Magnetoencephalography Measurements Research Articles

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.

BabyMEG: A whole-head pediatric magnetoencephalography system for human brain development research

We developed a 375-channel, whole-head magnetoencephalography (MEG) system ("BabyMEG") for studying the electrophysiological development of human brain during the first years of life. The helmet accommodates heads up to 95% of 36-month old boys in the USA. The unique two-layer sensor array consists of: (1) 270 magnetometers (10 mm diameter, ∼15 mm coil-to-coil spacing) in the inner layer, (2) thirty-five three-axis magnetometers (20 mm × 20 mm) in the outer layer 4 cm away from the inner layer. Additionally, there are three three-axis reference magnetometers. With the help of a remotely operated position adjustment mechanism, the sensor array can be positioned to provide a uniform short spacing (mean 8.5 mm) between the sensor array and room temperature surface of the dewar. The sensors are connected to superconducting quantum interference devices (SQUIDs) operating at 4.2 K with median sensitivity levels of 7.5 fT/√Hz for the inner and 4 fT/√Hz for the outer layer sensors. SQUID outputs are digitized by a 24-bit acquisition system. A closed-cycle helium recycler provides maintenance-free continuous operation, eliminating the need for helium, with no interruption needed during MEG measurements. BabyMEG with the recycler has been fully operational from March, 2015. Ongoing spontaneous brain activity can be monitored in real time without interference from external magnetic noise sources including the recycler, using a combination of a lightly shielded two-layer magnetically shielded room, an external active shielding, a signal-space projection method, and a synthetic gradiometer approach. Evoked responses in the cortex can be clearly detected without averaging. These new design features and capabilities represent several advances in MEG, increasing the utility of this technique in basic neuroscience as well as in clinical research and patient studies.

On the Potential of a New Generation of Magnetometers for MEG: A Beamformer Simulation Study.

Magnetoencephalography (MEG) is a sophisticated tool which yields rich information on the spatial, spectral and temporal signatures of human brain function. Despite unique potential, MEG is limited by a low signal-to-noise ratio (SNR) which is caused by both the inherently small magnetic fields generated by the brain, and the scalp-to-sensor distance. The latter is limited in current systems due to a requirement for pickup coils to be cryogenically cooled. Recent work suggests that optically-pumped magnetometers (OPMs) might be a viable alternative to superconducting detectors for MEG measurement. They have the advantage that sensors can be brought to within ~4 mm of the scalp, thus offering increased sensitivity. Here, using simulations, we quantify the advantages of hypothetical OPM systems in terms of sensitivity, reconstruction accuracy and spatial resolution. Our results show that a multi-channel whole-head OPM system offers (on average) a fivefold improvement in sensitivity for an adult brain, as well as clear improvements in reconstruction accuracy and spatial resolution. However, we also show that such improvements depend critically on accurate forward models; indeed, the reconstruction accuracy of our simulated OPM system only outperformed that of a simulated superconducting system in cases where forward field error was less than 5%. Overall, our results imply that the realisation of a viable whole-head multi-channel OPM system could generate a step change in the utility of MEG as a means to assess brain electrophysiological activity in health and disease. However in practice, this will require both improved hardware and modelling algorithms.

EP 58. Corticomuscular coherence during isotonic contractions with DBS and medication in PD patients

