Alzheimer's dementia (AD) and related neurodegenerative diseases are inflicting a global healthcare crisis. Neuropsychiatric symptoms including agitation and aggression affect up to 80% of patients with AD. Among these symptoms, agitation and aggression are the most burdensome for patients, families, caregivers, and the health care system. The mechanisms of agitation in AD are poorly understood and the current interventions are only modestly effective while having serious adverse effects.

In this study, the investigators propose to assess the mechanisms and treatment of neuropsychiatric symptoms in AD with the use of non-invasive, brain stimulation approaches. Agitation/aggression is associated with abnormalities in the balance of cortical excitation/inhibition. Transcranial magnetic stimulation (TMS) using single and paired pulse TMS paradigms such as long interval cortical inhibition (LICI) and short interval intracortical inhibition (SICI) can assess this cortical excitation/inhibition balance in vivo. The investigators will use TMS, combined with electroencephalography (EEG), to assess cortical excitation/inhibition balance in the dorsolateral prefrontal cortex (DLPFC) of participants with AD with and without agitation/aggression and age matched healthy comparators.

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that uses a very low intensity electric current to modulate cortical excitability and brain plasticity. tDCS can restore excitation and inhibition balance by altering GABA and glutamate activity in the brain. tDCS can be safely administered to awake persons and is very well tolerated. Studies in healthy individuals and patients with AD have shown that tDCS applied to frontal brain regions can enhance cognitive function. Further, tDCS applied to frontal brain regions can improve depressive symptoms. The effects of tDCS on neuropsychiatric symptoms are not known. In this study, the investigators will use tDCS applied to the frontal brain regions to target deficits in inhibition and symptoms of agitation/aggression in AD.

This study will be done over a period of 3 years at Centre for Addiction and Mental Health Geriatric Psychiatry Division and Temerty Centre for Therapeutic Brain Intervention. The investigators will recruit and enroll 30 individuals with AD + Agitation (mild to moderate agitation), 30 individuals with AD without agitation and 30 older healthy individuals. Medical work up will be done as part of clinical care prior to study enrollment to rule out medical causes of agitation. Clinical and cognitive assessments will be performed to characterize participants at baseline. The investigators will assess cortical inhibition using TMS EEG at baseline in the three groups. Individuals with AD + agitation will then receive a 2 week course of active/sham cathodal tDCS to frontal brain region in a double blind 1:1 randomized control design. Assessment of cortical inhibition using measures identical to baseline will be done at the end of tDCS course. The investigators will also repeat clinical and cognitive assessments after tDCS course and 2 weeks later. Measures of cortical inhibition will first be compared between HC and AD, and between AD and AD + Agitation groups. The treatment effects on clinical symptoms, cortical inhibition and the rate adverse events will be compared between active and sham tDCS groups.

During the Covid-19 pandemic, the study has been modified to be administered in a hybrid manner to accommodate both in-person and virtual assessments. Clinical and cognitive assessments can be done in-person or remotely. The initial tDCS course will be conducted in-person with the research staff. The remaining tDCS intervention will be completed onsite or at the participant's home with virtual supervision from the research team.