Dynamic Reconfiguration of the Language Network Preceding Onset of Speech in Picture Naming

Peer reviewe

Comparing MEG and EEG in detecting the ∼20-Hz rhythm modulation to tactile and proprioceptive stimulation

Modulation of the ∼20-Hz brain rhythm has been used to evaluate the functional state of the sensorimotor cortex both in healthy subjects and patients, such as stroke patients. The ∼20-Hz brain rhythm can be detected by both magnetoencephalography (MEG) and electroencephalography (EEG), but the comparability of these methods has not been evaluated. Here, we compare these two methods in the evaluating of ∼20-Hz activity modulation to somatosensory stimuli. Rhythmic ∼20-Hz activity during separate tactile and proprioceptive stimulation of the right and left index finger was recorded simultaneously with MEG and EEG in twenty-four healthy participants. Both tactile and proprioceptive stimulus produced a clear suppression at 300–350 ms followed by a subsequent rebound at 700–900 ms after stimulus onset, detected at similar latencies both with MEG and EEG. The relative amplitudes of suppression and rebound correlated strongly between MEG and EEG recordings. However, the relative strength of suppression and rebound in the contralateral hemisphere (with respect to the stimulated hand) was significantly stronger in MEG than in EEG recordings. Our results indicate that MEG recordings produced signals with higher signal-to-noise ratio than EEG, favoring MEG as an optimal tool for studies evaluating sensorimotor cortical functions. However, the strong correlation between MEG and EEG results encourages the use of EEG when translating studies to clinical practice. The clear advantage of EEG is the availability of the method in hospitals and bed-side measurements at the acute phase.Peer reviewe

The relationship between electrophysiological and hemodynamic measures of neural activity varies across picture naming tasks: A multimodal magnetoencephalography-functional magnetic resonance imaging study

Different neuroimaging methods can yield different views of taskdependent neural engagement. Studies examining the relationship between electromagnetic and hemodynamic measures have revealed correlated patterns across brain regions but the role of the applied stimulation or experimental tasks in these correlation patterns is still poorly understood. Here, we evaluated the across-tasks variability of MEG-fMRI relationship using data recorded during three distinct naming tasks (naming objects and actions from action images, and objects from object images), from the same set of participants. Our results demonstrate that the MEG-fMRI correlation pattern varies according to the performed task, and that this variability shows distinct spectral profiles across brain regions. Notably, analysis of the MEG data alone did not reveal modulations across the examined tasks in the timefrequency windows emerging from the MEG-fMRI correlation analysis. Our results suggest that the electromagnetic-hemodynamic correlation could serve as a more sensitive proxy for task-dependent neural engagement in cognitive tasks than isolated within-modality measures.Peer reviewe

Cortical beta burst dynamics are altered in Parkinson's disease but normalized by deep brain stimulation

Exaggerated subthalamic beta oscillatory activity and increased beta range cortico-subthalamic synchrony have crystallized as the electrophysiological hallmarks of Parkinson's disease. Beta oscillatory activity is not tonic but occurs in 'bursts' of transient amplitude increases. In Parkinson's disease, the characteristics of these bursts are altered especially in the basal ganglia. However, beta oscillatory dynamics at the cortical level and how they compare with healthy brain activity is less well studied. We used magnetoencephalography (MEG) to study sensorimotor cortical beta bursting and its modulation by subthalamic deep brain stimulation in Parkinson's disease patients and age-matched healthy controls. We show that the changes in beta bursting amplitude and duration typical of Parkinson's disease can also be observed in the sensorimotor cortex, and that they are modulated by chronic subthalamic deep brain stimulation, which, in turn, is reflected in improved motor function at the behavioural level. In addition to the changes in individual beta bursts, their timing relative to each other was altered in patients compared to controls: bursts were more clustered in untreated Parkinson's disease, occurring in 'bursts of bursts', and re-burst probability was higher for longer compared to shorter bursts. During active deep brain stimulation, the beta bursting in patients resembled healthy controls' data. In summary, both individual bursts' characteristics and burst patterning are affected in Parkinson's disease, and subthalamic deep brain stimulation normalizes some of these changes to resemble healthy controls' beta bursting activity, suggesting a non-invasive biomarker for patient and treatment follow-up.Peer reviewe

