Fully Complex Magnetoencephalography

Complex numbers appear naturally in biology whenever a system can be analyzed in the frequency domain, such as physiological data from magnetoencephalography (MEG). For example, the MEG steady state response to a modulated auditory stimulus generates a complex magnetic field for each MEG channel, equal to the Fourier transform at the stimulus modulation frequency. The complex nature of these data sets, often not taken advantage of, is fully exploited here with new methods. Whole-head, complex magnetic data can be used to estimate complex neural current sources, and standard methods of source estimation naturally generalize for complex sources. We show that a general complex neural vector source is described by its location, magnitude, and direction, but also by a phase and by an additional perpendicular component. We give natural interpretations of all the parameters for the complex equivalent-current dipole by linking them to the underlying neurophysiology. We demonstrate complex magnetic fields, and their equivalent fully complex current sources, with both simulations and experimental data.Comment: 23 pages, 1 table, 5 figures; to appear in Journal of Neuroscience Method

Dissociable Influences of Auditory Object vs. Spatial Attention on Visual System Oscillatory Activity

Given that both auditory and visual systems have anatomically separate object identification (“what”) and spatial (“where”) pathways, it is of interest whether attention-driven cross-sensory modulations occur separately within these feature domains. Here, we investigated how auditory “what” vs. “where” attention tasks modulate activity in visual pathways using cortically constrained source estimates of magnetoencephalograpic (MEG) oscillatory activity. In the absence of visual stimuli or tasks, subjects were presented with a sequence of auditory-stimulus pairs and instructed to selectively attend to phonetic (“what”) vs. spatial (“where”) aspects of these sounds, or to listen passively. To investigate sustained modulatory effects, oscillatory power was estimated from time periods between sound-pair presentations. In comparison to attention to sound locations, phonetic auditory attention was associated with stronger alpha (7–13 Hz) power in several visual areas (primary visual cortex; lingual, fusiform, and inferior temporal gyri, lateral occipital cortex), as well as in higher-order visual/multisensory areas including lateral/medial parietal and retrosplenial cortices. Region-of-interest (ROI) analyses of dynamic changes, from which the sustained effects had been removed, suggested further power increases during Attend Phoneme vs. Location centered at the alpha range 400–600 ms after the onset of second sound of each stimulus pair. These results suggest distinct modulations of visual system oscillatory activity during auditory attention to sound object identity (“what”) vs. sound location (“where”). The alpha modulations could be interpreted to reflect enhanced crossmodal inhibition of feature-specific visual pathways and adjacent audiovisual association areas during “what” vs. “where” auditory attention

Decreased event-related theta power and phase-synchrony in young binge drinkers during target detection: An anatomically-constrained MEG approach

Background: The prevalence of binge drinking (BD) has been on the rise in recent years. It is associated with a range of neurocognitive deficits among adolescents and young emerging adults who are especially vulnerable to alcohol use. Attention is an essential dimension of executive functioning and attentional disturbances may be associated with hazardous drinking. The aim of the study was to examine the oscillatory neural dynamics of attentional control during visual target detection in emerging young adults as a function of BD. Method: Fifty-one first-year university students (18±0.6 years) were assigned to light drinking (LD, N=26), and BD (N=25) groups based on their alcohol consumption patterns. High-density magnetoencephalography (MEG) signal was combined with structural magnetic resonance imaging (MRI) in an anatomically-constrained MEG model to estimate event-related source power in theta (4-7 Hz) frequency band. Phase-locked co-oscillations were further estimate between the principally activated regions during task performance. Results: Overall, the greatest event-related theta power was elicited by targets in the right inferior frontal cortex (rIFC) and it correlated with performance accuracy and selective attention scores. BDs exhibited lower theta power and dysregulated oscillatory synchrony to targets in the rIFC which correlated with higher levels of alcohol consumption. Conclusions: These results confirm that a highly interactive network in the rIFC subserves attentional control, revealing the importance of theta oscillations and neural synchrony for attentional capture and contextual maintenance. Attenuation of theta power and synchronous interactions in BDs may indicate early stages of suboptimal integrative processing in young, highly functioning BDsThis study was supported by the projects SPI/2010/134 and SPI/2010/051 from the Spanish Ministry of Health and Social Politics (National Plan of Drugs), and the National Institutes of Health, US, (R01-AA016624)S

The impact of MEG source reconstruction method on source-space connectivity estimation: A comparison between minimum-norm solution and beamforming.

