Effects of stroke on EEG originating from changes on brain tissue electrical activity and geometry

Stroke is the second leading cause of death in the world. Therefore, new and improved diagnosis or treatment methods need to be developed. Nowadays, diagnosis of the stroke is made by using magnetic resonance imaging or computed topography scan. Both methods have an excellent localization and recognition ability, but they are not 100% accurate. In some cases, these imaging methods are able to detect the brain defect within hours or days, based on how bad the injury is. Also, they cannot provide in-line follow up of the patient condition. In this thesis, effects of stroke on electroencephalography (EEG) is evaluated by searching subject related articles from several databases such as Google Scholar and Andor. Background from brain electrical activity, EEG, stroke and its subtypes are given. There are several clinical researches considering on changes on EEG during stroke. However, in most of the cases there were only few patients involved in the study. Therefore, more research must be done, so that the relationship between EEG changes and stroke could be fully understood. Several studies suggest that EEG would add crucial value to the early diagnosis of stroke in the future

The neuroscience of musical creativity using complexity tools

This project is heavily experimental and draws on a wide variety of disciplines from musicology and music psychology to cognitive neuroscience and (neuro)philosophy. The objective is to explore and characterise brain activity during the process of creativity and corroborating this with self-assessments from participants and external assessments from professional “judges”. This three-way experimental design bypasses the semantically difficult task of defining and assessing creativity by asking both participants and judges to rate ‘How creative did you think that was?’. Characterising creativity is pertinent to complexity as it is an opportunity to comprehensively investigate a neural and cognitive system from multiple experimental and analytical facets. This thesis explores the anatomical and functional system underlying the creative cognitive state by analysing the concurrent time series recorded from the brain and furthermore, investigates a model in the stages of creativity using a behavioural experiment, in more detail than hitherto done in this domain. Experimentally, the investigation is done in the domain of music and the time series is the recorded Electroencephalogram (EEG) of a pianist’s whilst performing the two creative musical tasks of ‘Interpretation’ and ‘Improvisation’ manipulations of musical extracts. An initial pilot study consisted of 5 participants being shown 30 musical extracts spanning the Classical soundworld across different rhythms, keys and tonalities. The study was then refined to only 20 extracts and modified to include 10 Jazz extracts and 8 participants from a roughly equal spread of Classical and Jazz backgrounds and gender. 5 external assessors had a roughly even spread of expertise in Jazz and Classical music. Source localisation was performed on the experimental EEG data collected using a software called sLORETA that allows a linear inverse mapping of the electrical activity recorded at the scalp surface onto deeper cortical structures as the source of the recorded activity. Broadman Area (BA) 37 which has previously been linked to semantic processing, was robustly related to participants from a Classical background and BA 7 which has previously been linked to altered states of consciousness such as hypnagogia and sleep, was robustly related to participants from a Jazz background whilst Improvising. Analyses exploring the spread, agreement and biases of ratings across the different judges and self-ratings revealed a judge and participant inter-rater reliability at participant level. There was also an equal agreement between judges when rating the different genres Jazz or Classical, across the different tasks of ‘Improvisation’ and ‘Interpretation’, increasing confidence in inter-genre rating reliability for further analyses on the EEG of the extracts themselves. Furthermore, based on the ratings alone, it was possible to partition participants into either Jazz or Classical, which agreed with phenomenological interview information taken from the participants themselves. With the added conditions of extracts that were deemed creative by objective judge assessment, source localisation analyses pinpointed BA 32 as a robust indicator of Creativity within the participants’ brain. It is an area that is particularly well connected and allows an integration of motoric and emotional communication with a maintenance of executive control. Network analysis was performed using the PLV index (Phase Locking Value) between the 64 electrodes, as the strength of the links in an adjacency matrix of a complex network. This revealed the brain network is significantly more efficient and more strongly synchronised and clustered when participants’ are playing Classical extracts compared to Jazz extracts, in the fronto-central region with a clear right hemispheric lateralization. A behavioural study explored the role of distraction in the ‘Incubation’ period for both interpretation and improvisation using a 2-back number exercise occupying working memory, as the distractor. Analysis shows that a distractor has no significant effect on ‘Improvisation’ but significantly impairs ‘Interpretation’ based on the self-assessments by the participants.Open Acces

