Large scale neurocognitive networks underlying episodic memory

Large-scale networks of brain regions are believed to mediate cognitive pro cesses, including episodic memory. Analyses of regional differences in brai n activity, measured by functional neuroimaging, have begun to identify put ative components of these networks. To more fully characterize neurocogniti ve networks, however, it is necessary to use analytical methods that quanti fy neural network interactions. Here, we used positron emission tomography (PET) to measure brain activity during initial encoding and subsequent reco gnition of sentences and pictures. For each type of material, three recogni tion conditions were included which varied with respect to target density ( 0%, 50%, 100%). Analysis of large-scale activity patterns identified a coll ection of foci whose activity distinguished the processing of sentences vs. pictures. A second pattern, which showed strong prefrontal cortex involvem ent, distinguished the type of cognitive process (encoding or retrieval). F or both pictures and sentences, the manipulation of target density was asso ciated with minor activation changes. Instead, it was found to relate to sy stematic changes of functional connections between material-specific region s and several other brain regions, including medial temporal, right prefron tal and parietal regions. These findings provide evidence for large-scale n eural interactions between material-specific and process-specific neural su bstrates of episodic encoding and retrieval.