Dementia is a prevalent condition with no known cure. It affects not only the person with dementia but also the family. This study will investigate the effects of applying different transcranial electrical stimulation (tES) waveforms to find the optimal stimulation when paired with cognitive exercise on older adults with dementia. It is a placebo-controlled double-blind study with statistical rigor. In addition, the investigators will investigate technological methods to monitor changes due to the intervention. These include functional near infrared spectroscopy (fNIRS), electroencephalography (EEG), electrovestibulography (EVestG), and balance measurements using accelerometers. fNIRS measures oxygenated and deoxygenated blood flow to the brain and it may help to understand neuronal changes due to intervention. EEG measures the electrical activity of the brain via electrodes placed on the scalp to study the brain's response to different electrical stimulations treatments. EVestG is a non-invasive measure of the vestibuloacoustic system in both background (no motion) and in response to passive whole-body tilt stimuli. We hypothesize there is a direct link between vestibular system deficiencies and Alzheimer's. Lastly, the investigators will measure the effects of electrical stimulation on balance by measuring postural sway using two accelerometers. This study investigates the effects of transcranial alternating current stimulation (tACS) and transcranial direct current stimulation (tDCS) with different protocols for frequencies to determine the most effective treatment to improve cognitive abilities and working memory for people with dementia. The global objective of the proposed study is to investigate the effects of different tES in improving the cognitive status of older adults with dementia, when paired simultaneously with cognitive training. The general objectives include the following: 1. Run a human research randomized and double-blind study to address the global objective of the study with statistical rigor. 2. Investigate the correlation of fNIRS outcomes with those of the primary outcome measures of the study in Objective 1 as well as its predictivity to the treatment at baseline. 3. Investigate the effects of different tES on EEG recordings as well as its predictivity to the treatment at baseline. 4. Investigate the effects of different tES on balance for older adults with cognitive impairment. 5. Investigate the correlation of EVestG outcomes with those of the primary outcome measures of the study in Objective 1 as well as its predictively to the treatment at baseline. 6. Disseminate the project results and design the next follow up project.