title: Theta-gamma phase-phase coupling during working memory maintenance in the human hippocampus.
abstract: The theta-gamma neural coding theory suggests that multiple items are represented in working memory (WM) by a superposition of gamma cycles on theta oscillations. To enable a stable, non-interfering representation of multiple items, such a theta-gamma neural code may be reflected by phase-phase coupling, i.e., a precise locking of gamma subcycles to specific theta phases. Recent data have indicated that the hippocampus critically contributes to multi-item working memory. Therefore, we investigated phase-phase coupling patterns in the hippocampus based on intracranial EEG recordings in presurgical epilepsy patients performing a variant of the serial Sternberg WM task. In accordance with predictions of the theta-gamma coding theory, we observed increased phase-phase coupling between theta and beta/gamma activity during working memory maintenance compared to inter-trial intervals. These phase-phase coupling patterns were apparent during maintenance of two and four items, but not during maintenance of a single item, where prominent lower coupling ratios occurred. Furthermore, we observed that load-dependent changes of coupling factors correlated with individual WM capacities. Our data demonstrate that multi-item WM is associated with changes in hippocampal phase-phase coupling between theta and beta/gamma activity.
authors.affiliation: a Department of Epileptology , University of Bonn , Bonn , Germany.
authors.forename: Leila
authors.initials: L
authors.lastname: Chaieb
authors.affiliation: a Department of Epileptology , University of Bonn , Bonn , Germany.
authors.forename: Marcin
authors.initials: M
authors.lastname: Leszczynski
authors.affiliation: b Department of Neuropsychology , Institute of Cognitive Neuroscience, Ruhr-University Bochum , Bochum , Germany.|c German Center for Neurodegenerative Diseases , Bonn , Germany.
authors.forename: Nikolai
authors.initials: N
authors.lastname: Axmacher
authors.affiliation: a Department of Epileptology , University of Bonn , Bonn , Germany.
authors.forename: Marlene
authors.initials: M
authors.lastname: Höhne
authors.affiliation: a Department of Epileptology , University of Bonn , Bonn , Germany.|d Life and Brain GmbH , Bonn , Germany.
authors.forename: Christian E
authors.initials: CE
authors.lastname: Elger
authors.affiliation: a Department of Epileptology , University of Bonn , Bonn , Germany.
authors.forename: Juergen
authors.initials: J
authors.lastname: Fell
mesh_terms: D000328:Adult; D004569:Electroencephalography; D000066499:Facial Recognition / Q000502:physiology; D005260:Female; D006624:Hippocampus / Q000502:physiology*; D006801:Humans; D008297:Male; D008570:Memory, Short-Term / Q000502:physiology*; D059629:Signal-To-Noise Ratio; D013826:Theta Rhythm / Q000502:physiology
keywords: Gamma activity; Hippocampus; Intracranial EEG; Theta activity; Working memory; n:m phase coupling