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

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.

No effect of whole-hand water flow stimulation on skill acquisition and retention during sensorimotor adaptation

“…Several studies have reported that the prior facilitation of S1 excitability facilitates skill acquisition during sensorimotor adaptation ( Celnik et al, 2007 ; Veldman et al, 2018 ). In contrast, our previous study found that whole-hand WI and WF did not affect S1 excitability ( Le Cong et al, 2022 ), although we did not directly measure S1 excitability in this study. Based on previous results, whole-hand WI and WF did not influence skill acquisition during sensorimotor adaptation because they did not affect S1 excitability.…”

“…Besides the tactile and pressure sensory inputs from water, it has also been confirmed to induce skin vibration at approximately 70 Hz (Sato et al, 2014b). Previous neurophysiological studies have found that whole-hand WF induces neural disinhibition in M1 (Sato et al, 2014b) and not in S1 (Le Cong et al, 2022), whereas whole-hand WI (involving only the hand in water) and non-immersed controls had no effect in either M1 or S1. The discrepancy in the response to whole-hand WF between M1 and S1 is presumably due to the lower plasticity of S1 compared to M1 (Nakatani-Enomoto et al, 2012).…”

“…However, the scientific evidence for improving movement through sensorimotor adaptation is limited. Recently, we developed a novel RSS using water flow (WF) called “whole-hand WF.” ( Sato et al, 2014b ; Le Cong et al, 2022 ) Unlike general local electrical stimulation, this RSS can provide sustained stimulation over a large site on the hand ( Godde et al, 1996 ; Rocchi et al, 2017 ). Besides the tactile and pressure sensory inputs from water, it has also been confirmed to induce skin vibration at approximately 70 Hz ( Sato et al, 2014b ).…”

“…The discrepancy in the response to whole-hand WF between M1 and S1 is presumably due to the lower plasticity of S1 compared to M1 ( Nakatani-Enomoto et al, 2012 ). Considering these distinct responses to whole-hand WF between S1 and M1 ( Sato et al, 2014b ; Le Cong et al, 2022 ) and that M1 involves memory retention during sensorimotor adaptation but not acquisition and consolidation ( Hamel et al, 2017 ), whole-hand WF-induced plasticity in M1 may modulate motor retention during sensorimotor adaptation.…”