The Effect of Inter-pulse Interval on TMS Motor Evoked Potentials in Active Muscles.

Objective: The time interval between transcranial magnetic stimulation (TMS) pulses affects evoked muscle responses when the targeted muscle is resting. This necessitates using sufficiently long inter-pulse intervals (IPIs). However, there is some evidence that the IPI has no effect on the responses evoked in active muscles. Thus, we tested whether voluntary contraction could remove the effect of the IPI on TMS motor evoked potentials (MEPs). Methods: In our study, we delivered sets of 30 TMS pulses with three different IPIs (2, 5, and 10 s) to the left primary motor cortex. These measurements were performed with the resting and active right hand first dorsal interosseous muscle in healthy participants (N = 9 and N = 10). MEP amplitudes were recorded through electromyography. Results: We found that the IPI had no significant effect on the MEP amplitudes in the active muscle (p = 0.36), whereas in the resting muscle, the IPI significantly affected the MEP amplitudes (p < 0.001), decreasing the MEP amplitude of the 2 s IPI. Conclusions: These results show that active muscle contraction removes the effect of the IPI on the MEP amplitude. Therefore, using active muscles in TMS motor mapping enables faster delivery of TMS pulses, reducing measurement time in novel TMS motor mapping studies.

Effects of Game Mode in Multiplayer Video Games on Intergenerational Social Interaction: Randomized Field Study.

BACKGROUND: Maintaining social relationships is a basic human need and particularly essential in old age, including when living in a retirement home. Multiplayer video games can promote positive social interactions among players from different generations while playing. Yet, such facilitation of positive social interactions depends on specific game design. To systematically investigate the effects of game design on social interaction between seniors and their coplayers, the game Myosotis FoodPlanet was developed in this study, and the impacts of 3 different game modes on social interaction were compared in a controlled field trial. OBJECTIVE: This study aims to compare the effects of 3 different game modes (competitive, cooperative, and creative) on social interactions (verbal and nonverbal communication) between seniors and their younger coplayers. METHODS: This study was conducted in a Swiss retirement home as a controlled field trial. Participants were residents of the retirement home (N=10; mean age 84.8 years, SD 5.9 years) and played in pairs with their caregivers. Each pair played 3 game modes in random order. This resulted in 30 game sequences of 20 minutes each. A within-subject design was applied with game mode as the within-factor and social interaction as the outcome variable. To assess the quality of social interaction, 30 video-recorded game sequences were analyzed based on an event sampling method. RESULTS: Analysis of variance for repeated measurements revealed significant effects: there was significantly more verbal communication in the creative mode than in the cooperative mode (P=.04) with a strong effect size (Cohen f=0.611). An examination of verbal communication revealed more game-related communication in the creative mode than in the cooperative mode (P=.01) and the competitive mode (P=.09) with marginally significant effects and strong effect sizes (Cohen f=0.841). In addition, significantly more biography-related communication occurred in the creative mode than in the cooperative mode (P=.03), with a strong effect size (r=0.707). Regarding nonverbal communication (eg, laughing together), analysis of variance for repeated measurements showed significant differences among the game modes (P=.02) with a strong effect size (Cohen f=0.758). Results showed that there was significantly more laughing together in the competitive mode (competitive>cooperative>creative). CONCLUSIONS: The results show that game mode can be an important factor for shaping the social interactions of players playing together. Compared with other modes, creative game modes can increase verbal communication. In contrast, competitive modes may stimulate more laughing together. This has important implications for game design and the use of computer games to promote social interaction between seniors and their coplayers in practice.

A probabilistic transcranial magnetic stimulation localization method.

Objective.Transcranial magnetic stimulation (TMS) can be used to safely and noninvasively activate brain tissue. However, the characteristic parameters of the neuronal activation have been largely unclear. In this work, we propose a novel neuronal activation model and develop a method to infer its parameters from measured motor evoked potential signals.Approach.The connection between neuronal activation due to an induced electric field and a measured motor threshold is modeled. The posterior distribution of the model parameters are inferred from measurement data using Bayes' formula. The measurements are the active motor thresholds obtained with multiple stimulating coil locations, and the parameters of the model are the location, preferred direction of activation, and threshold electric field value of the activation site. The posterior distribution is sampled using a Markov chain Monte Carlo method. We quantify the plausibility of the model by calculating the marginal likelihood of the measured thresholds. The method is validated with synthetic data and applied to motor threshold measurements from the first dorsal interosseus muscle in five healthy participants.Main results.The method produces a probability distribution for the activation location, from which a minimal volume where the activation occurs with 95% probability can be derived. For eight or nine stimulating coil locations, the smallest such a volume obtained was approximately 100 mm3. The 95% probability volume intersected the pre-central gyral crown and the anterior wall of the central sulcus, and the preferred direction was perpendicular to the central sulcus, both findings being consistent with the literature. Furthermore, it was not possible to rule out if the activation occurred either in the white or grey matter. In one participant, two distinct activations sites were found while others exhibited a unique site.Significance.The method is both generic and robust, and it lays a foundation for a framework that enables accurate analysis and characterization of TMS activation mechanisms.

