[show abstract][hide abstract] ABSTRACT: Recently, the signal space separation (SSS) method, based on the multipole expansion of the magnetic field, has become increasingly important in magnetoencephalography (MEG). Theoretical arguments and simulations suggest that increasing the asymmetry of the MEG sensor array from the traditional, rather symmetric geometry can significantly improve the performance of the method. To test this concept, we first simulated addition of tangentially oriented standard sensor elements to the existing 306-channel Elekta Neuromag® sensor array, and evaluated and optimized the performance of the new sensor configuration. Based on the simulation results, we then constructed a prototype device with 18 additional tangential triple sensor elements and a total of 360 channels. The experimental results from the prototype are largely in agreement with the simulations. In application of the spatial signal space separation method, the 360-channel device shows an approximately 100% increase in software shielding capability, while residual reconstruction noise of evoked responses is decreased by 20%. Further, the new device eliminates the need for regularization while applying the SSS method. In conclusion, we have demonstrated in practice the benefit of reducing the symmetry of the MEG array, without the need for a complete redesign.
IEEE transactions on bio-medical engineering 04/2013; · 2.15 Impact Factor
[show abstract][hide abstract] ABSTRACT: INTRODUCTION: Magnetoencephalography (MEG) measures magnetic fields generated by neuronal currents. MEG is complementary to EEG. Considerable body of evidence indicates that ictal MEG recordings can provide useful information for pre-surgical evaluation of epilepsy patients alongside the more established long-term ictal video-EEG. Ictal MEG is recorded in some epilepsy surgery centers. However, a wider adoption of ictal MEG is hampered by lack of tools for synchronized video-MEG recording similar to those of video-EEG. METHODS: We have augmented MEG with a synchronized behavioral video-recording system. To estimate its additional value in ictal recordings, we retrospectively analyzed recordings of 10 epilepsy patients with and without the video. RESULTS: In six patients out of ten, adding the video substantially changed the resulting interpretations. In all six cases the effect was considerable: the number of detected seizures changed by more than 50%. CONCLUSIONS: Synchronized video and audio recording capabilities are important for effective ictal MEG recordings of epilepsy patients.
[show abstract][hide abstract] ABSTRACT: Ultra-low-field (ULF) MRI (B 0 = 10-100 µT) typically suffers from a low signal-to-noise ratio (SNR). While SNR can be improved by pre-polarization and signal detection using highly sensitive superconducting quantum interference device (SQUID) sensors, we propose to use the inter-dependency of the k-space data from highly parallel detection with up to tens of sensors readily available in the ULF MRI in order to suppress the noise. Furthermore, the prior information that an image can be sparsely represented can be integrated with this data consistency constraint to further improve the SNR. Simulations and experimental data using 47 SQUID sensors demonstrate the effectiveness of this data consistency constraint and sparsity prior in ULF-MRI reconstruction.
PLoS ONE 01/2013; 8(4):e61652. · 3.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: The development of speech perception shows a dramatic transition between infancy and adulthood. Between 6 and 12 months, infants' initial ability to discriminate all phonetic units across the world's languages narrows-native discrimination increases while non-native discrimination shows a steep decline. We used magnetoencephalography (MEG) to examine whether brain oscillations in the theta band (4-8 Hz), reflecting increases in attention and cognitive effort, would provide a neural measure of the perceptual narrowing phenomenon in speech. Using an oddball paradigm, we varied speech stimuli in two dimensions, stimulus frequency (frequent vs. infrequent) and language (native vs. non-native speech syllables) and tested 6-month-old infants, 12-month-old infants, and adults. We hypothesized that 6-month-old infants would show increased relative theta power (RTP) for frequent syllables, regardless of their status as native or non-native syllables, reflecting young infants' attention and cognitive effort in response to highly frequent stimuli ("statistical learning"). In adults, we hypothesized increased RTP for non-native stimuli, regardless of their presentation frequency, reflecting increased cognitive effort for non-native phonetic categories. The 12-month-old infants were expected to show a pattern in transition, but one more similar to adults than to 6-month-old infants. The MEG brain rhythm results supported these hypotheses. We suggest that perceptual narrowing in speech perception is governed by an implicit learning process. This learning process involves an implicit shift in attention from frequent events (infants) to learned categories (adults). Theta brain oscillatory activity may provide an index of perceptual narrowing beyond speech, and would offer a test of whether the early speech learning process is governed by domain-general or domain-specific processes.
