Fig. 4. Single-layer model (SLM) vs. Dual-layer model (DLM) of GS.

a The SLM of GS suggests that the GS stems from physiological signal like respiration and cardiac activity, or subcortical areas like basal forebrain. The cortical topography of GS is a consequence of the representation of these subcortical-cortical sources in the brain. Therefore, cortical GS topography is considered to be a mere manifestation of the subcortical-cortical GS itself with both standing in a one-to-one correspondence. b The DLM of GS suggests that GS is a constellation of neural activities at both a more spatially extended global background layer and a more spatially restricted surface layer featuring co-activation pattern of different networks. The background layer is the global brain activity whose neural signals, through its subcortical-cortical phase-based infraslow fluctuations, are closely coupled with the fluctuations of the bodily physiological signals like respiration, cardiac activity, and are projected from subcortical to whole brain cortical regions. That, in turn, allows for (1) regulating the level of arousal, and (2) the structuring of the dynamic topography of the cortical instantaneous brain networks/co-activation patterns (CAPs) at the surface layer as basis for coordinating different forms of cognition. Co-activation patterns CAPs; Global signal GS.