Why the PCC keeps showing up when attention turns inward
Mind wandering is not one brain state but a family of them, and that is exactly why a single region is hard to pin down. One broad investigation found that the default mode network alone was not unambiguously linked to mind wandering; what mattered was the dynamic coupling between the default mode network and its antagonistic network, the system that normally supports outward focus [1]. The PCC appears to be what makes that coupling possible: its functional connections were described as especially enabling the integration of information across these networks, which is the basis for calling it a regulator of internal attention [1].
A separate line of evidence points at the same region from a different angle. When people monitored their own breathing and their responses drifted out of time, a sign of distraction or mind wandering, elevated alpha-band activity appeared in posterior default mode regions, and the isthmus portion of the PCC showed elevated activity specifically in people reporting depressive symptoms [3]. That study also found posterior default mode activity was inversely linked with connectivity to the fronto-parietal network, suggesting that interactions between these networks help regulate the posterior default mode during internal attention [3]. In plain terms, the PCC looks less like a switch that turns mind wandering on and like part of a balancing mechanism that decides how much attention stays inward.
Does the hub extend below the cortex?
The subcortical claim is the more tentative half of the answer. The same broad investigation used newer subcortical imaging and atlasing to look at understudied deep-brain regions and reported that subtler connections were spontaneously echoed in the subcortex, showing functional properties analogous to the PCC [1]. That is a claim about resemblance, not proof that subcortical structures are part of the same regulatory hub, and it is described as groundwork for exploring these regions rather than a settled map [1].
It is also worth being precise about what kind of evidence supports the hub idea. The PCC's role is inferred from functional connectivity, meaning regions whose activity rises and falls together, not from showing that the PCC directs other regions. The same investigation stresses that mind wandering is complex and heterogeneous, interacts with situational factors, and defies a unitary neural representation [1]. So the honest version of the answer is that the PCC is the best-supported common node across cortical findings, with subcortical involvement suggested but not established.
The PCC also tracks internal life that is not mind wandering
The PCC's involvement is not limited to drifting attention. In a study of mood in epilepsy patients with implanted electrodes, depressed mood correlated with increased theta power, most significantly in electrodes within the PCC, while elevated mood correlated with increased alpha power most significantly in the intraparietal sulcus, a region of the outward-attention network [2]. That study framed the PCC as a major default mode hub serving internal processes such as self-reflection, mind wandering, and memory, and argued it should be sampled in future work on mood [2]. The finding is about mood rather than about internal attention itself, so it broadens the picture without directly confirming the hub role.
A reading study adds a useful contrast. Retrieving personal memories, used as a stand-in for mind wandering, recruited core default mode regions including the PCC, while reading for comprehension recruited a different, more lateral part of the same network [4]. People who generated more mental content and comprehended less showed more decoupling between reading-related default mode sites and visual cortex, and retrieving personally relevant memories increased that decoupling [4]. Here the PCC marks the inward, memory-based mode, but the study does not single it out as a hub or connectivity node, so it supports the region's involvement in internal attention without settling the hub question [4].
What the hub idea does not explain
Not every PCC finding is about internal attention. In a video-game study, the ventral portion of the PCC was activated during anticipating an opponent's actions, and the strength of that activation correlated with gaming proficiency [5]. That work places the ventral PCC in the theory of mind network, the system for inferring other people's intentions, and says nothing about mind wandering or internal attention [5]. The contrast is a reminder that the PCC is a large, subdivided region, and findings from one part or one task should not be assumed to transfer to another.
Putting the pieces together, the most defensible answer is that the PCC is a well-supported hub for regulating internal attention in the cortical default mode system, with its connectivity to the opposing attention network doing much of the explanatory work [1][3]. Subcortical participation is suggested by analogous functional properties but remains exploratory [1], and the region's role in mood and social inference shows the hub is not dedicated to mind wandering alone [2][5]. What would sharpen the answer is work that directly tests whether subcortical regions couple with the PCC during off-task thought, rather than showing similar properties in parallel.
Sources used in this answer
Neural mechanisms of the wandering mind
Functional connections of the posterior cingulate cortex were described as especially enabling integration between the default mode network and its antagonistic network during mind wandering, substantiating its role in regulating internal attention, with subtler analogous connections echoed in the subcortex.
451 Posterior Cingulate Theta Power as a Novel Biomarker of Mood
Depressed mood correlated with increased theta power most significantly in posterior cingulate cortex electrodes, while elevated mood correlated with increased alpha power most significantly in the intraparietal sulcus, supporting a network-based view of mood physiology.
Posterior default mode network activity underlies distracted interoceptive attention and provides an objective basis for low mindfulness & depression
Distraction during a breath-monitoring task was associated with elevated posterior default mode alpha activity, with the isthmus region of the posterior cingulate cortex showing elevated activity on distracted trials in people with depressive symptoms.
Editor's evaluation: Perceptual coupling and decoupling of the default mode network during mind-wandering and reading
Autobiographical memory retrieval recruited core default mode regions including the posterior cingulate cortex, and greater decoupling between reading-related default mode sites and visual cortex was linked to more self-generated mental content and poorer comprehension while reading.
Neural substrates of attack event prediction in video games: the role of ventral posterior cingulate cortex and theory of mind network
The ventral posterior cingulate cortex, a hub of the theory of mind network, was activated during action anticipation in video games, and its activation strength correlated with gaming proficiency.