Synthetic Vasomotion for Glymphatic Enhancement

The goal of this pilot clinical trial is to learn whether a week-long study combining controlled breathing procedures, intermittent exposure to a low-concentration carbon dioxide gas mixture, and noninvasive vagus nerve stimulation can be carried out safely and comfortably in adults with early-stage Parkinson's disease and age-matched healthy adults. The study will also explore how these procedures affect physiological signals related to breathing, cardiovascular and autonomic function, brain activity, and physiological processes thought to be related to fluid movement in and around the brain. The main questions the study aims to answer are: * Can participants safely and comfortably complete the study procedures and at-home monitoring? * Do physiological responses differ when intermittent hypercapnia is paired with active noninvasive vagus nerve stimulation compared with sham stimulation? * Are changes measured during the intervention visits related to overnight physiological measurements and changes in blood-based biomarkers? Researchers will compare active and sham noninvasive vagus nerve stimulation within the same participants. Both conditions will be paired with guided breathing and intermittent hypercapnia, in which participants briefly breathe a controlled gas mixture containing 5% carbon dioxide. Participants will: * Complete an intake and baseline visit and several nights of at-home physiological monitoring. * Complete two intervention visits in randomized order, one using active vagus nerve stimulation and one using sham stimulation. * Complete guided breathing and intermittent hypercapnia procedures while respiratory, cardiovascular, and other physiological signals are monitored. * Provide blood samples before and after intervention procedures for exploratory biomarker measurements. This is an exploratory pilot study. It is not intended to diagnose, treat, or prevent Parkinson's disease or any other medical condition.