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. 2022 Oct 25:16:962936.
doi: 10.3389/fnhum.2022.962936. eCollection 2022.

Effect of whole-hand water flow stimulation on the neural balance between excitation and inhibition in the primary somatosensory cortex

Dat Le Cong  1   2   3 , Daisuke Sato  1   3 , Koyuki Ikarashi  1   2   3   4 , Tomomi Fujimoto  1   3 , Genta Ochi  1   3 , Koya Yamashiro  1   3
Affiliations

Effect of whole-hand water flow stimulation on the neural balance between excitation and inhibition in the primary somatosensory cortex

Dat Le Cong et al. Front Hum Neurosci. .

Abstract

Sustained peripheral somatosensory stimulations, such as high-frequency repetitive somatosensory stimulation (HF-RSS) and vibrated stimulation, are effective in altering the balance between excitation and inhibition in the somatosensory cortex (S1) and motor cortex (M1). A recent study reported that whole-hand water flow (WF) stimulation induced neural disinhibition in the M1. Based on previous results, we hypothesized that whole-hand WF stimulation would lead to neural disinhibition in the S1 because there is a strong neural connection between M1 and S1 and aimed to examine whether whole-hand WF stimulation would change the neural balance between excitation and inhibition in the S1. Nineteen healthy volunteers were studied by measuring excitation and inhibition in the S1 before and after each of the four 15-min interventions. The excitation and inhibition in the S1 were assessed using somatosensory evoked potentials (SEPs) and paired-pulse inhibition (PPI) induced by single- and paired-pulse stimulations, respectively. The four interventions were as follows: control, whole-hand water immersion, whole-hand WF, and HF-RSS. The results showed no significant changes in SEPs and PPI following any intervention. However, changes in PPI with an interstimulus interval (ISI) of 30 ms were significantly correlated with the baseline value before whole-hand WF. Thus, the present findings indicated that the whole-hand WF stimulation had a greater decreased neural inhibition in participants with higher neural inhibition in the S1 at baseline. Considering previous results on M1, the present results possibly show that S1 has lower plasticity than M1 and that the duration (15 min) of each intervention may not have been enough to alter the balance of excitation and inhibition in the S1.

Keywords: paired-pulse inhibition (PPI); primary somatosensory cortex (S1); repetitive somatosensory stimulation; somatosensory evoked potentials (SEP); whole-hand water flow stimulation.

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Conflict of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

FIGURE 1
FIGURE 1
Experimental procedure. Somatosensory evoked potential and paired-pulse inhibition were assessed before and after each intervention for 15 min. Each intervention was conducted in random order for each participant.
FIGURE 2
FIGURE 2
Single- and paired-pulse somatosensory evoked potential waveforms and subtracted waveform. Filled and open triangles show first and second electrical stimulation, respectively.
FIGURE 3
FIGURE 3
Sensory threshold before each intervention. There was no significant difference among the four interventions, as shown by the Friedman test. HF-RSS, high-frequency repetitive somatosensory stimulation.
FIGURE 4
FIGURE 4
Single-pulse N20-P25 amplitude before and after each intervention: (A) Control, (B) HF-RSS, (C) Whole-hand WI, and (D) Whole-hand WF. Generalized linear mixed model revealed that there was no significant interaction or main effect of “intervention” and “time,” except for the main effect of “intervention.” HF-RSS, high-frequency repetitive somatosensory stimulation.
FIGURE 5
FIGURE 5
PPI_5 ms of N20-P25 amplitude before and after each intervention: (A) Control, (B) HF-RSS, (C) Whole-hand WI, and (D) Whole-hand WF. Generalized linear mixed model revealed that there was no significant interaction or main effect of “intervention” and “time.” HF-RSS, high-frequency repetitive somatosensory stimulation.
FIGURE 6
FIGURE 6
PPI_30 ms of N20-P25 amplitude before and after each intervention: (A) Control, (B) HF-RSS, (C) Whole-hand WI, and (D) Whole-hand WF. Generalized linear mixed model revealed that there was no significant interaction or main effect of “intervention” and “time.” HF-RSS, high-frequency repetitive somatosensory stimulation.
FIGURE 7
FIGURE 7
Correlation between PPI_5 ms at pre-intervention and the ratio of before to after each intervention: (A) Control, (B) HF-RSS, (C) Whole-hand WI, and (D) Whole-hand WF. There was no significant correlation in any intervention. HF-RSS, high-frequency repetitive somatosensory stimulation.
FIGURE 8
FIGURE 8
Correlation between PPI_30 ms at pre-intervention and the ratio of before to after each intervention: (A) Control, (B) HF-RSS, (C) Whole-hand WI, and (D) Whole-hand WF. There was a significant negative correlation only in whole-hand water immersion, but not in other interventions. HF-RSS, high-frequency repetitive somatosensory stimulation.
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