Cerebral Cortex Advance Access originally published online on November 10, 2004
Cerebral Cortex 2005 15(6):749-759; doi:10.1093/cercor/bhh176
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Cerebral Cortex V 15 N 6 © Oxford University Press 2004; all rights reserved
Ketamine Disrupts Frontal and Hippocampal Contribution to Encoding and Retrieval of Episodic Memory: An fMRI Study
1 University of Cambridge, Department of Psychiatry, Cambridge, UK, 2 University of Cambridge, Department of Anaesthesiology, Cambridge, UK, 3 University of Washington/Harborview Medical Center, Department of Anesthesiology, Seattle, WA, USA and 4 Wolfson Brain Imaging Centre, University of Cambridge, UK
Address correspondence to Paul C. Fletcher, University of Cambridge, Department of Psychiatry, Addenbrooke's Hospital, Box 255, Cambridge CB3 8HG, UK. Email: pcf22{at}cam.ac.uk.
The N-methyl-D-aspartate (NMDA) receptor antagonist ketamine produces episodic memory deficits. We used functional magnetic resonance imaging to characterize the effects of ketamine on frontal and hippocampal responses to memory encoding and retrieval in healthy volunteers using a double-blind, placebo-controlled, randomized, within-subjects comparison of two doses of intravenous ketamine. Dissociation of the effects of ketamine on encoding and retrieval processes was achieved using two study-test cycles: in the first, items were encoded prior to drug infusion and retrieval tested, during scanning, on drug; in the second, encoding was scanned on drug, and retrieval tested once ketamine plasma levels had declined. We additionally determined the interaction of ketamine with the depth of processing that occurred at encoding. A number of effects upon task-dependent activations were seen. Overall, our results suggest that left frontal activation is augmented by ketamine when elaborative semantic processing is required at encoding. In addition, successful encoding on ketamine is supplemented by additional non-verbal processing that is incidental to task demands. The effects of ketamine at retrieval are consistent with impaired access to accompanying contextual features of studied items. Our findings show that, even when overt behaviour is unimpaired, ketamine has an impact upon the recruitment of key regions in episodic memory task performance.
Key Words: encoding • episodic memory • hippocampus • ketamine • NMDA antagonist • retrieval
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