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

In sensory and mid-level regions of the brain, stimulus information is often topographically organized; functional responses are arranged in maps according to features such as retinal coordinates, auditory pitch, and object animacy or size. However, such organization is typically measured during stimulus input, e.g., when subjects are viewing gratings or images. Much less is known about the possible spatial organization of function during episodic recall of real-world events, which seems to drive higher-order cortical regions in the default mode network, particularly in posterior midline areas. Prior studies have shown that when multiple people remember a common experience, event-specific activity patterns in the posterior medial cortex are similar across individuals. This indicates that spatially organized functional responses underlying episodic recall do exist. In this paper we leverage fMRI data collected during 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. 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. Furthermore, these states are significantly related to actions and social-affective features of events in the movies. Together, these findings elucidate the properties of a spatially organized code within the posterior default mode network which appears during natural recollection of memories.