Network analysis in episodic encoding and retrieval of word-pair associates: a PET study

The involvement of distributed brain regions in declarative memory has been hypothesized based on studies with verbal memory tasks. To characterize ep isodic declarative memory function further, 14 right-handed volunteers perf ormed a visual verbal learning task using paired word associates. The volun teers underwent positron emission tomography. O-15-butanol was used as a tr acer of regional cerebral blood flow (rCBF). Inter-regional functional inte ractions were assessed based on within-task, across-subject interregional r CBF correlations. Anatomical connections between brain areas were based on known anatomy. Structural equation modelling was used to calculate the path coefficients representing the magnitudes of the functional influences of e ach area on the ones to which it is connected by anatomical pathways. The e ncoding and the retrieval network elicit similarities in a general manner b ut also differences. Strong functional linkages involving visual integratio n areas, parahippocampal regions, left precuneus and cingulate gyrus were f ound in both encoding and retrieval; the functional linkages between poster ior regions and prefrontal regions were more closely linked during encoding , whereas functional linkages between the left parahippocampal region and p osterior cingulate as well as extrastriate areas and posterior cingulate gy rus were stronger during retrieval. In conclusion, these findings support t he idea of a global bihemispheric, asymmetric encoding/retrieval network su bserving episodic declarative memory. Our results further underline the rol e of the precuneus in episodic memory, not only during retrieval but also d uring encoding.