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Cerebral Cortex Advance Access published online on June 14, 2007
Cerebral Cortex, doi:10.1093/cercor/bhm084
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© The Author 2007. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

The Role of Sleep in Declarative Memory Consolidation—Direct Evidence by Intracranial EEG

Nikolai Axmacher1,2, Sven Haupt1, Guillén Fernández3, Christian E. Elger1,2 and Juergen Fell1

1 Department of Epileptology, University of Bonn, 53105 Bonn, Germany, 2 Life and Brain GmbH, 53127 Bonn, Germany, 3 F.C. Donders Center for Cognitive Neuroimaging and Department of Neurology, Radboud University Nijmegen, 6500 HB Nijmegen, The Netherlands

Address correspondence to Dr Nikolai Axmacher, Departmet of Epileptology, University of Bonn, Sigmund-Freud-Straße 25, D-53105 Bonn, Germany, Email: nikolai.axmacher{at}ukb.uni-bonn.de.

Two step theories of memory formation assume that an initial learning phase is followed by a consolidation stage. Memory consolidation has been suggested to occur predominantly during sleep. Very recent findings, however, suggest that important steps in memory consolidation occur also during waking state but may become saturated after some time awake. Sleep, in this model, specifically favors restoration of synaptic plasticity and accelerated memory consolidation while asleep and briefly afterwards. To distinguish between these different views, we recorded intracranial electroencephalograms from the hippocampus and rhinal cortex of human subjects while they retrieved information acquired either before or after a "nap" in the afternoon or on a control day without nap. Reaction times, hippocampal event-related potentials, and oscillatory gamma activity indicated a temporal gradient of hippocampal involvement in information retrieval on the control day, suggesting hippocampal–neocortical information transfer during waking state. On the day with nap, retrieval of recent items that were encoded briefly after the nap did not involve the hippocampus to a higher degree than retrieval of items encoded before the nap. These results suggest that sleep facilitates rapid processing through the hippocampus but is not necessary for information transfer into the neocortex per se.

Key Words: hippocampus • intracranial EEG • memory consolidation • plasticity • sleep


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