Award Date

5-1-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Kinesiology and Nutrition Sciences

First Committee Member

Brach Poston

Second Committee Member

Graham McGinnis

Third Committee Member

Jing Nong Liang

Fourth Committee Member

Sheniz Moonie

Number of Pages

148

Abstract

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that has emerged over the past 10-12 years as safe, affordable, portable, easy to use, and potentially efficacious adjuvant training intervention in several aspects of human performance. tDCS has been shown to be able to improve both cognitive and motor function in a wide variety of populations such as young adults, older adults, stroke, Parkinson’s disease, and many others. However, the majority of tDCS research has been conducted in the motor system of healthy young adults, which will also be the focus of this dissertation.The basic effects of tDCS applied to cortical areas of the brain were determined in the 1960s in animal experiments. However, complementary methods to study the effects of tDCS on human cortical areas were not available at the time. Therefore, the contemporary study of tDCS in humans did not begin until a little over 20 years ago when methods such as transcranial magnetic stimulation (TMS) had become available to quantify the effects of tDCS on human cortical excitability easily and non-invasively. Accordingly, the number of research studies and interest in tDCS started to increase exponentially about 10-12 years ago when the first studies involving human motor skill learning enhancement due to tDCS were published. In addition, initial studies confirmed in humans some of the most basic physiological effects of tDCS that were previously found in animals. The primary acute effect of tDCS when applied to cortical areas in humans is a modulation of neuronal excitability. This effect has been most studied and best characterized when tDCS is applied to the primary motor cortex (M1). The most basic finding is that the modulation of M1 excitability depends on the tDCS polarity. For example, electrode montages that place the anode over M1 (termed anodal tDCS) increases M1 excitability, whereas placement of the cathode over M1 (termed cathodal tDCS) decreases M1 excitability. Importantly, the currents applied through application of tDCS are very weak. Therefore, tDCS does not increase cortical excitability by directly causing action potentials, but rather by inducing small changes in the transmembrane potential of neurons, which increases or decreases their average spontaneous firing rates, leading to the overall observed changes in cortical excitability. In addition, other stimulation parameters influence the modulation of cortical excitability via tDCS. These parameters are the total dose of the stimulation, which is a combination of parameters of stimulation duration, intensity (current strength), and density. Furthermore, the temporal aspects of anodal tDCS have been elucidated and indicate that the excitability can increase immediately upon initiation of stimulation, can be detectable within 3-5 minutes, remains elevated during stimulation, and can persist for up to 30-90 minutes after stimulation ceases. Overall, the most common findings associated with the application of tDCS to M1 in humans are that anodal stimulation increases M1 excitability and increases motor performance, whereas cathodal stimulation decreases M1 excitability and has either no effects or negative effects on motor performance. Therefore, this dissertation is concerned only with anodal tDCS and all references hereafter to tDCS refer to anodal tDCS unless otherwise specified. The practical application of tDCS in research studies involves using a small battery-operated stimulator to apply tDCS to the scalp through two rubber small electrodes that are enclosed in saline-soaked sponges to impact a brain area of interest. For example, the most common M1- tDCS electrode montage involves placing the anode over M1 and the cathode over the contralateral supraorbital region (forehead). This is termed an M1-SO tDCS electrode montage and delivers anodal tDCS to M1 in an effort to increase motor performance and/or increase M1 excitability as mentioned previously. M1 is the most important brain region involved in the control of voluntary movement. Accordingly, the output of M1 projections to motor neurons in the spinal cord plays the predominant role the generation and execution of skilled movements, especially of the hand and arm. In addition, M1 is involved in many other aspects of motor control (e.g. motor planning) that are primarily accomplished by other brain regions, but integrated with the role of M1 in this process. Finally, M1 is arguably the most important brain region responsible the physiological adjustments that occur with motor skill acquisition and motor learning, although the cerebellum in particular also plays a major role. Accordingly, this is one of several major reasons why these brain areas have been the most targeted by tDCS in the research literature. There have been many recent advances in the understanding of the influence of tDCS on motor skill and motor learning in healthy young adults. Many of the basic physiological and behavior effects of tDCS have been identified and a basic understanding of the range of stimulation parameters has been identified to elicit positive effects on motor performance. However, much more research on these and related topics are needed to determine the viability of tDCS as an adjunct training intervention to increase motor skill in healthy adults. Thus, a few major interrelated gaps in the literature were addressed in this dissertation. Specifically, the ability of tDCS applied to different brain areas over multiple days to enhance complex motor tasks was the primary focus. This was due to observations that the overwhelming majority of previous tDCS studies have involved relatively simple motor tasks and a single stimulation session in participants who had usually never in the past performed the somewhat artificial laboratory tasks used in these studies. Relatedly, the projects within this dissertation were designed to address if the findings of previous multi-day tDCS studies that observed performance increases of 20-40% with multi-day tDCS application could be achieved in complex motor tasks. This knowledge is essential as most daily living activities as well as workplace, military, and sports settings entail complex tasks. Thus, tDCS must be able to show efficacy in improving performance on complex motor tasks to be a viable adjunct training intervention for various real-world applications. Therefore, the overall purposes of this dissertation were to examine the influence of tDCS applied to different brain regions over multiple days on motor learning in complex motor tasks. These purposes were accomplished through a series of three interrelated studies. In the first study (Chapter 2), the primary purpose was to examine the influence of M1-tDCS applied over multiple days on motor learning of a complex overhand throwing task. The secondary purpose was to examine the association between M1-tDCS induced