S93. Cortical connectivity modulation during sleep onset: A study via graph theory on EEG data

Introduction: Sleep is a complex physiological process, linked with
the function of every bodily organ, and its dysregulation has negative
impacts for well-being and health. Despite its importance, after decades
of research, much uncertainty remains with respect to brain
functional changes occurring when we fall asleep. The known point
is that there are remarkable changes between full awake and sleep
conditions and that the variety of human states of consciousness
and the wakefulness–sleep (W–S) transition are based on the
dynamic connectivity of brain regions that continuously interact
through complex neural networks with time- and task-varying
architecture.
Methods: Sleep onset is characterized by a specific and orchestrated
pattern of frequency and topographical EEG changes.
Conventional power analyses of electroencephalographic (EEG) and
computational assessments of network dynamics have described
an earlier synchronization of the centrofrontal areas rhythms and a
spread of synchronizing signals from associative prefrontal to posterior
areas. Here, we assess how ‘‘small world” characteristics of the
brain networks, as reflected in the EEG rhythms, are modified in
the wakefulness-sleep transition comparing the pre- and postsleep
onset epochs.
Results: The results show that sleep onset is characterized by a less
ordered brain network (as reflected by the higher value of small
world) in the sigma band for the frontal lobes indicating stronger
connectivity, and a more ordered brain network in the low frequency
delta and theta bands indicating disconnection on the remaining
brain areas. Our results depict the timing and topography of the
specific mechanisms for the maintenance of functional connectivity
of frontal brain regions at the sleep onset, also providing a possible
explanation for the prevalence of the frontal-to-posterior information
flow directionality previously observed after sleep onset.
Conclusion: This study opens interesting avenues for future
researches investigating eventual modifications of brain connectivity
and network organization in the evolution of sleep stages.