DMN is a collection of distributed and interconnected brain regions3 (Figure 1) that are typically suppressed when an individual is focused on external stimuli; however, in the absence of attention to external stimuli, the DMN switches or ‘‘defaults’’ to internally focused thought processes, such as self-reflection, daydreaming, mind wandering, recall of personal experiences, and envisioning the future.1,2,4 The DMN is also hypothesized to be active during periods of ‘‘rest’’ and quiet wakefulness, allowing us to reflect on our experiences, feelings, and social interactions.

system for internal ‘‘self-focused’’ mental processes. Notably, the origins of its discovery derive not from enhanced activation during cognition but rather from relative suppression or deactivation of its key nodes during cognitively demanding, externally focused tasks.2,7

In recent years, evidence has mounted for a more direct role for the DMN across multiple cognitive domains—most notably self-reference, social cognition, episodic memory, language and semantic memory, and mind wandering.

five key cognitive functions now widely associated with the DMN: self-reference, social cognition, episodic memory, language and semantic memory, and mind wandering.

Subjective experience suggests that we are continually engaged in unconstrained, stimulus-independent thoughts even when no external stimuli are being processed.11–13

cohesive, temporally coupled, network of distributed brain areas associated with high-level cognition

Jamesian notion of streams of consciousness—that the brain is constantly processing various types of information and experiences, and that these mental processes occur continuously and in parallel—loomed large.11

if DMN functions were indeed related to introspection, the self, and autobiographical memory, then the underlying neural mechanisms would be difficult to investigate in animals.

our understanding of key cognitive functions subserved by the DMN, including self-reference, social cognition, episodic memory, and its role in personally relevant aspects of remembered experiences, language comprehension and semantic memory, and mind wandering.

significant overlap between the DMN and language and semantic processing systems.55,58

individual nodes of the DMN have been implicated in a wide range of cognitive functions, including self-reference, social cognition, episodic memory, and language and semantic memory. I propose that the DMN integrates these cognitive functions to create a coherent internal narrative of our experiences. The model emphasizes the DMN’s role as a functional and structural hub in the brain, assimilating and transmitting representations of salient external and internal events through global brain activity patterns. These patterns are intermittently altered by latent switching mechanisms, leading to new brain states and mind wandering.

it would be erroneous to conceptualize the DMN as a static network with a unitary function. Moreover, the functional dynamics of the DMN are far from stationary,74 and multiplexed subnetworks associated with individual DMN nodes provide functional streams by which the DMN can dynamically couple with other networks.75–77 A common theme that emerges here is that the DMN does not operate in isolation, and its functions are shaped by dynamic interactions with other brain systems.

propose that the DMN is an integrative system that facilitates the construction of an ongoing internal narrative drawing on the key cognitive elements I have highlighted above. These narratives shape our understanding of our individual, highly personal, experiences. Here, it is well to ponder a gedanken experiment on what human cognition might look like if the DMN stopped functioning. I suggest that this would effectively end the narratives we tell ourselves. Our brains would not bind experiences to create a coherent internal narrative in the context of our experiences and shared social interactions. This narrative, part monolog and part dialog, is central to construction of the ‘‘epistemic self,’’ component processes of which include episodic memories; semantic knowledge of facts about one’s life and the

world; representations of individual values and beliefs; and the ability to experience and produce evaluative directions to our perceptions, actions, and reasoning.78,79 Such narratives necessarily involve integration of operations subserved by language, semantic, and memory systems that intersect in the DMN.

I further posit that the DMN integrates component processes and broadcasts them globally to construct and maintain an experience of subjective continuity. This integrate-and-broadcast process is briefly paused when stimuli in the external world need to be attended to. Once they are attended to, the inner narrative resumes with updated representations of events that are personally salient. This process allows us to build mental models of the world and integrate new information for further deliberation and reflection, constrained by our past experiences but removed from the exigencies of external stimuli and task contexts. Disruptions to any stage of this process can render the system vulnerable to a loss of sense of self, as seen most dramatically in psychiatric conditions such as dissociative disorders80 and

neurological disorders such as Alzheimer’s disease.19 I speculate that the DMN’s role in constructing such an inner narrative may have ontogenetic origins in early childhood. In his seminal work on thought and language, Vygotsky proposed that inner speech is the internalization of external verbal communication, a process by which we take in and internalize the words and language that we hear from others and then use this internalized language—now appropriated for the self—to think, reflect, and communicate with ourselves and others.81 Vygotsky articulated a developmental basis for this process in early childhood. He argued that children initially use self-directed speech to regulate their own actions and thoughts. Over time, children and adults internalize this self-directed private speech into a form of abstract voiceless inner monolog, which then becomes an efficient tool for self-regulation, planning, and social communication.82,83

the DMN comprises distinct subnetworks that subserve dedicated cognitive functions. However, these subnetworks are not completely segregated and likely contain convergence zones resulting in interdigitated and multiplexed circuits. These convergence zones may serve as connector hubs84,115 that integrate information across DMN subnetworks, thereby facilitating flexible and adaptive integration of self-referential, social, episodic memory, language, and semantic memory processes described in this review.

we are presented with a conundrum similar to one faced by physicists a century ago: internal mental processes cannot be studied without external stimuli, yet once exposed to external probes, internal mental and brain states are altered and disrupted, similar to the Schrodinger effect in physics. Improved experimental perspectives, neural recording methodologies, and computational models are required to address the many remaining open questions.

Recent studies have provided evidence that the DMN is not a singular, homogeneous system; instead, it comprises distinct subnetworks that perform different cognitive functions. The precise functional anatomy of subnetworks, the convergence zones that link them, and their distinct contributions to cognitive functions remain an area of active research. It is likely that some convergence zones reflect the processing of internally generated information, such as autobiographical memories or future-oriented thoughts, while others are involved in the integration of information across multiple sensory modalities.

The unifying model I have outlined posits that the DMN combines episodic memory, language, and semantic memory processes to generate an ongoing internal narrative and subjective continuity of internal mental thoughts. This narrative, which is unique to each of us, plays a critical role in the construction of a sense of self, encompassing the ‘‘individual self’’ and the ‘‘social self.’’118 The individual self embodies our ability to introspect and to be cognizant of our thoughts, emotions, and actions. The social self involves perspective taking, navigating social situations, forming and maintaining relationships, and understanding one’s behaviors within a social context. Together, they forge a multifaceted personal narrative that shapes how we perceive ourselves and interact with others. The inner narrative guides our stream of thoughts, feelings, and mental images; shapes our subjective experience of the world; and forms a vital component of human consciousness.