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Measuring functional connectivity with wearable MEG
Optically-pumped magnetometers (OPMs) offer the potential for a step change in magnetoencephalography (MEG) enabling wearable systems that provide improved data quality, accommodate any subject group, allow data capture during movement and potentially reduce cost. However, OPM-MEG is a nascent technology and, to realise its potential, it must be shown to facilitate key neuroscientific measurements, such as the characterisation of brain networks. Networks, and the connectivities that underlie them, have become a core area of neuroscientific investigation, and their importance is underscored by many demonstrations of their disruption in brain disorders. Consequently, a demonstration of network measurements using OPM-MEG would be a significant step forward. Here, we aimed to show that a wearable 50-channel OPM-MEG system enables characterisation of the electrophysiological connectome. To this end, we measured connectivity in the resting state and during a visuo-motor task, using both OPM-MEG and a state-of-the-art 275-channel cryogenic MEG device. Our results show that resting-state connectome matrices from OPM and cryogenic systems exhibit a high degree of similarity, with correlation values >70%. In addition, in task data, similar differences in connectivity between individuals (scanned multiple times) were observed in cryogenic and OPM-MEG data, again demonstrating the fidelity of the OPM-MEG device. This is the first demonstration of network connectivity measured using OPM-MEG, and results add weight to the argument that OPMs will ultimately supersede cryogenic sensors for MEG measurement.
Journal Article
Aviation : cool women who fly
Introduces the field of aviation, focusing on the lives and careers of three specific women.
Book
Cross-Axis projection error in optically pumped magnetometers and its implication for magnetoencephalography systems
•The presented manuscript describes an error source commonly encountered in optically pumped magnetometers (OPMs) in presence of remnant static fields that we call “Cross-Axis projection Error”.•Through theoretical analysis, the cross-axis projection error is studied and simulated, and through experiment, it is induced in the measured OPM response. The simulation and experiment results are in good agreement.•The impact of cross-axis projection error on the localization capability of OPM-based magnetoencephalography systems (OPM-MEG) is simulated and analyzed.•We find that cross-axis projection error can be alleviated by keeping the remnant field below ±1 nT.
Optically pumped magnetometers (OPMs) developed for magnetoencephalography (MEG) typically operate in the spin-exchange-relaxation-free (SERF) regime and measure a magnetic field component perpendicular to the propagation axis of the optical-pumping photons. The most common type of OPM for MEG employs alkali atoms, e.g. 87Rb, as the sensing element and one or more lasers for preparation and interrogation of the magnetically sensitive states of the alkali atoms ensemble. The sensitivity of the OPM can be greatly enhanced by operating it in the SERF regime, where the alkali atoms’ spin exchange rate is much faster than the Larmor precession frequency. The SERF regime accommodates remnant static magnetic fields up to ±5 nT. However, in the presented work, through simulation and experiment, we demonstrate that multi-axis magnetic signals in the presence of small remnant static magnetic fields, not violating the SERF criteria, can introduce significant error terms in OPM's output signal. We call these deterministic errors cross-axis projection errors (CAPE), where magnetic field components of the MEG signal perpendicular to the nominal sensing axis contribute to the OPM signal giving rise to substantial amplitude and phase errors. Furthermore, through simulation, we have discovered that CAPE can degrade localization and calibration accuracy of OPM-based magnetoencephalography (OPM-MEG) systems.
Journal Article
Toucan keep a secret
\"Meg Langslow is at Trinity Episcopal locking up after an event and checking on the toucan Meg's friend Rev. Robyn Smith is fostering in her office. After hearing a hammering in the columbarium (the small building where cremated remains are held), Meg finds an elderly parishioner lying dead on the floor of the crypt. Several niches have been chiseled open; several urns knocked out; and amid the spilled ashes is a gold ring with a huge red stone. The curmudgeonly victim had become disgruntled with the church and ranted all over town about taking back his wife's ashes. Did someone who had it in for him follow him to the columbarium? Or was the motive grave robbery? Or did he see someone breaking in and investigate? Why was the ruby left behind? While the Chief Burke investigates the murder, Robyn recruits Meg to contact the families of the people whose ashes were disturbed. During this task, Meg learns many secrets about Caerphilly's history--and finds that the toucan may play a role in unmasking the killer. Clues and events indicate that a thief broke into the church to steal the toucan the night of the murder, so Meg decides to set a trap for the would-be toucan thief--who might also be the killer\"-- Provided by publisher.
Book
Autoreject: Automated artifact rejection for MEG and EEG data
We present an automated algorithm for unified rejection and repair of bad trials in magnetoencephalography (MEG) and electroencephalography (EEG) signals. Our method capitalizes on cross-validation in conjunction with a robust evaluation metric to estimate the optimal peak-to-peak threshold – a quantity commonly used for identifying bad trials in M/EEG. This approach is then extended to a more sophisticated algorithm which estimates this threshold for each sensor yielding trial-wise bad sensors. Depending on the number of bad sensors, the trial is then repaired by interpolation or by excluding it from subsequent analysis. All steps of the algorithm are fully automated thus lending itself to the name Autoreject.