Despite numerous important contributions, the investigation of brain connectivity with magnetoencephalography (MEG) still faces multiple challenges. One critical aspect of source-level connectivity, largely overlooked in the literature, is the putative effect of the choice of the inverse method on the subsequent cortico-cortical coupling analysis. We set out to investigate the impact of three inverse methods on source coherence detection using simulated MEG data. To this end, thousands of randomly located pairs of sources were created. Several parameters were manipulated, including inter- and intra-source correlation strength, source size and spatial configuration. The simulated pairs of sources were then used to generate sensor-level MEG measurements at varying signal-to-noise ratios (SNR). Next, the source level power and coherence maps were calculated using three methods (a) L2-Minimum-Norm Estimate (MNE), (b) Linearly Constrained Minimum Variance (LCMV) beamforming, and (c) Dynamic Imaging of Coherent Sources (DICS) beamforming. The performances of the methods were evaluated using Receiver Operating Characteristic (ROC) curves. The results indicate that beamformers perform better than MNE for coherence reconstructions if the interacting cortical sources consist of point-like sources. On the other hand, MNE provides better connectivity estimation than beamformers, if the interacting sources are simulated as extended cortical patches, where each patch consists of dipoles with identical time series (high intra-patch coherence). However, the performance of the beamformers for interacting patches improves substantially if each patch of active cortex is simulated with only partly coherent time series (partial intra-patch coherence). These results demonstrate that the choice of the inverse method impacts the results of MEG source-space coherence analysis, and that the optimal choice of the inverse solution depends on the spatial and synchronization profile of the interacting cortical sources. The insights revealed here can guide method selection and help improve data interpretation regarding MEG connectivity estimation

MEG language mapping using a novel automatic ECD algorithm in comparison with MNE, dSPM, and DICS beamformer

IntroductionThe single equivalent current dipole (sECD) is the standard clinical procedure for presurgical language mapping in epilepsy using magnetoencephalography (MEG). However, the sECD approach has not been widely used in clinical assessments, mainly because it requires subjective judgements in selecting several critical parameters. To address this limitation, we developed an automatic sECD algorithm (AsECDa) for language mapping.MethodsThe localization accuracy of the AsECDa was evaluated using synthetic MEG data. Subsequently, the reliability and efficiency of AsECDa were compared to three other common source localization methods using MEG data recorded during two sessions of a receptive language task in 21 epilepsy patients. These methods include minimum norm estimation (MNE), dynamic statistical parametric mapping (dSPM), and dynamic imaging of coherent sources (DICS) beamformer.ResultsFor the synthetic single dipole MEG data with a typical signal-to-noise ratio, the average localization error of AsECDa was less than 2 mm for simulated superficial and deep dipoles. For the patient data, AsECDa showed better test-retest reliability (TRR) of the language laterality index (LI) than MNE, dSPM, and DICS beamformer. Specifically, the LI calculated with AsECDa revealed excellent TRR between the two MEG sessions across all patients (Cor = 0.80), while the LI for MNE, dSPM, DICS-event-related desynchronization (ERD) in the alpha band, and DICS-ERD in the low beta band ranged lower (Cor = 0.71, 0.64, 0.54, and 0.48, respectively). Furthermore, AsECDa identified 38% of patients with atypical language lateralization (i.e., right lateralization or bilateral), compared to 73%, 68%, 55%, and 50% identified by DICS-ERD in the low beta band, DICS-ERD in the alpha band, MNE, and dSPM, respectively. Compared to other methods, AsECDa’s results were more consistent with previous studies that reported atypical language lateralization in 20-30% of epilepsy patients.DiscussionOur study suggests that AsECDa is a promising approach for presurgical language mapping, and its fully automated nature makes it easy to implement and reliable for clinical evaluations

