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POSITRON emission tomography was used to investigate the functional anatomy of mental simulation of routes (MSR) in five normal volunteers. Normalized regional cerebral blood flow was measured while subjects mentally navigated between landmarks of a route which had been previously learned by actual navigation. This task was contrasted with both static visual imagery of landmarks (VIL) and silent Rest. MSR appears to be subserved by two distinct networks: a non-specific memory network including the posterior and middle parts of the hippocampal regions, the dorsolateral pre-frontal cortex and the posterior cingulum, and a specific mental navigation network, comprising the left precuneus, insula and medial part of the hippocampal regions.