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 evaluate the therapeutic effects of tDCS on improving cognitive function in patients with ASD, (2) to better understand the neural mechanisms underlying the neuromodulation effects of tDCS in patients with ASD, and (3) to determine whether resting-state functional connectivity measures can predict the therapeutic effects of active tDCS in individuals with ASD.

Methods: To assess the therapeutic effects and neural mechanisms of tDCS, 90 adolescents with ASD (age 12-22 years) will receive three stimulation conditions: cathodal tDCS of the left DLPFC, anodal tDCS of the left DLPFC and sham-tDCS with at least a week apart each time. During the active tDCS or sham-tDCS condition, the participants will be administered a computerized test battery (Cambridge Neuropsychological Test Automated Battery, CANTAB®) to evaluate their cognitive function. EEG before and after the tDCS to evaluate the tDCS-induced alteration in their neural activity and functional connectivity. Hypothesis: Drawing together the different evidence linking ASD with cortical hyper-excitability and disordered neural connectivity, as reviewed previously, the investigators hypothesize that, relative to a sham-tDCS condition, active cathodal (inhibitory) and anodal (excitatory) tDCS over the left DLPRC will induce stimulation-linked facilitation of learning and resultant improvement of cognitive functioning in patients with ASD. In addition to the therapeutic effects of tDCS, enhanced neural connectivity, as indexed by altered level EEG theta coherence in patients with ASD, will mediate the beneficial effects of tDCS, relative to sham tDCS, on improvements in cognitive function. Moreover, resting-state functional connectivity will moderate the beneficial effects of active tDCS on cognitive function, relative to sham tDCS, such that participants with greater pre-treatment resting state functional connectively will evidence greater/less response to tDCS, relative to participants receiving sham tDCS.