Uncoupling in intrinsic brain activity.

One of the remarkable discoveries in the history of functional neuroimaging was that a consistent set of brain regions show decreases in blood flow during performance of different tasks when compared to rest (1). The idea that this might be due to quiescence of an activated state during rest (e.g., due to “mind wandering”) was countered by the finding that the resting oxygen extraction fraction in these regions was not elevated (2). Rather, it was argued that organized intrinsic brain activity is present at baseline—a default mode—from which activity could be suppressed in select regions during task engagement (3). Countless studies since using blood oxygen level-dependent functional magnetic resonance imaging (BOLD-fMRI) have reproduced this fundamental property of brain activity and, moreover, have shown that these same regions—the default mode network (DMN)—are structurally and functionally connected (4). In vivo intracranial electrophysiology, performed to find a source of a patient’s epilepsy, also shows that high-frequency band electrophysiological activity is reduced in the DMN during tasks (5). So, it comes with some surprise that, in PNAS, Jonasson et al. (6) report a different result with [18F]fluorodeoxyglucose positron emission tomography (FDG-PET). Imaging of human brain activity is currently dominated by two techniques: FDG-PET and BOLD-fMRI. FDG-PET provides a method to estimate the regional cerebral metabolic rate of glucose, whereas BOLD-fMRI relies on the phenomenon of blood flow increasing beyond changes … [↵][1]1To whom correspondence may be addressed. Email: goyalm{at}wustl.edu. [1]: #xref-corresp-1-1