C. Bak

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.

Origin of Human Motor Readiness Field Linked to Left Middle Frontal Gyrus by MEG and PET

Combined magnetoencephalography and positron emission tomography identified a prior source of activity in the left middle frontal gyrus during uncued movements of the right index finger. Voluntary movements gave rise to a change in the cortical electrical potential known as the Bereitschaftspotential or Readiness Potential, recorded as early as 1500 ms before the onset of movement. The Readiness Field is the magnetic field counterpart to the Bereitschaftspotential. In the present study, magnetoencephalography identified four successively active sources of fluctuation in the Readiness Field in the period from 900 ms before, to 100 ms after, the onset of the movement. The first source to be active was registered between 900 and 200 ms prior to the onset of the movement. This source of initial activity was mapped by positron emission tomography to the middle frontal gyrus, Brodmann area 9. The three sources subsequently to be active were mapped to the supplementary motor area, premotor cortex, and motor cortex (M1), all in the left hemisphere.

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.

Parametric excitation of plasma oscillations in a Josephson tunnel junction

Experimental evidence for subharmonic parametric excitation of plasma oscillations in Josephson tunnel junctions is presented. The experiments described are performed by measuring the microwave power necessary to switch a Josephson−tunnel junction biased in the zero−voltage state to a finite−voltage state.

Auditory event-related magnetic fields in a tone-duration discrimination task. Source localization for the mismatch field and for a new component M2″

Auditory event related magnetic fields were measured using an odd‐ball paradigm in which the rare event was a tone of short duration, D2, and the frequent one a tone of longer duration, D1. The subjects were required to attend to and count the number of rare stimuli. In the average across target stimuli a mismatch field (MMF) occurs and the dependence of the MMF, especially its latency, on the tone duration D2 is examined in detail. The location of an equivalent current dipole for the MMF‐source is found and turns out to be at variance with earlier results. In addition to the MMF we propose a new component, here called MMF, which in time overlaps the magnetic equivalent of the P200 signal and which has a source location (equivalent current dipole) lying rather close to the MMF‐source. The two sources are, however, active at latencies differing by a time equal to D2. We speculate that MMF indicates the onset of the process: “evaluation of tone‐duration” while the MMF indicates the end of this process.

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.