Functional connectivity of semantic and default mode networks during narrative comprehension

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Abstract
Understanding narratives requires at least transient access to the semantic system, to decode incoming content, and prolonged access to the default mode network to maintain and manipulate the narrative model. Subregions within the integrative semantic hubs in bilateral anterior temporal lobe appear differentially sensitive to the need to rapidly decode external input (exogenous processing) versus reflecting on context stored in the narrative model (endogenous processing). The latter is most consistently reported in the middle temporal gyrus portion of the hub, suggesting that this region serves as a critical hinge point, dynamically interacting with the default mode network to facilitate endogenous processing. The present study investigated this by characterizing the functional connectivity profiles of anterior temporal lobe subregions during movie-viewing and examining content-evoked changes in these profiles. Compared to other anterior temporal lobe subregions, middle temporal gyrus was more functionally connected to the default mode network, and these connections were strengthened during moments with limited incoming information, providing viewers with a chance to reflect on the content. Rather than being functionally distinct networks, the semantic and default mode systems dynamically interact to facilitate reflection or endogenous semantic processing. Future work should further characterize how neural systems dynamically shift from integrated to segregated states in response to everyday processing demands.

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