Tremor in Parkinson's disease is a common and highly burdensome symptom. Recent evidence shows that areas in the brain that are underlying Parkinson's tremor overlap with those that respond to somatosensory stimulation. Applying such stimulation to the tremulous limb might therefore influence tremor-related brain activity and thereby potentially reduce tremor. In this study, the investigators explore this possibility and investigate whether tremor specific vibrotactile stimulation at the wrist of the most affected arm influences tremor severity.

Parkinson's disease (PD) is the second most common neurodegenerative disease, of which tremor is a common and highly burdensome symptom. The pathophysiology of tremor involves a cerebral network that consists of basal ganglia and a cerebello-thalamo-cortical motor circuit. Emerging evidence suggests that somatosensory input from the tremulous limb to the brain may influence this network. Specifically, tremor-related activity was observed in primary and secondary somatosensory cortex, and thalamic cells involved in tremor appear to be responsive to somatosensory stimulation as well. Somatosensory afferents may thus play a role in tremor pathophysiology and could therefore be used as treatment target. Here, the investigators will test this hypothesis by investigating the effect of modulating somatosensory input (with vibrotactile stimulation) on tremor severity.

The primary objectives are 1) to test whether rhythmic somatosensory input (vibrotactile stimulation) at tremor frequency reduces tremor power of Parkinson's tremor as compared to vibrotactile stimulation at 1.5 times tremor frequency and 2) whether continuous 80Hz vibrotactile stimulation reduces tremor power as compared to sham stimulation. As secondary objectives, the investigators will test whether the effect of vibrotactile stimulation on tremor power depends on the context in which tremor occurs (rest, cognitive coactivation, posture), whether stimulation at tremor frequency reduces tremor power as compared to sham stimulation, and whether vibrotactile stimulation has an effect on two other common PD symptoms: impaired gait and bradykinesia. The investigators will include 27 patients with Parkinson's disease and rest as well as postural tremor. The intervention involves subtle mechanical vibrations on the wrist or ankles. This is a non-invasive and painless way to provide somatosensory stimuli, in a continuous (80Hz) or rhythmic fashion (brief bursts of 80 Hz at tremor or step frequency).

The investigators expect that this study will provide mechanistic insights into if and how somatosensory (vibro-tactile) afferents influence the cerebral tremor circuit in Parkinson's disease, and may provide a solid basis for designing further clinical treatment studies.

Inclusion Criteria:

• 18-80 years old and able to provide informed consent
• Have a diagnosis of idiopathic PD made by a movement disorders specialist
• Medically optimized without planned medication changes for the duration of the study • Resting tremor subscore >/= 2 of the most-affected arm on the Movement Disorders Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS) item 2.10
• Postural tremor subscore >/= 1 of the most-affected arm on the MDS-UPDRS item 3.15
• Signed informed consent

Exclusion Criteria:

• The presence of additional neurologic diseases that might confound testing or the coexistence of PD and essential tremor together (action tremor that was present prior to the development of parkinsonism)
• Moderate to severe peripheral neuropathy (reduced vibratory sensation) at the upper extremities, quantified with a graduated tuning fork
• Montreal cognitive assessment (MoCA) score < 20 or previously documented dementia
• Unable to walk without walking aid