Effects Of Transcranial Alternating Current Stimulation On Working Memory And Motor Control During An Upper Limb Dual-Task

ObjectiveWorking Memory(WM)is the cognitive ability to quickly memorize and use the memory content for the current task.Studies have shown a specific correlation between working memory and academic scores,athletic performance and balance control of the elderly.The N-back test is currently the most commonly used working memory test method.This task requires the participant to determine whether the current stimulus is consistent with the previous N stimulus.However,working memory in real life is often used in a complex surrounding environment and accompanied by other tasks.Therefore,whether the N-back test completed in a quiet laboratory environment can truthfully reflect the working memory ability in real-world scenarios remains to be further demonstrated.In recent years,there have been more and more discussions on the methods of improving working memory.As a result,the regulation of non-invasive brain stimulation(NIBS)on the central nervous system has attracted more and more attention.This technology applies a weak current to the skull to achieve regulating nerve excitability.Transcranial alternating current stimulation(tACS),which is one of the main techniques of NIBS,can entrain a specific frequency that realizes a functionbased oscillation.This stimulation method is considered to have the potential of improving cognitive abilities.Existing research results show that tACS can improve cognitive abilities,including executive function,attention,and memory.However,whether tACS can improve working memory in real-world situations is also a question worthy of research.Based on the above background,this research is summarized for the following three purposes:(1)According to the N-back paradigm,design a dual-task test method that combines motor and cognitive tasks.Explore the correlation between the two tasks and verify the influence of memory load on the accuracy and reaction time of the N-back test and dual-task test.(2)Use the biomechanical method to describe the dual-task performance and compare the changes of upper limb movement characteristics and joint coordination under single-task(only motor task)and dual-task conditions.(3)Explore the effects of tACS on the accuracy and reaction time of N-back and the effects of upper limb movement characteristics and multi-joint coordination in dual-task situations.Verify the regulating effect of tACS technology in improving working memory.MethodsThis research is designed as a random double-blind crossover experiment.A total of 16 healthy male college students were recruited.Each participant visited the laboratory twice and randomly received active and sham tACS.N-back paradigm test and dual-task test were performed before and after the stimulation of each visit.The stimulation was performed using a transcranial electrical stimulation device from Neuroconn,Germany.A high-density stimulation protocol(4×1)was used(peak current2 m A,frequency 5 Hz,duration 20 minutes).The left dorsolateral prefrontal was chosen as the targeted brain area.According to the international 10-20 EEG system,the centre electrode(+2m A)is placed at point F3,and the surrounding electrodes(-0.5m A)are placed at FC3,F5,AF3,and F1.The N-back test paradigm is completed in a separate and quiet room and contains two difficulty levels(1-back and 2-back).The stimulation material uses capital English letters.In the 1-back task,participants need to determine whether the current letter is consistent with the previous letter.Use the E-prime system to collect and calculate the correct rate and reaction time.The dual-task test designed in this research is in the form of throwing(action task)superimposed on N-back(cognitive task),and two difficulty levels of 1-back and 2-back are also set.In addition,a single-task condition(only motion task)was also set up in the research.The tested stimulus materials are arrows in three directions: forward,left,and right.In the 1-back task,the participant needs to throw in the direction of the previous arrow when the current stimulus appears.Subjects with single-task conditions only need to throw according to the direction of the arrow currently appearing.Use Vicon motion capture system(sampling frequency 100Hz)to collect biomechanical data,and import the data into Visual3D software to calculate upper limb joint kinematics and joint coordination indexes.Different statistical methods were used based on the research purpose.Paired Sample t-Test was used to verify the effect of memory load on N-back and dual-task performance.Pearson Correlation Analysis was used to verify the correlation between N-back test performance and dual-task performance.One-way repeated-measures ANOVA was used to verify the effect of dual-task on motor performance and coordination.Two-way(time × group)repeated-measures ANOVA to verify the effect of transcranial alternating current stimulation on N-back test performance and dual-task performance.The statistical indicators are expressed in the form of Mean±SD,and the significance level is set at P<0.05.Results(1)The accuracy and reaction time of the N-back test are significantly affected by the memory load.With the increase of memory load,the accuracy rate was significantly reduced(p=0.001),and the reaction time was significantly prolonged(p=0.001).The impact of memory load on dual tasks is mainly reflected in the accuracy rate,decreasing significantly as the difficulty increases(p=0.01).The results of correlation analysis show that the correlation between the two tasks is low on the indicators of correctness and reaction time.(2)Compared with the single task,there are significant differences in the action time,throwing speed,maximum acceleration(acceleration),maximum acceleration(deceleration)and other indicators of the dual-task: the action duration under the 2-back condition is significantly increased compared with single-task(p=0.009);the throwing speed of 1-back and 2-back conditions is significantly reduced compared to single-task conditions(p=0.035,p=0.001);the maximum acceleration under 1-back conditions is significantly reduced(p=0.019);the maximum deceleration increases significantly under the 2-back condition(p=0.017).In addition,joint coordination changes under the1-back condition and the slope of the elbow-wrist joint angle-angle diagram fitting curve is significantly lower than that of the single-task condition(p=0.018).(3)The influence of tACS on the N-back test mainly accelerates the reaction time.Both the active and sham stimulation produced significant changes: 1-back(p=0.006,p=0.016),2-back(p=0.004,p=0.008).The effect of tACS on dual tasks is mainly manifested in increasing the accuracy rate,and only true stimulation is significant: 1-back(p=0.004),2-back(p=0.023).The maximum deceleration of the hand increased significantly after active stimulation(p=0.040),but this result only appeared under single-task conditions.The effect of tACS on the kinematics of the upper limbs is mainly presented in the 1-back task.The range of motion of the wrist joint(p=0.019)and the peak angular velocity of the wrist joint(p=0.025)increase significantly after active stimulation.The dual-task cost of the 2-back condition decreased significantly after the active stimulation(p=0.026).In the 1-back task,the slope of the fitting curve of the elbow-wrist joint angle-angle diagram increased significantly after the active stimulation(p<0.001).In the 2-back task,the change(pre vs post)in the active and sham stimulation showed a different trend.After active stimulation,the index significantly decreased(p<0.001),and after sham stimulation,the index increased significantly(p<0.001).Conclusion(1)This research designed a new test task combining throwing action and working memory,this task is different from the N-back task.Since the complexity of the task is closer to the real scene,the new task can be regarded as a more practical working memory test method to comprehensively assess the working memory of the subjects in the cognitive and action-interlaced environment.(2)The working memory test method designed in this research can be regarded as a cognitive-motor dual-task test.As the difficulty of cognitive tasks increases,the dual-task cost increases and the performance of motor tasks decreases.Specifically,it is manifested in the decrease of the accuracy rate,the slowing of the throwing speed,the increase of the action time,the slowing down of acceleration,the speeding up of deceleration,and the deterioration of elbow-wrist joint coordination.(3)The tACS program used in this study has a particular effect on improving working memory,which is reflected in the performance of the N-back task(speeding up the reaction time)and the performance of dual-task(improve the accuracy rate).In particular,the effect of promoting the dual-task is more obvious,which significantly improves the accuracy of throwing.In addition,the movement characteristics of the wrist joint also changed,which is reflected in the increase of the joint range of motion of the wrist joint and the increase of the peak joint angular velocity.As a result,the joint coordination between the elbow and wrist is significantly close to the single-task state,and the motion control is optimized.