Neural activity reveals interactions between episodic and semantic memory systems during retrieval

The distinction between memory for events and memory for facts rests on a range of findings indicating that different memory systems respectively subserve episodic and semantic memory functions. Whereas general knowledge clearly affects memory for events, the respective roles of encoding and retrieval processes in this interaction remain unclear. Using intracranial electroencephalography and multivariate classification techniques to decode memory states, we compared performance and brain activity during the study and free recall of semantically categorized and unrelated word lists. In addition to general biomarkers of encoding and retrieval, we identified specific neural signals, particularly at retrieval, that predicted recall performance and semantic clustering for categorized lists. In contrast, we observed biomarkers of temporal (i.e., episodic) organization of recall for unrelated lists only at encoding. These findings suggest that semantic memory processes flexibly interact with episodic retrieval to guide memory search.