Statistical analysis of the DIAMOND MI study by the multipole method.

We present a new method to describe the dynamics of the beat-to-beat RR time series. The classification of the phase-space plots obtained from RR time series is performed by a calculation of parameters which describe the features of the two-dimensional plot. We demonstrate that every parameter has its specific consequence on the evaluation of the state of the cardiac function. By applying the method to the DIAMOND MI study we demonstrate that these parameters have more prognostic power than previously suggested risk markers. The results suggest that the RR intervals constitute a highly complex time series which necessitates the use of refined mathematical-statistical methods in order to reveal pathologies in the heart rate.

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 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 M2", 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 M2" indicates the onset of the process: "evaluation of tone-duration" while the MMF indicates the end of this process.

Auditory evoked magnetic fields: analysis of the N100/P200 signal.

We propose a method to analyse the N100/P200 structure observed in auditory evoked magnetic fields. Based in linear cable theory for passive dendritic conduction we assume an expression for the time-course of the equivalent current dipole moment and show that good agreement can be obtained with experimental results. The analysis leads to realistic values for the membrane time constant (approximately 30 ms) and the reduced dendritic length (approximately 2).

On the evidence of auditory evoked magnetic fields as an objective measure of tinnitus.

The purpose of the present study was to determine the utility of auditory evoked magnetic fields as an objective measure of tinnitus. The auditory evoked magnetic fields of 14 patients with tinnitus and of 14 sex- and age-matched controls were measured by means of a 7-channel BTI neuromagnetometer. Stimuli were 1 kHz tone-bursts presented randomly. Tinnitus patients and controls were similar with respect to latencies and amplitudes of the N100m and P200m, and with respect to the locations and moments of the equivalent current dipoles. The present study could not support the notion that specific abnormalities of the auditory evoked magnetic fields are characteristic of patients with tinnitus.

[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.