Regional analysis of hippocampal activation during encoding and retrieval: An fMRI study.

Introduction

Two models suggest how sub-regions of the hippocampus work in episodic memory: (1) episodic memory is subserved by posterior two-thirds of the hippocampus, (2) anterior hippocampus serves memory encoding and posterior serves memory retrieval.

Within the context of these models, we examined activation difference by fMRI between hippocampal sub-regions during encoding and retrieval of words, and examined susceptibility artifacts that could confound the results.

Method

During fMRI scanning, 14 healthy adults (6 M/8 F, 18-48 years) were presented with 40 unique nouns (encoding), and then presented with 32 identical nouns and 16 new ones (retrieval). Control condition was the same two nouns alternating repeatedly.

Results

Encoding: Activation: bilateral—anterior and posterior to hippocampus midpoint.

Retrieval: Activation: mostly on the right, anterior and posterior to hippocampus midpoint

Conclusions

There was no evidence for regional anatomic differences in activation between encoding and retrieval. Susceptibility-induced signal loss on activation has biased previous fMRI results. Even after accounting for such a susceptibility artifact, both encoding and retrieval of verbal stimuli activated the middle and posterior hippocampus more strongly than the anterior hippocampus.

Greicius MD, Krasnow B, Boyett-Anderson JM, Eliez S, Schatzberg AF, Reiss AL, Menon V. (2003). "Regional analysis of hippocampal activation during encoding and retrieval: An fMRI study." Hippocampus 13:164-174. PDF