Retrieving memories via internal context requires the hippocampus.

Episodic memory encodes the unique contexts of events so that people can remember the details of an experience when cued by only a subset of event features (Tulving, 1972). In humans, the hippocampus is crucial for this kind of memory (Scoville and Milner, 1957; Vargha-Khadem et al., 1997). The present study tested whether the hippocampus was required for nonspatial, context-dependent memory retrieval in rats that were trained in a constant external environment to approach different nonspatial goal objects depending on their current internal motivational state (hunger or thirst). The rats learned to reliably approach the correct goal and thus used internal context to guide associative memory retrieval. Both fornix transection and selective neurotoxic hippocampal lesions severely impaired memory performance, but cue and motivational discrimination, as well as stimulus-reward associations, were preserved. The findings suggest that the hippocampus is required for using internal contextual information for flexible associative memory retrieval.