How default is the default mode of brain function? Further evidence from intrinsic BOLD signal fluctuations

Abstract

The default mode of brain function hypothesis and the presence of spontaneous intrinsic low-frequency signal fluctuations during rest have recently attracted attention in the neuroscience community. In this study we asked two questions: First, is it possible to attenuate intrinsic activity in the self-referential, default mode of brain function by directing the brains resources to a goal-oriented and attention-demanding task? Second, what effect does a sustained attention-demanding overt task performance have on the two intrinsically active networks in the brain, those being the task-negative, default-mode and the anticorrelated, task-positive network? We used functional magnetic resonance imaging to monitor spontaneous intrinsic activity during rest and sustained performance of a sequential two-back working memory task. We compared intrinsic activity during rest and the two-back task to the signal increases and decreases observed in an epoch-related version of the working memory task. Our results show that spontaneous intrinsic activity in the default-mode network is not extinguished but rather attenuated during performance of the working memory task. Moreover, we show that the intrinsic activity in the task-positive network is reorganized in response to the working memory task. The results presented here complements earlier work that have shown that task-induced signal deactivations in the default-mode regions is modulated by cognitive load to also show that intrinsic, spontaneous signal fluctuations in the default-mode regions persist and reorganize in response to changes in external work load.