[show abstract][hide abstract] ABSTRACT: Parkinsonian patients have abnormal oscillatory activity within the basal ganglia-thalamocortical circuitry. Particularly, excessive beta band oscillations are thought to be associated with akinesia. We studied whether cortical spontaneous activity is modified by deep brain stimulation (DBS) in advanced Parkinson's disease and if the modifications are related to the clinical symptoms.
We studied the effects of bilateral electrical stimulation of subthalamic nucleus (STN) on cortical spontaneous activity by magnetoencephalography (MEG) in 11 Parkinsonian patients. The artifacts produced by DBS were suppressed by tSSS algorithm.
During DBS, UPDRS (Unified Parkinson's Disease Rating Scale) rigidity scores correlated with 6-10Hz and 12-20Hz somatomotor source strengths when eyes were open. When DBS was off UPDRS action tremor scores correlated with pericentral 6-10Hz and 21-30Hz and occipital alpha source strengths when eyes open. Occipital alpha strength decreased during DBS when eyes closed. The peak frequency of occipital alpha rhythm correlated negatively with total UPDRS motor scores and with rigidity subscores, when eyes closed.
STN DBS modulates brain oscillations both in alpha and beta bands and these oscillations reflect the clinical condition during DBS.
MEG combined with an appropriate artifact rejection method enables studies of DBS effects in Parkinson's disease and presumably also in the other emerging DBS indications.
Clinical neurophysiology: official journal of the International Federation of Clinical Neurophysiology 04/2012; 123(10):2010-7. · 3.12 Impact Factor
[show abstract][hide abstract] ABSTRACT: We studied the degree of repeatability of auditory and somatosensory evoked fields (AEF and SEF). A single subject was studied
repeatedly with a 306-channel Elekta Neuromag® system over a period of six years. In 21 recordings the subject kept his head
as immobile as possible. Averaged AEF and SEF data were analyzed with equivalent current dipoles (ECDs). The mean 3D variations
of the dipole localizations were 5.6mm for SEF, 6.7mm for ipsilateral AEF, and 5.9mm for contralateral AEF.
In addition, 17 recordings were performed with the subject moving his head continuously. In some of these recordings, additional
interference was introduced by leaving the door of the magnetically shielded room open or by attaching magnetized pieces on
the scalp. The ECDs obtained from the corresponding static head position served as the reference. After applying spatiotemporal
signal space separation (tSSS) and head movement compensation (MC), the mean ECD localization differences to the reference
localizations were 3.5 ± 2.6mm for SEF, 4.0 ± 1.5 mm for ipsilateral AEF, and 4.0 ± 2.0 mm for contralateral AEF.
The combined tSSS and MC processing compensated effectively for the disturbances caused by head motion and additional interference
sources. All ECD parameters were similar to the reference results for SEFs, but AEF dipole amplitudes decreased by 30% during
head movement. Importantly, the dipole localization differences after tSSS and MC processing of data with motion and additional
interference are smaller than the localization variability in the reference recordings.
KeywordsRepeatability-Movement compensation-Signal space separation-AEF-SEF
[show abstract][hide abstract] ABSTRACT: Video-EEG is the gold standard in clinical epilepsy diagnostics. MEG systems have traditionally lacked video capabilities.
