Brain systems engaged in encoding and retrieval of word-pair associates independent of their imagery content or presentation modalities.

In this study, we aimed to characterize commonalities and differences of activation patterns during verbal episodic memory processes across different presentation modalities (visual or auditory) and different imagery content (low or high) of the presented verbal memory items. Twelve right-handed normal male volunteers took part in the study. Each subject underwent six O-15-butanol positron emission tomography scans. In six of the subjects the verbal material was presented visually, and in six subjects auditorily. The subjects had to encode and retrieve two sets of 12 word-pair associates of high (set 1) or low (set 2) imagery content (not semantically related). The presentation of nonsense words served as reference condition. Images were analyzed with statistical parametric mapping. Conjunction analysis was used to identify commonalities, and cognitive subtraction analysis was used to identify differences. The use of conjunction analyses enabled us to identify commonly activated regions involved in episodic encoding and retrieval of verbal material irrespective of the presentation modality or the imagery content. Our results add further evidence to recent findings that bilateral prefrontal activations are important for episodic retrieval and thus the role of the left prefrontal cortex has been underestimated during episodic retrieval. Furthermore, our results support the idea of functionally segregated areas in the prefrontal cortex. Finally, our results provide strong evidence that mesial parietal cortex (precuneus) involvement is not restricted to processes involving imagery.