Abstract
Theta oscillations in the medial temporal lobe (MTL) of mammals are involved in various functions such as spatial navigation, sensorimotor integration, and cognitive processing. While the theta rhythm was originally assumed to originate in the medial septum, more recent studies suggest autonomous theta generation in the MTL. Although coherence between entorhinal and hippocampal theta activity has been found to influence memory formation, it remains unclear whether these two structures can generate theta independently. In this study we analyzed intracranial electroencephalographic (EEG) recordings from 22 patients with unilateral hippocampal sclerosis undergoing presurgical evaluation prior to resection of the epileptic focus. Using a wavelet-based, frequency-band-specific measure of phase synchronization, we quantified synchrony between 10 different recording sites along the longitudinal axis of the hippocampal formation in the non-epileptic brain hemisphere. We compared EEG synchrony between adjacent recording sites (i) within the entorhinal cortex, (ii) within the hippocampus, and (iii) between the hippocampus and entorhinal cortex. We observed a significant interregional gap in synchrony for the delta and theta band, indicating the existence of independent delta/theta rhythms in different subregions of the human MTL. The interaction of these rhythms could represent the temporal basis for the information processing required for mnemonic encoding and retrieval.
Highlights
Brain oscillations in the medial temporal lobe (MTL) of higher vertebrates have been a focus of neuroscientific research over the past decades
We address the question whether the interplay between the human hippocampus and entorhinal cortex is governed by a common theta rhythm or whether both regions exhibit essentially independent theta rhythms
0 EC-EC EC-Hi Hi-Hi two structures revealed that entorhinal–hippocampal synchrony was lower than synchrony within either of these structures throughout different EEG bands, with this effect reaching statistical significance only in the delta and theta band
Summary
Brain oscillations in the medial temporal lobe (MTL) of higher vertebrates have been a focus of neuroscientific research over the past decades. Particular emphasis has been placed on the role of theta oscillations that have been associated with a variety of cognitive functions in animals and humans, among these sensorimotor integration, spatial navigation, and mnemonic processing (cf Buzsáki, 2005). Mediotemporal theta oscillations have been found to act as a timing mechanism for neural activity (e.g., Jacobs et al, 2007; O’Keefe and Recce, 1993; Siapas et al, 2005; Skaggs et al, 1996) and to modulate higher frequency activity in the gamma range both in rodents (Bragin et al, 1995; Chrobak and Buzsáki, 1998) and humans (Canolty et al, 2006; Demiralp et al, 2007; Mormann et al, 2005). By firing periodically but with a slight frequency mismatch relative to their local field theta activity, place cells in the rodent hippocampus create a temporal code – defined by the phase relationship to the concurrent theta cycle – that can represent the position of an animal more accurately and dissociated from the place cell’s firing rate (Huxter et al, 2003)
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