Abstract
High-definition transcranial direct current stimulation (HD-tDCS) seems to overcome a drawback of traditional bipolar tDCS: the wide-spread diffusion of the electric field. Nevertheless, most of the differences that characterise the two techniques are based on mathematical simulations and not on real, behavioural and neurophysiological, data. The study aims to compare a widespread tDCS montage (i.e., a Conventional bipolar montage with extracephalic return electrode) and HD-tDCS, investigating differences both at a behavioural level, in terms of dexterity performance, and a neurophysiological level, as modifications of alpha and beta power as measured with EEG. Thirty participants took part in three sessions, one for each montage: Conventional tDCS, HD-tDCS, and sham. In all the conditions, the anode was placed over C4, while the cathode/s placed according to the montage. At baseline, during, and after each stimulation condition, dexterity was assessed with a Finger Tapping Task. In addition, resting-state EEG was recorded at baseline and after the stimulation. Power spectrum density was calculated, selecting two frequency bands: alpha (8–12 Hz) and beta (18–22 Hz). Linear mixed effect models (LMMs) were used to analyse the modulation induced by tDCS. To evaluate differences among the montages and consider state-dependency phenomenon, the post-stimulation measurements were covariate-adjusted for baseline levels. We observed that HD-tDCS induced an alpha power reduction in participants with lower alpha at baseline. Conversely, Conventional tDCS induced a beta power reduction in participants with higher beta at baseline. Furthermore, data showed a trend towards a behavioural effect of HD-tDCS in participants with lower beta at baseline showing faster response times. Conventional and HD-tDCS distinctively modulated cortical activity. The study highlights the importance of considering state-dependency to determine the effects of tDCS on individuals.
Highlights
Transcranial direct current stimulation is a non-invasive brain stimulation (NIBS) technique that uses a constant weak electric current to modulate specific brain areas over which it is a pplied[1,2,3]
A further critical aspect is related to biophysical properties and concerns the spatial distribution of the electric field of bipolar Transcranial direct current stimulation (tDCS) montages
High-definition tDCS (HD-tDCS) montages use smaller electrodes than Conventional tDCS allowing the electric current to be delivered with increased density and focality
Summary
Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation (NIBS) technique that uses a constant weak electric current (generally 1–2 mA) to modulate specific brain areas over which it is a pplied[1,2,3]. A further critical aspect is related to biophysical properties and concerns the spatial distribution of the electric field of bipolar tDCS montages. The most used HD-tDCS montage is the 4 × 1 ring configuration, which consists of one active electrode placed on the area of interest and four return electrodes on the surroundings In this way, the delivered electric current is constrained and localised within the return electrodes[9]. Different electric field distributions have been found in computational studies concerning Conventional and HD-tDCS, no clear conclusion can be drawn on whether HD-tDCS can really produce a more focal and strong modulation than Conventional tDCS, both at behavioural and neurophysiological levels[15, 16]. These studies typically consider either the behavioural and EEG effects of HD-tDCS, without a direct comparison of this montage with a Conventional tDCS montage (for e xample17, 18) or compare in the same study both the montages, without providing EEG measures (for example[19, 20])
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