Background: Using conventional tDCS over the temporo-parietal junction (TPJ) we previously reported that it is possible to manipulate subjective visual vertical (SVV) and postural control. We also demonstrated that high-deﬁnition tDCS (HD-tDCS) can achieve substantially greater cortical stimulation focality than conventional tDCS. However, it is critical to establish dose-response effects using well-deﬁned protocols with relevance to clinically meaningful applications.
Objective: To conduct three pilot studies investigating polarity and intensity-dependent effects of HD-tDCS over the right TPJ on behavioral and physiological outcome measures in healthy subjects. We additionally aimed to establish the feasibility, safety, and tolerability of this stimulation protocol.
Methods: We designed three separate randomized, double-blind, crossover phase I clinical trials in different cohorts of healthy adults using the same stimulation protocol. The primary outcome measure for trial 1 was SVV; trial 2, weight-bearing asymmetry (WBA); and trial 3, electroencephalography power spectral density (EEG-PSD). The HD-tDCS montage comprised a single central, and 3 surround electrodes (HD-tDCS3x1) over the right TPJ. For each study, we tested 3x2min HD-tDCS3x1 at 1, 2 and 3mA; with anode center, cathode center, or sham stimulation, in random order across days.
Results: We found signiﬁcant SVV deviation relative to baseline, speciﬁc to the cathode center condition, with consistent direction and increasing with stimulation intensity. We further showed signiﬁcant WBA with direction governed by stimulation polarity (cathode center, left asymmetry; anode center, right asymmetry). EEGPSD in the gamma band was signiﬁcantly increased at 3mA under the cathode.
Conclusions: The present series of studies provide converging evidence for focal neuromodulation that can modify physiology and have behavioral consequences with clinical potential.