Introduction Deep brain stimulation (DBS) effectively alleviates the motor symptoms in Parkinson’s disease (PD) although its effect mechanism is still unclear. Corticomuscular coherence (CMC) is a marker of the functional connectivity between the pyramidal system and the periphery ( Hari and Salenius, 1999 ). CMC has been shown to be reduced in PD patients but restorable with medication ( Salenius et al., 2002 ). DBS, on the other hand, has been shown to variably affect CMC in advanced PD patients ( Airaksinen et al., 2015 ). Objectives We investigated movement related cortical oscillations with primary focus on beta CMC (13–30 Hz). The objective was to discern whether both medication and DBS affect CMC in similar manners in unison with their comparable effects on PD patients’ motor symptoms as measured by the Unified Parkinson’s Disease Rating Scale (UPDRS-III). Patients and methods We recorded magnetoencephalography (MEG) from three PD patients performing hand gripping during DBS ON and medicated (MED ON) conditions and from three age-matched healthy control. For the MED ON condition, patients were administered 200 mg of levodopa with MEG measurements one hour later. Participants performed isotonic contractions with their right hand (hand gripping), and electromyography (EMG) was recorded from the extensor digitorum communis muscle in a belly-tendon montage. We calculated the mean-squared coherence between MEG and the rectified EMG signals. Confidence intervals were set at 99% of the time-shifted MEG-EMG coherence (3 s delay of EMG signal) ( [Halliday et al., 1995] , [Airaksinen et al., 2015] ). For each group and condition, we selected the maximum CMC value in the beta range (13–30 Hz) within the average of an a priori selection of nine left sensorimotor gradiometer pairs in the grand averaged data for each group and condition. Results The grand averaged CMC results are shown in Fig. 1. Medication (MED ON) seemed to normalize beta CMC values in PD patients to comparable levels of the controls whereas DBS led to reduced beta CMC values while both treatments had similar effects on the patients’ motor symptoms (UPDRS-III). Conclusion Although both medication and DBS ameliorate clinical motor performance to similar extents, they seem to have differentiated effects on CMC which may point to differences in the effects mechanisms of the two treatments.

Neural Correlates of Auditory Figure-Ground Segregation Based on Temporal Coherence.

To make sense of natural acoustic environments, listeners must parse complex mixtures of sounds that vary in frequency, space, and time. Emerging work suggests that, in addition to the well-studied spectral cues for segregation, sensitivity to temporal coherence—the coincidence of sound elements in and across time—is also critical for the perceptual organization of acoustic scenes. Here, we examine pre-attentive, stimulus-driven neural processes underlying auditory figure-ground segregation using stimuli that capture the challenges of listening in complex scenes where segregation cannot be achieved based on spectral cues alone. Signals (“stochastic figure-ground”: SFG) comprised a sequence of brief broadband chords containing random pure tone components that vary from 1 chord to another. Occasional tone repetitions across chords are perceived as “figures” popping out of a stochastic “ground.” Magnetoencephalography (MEG) measurement in naïve, distracted, human subjects revealed robust evoked responses, commencing from about 150 ms after figure onset that reflect the emergence of the “figure” from the randomly varying “ground.” Neural sources underlying this bottom-up driven figure-ground segregation were localized to planum temporale, and the intraparietal sulcus, demonstrating that this area, outside the “classic” auditory system, is also involved in the early stages of auditory scene analysis.”

Temporal Integration Windows in Neural Processing and Perception Aligned to Saccadic Eye Movements

SummaryWhen processing dynamic input, the brain balances the opposing needs of temporal integration and sensitivity to change. We hypothesized that the visual system might resolve this challenge by aligning integration windows to the onset of newly arriving sensory samples. In a series of experiments, human participants observed the same sequence of two displays separated by a brief blank delay when performing either an integration or segregation task. First, using magneto-encephalography (MEG), we found a shift in the stimulus-evoked time courses by a 150-ms time window between task signals. After stimulus onset, multivariate pattern analysis (MVPA) decoding of task in occipital-parietal sources remained above chance for almost 1 s, and the task-decoding pattern interacted with task outcome. In the pre-stimulus period, the oscillatory phase in the theta frequency band was informative about both task processing and behavioral outcome for each task separately, suggesting that the post-stimulus effects were caused by a theta-band phase shift. Second, when aligning stimulus presentation to the onset of eye fixations, there was a similar phase shift in behavioral performance according to task demands. In both MEG and behavioral measures, task processing was optimal first for segregation and then integration, with opposite phase in the theta frequency range (3–5 Hz). The best fit to neurophysiological and behavioral data was given by a dampened 3-Hz oscillation from stimulus or eye fixation onset. The alignment of temporal integration windows to input changes found here may serve to actively organize the temporal processing of continuous sensory input.

