Spontaneous low‐frequency BOLD signal fluctuations: An fMRI investigation of the resting‐state default mode of brain function hypothesis

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Abstract
Recent neuroimaging studies have lead to the proposal that rest is characterized by an organized, baseline level of activity, a default mode of brain function that is suspended during specific goal‐oriented mental activity. Previous studies have shown that the primary function subserved by the default mode is that of an introspectively oriented, self‐referential mode of mental activity. The default mode of brain function hypothesis is readdressed from the perspective of the presence of low‐frequency blood oxygenation level‐dependent (BOLD) functional magnetic resonance imaging (fMRI) signal changes (0.012–0.1 Hz) in the resting brain. The results show that the brain during rest is not tonically active in a single mode of brain function. Rather, the findings presented here suggest that the brain recurrently toggles between an introspectively oriented mode (default mode) and a state‐of‐mind that tentatively might be interpreted as an extrospectively oriented mode that involves a readiness and alertness to changes in the external and internal environment. Hum. Brain Mapping, 2005.