During the active- and sham-tDCS conditions, the participants will receive current with ramp up and ramp down mode for 10 seconds, eliciting a tingling sensation on the scalp that fades over seconds. Following that, the tasks will be initiated five minutes subsequent to the stimulation mode and terminated prior to its end for active-tDCS condition. Whereas, the sham condition with the same placement and intensity will only receive 30s initial stimulation and then discontinue. Throughout the active / sham-tDCS condition, participants are required to sit still and focus their attention on a "+" displayed on a computer monitor during the five-minute rest. After that, they will undergo active-tDCS (1mA, 20 min) to the left DLPFC or sham stimulation over 10 sessions in 2 weeks, while performing the executive function training tasks.

Participants in the sham-tDCS group will receive 10-day active tDCS and assessments will be performed before and after the 10-tDCS session.

All tDCS responders (>10% reduction in SRS scores) will enrol on a 6 months follow-up phase in which they will be randomized to receive either bimonthly 20-minute booster tDCS sessions, or bimonthly 20-minute booster sham tDCS sessions for 3 months, followed by monthly 20-minute booster tDCS, monthly 20-minute booster sham tDCS, for another 3 months, with a maximum of 9 (sham) tDCS booster sessions.

For active-tDCS condition, participants will receive stimulation on the dorsolateral prefrontal cortex with ramp up and ramp down mode for 10 seconds, eliciting a tingling sensation on the scalp that fades over seconds. Following that, a twenty-minute executive functional training task will be initiated five minutes subsequent to the stimulation mode, and the stimulation will be terminated when the training task ends.

SRS-2 is a sensitive measure of social functioning in children that detects even subtle symptoms that are highly related to ASD. It uses a four-point scale and focuses on different aspects of socialization. The total score reflects the clinical effectiveness of tDCS, and higher scores indicate greater symptom severity. It has been shown that SRS-2 is sensitive to detect changes in social communication improvement related to improved cognitive functioning after treatment. SRS-2 assessments will be conducted 1 day before and 1 day after tDCS treatment.

Based on the tDCS outcome recorded 1 day after tDCS treatment, participants will be categorized into responders and non-responders based on the percentage of change in the total SRS score. Participants who show reductions of at least 10% in total SRS scores as compared to baseline scores will be considered responders. This percentage reduction benchmark was set with reference to the minimal clinically important difference (MCID) and calculated using the standard error measurement method from an ASD sample in a previous randomized controlled trial.