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The majority of functional neuroscience studies have focused on the brain's response
to a task or stimulus. However, the brain is very active even in the absence of explicit
input or output. In this Article we review recent studies examining spontaneous fluctuations
in the blood oxygen level dependent (BOLD) signal of functional magnetic resonance
imaging as a potentially important and revealing manifestation of spontaneous neuronal
activity. Although several challenges remain, these studies have provided insight
into the intrinsic functional architecture of the brain, variability in behaviour
and potential physiological correlates of neurological and psychiatric disease.