Changes in fronto-posterior functional coupling at sleep onset in humans

The aim of this study was to assess the functional coupling between anterior and posterior areas as induced by the sleep onset process. The functional coupling was indexed by an analysis of spectral coherence and directed transfer function (DTF) from electroencephalographic (EEG) data. As it has been reported that more anterior areas first synchronize sleep EEG activity, we hypothesized a fronto-posterior direction of the cortical functional coupling during the sleep onset process. Ten normal right-handed male students slept for two-nights (one adaptation, one baseline) in the laboratory, with standard polysomnographic recordings. Spectral coherence and DTF were computed on data recorded by anterior (FzA(1)) and posterior (PzA(1), OzA(1)) derivations. EEG coherence at the delta/theta band was higher during the presleep period than the sleep onset period, while EEG coherence at the alpha band was higher during the sleep onset period than the presleep period. The DTF findings indicated a prevalence of the occipital-to-frontal information flow at delta/theta and alpha bands during the presleep period and a prevalence of the frontal-to-parieto-occipital information flow at all bands during the sleep onset period. The coherent pattern of changes in EEG coherence and in DTF values at sleep onset lends further support to the notion of sleep as a local process, showing that the sleep onset process is subserved by the functional coordination of a cortical fronto-posterior network. In this network, prefrontal areas may play a leading role in the propagation of synchronizing signals conveyed at frequencies spanning delta to beta rhythms.