Brain oscillations, connectomes and neurophysiology

Magnetoencephalography (MEG) is a neuroimaging technique allowing the investigation of brain function non-invasively, by detecting changes in magnetic fields outside the head induced by ensembles of neurons firing synchronously. Oscillatory electrophysiological activity measured using MEG has been shown to support long range functional communication in the brain, showing close resemblance to known fMRI functional networks. Interest in functional connectivity is growing as disruptions in the functional connectome have been implicated in both neurological and mental health conditions. However, the connection between brain oscillations, functional connectivity and neurochemistry still remains largely unexplored. In this thesis I aim to shed light on this connection. The thesis begins with a description of the theory behind neural signals measurable with MEG (chapter 1) and the details of source localisation and functional connectivity (chapter 2). Following this, there are three experimental chapters: in chapter 3 I test how practical aspects of experimental design affect the intra-subject repeatability of the MEG functional connectome. The use of a foam head-cast, which is known to improve co-registration accuracy, is shown to increase significantly the between session repeatability of both beamformer reconstruction and functional connectivity estimation. Moreover longer recordings offer a large improvements in repeatability of functional connectivity, with analysis suggesting this result is caused by a genuine effect of brain state. In chapter 4 I present MEG data recorded during a sensory attention task, in which participants were asked to identify braille patterns presented to their fingers, whilst simultaneously switching attention between hands. Whilst a weight of evidence suggests that ‘low’ frequency (beta band) oscillations are representative of cortical inhibition, more recent studies have closely linked these same phenomena to functional connectivity. Results show that attentional modulation changes beta dynamics in primary sensory cortex, with attended stimuli generating lower post-stimulus responses (i.e. lower beta ‘rebound’); this effect is driven by the transient formation and dissolution of distributed networks, which form in response to unattended stimuli, and likely facilitate top down inhibitory influence on the primary sensorimotor cortex. The results are related to subject behaviour, with high pre-stimulus beta connectivity leading to poor task performance. Finally, in chapter 5, combining MEG and Magnetic Resonance Spectroscopy (MRS), I show that beta oscillations are directly related to GABAergic signalling. The results here presented offer a mechanistic interpretation of the role played by beta oscillations in mediating inhibition. This has implications for our understanding of neural oscillations and connectivity in both the healthy brain and a range of disorders

How Sensitive Are Conventional MEG Functional Connectivity Metrics With Sliding Windows to Detect Genuine Fluctuations in Dynamic Functional Connectivity?

Despite advances in the field of dynamic connectivity, fixed sliding window approaches for the detection of fluctuations in functional connectivity are still widely used. The use of conventional connectivity metrics in conjunction with a fixed sliding window comes with the arbitrariness of the chosen window lengths. In this paper we use multivariate autoregressive and neural mass models with a-priori defined ground truths to systematically analyse the sensitivity of conventional metrics in combination with different window lengths to detect genuine fluctuations in connectivity for various underlying state durations. Metrics of interest are the coherence, imaginary coherence, phase lag index, phase locking value and the amplitude envelope correlation. We performed analysis for two nodes and at the network level. We demonstrate that these metrics show indeed higher variability for genuine temporal fluctuations in connectivity compared to a static connectivity state superimposed by noise. Overall, the error of the connectivity estimates themselves decreases for longer state durations (order of seconds), while correlations of the connectivity fluctuations with the ground truth was higher for longer state durations. In general, metrics, in combination with a sliding window, perform poorly for very short state durations. Increasing the SNR of the system only leads to a moderate improvement. In addition, at the network level, only longer window widths were sufficient to detect plausible resting state networks that matched the underlying ground truth, especially for the phase locking value, amplitude envelope correlation and coherence. The length of these longer window widths did not necessarily correspond to the underlying state durations. For short window widths resting state network connectivity patterns could not be retrieved. We conclude that fixed sliding window approaches for connectivity can detect modulations of connectivity, but mostly if the underlying dynamics operate on moderate to slow timescales. In practice, this can be a drawback, as state durations can vary significantly in empirical data