Compact, ultra-low vibration, closed-cycle helium recycler for uninterrupted operation of MEG with SQUID magnetometers

A closed-cycle helium recycler was developed for continuous uninterrupted operation for magnetometer-based whole-head magnetoencephalography (MEG) systems. The recycler consists of a two stage 4K pulse-tube cryocooler and is mounted on the roof of a magnetically shielded room (MSR). A flexible liquid helium (LHe) return line on the recycler is inserted into the fill port of the MEG system in the MSR through a slotted opening in the ceiling. The helium vapor is captured through a line that returns the gas to the top of the recycler assembly. A high-purity helium gas cylinder connected to the recycler assembly supplies the gas, which, after it is liquefied, increases the level of LHe in the MEG system during the start-up phase. No storage tank for evaporated helium gas nor a helium gas purifier is used. The recycler is capable of liquefying helium with a rate of ∼17L/d after precooling the MEG system. It has provided a fully maintenance-free operation under computer control for 7months without refill of helium. Although the recycler is used for single-orientation operation at this initial testing site, it is designed to operate at ±20° orientations, allowing the MEG system to be tilted for supine and reclining positions. Vibration of the recycler is dampened to an ultra-low level by using several vibration isolation methods, which enables uninterrupted operation during MEG measurements. Recyclers similar to this system may be quite useful even for MEG systems with 100% magnetometers.

Skull Defects in Finite Element Head Models for Source Reconstruction from Magnetoencephalography Signals.

Magnetoencephalography (MEG) signals are influenced by skull defects. However, there is a lack of evidence of this influence during source reconstruction. Our objectives are to characterize errors in source reconstruction from MEG signals due to ignoring skull defects and to assess the ability of an exact finite element head model to eliminate such errors. A detailed finite element model of the head of a rabbit used in a physical experiment was constructed from magnetic resonance and co-registered computer tomography imaging that differentiated nine tissue types. Sources of the MEG measurements above intact skull and above skull defects respectively were reconstructed using a finite element model with the intact skull and one incorporating the skull defects. The forward simulation of the MEG signals reproduced the experimentally observed characteristic magnitude and topography changes due to skull defects. Sources reconstructed from measured MEG signals above intact skull matched the known physical locations and orientations. Ignoring skull defects in the head model during reconstruction displaced sources under a skull defect away from that defect. Sources next to a defect were reoriented. When skull defects, with their physical conductivity, were incorporated in the head model, the location and orientation errors were mostly eliminated. The conductivity of the skull defect material non-uniformly modulated the influence on MEG signals. We propose concrete guidelines for taking into account conducting skull defects during MEG coil placement and modeling. Exact finite element head models can improve localization of brain function, specifically after surgery.

New Perspectives on the Use of Spatial Filters in Magnetoencephalographic Array Processing

The increase in computer power of the last few decades has allowed the resurgence of the theory behind spatial filtering (a.k.a. beamforming) and its application to array signal processing. That is the case of magnetoencephalographic (MEG) data, which relies on dense arrays of detectors in order to measure the brain activity non-invasively. In particular, spatial filters are used in MEG signal processing to estimate the magnitude and location of the current sources within the brain. This is achieved by calculating different beamformer-based indexes which usually involve a large computational complexity. Here, a new perspective on how today’s computers make it possible to handle such complexity is presented, up to the point when new and ever more complex neural activity indexes can be developed. Such is the case of indexes based on eigenspace projections and reduced-rank beamformers, whose applicability is shown in this paper for the case of using real MEG measurements and realistic models.

Multimodal Diffusion-MRI and MEG Assessment of Auditory and Language System Development in Autism Spectrum Disorder.

Background: Auditory processing and language impairments are prominent in children with autism spectrum disorder (ASD). The present study integrated diffusion MR measures of white-matter microstructure and magnetoencephalography (MEG) measures of cortical dynamics to investigate associations between brain structure and function within auditory and language systems in ASD. Based on previous findings, abnormal structure-function relationships in auditory and language systems in ASD were hypothesized.Methods: Evaluable neuroimaging data was obtained from 44 typically developing (TD) children (mean age 10.4 ± 2.4 years) and 95 children with ASD (mean age 10.2 ± 2.6 years). Diffusion MR tractography was used to delineate and quantitatively assess the auditory radiation and arcuate fasciculus segments of the auditory and language systems. MEG was used to measure (1) superior temporal gyrus auditory evoked M100 latency in response to pure-tone stimuli as an indicator of auditory system conduction velocity, and (2) auditory vowel-contrast mismatch field (MMF) latency as a passive probe of early linguistic processes.Results: Atypical development of white matter and cortical function, along with atypical lateralization, were present in ASD. In both auditory and language systems, white matter integrity and cortical electrophysiology were found to be coupled in typically developing children, with white matter microstructural features contributing significantly to electrophysiological response latencies. However, in ASD, we observed uncoupled structure-function relationships in both auditory and language systems. Regression analyses in ASD indicated that factors other than white-matter microstructure additionally contribute to the latency of neural evoked responses and ultimately behavior. Results also indicated that whereas delayed M100 is a marker for ASD severity, MMF delay is more associated with language impairment.Conclusion: Present findings suggest atypical development of primary auditory as well as auditory language systems in ASD. Findings demonstrate the need for additional multimodal studies to better characterize the different structural features (white matter, gray matter, neurochemical concentration) that contribute to brain activity, both in typical development and in ASD. Finally, the neural latency measures were found to be of clinical significance, with M100 associated with overall ASD severity, and with MMF latency associated with language performance.

