Background: Transcranial Direct Current Stimulation (tDCS) is a form of non-invasive brain stimulation that has aroused increased interests in the past decade. Not only that it is transient with little side-effects, and can be well-tolerated by children, it is also affordable and readily accessible, making it an appealing treatment option for autism spectrum disorder (ASD).

Objective: (1) To assess the therapeutic effects of tDCS when combined with cognitive training for 10 consecutive weekdays on improving cognitive processing in children with ASD, relative to control group receiving sham-stimulation, and (2) to evaluate the associated neural mechanisms underlying the treatment effect of tDCS on children with ASD.

Methods: To assess the therapeutic effects of tDCS, 100 adolescents with ASD (age 8-12 years) will be randomly assigned to active- (n=50), or sham- (n=50) tDCS groups. Twenty-minute sessions of 1.5 mA tDCS stimulation to the left dorsolateral prefrontal cortex (DLPRC) in conjunction with cognitive training exercise will be provided on 10 consecutive weekdays. EEGs and neuropsychological tests will be administered before and immediate after the series of tDCS sessions. Hypothesis: The investigators hypothesize that children with ASD who are randomly assigned to receive a montage of prefrontal tDCS, with cathode (inhibitory) placed over left DLPFC and anode (excitatory) over right supraorbital region) will evidence greater improvement in executive function (primary outcome) than children with ASD who are randomly assigned to receive sham-tDCS. In addition to testing the primary clinical outcome, stated above, in planned exploratory analyses, the investigators will also examine the effects of tDCS on secondary outcome measures of cognitive function, including information processing speed, working memory, inhibitory control, and cognitive flexibility; and conduct exploratory mediation analyses to better understand the potential neurophysiological factors underlying the therapeutic effects of tDCS. This will include E/I ratio as exploratory mediator variables. As these secondary analyses are exploratory, the investigators will report them as such in presentations and published papers, and the investigators will not draw definitive conclusions from them. Rather, they will be used to better understand the potential impact of tDCS and the mechanisms underlying impact, and to inform future research.