Mild Traumatic Brain Injury Affects Cognitive Processing and Modifies Oscillatory Brain Activity during Attentional Tasks

Despite the high prevalence of mild traumatic brain injury (mTBI), current diagnostic tools to objectively assess cognitive complaints after mTBI continue to be inadequate. Our aim was to identify neuronal correlates for cognitive difficulties in mTBI patients by evaluating the possible alterations in oscillatory brain activity during a behavioral task known to be sensitive to cognitive impairment after mTBI. We compared oscillatory brain activity during rest and cognitive tasks (Paced Auditory Serial Addition Test [PASAT] and a vigilance test [VT]) with magnetoencephalography between 25 mTBI patients and 20 healthy controls. Whereas VT induced no significant differences compared with resting state in either group, patients exhibited stronger attenuation of 8- to 14-Hz oscillatory activity during PASAT than healthy controls in the left parietotemporal cortex (pPeer reviewe

Avbilding av språkliga funktioner i hjärnan med MEG och fMRI

This Thesis considers the cortical mechanisms underlying language function, as measured by magnetoencephalography (MEG) and functional Magnetic Resonance Imaging (fMRI). In MEG, interpretation of the data critically depends on the ability to estimate the underlying neural activity and localize it to a certain part of the brain. In this Thesis, the accuracy of this procedure is explored in localization of cortical rhythms and for different head conductor models. A comparison of different source modeling techniques shows that rhythmic activity can be identified reliably with a variety of tools, such as equivalent current dipoles (ECDs), minimum norm approaches (MCEFD, minimum current estimates in the frequency domain), and beamformers (DICS, dynamic imaging of coherent sources). The results show that DICS is more sensitive to weak sources than the two other methods, both in measured and in simulated data. Computer simulations also demonstrate that for source localization performed under normal noisy conditions, a simple spherically symmetric head conductor model is in most cases a sufficient model for the conductivity geometry of the head. This Thesis specifically considers the cortical processing of action and object naming. We investigate whether different cortical regions are activated when actions or objects are named from the same images, and how the content of the image affects the brain correlates of naming. The MEG and fMRI results presented in this Thesis indicate that verbs and nouns are processed within the same cortical network, and demonstrate that image category (action/object) has a stronger influence than naming category (verb/noun) on the activation pattern within this network. In addition, we consider the relationship between MEG evoked responses and fMRI BOLD (blood-oxygen-level-dependent) signals in language tasks. We demonstrate differences between these two measures in both picture naming and reading, and show that such differences do not depend on experimental procedures such as different participants, languages, or task. In particular, we demonstrate an opposite stimulus effect for symbols and letter strings in the left occipito-temporal cortex in MEG vs. fMRI in reading, although the simultaneously measured electroencephalogram (EEG) was similar. We argue that the observed differences within this region reflect different neural generation mechanisms of the MEG evoked response and fMRI BOLD signals.Denna avhandling använder funktionella avbildningstekniker, såsom magnetoenkelfalografi MEG) och funktionell magnetavbildning (fMRI, functional magnetic resonance imaging), förr att kartlägga hur språkliga funktioner bearbetas i hjärnan. Avhandlingen strävar efter att ge en mångfacetterad bild av både metoder och kognitiva frågor inom funktionell avbildning. Ett centralt problem inom MEG är att identifiera de områden i hjärnan som gett upphov till det uppmätta magnetfältet. Denna avhandling utvärderar olika metoder för identifgiering av kortikala rytmer. Resultaten visar att kortikala rytmel kan lokasiseras tillförtligt med hjälp av ett flertal olika metoder, såsom ekvivalenta dipolmodeller (ECD), minsta norm metoder (MCEFD= och spatialfilter (DICS).Jämförelsen visar att DICS kan identifiera områden med svaga kortikala rytmer som de övriga metoderna inte förmår indentifiera. Vidare demonstrerar en granskning av olika modeller för att beskriva huvudets konduktivitetsprofil att en enkel sfärisk modell oftast är tillräcklig då man löser det inversa problemet. Avhandlingen granskar särskilt bearbetning av verb och substantiv i hjärnan, samt hur olika typer av bilder som illustrerar en utförd handling eller ett enskilt föremål påverkar aktiviteten i hjärnan vid benämningen av en bild. Resultaten från både MEG oc fMRI påvisar att verb oc substantiv vearbetas i ett gemensamt nätverk av områden i hjärnan. Inom detta nätverk har dock bildens inneh¨ll (en hanling/ett föremål) en större betydelse för aktiviteten än vilken kategori av ord (verb/substantiv) som skall benämnas. Därtill undersöks förhållandet mellan BOLD (blood-oxygen-level-dependent) signalen i fMRI och framkallade responser i MEG. Resultaten påvisar skillnader mellan dessa metoder vid benämning av bilder och läsning som inte går att förklara med olikheter i experimentella tillbägagångssätt, t.ex. uppgift, språk, eller försökspersoner. Avhanlingen demonstrerar att MEG och fMRI kan uppvisa rakt mosatta effekter för symboler och bokstavskombinationer i vänstra temporal-occipitala hjärnbarken vid läsning, torts att det samtidigt uppmätta elektroenkelfalogrammet (EEG) inte påvisade skillnader. Resultaten tyder på att BOLD-signalen i fMRI och de framkallade responserna i MEG i detta fall hänför sig till skilda neurala mekanismer

