Egocentric spatial scaffolds organize cortical memory engrams
Episodic memory requires reconstructing the position of the self within a remembered environment, yet whether and how memory engrams incorporate self-referenced spatial information remains unknown. Using activity-dependent engram tagging, longitudinal calcium imaging and optogenetic perturbation in the retrosplenial cortex, we found that cortical memory engrams are enriched for egocentric and boundary-coding neurons that encode self-position relative to environmental boundaries. Longitudinal imaging revealed that future engram neurons are preferentially recruited from a pre-existing spatial scaffold rather than generated de novo during learning. During memory retrieval, scaffold populations underwent coordinated refinement and became transiently reorganized into a scaffold-engram network state whose dynamics tracked memory expression. Silencing scaffold neurons reduced memory expression while preserving recall dynamics, whereas engram silencing abolished recall dynamics while maintaining a stable low-memory state. Together, these findings identify a scaffold-engram architecture through which episodic memories are reconstructed by reinstating the self-referenced spatial representations within remembered space.