LAUSR

In a recent letter to the editor, Langenbach and colleagues [1] cited concerns about the repeated use of Ag/AgCl electrodes in 4x1 High Definition transcranial direct current stimulation (HDtDCS). Briefly, the authors reported increased impedance (with sudden, occasional cessation of stimulation) and visual evidence of oxidation (blackening of the electrode surface) after about 8 sessions inwhich the same electrodewas used as the anode (2mA) and four other “ring” electrodes served as cathodes for up to 20minutes per session. Relevant additional methodological information obtained via personal communication with Dr. Langenbach (April 3, 2020) includes the following: 1) stimulation used a neuroConn DC-Stimulator MC (software version 1.6.0), which allows for independent electrical current modulation of each electrode, 2) investigators used a standard EEG cap and placed gel directly into the holes (i.e., no specialized holders were used), 3) a variable amount of gel was used (both across electrodes and participants) but impedance was always <8kU, 4) total duration of use varied for each set of electrodes. Langenbach and colleagues discussed methods to reduce wear including electrode rotation as well as the need for increased transparency in reporting about electrode use/care. Given the above noted concerns, we felt it important to share our experiences with HD-tDCS (or multi-electrode tDCS) using data from our ongoing randomized trial as an example (NCT02155946 (see also ref 3)). In this study, participants with mild cognitive impairment (MCI) receive 5 consecutive daily sessions of active (2mA for 20 minutes/session) or sham HD-tDCS. As previously noted [3], the center anode is positioned at F5, while the ring cathodes are positioned at Fp1, F1, C5, and F9. Upon starting this line of research, our initial review of the literature [2] and recommendations from representatives at Soterix Medical Inc. (from whom units were acquired) indicated that electrodes are reusable for about 10 sessions at an intensity of 2mA if proper electrode rotation is maintained, such that no one electrode acts as the “center” for more than two sessions. Such recommendations precede, but ultimately support, Langenbach et al.‘s suggestion for electrode rotation. Fig. 1 is an example of our standard rotation, where each electrode is assigned a letter and teammembers are required to position electrodes at the pre-designated locations for each session. In this study [3], each set of electrodes is used for two participants (participant 1 1⁄4 sessions 1e5; participant 2 1⁄4 sessions 6e10) so that each electrode serves as the center for only two sessions. Our trial uses a Soterix Medical Inc. 4x1 HD-tDCS unit, which provides full amplitude to the center channel/electrode but passively splits current to the ring electrodes, and is powered by a Soterix Medical Inc. Clinical Trials (CT) unit. We use electrode