| Background:As a high-level cognitive process,executive functions enable individuals to deploy cognitive resources in a rational manner to efficiently complete complex tasks.Executive functions guide individuals’ behavior toward goals,including initiating and stopping actions.At the same time,executive functions monitor and change behavior,enabling individuals to predict future outcomes and adapt to new and changing situations.Several studies have shown that executive function is closely related to an individual’s health,academic performance,interpersonal relationships,and marital well-being.As the future pacesetters of the force,military medical college cadets face complex work tasks and a variety of challenges.Studies have shown that executive function is highly correlated with the successful completion of a number of security and combat readiness tasks,such as attempting unexpected missions and ladder evacuation of the sick and wounded on the battlefield.Therefore,the enhancement of executive function is critical for military medical college trainees.Currently,the mainstream methods to enhance executive function are cognitive training and Noninvasive Brain Stimulation(NIBS).Cognitive training is time-consuming and requires a high level of subject cooperation.Common NIBS such as Transcranial Direct Current Stimulation(t DCS)and Transcranial Magnetic Stimulation(TMS).Their safe,noninvasive,convenient and effective advantages have been widely recognized in the field of cognitive enhancement research.Studies in the literature show that t DCS requires multiple applications to achieve the desired intervention effect,while TMS has the risk of triggering suprathreshold stimulation and causing harm to the subject.Recently,some scholars combined transcutaneous aricular vagus nerve stimulation(ta VNS)with t DCS for immediate interventions in healthy adult populations.The brain imaging results showed that the "simultaneous t DCS combined with ta VNS" intervention method was more effective.It remains to be investigated whether repeated "simultaneous t DCS combined with t AVNS" interventions will have an effect on executive function enhancement in military medical college trainees.Therefore,this study aims to further investigate the behavioral characteristics and brain neural mechanisms of the repeated "synchronized t DCS combined with t AVNS" intervention on executive functions of military medical trainees.Subjects and Methods:This study consisted of two parts.The experiment was conducted by a randomized double-blind sham-stimulated control method.The subjects in both parts of the study were the same,all 72 undergraduate trainees from military medical universities,and were completely randomized into 3 groups of 24 trainees each.The specific groups were: Sham group(negative control group),t DCS group(positive control group),and combined group("simultaneous t DCS combined with ta VNS" intervention method,experimental group).Since the purpose of this study was to explore the effect of "simultaneous t DCS combined with t AVNS" on executive function,and t DCS is traditionally recognized as an effective NIBS method to improve executive function,it was used as the positive control group,while t AVNS was not significant for cognitive improvement in previous studies.A control group with ta VNS alone for intervention was not set up in the study.The specific intervention protocols were set as follows: Sham group with both t DCS and ta VNS in sham stimulation mode;t DCS group with t DCS in normal working mode and ta VNS in sham stimulation mode;and combined group with both t DCS and ta VNS in normal working mode.The intervention period was 3 days,twice a day(30 min each time,the first intervention of the day at 09:00 and the second intervention of the day at 16:00),for a total of 6 sessions.In the first part,subjects were tested at baseline levels of executive functioning behavior and then the corresponding intervention was administered according to the intervention protocol according to subgroups.A two-factor repeated measures ANOVA with3(three groups: Sham group,t DCS group,and combined group)× 4(four measurements:baseline,one day post-intervention,one week post-intervention follow-up,and one month post-intervention follow-up)was used to statistically analyze the behavioral scores of the three subsystems of executive function(working memory,cognitive flexibility,and inhibitory control)and to investigate whether the "simultaneous t DCS The behavioral characteristics of the "combined t DCS and ta VNS" intervention in executive function were investigated.In the second part,f NIRS was used as a neuroimaging tool to assess the resting state functional connectivity in the prefrontal and adjacent brain regions.A one-way(number of measurements: baseline,one day post-intervention,one week post-intervention follow-up,and one month post-intervention follow-up)repeated measures ANOVA was used to statistically analyze the resting state f NIRS data in each of the three groups of subjects.To investigate the neurological mechanisms underlying the "simultaneous t DCS combined with ta VNS" intervention in executive function.Research Results:Part I:(1)Compared with the Sham group,both the t DCS group and the combined group had significantly higher(P<0.05;P<0.001)correct rates(Accuracy Rate,ACC)for the executive function subsystem working memory letter 2-back behavioral task and significantly lower(P<0.05;P < 0.001);switching cost in the cognitive flexibility digit switching task were significantly decreased(P<0.05;P<0.001);inconsistent trial and neutral trial RT in the inhibitory control CW-Stroop task were significantly decreased(P<0.05;P<0.05,P<0.001),and the rest were not statistically different;(2)at the time of January follow-up,the joint group was significantly different from the t DCS group compared to the t DCS group in terms of working memory,cognitive flexibility level and cognitive processing speed(inhibition of control neutral trial versus inconsistent trial RT)still maintained the intervention advantage,and its enhancement effect was better than that of the t DCS group.In the second part:(1)the strength of functional connectivity in the prefrontal and adjacent brain regions was significantly weaker in the combined group after the intervention compared with that before the intervention;the change in the strength of functional connectivity before and after the intervention was smaller in the t DCS and Sham groups;(2)for the percentage of channel connectivity with significant change in the correlation coefficient before correction,the combined group was much higher than the t DCS and Sham groups by chi-square test(P<0.001);the P value for Bonferroni correction revealed a significant weakening of functional connectivity in the prefrontal and adjacent brain regions in the joint group and no significant changes in the t DCS and Sham groups.Research conclusions:Part I: Both the t DCS intervention and the "simultaneous t DCS combined with ta VNS" intervention improved working memory and cognitive flexibility,and to some extent,cognitive processing speed(reduced RT on CW-Stroop paradigm neutral trials and inconsistent trials).And the intervention effect was stronger and longer lasting in the combined group than in the t DCS group.The second part: the "simultaneous t DCS combined with ta VNS" intervention effectively reduced the functional connectivity strength in the prefrontal and adjacent brain regions,indicating that this intervention improved the connectivity efficiency in the prefrontal and adjacent brain regions.This study investigated the behavioral characteristics and neurological mechanisms of the "synchronized t DCS combined with ta VNS" intervention on the improvement of executive function in military medical students,using both behavioral measures of executive function and f NIRS resting state monitoring in prefrontal brain regions to provide a conceptual justification for the further extension of the "synchronized t DCS combined with ta VNS" intervention. |
PDF Full Text Request