Neuronal correlate of BOLD signal fluctuations at rest: Err on the side of the baseline

Functional MRI (fMRI) indirectly measures changes in neuronal activity because the blood oxygenation level-dependent (BOLD) signal is sensitive to changing concentrations of oxyhemoglobin (vs. deoxyhemoglobin) to support functional energy demand (1). Changes in the BOLD signal are usually interpreted from an unspecified baseline state (2). However, there is no true baseline because the brain is never actually at rest (3), measured either in terms of neuronal activity (4) or the energy that activities demand (5). Recently, however, investigators have begun to study the resting state using fMRI, but interpretation remains controversial because of questions about the relationship between the BOLD signal and neuronal activity. In PNAS, Scholvinck et al. (6) tackle this controversy by correlating slow modulations of neuronal activity in the resting state with spontaneous fluctuations in the BOLD signal.

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