Whole-head Magnetoencephalography System Research Articles

From cryogenic to on-scalp magnetoencephalography for the evaluation of paediatric epilepsy.

Magnetoencephalography (MEG) is a neurophysiological technique based on the detection of brain magnetic fields. Whole-head MEG systems typically house a few hundred sensors requiring cryogenic cooling in a rigid one-size-fits-all (commonly adult-sized) helmet to keep a thermal insulation space. This leads to an increased brain-to-sensor distance in children, because of their smaller head circumference, and decreased signal-to-noise ratio. MEG allows detection and localization of interictal and ictal epileptiform discharges, and pathological high frequency oscillations, as a part of the presurgical assessment of children with refractory focal epilepsy, where electroencephalography is not contributive. MEG can also map the eloquent cortex before surgical resection. MEG also provides insights into the physiopathology of both generalized and focal epilepsy. On-scalp recordings based on cryogenic-free sensors have demonstrated their use in the field of childhood focal epilepsy and should become a reference technique for diagnosing epilepsy in the paediatric population. WHAT THIS PAPER ADDS: Magnetoencephalography (MEG) contributes to the diagnosis and understanding of paediatric epilepsy. On-scalp MEG recordings demonstrate some advantages over cryogenic MEG.

The neural processing of sound in infants

Early auditory abilities play an important role in infants’ ability to acquire language, appreciate music, and navigate the complex acoustic environments around them. However, the neural mechanisms that support infant sound processing are not well understood. This study used magnetoencephalography (MEG) with recent advancements in movement compensation to obtain functional measures of auditory processing in awake infants. MEG responses were recorded longitudinally at 3, 6, and 11 months of age to an English bi-syllabic word and an amplitude-modulated complex tone as a non-speech but spectro-temporally complex control stimulus. MEG recordings were made with an Elekta Neuromag® whole-head 306-channel MEG system. The neural generators of the MEG signals were determined using an equivalent current dipole (ECD) model. High quality MEG data with good signal-to-noise ratios were recorded in infants as young as 3 months of age in both conditions. ECD field maps showed focal sources in the brain confirming the ECD model is a robust source localization approach that can fit auditory neural sources in infants. Our results show that dipole modeling of MEG signals in combination with advanced movement compensation offers a temporally precise method of investigating how the cortical processing of sound changes across the first year of life.

Frequency-specific equivalence of brain activity on motor imagery during action observation and action execution

Background Human motor imagery (MI), action execution, and action observation (AO) are functionally considered as equivalent. MI during AO can extensively induce activation of motor-related brain network in the absence of overt movement. The magnetoencephalography (MEG) provides an important technology to reveal and reflect human brain information processing in multi-frequency bands. Utilizing a MEG system, we aimed to quantitatively investigate the frequency-specific equivalent characteristics in brain processing patterns between MI during AO and action execution in multi-frequency bands, including delta, theta, alpha, beta, gamma, and high-frequency oscillations. Methods A total of 12 healthy subjects were studied with a whole-head MEG system during finger movement and MI during finger movement observation. We analyzed the brain activities in multi-frequency ranges of 1 Hz to 200 Hz. Results Both MI during AO and action execution evoked the distinctive brain activities in low frequency ranges (i.e. delta, theta, and alpha). Significant differences were found in global spectral power between finger movement and MI during AO in delta and alpha oscillations. Compared with finger movement, delta (1–4 Hz) oscillation power in MI during AO were obviously decreased in left and right frontals and occipitals, and theta (4–8 Hz) and alpha (8–13 Hz) oscillation power were obviously increased in frontal, parietal and occipital. Conclusion MEG power evoked by finger movement and MI during AO is mainly concentrated in the energy distribution below 13 Hz. Furthermore, finger movement and MI during AO might share frequency-specific equivalence of brain neural activation dependent on different MEG frequency ranges.

