Research

Navigated transcranial magnetic stimulation of motor function

Navigated transcranial magnetic stimulation (nTMS) was developed for anatomical targeting of the magnetic stimulation to the brain cortex. NTMS is particularly useful in evoking activation in the motor areas of the brain to induce muscle action. The technological development neuronavigation has made nTMS particularly suitable for mapping the cortical motor areas for assessing the functional status of the motor tracts healthy subjects and patients.

Using nTMS, individual neuromuscular excitability and plastic changes in muscle function can be quantitatively mapped and modulated. Motor responses following nTMS focused on the motor cortex can be measured. (Figure 1).


Figure 1. Navigated transcranial magnetic stimulation. Visualization of the stimulated location on the brain cortex is shown online with the induced electric field direction. Following nTMS targeted to the primary motor cortex, a motor evoked potential (MEP) can be measured from the muscle associated with the stimulation site using surface eletromyography (EMG).

Analysis of the EMG responses can be performed semiautomatically using customized tools (Figure 2). Quick analysis of large sets of data requires good computational help to enable semiautomatic analysis of our data sets. Typical responses measured from nTMS are the MEPs and they provide quantitative data on the nTMS-induced activation and inhibitory effect. The response information can be coupled with quantitative information from the navigation procedure.


Figure 2. We have made custom software to aid in a quick and semiautomated analysis of the nTMS data to analyze MEP responses, cortical map size, silent periods and effects related to response modulation.

Application of nTMS enables versatile assessment of the motor tract and intrinsic as well as extrinsic modulatory effects of the nervous system. Such assessment methods have been developed in our lab to enable more accurate, automated and quantitative assessment of the neuromuscular connections. (Figure 3).


Figure 3. Cortical silent period can be detected automatically and reliably from TMS-triggered EMG using our algorithm. Download our algorithm for automatic detection of cortical silent periods here.

Application of nTMS in combination with electroencephalography (EEG) provides a unique opportunite to map neural connection and the motor system network function upon activation. (Figure 4).


Figure 4. Motor cortical activation following nTMS targeting the motor cortex produces responses through the neuronal connections. This way the motor network can be mapped, and excitation of the motor network quantified combining nTMS and EEG.

For TMS threshold estimation using adaptive threshold hunting, we have created a mobile application, which makes it possible to quickly estimate motor thresholds, to log them and to share them. The algorithm has been validated against computational subjects. (Figure 5).


Figure 5. Our adaptive threshold hunting tool can be used to estimate motor thresholds and has been validated against computational subjects with known true thresholds ATH-tool can be downloaded for Android-devices from Google Play.


References:

  1. Julkunen P (2014), Methods for estimating cortical motor representation size and location in navigated transcranial magnetic stimulation. Journal of Neuroscience Methods, 232, 125-133.

  2. Julkunen P, Säisänen L, Danner N, Niskanen E, Hukkanen T, Mervaala E, Könönen M (2009), Comparison of navigated and non-navigated transcranial magnetic stimulation for motor cortex mapping, motor threshold and motor evoked potentials. NeuroImage, 44, 790-795.

  3. Julkunen P, Kallioniemi E, Könönen M, Säisänen L (2013), Feasibility of automated analysis and inter-examiner variability of cortical silent period induced by transcranial magnetic stimulation. Journal of Neuroscience Methods, 217, 75-81.

  4. Kallioniemi E, Könönen M, Säisänen L, Gröhn H, Julkunen P (2015), Functional neuronal anisotropy assessed with neuronavigated transcranial magnetic stimulation. Journal of Neuroscience Methods, 256, 82-90.

  5. Julkunen P (2019), Mobile application for adaptive threshold hunting in transcranial magnetic stimulation. IEEE in Transactions on Neural Systems & Rehabilitation Engineering, in press.

PhD theses of our present and former researchers:

Minna Pitkänen:
Characterization of motor cortical function with navigated transcranial magnetic stimulation

Elisa Kallioniemi:
Assessment of motor cortical excitation-inhibition balance and microstructure: studies combining navigated transcranial magnetic stimulation and magnetic resonance imaging

Mervi Könönen:
Functional human neuroimaging using clinical tools: studies of cortical motor areas

Nils Danner:
Characterization of cortical excitability in Unverricht-Lundborg disease (EPM1)

Laura Säisänen:
Human motor cortex function characterized by navigated transcranial magnetic stimulation

Florinda Ferreri:
TMS and TMS-EEG studies on the excitability, connectivity and plasticity of the human motor cortex

Most recent publications of our researchers:

Contact: +358 29 4451111 (Switch), email: bbc-l [at] lists.uef.fi