Optimising experimental design for MEG resting state functional connectivity measurement

The study of functional connectivity using magnetoencephalography (MEG) is an expanding area of neuroimaging, and adds an extra dimension to the more common assessments made using fMRI. The importance of such metrics is growing, with recent demonstrations of their utility in clinical research, however previous reports suggest that whilst group level resting state connectivity is robust, single session recordings lack repeatability. Such robustness is critical if MEG measures in individual subjects are to prove clinically valuable. In the present paper, we test how practical aspects of experimental design affect the intra-subject repeatability of MEG findings; specifically we assess the effect of co-registration method and data recording duration. We show that the use of a foam head-cast, which is known to improve co-registration accuracy, increased significantly the between session repeatability of both beamformer reconstruction and connectivity estimation. We also show that recording duration is a critical parameter, with large improvements in repeatability apparent when using ten minute, compared to five minute recordings. Further analyses suggest that the origin of this latter effect is not underpinned by technical aspects of source reconstruction, but rather by a genuine effect of brain state; short recordings are simply inefficient at capturing the canonical MEG network in a single subject. Our results provide important insights on experimental design and will prove valuable for future MEG connectivity studies

Altered temporal stability in dynamic neural networks underlies connectivity changes in neurodevelopment

Network connectivity is an integral feature of human brain function, and characterising its maturational trajectory is a critical step towards understanding healthy and atypical neurodevelopment. Here, we used magnetoencephalography (MEG) to investigate both stationary (i.e. time averaged) and rapidly modulating (dynamic) electrophysiological connectivity, in participants aged from mid-childhood to early adulthood (youngest participant 9 years old; oldest participant 25 years old). Stationary functional connectivity (measured via inter-regional coordination of neural oscillations) increased with age in the alpha and beta frequency bands, particularly in bilateral parietal and temporo-parietal connections. Our dynamic analysis (also applied to alpha/beta oscillations) revealed the spatiotemporal signatures of 8 dynamic networks; these modulate on a ∼100 ms time scale, and temporal stability in attentional networks was found to increase with age. Significant overlap was found between age-modulated dynamic networks and inter-regional oscillatory coordination, implying that altered network dynamics underlie age related changes in functional connectivity. Our results provide novel insights into brain network electrophysiology, and lay a foundation for future work in childhood disorders

Neural underpinnings of threat bias in relation to loss-of-control eating behaviors among adolescent girls with high weight

IntroductionLoss-of-control (LOC) eating, a key feature of binge-eating disorder, may relate attentional bias (AB) to highly salient interpersonal stimuli. The current pilot study used magnetoencephalography (MEG) to explore neural features of AB to socially threatening cues in adolescent girls with and without LOC-eating.MethodsGirls (12–17 years old) with overweight or obesity (BMI >85th percentile) completed an AB measure on an affective dot-probe AB task during MEG and evoked neural responses to angry or happy (vs. neutral) face cues were captured. A laboratory test meal paradigm measured energy intake and macronutrient consumption patterns.ResultsGirls (N = 34; Mage = 15.5 ± 1.5 years; BMI-z = 1.7 ± 0.4) showed a blunted evoked response to the presentation of angry face compared with neutral face cues in the left dorsolateral prefrontal cortex, a neural region implicated in executive control and regulation processes, during attention deployment (p < 0.01). Compared with those without LOC-eating (N = 21), girls with LOC-eating (N = 13) demonstrated a stronger evoked response to angry faces in the visual cortex during attention deployment (p < 0.001). Visual and cognitive control ROIs had trends suggesting interaction with test meal intake patterns among girls with LOC-eating (ps = 0.01).DiscussionThese findings suggest that girls with overweight or obesity may fail to adaptively engage neural regions implicated in higher-order executive processes. This difficulty may relate to disinhibited eating patterns that could lead to excess weight gain

