Objectives: We used a 3-compartment boundary element method (BEM) model from an averaged magnetic resonance image (MRI) data set (Montreal Neurological Institute) in order to provide simple access to realistically shaped volume conductor models for source reconstruction, as compared to individually derived models. The electrode positions were transformed into the model's coordinate system, and the best fit dipole results were transformed back to the original coordinate system. The localization accuracy of the new approach was tested in a comparison with simulated data and with individual BEM models of epileptic spike data from several patients.Methods: The standard BEM model consisted of a total of 4770 nodes, which describe the smoothed cortical envelope, the outside of the skull, and the outside of the skin. The electrode positions were transformed to the model coordinate system by using 3-5 fiducials (nasion, left and right preauricular points, vertex, and inion). The transformation consisted of an averaged scaling factor and a rigid transformation (translation and rotation). The potential values at the transformed electrode positions were calculated by linear interpolation from the stored transfer matrix of the outer BEM compartment triangle net. After source reconstruction the best fit dipole results were transformed back into the original coordinate system by applying the inverse of the first transformation matrix.Results: Test-dipoles at random locations and with random orientations inside of a highly refined reference BEM model were used to simulate noise-free data. Source reconstruction results using a spherical and the standardized BEM volume conductor model were compared to the known dipole positions. Spherical head models resulted in mislocation errors at the base of the brain. The standardized BEM model was applied to averaged and unaveraged epileptic spike data from 7 patients. Source reconstruction results were compared to those achieved by 3 spherical shell models and individual BEM models derived from the individual MRI data sets. Similar errors to that evident with simulations were noted with spherical head models. Standardized and individualized BEM models were comparable.Conclusions: This new approach to head modeling performed significantly better than a simple spherical shell approximation, especially in basal brain areas, including the temporal lobe. By using a standardized head for the BEM setup, it offered an easier and faster access to realistically shaped volume conductor models as compared to deriving specific models from individual 3-dimensional MRI data. q

Summary:Purpose: To determine the area of cortical generators of scalp EEG interictal spikes, such as those in the temporal lobe epilepsy.Methods: We recorded simultaneously 26 channels of scalp EEG with subtemporal supplementary electrodes and 46 to 98 channels of intracranial EEG in 16 surgery candidates with temporal lobe epilepsy. Cerebral discharges with and without scalp EEG correlates were identified, and the area of cortical sources was estimated from the number of electrode contacts demonstrating concurrent depolarization.Results: We reviewed ∼600 interictal spikes recorded with intracranial EEG. Only a very few of these cortical spikes were associated with scalp recognizable potentials; 90% of cortical spikes with a source area of >10 cm 2 produced scalp EEG spikes, whereas only 10% of cortical spikes having <10 cm 2 of source area produced scalp potentials. Intracranial spikes with <6 cm 2 of area were never associated with scalp EEG spikes.Conclusions: Cerebral sources of scalp EEG spikes are larger than commonly thought. Synchronous or at least temporally overlapping activation of 10-20 cm 2 of gyral cortex is common. The attenuating property of the skull may actually serve a useful role in filtering out all but the most significant interictal discharges that can recruit substantial surrounding cortex.

Summary: Purpose: To determine the intracranial EEG features responsible for producing the various ictal scalp rhythms, which we previously identified in a new EEG classification for temporal lobe seizures.Methods: In 24 patients, we analyzed simultaneous intracranial and surface ictal EEG recordings (64 total channels) obtained from a combination of intracerebral depth, subdural strip, and scalp electrodes.Results: Four of four patients with Type 1 scalp seizure patterns had mesial temporal seizure onsets. However, discharges confined to the hippocampus produced no scalp EEG rhythms. The regular 5-to 9-Hz subtemporal and temporal EEG pattern of Type l a seizures required the synchronous recruitment of adjacent inferolateral temporal neocortex. Seizure discharges confined to the mesiobasal temporal cortex produced a vertex dominant rhythm (Type lc) due to the net vertical orientation of dipolar sources located there. Ten of 13 patients with Type 2 seizures had inferolateral or lateral, temporal neocortical seizure onsets. Initial cerebral ictal activity was typically a focal or regional, low voltage, fast rhythm (20-40 Hz) that was often associated with widespread background flattening. Only an attenuation of normal rhythms was reflected in scalp electrodes. Irregular 2-to 4-Hz cortical ictal rhythms that commonly followed resulted in a comparably slow and irregular scalp EEG pattern (Type 2a). Type 2C seizures showed regional, periodic, 1-to 4-Hz sharp waves following intracranial seizure onset. Seven patients had Type 3 scalp seizures, which were characterized by diffuse slowing or attenuation of background scalp EEG activity. This resulted when seizure activity was confined to the hippocampus, when there was rapid seizure propagation to the contralateral temporal lobe, or when cortical ictal activity failed to achieve widespread synchrony.Conclusions: Type 1, 2, and 3 scalp EEG patterns of temporal lobe seizures are not a reflection of cortical activity at seizure onset. Differences in the subsequent development, propagation, and synchrony of cortical ictal discharges produce the characteristic scalp EEG rhythms. Key Words: EEG (intracrania1)-ECoG-EpilepsySeizures-Temporal lobe.Several studies report ictal scalp EEG to be reliable in localizing the side of seizure origin in patients with temporal lobe epilepsy being evaluated for surgery (1,2). However, scalp EEG patterns are usually not considered to be specific enough to localize seizure onsets to sublobar regions within the temporal lobe. The distinction between seizures that arise from mesial temporal structures versus lateral temporal cortex, for example, is clinically important, because patients with the former will often have their seizures eliminated by standard temporal lobectomies, whereas patients with neocortical disease may need invasive monitoring to tailor a resection (3).Accepted January 24, 1997. Address correspondence and reprint requests to Dr. J. S. Ebersole at Neurology Service, VA Medical Center, West Haven, CT 06516, U.S.A.We developed ...

Identifying patients whose complex partial seizures originate in temporal neocortex rather than in hippocampus is important because such patients have less favorable outcomes with standard anteromesial temporal resections. We reviewed scalp-recorded ictal EEGs of 93 epilepsy surgery candidates who either underwent intracranial EEG monitoring (n = 58) or who were referred directly for temporal lobectomy (n = 35). We definded seven patterns of early seizure discharges, grouped patients according to their seizure pattern, and correlated these with the site of seizure onset determined by intracranial EEG. Categorization by seizure pattern was also compared with brain magnetic resonance imaging (MRI) findings intracarotid amobarbital (Wada) testing. An initial, regular 5- to 9- Hz inferotemporal rhythm (type 1A) was most specific for hippocampal-onset seizures. Less commonly, a similar vertex/parasagittal positive rhythm (type 1B) or a combination of types 1B and 1A rhythms (type 1C) was recorded. Seizures originating in temporal neocortex were most often associated with irregular, polymorphic, 2- to 5-Hz lateralized activity (type 2A). This pattern was commonly followed by a type 1A theta rhythm (type 2B) or was preceded by repetitive, sometimes periodic, sharp waves (type 2C). Seizures without a clear lateralized EEG discharge (type 3) were most commonly of temporal neocortical origin. These associations between type of seizure pattern and probable site of cerebral origin were statistically significant. MRI and Wada testing did not have as much specificity as ictal patterns in differentiating among seizure origins. We conclude that the initial pattern of ictal discharge on scalp EEG can assist in distinguishing seizures of temporal neocortical onset from those of hippocampal onset. This information can be used to identify patients for invasive monitoring.