| Transcranial direct current stimulation(tDCS)as a noninvasive regulation activity of cerebral cortex stimulation technology,has been widely applied to regulate the cognitive function of brain in recent years.At the same time,the tDCS stimulating effect on the performance of working memory,which is the basis of complex cognitive functions,is discussed by a lot of research.Although many studies have shown that The anodal tDCS applied to the dorsolateral prefrontal cortex can effectively improve the performance of working memory,the improvement effect of anodal tDCS is still inconsistent across different studies,and there are still results indicating contradictory conclusion of that anodal tDCS is improving working memory.In addition,the effects of cathode tDCS stimulation on working memory are still irregular.Thus,how does tDCS of different polarity regulate the performance of working memory? What factors are associated with the effect of the stimulus? These problems have not been well explained,so the mechanism of tDCS regulating working memory still needs to be further explored.In view of the above problem,in this study,sham tDCS,anodal tDCS and cathode tDCS were applied to the left dorsolateral prefrontal cortex to further analyze the mechanism of transcranial direct current stimulation intervening working memory from the perspectives of behavioral data and EEG characteristics.The EEG signal features include brain function network features and event-related potential features.At the same time,the relationship between the stimulation effect of tDCS and task difficulty is discussed.The main contents and innovations of this paper are as follows :(1)under the theoretical support of complex network analysis,this study constructs the topology structure of brain functional network according to the phase synchronization of the EEG from brain regions,and analyzes the changes of characteristic parameters of brain functional network before and after tDCS stimulation with different polarity and under various memory load tasks.The results showed that the enhancement effect of anode tDCS stimulation on working memory is resulted from the enhanced cortical activity around the stimulation point which leads to its stronger connections with other brain regions,and the enhancement effect was the most significant in the four-picture load task and the worst in the five-picture load task.After the stimulation of cathode tDCS,the network connectivity characteristics decreased,but thebehavioral data was irregular.According to the changes of the brain function network characteristic parameters as well as its topological structure,cathode tDCS stimulation results in that the key brain regions are suppressed then the brain automatically chooses other brain regions for compensatory utilities.Additioanally,heterogeneous locations of function area and the individual differences will lead to the difficulty in obtaining consistent rules for cathode tDCS stimulus effects on cognitive function.In addition,the study also found that five-picture load task has the strongest inhibitory effect under cathode tDCS stimulation.(2)Event-related potential(ERP)can reflect the neuroelectrophysiological changes of the brain in the cognitive process,but there are still large variations in the results of the current studies related to working memory,so the potential significance of ERP characteristics still needs to be further discussed.Considering the fact that the brain function network and ERP can reflect the brain information processing ability and there is limited literature to analyze the correlation between the two,this article illustrates the neural function mechanism of ERP from the perspective of the brain function network.The experimental results show that the P300 amplitude and average degree,average clustering coefficient,average global efficiency are positively correlated through Pearson correlation analysis,which also means that the higher the individual P300 amplitude,the stronger the connectivity between different brain regions.These results further prove that ERP can be used as an effective indicator for evaluating personal working memory performance.(3)This study believes that the stimulation effect of tDCS may be affected by many factors,among which the workload of working memory tasks is one of them.Therefore,it is discussed through the combination of brain function network characteristics and ERP characteristics.The P300 amplitude results show that under the same tDCS stimulation conditions,the amplitude of P300 in three-picture and four-picture load tasks increases with the increase of load,but the amplitude of the P300 in the fivepicture load task decreased instead of increasing.Combined with previous researches,it can be seen that this is the performance of P300 amplitude in too difficult tasks.Considering the characteristic changes of the brain function network,it can be further concluded that the short-term effect of anodal tDCS has an upper limit so the anodal tDCS can not play a good role in too difficult tasks,and when the brain is dealing with too difficult tasks,it will involve more brain regions for information integration resulting in the manifested inhibitory effect of cathodic tDCS stimulation in more brain regions.The above work provides strong evidence for the effect of TDCS stimulation on working memory tasks,and enriches the relevant research results. |
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