Independent delta/theta rhythms in the human hippocampus
and entorhinal cortex

Mormann, Florian; Osterhage, Hannes; Andrzejak, Ralph Gregor; Weber, Bernd; Fernández, Guillén; Fell, Juerguen; Elger, Christian E.; Lehnertz, Klaus

doi:http://dx.doi.org/10.3389/neuro.09.003.2008

Independent delta/theta rhythms in the human hippocampus and entorhinal cortex

Citation

Mormann F, Osterhage H, Andrzejak RG, Weber B, Fernandez G, Fell J, Elger CE, Lehnertz K. Independent delta/theta rhythms in the human hippocampus and entorhinal cortex. Front Hum Neurosci. 2008 May 15(3). DOI: 10.3389/neuro.09.003.2008

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 infl uence 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-specifi c measure of phase synchronization, we quantifi ed 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 signifi cant 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.