[Preoperative and postoperative magnetoencephalography in a patient with partial epilepsy]. / Prae-og postoperativ magnetoencefalografisk undersøgelse af en patient med partiel epilepsi.

Magnetoencephalography (MEG) is a non-invasive method with a potential of clinical diagnostic use to localize epileptogenic foci in the brain. The aim of this study was to investigate whether the pathological focus localized by MEG was concurrent with the conventional preoperative examinations in a patient with medically intractable epilepsy who underwent surgery (left side uncohippocampectomy). A conventional preoperative test battery had already documented a left-hemisphere fronto-temporal epileptogenic focus. Postoperatively the patient was seizure free. MEG was performed three months before and ten months after the operation. The analysis of the MEG data indicated a left hemisphere fronto-temporal focus in agreement with results obtained by means of the conventional methods used for locating epileptic foci. Postoperatively the focus had disappeared.

Dependence of the auditory evoked magnetic field (100 msec signal) of the human brain on the intensity of the stimulus.

The intensity dependence of the 100 msec magnetic field signal evoked by contralateral application of a tone burst stimulus has been examined for both hemispheres and for a number of frequencies. In all cases the component of the magnetic field normal to the skull was measured; in some cases this component was oriented in the outward direction (group 1 and some group 2 subjects), in the other cases in the inward direction (group 2). The experimental results were analysed in terms of an equivalent current dipole model. The analysis gave rise to the introduction of a transit time (tau 0 approximately 60 msec) from the onset of the stimulus to the activation of the current dipole and to the introduction of a functional relationship between the dipole moment and a 'reduced' intensity, and between the latency and the 'reduced' intensity. Further, the reproducibility of the signal was verified.

Cortical magnetic fields evoked by frequency glides of a continuous tone.

Frequency glides from a continuous tone have been shown to produce activity from the human cortex that can be recorded as time-varying magnetic fields outside the scalp in the same way as simpler auditory stimuli such as clicks and tone bursts. Data analysis has been based on a model assuming an equivalent current dipole localized close to the skull surface. Recorded data have shown good agreement with such a model. Interhemispheric differences have been shown in the location of this dipole, as well as with regard to dipole moment and latencies of responses to contralateral stimulation. The location of the equivalent dipole for frequency glide stimulation is close to that previously reported for tone pulse stimulation. However, the results indicate that differences in location of the order of 10 mm may exist. Comparing previously reported electric responses to frequency glides indicates essentially qualitative agreement although some significant differences have also been found. This is interpreted as evidence that at least the major contributions to the two types of response are produced by the same generator in the temporal lobe of the human cortex.

Auditory magnetic fields from the human cerebral cortex: location and strength of an equivalent current dipole.

Auditory evoked cortical magnetic fields are recorded from human subjects by means of a SQUID gradiometer. The spatial and temporal distributions of the averaged evoked fields normal to the surface of the skull are measured from both hemispheres in response to contra- and ipsilateral 1 kHz stimulation. The evoked magnetic response can be separated into a dominant and a 'residual' signal and the former is analysed with a particular source model consisting of a single equivalent current dipole in each hemisphere. We find that the equivalent current dipoles are located near the superior surface of the temporal lobes approximately 20 mm below the surface of the skull. The dipoles are oriented in the superior-inferior direction. In the left hemisphere the dipole is located approximately 14 mm posterior to that in the right hemisphere, but otherwise no hemisphere/ear difference in dipole location or orientation is found. The strength of the dipole in the left hemisphere is found to be twice as great as that in the right hemisphere when stimulating the right ear, whereas no difference is found when stimulating the left ear. The strength of the dipole is greater in response to contralateral than ipsilateral stimulation. By means of a statistical experiment and using estimates of the variance of the recorded evoked fields we show that the model suggested is adequate to describe the experimental data and that the overall confidence of the extracted dipole parameters can be estimated.

Auditory magnetic fields: source location and 'tonotopical organization' in the right hemisphere of the human brain.

The late, acoustically evoked, averaged magnetic field from the right hemisphere of the human brain is composed of two signals. One is dominant, appears generated by an equivalent current dipole within or near the primary auditory cortex and shows a frequency dependent location and/or orientation (tonotopical organization). The other, denoted the 'residual' signal, resembles the electric T-complex and is possibly generated more diffusely in the auditory and adjacent cortical areas.

Auditory magnetic fields from the human cortex. Influence of stimulus intensity.

The late averaged magnetic field evoked by contra- and ipsilateral auditory stimulation is recorded by means of a SQUID magnetometer from both hemispheres in four normally hearing, right-handed male adults. The stimuli consist of 1 kHz, 500 ms tone pulses with intensities from 5 to 85 dB HL and averaging is based on 60 sweeps. Stimulating the right ear the averaged magnetic field from the left hemisphere is approx. twice as great as that from the right hemisphere, whereas stimulating the left ear no difference in magnitude is found. The amplitude input-output functions are steeply rising near threshold and more shallow at high intensities. The responses from contralateral stimulation are approx. 9 ms earlier than those from ipsilateral stimulation with no interhemispheric difference.

Magnetic auditory responses from the human brain. A preliminary report.

By means of a magnetic sensor, SQUID (Superconducting Quantum-Interference Device) the late acoustically evoked magnetic field was recorded from the right and left side of the skull in 5 humans in response to ipsi- and contralateral 1 kHz tone bursts at 80 dB SPL. The '100 ms' component of the magnetic field has opposite polarity on the two sides of the head and when crossing the primary auditory cortex at the Sylvian fissure in a posterior--anterior track, polarity inversion of this component takes place within a highly localized region. The evoked magnetic field is widely distributed across the scalp and seems to be produced by an equivalent magnetic dipole located in or near the primary auditory cortex. In the present experiment neither right--left hemisphere nor ipsi--contralateral differences could be demonstrated.