Low intensity transcranial direct current stimulation induces acute neuromodulation of steady-state visual evoked potentials: A stereoelectroencephalographic investigation in humans in-vivo

Abstract Quantitative investigation of cognitive evoked potentials (EP) neuromodulation due to tDCS is difficult to assess using scalp EEG recordings. Indeed, low SNR, instrumental artifacts and physiological EP variabilities (amplitude and latency) are all factors that limit a precise quantification. Here, our goal was to quantify precisely the tDCS effects on face perception EP using a robust and original methodology. Simultaneous tDCS, Fast Visual Periodic Stimulations (FPVS), and SEEG recordings were performed in nine drug-resistant epileptic patients. FPVS sessions were performed at 6Hz (base frequency, BF), with face pictures apparition at 1.2Hz (oddball frequency, OF), before, during, and after tDCS. TDCS was performed, using 2 HD-electrodes, at +2mA for 20min (Fig.1) over the right or left visual ventral stream (VVS). SEEG signals, from multi-contacts SEEG electrodes in the VVS, were analyzed in the frequency domain. SNR of the EP was estimated (boxplots) at BF and OF (+harmonics) and compared over time, frequencies, and montages. 244 (18 %) and 105 (8%) SEEG contacts showed significant EP before tDCS for OF and BF respectively. The SNR increases at both BF and OF weren’t significantly different over time. On average, tDCS increased the SNR at OF from 13 to 71% (Q1: 7-15%; Q3: 15-129%). For BF, this increase is lower with an average of 19-37% (Q1: 6-17%; Q3: 24-45%). The SNR increase was higher when tDCS was applied over the right VVS (mean: 19-71%; Q1: 6-16%; Q3: 25-130%) compared to the left (mean: 13-37%, Q1:7-18%; Q3: max15.77-44.61%) (Fig.2). These results in humans in-vivo showed direct evidence that strong and acute neuromodulation can occur using low intensity and only 2 HD-tDCS electrodes especially when they are placed over the right VVS. Interestingly, we demonstrated a selective effect of tDCS with a higher increase at the OF compared to BF in the VVS. Fig.1: Fig.2 : Research Category and Technology and Methods Clinical Research: 9. Transcranial Direct Current Stimulation (tDCS) Keywords: Transcranial electrical stimulation, Neuromodulation, Face Perception, Stereoelectroencephalography