Inter-individual variations in electric fields induced in the brain by exposure to uniform magnetic fields at 50 Hz.

The International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines and the Institute of Electrical and Electronics Engineers (IEEE) standard establish safety limits for human exposure to electromagnetic fields. At low frequencies, only a limited number of computational body models or simplified geometrical shapes are used to relate the internal induced electric fields and the external magnetic fields. As a consequence, both standard/guidelines derive the exposure reference levels for the external magnetic field without considering the variability between individuals. Here we provide quantitative data on the variation of the maximum electric field strengths induced in the brain of 118 individuals when exposed to uniform magnetic fields at 50 Hz. We found that individual characteristics, such as age and skull volume, as well as incident magnetic field direction, have a systematic effect on the peak electric field values. Older individuals show higher induced electric field strengths, possibly due to age-related anatomical changes in brain. Peak electric field strengths are found to increase for larger skull volumes, as well as for incident magnetic fields directed along the lateral direction. Moreover, the maximum electric fields provided by the anatomical models used by ICNIRP for deriving exposure limits are considerably higher than those obtained here. On the contrary, the IEEE elliptical exposure model produces a weaker peak electric field strength. Our findings are useful for the revision and harmonization of the current exposure standard and guidelines. The present investigation reduces the dosimetric uncertainty of the induced electric field among different anatomical induction models. The obtained results can be used as a basis for the selection of appropriate reduction factors when deriving exposure reference levels for human protection to low-frequency electromagnetic exposure.

Computational errors of the induced electric field in voxelized and tetrahedral anatomical head models exposed to spatially uniform and localized magnetic fields.

At low and intermediate frequencies, the strength of the induced electric field is used as dosimetric quantity for human protection in the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines. To compute the induced electric field, numerical methods based on anatomically realistic voxel models are commonly used. However, grid-based models introduce staircase approximation errors when curved surfaces are discretized with voxels, particularly in correspondence of boundaries with large differences in electrical conductivity. By contrast, those kind of artefacts are absent in tetrahedral meshes. Here, we investigate the computational errors that affect voxelized and tetrahedral head models when exposed to uniform magnetic fields at 50 Hz, and localized exposure due to transcranial magnetic stimulation. Five subjects were considered, and for each of them four voxel grids and four tetrahedral meshes were reconstructed with different resolutions. The differences in the results were characterized by comparing the induced electric fields computed using those meshes/grids. The results showed modest discrepancies in the overall electric field distributions between the various grids and meshes. However, the peak electric field strengths were erroneous for both tetrahedral and voxel models. Therefore, post-processing techniques are needed to suppress those numerical artefacts. For this purpose, the 99.99th, or lower, percentile of the electric field strength was found to remove the numerical errors. In addition, we found that spatially averaging the electric fields over 2 mm cubical volumes, as described by the ICNIRP, was effective in removing most of the spuriously large electric fields. When spatial averaging was used, relative coarse head models consisting of approximately 1 mm voxels or tetrahedral meshes with 2 mm average side length were sufficient to mitigate the artefacts. Nonetheless, the additional percentile filtering might still be needed to suppress the erroneous values completely.

A multi-scale computational approach based on TMS experiments for the assessment of electro-stimulation thresholds of the brain at intermediate frequencies.

In recent years, human exposure to electromagnetic fields (EMF) at intermediate frequencies (300 Hz-10 MHz) has risen, mainly due to the growth of technologies using these fields. The current safety guidelines/standards defined by international bodies (e.g. ICNIRP and IEEE) established basic restrictions for limiting EMF exposure. These limits at intermediate frequencies are derived from threshold values of the internal electric field that may produce transient effects, such as the stimulation of the nervous system. However, there are some discrepancies between the basic restrictions of those guidelines/standards. The aim of this study is to investigate the excitation thresholds of the nervous system exposed to intermediate-frequency electromagnetic fields, with the purpose of extrapolating the threshold-frequency curves which are compared with existing basic restrictions prescribed by the international guidelines/standards. Our investigation was based on transcranial magnetic stimulation (TMS) experiments, physiological measurements, and individualized MRI-based computer simulations for the determination of brain stimulation thresholds. The combined approach with established biological axon models enabled the extrapolation of the measured thresholds for sinusoidally varying electric fields. The findings reveal that the exposure limits are significantly conservative for the brain, especially at frequencies in the range of 300 Hz-5 kHz.