On-scalp MEG sensor localization using magnetic dipole-like coils: A method for highly accurate co-registration

Source modelling in magnetoencephalography (MEG) requires precise co-registration of the sensor array and the anatomical structure of the measured individual’s head. In conventional MEG, the positions and orientations of the sensors relative to each other are fixed and known beforehand, requiring only localization of the head relative to the sensor array. Since the sensors in on-scalp MEG are positioned on the scalp, locations of the individual sensors depend on the subject’s head shape and size. The positions and orientations of on-scalp sensors must therefore be measured at every recording. This can be achieved by inverting conventional head localization, localizing the sensors relative to the head - rather than the other way around.In this study we present a practical method for localizing sensors using magnetic dipole-like coils attached to the subject’s head. We implement and evaluate the method in a set of on-scalp MEG recordings using a 7-channel on-scalp MEG system based on high critical temperature superconducting quantum interference devices (high-Tc SQUIDs). The method allows individually localizing the sensor positions, orientations, and responsivities with high accuracy using only a short averaging time (≤ 2 ​mm, < 3° and < 3%, respectively, with 1-s averaging), enabling continuous sensor localization. Calibrating and jointly localizing the sensor array can further improve the accuracy of position and orientation (< 1 ​mm and < 1°, respectively, with 1-s coil recordings).We demonstrate source localization of on-scalp recorded somatosensory evoked activity based on co-registration with our method. Equivalent current dipole fits of the evoked responses corresponded well (within 4.2 ​mm) with those based on a commercial, whole-head MEG system.

Noise cancellation for a whole-head magnetometer-based MEG system in hospital environment

We describe a strategy of removing magnetic field interference for a whole-head pediatric magnetoencephalography (MEG) system (‘babyMEG’) installed in a hospital. The 375-channel sensor array of babyMEG consists entirely of magnetometers in two layers to maximize the sensitivity for detecting MEG signals from infants, toddlers, and young children. It is equipped with a continuously operating closed-cycle helium recycler to reduce the operating costs. These two features pose special challenges for noise cancellation. Our strategy uses a combination of several methods. The system is installed in a light-weight, magnetically shielded room (MSR) equipped with an active external shielding. In addition we employ two software-based techniques—a signal space projection (SSP) technique and a synthetic gradiometer (SG) method—for removing the environmental magnetic noise in real time and displaying the output online. The shielding effects are: passive shielding–36 dB, active shielding–12 dB, SSP–40 dB and SG–40 dB, for a combined maximum shielding of about 90 dB at 0.1 Hz. We evaluated the performance of the babyMEG after applying the noise cancellation techniques. The dipole localization errors were <3 mm after averaging 50 epochs with empty room noise in a simulation study for dipoles >10 nAm, which is in the low range of empirically observed dipole moments. In a phantom study with realistic environmental noise, we could clearly recover an evoked cortical magnetic field produced by a 20 nAm dipole after averaging 50 epochs. The localization error was ∼6 mm after averaging 20 epochs. In infants, we could clearly detect a somatic evoked field after averaging ∼20 responses. The unique two-layer sensor design combined with the SSP or SG provides effective noise suppression for a magnetometer-based pediatric MEG system in hospital environment with the closed-cycle helium recycler operating continuously during MEG measurements.

Non-invasive Investigation of Human Hippocampal Rhythms Using Magnetoencephalography: A Review.

Hippocampal rhythms are believed to support crucial cognitive processes including memory, navigation, and language. Due to the location of the hippocampus deep in the brain, studying hippocampal rhythms using non-invasive magnetoencephalography (MEG) recordings has generally been assumed to be methodologically challenging. However, with the advent of whole-head MEG systems in the 1990s and development of advanced source localization techniques, simulation and empirical studies have provided evidence that human hippocampal signals can be sensed by MEG and reliably reconstructed by source localization algorithms. This paper systematically reviews simulation studies and empirical evidence of the current capacities and limitations of MEG “deep source imaging” of the human hippocampus. Overall, these studies confirm that MEG provides a unique avenue to investigate human hippocampal rhythms in cognition, and can bridge the gap between animal studies and human hippocampal research, as well as elucidate the functional role and the behavioral correlates of human hippocampal oscillations.

