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Abstract
Navigated transcranial magnetic stimulation (nTMS) mapping of cortical muscle representations allows noninvasive assessment of the state of a healthy or diseased motor system, and monitoring changes over time. These applications are hampered by the heterogeneity of existing mapping algorithms and the lack of detailed information about their accuracy. We aimed to find an optimal motor evoked potential (MEP) sampling scheme in the grid-based mapping algorithm in terms of the accuracy of muscle representation parameters. The abductor pollicis brevis (APB) muscles of eight healthy subjects were mapped three times on consecutive days using a seven-by-seven grid with ten stimuli per cell. The effect of the MEP variability on the parameter accuracy was assessed using bootstrapping. The accuracy of representation parameters increased with the number of stimuli without saturation up to at least ten stimuli per cell. The detailed sampling showed that the between-session representation area changes in the absence of interventions were significantly larger than the within-session fluctuations and thus could not be explained solely by the trial-to-trial variability of MEPs. The results demonstrate that the number of stimuli has no universally optimal value and must be chosen by balancing the accuracy requirements with the mapping time constraints in a given problem.
Highlights
Mapping cortical motor representations of muscles using navigated transcranial magnetic stimulation is a valuable noninvasive method providing information about the motor system that is useful for research and clinical purposes [1,2,3]
We have studied the dependence of the accuracy of muscle representation parameters on the aspects of the grid-based transcranial magnetic stimulation (TMS) mapping experiment and data processing
The grid size impacted the completeness of the muscle representation coverage, and a square grid with a side of 53 mm centered at the hotspot covered on average 97.9% of the representation area for the abductor pollicis brevis (APB), extensor digitorum communis (EDC) and flexor digitorum superficialis (FDS) muscles
Summary
Mapping cortical motor representations of muscles using navigated transcranial magnetic stimulation (nTMS) is a valuable noninvasive method providing information about the motor system that is useful for research and clinical purposes [1,2,3]. A growing body of literature is concerned with the use of nTMS mapping for assessing the state of the motor system and its plastic changes during learning of new skills [6,7,8,9], in neurological diseases, such as stroke [10], dystonia [11], spinal cord injury [12,13], amyotrophic lateral sclerosis [14], as well as during the course of treatment [15]. The high variability of motor evoked potentials (MEPs), on which the TMS-maps are based, makes the accurate estimation of representation parameters challenging [16,17,18].
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