This study explores how **transcranial direct current stimulation (tDCS)** affects thinking skills in people with **mild cognitive impairment (MCI)**, which is a stage before more serious memory problems. tDCS is a gentle brain stimulation technique thought to help brain function. The study will involve 120 participants aged 45-80, who will be divided into groups to receive different types of tDCS or a fake treatment called a sham. - **Study Duration**: Participants will have 10 sessions over 14 days. - **Tests**: Various tests and brain scans (fMRI and EEG) will track changes in brain activity and memory. - **Eligibility**: Participants must be able to read and write, be between 45-80, and have MCI or Parkinson's with MCI. This study could help find better ways to maintain memory and thinking abilities as people age. Participants will be monitored at the start, after 14 days, and again after 90 days to see the long-term effects of the treatment.