2. Follow-up of five cases with Rasmussen encephalitis with magnetoencephalography

Magnetoencephalography (MEG) recordings of interictal and/or ictal discharges can provide valuable information about the location of the epileptogenic area in the brain. Rasmussen Encephalitis (RE) is characterized by intractable epilepsy, progressive hemiparesis, and unilateral hemispheric atrophy. The progression of the symptoms to significant neurological impairment usually occurs within months to a few years. Five patients (mean age at first observation 7.6 ± 3 years) fitting Bien’s criteria for the diagnosis of RE were enrolled for the study and follow-up for at least 1.5 years (4–7 MEG measures per patient), in order to evaluate correlations between the evolution of clinical picture, neuroradiological and MEG findings. MEG data were visually inspected and interictal spikes were selected for the source localization by means of equivalent current dipoles (ECDs). Cortical thickness was evaluated from Magnetic Resonance Imaging (MRI). All patients showed ictal/interictal multifocality, reduction of background cortical magnetic activity, and progressive focal cortical atrophy on MRI of the affected hemisphere. Moreover, our preliminary data showed that dipole clusters were found mainly in the most atrophic cortical regions, suggesting a close relationship between MRI and MEG alterations.

ID 66 – Effect of a medical food on macroscopic brain activity in mild Alzheimer’s disease: Results from an exploratory double blind, randomised controlled magnetoencephalography study

Objective The aim of the study was to explore the effect of a medical food intervention, developed to improve synaptic formation and function, on magnetoencephalography (MEG), as a proxy of macroscopic effects of synaptic changes, in patients with mild Alzheimer’s disease (AD). Methods In an exploratory 24-weeks, double blind, randomised controlled sub-study of the Souvenir II study, differences between the active ( n = 22) and control ( n = 27) drug-naive mild AD patients were computed for frequency analysis, functional connectivity and functional brain network measures in source space MEG. Results The groups were significantly different with respect to baseline mini-mental state examination (MMSE) (active: 23.8 ± 2.3 (mean ± SD); control 25.3 ± 2.6). No consistent statistically significant intervention effects were found on the MEG measures. Conclusions In the current subsample of mild AD patients we could not detect an intervention effect of the medical food on MEG measures as opposed to EEG that has been used in a larger sample and revealed beneficial intervention effects on functional connectivity and brain network organisation in mild AD. Key message Further research is needed with substantially larger sample sizes and better balanced study groups to further assess the role of MEG in intervention trials in AD.

Impact of electroconvulsive therapy on magnetoencephalographic correlates of dysfunctional emotional processing in major depression

In major depressive disorder (MDD), electrophysiological and imaging studies provide evidence for a reduced neural activity in parietal and dorsolateral prefrontal regions. In the present study, neural correlates and temporal dynamics of visual affective perception have been investigated in patients with unipolar depression in a pre/post treatment design using magnetoencephalography (MEG). Nineteen in-patients and 19 balanced healthy controls passed MEG measurement while passively viewing pleasant, unpleasant and neutral pictures. After a 4-week treatment with electroconvulsive therapy or 4-week waiting period without intervention respectively, 16 of these patients and their 16 corresponding controls participated in a second MEG measurement. Before treatment neural source estimations of magnetic fields evoked by the emotional scenes revealed a general bilateral parietal hypoactivation in depressed patients compared to controls predominately at early and mid-latency time intervals. Successful ECT treatment, as reflected by a decline in clinical scores (Hamilton Depression Scale; HAMD) led to a normalization of this distinct parietal hypoactivation. Effective treatment was also accompanied by relatively increased neural activation at right temporo-parietal regions. The present study indicates dysfunctional parietal information processing and attention processes towards emotional stimuli in MDD patients which can be returned to normal by ECT treatment. Since convergent neural hypoactivations and treatment effects have recently been shown in MDD patients before and after pharmacological therapy, this electrophysiological correlate might serve as a biomarker for objective treatment evaluation and thereby potentially advance treatment options and support the prediction of individual treatment responses.

