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. Author manuscript; available in PMC: 2014 Oct 15.
Published in final edited form as: Neuroimage. 2013 Apr 6;80:349–359. doi: 10.1016/j.neuroimage.2013.04.001

Figure 1. Origins of the BOLD signal.

Figure 1

A) The magnitude of the BOLD signal depends on the initial magnetisation, M0, and the decay time, T2*. Changes in the BOLD signal strength, S, can arise from changes in either M0, T2* or a combination of both. FMRI assumes that changes in T2* are solely due to neural activity-related changes in deoxyhaemoglobin concentration. B) A schematic of the relationship between a transient increase in neural activity and the corresponding BOLD signal is shown. When neural activity causes increased oxygen consumption (CMRO2), neurovascular coupling mechanisms alter the tone of the vasculature changing CBF and CBV. The complicated interaction between these 3 parameters leads to the BOLD signal as measured with FMRI. Changes in the BOLD signal accurately reflect neural activity fluctuations, only if the intermediary vascular steps are not significantly altered. Many of the confounds in resting-state FMRI originate from physiological changes in the vasculature.