This is a 24-week randomized, double-blind, placebo-controlled trial. We will enroll patients with aMCI or mild AD. All patients will receive 2 weeks of tDCS (5 sessions per week, 10 sessions in total) during the first 2 weeks of the study, and will also be allocated randomly to either of 2 treatment groups for 24 weeks: (1) DAOIB group; (2) placebo group. We will assess the patients every 8 weeks during the treatment period (weeks 0, 8, 16, and 24). We hypothesize that augmentation with DAOIB will yield better effect than tDCS alone in improving the cognitive function, global functioning and quality of life in patients with aMCI or mild AD.

Inclusion Criteria:

• Clinical diagnosis of Alzheimer's disease or mild cognitive impairment
• MMSE between 10-26
• CDR 1 or 0.5

Exclusion Criteria:

• Hachinski Ischemic Score > 4
• Substance abuse/dependence
• Parkinson disease, epilepsy, dementia with psychotic features
• Major depressive disorder
• Major physical illnesses
• Severe visual or hearing impairment