The Number or Type of Stimuli Used for Somatosensory Stimulation Affected the Modulation of Corticospinal Excitability

Abstract

Motor evoked potentials (MEPs) evoked by transcranial magnetic stimulation (TMS) a few milliseconds after this cortical activity following electrical stimulation (ES) result in an inhibition comparable to that by TMS alone; this is called short-latency afferent inhibition (SAI). Cortical activity is observed after mechanical tactile stimulation (MS) and is affected by the number of stimuli by ES. We determined the effects of somatosensory stimulus methods and multiple conditioning stimuli on SAI in 19 participants. In experiment 1, the interstimulus intervals between the conditioning stimulation and TMS were 25, 27 and 29 ms for ES and 28, 30 and 32 ms for MS. In experiment 2, we used 1, 2, 3 and 4 conditioning stimulations of ES and MS. The interstimulus interval between the ES or MS and TMS was 27 or 30 ms, respectively. In experiment 1, MEPs were significantly decreased in both the ES and MS conditions. In experiment 2, MEPs after ES were significantly decreased in all conditions. Conversely, MEPs after MS were significantly decreased after one stimulus and increased after four stimulations, indicating the SAI according to the number of stimuli. Therefore, the somatosensory stimulus methods and multiple conditioning stimuli affected the SAI.

Keywords: electrical stimulation; mechanical tactile stimulation; short-latency afferent stimulation; transcranial magnetic stimulation.

Figure 1

Schematic representation of somatosensory stimulation and transcranial magnetic stimulation (TMS). (A) Paradigm of experiment 1. Motor evoked potentials were measured by TMS alone (Aa: single), electrical stimulation conditions (Ab: ISI, 25 ms; Ac: ISI, 27 ms; Ad: ISI, 29 ms) and mechanical tactile stimulation conditions (Ab: ISI, 28 ms; Ac: ISI, 30 ms; Ad: ISI, 32 ms). (B) Paradigm of experiment 2. Motor evoked potentials were measured by TMS alone (Ba: single) and four different numbers of repetitions (Bb: 1 stim; Bc: 2 stim; Bd: 3 stim; Be: 4 stim) of each somatosensory stimulation; the interstimulus interval between the somatosensory stimulation and TMS was set to 27 ms (electrical stimulation) and 30 ms (mechanical tactile stimulation).

Figure 2

Mean motor evoked potentials (MEPs) of each condition. Correlation and comparison of the motor evoked potential (MEP) ratio after each conditioning stimulus. (A) Pearson’s correlation analysis revealed a significant correlation between the maximum inhibitory effects of electrical stimulation and mechanical tactile stimulation in each participant. (B) The inhibitory effects of MEP after electrical stimulation were significantly higher than those after mechanical tactile stimulation. Data are expressed as the mean ± standard error of the mean (SEM).

Figure 3

Correlation and comparison of the motor evoked potential (MEP) ratio after each conditioning stimulus. (A) Pearson’s correlation analysis revealed a significant correlation between the maximum inhibitory effects of electrical stimulation and mechanical tactile stimulation in each participant. (B) The inhibitory effects of MEP after electrical stimulation were significantly higher than those after mechanical tactile stimulation. Data are expressed as the mean ± standard error of the mean (SEM).

Figure 4

Modulation of motor evoked potentials (MEPs) according to the number of stimuli in each conditioning. Data are expressed as the mean ± standard error of the mean (SEM). (A) Modulation of MEP amplitudes with each number of electrical stimulations. The post hoc analysis showed a significant decrease in MEP in the presence of conditioning stimulation compared with that in the presence of single stimulation (1 stim, p < 0.001; 2 stim, p = 0.001; 3 stim, p < 0.001; 4 stim, p < 0.001). (B) The post hoc analysis showed a significant decrease in MEP at the conditioning stimulation of 1 stim compared with that at single stimulation (p = 0.030) and a significant increase in MEP at 4 stim (p = 0.015).