ИСТИНА

Motivation and aim of the study. Mapping cortical motor representations of muscles with navigated transcranial magnetic stimulation (nTMS) is a valuable noninvasive method of evaluating the function of the motor system in healthy individuals [1] and patient populations, including patients with mirror movements, cortical dysgenesis, neurodegenerative diseases, spinal cord injuries and traumatic limb amputations [2]. Whereas the application of nTMS mapping to brain surgery has become an established technique [3,4], its use for the assessment of neuroplasticity in healthy and diseased individuals is still an area with many open questions. Interpretation of motor maps obtained with nTMS is complicated by the significant variability of motor evoked potentials (MEPs), which limits the accuracy of determining motor representation parameters and contributes to their trial-to-trial variability. Moreover, different groups employ a variety of mapping and data processing protocols [5], which complicates the comparison of nTMS map parameters between studies in the absence of standard generally accepted algorithms. Thus, we aimed to find an optimal mapping and processing protocol based on the evaluation of the accuracy of motor representation parameters that can be achieved under different protocols. Methods. We performed nTMS mapping using the NBS eXimia Nexstim (Finland) stimulator under an extended protocol, providing data from which mappings with smaller numbers of stimuli can be simulated. Mapping of the APB muscle was performed in 8 heathy individuals using a stimulation point grid consisting of 7x7 square cells with a side of 7.7 mm, with each cortical location being stimulated 10 times in a pseudorandom order. Three mapping sessions were performed on consecutive days, with a fixed TMS intensity equal to 110% of the resting motor threshold determined in the first session. The study design was approved by the local ethical committee, and the volunteers signed a written informed consent form. Results and discussion. The results allowed a comparison of inter-session variability of motor representation parameters for different protocols. Protocols with 5 and 10 stimuli per grid point produced similar variability, which was significantly smaller than that with 3 or 1 stimuli per point. This suggests that a protocol with 5 stimuli per point can provide a good compromise between accuracy and mapping duration. The comparison of data processing algorithms showed better reproducibility for the inclusion of a point in the map if it produced at least one suprathreshold MEP, rather than based on the mean MEP amplitude (Fig. 1). Conclusion. The results can be used for choosing an optimal nTMS mapping protocol for a given study with its particular accuracy requirements and mapping time constraints. Acknowledgement. This study was supported by the Russian Science Foundation under grant 17-75-10062. [1] A. Pascual-Leone, J. Grafman, M. Hallett, Modulation of cortical motor output maps during development of implicit and explicit knowledge, Science, 1994, Vol. 263, Issue 5151, pp. 1287-1289, DOI: 10.1126/science.8122113. [2] A. Rotenberg, J.C. Horvath, A. Pascual-Leone (Eds.), Transcranial Magnetic Stimulation, Humana Press, 2014, doi: 10.1007/978-1-4939-0879-0. [3] Jean-Pascal Lefaucheur, Thomas Picht, The value of preoperative functional cortical mapping using navigated TMS, Clinical Neurophysiology, V. 46, Issue 2, 2016, pp. 125-133. [4] Sandro M. Krieg et al., Protocol for motor and language mapping by navigated TMS in patients and healthy volunteers; workshop report, Acta Neurochirurgica, 2017, V. 159, Issue 7, pp. 1187–1195. [5] Lüdemann-Podubecká J. Nowak D.A. Mapping cortical hand motor representation using TMS: A method to assess brain plasticity and a surrogate marker for recovery of function after stroke? Neurosci. Biobehav. Rev. 2016; 69: 239.