A new view of the medial temporal lobes and the structure of memory

Recent research in cognitive neuroscience has supported the idea that active rehearsal of information over short delays, or working memory maintenance, is accomplished by activating long-term memory representations. Nonetheless, it is widely assumed that although the human hippocampus and related medial temporal lobe structures may be critical for the formation of long-term memories, they are not involved in working memory maintenance. Here, we reconsider this issue and review evidence suggesting that humans and nonhuman primates with large medial temporal lobe lesions have difficulty retaining complex, novel information even across short delays. These results suggest that perirhinal and entorhinal regions, and under some circumstances, even the hippocampus, may be necessary for some forms of working memory as well as long-term memory. Moreover, neurophysiological and neuroimaging evidence suggests that all of these medial temporal regions exhibit activity associated with the active maintenance of novel information. Finally, we review a neurally plausible computational model of corticohippocampal interactions that points to a special role of the hippocampus in the representation of relational codes in memory. Our analyses suggest that the hippocampus plays this special role not only in episodic long-term memory, but also in working memory maintenance. Collectively, these results are consistent with the hypothesis that the active maintenance of complex, novel information is accomplished through the sustained activation of long-term memory representations bound together by the hippocampus and medial temporal cortical regions.