Dynamics of large-scale electrophysiological networks: a technical review

For several years it has been argued that neural synchronisation is crucial for cognition. The idea that synchronised temporal patterns between different neural groups carries information above and beyond the isolated activity of these groups has inspired a shift in focus in the field of functional neuroimaging. Specifically, investigation into the activation elicited within certain regions by some stimulus or task has, in part, given way to analysis of patterns of co-activation or functional connectivity between distal regions. Recently, the functional connectivity community has been looking beyond the assumptions of stationarity that earlier work was based on, and has introduced methods to incorporate temporal dynamics into the analysis of connectivity. In particular, non-invasive electrophysiological data (magnetoencephalography / electroencephalography (MEG/EEG)), which provides direct measurement of whole-brain activity and rich temporal information, offers an exceptional window into such (potentially fast) brain dynamics. In this review, we discuss challenges, solutions, and a collection of analysis tools that have been developed in recent years to facilitate the investigation of dynamic functional connectivity using these imaging modalities. Further, we discuss the applications of these approaches in the study of cognition and neuropsychiatric disorders. Finally, we review some existing developments that, by using realistic computational models, pursue a deeper understanding of the underlying causes of non-stationary connectivity

Brain oscillations, connectomes and neurophysiology

Magnetoencephalography (MEG) is a neuroimaging technique allowing the investigation of brain function non-invasively, by detecting changes in magnetic fields outside the head induced by ensembles of neurons firing synchronously. Oscillatory electrophysiological activity measured using MEG has been shown to support long range functional communication in the brain, showing close resemblance to known fMRI functional networks. Interest in functional connectivity is growing as disruptions in the functional connectome have been implicated in both neurological and mental health conditions. However, the connection between brain oscillations, functional connectivity and neurochemistry still remains largely unexplored. In this thesis I aim to shed light on this connection. The thesis begins with a description of the theory behind neural signals measurable with MEG (chapter 1) and the details of source localisation and functional connectivity (chapter 2). Following this, there are three experimental chapters: in chapter 3 I test how practical aspects of experimental design affect the intra-subject repeatability of the MEG functional connectome. The use of a foam head-cast, which is known to improve co-registration accuracy, is shown to increase significantly the between session repeatability of both beamformer reconstruction and functional connectivity estimation. Moreover longer recordings offer a large improvements in repeatability of functional connectivity, with analysis suggesting this result is caused by a genuine effect of brain state. In chapter 4 I present MEG data recorded during a sensory attention task, in which participants were asked to identify braille patterns presented to their fingers, whilst simultaneously switching attention between hands. Whilst a weight of evidence suggests that ‘low’ frequency (beta band) oscillations are representative of cortical inhibition, more recent studies have closely linked these same phenomena to functional connectivity. Results show that attentional modulation changes beta dynamics in primary sensory cortex, with attended stimuli generating lower post-stimulus responses (i.e. lower beta ‘rebound’); this effect is driven by the transient formation and dissolution of distributed networks, which form in response to unattended stimuli, and likely facilitate top down inhibitory influence on the primary sensorimotor cortex. The results are related to subject behaviour, with high pre-stimulus beta connectivity leading to poor task performance. Finally, in chapter 5, combining MEG and Magnetic Resonance Spectroscopy (MRS), I show that beta oscillations are directly related to GABAergic signalling. The results here presented offer a mechanistic interpretation of the role played by beta oscillations in mediating inhibition. This has implications for our understanding of neural oscillations and connectivity in both the healthy brain and a range of disorders

Tracking dynamic brain networks using high temporal resolution MEG measures of functional connectivity

