Does the Default Mode Network Mediate an Ongoing Internal Narrative? An Evaluation of Menon's (2023) Hypothesis.

Abstract

The default mode network (DMN) is a collection of interconnected transmodal brain regions that is engaged during internally oriented thought processes. It has been linked with multiple functions, including self-referential judgment, social cognition, episodic memory, and semantic memory. In an effort to identify a unitary overarching purpose of the DMN, Menon [Menon, V. 20 years of the default mode network: A review and synthesis. Neuron, 111, 2469-2487, 2023] proposed that it uses its diverse capacities to create an "ongoing internal narrative" that represents dynamically shifting frames of thought and develops ontogenetically from self-directed overt speech during childhood. My aim was to evaluate this hypothesis. The core of the hypothesis is the notion of an ongoing internal narrative, but this is open to different interpretations. If it is interpreted rather narrowly as referring to a story-like stream of inner speech (IS), then Menon's hypothesis faces at least five challenges. First, regarding its developmental assumptions, research suggests that IS does not originate from self-directed overt speech. Second, there are huge individual differences in the frequency of IS, which implies that if the DMN does mediate an internal narrative, this narrative is not ongoing for everyone; instead, its rate of occurrence varies greatly across the population. Third, rodents and nonhuman primates possess a putative DMN, but they lack language and hence also IS, so the function of their DMN is left unclear. Fourth, IS often has a condensed, note-like form rather than being experienced as a full-fledged narrative. And fifth, so far only a couple neuroscientific studies support DMN engagement during IS. It is also possible, however, to interpret Menon's key notion of an ongoing internal narrative more broadly as involving a coherent sequence of situation models that are strongly influenced by language but not completely dependent on it. This interpretation not only obviates most of the problems just mentioned but also brings Menon's hypothesis more in line with other recent proposals regarding the DMN. Further work is needed, however, to refine and test this version of the hypothesis.