The purpose of this research is to demonstrate that mild stimulation of a nerve (trigeminal nerve) in the head can modulate blood flow in the brain. The modulated blood flow will restore the flow of cerebrospinal fluid (CSF) and this in turn can help improve waste clearance in the brain and prevent build up that may lead to disease.

The goal of this clinical trial is to demonstrate that non-invasive stimulation of the trigeminal nerve can be used to restore blood flow and consequently modulate CSF flow into the brain parenchyma to facilitate brain waste clearance that is impaired in patients with AD. This study will collect brain MRI data on 20 healthy cognitively unimpaired adults receiving intermittent trigeminal nerve branch (V1) stimulation. Participants with recent mild traumatic brain injuries will also be enrolled to establish feasibility within a clinically relevant population.

Inclusion Criteria - Healthy Participants :

• Age 20-85 years
• Willing and able to undergo all procedures
• Free of any neurological (e.g., epilepsy, stroke, Parkinson's, brain tumors, moderate or severe traumatic brain injury), neuropsychiatric (e.g., schizophrenia, substance abuse, clinical depression), or developmental (e.g., autism, ADHD) or any other medical condition at the discretion of the PI and co-investigators

Inclusion Criteria - mTBI :

• Age 20-65
• Diagnosed with mild traumatic brain injury and be within 1-52 weeks post-injury
• Willing and able to undergo all procedures

Exclusion Criteria - Healthy Participants:

• Contraindication to MRI
• Known allergies to common electrode materials

Exclusion Criteria - mTBI:

• Contraindications for MRI
• Known allergies to common electrode materials
• Candidates currently on cardiovascular medications which, in the opinion of the investigators, would introduce a confounding factor to glymphatic system function (i.e. ACE-inhibitors, angiotensin receptor blockers, calcium channel blockers)
• Candidates who have chronic infectious diseases (e.g. hepatitis, HIV, TB)
• Candidates diagnosed with small vessel disease, vascular associated dementia (e.g. VCID), or renal disease
• Candidates retinopathies or neuropathies that, in the opinion of the investigators, would prevent them from being able to complete the study or would introduce a confounding factor to glymphatic system function
• Candidates with neurological disorders other than those attributed to their primary diagnosis (e.g. MS, PD, ALS, AD or other dementia, uncontrolled pain)
• Candidates who have had a penetrating injury, craniotomy (with the exception of a burr hole (trephination) for resolution of acute subdural hematoma), or refractory subdural hematoma
• Candidates with a history of seizures (except those in the acute or post-acute phases and are controlled)
• Candidates who experienced a loss of consciousness greater than 24 hours as a result of their TBI
• Candidates who, in the opinion of the investigators, are unable to feel a sensory referred sensation from the stimulation or successfully complete the electrode placement and testing fNIRs