Akira Hyodo

Nerve excitation model for localized magnetic stimulation of finite neuronal structures

We focus on a membrane excitation model of finite and bent nerves elicited by magnetically induced electric fields. We simulated nerve excitation by changing the length of an axon and the distance between the coil and the nerve. The computer simulation suggests that the termination points of nerves or the bending part of an axon are low threshold stimulation sites when magnetically stimulated with a figure-eight coil. We also suggest that a short axon is more easily excited than an infinitely long axon of the same thickness. These results give useful information in understanding neuronal responses to magnetic stimulation of both central and peripheral nervous systems.

Effects of Coil Parameters on the Stimulated Area by Transcranial Magnetic Stimulation

In this study, we investigate the relationships between the eddy current density and the coil configuration of transcranial magnetic stimulation (TMS). The aim of this study is to determine the coil parameters such as the radius of the coil and the bending angle of the coil to stimulate specified area such as the dorsolateral prefrontal cortex (DLPFC). We used a realistic 3-D human head model with inhomogeneous conductivity to obtain accurate eddy current distributions. In the TMS model, eddy current distributions were obtained for figure-eight coils with radius of 30, 40 and 50 mm, and a bending angle of the coil changed from 0 to 30 degrees. Computer simulations show that the maximum value of the eddy current increases linearly with the increase of the radius and the bending-angle of the coil. The maximum eddy current density was 87.6 A/m2 under the case where the bending-angle was 30 degrees and the radius was 50 mm. The stimulated area increased with the increase of the radius and bending-angle of the coil. It is possible to determine coil parameters to stimulate target area appropriately.

Measurements of evoked electroencephalograph by transcranial magnetic stimulation applied to motor cortex and posterior parietal cortex

To investigate the functional connectivity, the evoked potentials by stimulating at the motor cortex, the posterior parietal cortex, and the cerebellum by transcranial magnetic stimulation (TMS) were measured. It is difficult to measure the evoked electroencephalograph (EEG) by the magnetic stimulation because of the large artifact induced by the magnetic pulse. We used an EEG measurement system with sample-and-hold circuit and an independent component analysis to eliminate the electromagnetic interaction emitted from TMS. It was possible to measure EEG signals from all electrodes over the head within 10 ms after applying the TMS. When the motor area was stimulated by TMS, the spread of evoked electrical activity to the contralateral hemisphere was observed at 20 ms after stimulation. However, when the posterior parietal cortex was stimulated, the evoked electrical activity to the contralateral hemisphere was not observed. When the cerebellum was stimulated, the cortical activity propagated from the stimu...

The numeric calculation of eddy current distributions in transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is a method to stimulate neurons in the brain. It is necessary to obtain eddy current distributions and determine parameters such as position, radius and bend-angle of the coil to stimulate target area exactly. In this study, we performed FEM-based numerical simulations of eddy current induced by TMS using three-dimentional human head model with inhomogeneous conductivity. We used double-cone coil and changed the coil radius and bend-angle of coil. The result of computer simulation showed that as coil radius increases, the eddy current became stronger everywhere. And coil with bend-angle of 22.5 degrees induced stronger eddy current than the coil with bendangle of 0 degrees. Meanwhile, when the bend-angle was 45 degrees, eddy current became weaker than these two cases. This simulation allowed us to determine appropriate parameter easier.

Long latency motor evoked responses to magnetic stimulation of the spinal roots in the human neck

Long latency responses (LLR) to magnetic stimulation of the spinal roots in the human neck were obtained using single pulsed magnetic nerve stimulation. The effects of repetitive magnetic stimulation of median nerves on the LLR were also investigated. When the repetitive magnetic stimulation was added to the median nerve, the latency of motor evoked potentials (MEPs) of the LLR were changed in 50-100 ms. >

Influence of coil current configuration in magnetic stimulation of a nerve fiber in inhomogeneous and anisotropic conducting media

In this study, we used a computer simulation to investigate the effects of the coil current waveform and direction on the excitation processes of the nerve axon in inhomogeneous and anisotropic conducting media in magnetic stimulation. We assumed that the nerve axon was located in the media with 2 regions having different conductivities or electrical anisotropy that simulate different tissue types. The distribution of induced electric fields was calculated with the finite element method (FEM). The nerve fiber was modeled after equivalent electrical circuits having active nodes of Ranvier. The direction of the coil current at the intersection of a figure-eight coil was assumed to flow perpendicular to the nerve axon. We observed the excitation threshold when the coil current waveform and direction are changed with varying the electrical properties such as tissue electrical conductivity and anisotropy. The simulation results show that the threshold decreases with the increase of conductivity ratio between 2 regions and it also depends on the coil current waveform and direction. Biphasic coil current has lower threshold than monophasic one when the current direction is the same in both waveforms. The results also suggest that the tissue anisotropy strongly affects the excitation threshold. The threshold increases with the increase of tissue anisotropic ratio of longitudinal direction to the transverse one respect to the nerve axon. The results in this study give useful information to explain the experimental results of the magnetic stimulation of human peripheral nervous systems and the theoretical model is applicable to understand the characteristics in magnetic stimulation of both peripheral and central nervous systems.

