Global Signal Topography of the Human Brain: A Novel Framework of Functional Connectivity for Psychological and Pathological Investigations

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.

Beyond noise to function: reframing the global brain activity and its dynamic topography

“…Furthermore, various studies show that GS is represented reliably in different degrees in different regions, i.e., it displays a dynamic topography (Fig. 1) 10,17,[19][20][21][22][41][42][43][44] (see Box 1 for different ways of calculating the spatial pattern, in terms of GS topography). The potential behavioral and cognitive relevance of GS topography is supported by the observation of topographical changes in various neurologic and psychiatric disorders as these show major alterations in perception and cognition (see below for details).…”

The temporal dedifferentiation of global brain signal fluctuations during human brain ageing

“…Although early studies have found prominent artifacts (i.e. head motion, respiration) in the GS 5 , 6 , numerous recent studies have tied GS fluctuations to vigilance 7 , behavioral traits 8 , brain states 9 , and mental disorders 3 , 10 , suggesting that the GS conveys particular physiological, psychological, and pathological information 11 . A recent study found a close relationship between the GS of rs-fMRI and the global EEG signal at multiple frequency bands 12 .…”

Altered Global Signal Topography in Alcohol Use Disorders

“…Hence, significant research studies were carried out to develop GST to analyze the correlation between local and global signals in the rs-fMRI study. It has been observed that the lower GST correlates with different cortical locations, such as the higher-order cortices and sensory cortex (Ao et al, 2021 ). Moreover, GST can be modulated by individual factors, like conscious state and attention-demanding tasks (Ao et al, 2021 ).…”

“…It has been observed that the lower GST correlates with different cortical locations, such as the higher-order cortices and sensory cortex (Ao et al, 2021 ). Moreover, GST can be modulated by individual factors, like conscious state and attention-demanding tasks (Ao et al, 2021 ). In addition, abnormal GST has been detected in patients with bipolar disorder, schizophrenia, epilepsy, and major depressive disorder (Li et al, 2019 , 2021 ; Wang et al, 2021 ).…”