Localization of the epileptic focus by low-resolution electromagnetic tomography in patients with a lesion demonstrated by MRI.

Patients with medically intractable partial epilepsy and well-defined symptomatic MRI lesions were studied using phase-encoded frequency spectral analysis (PEFSA) combined with low-resolution electromagnetic tomography (LORETA). Ten patients admitted to the epilepsy monitoring unit with MRI-identified lesions and intractable partial epilepsy were studied using 31-electrode scalp EEG. The scalp electrodes were located in three-dimensional space using a magnetic digitizer and coregistered with the patient's MRI. PEFSA was used to obtain a phase-encoded scalp map for the ictal frequencies. The ictal generators were obtained from the scalp map using LORETA. In addition, the generators of interictal epileptogenic spikes were identified using time-domain LORETA. The LORETA generators were rostral to the MRI lesion in 87% (7/8) of patients with temporal lobe lesions, but all were located in the mesial temporal lobe in concordance with the patients' MRI lesions. In patients with frontal lobe epilepsy, the ictal generators at the time that the spectral power was maximal localized to the MRI lesions. Eight of 10 patients had interictal spikes, of which 4 were bilateral independent temporal lobe spikes. Only generators of the interictal spikes that were ipsilateral to seizure onset correlated with the ictal generators. LORETA combined with PEFSA of the ictal discharge can localize ictal EEG discharges accurately and improve correlation with brain anatomy by allowing coregistration of the ictal generator with the MRI. Analysis of interictal spikes was less useful than analysis of the ictal discharge.

Characterization and comparison of local onset and remote propagated electrographic seizures recorded with intracranial electrodes.

PURPOSE: To compared the ictal discharge patterns between local onset and remote propagated electrographic seizures recorded with chronic intracranial electrodes. METHODS: The electrophysiological data from 88 consecutive patients who underwent chronic intracranial EEG monitoring were retrospectively reviewed. The early and late discharge patterns of electrographic seizures at local onset and distant propagated sites were determined by blinded visual inspection and computerized analysis. RESULTS: Four early and three late electrographic seizure patterns were observed at the local onset sites. The four early patterns consisted of a rhythmic discharge in the beta range ("beta buzz"), rhythmic alpha-theta activity, rhythmic sharp waves in the delta range, and an irregular spike discharge. The three distinct late-discharge patterns consisted of a late beta buzz, rhythmic sharp theta activity, and a rhythmic polyspike and wave discharge. At remote propagated sites, electrographic seizures could be divided into two different types according to their early discharge pattern. The first was unique to remote propagated electrographic seizures and consisted of a rhythmic theta-delta activity correlated with the concurrent activity at the local-onset site. The second remote initiation type consisted of patterns indistinguishable from the earlier discharge patterns recorded at the local onset site. CONCLUSIONS: The initial ictal discharge pattern recorded with intracranial electrodes can assist in differentiating local onset and remote propagated electrographic seizures, with rhythmic round theta-delta activity being unique to distant propagated sites. Nevertheless, the initial discharge of a subclass of remote propagated electrographic seizures consists of an independent pattern indistinguishable from that observed at local onset sites.

Subclinical rhythmic electrographic discharges of adults (SREDA) revisited: a study using digital EEG analysis.

Previous descriptions of the subclinical rhythmic electrographic discharges of adults (SREDA) have been based entirely on visual analysis of analog electroencephalographic (EEG) recordings. The introduction of digital electroencephalograms (EEGs) and advances in digital signal processing provide an opportunity to restudy in more depth the nature of SREDA. We identified nine patients who had SREDA diagnosed on a routine EEG recording since the introduction of digital EEG to our laboratory in August 1995. Following careful rereview using standard montages, six of these patients were determined to fulfill the traditional requirements for the diagnosis of SREDA, whereas three were believed to have other benign discharges. Review with Laplacian montages demonstrated that the site of the SREDA activity was maximal in the parietal region or parietocentrotemporal regions, whereas it was maximal in the temporal or frontotemporal regions in the non-SREDA discharges. Frequency analysis, using both the conventional fast Fourier transform (FFT) and time-frequency mapping with the Wigner FFT variant, demonstrated that the SREDA consisted of a complex mixture of multiple rapidly shifting frequencies which showed little spatial and temporal correlation. In contrast, the non-SREDA all consisted of a single dominant well-organized rhythmic frequency spectrum that remained stable throughout space and time.

