The Role of the Posterior Medial Network in Language Comprehension: Dissociating Construction of Episodic versus Semantic Representations

Language comprehension involves the construction of complex mental representations, i.e., "event representations", reflecting current events or situation models. The construction of these representations requires manipulation of both semantic and episodic content and has been widely associated with the functioning of the posterior medial network, a subsystem of the default network. However, it is still unknown the extent to which activity in posterior medial network reflects construction of the variable episodic versus semantic content of event representations. In this fMRI study, we establish the unique neural correlates of (1) episodic richness and (2) semantic coherence measured during a narrative reading task. Interestingly, we observed a functional fractionation within the posterior medial network in terms of brain regions whose activity was modulated by semantic or episodic content. Specifically, the number of episodic details in the narratives modulated the activity in the left angular gyrus and the retrosplenial cortex/ventral posterior cingulate cortex. Semantic coherence, in contrast, modulated neural responses in the right anterior temporal lobe/middle temporal gyrus, but also in brain regions within the posterior medial network, such as the dorsal posterior cingulate cortex. These results provide the first demonstration of functional dissociations within the posterior medial network in terms of brain regions involved in the construction of semantic versus episodic representational content during a language comprehension task. Significance StatementThe construction of "event representations", which is crucial to understand the world around us, predict the future and make plans, requires manipulation of both semantic and episodic information. The functioning of the posterior medial network has been tightly linked to formation of event representations. However, it is unclear the extent to which activity in this network reflects construction of the variable episodic versus semantic content of event representations. The present study provides the first demonstration of functional dissociations within posterior medial network in terms of brain regions involved in construction of semantic versus episodic representational content during language comprehension. These findings represent a first step towards understanding how episodic and semantic memory systems operate during the construction of event representations.