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Reliability of TMS measurements of the motor cortex




Causer, Laurie, author
Malcolm, Matt P., advisor
Davies, Patricia, committee member
Seger, Carol A., committee member

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BACKGROUND: Transcranial magnetic stimulation (TMS) was introduced in 1985 and has been used to study the human motor system through a variety of applications including single pulse, paired pulse and repetitive pulse stimulation parameters. Paired pulse TMS studies assess motor cortical excitability, in which the first (conditioning) stimulus (CS) modifies the response to the second (test) stimulus (TS) (Maeda, Gangitano, Thall, & Pascual-Leone, 2002). The time between pulses, or the interstimulus interval, is the distinguishing factor between the application of paired pulse TMS to investigate intracortical inhibition (ICI) or intracortical facilitation (ICF). Studies of cortical excitability using paired pulse TMS can provide novel insights into the pathophysiology of various neurological and psychiatric disorders (Maeda, et ah, 2002) and have begun to be utilized as outcome measures to document changes in cortical excitability in response to repetitive TMS. The stability of the muscle responses known as motor evoked potentials (MEPs) elicited in response to paired pulse stimulation has not been well documented in the literature to date. As such, the primary goal of this study was to establish the test-retest reliability of two paired pulse measures of the motor cortex, ICI and ICF, in two muscle representations; first dorsal interossei (FDI) and abductor pollicis brevis (APB). METHODS: Fifteen healthy individuals, age 19-37 years old, participated in two identical testing sessions held exactly one week apart from each other. Four different types of stimulation (CS, TS, 2ms, and 15ms) were delivered over the motor cortex 20 times in a random order. The corresponding MEPs were recorded and their size were documented using two common methods found in the literature; area under the curve and peak to peak amplitude. RESULTS: Reliability was determined using intra-class correlation coefficients (ICCs). Poor reliability was documented in both methods of analysis; whether twenty trials or ten trials were averaged, and even still after normalizing data, with ICCs ranging from (-.508 - .347). CONCLUSION: Additional studies investigating the test-retest reliability of paired pulse measures of the motor cortex need to be conducted to document the stability of MEPs. Potential sources of variation in MEPs size include electrode placement variation, stimulation intensity changes, coil placement variability, state of the overall nervous system, and the state of the individual muscle (contracted/relaxed). Until the reliability of paired pulse stimulation is established, researchers should use caution linking the changes in the size of MEPs in response to paired pulse stimulation to interventions, disease, or other external factors.


Covers not scanned.
Print version deaccessioned 2022.

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Magnetic brain stimulation
Motor cortex


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