increases in cortical excitability and the magnitude of motor learning. The study utilized a double-blind, randomized, between- subjects, SHAM-controlled, experimental design. Two separate groups of participants completed three consecutive practice sessions on three consecutive days that involved practice of the overhand throwing task simultaneous with application of M1-tDCS or SHAM stimulation. The first hypothesis was that M1-tDCS would improve endpoint accuracy in the complex overhand throwing task to a greater extent than practice alone (SHAM stimulation). The secondary hypothesis was that the increase in MEP amplitude following tDCS would be positively associated with the degree of motor learning exhibited by participants in the tDCS group. The study produced four main findings. First, endpoint error significantly decreased over the three days of practice in the tDCS group, but not in the SHAM group. Second, the decreases in endpoint error were due to both online (within-session) and offline (between-session) effects in the tDCS group. Third, MEP amplitude was significantly increased in the tDCS group, but not in the SHAM group. Fourth, the increases in MEP amplitude in the tDCS group were not positively associated with the degree of improvements in motor learning. Taken together, the findings indicated that M1-tDCS applied over three consecutive days can improve motor learning in a complex motor task in young adults, but these improvements are not associated with tDCS induced changes in M1 excitability. In the second study (Chapter 3), the exact same experimental methodology was repeated as in the first study (Chapter 2), with the exception that cerebellar tDCS (c-tDCS) was applied instead of M1-tDCS. This was primarily based on studies by other research groups that c-tDCS could potentially be as effective or almost as effective as M1-tDCS. In Chapter 3, the primary purpose to determine the influence of c-tDCS applied over multiple days on motor learning in a complex overhand throwing task. The secondary purpose was to determine if c-tDCS could enhance M1 excitability and if any of the excitability enhancements would be positively correlated with the amount of motor learning displayed by the c-tDCS group. The study utilized a double-blind, randomized, between-subjects, SHAM-controlled, experimental design. Two separate groups of participants completed three consecutive practice sessions on three consecutive days that involved practice of the overhand throwing task simultaneous with application of c-tDCS or SHAM stimulation. The first hypothesis was that c-tDCS application over multiple days would enhance accuracy in a complex overhand throwing task to a greater degree than practice alone (SHAM stimulation). The secondary hypothesis was that the increase in MEP amplitude following c-tDCS would be positively associated with the degree of motor learning exhibited by participants in the c-tDCS group. The study produced four main findings. First, overhand throwing accuracy improved over the three days of practice, but the magnitude of reduction in endpoint error achieved at the end of practice was not significantly different between the c-tDCS and SHAM stimulation groups. Second, the relative influences of online and offline learning to the total motor learning was also similar between the two groups Third, M1 excitability was increased for both the c-tDCS and SHAM groups, but the increases in M1 excitability were similar for the two groups. Fourth, increases in endpoint accuracy were not associated with increases in MEP amplitude even when comparisons were restricted to participants in either group that displayed both increases in endpoint accuracy and MEP amplitude. Therefore, the main findings of the study were contrary to the original two hypotheses and collectively indicated that three consecutive daily applications of c-tDCS does not improve motor learning in a very complex motor task to a greater degree than practice alone or significantly increase M1 excitability. In the third study (Chapter 4), the overall general research theme was consistent with the prior two studies and chapters as the influence of tDCS on a complex motor skill was investigated. However, this study had several differences compared with the first two studies. Specifically, it was a case series that used a cross-over within-subjects experimental design and involved elite performers executing a 10-meter rifle shooting task while tDCS was applied to the DLPFC. Therefore, the purpose of this chapter was to determine the influence of DLPFC-tDCS applied over multiple days on motor learning in 10-meter air rifle shooting performance in elite Deaflympic athletes. The Deaflympic athletes are considered competitors with a hearing loss of no less than 55 dB in their better hearing ear. Due to the difficulty of recruiting elite performers (athletes) for research studies this study was a case series (4 participants) that utilized a randomized, double-blind, SHAM-controlled, within-subjects, cross-over design. Participants complete a set of practice sessions in a DLPFC-tDCS condition and a SHAM condition in a cross-over design with a week washout period. The hypothesis was that DLPFC-tDCS would enhance shooting performance more than practice alone (SHAM stimulation). Additionally, it was hypothesized that shooting performance would progressively improve over the three days of DLPFC-tDCS application, whereas shooting performance would remain relatively constant over the three days of SHAM stimulation. The study produced three main findings. First, DLPFC- tDCS applied concurrently with practice over three practice sessions did not improve total points or endpoint error relative to SHAM stimulation. Second, total points and endpoint error were similar in the DLPFC-tDCS condition and the SHAM condition in the post-test blocks performed after stimulation on each of the three days. Thus, DLPFC did not elicit any significant after-effects. Third, shooting performance remained relatively constant across all practice days and practice blocks in both stimulation conditions and near the highest levels attained by these athletes in training and competition. Taken together, the findings indicate that DLPFC-tDCS applied concurrent with practice for three consecutive days does not improve shooting performance in elite athletes beyond performance ceiling levels reached through extensive practice using traditional training approaches. In summary, this dissertation examined the influence of tDCS applied to different brain regions over multiple days on motor learning in complex motor tasks. The main overall findings were that M1-tDCS can improve motor learning in a complex motor task when applied over three consecutive days, whereas c-tDCS and DLPFC do not appear to improve motor learning to a greater extent than practice alone (SHAM stimulation).

Keywords

motor skill; transcranial direct current stimulation; transcranial magnetic stimulation

Disciplines

Medicine and Health Sciences

File Format

pdf

File Size

1978 KB

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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