Musical training is related to altered functional connectivity during statistical learning: an MEG study

Musical training is related to altered functional connectivity during statistical learning: an MEG study

Maturation of auditory neural processes in autism spectrum disorder - A longitudinal MEG study.

BackgroundIndividuals with autism spectrum disorder (ASD) show atypical brain activity, perhaps due to delayed maturation. Previous studies examining the maturation of auditory electrophysiological activity have been limited due to their use of cross-sectional designs. The present study took a first step in examining magnetoencephalography (MEG) evidence of abnormal auditory response maturation in ASD via the use of a longitudinal design.MethodsInitially recruited for a previous study, 27 children with ASD and nine typically developing (TD) children, aged 6- to 11-years-old, were re-recruited two to five years later. At both timepoints, MEG data were obtained while participants passively listened to sinusoidal pure-tones. Bilateral primary/secondary auditory cortex time domain (100 ms evoked response latency (M100)) and spectrotemporal measures (gamma-band power and inter-trial coherence (ITC)) were examined. MEG measures were also qualitatively examined for five children who exhibited “optimal outcome”, participants who were initially on spectrum, but no longer met diagnostic criteria at follow-up.ResultsM100 latencies were delayed in ASD versus TD at the initial exam (~ 19 ms) and at follow-up (~ 18 ms). At both exams, M100 latencies were associated with clinical ASD severity. In addition, gamma-band evoked power and ITC were reduced in ASD versus TD. M100 latency and gamma-band maturation rates did not differ between ASD and TD. Of note, the cohort of five children that demonstrated “optimal outcome” additionally exhibited M100 latency and gamma-band activity mean values in-between TD and ASD at both timepoints. Though justifying only qualitative interpretation, these “optimal outcome” related data are presented here to motivate future studies.ConclusionsChildren with ASD showed perturbed auditory cortex neural activity, as evidenced by M100 latency delays as well as reduced transient gamma-band activity. Despite evidence for maturation of these responses in ASD, the neural abnormalities in ASD persisted across time. Of note, data from the five children whom demonstrated “optimal outcome” qualitatively suggest that such clinical improvements may be associated with auditory brain responses intermediate between TD and ASD. These “optimal outcome” related results are not statistically significant though, likely due to the low sample size of this cohort, and to be expected as a result of the relatively low proportion of “optimal outcome” in the ASD population. Thus, further investigations with larger cohorts are needed to determine if the above auditory response phenotypes have prognostic utility, predictive of clinical outcome.

Analysis of simultaneous MEG and intracranial LFP recordings during Deep Brain Stimulation: a protocol and experimental validation

BackgroundDeep Brain Stimulation (DBS) is an effective treatment for several neurological and psychiatric disorders. In order to gain insights into the therapeutic mechanisms of DBS and to advance future therapies a better understanding of the effects of DBS on large-scale brain networks is required. New methodIn this paper, we describe an experimental protocol and analysis pipeline for simultaneously performing DBS and intracranial local field potential (LFP) recordings at a target brain region during concurrent magnetoencephalography (MEG) measurement. Firstly we describe a phantom setup that allowed us to precisely characterise the MEG artefacts that occurred during DBS at clinical settings. ResultsUsing the phantom recordings we demonstrate that with MEG beamforming it is possible to recover oscillatory activity synchronised to a reference channel, despite the presence of high amplitude artefacts evoked by DBS. Finally, we highlight the applicability of these methods by illustrating in a single patient with Parkinson's disease (PD), that changes in cortical-subthalamic nucleus coupling can be induced by DBS. Comparison with existing approachesTo our knowledge this paper provides the first technical description of a recording and analysis pipeline for combining simultaneous cortical recordings using MEG, with intracranial LFP recordings of a target brain nucleus during DBS.

An MEG-Compatible Electromagnetic-Tracking System for Monitoring Orofacial Kinematics.

