Reconstruction of quasi-radial dipolar activity using three-component magnetic field measurements.

OBJECTIVE: While standard magnetoencephalographic systems record only one component of the biomagnetic field, novel vector-biomagnetometers enable measurement of all three components of the field at each sensing point. Because information content in standard one-component magnetoencephalography (MEG) is often not adequate to reconstruct quasi-radial dipolar activity, we tested the hypothesis that quasi-radial activity can be estimated using three-component MEG. METHODS: We stimulated the right median nerve in 11 healthy volunteers and recorded the somatosensory evoked fields over the contralateral hemisphere using a novel vector-biomagnetometer system comprised of SQUID-based magnetometer triplets. Source reconstruction for the early cortical components N20m and P25m was subsequently performed. RESULTS: Both tangential and quasi-radial dipolar activity could be reconstructed in 10 of the 11 participants. Dipole locations were found in the vicinity of the central sulcus, and dipole orientations were predominantly tangential for N20m and quasi-radial for P25m. The mean location difference between the tangential and quasi-radial dipoles was 11.9 mm and the mean orientation difference was 97.5°. CONCLUSIONS: Quasi-radial dipolar activity can be reconstructed from three-component magnetoencephalographic measurements. SIGNIFICANCE: Three-component MEG provides higher information content than does standard MEG.

Dyslexia: the possible benefit of multimodal integration of fMRI- and EEG-data.

Biological research about dyslexia has been conducted using various neuroimaging methods like functional Magnetic Resonance Imaging (fMRI) or Electroencephalography (EEG). Since language functions are characterized by both distributed network activities and speed of processing within milliseconds, high temporal as well as high spatial resolution of activation profiles are of interest: "where" can dyslexia specific activations be detected and "when" do language processes start to diverge between dyslexics and controls? Due to the network character of language processing, fMRI-constrained distributed source models based on EEG-data were computed for multimodal data integration. First single-case results show that this method could be a promising approach for the understanding of a repeatedly described experimental finding for dyslexia like that of an overactivation in inferior frontal language areas. Multimodal data analysis for the subjects presented here could probably demonstrate that inferior frontal overactivations are the consequence of a phonological deficit and could represent ongoing articulation processes used to solve phonologically challenging tasks.

Rapid functional plasticity of the somatosensory cortex after finger amputation.

Recent research indicates that areas of the primary somatosensory (SI) and primary motor cortex show massive cortical reorganization after amputation of the upper arm, forearm or fingers. Most of these studies were carried out months or several years after amputation. In the present study, we describe cortical reorganization of areas in the SI of a patient who underwent amputation of the traumatized middle and ring fingers of his right hand 10 days before cortical magnetic source imaging data were obtained. Somatosensory-evoked magnetic fields (SEF) to mechanical stimuli to the finger tips were recorded and single moving dipoles were calculated using a realistic volume conductor model. Results reveal that the dipoles representing the second and fifth fingers of the affected hand were closer together than the comparable dipoles of the unaffected hand. Our findings demonstrate that neural cell assemblies in SI which formerly represented the right middle and ring fingers of this amputee became reorganized and invaded by neighbouring cell assemblies of the index and little finger of the same hand. These results indicate that functional plasticity occurs within a period of 10 days after amputation.

Source localization and possible causes of interictal epileptic activity in tumor-associated epilepsy.

Electrophysiological studies in gliomas have demonstrated action potentials in neoplastic cells. These "spiking tumor cells" are, however, an enigma. In attempt to find evidences for spikes within tumoral borders, 21 patients with different intracerebral tumors were preoperatively screened for the occurrence of epileptogenic discharges using multichannel MEG and EEG. A correlation between histopathology and the distance between dipole and tumor border could be found. Glioma patients showed epileptic activities closer to the border than those with mixed glioneuronal neoplasms and metastases. Four glioma patients demonstrated epileptic activity within the tumor boundary, however, not in the deep center of the tumor. Patch-clamping of cells from acute glioma slices did not yield a correlation between the presence of voltage-gated sodium channels in tumor cells and the MEG/EEG data. Our results demonstrate that the zone with the highest epileptogenic potential is different in gliomas and other brain tumors. However, our data do not strongly suggest that glioma cells are directly involved in the generation of tumor-associated epilepsy in vivo via their capability to generate action potentials.

Effects of passive tactile co-activation on median ulnar nerve representation in human SI.

In animals simple passive co-activation causes a fusion and expansion of the involved cortical representations. We used passive tactile finger co-activation for 40 min to investigate cortical representational changes in the human somatosensory cortex. Magnetic source imaging revealed that the euclidean distance between median and ulnar nerve somatosensory evoked fields (SEF) was significantly reduced after application of 600 synchronous airpuff stimuli to the fingertips of four fingers. In the control experiment without co-activation no significant change in distance was observed. Perception threshold and spatial two-point discrimination were not affected by the synchronous stimulation. This is in contrast to blind three-finger Braille readers who frequently mislocalize stimuli applied to the reading fingers. This points to a lack of behavioural relevance or the short duration of co-activation.

A 16-channel SQUID-device for biomagnetic investigations of small objects.