In order to assess the practical significance of the algorithm, we conducted extensive validation and comparisons with state-of-the-art methods on four public datasets containing MEG and EEG recordings from more than 200 subjects. The comparisons include purely qualitative efforts as well as quantitatively benchmarking against human supervised and semi-automated preprocessing pipelines. The algorithm allowed us to automate the preprocessing of MEG data from the Human Connectome Project (HCP) going up to the computation of the evoked responses. The automated nature of our method minimizes the burden of human inspection, hence supporting scalability and reliability demanded by data analysis in modern neuroscience.
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•A strategy for artifact rejection in M/EEG using peak-to-peak thresholds is proposed•The thresholds are estimated using cross-validation with a robust error metric•The method detects and repairs outlier data segments for each sensor•Comparison with competing methods on 200 subjects with ground truth responses
Journal Article
Gone gull
\"Gone Gull brings readers yet another knee-slapping adventure filled with New York Times bestselling author Donna Andrews' cast of wacky characters. Meg Langslow is spending the summer at the Biscuit Mountain Craft Center, helping her grandmother Cordelia run the studios. But someone is committing acts of vandalism, threatening to ruin the newly-opened center's reputation. Is it the work of a rival center? Have the developers who want to build a resort atop Biscuit Mountain found a new tactic to pressure Cordelia into selling? Or is the real target Meg's grandfather, who points out that any number of environmentally irresponsible people and organizations could have it in for him? While Meg is trying to track down the vandal, her grandfather is more interested in locating a rare gull. Their missions collide when a body is found in one of the classrooms. Can Meg identify the vandal and the murderer in time to save the center's name--while helping her grandfather track down and rescue his beloved gulls?\"-- Provided by publisher.
Book
Mouth magnetoencephalography: A unique perspective on the human hippocampus
Traditional magnetoencephalographic (MEG) brain imaging scanners consist of a rigid sensor array surrounding the head; this means that they are maximally sensitive to superficial brain structures. New technology based on optical pumping means that we can now consider more flexible and creative sensor placement. Here we explored the magnetic fields generated by a model of the human hippocampus not only across scalp but also at the roof of the mouth. We found that simulated hippocampal sources gave rise to dipolar field patterns with one scalp surface field extremum at the temporal lobe and a corresponding maximum or minimum at the roof of the mouth. We then constructed a fitted dental mould to accommodate an Optically Pumped Magnetometer (OPM). We collected data using a previously validated hippocampal-dependant task to test the empirical utility of a mouth-based sensor, with an accompanying array of left and right temporal lobe OPMs. We found that the mouth sensor showed the greatest task-related theta power change. We found that this sensor had a mild effect on the reconstructed power in the hippocampus (~10% change) but that coherence images between the mouth sensor and reconstructed source images showed a global maximum in the right hippocampus. We conclude that augmenting a scalp-based MEG array with sensors in the mouth shows unique promise for both basic scientists and clinicians interested in interrogating the hippocampus.
Journal Article
Die like an eagle
Meg Langslow is Team Mom and Michael is coach of their twin sons' youth baseball team, the Caerphilly Eagles. Meg tangles with Biff Brown, the petty, vindictive league head. On opening day, Biff's lookalike brother is found dead in the porta-potty at the ball field. So many people think Biff's scum that it would be easy to blame him, but he has an alibi and Meg suspects he may actually have been the intended victim. With Die Like an Eagle, readers can look forward to another zany Meg Langslow mystery this one filled with the spirit of America's pastime and Donna's eagle eye.Like Meg Langslow, the blacksmith heroine of her series, Donna Andrews was born and raised in Yorktown, Virginia. She introduced Meg to readers in her Malice Domestic Contest-winning first mystery, Murder with Peacocks, and readers are still laughing. This novel swept up the Agatha, Anthony, Barry, and a Romantic Times award for best first novel, and a Lefty for funniest mystery\"-- Provided by publisher.
Book
Real-time, model-based magnetic field correction for moving, wearable MEG
•Zero-field OPMs operate within a limited magnetic field range.•We correct for background field changes in real-time using coils on-board the OPMs.•We used a model of the background field based on low-order regular solid harmonics.•We were able to record auditory evoked fields during movements of 1.5m - 2m.
Most neuroimaging techniques require the participant to remain still for reliable recordings to be made. Optically pumped magnetometer (OPM) based magnetoencephalography (OP-MEG) however, is a neuroimaging technique which can be used to measure neural signals during large participant movement (approximately 1 m) within a magnetically shielded room (MSR) (Boto et al., 2018; Seymour et al., 2021). Nevertheless, environmental magnetic fields vary both spatially and temporally and OPMs can only operate within a limited magnetic field range, which constrains participant movement. Here we implement real-time updates to electromagnetic coils mounted on-board of the OPMs, to cancel out the changing background magnetic fields. The coil currents were chosen based on a continually updating harmonic model of the background magnetic field, effectively implementing homogeneous field correction (HFC) in real-time (Tierney et al., 2021). During a stationary, empty room recording, we show an improvement in very low frequency noise of 24 dB. In an auditory paradigm, during participant movement of up to 2 m within a magnetically shielded room, introduction of the real-time correction more than doubled the proportion of trials in which no sensor saturated recorded outside of a 50 cm radius from the optimally-shielded centre of the room. The main advantage of such model-based (rather than direct) feedback is that it could allow one to correct field components along unmeasured OPM axes, potentially mitigating sensor gain and calibration issues (Borna et al., 2022).
Journal Article