Magnetic phenomena of the central nervous system

Magnetic detection of sleep spindles in normal subjects

The magnetoencephalogram (MEG) and electroencephalogram (EEG) were recorded simultaneously from 3 normal subjects during light sleep. The extracranial magnetic field patterns associated with the ionic currents within the brain (MEG) were measured using a SQUID magnetometer in a magnetically shielded room. Although a previous report indicated that there are few sleep spindles in MEG records, we observed many in all 3 subjects. We believe this is because measurements were made vertically at the vertex region.

Spatial filtering in multichannel magnetoencephalography

Partial differential equations in boundary-value problems have been studied in order to estimate the influence of several geometrical and physical parameters involved in the outward transmission of the brain's magnetic field. Explicit Green kernels are used to obtain integral forms of generalized solutions which can be deduced from each other, as expressed over concentric spherical surfaces. That leads to numerical applications dealing with the radial component of the magnetic field. From this study, a new spatial filtering is proposed as a possible improvement in two-dimensional magnetoencephalographic mapping using large multisensors.

Polarization of nerve regeneration (electrotaxis)

Regeneration of both median nerves was studied in the rat in three different experimental models of centrocentral anastomosis through an interposed segment of pre-degenerated tibial nerve after denervation by spinal root transection. Different patterns of regeneration were observed in the anastomoses. These patterns suggest a bio-electrical polarity related to neuronal function. The present experimental model appears to offer a new opportunity to study neuronal regeneration under the influence of defined bioelectrical conditions.

Structural and regenerative changes in deafferented and deefferented ulnar nerves

Both brachial plexuses of Wistar rats were deafferented or deefferented by disrupting their dorsal or ventral roots. At the same time, the ulnar nerves were transected and sutured. A sham group and another group in which the upper (C-5) and lower (T-1) spinal nerves were transected on both sides were included as control groups. Clear differences in the ulnar nerve regeneration rate were found between the deafferented and the deefferented animals. Ultrastructural studies disclosed signs of transient axonopathy in the nontransected fibers of the proximal segment of the ulnar nerve. The possible role of bioelectricity in the genesis of these changes is discussed.

Magnetic determination of the spatial extent of a single cortical current source: a theoretical analysis

In this paper a model, originally developed for calculating the magnetic field produced by a single nerve fiber, is used to compare the ability of two magnetometers with different spatial resolutions to map the magnetic field distribution from a highly localized current source in the cerebral cortex. It is concluded that whenever the source-to-coil distance is much larger than the dimensions of the source it will be difficult, if not impossible, to use an inverse calculation to determine the spatial extent of the source from the magnetic field. When the source-to-coil distance is comparable to the extent of the source, an inverse calculation can provide information regarding the axial length of a localized dipolar current source.

Magnetic localization of a dipolar current source implanted in a sphere and a human cranium

Magnetic fields produced by a dipolar source implanted in a spherical conductor and a human cranial specimen were measured in the magnetoencephalogram (MEG). The location of the source was accurately computed in the spherical conductor from the identified magnetic field extrema using equations for a current dipole in a sphere. This same method was insufficient for localizing the source in a human cranium, where magnetic field maps appeared as distortions from the classical dipolar pattern. A more complete computer modeling procedure was used, adjusting for the non-spherical dimensions of the recording matrix on the cranium. By fitting the gradient of computer simulated fields to those measured outside the cranium, the accuracy of source localization was substantially improved. The greatest distortion of the extracranial magnetic field was an inequality in the measured amplitude of the two extrema, produced by an increased distance and angle of the MEG probe when recording over the lower face and ear. However, gross heterogeneities in the resistance of the skull due to a craniectomy and an implanted insulating balloon had a negligible effect on the extracranial magnetic field pattern.

Determinative factors in human tonotopy: tentative analysis

Two main parameters of stimulation were checked as an attempt to investigate the limits of the tonotopic organization in the human auditory cortex. Auditory Evoked magnetic Field (AEF) experiments from a dozen normal subjects are reported. Five of them clearly showed tonotopic AEF responses to long tone bursts stimuli, which disappeared when equivalent white noise was superimposed to the tone. Discrepancies from previous reports and high inter-individual variability are discussed.

