Objectives. We based our study on 2 aspects: (i) the interestin the functional role of oscillatory brain activity in specificfrequency bands and the coherence analysis between EEGtraces; (ii) the increasing use of non-invasive stimulation ofthe human brain via transcranial direct current stimulation(tDCS). The aim of the study was to assess the potentialeffects of the combination of these two lines of research ona group of patients in vegetative state (VS).Materials. Five patients in VS and 5 age-matched healthycontrols received anodal tDCS over the central (C3),parietal (P3), dorso-lateral prefrontal areas (DLPF), anda sham stimulation. tDCS was applied for 20 min at 200microA. 19 channels-EEG was recorded before and aftereach stimulation session.Method. EEG was filtered between 0.5 and 30 Hz by ellipticfilters. Fast Fourier Transformation was performed on2 sec-epochs. For each stimulation site, coherence valueswere estimated within four frequency bands: Delta (0.5-3.5 Hz), Theta (4-7.5 Hz), Alpha (8-12.5 Hz), and Beta(13-30 Hz). Each coherence map was proportionallythresholded, preserving 50% of the strongest coherencevalues, to produce a weighted adjacency matrix. Theestimated functional connectivity patterns were characterizedby means of two global network metrics derived from graph theory: modularity and global efficiency.Modularity measures how the network is organized intomodules with high level clustering. Global efficiencymeasures how efficient the network is in exchanginginformation at the global level. For each frequency bandand site of effective stimulation, we performed a repeatedmeasure analysis of variance (RMANOVA) on eachgraph measure, using Stimulation (Effective vs. Sham)and tDCS (Pre vs. Post) as factors.Results. RM-ANOVA computed on modularity in thealpha frequency band revealed a significant interactionbetween the factors Stimulation and tDCS (p = .035)when applying tDCS on DLPF of healthy participants.Two paired t-tests revealed a significant increase of thenetwork modularity (pre tDCS: .16 ± .007; post tDCS: .21± .014) when stimulating in DLPF (p = .009) comparedto SHAM (p = .76). No significant effect was revealedon other frequency bands and stimulation site on bothhealthy participants and VS patients.Discussion. The tDCS-related modifications of the coherencein the alpha frequencies after DLPF stimulationcould indicate that the alpha range is important to transmitinformation between cortical areas in healthy controls. Theincreased modularity suggests that the coherently synchronousalpha activities can occur within different modularbrain areas and appear substantially independent of oneanother. This mechanism seems to be lack in VS patients.Conclusion. Our preliminary study revealed that tDCSbut not sham stimulation elevates EEG alpha power andthus demonstrates the feasibility of tDCS to modulatespecific oscillatory brain activity. tDCS could be considereda powerful tool for diagnosis/prognosis in alteredstate of consciousness.
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