Modulation of motor cortex excitability by peripheral magnetic stimulation of different stimulus sites and frequencies.

Peripheral stimulation is known to influence the state of cortical excitability. The purpose of this study is to investigate whether peripheral magnetic stimulation has similar effects on cortical excitability to transcranial magnetic stimulation (TMS). A magnetic stimulator with a flat figure-of-eight coil was used for both TMS, and peripheral magnetic stimulation applied to the bilateral forearms. TMS was performed on the left primary motor cortex to evaluate influence of the peripheral magnetic stimulation, and motor evoked potential (MEP) was measured from the right first dorsal interosseous. Peripheral magnetic stimulation was performed at a stimulus frequency of 1 Hz or 10 Hz, to the stimulus sites on the right and left supination of the forearm. The effects of peripheral magnetic stimulation were evaluated by comparing the mean MEP amplitude elicited by TMS before and after peripheral magnetic stimulation. We found that cortical excitability varied according to the stimulation site and frequency of the peripheral magnetic stimulation. The inhibition of cortical excitability was observed following 1 Hz peripheral magnetic stimulation over the right forearm (p<;0.001). In contrast, increased cortical excitability was observed using 1 Hz peripheral magnetic stimulation over the left forearm and 10 Hz stimulation over either the right or left forearms. We suggest that peripheral magnetic stimulation has a similar effect to TMS, and can induce both facilitation and inhibition of cortical excitability.

The impact of rTMS over the dorsolateral prefrontal cortex on cognitive processing.

The purpose of the present study was to use event-related potentials (ERP) to clarify the effect of magnetic stimulation on cognitive processing. A figure eight-shaped flat repetitive transcranial magnetic stimulation (rTMS) coil was used to stimulate either the region over the left or the right dorsolateral prefrontal cortex, which is considered to be the origin of the P300 component. Stimulus frequencies were 1.00, 0.75 and 0.50 Hz rTMS. The strength of the magnetic stimulation was set at 80% of the motor threshold for each participant. The auditory oddball task was used to elicit P300s before and shortly after rTMS, and comprised a sequence of sounds containing standard (1 kHz pure tone, 80% of trials) and deviant (2 kHz pure tone, 20% of trials) stimuli. We found that a 1.00 Hz rTMS pulse train over the left dorsolateral prefrontal cortex increased P300 latencies by 8.50 ms at Fz, 12.85 ms at Cz, and 11.25 ms at Pz. In contrast, neither 0.75 and 0.50 Hz rTMS pulse trains over the left dorsolateral prefrontal cortex nor 1.00, 0.75 and 0.50 Hz rTMS pulse trains over the right dorsolateral prefrontal cortex altered P300 latencies. These results indicate that rTMS frequency affects cognitive processing. Thus, we suggest that the effects of rTMS vary according to the activity of excitatory and inhibitory neurons in the cerebral cortex.

Predicting rTMS effect for deciding stimulation parameters.

Repetitive transcranial magnetic stimulation (rTMS) is used in the medical field to modulate cortical excitability. However, when applied in this setting, rTMS stimulation parameters are not usually decided objectively. The aim of this study is to make a model that predicts the rTMS effect, allowing stimulation parameters (intensity and pulse number) to be easily determined before use. First, we investigated the relationship between stimulation condition and rTMS outcome. rTMS delivered at 1 Hz was applied with stimulation intensities of 85%, 100%, or 115% resting motor threshold (RMT) over the primary motor cortex in the left hemisphere. Motor-evoked potentials (MEPs) were measured before rTMS and after every 200 rTMS pulses. Eighteen hundred pulses were applied for each stimulation condition. Results showed that more pulses and stronger intensities lead to a larger decrease in MEP amplitude. An initial prediction model was then made by applying multiple regression analysis over the experimental data. We then adjusted the model depending on the size of the initial MEP amplitude before rTMS, and confirmed the improvement.