Functional mapping of the human cerebellum with positron emission tomography

Alterations of local neuronal activity induced within the human cerebellum by tactile stimulation and voluntary movement were mapped with positron emission tomographic measurements of brain blood flow. Finger movements produced bilateral, parasagittal blood-flow increases in anterior, superior hemispheric cortex of the cerebellum. Responses to tactile finger stimulation were coextensive with responses to voluntary finger movements but were less intense. Saccadic eye movements produced midline blood-flow increases in the posterior vermis of the cerebellum. Positron emission tomography now permits investigation of functional-anatomical relations within the human cerebellum.

Electrical sources in human somatosensory cortex: identification by combined magnetic and potential recordings

Magnetic fields and electrical potentials produced by neuronal activity have different properties that can be used for the identification of electrical sources in the human brain. Fields and potentials occurring 20 to 30 milliseconds after median nerve stimulation in human subjects were compared in order to investigate the sources of evoked potential components that have been attributed by different investigators to the thalamus or thalamocortical afferents, to separate radial sources in somatosensory cortex and motor cortex, or to a tangential source in somatosensory cortex. The magnetic and potential wave forms were highly similar in morphology, and their spatial distributions were centered over sensorimotor cortex, were dipolar in shape, and differed in orientation by approximately 90 degrees; distances between the minimum and maximum of the magnetic distributions were about 60 percent of those of the potential distributions. These results cannot be accounted for by thalamic sources or radial cortical sources alone, but are consistent with a tangential source in somatosensory cortex, with an additional smaller contribution from radial sources.

Neuromagnetic studies of somatosensory system: principles and examples

A review is given about the basic principles of neuromagnetic recordings and somatically evoked magnetic fields with examples from the authors' own work. MEG provides good spatial resolution for activity in the fissural cortex. It has, for example, allowed differentiation of current sources in the primary and secondary somatosensory cortices. Some cortical areas activated by painful stimuli have also been localized by means of MEG.

Fast and slow magnetic phenomena in focal epileptic seizures

The magnetic fields associated with penicillin-induced focal epilepsy were measured in laboratory rats. Interictal magnetic spikes were similar to those previously observed in humans with focal seizure disorders. The magnetic fields of the seizure itself displayed both slow and fast phenomena, reversing in direction on opposite sides of the head.

Somatosensory evoked cerebral magnetic fields from SI and SII in man

We have recorded cerebral magnetic fields elicited by electrical stimulation of median and peroneal nerves. Field mapping indicates that the deflections at 30-80 and 150-180 msec are due to activity at SI. Additional activity at 90-125 msec is generated at SII, on the superior bank of the sylvian fissure. At SI, the source locations are in agreement with the known somatotopy. Only contralateral stimuli evoke responses at SI, whereas both ipsi- and contralateral stimuli elicit responses at SII.

Neuromagnetic evidence of spatially distributed sources underlying epileptiform spikes in the human brain

Neuromagnetic measurements were performed on 17 subjects with focal seizure disorders. In all of the subjects, the interictal spike in the scalp electroencephalogram was associated with an orderly extracranial magnetic field pattern. In eight of these subjects, multiple current sources underlay the magnetic spike complex. The multiple sources within a given subject displayed a fixed chronological sequence of discharge, demonstrating a high degree of spatial and temporal organization within the interictal focus.

Neuromagnetic localization of cortical activity evoked by painful dental stimulation in man

We have recorded cerebral magnetic fields evoked by painful dental stimulation. The field pattern indicates a current source at the upper bank of the anterior Sylvian Fissure, corresponding to the anterior end of the secondary somatosensory cortex. This finding suggests cortical representation of tooth pulp in man. The neuromagnetic technique, allowing the investigation of this cortical area, thus provides a new non-invasive tool for pain research.

Neuromagnetic responses from the second somatosensory cortex in man

Cerebral magnetic fields elicited by electric stimulation of peripheral nerves were studied in man. Responses were found over the Sylvian fissure at latencies of 95-125 ms for both contra- and ipsilateral stimuli. The field distribution indicated that the responses are generated in the second somatosensory cortex SII at the upper bank of the Sylvian fissure. These responses seem to provide the first non-invasive tool to study the function of SII in man.

