Improved transcranial magnetic stimulation protocols to locate brain activations

Noora Matilainen

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

Transcranial magnetic stimulation (TMS) is a non-invasive neurostimulation technique used in clinical treatment and research. It is a technique that provides essential information about brain activity and function, as well as effective treatment for certain neurological disorders. The use of TMS is however still limited by several fundamental uncertainties. For example, it remains uncertain which forms of stimulation are required to elicit specific responses. In addition, the TMS procedure itself can be time consuming and is prone to errors. This summary offers new knowledge of how TMS parameters affect neurostimulation and what they stimulate. Publication I examines the effect of the TMS inter-pulse interval (IPI) on motor evoked potential (MEP) amplitude in active and resting muscles. Previous research has shown that MEP amplitudes are significantly influenced by IPI in resting muscles, with shorter intervals generally leading to decreased amplitudes. This study, however, reveals that active muscle contraction during TMS eliminates the modulating effect of IPI, allowing the use of shorter IPIs which speeds up TMS procedures. Publication II investigates the accuracy of a three-point navigated TMS, still a commonly used approach for neuronavigation. The findings reveal that errors in landmark pointing can significantly impact the accuracy of coil positioning and the induced electric field, highlighting the importance of minimizing such errors in TMS research. Publication III explores the use of computational dosimetry to predict the optimal coil positioning and to estimate motor threshold values in TMS. While the study shows promising results in predicting optimal coil locations, the accuracy of predicting hotspots is slightly less than the hypothetical target of 1 cm. Nevertheless, the method is possibly useful in clinical practise, offering potential improvements in the speed and reliability of TMS hotspot-finding procedures. Publication IV contributes to TMS localization and investigates the differences between posteroanterior (PA) and anteroposterior (AP) coil current directions. The study suggests that PA-TMS primarily activates the precentral gyrus, while AP-TMS is more likely to activate the postcentral gyrus, with both directions showing a higher likelihood of white matter activation. Together, these four studies contribute to a deeper understanding of TMS mechanisms, the optimization of stimulation protocols, and improved accuracy in TMS procedures, with implications for both research and clinical applications.
Translated title of the contributionParannellut transkraniaalisen magneettistimulaation toimintatavat aivotoiminnan paikantamiseen
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Laakso, Ilkka, Supervising Professor
  • Laakso, Ilkka, Thesis Advisor
Publisher
Print ISBNs978-952-64-2088-2
Electronic ISBNs978-952-64-2089-9
Publication statusPublished - 2024
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • transcranial magnetic stimulation
  • computational modelling
  • primary motor cortex
  • motor-evoked potential
  • motor mapping

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