Can tDCS Enhance Efficacy of Rehabilitative Intervention for Freezing of Gait in Parkinson's Disease?

Inclusion Criteria: * men and women aged 30 to 80 years with DOPA-responsive PD Hoehn and Yahr (HY) grade of 2 to 4 while "off" * must be on a regimen including levodopa * total dose of levodopa and dopamine agonists (using dopamine equivalents) has to be equal to or more than 300 milligrams per day * Gait difficulties with Freezing of Gait as defined by MDS-UPDRS I score ≥ 2 in FOG * Optimal conventional PD medication for \> 1 month prior to screening * scheduled for rehabilitative intervention for the treatment of freezing of gait Exclusion Criteria: * significant concurrent medical or psychiatric disease * history of seizures and epilepsy * Dementia or other neurodegenerative disease (besides PD) * pallidotomy, implanted electrodes and generator for deep brain stimulation * pregnancy * surgically or traumatically implanted foreign bodies such as an implanted medical pump, implanted hearing aids, metal plate in the skull, or metal implant in the skull or eyes (other than dental appliances or fillings) that may pose a physical hazard during tDCS. * Study would cause undue risk or stress for reasons such as tendency to fall, excessive fatigue, general frailty, or excessive apprehensiveness. * significant postural instability with daily falls, inability to walk the parcours or inability to walk 10 meters. * presence of significant cognitive dysfunction as determined by Montreal Cognitive Assessment (MOCA) \<20 or mentally impaired patients having no capacity to provide their own consent (the physician establishing the diagnosis and applying UPDRS will evaluate patient's mental capacity using conventional clinical interview). * presence of other co-morbid conditions that can contribute to gait dysfunction (orthopedic, rheumatologic, cardiac, other) * presence of clinically significant hallucinations * participation in any rehabilitation therapy for FOG within the last six months prior to screening