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Circuit mechanisms underlying memory encoding and retrieval in the long axis of the hippocampal formation

Abstract

Circuits within the hippocampal formation are active during memory processing. Here we used functional magnetic resonance imaging (fMRI) to examine multiple sites across the long axis of the hippocampal formation while subjects performed different phases of an associative memory task, learning to associate faces with names. Viewing faces and hearing names in isolation resulted in separate hippocampal activation patterns. Pairing faces with names resulted a spatially redistributed activation pattern, rather than a simple summation of the activation patterns resulting from viewing faces and hearing names in isolation. Recalling names when cued with faces reactivated a pattern similar to that found during paired training. Finally, the activation patterns representing faces and names were found to be experience dependent, emerging with repeated exposure. Interpreted in the context of hippocampal anatomy and physiology, these findings reveal hippocampal circuit mechanisms that underlie memory encoding and retrieval.

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Figure 1: Experimental design for the three studies.
Figure 2: Group data for hippocampal activation patterns associated with each condition from the first study (left) and the second study (right).
Figure 3: Group data from the first and second studies represented by percent change in signal intensity.
Figure 4: Hippocampal activation patterns associated with each condition; an individual example from the first study.
Figure 5: Results from the second study showing that activation patterns measured during the first block were coarsely tuned to faces and/or names.
Figure 6: Results from the second study showing that activation patterns measured during the third block were more finely tuned to faces and/or names.
Figure 7: Results from the second and third experiments that control for stimulus load shown with pixel counts (left) and with percent change in signal (right).

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Acknowledgements

We thank B. Rakitin for his assistance in data analysis. This work was supported in part by federal grants AG08702 and AG00946, the Beeson Faculty Scholar Award from the American Federation of Aging and the Charles S. Robertson gift from the Banbury Fund.

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Correspondence to Scott A. Small.

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Small, S., Nava, A., Perera, G. et al. Circuit mechanisms underlying memory encoding and retrieval in the long axis of the hippocampal formation. Nat Neurosci 4, 442–449 (2001). https://doi.org/10.1038/86115

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