Avbilding av språkliga funktioner i hjärnan med MEG och fMRI

This Thesis considers the cortical mechanisms underlying language function, as measured by magnetoencephalography (MEG) and functional Magnetic Resonance Imaging (fMRI). In MEG, interpretation of the data critically depends on the ability to estimate the underlying neural activity and localize it to a certain part of the brain. In this Thesis, the accuracy of this procedure is explored in localization of cortical rhythms and for different head conductor models. A comparison of different source modeling techniques shows that rhythmic activity can be identified reliably with a variety of tools, such as equivalent current dipoles (ECDs), minimum norm approaches (MCEFD, minimum current estimates in the frequency domain), and beamformers (DICS, dynamic imaging of coherent sources). The results show that DICS is more sensitive to weak sources than the two other methods, both in measured and in simulated data. Computer simulations also demonstrate that for source localization performed under normal noisy conditions, a simple spherically symmetric head conductor model is in most cases a sufficient model for the conductivity geometry of the head. This Thesis specifically considers the cortical processing of action and object naming. We investigate whether different cortical regions are activated when actions or objects are named from the same images, and how the content of the image affects the brain correlates of naming. The MEG and fMRI results presented in this Thesis indicate that verbs and nouns are processed within the same cortical network, and demonstrate that image category (action/object) has a stronger influence than naming category (verb/noun) on the activation pattern within this network. In addition, we consider the relationship between MEG evoked responses and fMRI BOLD (blood-oxygen-level-dependent) signals in language tasks. We demonstrate differences between these two measures in both picture naming and reading, and show that such differences do not depend on experimental procedures such as different participants, languages, or task. In particular, we demonstrate an opposite stimulus effect for symbols and letter strings in the left occipito-temporal cortex in MEG vs. fMRI in reading, although the simultaneously measured electroencephalogram (EEG) was similar. We argue that the observed differences within this region reflect different neural generation mechanisms of the MEG evoked response and fMRI BOLD signals.Denna avhandling använder funktionella avbildningstekniker, såsom magnetoenkelfalografi MEG) och funktionell magnetavbildning (fMRI, functional magnetic resonance imaging), förr att kartlägga hur språkliga funktioner bearbetas i hjärnan. Avhandlingen strävar efter att ge en mångfacetterad bild av både metoder och kognitiva frågor inom funktionell avbildning. Ett centralt problem inom MEG är att identifiera de områden i hjärnan som gett upphov till det uppmätta magnetfältet. Denna avhandling utvärderar olika metoder för identifgiering av kortikala rytmer. Resultaten visar att kortikala rytmel kan lokasiseras tillförtligt med hjälp av ett flertal olika metoder, såsom ekvivalenta dipolmodeller (ECD), minsta norm metoder (MCEFD= och spatialfilter (DICS).Jämförelsen visar att DICS kan identifiera områden med svaga kortikala rytmer som de övriga metoderna inte förmår indentifiera. Vidare demonstrerar en granskning av olika modeller för att beskriva huvudets konduktivitetsprofil att en enkel sfärisk modell oftast är tillräcklig då man löser det inversa problemet. Avhandlingen granskar särskilt bearbetning av verb och substantiv i hjärnan, samt hur olika typer av bilder som illustrerar en utförd handling eller ett enskilt föremål påverkar aktiviteten i hjärnan vid benämningen av en bild. Resultaten från både MEG oc fMRI påvisar att verb oc substantiv vearbetas i ett gemensamt nätverk av områden i hjärnan. Inom detta nätverk har dock bildens inneh¨ll (en hanling/ett föremål) en större betydelse för aktiviteten än vilken kategori av ord (verb/substantiv) som skall benämnas. Därtill undersöks förhållandet mellan BOLD (blood-oxygen-level-dependent) signalen i fMRI och framkallade responser i MEG. Resultaten påvisar skillnader mellan dessa metoder vid benämning av bilder och läsning som inte går att förklara med olikheter i experimentella tillbägagångssätt, t.ex. uppgift, språk, eller försökspersoner. Avhanlingen demonstrerar att MEG och fMRI kan uppvisa rakt mosatta effekter för symboler och bokstavskombinationer i vänstra temporal-occipitala hjärnbarken vid läsning, torts att det samtidigt uppmätta elektroenkelfalogrammet (EEG) inte påvisade skillnader. Resultaten tyder på att BOLD-signalen i fMRI och de framkallade responserna i MEG i detta fall hänför sig till skilda neurala mekanismer