Brain Activity on Observation of Another Person's Action: A Magnetoencephalographic Study.

Brain activity was recorded using a whole-head magnetoencephalography system followed by coherence analysis to assess neural connectivity in 10 healthy right-handed adults to clarify differences in neural connectivity in brain regions during action observation from several perspectives. The subjects were instructed to observe and memorize or imitate the hand action from a first-person or second-person visual perspective. The brain activity in coherence was modified among frontal and central, sensorimotor, and mirror neuron system-related regions based on the visual perspectives of finger movements. The regional activity in coherence changed similarly under the imitation and observation tasks compared with the condition of observing static hand figures. The information from different visual perspectives of body movements was processed in the frontal-central regions related to sensorimotor processes and partially in mirror neuron system.

Feed-forward-related neural activity for vocalization: A pilot study using magnetoencephalography

The present pilot study was aimed to clarifying neural activity related with the feed-forward function, sensory-motor integration processed before motor performance. Brain magnetic fields were recorded using a whole-head magnetoencephalography system before vocalization under various levels of surrounding noise in nine healthy volunteers. Participants were instructed to read a word presented on a monitor under auditory conditions with white noise of 60, 80, and 100-dB. The vocal strength of the participants significantly increased with an increase in the intensity of environmental noise. Neural connectivity associated with coherence was calculated among the vocalization-related brain areas selected: truncal area in the primary motor area (M1), premotor area (PM), supplementary motor area (SMA), posterior inferior frontal area (pIF), and posterior part of the superior temporal area (pST). Coherence between M1 and pIF in the frontal cortex without vocalization and auditory pST in the temporal area before vocalization was negatively correlated with the level of environmental noise and vocal strength, respectively. We considered that the findings in this pilot study are, at least partially, relevant to the feed-forward function for vocalization.

Bilateral Transcranial Direct Current Stimulation Reshapes Resting-State Brain Networks: A Magnetoencephalography Assessment.

Transcranial direct current stimulation (tDCS) can noninvasively induce brain plasticity, and it is potentially useful to treat patients affected by neurological conditions. However, little is known about tDCS effects on resting-state brain networks, which are largely involved in brain physiological functions and in diseases. In this randomized, sham-controlled, double-blind study on healthy subjects, we have assessed the effect of bilateral tDCS applied over the sensorimotor cortices on brain and network activity using a whole-head magnetoencephalography system. Bilateral tDCS, with the cathode (−) centered over C4 and the anode (+) centered over C3, reshapes brain networks in a nonfocal fashion. Compared to sham stimulation, tDCS reduces left frontal alpha, beta, and gamma power and increases global connectivity, especially in delta, alpha, beta, and gamma frequencies. The increase of connectivity is consistent across bands and widespread. These results shed new light on the effects of tDCS and may be of help in personalizing treatments in neurological disorders.

The significance of brain oscillations in motor sequence learning: Insights from Parkinson's disease.

Motor sequence learning plays a pivotal role in various everyday activities. Motor-cortical beta oscillations have been suggested to be involved in this type of learning. In Parkinson's disease (PD), oscillatory activity within cortico-basal-ganglia circuits is altered. Pathologically increased beta oscillations have received particular attention as they may be associated with motor symptoms such as akinesia. In the present magnetoencephalography (MEG) study, we investigated PD patients and healthy controls (HC) during implicit motor sequence learning with the aim to shed light on the relation between changes of cortical brain oscillations and motor learning in PD with a particular focus on beta power. To this end, 20 PD patients (ON medication) and 20 age- and sex-matched HC were trained on a serial reaction time task while neuromagnetic activity was recorded using a 306-channel whole-head MEG system. PD patients showed reduced motor sequence acquisition and were more susceptible to interference by random trials after training on the task as compared to HC. Behavioral differences were paralleled by changes at the neurophysiological level. Diminished sequence acquisition was paralleled by less training-related beta power suppression in motor-cortical areas in PD patients as compared to HC. In addition, PD patients exhibited reduced training-related theta activity in motor-cortical areas paralleling susceptibility to interference. The results support the hypothesis that the acquisition of a new motor sequence relies on suppression of motor-cortical beta oscillations, while motor-cortical theta activity might be related to stabilization of the learned sequence as indicated by reduced susceptibility to interference. Both processes appear to be impaired in PD.