We describe a novel motion-tracking system, called MASK (magnetoarticulography for the assessment of speech kinematics) designed to track detailed orofacial movements during magnetoencephalographic (MEG) measures of human brain activity. A three-dimensional electromagnetic-tracking method was employed using lightweight coils energized with high-frequency sinusoidal currents, creating magnetic dipoles that can be continuously localized by the MEG sensors. In addition to being compatible with commercial MEG devices, this system has advantages over optical or video methods in that it can record nonline-of-sight movements (e.g., tongue movements) and advantages over surface electromyographic recordings, which are prone to movement-related artifacts and signal crosstalk. Static and dynamic tracking accuracy was evaluated using calibration devices with fixed intercoil distances. MEG data were collected in two healthy adult volunteers to test feasibility of tracking movements during tongue and facial movement, and during overt speech. The MASK system was shown to have sufficient static and dynamic accuracy to track orofacial movements within the MEG helmet. We successfully acquired spatially precise kinematic information time-locked to brain activity with high temporal resolution. We demonstrated successful tracking of oromotor and speech movements together with brain activity using the MASK system. This novel technology will provide an innovative tool in support of research and clinical applications for individuals with speech and other oromotor disorders.

Current Source Mapping by Spontaneous MEG and ECoG in Piglets Model.

The previous research reveals the presence of relatively strong spatial correlations from spontaneous activity over cortex in Electroencephalography (EEG) and Magnetoencephalography (MEG) measurement. A critical obstacle in MEG current source mapping is that strong background activity masks the relatively weak local information. In this paper, the hypothesis is that the dominant components of this background activity can be captured by the first Principal Component (PC) after employing Principal Component Analysis (PCA), thus discarding the first PC before the back projection would enhance the exposure of the information carried by a subset of sensors that reflects the local neuronal activity. By detecting MEG signals densely (one measurement per 2×2 mm2) in three piglets neocortical models over an area of 18×26 mm2 with a special shape of lesion by means of a μSQUID, this basic idea was demonstrated by the fact that a strong activity could be imaged in the lesion region after removing the first PC in Delta, Theta and Alpha band, while the original recordings did not show such activity clearly. Thus, the PCA decomposition can be employed to expose the local activity, which is around the lesion in the piglets' neocortical models, by removing the dominant components of the background activity.

Brain-Source Imaging: From sparse to tensor models

A number of application areas such as biomedical engineering require solving an underdetermined linear inverse problem. In such a case, it is necessary to make assumptions on the sources to restore identifiability. This problem is encountered in brain-source imaging when identifying the source signals from noisy electroencephalographic or magnetoencephalographic measurements. This inverse problem has been widely studied during recent decades, giving rise to an impressive number of methods using different priors. Nevertheless, a thorough study of the latter, including especially sparse and tensor-based approaches, is still missing. In this article, we propose 1) a taxonomy of the algorithms based on methodological considerations; 2) a discussion of the identifiability and convergence properties, advantages, drawbacks, and application domains of various techniques; and 3) an illustration of the performance of seven selected methods on identical data sets. Directions for future research in the area of biomedical imaging are eventually provided.

MEG—measured auditory steady-state oscillations show high test–retest reliability: A sensor and source-space analysis

Stability of oscillatory signatures across magnetoencephalography (MEG) measurements is an important prerequisite for basic and clinical research that has been insufficiently addressed. Here, we evaluated the test–retest reliability of auditory steady-state responses (ASSRs) over two MEG sessions. The study required participants (N=13) to detect the rare occurrence of pure tones interspersed within a stream of 5Hz or 40Hz amplitude-modulated (AM) tones. Intraclass correlations (ICC; Shrout and Fleiss, 1979) were derived to assess stability of spectral power changes and the inter-trial phase coherence (ITPC) of task-elicited neural responses. ASSRs source activity was estimated using eLORETA beamforming from bilateral auditory cortex. ASSRs to 40Hz AM stimuli evoked stronger power modulation and phase-locking than 5Hz stimulation. Overall, spectral power and ITPC values at both sensor- and source-level showed robust ICC values. Notably, ITPC measures yielded higher ICCs (~0.86–0.96) between sessions compared to the assessment of spectral power change (~0.61–0.82). Our data indicate that spectral modulations and phase consistency of ASSRs in MEG data are highly reproducible, providing support for MEG-measured oscillatory parameters in basic and clinical research.