Award Date

August 2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Interdisciplinary Programs

First Committee Member

Brach Poston

Second Committee Member

Merrill Landers

Third Committee Member

Graham McGinnis

Fourth Committee Member

Hyunhwa Lee

Number of Pages

158

Abstract

Transcranial direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS) are promising non-invasive brain stimulation techniques, particularly for clinical applications involving altered or impaired motor function, such as in stroke and Parkinson’s disease. Although tDCS and tACS are different modalities, both forms of stimulation operate on the same principle - weak electrical currents are delivered through electrodes placed on the scalp in specific configurations known as “montages”. The most commonly used montage for targeting the primary motor cortex (M1) is the M1-SO configuration. This montage leverages the established observation that cortical regions directly beneath the anode generally increase in excitability, while those beneath the cathode generally decrease in excitability. Consequently, the M1-SO montage places the dominant M1 under anodal stimulation, with the cathode positioned on the contralateral supraorbital area. Many publications using the M1-SO montage report enhancements in motor performance, motor learning, skill retention, fatigue resistance, and increases in M1 excitability. However, despite the abundance of data collected using the M1-SO montage over the last 20 years, several important gaps in knowledge have recently emerged. 1) While many studies claim that twenty minutes of anodal stimulation to the dominant M1 produces an increase in cortical excitability, as quantified by MEPs, the changes that occur in the contralateral, non-stimulated M1 during this concurrent tDCS remain unclear. 2) It has been suggested that tACS does not increase cortical excitability as effectively as tDCS. However, due to its unique ability to induce neuronal entrainment, tACS may be more effective at modulating indirectly targeted brain regions, potentially enhancing clinical outcomes. This raises the question of what changes in cortical excitability, as quantified via MEPs, are observed in the contralateral, non-stimulated M1 when tACS is applied to the dominant M1 concurrently for twenty minutes. 3) While literature suggests that both tDCS and tACS can accelerate motor learning in both healthy and clinical populations, most studies erroneously infer these assumptions based on single-day stimulation protocols and often measure simple, single-joint motor skills as the basis of motor learning. Few publications explore the effects of brain stimulation, especially tACS, over multiple days of stimulation using a complex multi-joint motor skill. Therefore, it is critical to investigate whether multiple days of M1 anodal tACS can enhance motor learning of a complex multi-joint task, such as an overhand throw. In order to address the aforementioned gaps in current literature, the subsequent paragraphs will briefly summarize each chapter of this dissertation.Chapter 1 provides a comprehensive overview of the history, evolution, mechanisms, theories, and specific applications of non-invasive brain stimulation, and introduces relevant motor learning principles to establish a foundation of essential knowledge. The chapter is intentionally written in a clear, non-technical style to ensure that readers, regardless of their initial understanding of brain stimulation and motor learning, can grasp the technical content presented in chapters two through four. The study found in chapter 2 aimed to investigate the effect of tDCS applied to the dominant M1 on the excitability of the contralateral non-dominant M1. Utilizing a double-blind, randomized, SHAM-controlled, within-subjects, crossover design, eighteen young adults participated in two experimental sessions (tDCS and SHAM) in a counterbalanced order with a one-week washout period between sessions. Transcranial magnetic stimulation (TMS) was employed to measure the excitability of the contralateral M1 during 20 minutes of anodal tDCS at a current intensity of 1 mA. TMS assessments were conducted in five test blocks (Pre, D5, D10, D15, and Post). The Pre and Post test blocks were administered immediately before and after tDCS application, while the D5, D10, and D15 blocks were performed at 5, 10, and 15 minutes into the stimulation, respectively. The primary outcome was the 1 mV motor evoked potential (MEP) amplitude. Data were analyzed using a 2 condition (tDCS, SHAM) x 5 test (Pre, D5, D10, D15, Post) within-subjects ANOVA. Results showed no statistically significant main effects for condition (P = 0.213) or test (P = 0.502), nor was there a significant condition x test interaction (P = 0.860). These findings suggest that tDCS does not alter contralateral M1 excitability during or immediately following the stimulation under the tested parameters. Chapter 3 investigates the impact of tACS on the excitability of the non-dominant right M1 when tACS is applied to the dominant left M1. A double-blind, randomized, SHAM-controlled, within-subjects, crossover design was used. Eighteen young adults participated in both tACS and SHAM conditions on separate days in a counterbalanced order with a one-week washout period between sessions. Transcranial magnetic stimulation (TMS) was employed to measure the excitability of the contralateral right M1 while tACS was administered to the left M1. TMS measurements were taken in five blocks (Pre, D5, D10, D15, and Post) relative to a 20-minute session of tACS (70 Hz, 1 mA). The Pre and Post TMS blocks were conducted immediately before and after tACS, respectively, while the D5, D10, and D15 blocks occurred at 5, 10, and 15 minutes during the stimulation. The primary outcome was the 1 mV motor evoked potential (MEP) amplitude. A 2 condition (tACS, SHAM) x 5 test (Pre, D5, D10, D15, Post) within-subjects ANOVA was performed on the MEP data. Results showed no significant main effect for condition (P = 0.704) or for the condition x test interaction (P = 0.349). Although there was a significant main effect for test (P = 0.003), post hoc analysis revealed that none of the pairwise comparisons were statistically significant. These findings suggest that tACS applied to the left M1 does not significantly alter the excitability of the contralateral right M1 during or immediately following the stimulation, at least under the parameters used in this study. Finally, in chapter 4 the primary objective of this study was to evaluate the effect of M1-tACS applied over three consecutive days on the motor learning of a complex overhand throwing task in young adults. Additionally, the study aimed to investigate the impact of M1-tACS on M1 excitability. A double-blind, randomized, SHAM-controlled, between-subjects experimental design was employed. Twenty-four healthy young adults were assigned to either tACS or SHAM groups and participated in three identical experimental sessions. Each session involved blocks of overhand throwing trials with the right dominant arm while tACS was applied to the left M1. Performance was measured by endpoint error. To assess changes in M1 excitability, motor evoked potentials (MEPs) were recorded from the right first dorsal interosseous (FDI) muscle using transcranial magnetic stimulation (TMS). Results showed a significant reduction in endpoint error from pre-test to post-test across the three days of practice (P = 0.046), but no significant difference between the tACS and SHAM groups (P = 0.474). MEP amplitudes increased from pre-test to post-test (P = 0.003), yet these increases did not significantly differ between the groups (P = 0.409). Overall, the findings suggest that M1-tACS over multiple days does not enhance motor learning of a complex task more than practice alone (SHAM).

Keywords

Cortical excitability; Motor control; Motor learning; Motor skill; Non-invasive brain stimulation

Disciplines

Medical Neurobiology | Neuroscience and Neurobiology | Neurosciences

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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