The "two-brain" approach reveals the positive role of task-deactivated default mode network in narrative speech comprehension

During speech comprehension, as listeners need to keep tracking the external audio streams, the default mode network (DMN) is often de-activated and anticorrelated with task-positive networks. Such a pattern has been interpreted as the suppression of the DMN to support externally-oriented cognitive processes. Taking a "two-brain" approach, the current study demonstrated that, despite exhibiting deactivation and anticorrelated with the language network and executive control network, the DMN was not suppressed but played an active role in spoken narrative comprehension. This was evidenced by significant listener-speaker neural couplings in both the posterior and anterior DMN and the positive correlation between the coupling strength and listeners speech comprehension. Moreover, we demonstrated that the functionality of posterior DMN depended on its interaction with the executive control network, rather than its level of activation. Finally, Dynamic Causal Modeling together with the two-brain results indicates the language and executive control networks, the anterior DMN, and the posterior DMN occupied the bottom, intermediate and top layers of a hierarchical system, respectively. These results suggest that the DMN may primarily serve as an internally-oriented system that cooperates with the externally-oriented networks, which may allow the transformation of external acoustic signals into internal mental representations during language comprehension.