Disturbance of visual search by stimulating to posterior parietal cortex in the brain using transcranial magnetic stimulation

In this study, we applied a transcranial magnetic stimulation (TMS) to investigate the temporal aspect for the functional processing of visual attention. Although it has been known that right posterior parietal cortex (PPC) in the brain has a role in certain visual search tasks, there is little knowledge about the temporal aspect of this area. Three visual search tasks that have different difficulties of task execution individually were carried out. These three visual search tasks are the “easy feature task,” the “hard feature task,” and the “conjunction task.” To investigate the temporal aspect of the PPC involved in the visual search, we applied various stimulus onset asynchronies (SOAs) and measured the reaction time of the visual search. The magnetic stimulation was applied on the right PPC or the left PPC by the figure-eight coil. The results show that the reaction times of the hard feature task are longer than those of the easy feature task. When SOA=150 ms, compared with no-TMS condition, there was...

Inhibition of the neural activity by paired stimulation in somatosensory evoked potentials and high frequency oscillations

This study evaluated the inhibition of neural re- action induced by paired stimulation using somatosensory- evoked potentials (SEPs) and high frequency oscillations (HFOs) in healthy subjects. The paired stimulations were applied to the subject at the specific interval. We used three interstimulus intervals (ISIs: 60, 100 and 300 ms) to analyze SEPs components (P15, N20 and P25) and HFOs caused paired electrical stimuli which are delivered to right median nerve. SEPs has high-frequency component (about 600Hz) is called HFOs in the intrinsic latency near N20. Shorter ISIs (less than 100ms) decreased the P15-N20 and N20-P25 amplitude signifi- cantly. Furthermore, also the maximum amplitude and the intensity of HFOs (both early and late components) decreased significantly as the ISIs shorten. Hashimoto et al and Curio el al suggested early HFOs reflect population spikes in thalamo- cortical axons, and later may reflect in S1 within grey matter. Therefore, we hypothesis that population spike in the time series variation of neural population spikes decreased by paired stimulation to peripheral nerve. It may be non-invasive technique to investigate neural activity in human cortex.

Effects of stimulus parameters and tissue inhomogeneity on nerve excitation processes in magnetic stimulation of the brain: A simulation study

In this study, we used a computer simulation to investigate the nerve excitation processes of the nerve axon in an inhomogeneous volume conductor in magnetic stimulation. We assumed that the nerve axon was located in an inhomogeneous conducting medium with two regions having different conductivities that simulate different tissue types. The distribution of induced electric fields was calculated with the finite element method. The nerve fiber was modeled after equivalent electrical circuits having active nodes of Ranvier. We observed the excitation threshold when the coil current waveforms and direction are changed with varying the electrical properties of the tissue. The simulation results show that the threshold is lower when biphasic waveforms are used and that the optimal current direction depends on tissue conductivity. The results also suggest that the nerve is excited even when the coil current flow is perpendicular to the axon in inhomogeneous conducting media. The results in this study give useful...

Detection of a diabetic sural nerve from the magnetic field after electric stimulation

In this study, we proposed a new diagnostic technique for diabetic neuropathy using biomagnetic measurement. Peripheral neuropathy is one of the most common complications of diabetes. To examine the injury, the skin potential around the nerve is often measured after electric stimulation. However, measuring the magnetic field may reveal precise condition of the injury. To evaluate the effect of measuring the magnetic field, a simulation study was performed. A diabetic sural nerve was simulated as a bundle of myelinated nerve fibers. Each fiber was modeled as an electric cable of Ranvier’s nodes. Anatomical data were used to determine the number of nerve fibers and distribution of nerve fiber diameters. The electric potential and the magnetic field on the skin after electric stimulation were computed to the boundary element method. Biphasic time courses were obtained as the electric potential and the magnetic flux density at measurement points. In diabetic nerves, the longer interpeak latency of the electri...