Altered resting-state functional connectivity in patients with obsessive–compulsive disorder: A magnetoencephalography study

Aberrant cortical–striatal–thalamic–cortical circuits have been implicated in the pathophysiology of obsessive−compulsive disorder (OCD). However, the neurobiological basis of OCD remains unclear. We compared patterns of functional connectivity in patients with OCD and in healthy controls using resting-state magnetoencephalography (MEG). Participants comprised 24 patients with OCD (21 men, 3 women) and 22 age- and sex-matched healthy controls (19 men, 3 women). Resting-state measurements were obtained over a 6-min period using a 152-channel whole-head MEG system. We examined group differences in oscillatory activity and distribution of functional cortical hubs based on the nodal centrality of phase-locking value (PLV) maps. Differences in resting-state functional connectivity were examined through PLV analysis in selected regions of interest based on these two findings. Patients with OCD demonstrated significantly lower delta band activity in the cortical regions of the limbic lobe, insula, orbitofrontal, and temporal regions, and theta band activity in the parietal lobe regions than healthy controls. Patients with OCD exhibited fewer functional hubs in the insula and orbitofrontal cortex and additional hubs in the cingulate and temporo-parietal regions. The OCD group exhibited significantly lower phase synchronization among the insula, orbitofrontal cortex, and cortical regions of the limbic lobe in all band frequencies, except in the delta band. Altered functional networks in the resting state may be associated with the pathophysiology of OCD. These MEG findings indicate that OCD is associated with decreased functional connectivity in terms of phase synchrony, particularly in the insula, orbitofrontal cortex, and cortical regions of the limbic lobe.

Spatiotemporal Pattern of Human Cortical and Subcortical Activity during Early-Stage Odor Processing

The dynamics of early-stage cortical and subcortical responses in the human brain to odor stimulation are currently unknown. The objective of the present study was to analyze spatiotemporal patterns of human brain activity during odor perception using magnetoencephalography (MEG). In 12 normosmic healthy subjects, we investigated the onset of brain activity in relation to ipsilateral and contralateral stimulation with 2 odorants. Olfactory stimuli (200ms duration) were applied using an olfactometer, and brain activity was recorded with a 248-magnetometer whole-head MEG system. Olfactory responses were identified shortly (within 150ms) after stimulus onset in both hemispheres. Stimulation on the ipsilateral side yielded signals earlier (starting at 90ms) compared with contralateral stimulation in the primary olfactory cortex, hippocampus, parahippocampal gyrus, amygdala, and orbitofrontal cortex ( P < 0.001). The duration and peak amplitude of olfactory evoked magnetic fields were found to increase with increasing poststimulus time in the majority of the investigated cortical structures ( P ≤ 0.019 and P ≤ 0.021). The study showed the locations of early olfactory brain activity in humans within 150ms after the onset of stimuli. Olfactory activation is processed on the ipsilateral side of stimulation in early stages. After a short delay of 34ms a corresponding pattern of activation was also seen in the contralateral hemisphere, indicating the functional connectivity between the 2 hemispheres in the anterior commissure.

Increased Frontal Lobe Activation After Aneurysmal Subarachnoid Hemorrhage.

