Neuroimaging and neuropsychological data suggest that episodic and semantic memory may be mediated by distinct neural systems [Cabeza, R., & Nyberg, L. Imaging cognition II: An empirical review of 275 PET and fMRI studies. Journal of Cognitive Neuroscience, 12, 1–47, 2000; Gabrieli, J. D. Disorders of memory in humans. Current Opinion in Neurology and Neurosurgery, 6, 93–97, 1993; Gabrieli, J. D. Cognitive neuroscience of human memory. Annual Review of Psychology, 49, 87–115, 1998; Squire, L. R. The organization and neural substrates of human memory. International Journal of Neurology, 21–22, 218–222, 1987; Squire, L. R., & Zola, S. M. Structure and function of declarative and nondeclarative memory systems. Proceedings of the National Academy of Sciences, U.S.A., 93, 13515–13522, 1996; Tulving, E. Multiple memory systems and consciousness. Human Neurobiology, 6, 67–80, 1987]. However, an alternative perspective is that episodic and semantic memory represent different modes of processing within a single declarative memory system. To examine whether the multiple or the unitary system view better represents the data we conducted a network analysis using multivariate partial least squares (PLS) activation analysis followed by covariance structural equation modeling (SEM) of positron emission tomography data obtained while healthy adults performed episodic and semantic verbal retrieval tasks [Duzel, E., Cabeza, R., Picton, T. W., Yonelinas, A. P., Heinze, H. J., Scheich, H., & Tulving, E. Task-related and item related processes in episodic and semantic retrieval: A combined PET and ERP study. Proceedings of the National Academy of Sciences, U.S.A., 96, 1794–1799, 1999]. It is argued that if performance of episodic and semantic retrieval tasks are mediated by different memory systems, then there should differences in both regional activations and interregional correlations related to each type of retrieval task, respectively. The PLS results identified brain regions that were differentially active during episodic retrieval versus semantic retrieval. Regions that showed maximal differences in regional activity between episodic retrieval tasks were used to construct separate functional models for episodic and semantic retrieval. Omnibus tests of these functional models failed to find a significant difference across tasks for both functional models. The pattern of path coefficients for the episodic retrieval model were not different across tasks, nor were the path coefficients for the semantic retrieval model. The SEM results suggest that the same memory network/system was engaged across tasks, given the similarities in path coefficients. Therefore, activation differences between episodic and semantic retrieval may reflect variation along a continuum of processing during task performance within the context of a single memory system.

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