Biomagnetic investigations in basic physiological research using animals require measurement devices different from commercial biomagnetometers used in human investigations. Two major problems have to be tackled in the design of such biomagnetometers. First, the spatial sampling needs to be much higher. Second, the distance between pick-up coils and the sources needs to be much shorter in order to compensate the worse signal-to-noise ratio (SNR) due to the smaller pick-up coils. We designed and built a 16-channel biomagnetic measurement system meeting these design criteria. The pick-up coil diameter of this new biomagnetometer is 6.7 mm, thus allowing 16 channels on an area of 3.2x3.2 cm2. The pick-up coils are located 3 mm above the dewar outer bottom, hence the closest distance to the cortical surface can be a few millimetres. We provide as an example of first measurements performed with the new biomagnetometer investigations of epileptic spikes in adult rabbits by simultaneous magnetoencephalogram (MEG) and electrocorticogram (ECoG) recordings. The high SNR of the recorded MEG and the simultaneously detected electric potentials allow investigations of the spatio-temporal pattern of neuronal processes of epileptiform spikes with signal strengths of about 3.5 pT.

[Sex specific differences in hemispheric lateralization in schizophrenia? An MEG-MRI study]. / Geschlechtsspezifische Unterschiede der Hemisphärenlateralisation bei Schizophrenien? Eine MEG-MRT-Studie.

In this magnetoencephalography study the issue of hemispheric lateralisation in patients with schizophrenia was addressed using acoustically evoked neuromagnetic fields. The characteristics of dipoles in the superior temporal gyrus, the primary auditory cortex, were calculated. In contrast to other studies, alterations did not concern the localisation, but rather the orientation of dipoles. Of pathophysiological interest was that the dipoles abnormalities were found left-hemispherically in male (p = 0.02) and right-hemispherically in female patients with schizophrenia (p = 0.01) when compared to controls. The findings suggest gender-specific alterations of hemispheric lateralisation in schizophrenia.

Reorganization of the somatosensory cortex after amputation of the index finger.

Cortical reorganization occurs within the primary somatosensory and the primary motor cortex after amputation of the arm or forearm. Here we report on a patient showing cortical reorganization after amputation of his right index finger. Our findings indicate that the neural networks within the area of the amputated finger in the somatosensory cortex (SI) were invaded by neighbouring structures, i.e. of neural cell assemblies that subserve the thumb and middle finger of his right hand.

Reliability of dipole localization for the movement-evoked field component MEF I.

The movement-evoked field I (MEF I) component is the largest and most stable neuromagnetic component accompanying self-paced movements. In order to use MEG for studying dynamic changes in the cortical organization of movements, data about the reliability and variability of these neuromagnetic components for individual subjects must be established during different sessions. For this aim, three male subjects were requested to perform self-paced flexions of their index finger and thumb in repeated sessions while the MEG was recorded by a 31 channel system. The MEF I was identified for each session and a single equivalent dipole was calculated for this component. The dipole localizations of the various sessions were compared. The standard deviation of the localization for all persons and all values amounts to 4.0-5.2 mm for the three spatial dimensions. Our data suggest that the spatial distance between two single focal sources fitted to the MEF I must be greater than 14 mm to be interpreted as distinct. However, the neuromagnetic field structure and the resulting dipole localization of the MEF I component are quite stable and could be used for the evaluation of cortical plasticity.

Abnormalities of auditory evoked magnetic fields and structural changes in the left hemisphere of male schizophrenics--a magnetoencephalographic-magnetic resonance imaging study.

Functional and structural changes in 10 DSM-III-R male schizophrenics and 10 healthy volunteers were investigated using magnetoencephalographically (MEG) detected long-latency (N100 m) auditory evoked fields (AEFs) and magnetic resonance imaging (MRI). The AEFs were characterized by single moving equivalent dipoles, which were superimposed on MRIs. There were significant differences in dipole orientations and in AEF latencies in the left hemisphere of schizophrenics, when compared to the controls. The MEG-detected alterations were found to be associated with a bilateral volume reduction of the posterior superior temporal gyrus (pSTG), which was more pronounced in the left hemisphere. Separate analysis of white and gray matter has shown that the pSTG volume reduction resulted from decreased gray matter volumes without white matter changes. Both the functional and the morphological data indicate a left-hemispheric disturbance in our patients.

Abnormalities of auditory evoked magnetic fields in the right hemisphere of schizophrenic females.

Eight DSM-III-R female schizophrenics were investigated by magneto-encephalographically (MEG) detected long latency (N100m) auditory evoked fields (AEFs). In contrast to controls, in schizophrenics a lack of interhemispheric asymmetry was present, which was found to be due to alterations of the right hemisphere.

Multichannel magnetography in unshielded environments.

Any biomagnetic instrumentation requires a very sensitive sensor. As the strength of the magnetic field of interest ranges from about 10 fT to 50 pT, the only field sensor having the required sensitivity and small sampling volume is the superconducting quantum interference device (SQUID) e.g. thin-film DC SQUIDs. For transforming the signal from the antenna to the SQUID, a thin-film coupling coil is used. The SQUID itself is shielded and therefore insensitive to external noise. In a five-channel system second-order gradiometers are used in an unshielded environment. The measuring system enables us to balance each channel by means of lead plates without removing it from liquid helium. The lead plates can be handled by a revolver system from outside the dewar. By an iterative balancing procedure inside an artificial uniform field, imbalances less than 10(-4) could be achieved. These results are confirmed by mathematical calculation of mechanical balancing. Sensitivities down to 20 fT Hz-1/2 could be achieved during 'quiet' hours. Another method for the suppression of disturbances is electronic balancing. One of the most important problems in the multichannel system is the cross talk between the single channels. With respect to the geometry of our five-channel device the cross talk coefficient was calculated to be 3.9% and measured at 3.8%.