Mediotemporale Gamma-Aktivität und deklaratives Gedächtnis

Zusammenfassung

Einleitung: Für das deklarative Gedächtnis, d. h. das bewusst zugängliche Langzeitgedächtnis für Ereignisse und Fakten, sind der Hippokampus und der rhinale Kortex, zwei Strukturen im medialen Temporallappen (MTL), entscheidend. Bisher gab es noch keine direkte Evidenz für eine Interaktion dieser beiden Strukturen bei der Gedächtnisbildung. Eine transiente Kopplung neuronaler Verbände kann durch Phasensynchronisation der so genannten Gamma-Aktivität, d. h. der EEG-Aktivität im Frequenzbereich oberhalb von 20 Hz, ermöglicht werden. Methodik: Bei einer Gruppe von 9 Patienten mit unilateralen MTL-Epilepsien wurde das mediotemporale ereigniskorrelierte EEG mittels Tiefenelektroden während einer Wort-Gedächtnisaufgabe abgeleitet. Dann wurden die EEG-Antworten des nicht-pathologischen MTL für die später erinnerten und die vergessenen Wörter verglichen. Zusätzlich wurde bei einer Gruppe von 8 Patienten das kontinuierliche EEG während des Schlafes mittels Tiefen- und Oberflächenelektroden abgeleitet. Als Kopplungsmasse wurden die Phasensynchronisation und die spektrale Kohärenz quantifiziert. Ergebnisse: Die erfolgreiche Einspeicherung von Gedächtnisinhalten geht mit einer stimulusbezogenen anfänglichen Erhöhung der Phasensynchronisation zwischen rhinalem Kortex und Hippokampus und einer späteren Desynchronisation einher. Diese gedächtnisbezogenen Synchronisationsänderungen sind interindividuell hoch korreliert mit Erhöhungen der rhinal-hippokampalen Kohärenz im Theta-Band (4 - 7 Hz). Im Vergleich zum Wachzustand beobachteten wir während des Schlafes insbesondere im Gamma-Band eine deutliche Verringerung der rhinal-hippokampalen Kohärenz. Diskussion: Vermutlich spiegeln die Veränderungen der Theta-Kohärenz eine langsam modulierte Konnektivität zwischen rhinalem Kortex und Hippokampus wider, während die Phasensynchronisation im Gamma-Bereich schnelle Kopplungs- und Entkopplungsvorgänge bewerkstelligt, die den Informationsübertrag zwischen den beiden mediotemporalen Strukturen vorbereiten und abschließen. Die verringerte rhinal-hippokampale Kopplung während des Schlafes könnte ein indirektes elektrophysiologisches Korrelat der verminderten Fähigkeit zur Gedächtnisbildung während des Schlafes repräsentieren.

Abstract

Purpose: Human declarative memory, i. e. the consciously accessible long-term memory for events and facts, crucially depends on two structures within the medial temporal lobe (MTL), the rhinal cortex and the hippocampus. However, so far there was no direct evidence for an interaction between these two structures during memory formation. Transient coupling of neural assemblies can be accomplished by phase synchronisation of gamma activity, i. e. EEG activity in the frequency range above 20 Hz. Method: In a group of 9 patients with unilateral MTL-epilepsies the event-related EEG was recorded with depth electrodes during a word-memory task. The EEG responses of the non-pathological MTL for later remembered and forgotten words were compared. Additionally, the continuous EEG was recorded during sleep with depth and scalp electrodes in a group of 8 patients. Phase synchronisation and spectral coherence were quantified as measures for a neural coupling. Results: Successful memory formation is accompanied by an initial stimulus-related increase of phase synchronisation between rhinal cortex and hippocampus and a later decrease. These memory-related synchronisation changes are interindividually correlated with increases of rhinal-hippocampal theta (4 - 7 Hz) coherence. Compared to the waking state rhinal-hippocampal coherence decreases during sleep, most pronounced within the gamma-band. Discussion: The changes of theta coherence probably reflect a slowly modulated connectivity between rhinal cortex and hippocampus, while gamma synchronisation accomplishes the fast coupling and decoupling processes, which initiate and later terminate the information transfer between both structures. The reduced rhinal-hippocampal coupling during sleep may represent an indirect electrophysiological correlate of the diminished ability to encode memories during sleep.

Key words

Declarative memory - medial temporal lobe - hippocampus - gamma activity - synchronisation

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