Citation: de Albuquerque, L.L.; Pantovic, M.; Clingo, M.; Fischer, K.; Jalene, S.; Landers, M.; Mari, Z.; Poston, B. A Single Application of Cerebellar Transcranial Direct Current Stimulation Fails to Enhance Motor Skill Acquisition in Parkinson’s Disease: A Pilot Study. Biomedicines 2023, 11, 2219. https:// doi.org/10.3390/biomedicines 11082219 Academic Editors: Jun Lu, Masaru Tanaka and Eleonóra Spekker Received: 13 June 2023 Revised: 1 August 2023 Accepted: 4 August 2023 Published: 8 August 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). biomedicines Article A Single Application of Cerebellar Transcranial Direct Current Stimulation Fails to Enhance Motor Skill Acquisition in Parkinson’s Disease: A Pilot Study Lidio Lima de Albuquerque 1 , Milan Pantovic 2 , Mitchell Clingo 3 , Katherine Fischer 2 , Sharon Jalene 2 , Merrill Landers 4 , Zoltan Mari 5 and Brach Poston 2, * 1 School of Health and Applied Human Sciences, University of North Carolina Wilmington, Wilmington, NC 28403, USA; limadeal@uncw.edu 2 Department of Kinesiology and Nutrition Sciences, University of Nevada Las Vegas, Las Vegas, NV 89154, USA; milan.pantovic@unlv.edu (M.P.); fische74@unlv.nevada.edu (K.F.); sharon.jalene@unlv.edu (S.J.) 3 School of Medicine, University of Nevada Las Vegas, Las Vegas, NV 89154, USA; clingom@unlv.nevada.edu 4 Department of Physical Therapy, University of Nevada Las Vegas, Las Vegas, NV 89154, USA; merrill.landers@unlv.edu 5 Movement Disorders Program, Cleveland Clinic Lou Ruvo Center for Brain Health, Las Vegas, NV 89106, USA; mariz@ccf.org * Correspondence: brach.poston@unlv.edu Abstract: Parkinson’s disease (PD) is a progressive neurodegenerative disorder that leads to numer- ous impairments in motor function that compromise the ability to perform activities of daily living. Practical and effective adjunct therapies are needed to complement current treatment approaches in PD. Transcranial direct current stimulation applied to the cerebellum (c-tDCS) can increase motor skill in young and older adults. Because the cerebellum is involved in PD pathology, c-tDCS applica- tion during motor practice could potentially enhance motor skill in PD. The primary purpose was to examine the influence of c-tDCS on motor skill acquisition in a complex, visuomotor isometric precision grip task (PGT) in PD in the OFF-medication state. The secondary purpose was to deter- mine the influence of c-tDCS on transfer of motor skill in PD. The study utilized a double-blind, SHAM-controlled, within-subjects design. A total of 16 participants completed a c-tDCS condition and a SHAM condition in two experimental sessions separated by a 7-day washout period. Each session involved practice of the PGT concurrent with either c-tDCS or SHAM. Additionally, motor transfer tasks were quantified before and after the practice and stimulation period. The force error in the PGT was not significantly different between the c-tDCS and SHAM conditions. Similarly, transfer task performance was not significantly different between the c-tDCS and SHAM conditions. These findings indicate that a single session of c-tDCS does not elicit acute improvements in motor skill acquisition or transfer in hand and arm tasks in PD while participants are off medications. Keywords: Parkinson’s disease; transcranial direct current stimulation; motor skill; cerebellum; cerebellar stimulation; motor learning; manual dexterity; dopamine; basal ganglia; transfer of motor learning 1. Introduction Parkinson’s disease (PD) is the second most common neurodegenerative disorder and affects approximately one million people in the United States with annual costs approaching USD 11 billion [1]. The cardinal pathologic feature of PD is the loss of dopaminergic neurons in the substantia nigra pars compacta, which leads to striatal dopamine depletion. The decrease in dopamine is associated with a variety of motor deficits such as rigidity, bradykinesia, tremor, and postural instability that lead to severe impairments in the ability to perform daily living activities. Current surgical and pharmacological treatments may Biomedicines 2023, 11, 2219. https://doi.org/10.3390/biomedicines11082219 https://www.mdpi.com/journal/biomedicines