conditions obtained from 31 MUs that were investigated with the NERVE set at 1.0 × MT (I, J) and 20 MUs that were investigated with the NERVE set at 0.75 × MT (K, L). The error bars represent 1 standard deviation. The analysis window was set at a predefined period (1.0 ms duration) that started 1.0 ms after the onset of the TMS-induced excitatory peak in the PSTH. **p < 0.01.