Table 1.

Summary of Potential Contributors to the Global Signal (GS). Additional references are provided in the text. A detailed review of sources and mechanisms can be found in (Liu, 2016).

Source	Description	References
MRI System	Drifts and instabilities in radiofrequency, gradient, and shim subsystems and components can create artifactual signals. Contributions should be minimal for a well maintained system.	Foerster et al. (2005); Power et al. (2016)
Physiological Drift	Slowly varying physiological processes (e.g. fatigue). Contribution is not well characterized.	Evans et al. (2015); Yan et al. (2009)
Motion	Time-varying displacement of tissue components causes signal changes and spin history effects.	Hajnal et al. (1994); Power et al. (2015, 2016)
Cardiac	Fluctuations in cardiac rate drive changes in blood flow and oxygenation.	Shmueli et al. (2007); Chang et al. (2009)
Respiration	Modulates magnetic field, carbon dioxide levels, cerebral blood flow, cardiac rate, and pulse pressure.	Windischberger et al. (2002); Birn et al. (2006); Wise et al. (2004); Berntson et al. (1993)
Vigilance	GS amplitude decreases as mean vigilance levels rise. Temporal fluctuations in GS are negatively correlated with vigilance fluctuations.	Wong et al. (2013, 2015); Chang et al. (2016); Falahpour et al. (2016)