Inclusion Criteria:  For the group of healthy older adults:  MEC score greater than 26 points. GDS between 1 and 3.  For the MCI group:  MEC score between 21 and 26 points. GDS between 1 and 3.  For the mild EA group:  MEC score between 18 and 23 points. GDS between 3 and 4.  Exclusion Criteria:  Presentation of contraindications to tDCS (presence of intracranial metal implants, intracranial hypertension, comitial risk). Significant asymptomatic neurovascular disease History of previous symptomatic stroke Alcohol or drug abuse/dependence Severe psychiatric symptoms Depressive symptoms higher than mild