J. Lebech

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.

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 evoked magnetic fields from the human brain. Source localisation in a single-dipole approximation

Abstract Experimental results on late auditory evoked magnetic fields from the right side of the human brain are presented. It is shown that the results can be described by means of a source model consisting of a single, equivalent current-dipole with a dipole moment of ≈ 10−8 A m and a location close to the electrode position T4 and between 10 and 25 mm below the surface of the skull.

Observation of transmission of 45 GHz microwaves through a thin Mo-disk with and without the presence of a magnetic field

Abstract Transmission of microwaves of a frequency of 45 GHz through a high-purity, plane-parallel, single-crystalline molybdenum sample of a thickness of 0.15 mm and 0.27 mm, respectively, has been observed, both in the presence of a magnetic field and without a magnetic field. Azbel-Kaner type cyclotron resonance was seen in the signal transmitted through the sample.

Observation of electron time-of-flight effects in the skin-layers of a thin metallic disk

Abstract A time-of-flight effect for electrons in the skin-layers of a thin single-crystalline disk of silver has been observed in connection with microwave transmission experiments. A numerical estimate of the thickness of the region in which electrons interact with the surface field is given.

Exact solution of the dispersion equation for cyclotron waves for a cylindrical Fermi-surface

Abstract An exact solution of the dispersion equation for extraordinary cyclotron waves propagating in a metal with a cylindrical Fermi-surface is presented. The existence of a hitherto unnoticed mode is pointed out.

Microwave-transmission through thin metallic slabs discrete waves

Abstract Observed fine-structure in microwave transmission through thin single-crystal silver slabs is tentatively interpreted in terms of discrete waves. A connection to time-of-flight effects is indicated. It is shown that Fermi velocities may be directly determined.

Time-of-flight effects in microwave transmission through a thin silver slab

Abstract Experimental results of investigations on time-of-flight effects in 45 GHz microwave transmission through thin silver slabs are presented. New results for B  [100] are given.

Selective Averaging in Auditory Evoked Magnetic Field Experiments

Since the first observation by Reite et al. (1978) of auditory evoked magnetic fields (AEF) from the human cortex several research groups have studied especially the late response occurring around 100 ms after application of the auditory stimulus. There appears by now to exist a general consensus that the 100-ms signal can be accounted for in terms of a model consisting of an equivalent current dipole (ECD) located in or close to the primary auditory cortex. The method normally used in analyzing experimental data (magnetic fields or electrical potentials) on evoked responses has been to collect a - large - number of single epochs followed by a simple averaging procedure, possibly after artifact rejection (see one of several reviews, e.g. Hari (1986)). The formation of a simple average is based on the assumption that the evoked response is superimposed on a noise background consisting of alpha-waves, theta-waves, instrumental noise etc. and that this noise averages out for a sufficiently large number of recorded epochs. Thus, in particular the evoked response and the spontaneous brain waves are considered as independent signals. This, however, need not be strictly true, and we give two remarks to elucidate this point. Firstly, earlier non published recordings of auditory evoked responses, (tone-burst, l kHz,random ISI) showed small oscillations, which appears to be time-locked to the stimulus.

Pre- and Postoperative Magnetoencephalography in Partial Epilepsy. A Case Story

Magnetoencephalographic (MEG) recordings of spontaneous brain waves have in recent years been repeatedly applied in examinations of pathological activity in epileptic patients with the aim of a (3-D) localization of epileptic foci. In particular, Ricci et al. (see, e.g. Ricci et al. 1981, 1985a, 1985b) have used MEG-recordings in the examination of numerous epileptic patients. In their analysis of the experimental data they use the relative covariance method (see, e.g. Chapman 1989) for the detection of epileptic foci. In a number of cases they also determine a focus on the basis of an observed spatial distribution of spikes. The latter method was first used by Barth et al. (1982, 1984) and was also used in Sutherling et al. (1985). In the present paper we present some results of an MEG examination of an epileptic patient for whom a conventional preoperative test battery had already documented a left hemisphere frontotemporal epileptogenic focus. The MEG data were recorded before operation as well as after operation. The results of an analysis based on a relative covariance method are in agreement with the already documented focus.