Encodage d'un signal audio dans un électroencéphalogramme

Les interfaces cerveau-machine visent à établir un lien de communication entre le cerveau et un système externe à ce dernier. Les électroencéphalogrammes (EEG), dans ce contexte, ont l’avantage d’être non invasifs. Par contre, l’information sensorielle qui se retrouve dans un signal EEG est beaucoup moins ciblée que dans un signal neuronal acquis par une méthode invasive. De plus, étant donné que le cortex auditif est situé dans des repliements du tissu cortical, les neurones qui déchargent, suite à un stimulus auditif, sont parallèles à la surface corticale sur laquelle les EEG sont enregistrés. Par conséquent, l’information auditive qui se retrouve dans le canal EEG situé vis-à-vis du cortex auditif est faible. L’objectif principal de ce projet de recherche consiste donc à étudier la répartition de l’information auditive dans l’ensemble des canaux EEG. Pour ce faire, nous utilisons deux approches. Dans la première, nous tenterons d’estimer l’activité corticale sous-jacente à partir des signaux EEG en utilisant un modèle de couplage bande fréquence. En effet, certaines bandes de fréquences sont des bons prédicteurs des décharges neuronales. Cependant, cette approche n’a pas été validée pour le système auditif, nous confronterons donc l’estimation obtenue à une autre estimation en ayant recours à un modèle spécialisé pour l’encodage du signal de parole faisant appel aux processus ponctuels. Ce modèle prend en compte les dynamiques intrasèques des neurones et également des propriétés spectrotemporelles du stimulus d’entrée. Dans la seconde approche, nous étudierons la possibilité de classifier 3 voyelles (a, i et u) en fonction du nombre de canaux EEG utilisés ainsi que leur répartition sur le cuir chevelu. Nous aurons recours, pour cela, à un réservoir de neurone à décharge récurrent activé en entrée par les données EEG. Les résultats démontrent que l’information auditive se retrouve en fait dans l’ensemble des canaux EEG et qu’elle n’est pas confinée à un nombre restreint d’électrodes. Il est également montré que lorsque l’on utilise les 64 électrodes que comporte l’EEG pour classifier les 3 voyelles, on obtient une classification de l’ordre de 80%, mais aussi qu’un nombre limité de 10 électrodes suffit pour obtenir une classification satisfaisante et, qu’en plus, la position de ces électrodes sur le cuir chevelu est peu importante

Der Einfluss räumlich selektiver Aufmerksamkeit auf die bewusste Wahrnehmung und kortikale Verarbeitung somatosensorischer Reize

Zahlreiche Untersuchungen belegen, dass räumlich selektive Aufmerksamkeit visuelle und auditive Reizverarbeitung beeinflusst. Bestehende Modellvorstellungen sind, aufgrund der geringen Kenntnisse vergleichbarer somatosensorischer Effekte, schwer zu einem allgemeinen Mechanismus generalisieren. Mittels zeitlich-räumlich hoch aufgelöster Messmethoden wurde in dieser Dissertation Effekte räumlich selektiver Aufmerksamkeit auf die bewusste Wahr-nehmung und kortikale Verarbeitung somatosensorischer Reize untersucht. Im Einzelnen wurde gezeigt, dass die räumlich selektive Aufmerksamkeit die Maskierung eines überschwelli-gen Reizes an einer Hand durch einen starken Reiz an der anderen Hand moduliert. Mittels Elektroenzephalografie (EEG) wurde nachgewiesen, dass nach der Stimulation die Verarbei-tung in einem fronto-parietalen Netzwerk den Zugang ins Bewusstsein signalisiert. Der Be-fund einer der bewussten Wahrnehmung zeitlich vorausgehenden neuronalen Desynchronisa-tion im frontalen Kortex und in S1 erlaubt eine Erweiterung bestehender Modellvorstellun-gen. In einer simultanen EEG-funktionelle Magnetresonanztomografie (fMRT) -Studie wurde gezeigt, dass räumlich selektive Aufmerksamkeit die Signalverarbeitung während einer frühen sensorischen Phase der Reizverarbeitung beeinflusst (50 ms). Dieser Effekt korrelierte mit den Blutflußveränderungen in S1. Zusammenfassend zeigen die Studien, dass räumlich selektive Aufmerksamkeit zwar frühe somatosensorische Aktivität in S1 sowie die Wahrnehmung so-matosensorischer Reize moduliert, dies jedoch keine hinreichende Bedingung für die bewusste Wahrnehmung ist. Hingegen ist die attentional kontrollierte Desynchronisation somatosenso-rischer Rhythmen vor der Stimulation, die eine verstärkte fronto-parietale Reizverarbeitung nach sich zieht, hierfür entscheidend.Numerous studies have shown that selective orientation of attention to a stimulus location modulates visual and auditory stimulus processing. Due to the relatively little knowledge about comparable effects of attention in the somatosensory system, existing models can barely be assigned to general cortical mechanisms. The studies conducted in this dissertation should therefore contribute to this knowledge. Effects of spatial selective attention on conscious per-ception and cortical processing of somatosensory stimuli have been investigated by applying recording methods with high temporal and spatial resolutions. Specifically, it was shown that spatial selective attention modulates masking of supra-threshold stimulus on one hand by a strong stimulus applied to the other hand. Using electroencephalography (EEG), it was dem-onstrated that processing in a fronto-parietal network but not early S1-activation signals the entry into conscious perception. The finding of neuronal desynchronisation in the frontal cor-tex and S1 preceding conscious stimulus perception permits the extension of the existing models. With the aim of localizing the temporal effects of spatial selective attention, a simul-taneous EEG-functional magnetic resonance imaging (fMRI)-study was conducted. In con-trast to findings of visual attention, it was shown that orientation of attention enhances soma-tosensory processing at an early stage of stimulus processing (50 ms). This effect correlated with the changes of cortical blood flow in S1. Together, these studies show that spatial-selective attention modulates early activity in S1 as well as conscious perception of somatosen-sory stimuli. Nevertheless, this is not sufficient for an entrance into conscious perception. Instead, attentionally controlled pre-stimulus desynchronisation of somatosensory rhythmic activity, followed by an increased fronto-parietal stimulus processing are necessary prerequi-sites for conscious perception