Differential Effects of Low-Frequency rTMS on Motor Performance, Cortical Excitability and Inhibition in Aged Healthy Participants Open Access

Edwards, Lauren (Summer 2020)

Permanent URL: https://etd.library.emory.edu/concern/etds/2r36tz66r?locale=en


Low frequency repetitive transcranial magnetic stimulation (LF-rTMS) of the primary motor cortex (M1) alone, or when combined with sensorimotor training, can induce changes in neuronal activity at both the stimulated location and in remotely connected cortical areas. Level B evidence supports the use of LF-rTMS over the contralesional M1 for the treatment of post-stroke hemiparesis, an effect that may be mediated through interhemispheric connections with the non-stimulated ipsilesional M1. However, the effects of LF-rTMS are variable and the biological substrate underlying these therapeutic effects remains controversial. Furthermore, despite the highest prevalence of stroke in individuals above 65 years old, many current rTMS studies in healthy adults consist of younger cohorts. The objective of the present study is to characterize the modulatory effects of LF-rTMS on motor performance and cortical excitability, in both the stimulated and non-stimulated M1, and between each M1, in an older healthy population. Twenty right-handed healthy older adults (60 ± 7.2 years old, 13 females) with normal cognition and brain structure underwent 3 LF-rTMS experiments where rTMS was applied at 1 Hz (15 minutes, 900 pulses) over the left M1. Three different intensities of rTMS were used: 80% of resting motor threshold (RMT), 90% of RMT, and sham stimulation using a placebo coil. To evaluate the effects of LF-rTMS on M1 excitability, motor evoked potentials (MEPs) were collected from the extensor carpi ulnaris muscle before and after administration of LF-rTMS. Single- and paired-pulse TMS paradigms were used to assess general corticospinal excitability, intracortical inhibition and inter-hemispheric inhibition. A unimanual pointing task was used to assess motor performance of each hand. Results demonstrated that LF-rTMS did not significantly impact excitability or inhibition of either the stimulated or non-stimulated M1. LF-rTMS did differentially impact motor performance by hand with the contralateral hand showing no rTMS-related effects, but the ipsilateral hand having a reduction in improvement after LF-rTMS at 90% of RMT. A single administration of LF-rTMS does not significantly modify cortical excitability and has modest effects on motor performance in older adults, which is important for LF-rTMS application in the treatment of stroke.

Table of Contents

Introduction 1

Stroke Overview 1

Normal sensorimotor control of voluntary movement 1

Abnormal sensorimotor control of voluntary movement after stroke 3

Investigating neural mechanisms of sensorimotor control in humans with non-invasive brain stimulation 5

Table 1 – A detailed summary of rTMS studies evaluating healthy controls¹ 11

Chapter I – Neuromodulation as a probe of excitability and motor performance in healthy aged individuals 16

Introduction 16

Materials and Methods 18

Figure 1. Pointing task depiction. 20

Data Analysis 24

Figure 2. Extraction of curve parameters from SRCs. 26

Statistical Analysis 28

Results 30

Figure 3. Differential effects of LM1 LF-rTMS on left hand accuracy. 30

Figure 4. Changes in movement time of the left hand after LM1 LF-rTMS. 31

Figure 5. Illustration of group stimulus response curves by LF-rTMS interventions. 32

Figure 6. Effects of LM1 LF-rTMS on short-interval intracortical inhibition (SICI). 33

Figure 7. Effects of LM1 LF-rTMS on resting interhemispheric inhibition (rIHI). 34

Discussion 35

Conclusion 42

Discussion 42

Ongoing Studies – Assessments Post-Stroke 44

Future Direction – A Sensorimotor Approach 45

Conclusion 48

References 49

Appendix A 58

Table 2. Statistical Results 58

Figure 8. Effect of LM1 rTMS on Curve parameters extracted from SRC. 61

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