Despite advances in understanding the cellular and molecular processes underlying memory and cognition, and recent successful modulation of cognitive performance in brain disorders, the neurophysiological mechanisms remain underexplored. High frequency oscillations beyond the classic electroencephalogram spectrum have emerged as a potential neural correlate of fundamental cognitive processes. High frequency oscillations are detected in the human mesial temporal lobe and neocortical intracranial recordings spanning gamma/epsilon (60-150 Hz), ripple (80-250 Hz) and higher frequency ranges. Separate from other non-oscillatory activities, these brief electrophysiological oscillations of distinct duration, frequency and amplitude are thought to be generated by coordinated spiking of neuronal ensembles within volumes as small as a single cortical column. Although the exact origins, mechanisms and physiological roles in health and disease remain elusive, they have been associated with human memory consolidation and cognitive processing. Recent studies suggest their involvement in encoding and recall of episodic memory with a possible role in the formation and reactivation of memory traces. High frequency oscillations are detected during encoding, throughout maintenance, and right before recall of remembered items, meeting a basic definition for an engram activity. The temporal coordination of high frequency oscillations reactivated across cortical and subcortical neural networks is ideally suited for integrating multimodal memory representations, which can be replayed and consolidated during states of wakefulness and sleep. High frequency oscillations have been shown to reflect coordinated bursts of neuronal assembly firing and offer a promising substrate for tracking and modulation of the hypothetical electrophysiological engram.

Behavioural and Social Neuroscience Research Group CEITEC Central European Institute of Technology Masaryk University Brno 625 00 Czech Republic

Bioelectronics Neurophysiology and Engineering Laboratory Mayo Clinic Departments of Neurology and Biomedical Engineering and Physiology Mayo Clinic Rochester MN 55902 USA

BioTechMed Center Brain and Mind Electrophysiology laboratory Department of Multimedia Systems Faculty of Electronics Telecommunications and Informatics Gdansk University of Technology Gdansk 80 233 Poland

Brno Epilepsy Center 1th Department of Neurology St Anne's University Hospital and Medical Faculty of Masaryk University member of the ERN EpiCARE Brno 602 00 Czech Republic

Department of Biomedical Engineering St Anne's University Hospital in Brno and International Clinical Research Center Brno 602 00 Czech Republic

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