The hippocampus remains activated over the long term for the retrieval of truly episodic memories

PLoS One. 2012;7(8):e43495. doi: 10.1371/journal.pone.0043495. Epub 2012 Aug 24.


The role of the hippocampus in declarative memory consolidation is a matter of intense debate. We investigated the neural substrates of memory retrieval for recent and remote information using functional magnetic resonance imaging (fMRI). 18 young, healthy participants learned a series of pictures. Then, during two fMRI recognition sessions, 3 days and 3 months later, they had to determine whether they recognized or not each picture using the "Remember/Know" procedure. Presentation of the same learned images at both delays allowed us to track the evolution of memories and distinguish consistently episodic memories from those that were initially episodic and then became familiar or semantic over time and were retrieved without any contextual detail. Hippocampal activation decreased over time for initially episodic, later semantic memories, but remained stable for consistently episodic ones, at least in its posterior part. For both types of memories, neocortical activations were observed at both delays, notably in the ventromedial prefrontal and anterior cingulate cortices. These activations may reflect a gradual reorganization of memory traces within neural networks. Our data indicate maintenance and strengthening of hippocampal and cortico-cortical connections in the consolidation and retrieval of episodic memories over time, in line with the Multiple Trace theory (Nadel and Moscovitch, 1997). At variance, memories becoming semantic over time consolidate through strengthening of cortico-cortical connections and progressive disengagement of the hippocampus.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Female
  • Hippocampus / physiology*
  • Humans
  • Magnetic Resonance Imaging
  • Male
  • Memory / physiology*
  • Memory, Episodic*
  • Young Adult

Grant support

The study was funded by Institut National de la Santé et de la Recherche Médicale (INSERM). The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.