Absence of EEG gamma coherence in a local activated cortical state: a conserved trait of REM sleep

Abstract

During cognitive processes, there are extensive interactions between various regions of the cerebral cortex. Oscillations in the gamma frequency band (30-100 Hz) of the electroencephalogram are involved in the binding of spatially separated but temporally correlated neural events, which results in a unified perceptual experience. Like wakefulness, REM sleep is characterized by gamma oscillations in the EEG. Dreams, that are considered a special type of cognitive activity or proto-consciousness, mostly occur during this state. The power of the gamma band, assessed by the fast Fourier transform, reflects the local degree of synchronization at that frequency. On the other hand, the extent of interactions between different cortical areas at the gamma frequency band can be explored by means of a mathematical function called 'coherence', which reflects the 'strength' of functional interactions between cortical areas. The objective of the present report was to study in the rat the dynamic relationship between gamma power and coherence in the low (30-48 Hz) and high (52-98 Hz) gamma bands during waking and sleep, in occipital, parietal, and frontal neocortical areas, as well as in the olfactory bulb, that is a critical site of gamma rhythm-genesis. In addition, we re-analyzed previous recordings in cats, in order to evaluate the same dynamic relationship as in rats. In both species, the main result was that during REM sleep, gamma power increased, while gamma coherence between distant neocortical areas decreased. The fact that this profile is present in rodenthia as well as in carnivora suggests that this is a trait that characterize REM sleep in mammals.