Finding brain stimulation targets for low motivation

Inclusion Criteria: * Diagnosis of idiopathic Parkinson Disease. * At least 4 years of symptoms. * On dopaminergic medication for Parkinson Disease. * Stable on dopaminergic medication and other medications which may influence apathy (such as selective serotonin re-uptake inhibitors, stimulant medications) for at least 4 weeks prior to first study visit and remain stable throughout the study period. * Hospital's study-specific informed consent must be obtained. * Must have capacity to provide informed consent in English. * For female participants, confirmation that they have not had a menstrual period in over 12 months, or that they will use an effective form of contraception during the study. Exclusion Criteria: * Inability to provide informed consent. * Inability to perform effort task (determined during the titration session). * Presence of dementia (Montreal Cognitive Assessment (MoCA) score \< 21). * History of epilepsy or brain surgery. * Severe tremor or dyskinesia that would interfere with EEG (determined by the PI). * Patients with clinically significant medical or neurological conditions which may be an alternative cause of parkinsonism such as repeated brain injury, anti-dopaminergic medications, anoxic brain injury, or significant basal ganglia strokes. * Presence of other known central nervous system disease that may interfere with performance or interpretation of EEG or TMS. * Presence of any implanted metal devices including, but not limited to, pacemakers, deep brain stimulators, vagal nerve stimulators, bladder stimulators, or cochlear implants. * Presence of medical contraindications to TMS such as implanted stimulators, history of mania or bipolar disorder, history of epilepsy.