Demonstration of useful differences between magnetoencephalogram and electroencephalogram

For a dipole source, theory predicts 3 useful differences between the MEG and EEG spatial patterns over the head. These are seen when a comparison is made between theoretical MEG and EEG maps, due to the dipole in a spherical model of the head. If true, these differences would allow the MEG to better localize or differentiate neural sources in some ways than does the EEG. A first experimental test of the differences is made here. A comparison is made between MEG and EEG maps due to a neural source which appears to behave as a dipole (N20 of the somatic evoked response). The same 3 differences are seen, therefore the predicted differences are confirmed experimentally. The first 2 differences, due only to the tangential component of the dipole, are that the MEG pattern is rotated by 90 degrees from the EEG pattern and is one-third tighter. The first allows the MEG to localize a tangential dipole better in a preferred direction, across the dipole (while the EEG does so along the dipole); the second allows the MEG to localize somewhat better in its preferred direction than the EEG does in its preferred direction. The third difference is due only to the radial component of the dipole; while the MEG receives no contribution from this component, the EEG pattern is asymmetrical and is heavily weighted by it. This allows the MEG to reveal tangential sources which are obscured by the radial sources in the EEG. For sources which cannot be approximated by a dipole, the MEG-EEG differences will depend on the particular case; however, the spherical model can now be used with more confidence to predict differences in these cases.

The hippocampal formation as a source of the slow endogenous potentials

Magnetic fields were detected with a SQUID sensor at the temporal and occipital areas of the head in response to a frequent and an infrequent attended visual stimulus. The time-course of the magnetic field for the infrequent stimulus correlated highly with the simultaneously measured electrical potential that showed the commonly observed N2-P3 complex. Analysis of the pattern of the magnetic field showed that the sources of N2 and P3 lay deep in the brain within the hippocampal formation.

Event related brain electrical and magnetic activity: toward predicting on-job performance

Personnel assessment has depended on paper and pencil tests. These tests are able to predict academic performance fairly well, but have been criticized for their ineffectiveness in predicting on-job performance. Research on brain function which emphasizes "process" rather than "content" variables, may be able to predict on-job performance better than traditional tests. Relationships have been found between event related brain potentials (ERPs) and performance in fighter aircraft and on a sonar simulator, as well as enlistees promotions and attrition. Research has suggested that ERP records are better able to discriminate and classify performance groups than paper-and-pencil test scores. Biomagnetic data are described from heart and brain. These data suggest increased sensitivity to individual differences, and may offer greater opportunity for improving prediction of on-job performance, than ERP records or paper-and-pencil tests.

Biomagnetic measurements of spontaneous brain activity in epileptic patients

In the last few years there has been an increasing interest in the magnetic activity due to bioelectrical currents flowing in the brain. In this paper preliminary results are reported concerning spontaneous magnetic brain activity in 36 patients affected by different kinds of brain disease; in most of these cases the symptoms were induced by localized pathology (atrophies, scars, tumors). Measurements were carried out with the simultaneous recording of the EEG. At present one of the most interesting features of magnetic detection seems to be its high localizing ability in cases of cortical foci, and sometimes its ability to show activities not evident in the EEG. These features seem to be very encouraging for the search for technical improvements, with the aim of making the magnetic technique a candidate for current diagnostic purposes.

Tonotopic organization of the human auditory cortex

Neuromagnetic measurements of responses to auditory stimuli consisting of pure tones amplitude-modulated at a low frequency have been used to deduce the location of cortical activity. The evoked field source systematically increased in depth beneath the scalp with increasing frequency of the tone. The tonotopic progression can be described as a logarithmic mapping.

Modulation transfer functions of the human visual system revealed by magnetic field measurements

Properties of a neural source of magnetic field localized in the occipital lobe was measured in a steady-state paradigm using contrast reversing gratings. Comparisons with scalp potentials provided evidence that the evoked field was associated with intracellular currents. Its modulation transfer functions were similar to the analogous functions for the scalp potential and the detection of a grating. Moreover, the amplitude of the evoked field was linearly related to the potential amplitude and their phases were nearly identical. An analysis of the results in terms of theoretical relations between evoked field and potential led us to conclude that these two measures may yield a similar characterization of the source when one dipolar source predominantly gives rise to both measures, but they may yield complementary information when multiple sources contribute to the measures.

Magnetic auditory evoked fields: interhemispheric asymmetry

Magnetic auditory evoked fields (MAEFs) were recorded from the right (11 subjects) and the left (7 subjects) hemisphere following 128 click stimuli delivered to contralateral, ipsilateral and both (bilateral) ears. Right hemisphere MAEFs were of higher amplitude following contralateral, compared to ipsilateral, stimulation in 9 of 11 subjects; mean contralateral response amplitude was 135 +/- 33% (S.D) of ipsilateral response amplitude. Left hemisphere MAEFs were of higher amplitude following contralateral stimulation in 7 of 7 subjects; mean contralateral response amplitude was 145 +/- 44% of ipsilateral response amplitude. These observations are compatible with evidence that a majority of centripetal auditory input is crossed, and/or that contralateral auditory stimulation activates a larger area of cortex than does ipsilateral stimulation.

