Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by disturbances in communication, poor social skills, and aberrant behavior. To date, ASD has no known cure, and the disorder remains a highly disabling condition. Recently, transcranial direct-current stimulation (tDCS), a non-invasive brain stimulation technique, has shown great promise as a potentially effective and cost-effective tool for reducing the core symptoms in patients with autism, such as anxiety, aggression, impulsivity, and inattention. Although the preliminary findings in patients with ASD are encouraging, it remains to be determined whether this experimental data can translate into benefits in real life. Further studies are needed to determine the factors that can lengthen the therapeutic effects or cognitive benefits of tDCS, and to determine possible risk factors associated with relapse in patients with ASD. Booster sessions of tDCS is an important component of treatment planning and prognosis and may promote better outcomes to control for resurgence of symptoms. This study has three aims. First, the investigators aim to evaluate the therapeutic effects of tDCS on improving cognitive function in patients with ASD. Second, the investigators aim to better understand the neural mechanisms underlying the neuro-enhancing effects of tDCS in patients with ASD. Third, the investigators aim to assess the effectiveness of booster treatment cycles of tDCS for enhancing cognitive and social functions in individuals with ASD.