Reti complesse e analisi del segnale elettroencefalografico

The identification of subject-specific traits extracted from patterns of brain activity still represents an important challenge. The need to detect distinctive brain features, which is relevant for biometric and brain computer interface systems, has been also emphasized in monitoring the effect of clinical treatments and in evaluating the progression of brain disorders. Graph theory and network science tools have revealed fundamental mechanisms of functional brain organization in resting-state M/EEG analysis. Nevertheless, it is still not clearly understood how several methodological aspects may bias the topology of the reconstructed functional networks. In this context, the literature shows inconsistency in the chosen length of the selected epochs, impeding a meaningful comparison between results from different studies. In this study we propose an approach which aims to investigate the existence of a distinctive functional core (sub-network) using an unbiased reconstruction of network topology. Brain signals from a public and freely available EEG dataset were analyzed using a phase synchronization based measure, minimum spanning tree and k-core decomposition. The analysis was performed for each classical brain rhythm separately. Furthermore, we aim to provide a network approach insensitive to the effects that epoch length has on functional connectivity (FC) and network reconstruction. Two different measures, the phase lag index (PLI) and the Amplitude Envelope Correlation (AEC), were applied to EEG resting-state recordings for a group of eighteen healthy volunteers. Weighted clustering coefficient (CCw), weighted characteristic path length (Lw) and minimum spanning tree (MST) parameters were computed to evaluate the network topology. The analysis was performed on both scalp and source-space data. Results about distinctive functional core, show highest classification rates from k-core decomposition in gamma (EER=0.130, AUC=0.943) and high beta (EER=0.172, AUC=0.905) frequency bands. Results from scalp analysis concerning the influence of epoch length, show a decrease in both mean PLI and AEC values with an increase in epoch length, with a tendency to stabilize at a length of 12 seconds for PLI and 6 seconds for AEC. Moreover, CCw and Lw show very similar behaviour, with metrics based on AEC more reliable in terms of stability. In general, MST parameters stabilize at short epoch lengths, particularly for MSTs based on PLI (1-6 seconds versus 4-8 seconds for AEC). At the source-level the results were even more reliable, with stability already at 1 second duration for PLI-based MSTs. Our results confirm that EEG analysis may represent an effective tool to identify subject-specific characteristics that may be of great impact for several bioengineering applications. Regarding epoch length, the present work suggests that both PLI and AEC depend on epoch length and that this has an impact on the reconstructed network topology, particularly at the scalp-level. Source-level MST topology is less sensitive to differences in epoch length, therefore enabling the comparison of brain network topology between different studies

Perfiles de actividad magnética cerebral de jóvenes con consumo intensivo de alcohol

