Detection and Magnetic Source Imaging of Fast Oscillations (40–160 Hz) Recorded with Magnetoencephalography in Focal Epilepsy Patients

We present a framework to detect fast oscillations (FOs) in magnetoencephalography (MEG) and to perform magnetic source imaging (MSI) to determine the location and extent of their generators in the cortex. FOs can be of physiologic origin associated to sensory processing and memory consolidation. In epilepsy, FOs are of pathologic origin and biomarkers of the epileptogenic zone. Seventeen patients with focal epilepsy previously confirmed with identified FOs in scalp electroencephalography (EEG) were evaluated. To handle data deriving from large number of sensors (275 axial gradiometers) we used an automatic detector with high sensitivity. False positives were discarded by two human experts. MSI of the FOs was performed with the wavelet based maximum entropy on the mean method. We found FOs in 11/17 patients, in only one patient the channel with highest FO rate was not concordant with the epileptogenic region and might correspond to physiologic oscillations. MEG FOs rates were very low: 0.02–4.55 per minute. Compared to scalp EEG, detection sensitivity was lower, but the specificity higher in MEG. MSI of FOs showed concordance or partial concordance with proven generators of seizures and epileptiform activity in 10/11 patients. We have validated the proposed framework for the non-invasive study of FOs with MEG. The excellent overall concordance with other clinical gold standard evaluation tools indicates that MEG FOs can provide relevant information to guide implantation for intracranial EEG pre-surgical evaluation and for surgical treatment, and demonstrates the important added value of choosing appropriate FOs detection and source localization methods.Facultad de IngenieríaInstituto de Investigaciones en Electrónica, Control y Procesamiento de Señale

Detection and Magnetic Source Imaging of Fast Oscillations (40–160 Hz) Recorded with Magnetoencephalography in Focal Epilepsy Patients

We present a framework to detect fast oscillations (FOs) in magnetoencephalography (MEG) and to perform magnetic source imaging (MSI) to determine the location and extent of their generators in the cortex. FOs can be of physiologic origin associated to sensory processing and memory consolidation. In epilepsy, FOs are of pathologic origin and biomarkers of the epileptogenic zone. Seventeen patients with focal epilepsy previously confirmed with identified FOs in scalp electroencephalography (EEG) were evaluated. To handle data deriving from large number of sensors (275 axial gradiometers) we used an automatic detector with high sensitivity. False positives were discarded by two human experts. MSI of the FOs was performed with the wavelet based maximum entropy on the mean method. We found FOs in 11/17 patients, in only one patient the channel with highest FO rate was not concordant with the epileptogenic region and might correspond to physiologic oscillations. MEG FOs rates were very low: 0.02–4.55 per minute. Compared to scalp EEG, detection sensitivity was lower, but the specificity higher in MEG. MSI of FOs showed concordance or partial concordance with proven generators of seizures and epileptiform activity in 10/11 patients. We have validated the proposed framework for the non-invasive study of FOs with MEG. The excellent overall concordance with other clinical gold standard evaluation tools indicates that MEG FOs can provide relevant information to guide implantation for intracranial EEG pre-surgical evaluation and for surgical treatment, and demonstrates the important added value of choosing appropriate FOs detection and source localization methods.Facultad de IngenieríaInstituto de Investigaciones en Electrónica, Control y Procesamiento de Señale

Technique de parcellisation et de localisation des sources cérébrales à partir des signaux MEG

Ce travail consiste au développement d'une technique de résolution du problème inverse concernant la localisation de l'activité neuro-cérébrale à partir des données MEG (Magnétoencéphalographie). Cette technique se base sur la parcellisation sélective des sources du modèle corticale afin d'améliorer la régularisation du problème inverse, ensuite procéder à sa résolution. La parcellisation sélective est basée sur deux principes; la pré-localisation des sources et la formation des parcelles à partir des sources sélectionnées. Pour la pré-localisation des sources on exploite la technique MSP (Multivariate Sources Prelocalisation) qui permet d'estimer le degré de l'activation des sources. Pour la sélection des sources on utilise la technique de test d'hypothèse FDR (False Discovery Rate) qui permet de définir un seuil de sélection sur les informations de pré-localisation des sources. Après la parcellisation sélective, on procède à la résolution du problème inverse en exploitant la technique LCMV (Linearly Constrained Minimum Variance) adaptée pour un filtrage spatial sur des parcelles. Les résultats obtenus montrent que cette technique permet de localiser les sources avec une bonne sensibilité et spécificité. Il est aussi important de mentionner que cette technique est modulaire; ainsi, on peut exploiter un module séparément des autres. Par exemple, on peut combiner la technique de parcellisation sélective avec la technique MEM (Maximum Entropy Method) (Amblard C 2004)