Spatial filtering of multichannel electroencephalographic recordings through principal component analysis by singular value decomposition.

Principal component analysis (PCA) by singular value decomposition (SVD) may be used to analyze an epoch of a multichannel electroencephalogram (EEG) into multiple linearly independent (temporally and spatially noncorrelated) components, or features; the original epoch of the EEG may be reconstructed as a linear combination of the components. The result of SVD includes the components, expressible as time series waveforms, and the factors that determine how much each component waveform contributes to each EEG channel. By omission of some component waveforms from the linear combination, a new EEG can be reconstructed, differing from the original in useful ways. For example, artifacts can be removed and features such as ictal or interictal discharges can be enhanced by suppressing the remainder of the EEG. We developed a variation of this technique in which the factors that reconstruct the modified EEG from the original are stored as a matrix. This matrix is applied to multichannel EEG at successive times to create a new EEG continuously in real time, without redoing the time-consuming SVD. This matrix acts as a spatial filter with useful properties. We successfully applied this method to remove artifacts, including ocular movement and electrocardiographic artifacts. Removal of myogenic artifacts was much less complete, but there was significant improvement in the ability to visualize underlying activity in the presence of myogenic artifacts. The major limitations of the method are its inability to completely separate some artifacts from cerebral activity, especially when both have similar amplitudes, and the possibility that a spatial filter may distort the distribution of activities that overlap with the artifacts being removed.

Interelectrode coherences from nearest-neighbor and spherical harmonic expansion computation of laplacian of scalp potential.

Interchannel coherence is a measure of spatial extent of and timing relationships among cerebral electroencephalogram (EEG) generators. Interchannel coherence of referentially recorded potentials includes components due to volume conduction and reference site activity. The laplacian of the potential is reference independent and decreases the contribution of volume conduction. Interchannel coherences of the laplacian should, therefore, be less than those of referentially recorded potentials. However, methods used to compute the laplacian involve forming linear combinations of multiple recorded potentials, which may inflate interchannel coherences. WE compared 3 methods of computing the laplacian: (1) modified Hjorth (4 equidistant neighbors to each electrode), (2) Taylor's series (4 nonequidistant neighbors), and (3) spherical harmonic expansion (SHE). Average interchannel coherence introduced by computing the laplacian was less for nearest-neighbor methods (0.0207 +/- 0.0766) but still acceptable for the SHE method (0.0337 +/- 0.0865). Average interchannel coherence for simulated EEG (random data plus a common 10 Hz signal) was less for laplacian than for referential data because of removal of the common referential signal. Interchannel coherences of background EEG and partial seizure activity were less with the laplacian (any method) than with referential recordings. Laplacians calculated from the SHE do not demonstrate excessively large interchannel coherences, as have been reported for laplacians from spherical splines.

Determination of 10-20 system electrode locations using magnetic resonance image scanning with markers.

We determined locations of 33 scalp electrodes used for electroencephalographic (EEG) recording by placing markers in the positions determined by the 10-20 system and performing magnetic resonance image (MRI) scanning on volunteer subjects. Small Vaseline-filled capsules glued on the scalp with collodion produced easily delineated regions of increased signal on standard MRI head images. Measurements of each capsule's coordinates in 3 dimensions were made from MRI scans. A spherical surface was fitted through the marker positions, giving an average radius and an origin (center of sphere). The coordinate axes were rotated to ensure that electrode Cz was on the z-axis and that the y-axis was oriented in the posterior-anterior direction. Two spherical (angular) coordinates were determined for each electrode. Spherical electrode coordinates for different subjects differed by less than 20 degrees in all cases. An average and standard deviation of the spherical coordinates were calculated for each electrode. Standard deviations of several degrees were obtained. The average spherical coordinates obtained were close to those expected on the basis of applying the 10-20 system of placement to an ideal sphere. These measurements provide data necessary for various analyses of EEG performed to help localize epileptic foci.