The purpose of this study is to evaluate the effect of 10 sessions of anodal transcranial Direct Current Stimulation (tDCS - 1 mA) applied to left Cortex DorsoLateral PreFrontal (CDLPF) of Alzheimer's or Primary Progressive Aphasia (PPA) patients compared to the application of a placebo tDCS (sham procedure) on cognitive functions, which are evaluated at short term (1 week post-treatment) and mild term (3 weeks post-treatment).