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Psicología, leída el 24-01-2017El patrón de consumo de alcohol binge drinking se caracteriza por la ingesta intermitente de grandes cantidades del alcohol en un corto espacio de tiempo alternándose con periodos de abstinencia. En España, este tipo de consumo de alcohol se asocia al conocido efecto botellón en el que los jóvenes se reúnen en espacios públicos, principalmente los fines de semana, teniendo el alcohol como protagonista. La adolescencia, edad en la que se inicia este tipo de consumo, es considerada un periodo crítico de desarrollo en el que el cerebro experimenta grandes cambios madurativos, fundamentalmente en los lóbulos frontales. Dada su inmadurez, el cerebro adolescente muestra mayor vulnerabilidad ante el efecto neurotóxico del alcohol que el cerebro adulto. Debido a la alta prevalencia que este tipo de consumo presenta entre los jóvenes, la comunidad científica ha mostrado interés en las últimas décadas por estudiar las posibles consecuencias que puede tener en la estructura y funcionamiento del cerebro de jóvenes que beben de este modo. Sin embargo, hasta el momento no existían estudios que evaluasen el efecto del binge drinking en la actividad magnética cerebral. La Magnetoencefalografía es una técnica no invasiva que mide las corrientes magnéticas generadas por las pequeñas corrientes neurales que producen las neuronas. La presente tesis ha utilizado esta técnica a lo largo de los tres experimentos en los que se estudió: 1) la actividad magnética cerebral en el espacio de los sensores asociada del estado de reposo de jóvenes universitarios de 18-19 años con el patrón binge drinking y un grupo control, 2) la actividad magnética cerebral en espacio de las fuentes del estado de reposo y la conectividad estructural de los mismos jóvenes dos años más tarde, con 20-21 años; y 3) la actividad magnética cerebral en espacio de las fuentes asociada a una tarea Go/NoGo de los mismos jóvenes durante la primera fase del estudio, cuando tenían 18-19 años...The alcohol consumption binge drinking pattern is characterized by intermittent intake of large amounts of alcohol in a short space of time, alternated with periods of abstinence. In Spain, this type of alcohol consumption is associated with the well-known “efecto botellón” where young people gather in public spaces, especially on weekends, having the alcohol as the protagonist. Adolescence, the age in which this type of consumption begins, is considered a critical period of neural development in which the brain undergoes maturational changes, mainly in the frontal lobes. Given its immaturity, adolescent brain is more vulnerable to the neurotoxic effects of alcohol than the adult brain. Because of its high prevalence among young adolescents, since last decades the scientific community has shown increasing interest to study the possible consequences that binge drinking may have on the structure and functioning of the brain. However, so far there are no studies assessing the effect of binge drinking with Magnetoencephalography. Magnetoencephalography is a noninvasive technique that measures the magnetic currents generated by neural currents produced by pyramidal neurons. The present dissertation has used this technique over the three experiments, studying: 1) brain magnetic activity in the sensor space associated to resting state of university students of aged 18-19 years old with alcohol binge drinking pattern and also a control group, 2) brain magnetic activity in the source space also associated to resting state and structural connectivity of the same young students two years later, with 20-21 years old; and finally, 3) the brain magnetic activity in source space associated with a Go/NoGo task in the first phase of the study, when the participants were 18-19...Fac. de PsicologíaTRUEunpu

Entwicklung eines neuen Paradigmas zur Evaluation des Raumrichtungshörens bei Normalhörenden und Patientinnen und Patienten mit Cochleaimplantat – eine hochauflösende EEG-Studie

