Interictal spike localization using a standard realistic head model: simulations and analysis of clinical data.

OBJECTIVES: In this paper realistic and standard realistic head models were applied to neural source localization. METHODS: Three different triangulated head structures; the brain, the skull and the scalp were constructed from MRI information of each patient. For each subject the exact positions of the electrodes were digitized. RESULTS: The influence of the number of triangles and of the skull conductivity on the accuracy of the method was tested. The use of a standard realistic head model instead of spherical models is proposed in cases where detailed MRI information is not available, and the accuracy of this procedure is tested with dipole simulations. These techniques were applied also to EEG signals from 3 patients with focal epilepsy. In all cases the neural activity was assumed to be confined to a small portion of cortical tissue, so that the neural generator was approximated to a current dipole. The realistic head model localization is discussed on the basis of neuroimaging information. CONCLUSIONS: We show that the standard realistic head model is two or 3 times better than the spherical model for dipole localization and we propose it as a good alternative to the spherical model for EEG data processing, in cases where full MRI information is not available.

Correlation dimension maps of EEG from epileptic absences.

PURPOSE: The understanding of brain activity, and in particular events such as epileptic seizures, lies on the characterisation of the dynamics of the neural networks. The theory of non-linear dynamics provides signal analysis techniques which may give new information on the behaviour of such networks. METHODS: We calculated correlation dimension maps for 19-channel EEG data from 3 patients with a total of 7 absence seizures. The signals were analysed before, during and after the seizures. Phase randomised surrogate data was used to test chaos. RESULTS: In the seizures of two patients we could distinguish two dynamical regions on the cerebral cortex, one that seemed to exhibit chaos whereas the other seemed to exhibit noise. The pattern shown is essentially the same for seizures triggered by hyperventilation, but differ for seizures triggered by light flashes. The chaotic dynamics that one seems to observe is determined by a small number of variables and has low complexity. On the other hand, in the seizures of another patient no chaotic region was found. Before and during the seizures no chaos was found either, in all cases. CONCLUSIONS: The application of non-linear signal analysis revealed the existence of differences in the spatial dynamics associated to absence seizures. This may contribute to the understanding of those seizures and be of assistance in clinical diagnosis.

[Localization of brain electric sources in patients with focal epilepsy]. / Localização de fontes eléctricas cerebrais em doentes com epilepsia focal.

In this paper we discuss a non-invasive method to localize neural electrical sources using EEG data. In this method, the human head is modelled by a set of four concentric spheres with different conductivities which represent the scalp, the skull, the CSF and the brain or by three triangulated surfaces which approximate the exact head shape (in this model we do not consider the CSF layer) using NMR images. In this case the computer effort is very high, since the calculations imply thousands of equations. Therefore, the number of research groups working with this improved model, in the world, is very small. In both models, we assume that the neural source is a current dipole. This makes the model suitable for cases where the active brain areas are limited and localized. We discuss some error factors associated with the method, as the geometry of the head, the conductivity of the different layers and the number of electrodes used in the EEG measurements. Comparing the more realistic head model, with the spherical one we often have differences of 1-2 cm. However, we can reach even more pronounced differences in the frontal areas. Concerning the skull conductivity, we realized that it could introduce errors of 1-2 cm. We observed that at least 50 electrodes should be used only since 21 electrodes could imply errors of about 0.5 cm. The method was applied, both in the spherical version and the realistic one, to clinical cases of focal epileptic patients. The results are discussed in terms of the other clinical information available and they are coherent with the remaining clinical data.

Kluver-Bucy syndrome in systemic lupus erythematosus.

The Kluver-Bucy syndrome has not been previously reported as a complication of systemic lupus erythematosus (SLE). A 48-year-old female is described who sustained several cerebral infarcts, some of which were bitemporal, due to SLE vasculopathy. She developed a complex behavioural picture consisting of global aphasia, left-side neglect, hyperorality, hypermetamorphosis and hypersexuality. She displayed appropriate emotional reaction to visually presented objects, indicating that her Kluver-Bucy syndrome could not be explained by lack of visual recognition.