A 1mA direct current will be delivered through two 5cm x 5cm saline-soaked surface sponge electrodes by a battery-driven, constant-current stimulator (Neuroconn DC stimulator). First, the SMA will be identified based on the guidelines of the international 10-20 electrode system. The tDCS will then be applied using the following procedure:

1. The head of participants will be measured to find the 'vertex' (top) of the head. This point will be used as a landmark to locate the SMA. This region corresponds to the FCZ (e.g., Legon et al., 2013).
2. The return electrode will be placed over the mastoids. Once the electrodes are in position, cathodal tDCS will be applied at 1mA for 15 minutes, 2 times a day with a resting period of 20 minutes in between treatment periods, for 5 consecutive days.

The Yale Global Tic Severity Scale is the most extensively deployed scale worldwide for measuring tic severity. It requires an experienced clinician using all available information to rate motor and phonic tic severity during the previous week. Five dimensions (number, frequency, intensity, complexity, and interference) of motor and phonic tics are rated separately on a six point Likert-type scale (zero-five), with each point anchored to descriptive statements and relevant examples. The global score has a zero-one hundred range, composed by severity and overall impairment scores, each of which having a zero-fifty range. Higher values indicate worse outcome.

The Yale Global Tic Severity Scale is the most extensively deployed scale worldwide for measuring tic severity. It requires an experienced clinician using all available information to rate motor and phonic tic severity during the previous week. Five dimensions (number, frequency, intensity, complexity, and interference) of motor and phonic tics are rated separately on a six point Likert-type scale (zero-five), with each point anchored to descriptive statements and relevant examples. The global score has a zero-one hundred range, composed by severity and overall impairment scores, each of which having a zero-fifty range. Higher values indicate worse outcome.

Tic suppression will be measured using the Modified Rush Video-Based Tic Rating Scale through a 5 min video recording (2.5 min head and shoulders and 2.5 min whole body view). The scoring method consists of combining scores rating tic distribution (number of body areas involved, from 0 to 4), tic frequency (number of motor and phonic tics expressed from 0 to 4), and tic severity (for motor tics and for phonic tics, from 0 to 4). The total score range is 0 to 20, with higher values indicating worse outcome. Tic suppression will be expressed as suppression potency (IP), defined as follows: IP = RF-RI/RF, where RF is the Rush score during 'free' ticcing and RI the Rush score during tic suppression. Thus, the higher the IP value, the more efficient the suppression potency.