Solution logicielle pour la localisation de l'activité cérébrale à partir de mesures MEG/EEG conjointes

Si l’instrumentation a progressé, les méthodes de localisation de l’activité cérébrale sont encore du domaine de la recherche. Dans le cadre de la neuro-imagerie, le travail de ce mémoire permet de calculer et visualiser des estimations de l’activité cérébrale en fonction des mesures EEG (électroencéphalographie) et MEG (magnétoencéphalographie). Ces deux technologies mesurent respectivement le potentiel électrique sur le crâne et le champs magnétique produit par la matière grise en activité. Les deux modalités (EEG MEG) se modélisent de la même façon avec, d’un côté l’espace des sources, représentant le cortex, et d’un autre côté l’espace des mesures enregistrées par des capteurs. Ces deux espaces ne sont pas indépendants et sont reliés par ce que l’on appelle une matrice de gain. Une difficulté majeure est d’obtenir une matrice de gain fiable qui modélise le processus bioélectromagnétique de la tête. L’activation des sources vers les capteurs transite par la matrice de gain dans le processus nommé le problème direct. Vice et versa, à partir des mesures cette matrice servira à estimer l’activité cérébrale dans le problème inverse. Ce mémoire utilise une modélisation des tissus biologiques de la tête en utilisant la méthode BEM (Boundary Elements Method) pour le calcul de la matrice de gain. Un logiciel MATLAB, programmé en s’inspirant du langage objet, utilise deux méthodes de résolution du problème inverse. Un simulateur y est aussi intégré. Au travers de ce travail, il sera détaillé des solutions pour reconstruire l’activité cérébrale en fonction des mesures EEG, MEG ou les deux combinées

Radiant Sites: Projection and the Mobile Spectator in Contemporary Moving-Image Installations

This dissertation examines contemporary moving-image installations that use projected images to expand and elaborate upon the cinematic experience. It focuses on works by Douglas Gordon (b. 1966), Jim Campbell (b. 1956), and the partnered artists Janet Cardiff (b. 1957) and George Bures-Miller (b. 1960), all of whom have reconfigured the classical cinematic system of viewing since the 1990s. Through their works, I trace the term “expanded cinema” as a literal extension of projected light from the screen into the open gallery and beyond. I argue that the term “projection” – as thrown light, mental anticipation, and moving bodies – brings together cinema’s apparatus, text, and reception as a cohesive experience. These artists transport their light-based images to the gallery, exposing the projected image to mobile spectators, as well as to lighting conditions less conducive to a clear picture. However, the works I will discuss also maintain an explicit connection to the theatrical projection of narrative film. As these artists expand the exhibition spaces of cinema from theater to gallery, they also converge numerous cinematic formats, including celluloid film, magnetic videotape, digital video, still photography, and dynamic audio. I offer “projection” as a term which tethers the myriad trajectories of cinema’s expansion back to its apparatus, and even to the mobile spectator. Beyond the light phenomenon, I also draw from psychoanalytic theories of projection, especially relevant given its foundational contribution to film theory since the 1970s by authors such as Laura Mulvey, Christian Metz, and Mary Ann Doane. Furthermore, the “suture” theory of Kaja Silverman and others offers a link between classical Hollywood editing conventions and the spatial orientation of the gallery spectator. From these theorists, I ultimately offer a notion of the projecting viewer, a physically active version of the “embodied viewer” as conceived by phenomenological film theorist Vivian Sobchack. By observing their similarities to the common multiplex, art house, or even living rooms, these otherwise uprooted screens reveal in fleeting flickers traces of what we might intuitively call “the cinematic.