Neurocognitive deficits are common among survivors of aneurysmal subarachnoid hemorrhage, even among those with good outcomes and no structural lesions. This study aims to probe the neurophysiological underpinnings of cognitive dysfunction among patients with ruptured intracranial aneurysms using magnetoencephalography (MEG). Thirteen patients who had undergone uncomplicated coiling for aneurysmal subarachnoid hemorrhage and 13 matched controls were enrolled. Neuropsychological tests were done before magnetoencephalography scans. Magnetoencephalography data were acquired in a 151-channel, whole-head magnetoencephalography system for resting state and 2 cognitive tasks (go-no-go and set-shifting). Mean time from treatment to test was 18.8 months. Cognitive tasks of inhibition (go-no-go) indicated greater activation in the right anterior cingulate and inferior frontal gyrus, and cognitive set-shifting tasks (mental flexibility) indicated greater activity in the bilateral anterior cingulate cortex and right medial frontal gyrus among aneurysmal subarachnoid hemorrhage patients, with significantly different timing of activation between groups. Resting-state, beta-band connectivity of the anterior cingulate correlated negatively with Montreal Cognitive Assessment scores (left: r=-0.56; P<0.01 and right: r=-0.55; P<0.01): higher connectivity of this region was linked to poorer cognitive test performance. We have shown increased activation in areas of the anterior cingulate gyrus and frontobasal regions during the execution of more demanding tasks in good grade. The degree of activation in the anterior cingulate gyrus has a negative correlation with cognitive (Montreal Cognitive Assessment) scores. These subtle differences may be related to the common neurocognitive and behavioral complaints seen in this patient population.

Inhibitory effect of intensity and interstimulus interval of conditioning stimuli on somatosensory evoked magnetic fields.

Magnetoencephalography (MEG) recordings were performed to investigate the inhibitory effects of conditioning stimuli with various types of interstimulus intervals (ISIs) or intensities on somatosensory evoked magnetic fields (SEFs) using a 306-ch whole-head MEG system. Twenty-three healthy volunteers participated in this study. Electrical stimuli were applied to the right median nerve at the wrist. Six pulse trains with ISIs of 500ms were presented in Experiment 1. A paired-pulse paradigm with three kinds of conditioning stimulus (CON) intensities, 500ms before the test stimulus (TS), was applied in Experiment 2. Finally, three CONs 500 or 1000ms before TS were presented in Experiment 3. Three main SEF deflections (N20m, P35m, and P60m) were observed, and the source activities of P35m and P60m significantly decreased after the 2nd pulse of a six pulse trains. These source activities also significantly decreased with increasing intensity of CON. In addition, these attenuations of source activities were affected by CON-CON or CON-TS intervals. These results indicated that the source activities were modulated by the intensity and ISIs of CONs. Furthermore, P35m after the stimulation were very sensitive to CONs; however, the attenuation of P60m after the stimulation lasted for a longer period than that of P35m. Our findings suggest that the conditioning stimulation had inhibitory effects on subsequent evoked cortical responses for more than 500ms. Our results also provide important clues about the nature of short-latency somatosensory responses in human studies.

Abnormal resting-state brain activity in headache-free migraine patients: A magnetoencephalography study.

The aim of this study is to quantitatively assess the resting-state brain activity in migraine patients during the headache-free phase with magnetoencephalography (MEG). A total of 25 migraine patients during the headache-free phase and 25 gender- and age-matched control patients were studied with a whole-head MEG system at eyes-closed resting-state. MEG data were analyzed in multifrequency range of 4-200Hz. In a regional cortex analysis, the spectral power of gamma oscillations in left frontal and left temporal regions was significantly increased in migraine patients as compared to controls (all p<0.001), but no significant difference was found between the two groups for the global channels. Analyses of source location showed that there were significant differences in the distribution of gamma oscillation between migraine subjects and controls (p<0.025). Migraine patients in resting-state had altered brain activities in spectral power value and source distribution that can be detected and analyzed by MEG. Abnormal brain activities in the left frontal and temporal regions may be involved in pain modulation and processing of migraine. These findings provide new insights into the possible mechanisms of migraine attacks.

The maturation of auditory responses in infants and young children: a cross-sectional study from 6 to 59 months.

