The Neural Mechanism Of Generation, Execution And Inhibition Of Motor Intention

ObjectiveMotor intention is an internal goal occurs before or during the movement. Unlike reflective movements, intentional movements meet one's goals consciously. Neuroelectrophysiology as an effective method has contributed to the research of the neural mechanisms of motor intention.For the convenience of experimental researches, motor intention was decomposed into three aspects:what, when and whether. "What" means one can decide freely to do what, including which side of limbs and task rules. "When "means one can decide freely when to execute movements and "whether" means one can decide freely whether to execute the movements or not. Previous researches focus more on the first two aspects and less on the "whether" because of methodology limitation. Contrast to response inhibition that is triggered by external signals, Libet argued in 1980s'that one has the ability to suppress his/her movements voluntarily. But little is known about its mechanisms. Previous researches suggested that right inferior frontal gyrus was an important area being involved in response inhibition. Recently functional neuroimaging researches showed that left medial frontal cortex had more activation under voluntary inhibition condition than that under voluntary action condition, indicated that this area was also a critical area being involved in voluntary inhibition. Experimental evidence indicates that there are different neural mechanisms not only between voluntary inhibition and response inhibition, but also between voluntary inhibition and voluntary action. There are two kinds of voluntary inhibition:early inhibition stops the actions that have not yet been prepared, and late inhibition stops the prepared actions. Whether the two kinds of inhibition have a common mechanism or not is unknown yet.Even-related potential (ERP) has high temporal resolution. It can be used to reveal dynamic neural correlates. We combined ERP with a two-staged Go/NoGo paradigm to explore the neural mechanism of voluntary inhibition. We hypotheses (1) there are different ERP effects between voluntary inhibition and voluntary; (2) early voluntary inhibition and late voluntary inhibition involve in different brain areas.MethodSixteen undergraduates or graduates were recruited. We gained consents from all of participants before the experiments. All the subjects had normal or corrected-to-normal vision. No subject had a history of neurological or psychiatric disorders. Two subjects were excluded from the analysis due to strong eye movements and technical problems. The remaining 14 subjects consist of 7 females and 7 males with mean age 24.79±3.22 years-old, range 21 to 31. Each subject participated this study received￥40.A two-staged Go/NoGo paradigm was used. Gray symbols and color letters were presented on screen. During the first stage, the participants generate their reaction intentions according following cues:(1)gray symbol "<<<<" or ">>>>" indicating to prepare button-pressing by left or right hand; (2)gray symbol "<<>>" indicating one can decide which hand to press voluntarily; (3)gray symbol "****" indicating to wait for the cue of the second stage. During the second stage, one should execution or inhibit its intention:(1) green letters "OOOO" indicating to execute the prepared plan; (2) red letters "XXXX" indicating to withdraw the prepared plan; (3)blue letters "0000" indicating one can decide action or inhibition voluntarily. Every stimulus presented for 500 ms, the inter-stimuli interval was 1250 ms. There were 1200 trials in total. All the task conditions were presented in a pseudo-randomized order throughout the experiment. We used an event-related potential system with 19 channels to record EEG while the subjects performed the tasks. The reaction time (RT) and the correct rate (%) were also recorded. Continuous EEG epochs of 1350 (ms) duration including a 100-msec prestimulus baseline were analyzed. The automatic rejection amplitude was set to 70μV. The event-related EEG segments with large eye or muscle artifacts were discarded manually. Finally we averaged EEG segments to obtain ERP.We used SPSS 13.0 package to analyze behavioral results. A 2(whether: voluntary, instructed)×3 (what:instructed, free, controlled) repeated-measures ANOVA was conducted for reaction time. EEG data were analyzed with ERP system. We conducted a one-way ANOVA with three levels to analyze ERPs locked to the fist stimulus, degree of freedom was corrected with Lower-bound method. ERPs locked to the second stimulus were analyzed by conducting a 2(freedom:free, instructed)×2 (execution:reaction, inhibition) repeated-measures ANOVA. We used paired t-test to compare conditions further. The statistical analysis result of ERP was presented in the form of (SPM) of F-value and t-value of two-sides with significance levelα=0.05.Results1. behavioral performanceParticipants chose action in 57.2%(38.6%～79.8%) trials on voluntary action condition, left side 29.6%(20.2%～41%), right side 27.6%(21%～40.5%). RT MANOVA revealed significant main effect of "What" factor [F(2,26)=23.706, p=0.000] and main effect of "whether" [F(1,13)=13.289,p=0.003], the interaction between them is also significant [F(2,26)=8.262,p=0.002].2. Grand average waveforms of ERP and SPM (1) First stage of intention generation:SPM(F) showed significant main effects of "what" on bilateral frontal areas, parietal-frontal-centre areas, right temporal area. The significant ERP difference between instructed intention and free intention was revealed in frontal cortex bilaterally (150-200 ms), extensive front-parietal area (300-700 ms) and right temporal area at the stage of intention generation (300-700 ms).(2) Second stage of intention execution:Account to late inhibition condition, different ERPs, interaction effects and execution main effect were observed in left prefrontal area (160-220 ms), front-parietal central area and right prefrontal parietal area (300-550 ms). Under the condition of early inhibition, areas presented SPM(F) significant interaction effects between what and whether resembled on late inhibition, different ERPs elicited by voluntary inhibitions and voluntary actions were observed in right parietal-frontal areas.Conclusion(1) Similar to previous literatures, we observed dorsolateral prefrontal cortex was involved in "what" intention processes at the stage of generation of intention.(2) Left prefrontal and medial prefrontal cortex are involved in late voluntary inhibition, resembling previous fMRI results, indicate that voluntary inhibition need more internal control than voluntary action.(3) Early voluntary inhibition involves right frontal-parietal cortex.(4) Voluntary inhibition as well as response inhibition can elicit NoGo P3 involving bilateral prefrontal cortex, parietal cortex, central cortex and occipital cortex.