Browsing by Author "Reijonen, Jusa"

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  • Automatic estimation of infant encephalographic data quality
    (2017-05-08) Reijonen, Jusa
     Perustieteiden korkeakoulu | Master's thesis
    The electroencephalogram (EEG) is a commonly used signal for measuring infants' brain activity. The developmental processes of the brain can be studied with event-related potentials (ERPs), which are obtained by epoching the EEG signal into trials time-locked to a specific event and calculating the average of the trial epochs. The quality of EEG and ERPs is diminished by artifacts, which are signals originating from non-neural sources. Identifying and eliminating the artifacts is essential for revealing the underlying brain activity. This has traditionally been done by visual inspection, which is slow and poorly repeatable due to its subjective nature. More efficient and reliable artifact processing methods are thus needed. This thesis proposes a two-step method for artifact processing. First, the contaminated channels are detected by statistical thresholding based on several parameters extracted from the epoched data. Second, the contaminated epochs are identified by setting thresholds for features that have abnormal values in the presence of the most common types of artifacts. The performance of this approach is evaluated by comparing the detection results to a visual detection performed by EEG experts. Channel detection was found effective for subjects with good and moderate general data qualities, becoming weaker as the general data quality decreased. The results also showed that epoch detection with the selected thresholds was very effective. Further research is needed in order to verify the results as well as to evaluate the performance with different parameter and feature sets which can potentially improve the suggested method.
  • Biomimetic Materials with Electrotuneable Mechanical Properties
    (2015-01-19) Reijonen, Jusa
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Minimum-norm Estimation of TMS-activated Motor Cortical Sites in Realistic Head and Brain Geometry
    (2022) Reijonen, Jusa; Saisanen, Laura; Pitkanen, Minna; Julkunen, Meri; Ilmoniemi, Risto J.; Nieminen, Petteri; Julkunen, Petro
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Navigated transcranial magnetic stimulation (nTMS) is a widely used tool for motor cortex mapping. However, the full details of the activated cortical area during the mapping remain unknown due to the spread of the stimulating electric field (E-field). Computational tools, which combine the E-field with physiological responses, have potential for revealing the activated source area. We applied the minimum-norm estimate (MNE) method in a realistic head geometry to estimate the activated cortical area in nTMS motor mappings of the leg and hand muscles. We calculated the MNE also in a spherical head geometry to assess the effect of the head model on the MNE maps. Finally, we determined optimized coil placements based on the MNE map maxima and compared these placements with the initial hotspot placement. The MNE maps generally agreed well with the original motor maps: in the realistic head geometry, the distance from the MNE map maximum to the motor map center of gravity (CoG) was 8.8 ± 4.6 mm in the leg motor area and 6.6 ± 2.5 mm in the hand motor area. The head model did not have a significant effect on these distances; however, it had a significant effect on the distance between the MNE CoG and the motor map ( {p} < 0.05 ). The optimized coil locations were < 1 cm from the initial hotspot in 7/10 subjects. Further research is required to determine the level of anatomical detail and the optimal mapping parameters required for robust and accurate localization.
  • Spatial extent of cortical motor hotspot in navigated transcranial magnetic stimulation
    (2020-12-01) Reijonen, Jusa; Pitkänen, Minna; Kallioniemi, Elisa; Mohammadi, Ali; Ilmoniemi, Risto J.; Julkunen, Petro
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Background: Motor mapping with navigated transcranial magnetic stimulation (nTMS) requires defining a “hotspot”, a stimulation site consistently producing the highest-amplitude motor-evoked potentials (MEPs). The exact location of the hotspot is difficult to determine, and the spatial extent of high-amplitude MEPs usually remains undefined due to MEP variability and the spread of the TMS-induced electric field (E-field). Therefore, here we aim to define the hotspot as a sub-region of a motor map. New method: We analyzed MEP amplitude distributions in motor mappings of 30 healthy subjects in two orthogonal directions on the motor cortex. Based on the widths of these distributions, the hotspot extent was estimated as an elliptic area. In addition, E-field distributions induced by motor map edge stimulations were simulated for ten subjects, and the E-field attenuation was analyzed to obtain another estimate for hotspot extent. Results: The median MEP-based hotspot area was 13 mm2 (95% confidence interval (CI) = [10, 18] mm2). The mean E-field-based hotspot area was 26 mm2 (95% CI = [13, 38] mm2). Comparison with existing methods: In contrast to the conventional hotspot, the new definition considers its spatial extent, indicating the most easily excited area where subsequent nTMS stimuli should be targeted for maximal response. The E-field-based hotspot provides an estimate for the extent of cortical structures where the E-field is close to its maximum. Conclusions: The nTMS hotspot should be considered as an area rather than a single qualitatively defined spot due to MEP variability and E-field spread.