Abstract
The global signal (GS), which was once regarded as a nuisance of functional magnetic resonance imaging, has been proven to convey valuable neural information. This raised the following question: what is a GS represented in local brain regions? In order to answer this question, the GS topography was developed to measure the correlation between global and local signals. It was observed that the GS topography has an intrinsic structure characterized by higher GS correlation in sensory cortices and lower GS correlation in higher-order cortices. The GS topography could be modulated by individual factors, attention-demanding tasks, and conscious states. Furthermore, abnormal GS topography has been uncovered in patients with schizophrenia, major depressive disorder, bipolar disorder, and epilepsy. These findings provide a novel insight into understanding how the GS and local brain signals coactivate to organize information in the human brain under various brain states. Future directions were further discussed, including the local-global confusion embedded in the GS correlation, the integration of spatial information conveyed by the GS, and temporal information recruited by the connection analysis. Overall, a unified psychopathological framework is needed for understanding the GS topography.
Highlights
In fMRI studies, the global signal (GS), as the grand average of brain signals, is the largest scale of signal integration in the whole brain
Valuable information in the GS topography was uncovered in various situations, such as different conscious states, cognitive tasks, and brain disorders, shedding new light on the psychopathological theory
Some essential issues such as the local-global confusion and the integration of spatiotemporal information are to be resolved in order to clarify the psychological, physiological, and pathological significances of GS topography
Summary
In fMRI studies, the global signal (GS), as the grand average of brain signals, is the largest scale of signal integration in the whole brain. The GS topography has been established to measure the correlation between local brain signals and the GS, i.e., GSCORR. The cortical organization of functional brain networks has been revealed to be largely consistent across resting and various task states, suggesting the existence of an intrinsic architecture of functional networks (Cole et al, 2014, 2016; Gratton et al, 2018).
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