Here we describe the addition of a synchronized digital video recording (DVR) system to a standard Elekta Neuromag® 306-channel
MEG device operated at BioMag laboratory of the Helsinki University Central Hospital. We also provide some insights obtained
from using the system for ictal MEG recordings. Video and audio were recorded simultaneously with MEG, using a separate digital
video recording station. Synchronization was achieved by timestamping the video, audio and MEG streams and synchronizing all
parts of the system via Network Time Protocol (NTP). Video was recorded at 25 fps with an infrared-sensitive camera necessary
to study patients that have seizures during sleep. MEG and video are synchronized to the maximum achievable accuracy of one
video frame. At this time, five epileptic patients have undergone video-MEG recordings with the setup described above. In
three patients, ictal recordings were obtained. In two cases the video con siderably facilitated the exact temporal localization
of the seizure onset, thus reducing the amount of time required for data analysis and improving the reliability of the spatial
[show abstract][hide abstract] ABSTRACT: We have combined Signal Space Separation and beamformers (SSS beamformer). The SSS beamformer was tested by simulation in the presence of simulated brain noise. The SSS beamformer performs at least as well as the conventional beamformer, provided that the expansion order is sufficiently high. For beamformer outputs which depend on power or power difference normalized by the projected noise, the spatial resolution of the SSS beamformer is significantly better than that of the conventional beamformers if the sources are deeper, and about the same as that of the conventional beamformer when the sources are superficial. For beamformer outputs which depend on the ratio of powers, the spatial resolutions of the SSS and conventional beamfomers are the same. The sensor noise covariance matrix in the SSS basis is non-diagonal. The SSS beamformers with diagonalized noise covariance matrix exhibit better spatial resolution than that with non-diagonal noise covariance matrix. The SSS beamformers are computationally more efficient than the conventional beamformers.
[show abstract][hide abstract] ABSTRACT: Head, jaw and tongue movements contribute to speech artifacts in magnetoencephalography (MEG). Their sources lay close to MEG sensors, therefore, the spatio-temporal signal space separation method (tSSS), specifically suppressing nearby artifacts, can be used for speech artifact suppression. After data reconstruction by signal space separation (referred as SSS), tSSS identifies artifacts by their correlated temporal behavior inside and outside the sensor helmet. The artifacts to be eliminated are thresholded by the quantitative level of this correlation determined by correlation limit (CL). Unnecessarily high CL value may result in suboptimal interference suppression. We evaluated the performance of tSSS with different CLs on MEG data containing speech artifacts. MEG was recorded with 204 planar gradiometers and 102 magnetometers in two subjects counting aloud. The recorded data were processed by tSSS using CLs 0.98, 0.8 and 0.6, and traces were compared. The speech artifact was increasingly suppressed with decreasing CL, but sufficient suppression was achieved at different CL in each subject. Alpha rhythm was not suppressed with CL 0.98 or 0.8; some amplitude reduction with CL 0.6 occurred in one subject. The tSSS is a robust tool suppressing MEG artifacts. It can be fine tuned for challenging artifacts which, after insufficient rejection might resemble brain signals.
Journal of Neuroscience Methods 11/2008; 177(1):203-11. · 2.11 Impact Factor
[show abstract][hide abstract] ABSTRACT: In adults, neural responses to the acoustic properties of native and non-native speech sounds differ. Recent magnetoencephalography (MEG) studies indicate that two regions of the brain, the superior temporal area and inferior parietal area (Broca's), in both the left and right hemispheres, may contribute to how the adult brain responds to the acoustic properties of speech. The aim of the present study was to investigate neural activation in adults when listening to native and non-native speech, and to non-speech analogs of speech. We used MEG to examine activation of the superior temporal and inferior parietal areas. Native speakers of Finnish were presented with Finnish (pa and ta) and Chinese (t "looped c" "superscript h" i and "looped c" i) syllables, and sine-wave replicas of pa and ta, in a passive oddball paradigm. Our analysis examines the properties of localized neural activities to determine how activation of sensory and motor speech areas differs for speech and non-speech, and the role of language experience in brain activation.