Time-dependent effects of low-frequency repetitive transcranial magnetic stimulation of the supramarginal gyrus.

In this paper, we report our studies of the effects of stimulating the bilateral supramarginal gyrus (SMG) with low-frequency transcranial magnetic stimulation (rTMS) or short-term rTMS on brain excitability in humans. We analyzed the effects of various durations of stimulation on P300 latencies of the event-related potential (ERP). Magnetic pulses were delivered using a figure-eight flat coil. The intensity of rTMS was set to 80 % of the subject's motor threshold. In each round of rTMS, 100 magnetic pulses were applied over the scalp at frequencies of 1.00, 0.75, and 0.50 Hz. ERPs were measured prior to magnetic stimulation as a control. The effects of magnetic stimulation were then determined by measuring its effects on P300 latencies elicited by an odd-ball task. These latencies were measured before and 0, 5, 10, and 15 min after the magnetic stimulation. 1.00 Hz low-frequency rTMS of the left SMG decreased P300 latencies for approximately 10 min. In contrast, 0.50 Hz rTMS of the left SMG resulted in delayed P300 latencies for approximately 15 min. We furthermore found that 0.75 Hz rTMS of the left SMG and 1.00, 0.75 and 0.5 Hz rTMS of the right SMG did not affect P300 latencies. These results suggest that the duration of the effects of rTMS depend on the frequency of stimulation.

Aging curve of neuromotor function by pronation and supination of forearms using three-dimensional wireless acceleration and angular velocity sensors.

We have developed an evaluation system for pronation and supination of forearms. The motion of pronation and supination of the forearm is used as a diagnosis method of developmental disability, etc. However, this diagnosis method has a demerit in which diagnosis results between doctors are not consistent. It is hoped that a more quantitative and simple evaluation method is established. Moreover it is hoped a diagnostic criteria obtained from healthy subjects can be established to diagnose developmental disorder patients. We developed a simple and portable evaluation system for pronation and supination of forearms. Three-dimensional wireless acceleration and angular velocity sensors are used for this system. In this study, pronation and supination of forearms of 570 subjects (subjects aged 6-12, 21-100) were examined. We could obtain aging curves in the neuromotor function of pronation and supination. These aging curves obtained by our developed system, has the potential to become diagnostic criteria for a developmental disability, etc.

Relationship between pulse number of rTMS and inter reversal time of perceptual reversal.

The aim of this study is to investigate the stimulus parameter which affects the repetitive Transcranial Magnetic Stimulation (rTMS) effect. It is said that the condition under 1Hz rTMS induces the inhibition effect. On the other hand, the condition over 1 Hz rTMS induces the facilitation effect. However the number of pulses of rTMS is also important factor. In this study, we focused on the number of pulses. We used the cognitive task of perceptual reversal and compared the rTMS effects of different condition under 1 Hz which is the inhibition condition. It has been known that the right superior parietal lobule (SPL) has a role in perceptual reversal. We applied rTMS over the SPL and measured the inter-reversal time (IRT) of perceptual reversal. The results showed that when 0.25 Hz 60 pulses, 0.5 Hz 60 pulses and 1 Hz 60 pulses of rTMS was applied over the right SPL, the IRT was significantly smaller. On the other hand, when 1 Hz 240 pulses of rTMS was applied over the right SPL, the IRT was significantly longer. When 0.25 Hz 12 0 pulses, 0.5 Hz 120 pulses and 1Hz 120 pulses of rTMS was applied over the right SPL, there were no significant differences. Furthermore, to investigate the rTMS effects, when rTMS are applied over the motor area, we measured the motor evoked potential (MEP). The more pulses of rTMS was applied, the smaller the amplitude of MEP became. From these results, it was found that the IRT of perceptual reversal and the amplitude of MEP primarily affected by the number of pulses of rTMS.

Time change of perceptual reversal of ambiguous figures by rTMS.

