| The global signal(GS)is the grand average of the brain signals,which represents the general trend of brain activity.Functional magnetic resonance imaging(f MRI)studies have demonstrated that global mental states such as consciousness,arousal combined with brain network connectivity fluctuate with the phase of GS,implying that the phase of GS affects the work efficiency of the brain and thus the efficiency of cognitive process.Therefore,the present study used f MRI data from the Human Connectome Project(HCP)dataset to examine the relationship between GS phase and efficiencies of brain network and working memory based on resting state and N-back task.The first study explored the relationship between GS phase and brain network efficiency.The 82 healthy subjects from the HCP resting-state dataset were recruited.Then we calculated the GS phase in five narrow frequency bands from Slow-1 to Slow-5,and the global topographies of each phase.Based on the similarity of GS topographies,we divided the GS phase into four phases: peak,trough,rise and fall.Using ANOVA,the differences of global and local network efficiency in each of the four phases were examined.It was found that the global and local efficiencies of brain networks were significantly higher in the peak and trough than that in the rise and fall in the Slow-1,Slow-3,Slow-4 and Slow-5,indicating that the brain network efficiency was affected by the frequency and phase of GS.In addition,the local efficiencies of the brain visual and sensorimotor networks were higher in all frequency bands but lower in the default network,indicating that the GS topography remained relatively stable while fluctuating.These results suggest that although the GS topography is stable across frequency and phase,the brain network efficiency fluctuates with the change of GS phase.The second study explored the relationships between GS phase and efficiencies of brain network and working memory in the task state.The N-back task-state data of the above 82 subjects was used to explore whether the relationship between GS phase and brain network efficiency in the resting state still existed in the task state,and further examine the relationships between working memory efficiency and the above both.The GS phase and its topography were analyzed in the same procedure as the first study.It was found that the four phases of GS,peak,trough,rise and fall,could be divided only in the Slow-3,Slow-4 and Slow-5 frequency bands.However,the brain network efficiency is still significantly higher in the peak and trough than it in the rise and fall phases,which is consistent with the resting-state result.The 0-back and 2-back reaction times and accuracy were calculated for each phase,and then ANOVA was performed.The results showed that the main effect and interaction between task and phase were significant,indicating that the efficiencies of working memory load and updating varied dynamically with fluctuation of GS phase.Global efficiency and local efficiency of the association cortex were negatively correlated with reaction time and accuracy with frequency specificity,suggesting that working memory may reinforce task-specific local connectivity,which also fluctuates with GS phase.In summary,this paper investigated the relationships between GS phase and efficiencies of brain network and working memory both in resting-state and task-state data.The results support the GS,as a global factor,modulates local networks,and demonstrate that the working memory efficiency,as a composite cognitive process,fluctuating between trials can be traced to the global low-frequency activity of the brain,enriching and developing the neural oscillation theory.Moreover,the frequency-dependent GS topography implies that the brain relates to cognitive functions with a specific spatiotemporal structure,supporting the spectral fingerprint hypothesis.All in all,this study illustrates the function of GS phase and reveals the characteristics of spatiotemporal dynamics of brain efficiency from a global perspective. |
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