Research ArticleExperimental-design Specific Changes in Spontaneous EEG and During Intermittent Photic Stimulation by High Definition Transcranial Direct Current Stimulation
Introduction
Even as trials of transcranial direct current stimulation (tDCS) for clinical rehabilitation and cognitive enhancement progress (Brunoni et al., 2012, Bikson et al., 2016, Antal et al., 2017, Baudewig et al., 2018, Brunelin et al., 2018, David et al., 2018, Osoegawa et al., 2018), questions remain about the most robust biomarkers of neuromodulation, including application-specific target engagement (Bikson et al., 2018). The use of electroencephalography (EEG) with the application of tDCS is compelling mechanically, as both techniques integrate into a single head-gear (Woods et al., 2016); and biophysically, through the principle of reciprocity (Dmochowski et al., 2017). Measurement of EEG “online” during tDCS is complicated by inherent artifacts (Gebodh et al., 2019). Several trials have measured EEG “offline” comparing post- to pre-tDCS (Fregni et al., 2006, Zaehle et al., 2011, Faria et al., 2012, Maeoka et al., 2012, Auvichayapat et al., 2013, Schestatsky et al., 2013, Castillo-Saavedra et al., 2016, Liu et al., 2016, Kang et al., 2018), including one trial testing changes by a sham protocol (Holgado et al., 2018). High Definition transcranial Direct Current Stimulation (HD-tDCS) is advantageous over conventional sponge-pad tDCS since HD-tDCS allows for seamless mechanical integration (both techniques use gel cups, with no risks of saline leaking as with sponge-pads) and for focal stimulation using stimulation montages like the ‘4-cathode × 1-anode’ (4 × 1) ring (Datta et al., 2008, Caparelli-Daquer et al., 2012, Edwards et al., 2013, Kuo et al., 2013).
Any study of EEG changes by tDCS is limited by technical implementation (e.g. stimulation and recording hardware interactions (Gebodh et al., 2019); but also experimental design; the latter is addressed in detail here. In this exploratory study, we examined neurophysiological outcomes that indicated topographically restricted (focal) effects of HD-tDCS on EEG, namely locations inside and outside the stimulation region. In addition to background EEG, intermittent photic stimulation (IPS) evoked EEG responses were assessed, which can reveal latent bioelectrical oscillators not present or weak in spontaneous EEG during resting state (Lazarev et al, 2001). Our objective was not to identify explicit EEG responses to tDCS but to explore methodological issues surrounding the reproducible detection of real changes in EEG including: the validity of traditional sham approaches, the comparison of background EEG vs. IPS, the consideration of multiple time-points and EEG frequency bands, and the use of focal HD stimulation to resolve emergent asymmetries.
Section snippets
Participants
Twenty-five healthy individuals (eight males) aged 20–57 years (mean ± SD: 33.6 ± 11.1) with no history of neurological, psychiatric, chronic or drug-related illness volunteered for the study, without financial compensation. Subjects included, but were not limited to, staff and students at the Fernandes Figueira Institute and the Hospital Universitário Gaffrée e Guinle. All subjects underwent neurological and psychiatric evaluations by a neuropsychiatrist through an oral interview and physical
No-stimulation condition: background EEG
For the comparison of the BGR2 with BGR3, a 20-min intervention with no current resulted in an increase in central area of alpha and beta1 amplitude spectra (30–36% increase CL and CR in the alpha band, p < 0.01; 8% CR in the beta1 band, p = 0.02) with emergence of right-biased asymmetry in the central areas in the theta (p = 0.02), alpha (p = 0.02) and beta1 (p = 0.02) bands (Fig. 2). In the occipital leads, from BGR2–BGR3, a pronounced amplitude increase without asymmetry was observed in the
Discussion
Across intervention arms (active HD-tDCS, sham HD-tDCS, and no stimulation HD-tDCS), distinct effects on EEG were observed specific to temporal comparisons, regions, and frequency bands. Analysis of the HD-tDCS outcomes illustrate that the selection of comparison windows, in this case differing BGR EEG periods, can sway statistical tests (alpha/beta error). This exploratory analysis is not intended to draw definitive conclusions about temporal interactions (present in all arms) with perceptible
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