Die vorliegende Arbeit beschäftigt sich mit den neuronalen Mechanismen des Raumrichtungshörens. Um diese zu untersuchen wurde im Rahmen einer Pilotstudie ein neues Paradigma entwickelt. Mittels dieses Paradigmas soll ein neuronaler Marker für ein intaktes Vermögen zum Raumrichtungshören etabliert werden. Außerdem soll das Paradigma Aufschluss auf die am Raumrichtungshören beteiligten Hirnareale ermöglichen. Zusätzlich zu den Untersuchungen der normal hörenden Probandinnen und Probanden führte ich Messungen an CI-Trägerinnen und CI-Trägern durch, um Informationen über die klinische Anwendbarkeit des Markers bei Höreinschränkung zu erhalten. Die Erforschung des Raumrichtungshören bei CI-Trägerinnen und CI-Trägern ist von hoher Relevanz, da in diesem Bereich ein noch großes Verbesserungspotenzial des Hörvermögens besteht und die zu Grunde liegenden neuronalen Mechanismen bisher wenig verstanden sind. Das im Rahmen der Studie entwickelte Paradigma ist ein multimodales Repetitions-Suppressions-Design (MMRS-Paradigma), in welchem auf vier audiovisuelle Habituationsstimuli aus derselben Raumrichtung ein rein auditorischer Teststimulus folgte. Dieser kam entweder aus derselben Richtung wie die vorausgegangenen Stimuli („same“ Bedingung) oder aus einer benachbarten Richtung („different“ Bedingung). Die kortikale Aktivität maßen wir mittels eines hochauflösenden EEGs. In der „different“ Bedingung wurde in einem Zeitfenster von 100 bis 350 ms nach Einsetzen des Teststimulus ein Signal hervorgerufen, das MMRS-Signal, das die Wahrnehmung der veränderten Raumrichtung des Teststimulus in dieser Bedingung widerspiegelt. Ich schlage vor, MMRS-Signal als neurophysiologischen Marker für das Vermögen zum Raumrichtungshören einzuführen. Regionen, die signifikante Aktivitätsunterschiede zwischen „same“ und „different“ Bedingung zeigten, lagen im Bereich des rechten Frontallappens (C1, C3, C4, C7) und rechten Parietallappens (C3, C5, C8), im Bereich der rechten parieto-temporo-okzipitalen Junktionszone (C2), sowie in Arealen des linken Frontallappens und präfrontalen Kortex einschließlich (prä)motorischer und supplementär motorischer Areale (C6, C9). Die Beteiligung vieler verschiedener Hirnareale weist darauf hin, dass das MMRS-Signal die globale Verarbeitung widerspiegelt. Während des Raumrichtungshörens war die rechte Hemisphäre insgesamt stärker aktiv als die linke. Die Analyse der Aktivität für die einzelnen Raumrichtungen des Teststimulus zeigte eine Dominanz der zum Stimulus kontralateralen Hemisphäre. Diese Ergebnisse stimmen mit den Erkenntnissen aus vorangegangen Studien anderer Autoren überein und bestätigten die Stabilität und Reliabilität der Befunde. Außerdem untersuchten wir in Korrelationsanalysen den Zusammenhang zwischen der Lokalisationsleistung der CI-Trägerinnen und CI-Trägern in der Aufgabe zum Raumrichtungshören und der Ähnlichkeit der MMRS-Differenzkurve von Normalhörenden zu der von CI-Trägerinnen und CI-Trägern. CI-Trägerinnen und CI-Träger mit besserer Lokalisationsleistung zeigten eine den Normalhörenden ähnlichere MMRS-Differenzkurve in Regionen der temporo-parieto-okzipitalen Junktionszone (C2, 2150-2200 ms). Dies weist einerseits darauf hin, dass sich das MMRS-Signal als Marker für die Evaluation des Raumrichtungshörens bei Patientinnen und Patienten mit Hörschädigung einsetzen lässt. Andererseits deutet dies auf die wichtige Rolle der temporo-parieto-okzipitalen Junktionszone für das Raumrichtungshören hin. Das in der Pilotstudie entwickelte Paradigma lässt sich also zur Evaluation des Raumrichtungshörens und zur Erforschung neuronaler Prozesse des Raumrichtungshörens anwenden. Mögliche zukünftige Anwendungen des MMRS-Designs liegen unter Anderem im Bereich der rehabilitativen Medizin und der Diagnostik. Das MMRS-Paradigma ermöglicht nicht nur ein Training des Raumrichtungshörens, sondern auch das Monitoring des Trainingserfolgs oder die Evaluation des Raumrichtungshörens bei Kindern und Patientinnen und Patienten mit kognitiven Beeinträchtigungen

Methods for functional brain imaging

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2011.Cataloged from PDF version of thesis.Includes bibliographical references.Magnetic resonance imaging (MRI) has demonstrated the potential for non-invasive mapping of structure and function (fMRI) in the human brain. In this thesis, we propose a series of methodological developments towards improved fMRI of auditory processes. First, the inefficiency of standard fMRI that acquires brain volumes one slice at a time is addressed. The proposed single-shot method is capable, for the first time, of imaging the entire brain in a single-acquisition while still maintaining adequate spatial resolution for fMRI. This method dramatically increases the temporal resolution of fMRI (20 fold) and improves sampling efficiency as well as the ability to discriminate against detrimental physiological noise. To accomplish this it exploits highly accelerated parallel imaging techniques and MRI signal detection with a large number of coil elements. We then address a major problem in the application of fMVIRI to auditory studies. In standard fMRI, loud acoustic noise is generated by the rapid switching of the gradient magnetic fields required for image encoding, which interferes with auditory stimuli and enforces inefficient and slow sampling strategies. We demonstrate a fMRI method that uses parallel imaging and redesigned gradient waveforms to both minimize and slow down the gradient switching to substantially reduce acoustic noise while still enabling rapid acquisitions for fMRI. Conventional fMRI is based on a hemodynamic response that is secondary to the underlying neuronal activation. In the final contribution of this thesis, a novel image contrast is introduced that is aimed at the direct observation of neuronal magnetic fields associated with functional activation. Early feasibility studies indicate that the imaging is sensitive to oscillating magnetic fields at amplitudes similar to those observed by magnetoencephalography.by Thomas Witzel.Ph.D