The Journal of the Acoustical Society of America 06/2008; 123(5):3333. · 1.65 Impact Factor
[show abstract][hide abstract] ABSTRACT: Magnetoencephalography (MEG) provides a safe, noninvasive method for studying the developing brain by offering reliable localization of the brain regions activated during speech processing. However technical challenges make recording awake infants difficult. The small size of the infant head in the adult-sized helmet results in a low signal-to-noise ratio. Head and limb movement, which is typical of young infants, produces signal artifact that is difficult to overcome during signal processing. This study used MEG to study phonetic processing in awake, non-sedated typically developing infants from 5 to 16 months. The recordings were made using the Elekta Neuromag(R) 306-channel instrument at BioMag Laboratory, Helsinki University Central Hospital, Finland. Infants listened to speech syllables produced by a loudspeaker inside the magnetically shielded room. Newly developed signal processing methods and behavioral entertainment greatly improved the quality of the data, producing 29 successful infant recordings out of 35 attempts. We describe the methods, as well as removal of movement-modulated artifacts, efficient interference suppression, and movement compensation during data analysis. Whole-head MEG recordings in awake babies a few months old are now feasible.
The Journal of the Acoustical Society of America 06/2008; 123(5):3742. · 1.65 Impact Factor
[show abstract][hide abstract] ABSTRACT: A 122-channel d.c. SQUID magnetometer with a helmet-shaped detector array covering the subject's head has been operational in the Low Temperature Laboratory of the Helsinki University of Technology since June 1992. The new system allows simultaneous recording of magnetic activity all over the head. The probe employs 122 planar first-order thin-film gradiometers in dual units with two exactly orthogonal channels at 61 measurement sites. The performance of the device is analyzed and compared with more conventional axial gradiometer arrays by considering signal-to-noise ratios, spatial sampling theory, confidence intervals for the estimated equivalent current dipole positions, and information-theoretical channel capacity. The signal-to-noise ratio and the resolution of the planar and axial arrays with the same number of channels are found practically equal. The number of channels and their spacing in our new Neuromag-122 system are found fully adequate for neuromagnetic measurements. An example of whole cortex recordings of auditory evoked brain activity is presented and analyzed.
Physica Scripta 01/2007; 1993(T49A):198. · 1.03 Impact Factor
[show abstract][hide abstract] ABSTRACT: Abnormal low-frequency entrainment in sensorimotor cortical areas of Parkinson's disease (PD) patients has been observed in MEG recordings. Efficacy of high-frequency stimulation of subthalamic nucleus (STN) in treating PD may result from blocking such abnormal oscillations. Artifacts elicited by stimulation have precluded MEG study of such patients. We recorded spontaneous MEG activity with a 306-channel Elekta Neuromag® instrument from a Parkinsonian patient with an implanted neurostimulator, with electrodes in both subthalamic nuclei when 130-Hz STN stimulation was on and off. Auditory evoked fields (AEFs) were recorded as well. Brain activity was revealed by applying temporally extended signal space separation on the signals strongly contaminated by stimulation-related artifacts.Bipolar STN stimulation decreased spontaneous activity in the 3–20 Hz range over the right sensorimotor cortex both when eyes were open and closed. Less systematic changes were observed over the left hemisphere where monopolar stimulation was associated with an increase of the low-frequency activity. Suppression of occipital alpha activity in eyes open condition as well as AEF source locations corresponded with those in normal subjects. The results suggest that STN stimulation modifies spontaneous activity over the sensorimotor cortices.
International Congress Series 01/2007; 1300:345-348.
[show abstract][hide abstract] ABSTRACT: Discriminative responses to tones, harmonics, and syllables in the left hemisphere were measured with magnetoencephalography in neonates, 6-month-old infants, and 12-month-old infants using the oddball paradigm. Real-time head position tracking, signal space separation, and head position standardization were applied to secure quality data for source localization. Minimum current estimates were calculated to characterize infants' cortical activities for detecting sound changes. The activation patterns observed in the superior temporal and inferior frontal regions provide initial evidence for the developmental emergence early in life of a perceptual-motor link for speech perception that may depend on experience.