The aim of this study was to investigate the effect of stimulus frequency and number of pulses during rTMS (repetitive transcranial magnetic stimulation) on the phenomenon of perceptual reversal. Particularly, we focused on the temporal dynamics of perceptual reversal in the right SPL (superior parietal lobule), using the spinning wheel illusion. We measured the IRT (inter-reversal time) of perceptual reversal. To investigate whether stimulus frequency or the number of pulses is critical for the rTMS effect, we applied the following schedules over the right SPL and the right PTL (posterior temporal lobe): 0.25Hz 60 pulses, 0.25Hz 120pulses, 0.5Hz 120 pulses, and 1Hz 120 pulses biphasic rTMS at 90% of the resting motor threshold. As a control, we included a No-TMS condition. The results showed that rTMS with 0.25Hz 60 pulses over the right SPL caused shorter IRT. There were no significant differences between IRTs for rTMS with 0.25Hz 120 pulses, 0.5Hz 120 pulses or 1Hz 120 pulses over the right SPL. Comparing these results with those of a previous study, we found that an rTMS condition with 60 pulses causes shorter IRT; 240 pulses causes longer IRT; and 120 pulses does not change IRT. Therefore, when applying rTMS over the right SPL, the IRT of perceptual reversal is primarily affected by the number of pulses.

Effect of transcranial magnetic stimulation on P300 of event-related potential.

When the odd stimulation is presented, the positive component of electroencephalograph is induced at around 300 ms after the odd stimulation. This positive component is called P300. Many studies suggest that P300 may result from the summation of activity from multiple generators located in widespread cortical and subcortical areas. However, there is still no conclusive indication of the sources of P300. In this paper, we focus on the left supramaginal gyrus as one of the sources of P300. We investigated the temporal aspect of this area using TMS (transcranial magnetic stimulation). We investigated the relationship between the latency of the P300 and an effect of TMS when the left supramarginal gyrus was stimulated by TMS. In our previous study, we reported a method of removing stimulus artifact during TMS with Sample-and-Hold circuit and electroencephalogram (EEG) activity evoked by TMS could be measured successfully. In addition to this method, independent component analysis (ICA) was also applied to recorded EEG data in order to remove the stimulus artifact by off-line analysis. By using these methods, short latency (< 15 ms) EEG responses to TMS could be obtained. We stimulated the left supramarginal gyrus using a figure-eight coil during auditory oddball task. The TMS at 150 ms and 200 ms after the oddball sounds were presented. When the TMS was applied at 200 ms after the oddball stimulation, the peak response of P300 was delayed around 50 ms. Difference of the peak latency between the control measurement and the case of TMS applying at 150 ms was not significant. However, the differences of the peak latency of the control measurement and the peak latency of the measurement in the cases of TMS applying at 200 ms and 250 ms was significant (p<0.05). We considered that this delay was due to inhibiting to recognize the target stimulation.

Differences in evoked EEG by transcranial magnetic stimulation at various stimulus points on the head.

The combination of transcranial magnetic stimulation (TMS) and electroencephalogram (EEG) is an effective tool for investigating the cortical reactivity and the functional connectivity in the brain. In our previous study, we reported a method of removing stimulus artifact during TMS with Sample-and-Hold circuit and EEG activity evoked by TMS could be measured successfully. In addition to this method, independent component analysis (ICA) was also applied to recorded EEG data in order to remove the stimulus artifact from for off-line analysis. By using these methods, short latency ( 15 ms) EEG responses to TMS could be obtained. In this paper, we focused on the propagation of EEG activity elicited by TMS. We observed both the EEG topography and the distribution of the current density over the whole head by changing the stimulus site. When motor cortex was stimulated, the propagation of EEG activity to contralateral hemisphere could be clearly observed. However, when posterior parietal cortex was stimulated, no or less propagation of EEG responses could be recognized. These results suggest that the responses evoked by TMS over motor cortex propagate to contralateral hemisphere along the axon through the corpus callosum.

