P 235. Focal EEG effects of High Definition tDCS (HD-tDCS) detected by EEG photic driving

Abstract

HD-tDCS using a 4 × 1-ring configuration has been proposed as an alternative to conventional tDCS, capable to restrict effective current to an area within the ring perimeter. Previously we observed that HD-tDCS stimulation achieves changes in TMS-evoked motor responses comparable to conventional tDCS. In order to investigate focality predicted by computational models, here we used EEG to detect neuronal changes both inside and outside stimulated region. On 15 normal subjects (5 males, 21–57 years), an EEG cap with special electrode holders arranged according to the 4 × 1-ring configuration was placed with the central electrode holder over central area of the left hemisphere (C3). The peripheral holders were spaced 5 cm radially around the central holder at the corners of a square. Monopolar EEG (linked earlobes reference, eyes closed) was recorded inside stimulation area in the central 4 × 1 (anode-An-C3), post-central (medial cathode-Ct-C3p), pre-central (between the anode and two anterior cathodes-Int-C3a) positions, outside the stimulation area in the left occipital lead (O1) and at the homologous right hemisphere points (C4a, C4, C4p and O2). EEG recordings were performed during intermittent photic stimulation (IPS) of fixed frequencies 3, 5, 10 and 21 Hz with 30s intervals between stimulation runs as well as during 3 min resting states before (1st) and after (2nd background) IPS sessions. After the initial EEG recording of 9.5–10 min duration, stimulation was delivered through a battery-driven constant current stimulator connected to a HD-tDCS adaptor device (Soterix Medical Inc., New York, NY). The current was delivered with a ramp-up time of 10s, held at 2 mA for 20 min, and then ramped down over 10 s. EEG was recorded 2–3 min after HD-tDCS. EEG amplitude spectra before and after HD-tDCS were compared for the background states in the 6 standard frequency bands and for the IPS states at all the EEG frequencies corresponding to those of IPS and its harmonics up to 30 Hz. A generalized increase in amplitude spectra after HD-tDCS was observed for the IPS of 3 Hz at the EEG frequencies of 3, 6, 9 and 24 Hz (fundamental frequency and 2nd, 3rd and 8th harmonics), this effect being significantly lower in the stimulation area of the left hemisphere (An-C3, Ct-C3p and Int-C3a) as compared to the homologous leads of the right hemisphere, with lower increase in the An-C3 as compared to the Ct-C3p at 6 and 24 Hz. No interhemispheric asymmetry was observed between occipital regions (O1 and O2). Similar increase was also observed for the 2nd background state in relation to the same state before HD-tDCS) in An-C3, Int-C3a, C4, and C4a in the theta band, in all the right hemisphere leads and in An-C3 and Ct-C3p in the alpha band with significant right-side prevalence, and in C4, C4a, and C4p in the beta-1 band. The earlier 1st background state after HD-tDCS did not show significant changes. These results are in accordance with HD-tDCS producing focal changes in brain function.