Default-Mode Activity during a Passive Sensory Task: Uncoupled from Deactivation but Impacting Activation

TLDR

The default‑mode network, comprising regions such as the posterior cingulate and medial prefrontal cortex, is typically deactivated during low‑demand or rest periods compared with high‑demand tasks. Using a passive block‑design sensory task, the authors compared rest versus stimulus epochs and found the default‑mode network undetectable in one run and only partially in another, but independent component analysis revealed the full network in both runs. The study showed that the default‑mode network persisted across rest and stimulus epochs, was largely uncoupled from task activity, included the hippocampus, and that stronger default‑mode activity during rest predicted greater visual and auditory activation, suggesting that default‑mode activity can survive when tasks are not sufficiently challenging.

Abstract

Abstract Deactivation refers to increased neural activity during low-demand tasks or rest compared with high-demand tasks. Several groups have reported that a particular set of brain regions, including the posterior cingulate cortex and the medial prefrontal cortex, among others, is consistently deactivated. Taken together, these typically deactivated brain regions appear to constitute a default-mode network of brain activity that predominates in the absence of a demanding external task. Examining a passive, block-design sensory task with a standard deactivation analysis (rest epochs vs. stimulus epochs), we demonstrate that the default-mode network is undetectable in one run and only partially detectable in a second run. Using independent component analysis, however, we were able to detect the full default-mode network in both runs and to demonstrate that, in the majority of subjects, it persisted across both rest and stimulus epochs, uncoupled from the task waveform, and so mostly undetectable as deactivation. We also replicate an earlier finding that the default-mode network includes the hippocampus suggesting that episodic memory is incorporated in default-mode cognitive processing. Furthermore, we show that the more a subject's default-mode activity was correlated with the rest epochs (and “deactivated” during stimulus epochs), the greater that subject's activation to the visual and auditory stimuli. We conclude that activity in the default-mode network may persist through both experimental and rest epochs if the experiment is not sufficiently challenging. Time-series analysis of default-mode activity provides a measure of the degree to which a task engages a subject and whether it is sufficient to interrupt the processes—presumably cognitive, internally generated, and involving episodic memory—mediated by the default-mode network.