A study on some optical illusions based upon the theory of inducing field.

The study of optical illusion is an important method to elucidate the mechanism of visual perception. However, many details about the cause of optical illusions are still unclear. In this research, based on the characteristic of the physiological structure of the retina, we proposed an on-center receptive field model of the retina. Using this model, we simulated the distributions of the inducing field of some visual stimulus. Comparing to the past studies' results, the validity of the proposed model was proofed. Furthermore, we simulated the distributions of the inducing field of some typical illusions. The simulation results can explain these illusion phenomenon rationally. Therefore, it suggested that some of illusions are probably engendered by the distributions of the inducing field in the retina which generated by the illusions stimuli. The practicality of the proposed model was also verified.

Brain lateralization for mismatch response to across- and within-category change of vowels.

Differences in hemispheric predominance between across- and within-category change perception of vowels were assessed using a whole-head magnetoencephalography. The magnetic mismatch responses (MMNm) to pure-tone and vowel within-category changes were significantly predominant in the right hemisphere; on the other hand, vowel across-category MMNm did not differ in power between hemispheres. The results suggest that both hemispheres are symmetrically activated in the preattentive across-category change perception of vowels, while the within-category change of a vowel is analyzed as the change in physical features of the stimuli, thus predominantly activating the right hemisphere. Thus, the relative contribution of the left auditory cortex in the preattentive speech processing may occur only at the level of perception of the vowel across-category change.

Human middle latency auditory evoked magnetic fields.

The magnetic equivalents of SN10, Po, Na, Pa, Nb and Pb (SN10m, Pom, Nam, Pam, Nbm, and Pbm) in short and middle latency auditory evoked potentials were measured with a 7-channel DC superconducting quantum interference device (SQUID). The sources of Pom, Nam, Pam, Nbm and Pbm responses were estimated to be located in the auditory cortex, while the source of SN10m was considered to be in a deeper part of the brain. In addition, the source of Pam was estimated to be in the vicinity of the moving N100m source. The source of Pbm was considered to be in a separate area, anterior to the source of Pam and N100m, which suggested that source of Pam was located in the primary auditory cortex, while the source of Pbm was located in the secondary auditory cortex. The source of N100m was considered to spread from the primary auditory cortex to the secondary auditory cortex.

Source estimation of spontaneous MEG activity and auditory evoked responses in normal subjects during sleep.

This study focuses on source estimation of spontaneous MEG activity and auditory evoked responses during sleep. Sources of K-complexes and auditory evoked responses were investigated by magnetoencephalograph (MEG) and electroencephalograph (EEG) measurements, simultaneously. Sources of K-complexes during stage 2 sleep were investigated. The MEG results suggested that the sources of K-complexes can be modeled by two current dipoles. Dipoles for the K-complexes were estimated to be located 5 mm away from the sources of the N100 components of auditory evoked responses during wakefulness. Sources of auditory evoked responses during each sleep stage were also investigated to clarify the origins of the K-complex, the vertex sharp transient, and delta waves. Estimated dipoles for the N100 component for each sleep stage were estimated to be at slightly different locations in the auditory area. Based upon results of the MEG measurements and the EEG topographies, sources of the N330 component can be modeled by multiple current dipoles, which are seen to be distributed diffusely throughout the cerebral cortex.

Source models of sleep spindles using MEG and EEG measurements.

MEG measurements can detect brain sources that are difficult to detect with EEG measurements. The purpose of this study was to investigate models of sleep spindles using both MEG and EEG activities that had been recorded simultaneously. The components of magnetic fields perpendicular to the surface of the head were measured using a DC-SQUID with a first-derivative gradiometer. We propose three models for sleep spindles. In the first model, the source slides into the superficial region of the head so as to be perpendicular to it's surface, and with this model, the power spectrum of the MEG is decreased. In the second model, the source slides into the deeper structures, so that it is perpendicular to the surface. Here, the power spectra of both the MEG and EEG are decreased. The third model has source perpendicular to the surface, leaning and sliding into the deeper structures. Here, the power spectrum of the EEG is decreased but that of the MEG is not.