On the relation between somatic evoked potentials and fields

Recently Okada et al. (1981) reported that stimulation of the median nerve with a brief electrical impulse at the wrist evoked a transient change in the brain's magnetic field. This somatic evoked field (SEF) is similar in its temporal waveform to the response to the same stimulus reported for the electrical potential recorded on the pial surface of the exposed brain. Moreover, both measures differ substantially from the somatic evoked potential (SEP) recorded at the scalp. The present paper describes a more detailed account of the SEF as well as an analysis of its relation to the SEP and to the somatic pial response (SPR). Its purpose of the use the three measures in clarifying our understanding of the nature and locations of sources of the SEF. This paper is divided into three sections. The first is a background section which reviews basic principles and models that are widely used in deducing the locations of sources of evoked potentials and fields. It indicates the types of currents which may give rise to the SEF, and distinguishes between them and the current which is associated with the SEP. It concludes with a rationale for the experiments described in the next section. The experiments described in the second section determined how the SEF varies with the position from which it is recorded at the scalp. These variations turn out to be essential to our understanding of the nature and location of the sources of the SEF. The third section summarizes the results of the experiments and makes clear how they affect theories of the origin of the SEF. The findings also have implications for our understanding of the SEP and SPR. The most salient findings are: (1) The SEF recorded normal to the head provides essentially the same information as that provided by reported potential recordings from the exposed surface of the brain (the SPR). (2) The SEF originates in the cortex of the cerebrum in the vicinity of the central sulcus. (3) The currents that account for identifiable components of the SEF are opposite in direction to those that account for corresponding components of the SPR. This result is consistent with models that ascribe the detected field normal to the scalp to intracellular currents, whereas the VEP is associated with extracellular currents flowing in the opposite direction.

Auditory evoked transient and sustained magnetic fields of the human brain. Localization of neural generators

A long auditory stimulus elicits a magnetic evoked response in the human brain, consisting of transient deflections followed by a sustained response. The distributions of the magnetic fields indicate that the auditory evoked transient response at a latency of 100 ms as well as the auditory sustained response are generated at and around the primary auditory cortex.

Magnetic field of a nerve impulse: first measurements

The magnetic field of the action potential from an isolated frog sciatic nerve was measured by a SQUID magnetometer with a novel room-temperature pickup coil. The 1.2 x 10(-10) tesla field was measured 1.3 millimeters from the nerve with a signal-to-noise ratio of 40 to 1.

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.

Comparison of the magnetoencephalogram and electroencephalogram

The spatial response of the magnetoencephalogram (MEG) to sources in the brain's cortex is compared with that of the electroencephalogram (EEG). This is done using computer modeling of the head which is approximated by 4 concentric spherical regions that represent the brain and surrounding bone and tissue. Lead fields are calculated at points on the cortex for unipolar, bipolar and quadrupolar MEG and EEG measurements. Since lead fields are patterns of the sensitivity of these measurements to a source at various locations and orientations, they provide a convenient means for comparison. It is found that a unipolar MEG has a very different lead field than a unipolar EEG. Hence, this type of MEG detects sources at different locations and orientations than this EEG. Although bipolar MEG and EEG lead fields are found to have similar patterns, the MEG lead field is narrower than that of the EEG and hence 'sees' a smaller area on the cortex than the EEG. This is because the potentials measured by the EEG are 'smeared' by the low-conductivity skull; the magnetic fields measured by the MEG are not smeared. Quadrupolar MEG and EEG lead fields are found to be about the same. The responses of bipolar MEGs and EEGs to distributed sources, which are composed of aligned and randomly oriented dipoles, are compared. It is found that for both types of sources, the MEG 'sees' an area on the cortex which is approximately 0.3 times that for the EEG. Hence, the MEG appears to be useful for detecting a more restricted group of sources than the EEG.

Human magnetic auditory evoked fields

Magnetoencephalographic (MEG) averaged auditory evoked fields to click stimuli (N = 512) were recorded from four human subjects. The MEG was recorded with an asymmetric second derivative SQUID gradiometer located in an aluminum shielded room. Unlike conventional EEG auditory evoked potentials, which have a widespread distribution, evoked magnetic fields appear to be localized to the general area of the primary auditory cortex and diminish rapidly in amplitude as the gradiometer is moved away in any direction.