Fluctuations in functional interactions between brain regions typically occur at the millisecond time scale. Conventional connectivity metrics are not adequately time-resolved to detect such fast fluctuations in functional connectivity. At the same time, attempts to use conventional metrics in a time-resolved manner usually come with the selection of sliding windows of fixed arbitrary length. In the current work, we evaluated the use of high temporal resolution metrics of functional connectivity in conjunction with non-negative tensor factorisation to detect fast fluctuations in connectivity and temporally evolving subnetworks. To this end, we used the phase difference derivative, wavelet coherence, and we also introduced a new metric, the instantaneous amplitude correlation. In order to deal with the inherently noisy nature of magnetoencephalography data and large datasets, we make use of recurrence plots and we used pair-wise orthogonalisation to avoid spurious estimates of functional connectivity due to signal leakage. Firstly, metrics were evaluated in the context of dynamically coupled neural mass models in the presence and absence of delays and also compared to conventional static metrics with fixed sliding windows. Simulations showed that these high temporal resolution metrics outperformed conventional static connectivity metrics. Secondly, the sensitivity of the metrics to fluctuations in connectivity was analysed in post-movement beta rebound magnetoencephalography data, which showed time locked sensorimotor subnetworks that modulated with the post-movement beta rebound. Finally, sensitivity of the metrics was evaluated in resting-state magnetoencephalography, showing similar spatial patterns across metrics, thereby indicating the robustness of the current analysis. The current methods can be applied in cognitive experiments that involve fast modulations in connectivity in relation to cognition. In addition, these methods could also be used as input to temporal graph analysis to further characterise the rapid fluctuation in brain network topology

Differential classification of states of consciousness using envelope- and phase-based functional connectivity

The development of sophisticated computational tools to quantify changes in the brain's oscillatory dynamics across states of consciousness have included both envelope- and phase-based measures of functional connectivity (FC), but there are very few direct comparisons of these techniques using the same dataset. The goal of this study was to compare an envelope-based (i.e. Amplitude Envelope Correlation, AEC) and a phase-based (i.e. weighted Phase Lag Index, wPLI) measure of FC in their classification of states of consciousness. Nine healthy participants underwent a three-hour experimental anesthetic protocol with propofol induction and isoflurane maintenance, in which five minutes of 128-channel electroencephalography were recorded before, during, and after anesthetic-induced unconsciousness, at the following time points: Baseline; light sedation with propofol (Light Sedation); deep unconsciousness following three hours of surgical levels of anesthesia with isoflurane (Unconscious); five minutes prior to the recovery of consciousness (Pre-ROC); and three hours following the recovery of consciousness (Recovery). Support vector machine classification was applied to the source-localized EEG in the alpha (8–13 Hz) frequency band in order to investigate the ability of AEC and wPLI (separately and together) to discriminate i) the four states from Baseline; ii) Unconscious (“deep” unconsciousness) vs. Pre-ROC (“light” unconsciousness); and iii) responsiveness (Baseline, Light Sedation, Recovery) vs. unresponsiveness (Unconscious, Pre-ROC). AEC and wPLI yielded different patterns of global connectivity across states of consciousness, with AEC showing the strongest network connectivity during the Unconscious epoch, and wPLI showing the strongest connectivity during full consciousness (i.e., Baseline and Recovery). Both measures also demonstrated differential predictive contributions across participants and used different brain regions for classification. AEC showed higher classification accuracy overall, particularly for distinguishing anesthetic-induced unconsciousness from Baseline (83.7 ± 0.8%). AEC also showed stronger classification accuracy than wPLI when distinguishing Unconscious from Pre-ROC (i.e., “deep” from “light” unconsciousness) (AEC: 66.3 ± 1.2%; wPLI: 56.2 ± 1.3%), and when distinguishing between responsiveness and unresponsiveness (AEC: 76.0 ± 1.3%; wPLI: 63.6 ± 1.8%). Classification accuracy was not improved compared to AEC when both AEC and wPLI were combined. This analysis of source-localized EEG data demonstrates that envelope- and phase-based FC provide different information about states of consciousness but that, on a group level, AEC is better able to detect relative alterations in brain FC across levels of anesthetic-induced unconsciousness compared to wPLI