A positive role of task-deactivated default mode network in narrative speech comprehension

Psychological theories have implicated an active role of the default mode network (DMN) in natural speech comprehension. However, as listeners need to keep tracking the external audio streams, the DMN is regularly de-activated and anticorrelated with externally-oriented networks. Such a pattern has been interpreted as the suppression of the DMN to support externally-oriented cognitive processes. The current study aims to resolve this seeming contradiction. Brain activities from a speaker telling autobiographical stories and a group of participants (N = 62) listening to the recordings were collected with fMRI. By analyzing the listeners' brains alone, we found the DMN was deactivated during speech listening relative to a fixation period and anticorrelated with the task-positive perisylvian language network (pLN). Dynamic Causal Modeling showed the pLN had inhibitory influence on the DMN, whereas the DMN had excitatory influence on the pLN. Further between-brain analyses revealed the activities of DMN in the listener's brain were tightly coupled with the activities of the homologous network in the speaker's brain. Significant interbrain couplings were also observed in the pLN, but were weaker and faded quicker. Moreover, listeners showing stronger coupling responses to the speaker in the DMN understood the speech better, and tended to exhibit more positive DMN -to-pLN effective connections. We conclude that the DMN may occupy an internal system that works cooperatively with the externally-oriented pLN to support narrative speech comprehension.