Effects of Transcutaneous Cervical Vagus Nerve Stimulation on Non-motor Symptoms in Parkinson's Disease

The goal of this clinical trial is to learn if home-based transcutaneous cervical vagus nerve stimulation (tcVNS) can improve symptoms in adults with Parkinson's disease. tcVNS uses a small portable device placed on the skin of the left side of the neck to deliver mild electrical stimulation. It does not require surgery or anesthesia. The main questions it aims to answer are: * Does tcVNS reduce non-motor symptoms of Parkinson's disease, such as sleep problems, low mood, anxiety, digestive problems, and fatigue? * Does tcVNS change movement symptoms, memory and thinking, brain wave activity, heart rhythm patterns, and markers of inflammation in the blood? * Can participants use the device safely on their own at home every day, and what medical problems do they have while using it? Everyone in this study will use the same stimulation device. There is no placebo group. Researchers will compare each participant's own measurements taken before stimulation, after 4 weeks of stimulation, and again after a 4-week break, to see whether symptoms change and whether any change lasts. Participants will: * Use the stimulation device on the neck for 30 minutes once a day, at home, every day for 4 weeks. The first session is done at the clinic with study staff, who teach participants how to use the device. * Visit the clinic 6 times over about 9 weeks for physical and neurological checkups, questionnaires about their symptoms, memory and thinking tests, brain wave recordings (EEG), and heart rhythm recordings (HRV). * Give a blood sample of about 10 mL at 3 of the visits (about 30 mL in total) so researchers can measure markers of inflammation. * Have a brain MRI scan at 2 of the visits, at centers where MRI is available. * Keep a record of their device use in a paper diary or a smartphone app. The device also records use times automatically.