The effect of alertness and attention on the modulation of the beta rhythm to tactile stimulation

Beta rhythm modulation has been used as a biomarker to reflect the functional state of the sensorimotor cortex in both healthy subjects and patients. Here, the effect of reduced alertness and active attention to the stimulus on beta rhythm modulation was investigated. Beta rhythm modulation to tactile stimulation of the index finger was recorded simultaneously with MEG and EEG in 23 healthy subjects (mean 23, range 19-35 years). The temporal spectral evolution method was used to obtain the peak amplitudes of beta suppression and rebound in three different conditions (neutral, snooze, and attention). Neither snooze nor attention to the stimulus affected significantly the strength of beta suppression nor rebound, although a decrease in suppression and rebound strength was observed in some subjects with a more pronounced decrease of alertness. The reduction of alertness correlated with the decrease of suppression strength both in MEG (left hemisphere r = 0.49; right hemisphere r = 0.49, *p < 0.05) and EEG (left hemisphere r = 0.43; right hemisphere r = 0.72, **p < 0.01). The results indicate that primary sensorimotor cortex beta suppression and rebound are not sensitive to slightly reduced alertness nor active attention to the stimulus at a group level. Hence, tactile stimulus-induced beta modulation is a suitable tool for assessing the sensorimotor cortex function at a group level. However, subjects' alertness should be maintained high during recordings to minimize individual variability.Peer reviewe

Analysis of functional connectivity and oscillatory power using DICS

Communication between brain regions is thought to be facilitated by the synchronization of oscillatory activity. Hence, large-scale functional networks within the brain may be estimated by measuring synchronicity between regions. Neurophysiological recordings, such as magnetoencephalography (MEG) and electroencephalography (EEG), provide a direct measure of oscillatory neural activity with millisecond temporal resolution. In this paper, we describe a full data analysis pipeline for functional connectivity analysis based on dynamic imaging of coherent sources (DICS) of MEG data. DICS is a beamforming technique in the frequency-domain that allows the study of the cortical sources of oscillatory activity and synchronization between brain regions. All the analysis steps, starting from the raw MEG data up to publication-ready group-level statistics and visualization, are discussed in depth, including methodological considerations, rules of thumb and tradeoffs. We start by computing cross-spectral density (CSD) matrices using a wavelet approach in several frequency bands (alpha, theta, beta, gamma). We then provide a way to create comparable source spaces across subjects and discuss the cortical mapping of spectral power. For connectivity analysis, we present a canonical computation of coherence that facilitates a stable estimation of all-to-all connectivity. Finally, we use group-level statistics to limit the network to cortical regions for which significant differences between experimental conditions are detected and produce vertex- and parcel-level visualizations of the different brain networks. Code examples using the MNE-Python package are provided at each step, guiding the reader through a complete analysis of the freely available openfMRI ds000117 "familiar vs. unfamiliar vs. scrambled faces" dataset. The goal is to educate both novice and experienced data analysts with the "tricks of the trade" necessary to successfully perform this type of analysis on their own data.Peer reviewe