Biomedical subjects

K Iramina

Publications and source records attributed to K Iramina.

7 recordsLinked to original sources

PubMed1996

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.

Adult↗

PubMed1996

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.

Acoustic Stimulation↗

PubMed1996

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.

Brain↗

PubMed1996

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.

Brain↗

PubMed1995

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.

Acoustic Stimulation↗

PubMed1994

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.

Acoustic Stimulation↗

PubMed1990

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.

Computer Simulation↗