HIPPOCAMPAL THETA OSCILLATIONS LINK NEURONAL ASSEMBLIES TO EPISODIC MEMORIES: HERITAGE OF THE GRASTYÁN SCHOOL
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Two major structural designs characterize the cerebral cortex: the scalable, modular neocortex and the single-module hippocampus. Functions attributed to the hippocampal formation have varied over the past several decades and include episodic memory in human lesion studies, spatial mapping in single unit recordings and voluntary exploration of the environment in field recording studies in animals. The author suggests that the common thread across these parallel developments is that each captures the essence of episodic coding: items are organized in spatio-temporal context. He suggests that theta oscillations, studied extensively in the Grastyan school in Pécs, is the key temporal metric in this computation. Ordered sequences of items are encoded by the strict temporal relations of hippocampal cell assemblies nesting within cycles of theta oscillation. Such a temporal compression mechanism brings neuronal assemblies together in the time window of synaptic plasticity and allows the linking of first order (neighbor) and higher order relations. Seven to nine interleaving assemblies, representing overlapping past, present and future items, can be combined into an episode in a single theta cycle. During recall, the entire hippocampal connection matrix can be searched in the time period of the theta cycle (120 -140 msec). The author suggests that the hippocampus is an efficient search engine for the reconstruction of complex episodes from fragmentary information.
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