Abstract
As the brain is a complex system with occurrence of self-similarity at different levels, a dedicated analysis of the complexity of brain signals is of interest to elucidate the functional role of various brain regions across the various stages of vigilance. We exploited intracranial electroencephalogram data from 38 cortical regions using the Higuchi fractal dimension (HFD) as measure to assess brain complexity, on a dataset of 1772 electrode locations. HFD values depended on sleep stage and topography. HFD increased with higher levels of vigilance, being highest during wakefulness in the frontal lobe. HFD did not change from wake to stage N2 in temporo-occipital regions. The transverse temporal gyrus was the only area in which the HFD did not differ between any two vigilance stages. Interestingly, HFD of wakefulness and stage R were different mainly in the precentral gyrus, possibly reflecting motor inhibition in stage R. The fusiform and parahippocampal gyri were the only areas showing no difference between wakefulness and N2. Stages R and N2 were similar only for the postcentral gyrus. Topographical analysis of brain complexity revealed that sleep stages are clearly differentiated in fronto-central brain regions, but that temporo-occipital regions sleep differently.
Highlights
The study of brain organization and the relation between its anatomy and function has been of interest to neuroscientists since more than a century
Higuchi fractal dimension (HFD) provides additional information to that provided by traditional spectral analysis that does not allow to capture non-linear phenomena characteristic for complex nervous systems and does not take into account the instability of the EEG signal
Materials and methods Data acquisition. intracranial EEG (iEEG) data were recorded from 1520 channels with stereo-EEG electrodes and from 265 channels with cortical grids and strips from 38 brain regions during the awake state with eyes closed and during different sleep stages (R, N2 and N3) in 106 patients with drug-resistant focal epilepsy obtained from three centers: Montreal Neurological Institute and Hospital (MNI), Centre Hospitalier de l’Université de Montreal, and Grenoble-Alpes University Hospital
Summary
The study of brain organization and the relation between its anatomy and function has been of interest to neuroscientists since more than a century. In the first decade of 2000s Klonowski and Olejarczyk, followed by their collaborators and other investigators, successfully disseminated the HFD method They showed that this single measure of the complexity may be useful in the differentiation of various physiological and pathological states, including sleep21–26, anesthesia25,27–30, epilepsy25,31,32, stroke33,34, depression35–38, schizophrenia[39,40], Alzheimer’s disease[41,42,43], or to study changes in EEG complexity depending on age[44], and under the influence of the external electromagnetic field[45].
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