Comparison of PET and fMRI activation patterns during declarative memory processes

Aim: In this study neuronal correlates of encoding and retrieval in paired association learning were compared using two different neuroimaging methods : positron emission tomography (PET) and functional magnetic resonance imag ing (fMRI). Methods: 6 right-handed normal male volunteers took part in the study. Each subject underwent six 0-15-butanol PET scans and on fMRI study comprising four single epochs on a different day. The subjects had to lear n and retrieve 12 word pairs which were visually presented (highly imaginab le words, not semantically related). Results: Mean recoil accuracy was 93% in the PET as well as in the fMRI experiment. During encoding and retrieval we found anterior cingulate cortex activation, and bilateral prefrontal co rtex activation in both imaging modalities. Furthermore, we demonstrate the importance of the precuneus in episodic memory. With PET the results demon strate frontopolar activations whereas fMRI foils to show activations in th is area probably due to susceptibility artifacts. In fMRI we found addition ally parahippocampal activation and due to the whole-brain coverage cerebel lar activation during encoding. The distance between the center-of-moss act ivations in both modalities was 7.2 +/- 6.5 mm. Conclusion: There is a prep onderance of commonalities in the activation patterns yielded with fMRI and PET. However, there ore also important differences. The decision to choose one or the other neuroimaging modality should among other aspects depend o n the study design (single subject vs. group study) and the task of interes t.