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Poster A50, Wednesday, November 8, 10:30 – 11:45 am, Harborview and Loch Raven Ballrooms

Effects of laterality, handedness, and coil orientation on size and morphology of Motor Evoked Potentials (MEPs) recorded from lip muscles.

Patti Adank1, Dan Kennedy-Higgins1, Helen Nuttall1,2;1Department of Speech, Hearing and Phonetic Sciences, University College London, Chandler House, 2 Wakefield Street, London, UK, WC1N 1PF, 2Department of Psychology, Lancaster University, Lancaster, UK, LA1 4YF

Recent research indicates that brain areas once believed to be exclusive to speech production are also recruited for speech perception. The role of speech production areas, specifically the lip area in primary motor cortex, has been studied by collecting Motor Evoked Potentials (MEPs) as a measure of relative cortical excitability through the application of single-pulse Transcranial Magnetic Stimulation (TMS) [1-3]. MEP acquisition procedures of speech MEP studies tend to be based on methodological studies conducted mostly in hand muscles, e.g., in the first interosseous dorsalis (FDI), which is innervated by the corticospinal tract [4]. Facial muscles, such as OO, are innervated by the corticobulbar tract and it is unclear to which extent specifics of the corticospinal tract can be extrapolated to the corticobulbar tract. This study aimed to address this issue by characterising effects of laterality, handedness, and coil orientation on the size and shape of MEPS collected from ipsi- and contralateral OO or FDI in left-handed (7) and right-handed (10) participants for eight orientations (0, 45, 90, 135, 180, 225, 270, and 315 degrees). MEPs from OO were evoked consistently in six out of eight orientations (0, 45, 90, 135, 270, and 315 degrees), with large inter-individual variability in optimal orientation. Similar-sized OO MEPs were also recorded from the ipsi- and contralateral sites after left-sided stimulation only. Hand MEPs were recorded consistently with lower inter-individual variability for the 45 degree orientation from the contralateral hand only. We found no effects of handedness or effector on (standardised) MEPs. It is thus feasible to obtain lip MEPs using the ‘standard’ orientation of 45 degrees. Based on these results, it may be recommended that researchers adopt a modified thresholding procedure, where coil orientation, in addition to TMS intensity, is varied in order to detect similar populations of neurons that are oriented in potentially different directions across participants. This may promote a more accurate understanding of activation of the motor system for speech production, during speech perception. References: 1. Watkins, Strafella, and Paus, Seeing and hearing speech excites the motor system involved in speech production. Neuropsychologia, 2003. 41(8):989-94. 2. Nuttall, et al., The effect of speech distortion on the excitability of articulatory motor cortex Neuroimage, 2016. 128:218-226. 3. Murakami, Restle, and Ziemann, Observation-execution matching and action inhibition in human primary motor cortex during viewing of speech-related lip movements or listening to speech. Neuropsychologia, 2011. 49(7):2045-2054. 4. Rossini, et al., Non-invasive electrical and magnetic stimulation of the brain, spinal cord, roots and peripheral nerves: basic principles and procedures for routine clinical and research application. An updated report from an I.F.C.N. Committee. Clinical Neurophysiology, 2015. 126:1071-1107. 5. Adank, P., Nuttall, and Kennedy-Higgins, Transcranial Magnetic Stimulation (TMS) and Motor Evoked Potentials (MEPs) in Speech Perception Research. Language, Cognition & Neuroscience, 2016:1-10.

Topic Area: Methods

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