A Behavioral And F Nirs Study Of The Effectiveness Of Dual-Site HD-tDCS On Enhancing Response Inhibition Ability Of Military College Students

Background:Response inhibition is the ability to inhibit inappropriate or irrelevant actions so that one can make flexible,goal-directed behavioral responses based on environmental changes,which is one of the most important parts of cognitive ability.Response inhibition is not only vital for the executive control of healthy individuals’ behaviors,but also has important military significance.Previous studies revealed that the cortical neural substrates of response inhibition are mainly located in the right inferior frontal gyrus(rIFG)and the pre-supplementary motor area(pre-SMA).Transcranial direct current stimulation(tDCS)is a non-invasive brain stimulation and cognitive enhancement technique.Previous studies have shown that tDCS targeting rIFG or pre-SMA can improve response inhibition.However,there are still some limitations in the previous studies:(1)the results are heterogeneous and the effectiveness of tDCS needs more supporting evidence;(2)the spatial precision of conventional tDCS used in previous studies needs to be improved;(3)neuroimaging techniques were rarely used in previous studies;(4)the resting state functional connectivity(RSFC)changes of response-inhibition related cortices before and after tDCS intervention were unknown;(5)previous studies have focused on single-target stimulation over rIFG or pre-SMA,and the effect of dual-site anodal stimulation over both brain regions has never been investigated;(6)most of the previous studies were single-session stimulation,and few studies have investigated the effects of repeated sessions of tDCS or combined with concurrent cognitive training on improving response inhibition,hence,its effect needs to be explored.The purposes of this study were as follows:(1)to investigate changes in RSFC and brain network metrics in the response-inhibition related cortices(i.e.,rIFG and pre-SMA)after anodal high-definition tDCS(HD-tDCS)on the rIFG using resting-state functional near-infrared spectroscopy(f NIRS);(2)to investigate the behavioral and neurophysiological effects of different forms of anodal HD-tDCS on improving response inhibition,including HD-tDCS over the rIFG or pre-SMA and dual-site HD-tDCS over both areas;(3)to investigate the effects of 10 consecutive sessions of dual-site anodal HD-tDCS targeting rIFG and pre-SMA combined with concurrent cognitive training on improving response inhibition compared with 10 repeated sessions of dual-site stimulation or training alone,including long-lasting effects and transfer effects.Methods:This study included three parts.In the first part,the experiment followed a 2(stimulation type: rIFG stimulation and sham stimulation)× 2(time: pretest and posttest)two-factor mixed design.A group of 48 healthy military college students participated in the study and were randomly divided into two groups:(1)rIFG stimulation,n = 24;(2)sham stimulation,n = 24.Each participant received resting-state f NIRS for 7 min before and after either real or sham single-session HD-tDCS(1.25 m A,20 min)over the rIFG.ROI2 ROI FC,FC matrices,and efficiency-related parameters were analyzed for rIFG and pre-SMA based on the HbO,HbR,and HbT signals.In the second part of this study,the experiment followed a 4(stimulation type: rIFG +pre-SMA,rIFG,pre-SMA,and sham stimulation)× 2(time: pretest and posttest)two-factor mixed design.A total of 92 healthy military college students were randomly assigned to receive single-session(20 min)anodal HD-tDCS over rIFG + pre-SMA,rIFG,pre-SMA,or sham stimulation.Before and immediately after tDCS intervention,participants completed a stop-signal task(SST)and a go/nogo task(GNG).Their cortical activity was recorded using f NIRS during the go/nogo task.In the third part,the experiment followed a 4(intervention type: stimulation +training,stimulation,sham stimulation + training,and sham stimulation)× 3(time: pretest,posttest,and follow-up test)two-factor mixed design.Ninety-four healthy military college students were randomly assigned to undergo different interventions,including real stimulation combined with SST training,real stimulation,sham stimulation combined with SST training,and sham stimulation.Each intervention lasted 20 min daily for 10 consecutive days,and the stimulation protocol was from the second experiment,targeting rIFG and re-SMA simultaneously with a total current intensity of 2.5 m A.Performance on SST and possible transfer effects to Stroop task,attention network test,and N-back task were measured before and one day and one month after completing the intervention course.Results:Part one: The groups were matched in basic characteristics.For analyses based on HbR signals,the results showed that the ROI2 ROI FC was significantly elevated after rIFG stimulation relative to sham stimulation;the results of the FC matrix revealed that the FC between channel 2 and channel 24 was improved after rIFG stimulation.Based on HbT signals,the graph theory results showed that the majority of the stimulated nodes(1/5/8/12/19/26)in the rIFG stimulation group showed significantly elevated nodal efficiency(delta value)compared to the sham stimulation group.Part two: There were no significant differences in demographic and basic characteristics between the groups and hence the groups were matched.Behavioral data showed a significant decrease in SSRT after dual-site HD-tDCS that did not exist in the other conditions.Further analysis showed that significant reductions in SSRT were present in both the low-performance(LP)and high-performance(HP)subgroups after dual-site stimulation,as well as in the LP subgroup after rIFG stimulation.f NIRS data showed that only dual-site stimulation produced a significantly lower △HbO in the pre-SMA.However,pre-SMA tDCS modulated neither SSRT nor f NIRS signals significantly from pretest to posttest.The other indices in SST or GNG were not substantially altered for the real stimulation conditions relative to the sham stimulation condition.Part three: There were no significant differences in demographic and basic characteristics between the groups and hence the four groups were matched.The main findings showed that the combined protocol and the stimulation alone significantly reduced SSRT in the posttest and follow-up test compared to the pretest.However,training alone only decreased SSRT in the posttest.The sham control exhibited no changes.Subgroup analysis revealed that the combined protocol and the stimulation alone induced a decrease in the SSRT of the LP subgroup at the posttest and follow-up test compared with the pretest.However,only the combined protocol,but not the stimulation alone,improved the SSRT of the HP subgroup.The transfer effects were absent.Conclusions:This study demonstrates that anodal HD-tDCS on the rIFG can significantly enhance the functional connectivity between the rIFG and pre-SMA and increase the nodal efficiency of the response-inhibition cortical network more than sham stimulation.This study provides a new insight,based on the resting-state network of response inhibition,into the neural mechanism of the effects of tDCS at improving response inhibition.This study also provides important preliminary behavioral and neuroimaging evidence that dual-site HD-tDCS over the rIFG and pre-SMA is likely to be the most potent protocol for enhancing response inhibition ability in healthy individuals compared with commonly used single-target stimulation over rIFG or pre-SMA.Furthermore,this study provides supportive evidence for the synergistic effect of combining repeated tDCS and concurrent SST cognitive training,indicating better response inhibition improvement effects than the single intervention method.In addition,the long-term after-effects can persist for up to at least one month.Our findings provide a preliminary insight into strategies to enhance response inhibition ability for both psychiatric and non-psychiatric populations.