Magnetoencephalographic mapping of interictal spike propagation: a technical and clinical report.

Distinguishing propagated epileptic activity from primary epileptic foci is of critical importance in presurgical evaluation of patients with medically intractable focal epilepsy. We studied an 11-year-old patient with complex partial epilepsy by using simultaneous magnetoencephalography (MEG) and electroencephalography (EEG). In EEG, bilateral interictal discharges appeared synchronous, whereas MEG source analysis suggested propagation of spikes from the right to the left frontal lobe.

The value of multichannel MEG and EEG in the presurgical evaluation of 70 epilepsy patients.

OBJECTIVE: To evaluate the sensitivity of a simultaneous whole-head 306-channel magnetoencephalography (MEG)/70-electrode EEG recording to detect interictal epileptiform activity (IED) in a prospective, consecutive cohort of patients with medically refractory epilepsy that were considered candidates for epilepsy surgery. METHODS: Seventy patients were prospectively evaluated by simultaneously recorded MEG/EEG. All patients were surgical candidates or were considered for invasive EEG monitoring and had undergone an extensive presurgical evaluation at a tertiary epilepsy center. MEG and EEG raw traces were analysed individually by two independent reviewers. RESULTS: MEG data could not be evaluated due to excessive magnetic artefacts in three patients (4%). In the remaining 67 patients, the overall sensitivity to detect IED was 72% (48/67 patients) for MEG and 61% for EEG (41/67 patients) analysing the raw data. In 13% (9/67 patients), MEG-only IED were recorded, whereas in 3% (2/67 patients) EEG-only IED were recorded. The combined sensitivity was 75% (50/67 patients). CONCLUSION: Three hundred and six-channel MEG has a similarly high sensitivity to record IED as EEG and appears to be complementary. In one-third of the EEG-negative patients, MEG can be expected to record IED, especially in the case of lateral neocortical epilepsy and/or cortical dysplasia.

Spatiotemporal activity of a cortical network for processing visual motion revealed by MEG and fMRI.

A sudden change in the direction of motion is a particularly salient and relevant feature of visual information. Extensive research has identified cortical areas responsive to visual motion and characterized their sensitivity to different features of motion, such as directional specificity. However, relatively little is known about responses to sudden changes in direction. Electrophysiological data from animals and functional imaging data from humans suggest a number of brain areas responsive to motion, presumably working as a network. Temporal patterns of activity allow the same network to process information in different ways. The present study in humans sought to determine which motion-sensitive areas are involved in processing changes in the direction of motion and to characterize the temporal patterns of processing within this network of brain regions. To accomplish this, we used both magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI). The fMRI data were used as supplementary information in the localization of MEG sources. The change in the direction of visual motion was found to activate a number of areas, each displaying a different temporal behavior. The fMRI revealed motion-related activity in areas MT+ (the human homologue of monkey middle temporal area and possibly also other motion sensitive areas next to MT), a region near the posterior end of the superior temporal sulcus (pSTS), V3A, and V1/V2. The MEG data suggested additional frontal sources. An equivalent dipole model for the generators of MEG signals indicated activity in MT+, starting at 130 ms and peaking at 170 ms after the reversal of the direction of motion, and then again at approximately 260 ms. Frontal activity began 0-20 ms later than in MT+, and peaked approximately 180 ms. Both pSTS and FEF+ showed long-duration activity continuing over the latency range of 200-400 ms. MEG responses in the region of V3A and V1/V2 were relatively small, and peaked at longer latencies than the initial peak in MT+. These data revealed characteristic patterns of activity in this cortical network for processing sudden changes in the direction of visual motion.

Parieto-occipital approximately 10 Hz activity reflects anticipatory state of visual attention mechanisms.

High-density eeg recordings revealed sensory specific modulation of anticipatory parieto-occipital approximately 10 Hz oscillatory activity when visually presented word cues instructed subjects in an intermodal selective attention paradigm. Cueing attention to the auditory features of imminent compound audio-visual stimuli resulted in significantly higher approximately 10 Hz amplitude in the period preceding onset of this stimulus than when attention was cued to the visual features. We propose that this parieto-occipital approximately 10 Hz activity reflects a disengaged visual attentional system in preparation for anticipated auditory input that is attentionally more relevant. Conversely, lower approximately 10 Hz activity during the attend-visual condition may reflect active engagement of parieto-occipital areas in the anticipatory period. These results support models implicating parieto-occipital areas in the directing and maintenance of visual attention.

Dynamic neuroimaging of brain function.

To fully characterize the brain processes underlying sensorimotor and cognitive function, the spatial distribution of active regions, their interconnected regions must be measured. We describe methods for imaging brain sources from surface-recorded EEG and magnetoencephalographic data, called electromagnetic source imaging (EMSI). EMSI provides brain source locations within the common framework of magnetic resonance (MR) images of brain anatomy. This allows integration of data from other functional brain imaging methods, like positron emission tomography and functional MR imaging, which can improve the accuracy of EMSI localization. EMSI also provides submillisecond temporal resolution of the dynamic processes within brain systems. Examples are given of applications to visual perceptual and attentional studies.

The effect of stimulation rate on the signal-to-noise ratio of evoked responses.

The signal-to-noise ratio (SNR) in averaged evoked responses is proportional to the signal amplitude and to the square root of the stimulation frequency. If the SNR-stimulation-rate dependence is known for some specified component or feature of the response it is possible to select a rate that maximizes the SNR of that component within a given measurement time. The same stimulation rate also minimizes the acquisition time for a given SNR.

Estimates of visually evoked cortical currents.

We have measured magnetic fields evoked by the onset of checkerboard-like sectorial patterns presented at 16 locations near the center of the visual field. Small stimuli (less than 2 degrees), which, nevertheless, gave sufficiently strong responses to enable source localization, were used to limit cortical activation to a small area, thus simplifying the analysis of the magnetic field data. We focused on optimizing the experimental design: cortical sources could be located from measurements at just one position of our 24-channel magnetometer and with as few as 15-20 repetitions of the stimulus. Minimum-norm-estimate maps calculated from even a single response showed reproducible features of the current distribution, which was, 80-100 msec after the pattern onset, retinotopically organized in the occipital lobe. Since magnetoencephalography can reveal cortical locations with a precision of 2-3 mm, our procedure appears promising for further studies of cortical retinotopy and visual field defects.