Time course of focal slow wave activity in transient ischemic attacks and transient global amnesia as measured by magnetoencephalography

In this longitudinal study multichannel MEG was used to localize and to quantify focal pathological spontaneous neuromagnetic activity in six patients with transient ischemic attacks (TIA) and two patients with transient global amnesia (TGA). Slow (2-6 Hz) and beta (14-30 Hz) activity were monitored up to 10 weeks. Results were compared with normative data, and changes over time were statistically analyzed. MEG detected pathological activity that persisted clinical symptoms. Focal slow activity originating from sensorimotor (TIA) and mesiotemporal (TGA) cortices exceeded normal values up to 14 times during the first hours after the attack and recovered to normal within 11 days. Focal beta activity was not useful to monitor the time course of TIA or TGA.

Focal slow and beta brain activity in patients with multiple sclerosis revealed by magnetoencephalography

In multiple sclerosis (MS) inflammatory infiltrations cause white matter lesions. Magnetoencephalography (MEG) offers the opportunity to localize abnormal electric activity of neurons with a high spatio-temporal resolution. In this study, we investigated patients with MS in order to find if abnormal cortical activity is associated with (subcortical) MS lesions using simultaneous bilateral recording of biomagnetic activity. Eight patients suffering from definite laboratory-supported MS with mainly somatosensory deficits and multiple bihemispheric plaques revealed by MRI were included in the study. To obtain normative data, 8 healthy volunteers were investigated following the same measuring protocol. Spontaneous magnetic brain activity was recorded using a 2x37-channel biomagnetic system (BTI, USA). Offline analysis included digital filtering (to separately investigate slow and beta wave activity), a Principle Component Analysis and the Dipole Density Plot. Localization results were inserted into MR images using our contour fit procedure. The dipole distribution in the brain was quantified and compared between the groups by statistical analysis. In all MS patients, the maximum of focal abnormal activity was localized in cortical areas adjacent to the fiber lesions. In the healthy subjects, no focal abnormal brain activity could be found. However, the standardized maximum concentrations of dipoles were significantly higher in the MS patients than in the healthy control group both in the slow and in the beta wave analysis. These results let assume that subcortical lesions can occur together with abnormal cortical neuronal activity. The results are discussed in respect to their impact on the interpretation of the analysis of spontaneous magnetic brain activity.

Motor, somatosensory and auditory cortex localization by fMRI and MEG

Functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) were performed in six subjects during self-paced finger movement performance, tactile somatosensory stimulation and binaural auditory stimulation using identical stimulation paradigms. Both functional imaging modalities localized brain activity in adjacent areas of anatomically correct cortex. The mean distances measured between fMRI activity and the corresponding MEG dipoles were 10.1 mm (motor), 10.7 mm (somatosensory), 13.5 mm (auditory right hemisphere) and 14.3 mm (auditory left hemisphere). The distances found may reflect the correlation between electrophysiological and hemodynamic responses due to the different underlying substrates of neurophysiology measured by fMRI and MEG: BOLD contrast vs neuronal biomagnetic activity.

Sources of spontaneous slow waves associated with brain lesions, localized by using the MEG

Electric or magnetic slow wave brain activity can be associated with brain lesions. For an accurate source localization we transformed the magnetoencephalographic (MEG) coordinate system to the magnetic resonance imaging (MRI) system by using a surface fit of the digitally measured head surface and the reconstructed surface of the MRI scan. Furthermore we solved the problem to separate sources of focal activity from other multiple sources by introducing a spatial average, the Dipole Density Plot (DDP). The DDP shows in a quantified manner concentrations of dipoles across time. The DDP uses the single dipole model adequately, because only those signal sections will be analyzed, where one component contributes to the signal predominantly. In all cases, where multiple sources concurrently active are to be localized, a current distribution analysis will be used, the Current Localization by Spatial Filtering (CLSF). All source localization procedures were tested using structural brain lesions, which were verified by imaging techniques (MRI or CT), showing the results in close topographical relation to the lesions. The results so far let us assume, that the DDP and the CLSF are valuable tools to localize sources of focal spontaneous slow wave electrical brain activity.