MEG and EEG topography of frontal midline theta rhythm and source localization.

In this paper, we report on our study of frontal midline theta (Fm theta) activity in human subjects, recorded during mental processes such as arithmetic calculation. The Fm theta is a 6-7 Hz rhythmic wave with a duration of few seconds. The Fm theta activity is observed in the central region at the front of the head. EEGs and MEGs of Fm theta were measured simultaneously during mental calculation, and we analyzed these waveforms based on both topographic EEG maps and magnetic fields measurements. A single dipole simulated the EEG topography adequately, but there are many other dipole models which can generate a similar EEG pattern. It is difficult to estimate the source location of the Fm theta from the EEG topography alone because the EEG technique has a certain ambiguity associated with source estimation. Therefore, we considered the spatial relationships between the sources and the patterns of EEG and MEG that were simulated. Although it is not possible to obtain a unique solution for the source location of Fm theta from the EEG data alone, the simultaneous recording of MEGs from a large scalp area may result in an unambiguous solution. We therefore conclude that the simultaneous recording of both MEG and EEG data is more useful for accurate localization, than the EEG alone.

Source localization of middle latency auditory evoked magnetic fields.

The magnetic counterparts of middle latency auditory evoked responses (MLR) were measured for seven normal subjects with a 7-channel de superconducting quantum interference device (SQUID). The source of each component (Na, Pa, Nb and Pb) was estimated and plotted onto the individual magnetic resonance images (MRI). The source of Na, as well as those of Pa, Nb and Pb, was estimated to be in the supratemporal auditory cortex. The positions of Pa, Nb and Pb sources were compared with one another. No significant difference was observed between the positions of Pa and Nb sources. On the other hand, the source of Pb was found to be anterior to the sources of both Pa and Nb. It was suggested that there are more than two separate areas activated in the human auditory cortex during MLR.

Effects of stimulation side on human middle latency auditory evoked magnetic fields.

We investigated the effects of the stimulation side on middle latency auditory evoked responses (MLR) with neuromagnetic methods. Middle latency auditory evoked magnetic fields elicited by 1.0 ms clicks presented at ipsilateral and contralateral ear were recorded with a 7-channel DC SQUID. The amplitudes and latencies of the magnetic equivalents of Pa and Pb responses, which are the two main peaks of MLR, were measured. The amplitude of the contralateral Pa response was found to be significantly larger than the ipsilateral Pa response. On the other hand, no significant difference was observed in the latencies of Pa and Pb between the ipsi- and contralateral responses.

Modeling and source localization of MEG activities.

During the past decade, substantial advances in the understanding of the functional organization of the human brain have been made through the technique of MEG topographic mapping. Most of these investigations were concerned with the estimation and localization of sources which were modeled as single current dipoles positioned in a semi-infinite volume conductor with homogeneous conductivity. However, the sources in the brain are complex, and the head as a volume conductor consists of different materials with different electrical conductivities. The influence of these inhomogeneities on the MEG topography is studied by a computer simulation, modeling the sources as single or multiple dipoles located in inhomogeneous volume conductors. The computer simulation suggests some important aspects in estimation of source localization. The sources of MEG activities in human subject during sleep are also studied. A comparison of simulated MEG topographic patterns with measured data suggests that the sources of K-complexes can be modeled by two current dipoles. Sources for delta waves are analyzed by the FFT technique. The results show that the frequency distributions are different for delta waves measured by MEG and EEG techniques, leading us to conclude that at least two different sources are present. The MEG measurements have an advantage to provide important information concerning brain function which cannot be obtained using the EEG measurements.