Keiji Iramina

Neuronal current distribution imaging using magnetic resonance

A new functional magnetic resonance imaging (fMRI) technique to visualize the distribution of neuronal currents in the human brain was developed Measurements of the internal magnetic field deformation caused by an electric current dipole in a phantom were performed using a method based on the microscopic magnetic resonance imaging technique. The minimal value of the current dipole moment detected by the present method was determined to be 90 nAm. The technique was applied to obtain maps of human brain activity by using motor and sensory stimulus paradigms. Measurements were made with an EPI sequence at 1.5 T. Intensity changes, resulting from causes other than neuronal currents, were eliminated by editing functional images obtained with field gradients of different polarities. MRI mapping of the neuronal currents in the brain during middle finger and thumb tapping was clearly obtained.

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.

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.

Modeling and source localization of MEG activities

SummaryDuring 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.

Effects of transcranial magnetic stimulation on EEG activities

The combination of transcranial magnetic stimulation (TMS) and an electroencephalogram (EEG) is an effective tool for investigating the functional connectivity in the brain. This paper investigated cortical reactivity and connectivity by the combination of TMS and EEG. The spontaneous activity of the brain was measured before and after magnetic stimulation. The effect of TMS on alpha activity was investigated. The alpha wave was suppressed for a few seconds after stimulation of the occipital area. No differences in the suppression of the alpha waves were found with and without auditory masking. Furthermore, the coil click of TMS had no effect on the alpha wave. The alpha wave was significantly suppressed by the occipital stimulation, whereas slight suppression was observed in other areas of stimulation. The alpha wave was increasingly suppressed as the stimulation magnitude became more intense. In order to investigate the evoked response by TMS, evoked potentials generated by TMS were measured. It was observed that more evoked responses spread to the center of the brain when the cerebellum was stimulated than at other areas of stimulation. These results indicated that TMS blocked the neural connections within alpha wave generation, and the electrical currents produced by TMS affected the neural activities.

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

SummaryIn this paper, we report on our study of frontal midline theta (Fmθ) activity in human subjects, recorded during mental processes such as arithmetic calculation. The Fmθ is a 6–7 Hz rhythmic wave with a duration of few seconds. The Fmθ activity is observed in the central region at the front of the head. EEGs and MEGs of Fmθ 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 Fmθ 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 Fmθ 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.

Measurement of evoked electroencephalography induced by transcranial magnetic stimulation

This study focused on the measurement of evoked potentials induced by transcranial magnetic stimulation (TMS) for observing the neuronal connectivity in the brain. We developed an electroencephalography (EEG) measurement system to eliminate the electromagnetic interaction emitted from TMS. EEG activities 5 ms after TMS stimulation were measured. Using this artifact free amplifier, we investigated the intensity dependence of brain activation induced by TMS. When the stimulus intensity was changed at three levels, TMS-evoked EEG responses were measured. Several components of the evoked potential appeared at 9 ms, 20 ms, and 50 ms after stimulation. A large response appeared at about 9 ms after cerebellar TMS. There was a significant dependence of these responses on the stimulus intensity. During right-hand side motor area stimulation, there was no clear peak of the wave forms within 10 ms latency. Occipital stimulation caused more evoked responses to spread to the center of the brain than at other areas of ...

Source models of sleep spindles using MEG and EEG measurements

SummaryMEG 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 activity 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 its 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 the EEG are decreased. The third model has the 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.

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.

Measurements of biomagnetic fields using a high-resolution dc superconducting quantum interference device magnetometer

We have developed a multichannel high-resolution superconducting quantum interference device magnetometer for measuring biomagnetic fields produced by small animals. We measured the magnetocardiogram produced by a rat. Topographies of the P wave, QRS wave, and T wave of the rat magnetocardiogram were obtained. We also measured the magnetoretinogram, visually evoked magnetic fields, and auditory evoked magnetic fields of the rat. The signals of the magnetoretinogram appeared at 60 ms latency and the field intensities were about 0.6 pT. It was not possible to obtain the clear visually evoked fields because those signals were hidden behind the magnetoretinogram. It was possible to obtain auditory evoked magnetic fields of the rat. The significant differences of the wave forms were observed in both sides of the right ear, which are separated by 15 mm. Our system has adequate spatial resolution for measurement of the magnetocardiogram and auditory evoked magnetic fields produced by small animals.