A core set of neural states underlying naturalistic memory reactivation in the posterior medial cortex

In the posterior midline default mode network, spatial activity patterns similar to those during the initial experience are reactivated during the successful recall of past events. Prior studies have shown that these event-specific activity patterns are consistent across individuals recalling a shared experience, suggesting that common functional responses underlying episodic recall do exist. However, the spatial organization of function during episodic encoding and subsequent recall, especially in the absence of external stimuli, remains poorly understood. To address this, we leverage fMRI data collected during the encoding and recall of naturalistic movies to identify a core set of neural states in the posterior medial cortex. These states are stimulus-locked, reactivated during recall, and have a shared spatial organization across brains (i.e., individuals). We show that a surprisingly small number of these states (16 states across hemispheres) is sufficient to achieve the same levels of reactivation in the posterior medial cortex as when using the standard methods of the field. Additionally, these states are linked to actions and social-affective features of events in the movies. Our findings elucidate the properties of a common, spatially organized code within the posterior default mode network which appears during natural recollection of memories.