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The Journal of Neuroscience, July 18, 2007, 27(29):7807-7816; doi:10.1523/JNEUROSCI.0962-07.2007

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Behavioral/Systems/Cognitive
Sustained Neural Activity Patterns during Working Memory in the Human Medial Temporal Lobe

Nikolai Axmacher,^1,2 Florian Mormann,¹ Guillén Fernández,³ Michael X Cohen,^1,2,4 Christian E. Elger,^1,2 and Juergen Fell¹

¹Department of Epileptology, University of Bonn, 53105 Bonn, Germany, ²Life and Brain GmbH, 53127 Bonn, Germany, ³F. C. Donders Center for Cognitive Neuroimaging and Department of Neurology, Radboud University Nijmegen, 6500 HB Nijmegen, The Netherlands, and ⁴Department of Psychology, University of California, Davis, Davis, California 95616

Correspondence should be addressed to Dr. Nikolai Axmacher, Department of Epileptology, University of Bonn, Sigmund-Freud-Strasse 25, D-53105 Bonn, Germany. Email: nikolai.axmacher{at}ukb.uni-bonn.de

In contrast to classical findings that the medial temporal lobe (MTL) specifically underlies long-term memory, previous data suggest that MTL structures may also contribute to working memory (WM). However, the neural mechanisms by which the MTL supports WM have remained unknown. Here, we exploit intracranial EEG to identify WM-specific sustained activity patterns with the highest temporal and spatial resolution currently available in humans. Using a serial Sternberg paradigm, we found a positive shift of the direct current (DC) potential and a long-lasting decrease in MTL gamma-band activity during maintenance of a single item, reflective of a sustained reduction in neural activity. Maintenance of an increasing number of items elicited an incrementally negative shift of the DC potential and an increase in MTL gamma-band activity. In addition, the paradigm was conducted in healthy control subjects using functional magnetic resonance imaging. This confirmed that our results were not caused by pathological processes within the MTL, and that this region was indeed specifically activated during the task. Our results thus provide direct evidence for sustained neural activity patterns during working memory maintenance in the MTL, and show that these patterns depend on WM load.

Key words: working memory; intracranial EEG; fMRI; medial temporal lobe; hippocampus; DC shifts; gamma band activity

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Received Oct. 10, 2006; revised April 14, 2007; accepted May 3, 2007.

Correspondence should be addressed to Dr. Nikolai Axmacher, Department of Epileptology, University of Bonn, Sigmund-Freud-Strasse 25, D-53105 Bonn, Germany. Email: nikolai.axmacher{at}ukb.uni-bonn.de

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