Background: An understanding of the maturation of auditory cortex responses in typically developing infants and toddlers is needed to later identify auditory processing abnormalities in infants at risk for neurodevelopmental disorders. The availability of infant and young child magnetoencephalography (MEG) systems may now provide near optimal assessment of left and right hemisphere auditory neuromagnetic responses in young populations. To assess the performance of a novel whole-head infant MEG system, a cross-sectional study examined the maturation of left and right auditory cortex responses in children 6- to 59-months of age.Methods: Blocks of 1000 Hz (1st and 3rd blocks) and 500 Hz tones (2nd block) were presented while MEG data were recorded using an infant/young child biomagnetometer (Artemis 123). Data were obtained from 29 children (11 males; 6- to 59-months). Latency measures were obtained for the first positive-to-negative evoked response waveform complex in each hemisphere. Latency and age associations as well as frequency and hemisphere latency differences were examined. For the 1000 Hz tone, measures of reliability were computed.Results: For the first response—a response with a “P2m” topography—latencies decreased as a function of age. For the second response—a response with a “N2m” topography—no N2m latency and age relationships were observed. A main effect of tone frequency showed earlier P2m responses for 1st 1000 Hz (150 ms) and 2nd 1000 Hz (148 ms) vs. 500 Hz tones (162 ms). A significant main effect of hemisphere showed earlier N2m responses for 2nd 1000 Hz (226 ms) vs. 1st 1000 Hz (241 ms) vs. 500 Hz tones (265 ms). P2m and N2m interclass correlation coefficient latency findings were as follows: left P2m (0.72, p < 0.001), right P2m (0.84, p < 0.001), left N2m (0.77, p < 0.001), and right N2m (0.77,p < 0.01).Conclusions: Findings of strong age and latency associations, sensitivity to tone frequency, and good test-retest reliability support the viability of longitudinal infant MEG studies that include younger as well as older participants as well as studies examining auditory processing abnormalities in infants at risk for neurodevelopmental disorders.

Current and emerging potential for magnetoencephalography in pediatric epilepsy

Magnetoencephalography (MEG) is a noninvasive neuroimaging tool that is increasingly becoming useful for presurgical delineation of epileptogenic zones and eloquent cortex in both lesional and non-lesional pediatric cases. During the past 10 yrs, the use of MEG in pediatric epilepsy research has increased. This paper starts with a review of the use of MEG in pediatric epilepsy. We then describe the protocol used for epilepsy patients at the pediatric MEG facility in Boston Children’s Hospital and present two case studies of intractable epilepsy obtained in our laboratory -cortical dysplasia and tuberous sclerosis complex -to illustrate our methodology in localizing epileptiform generators. In both cases, we are able to localize generators of interictal spikes in the irritative zone just outside the lesion. We also present results on localization of the somatosensory cortex based on our pediatric MEG system to illustrate the utility of MEG for identification of the eloquent cortex. We complete this review by considering advantages and limitations of MEG in children with epilepsy, its future developments and research applications. Application of MEG in pediatric epilepsy will accelerate during the coming years as different types of pediatric whole-head MEG systems and more advanced data analysis methods become available to the researchers and clinicians. These advances will lead to greater use of MEG as a complement to clinical electroencephalography, with improved noninvasive delineation of the epileptogenic zone.

Artemis 123: development of a whole-head infant and young child MEG system

Background: A major motivation in designing the new infant and child magnetoencephalography (MEG) system described in this manuscript is the premise that electrophysiological signatures (resting activity and evoked responses) may serve as biomarkers of neurodevelopmental disorders, with neuronal abnormalities in conditions such as autism spectrum disorder (ASD) potentially detectable early in development. Whole-head MEG systems are generally optimized/sized for adults. Since magnetic field produced by neuronal currents decreases as a function of distance2 and infants and young children have smaller head sizes (and thus increased brain-to-sensor distance), whole-head adult MEG systems do not provide optimal signal-to-noise in younger individuals. This spurred development of a whole-head infant and young child MEG system – Artemis 123.Methods:In addition to describing the design of the Artemis 123, the focus of this manuscript is the use of Artemis 123 to obtain auditory evoked neuromagnetic recordings and resting-state data in young children. Data were collected from a 14-month-old female, an 18-month-old female, and a 48-month-old male. Phantom data are also provided to show localization accuracy.Results:Examination of Artemis 123 auditory data showed generalizability and reproducibility, with auditory responses observed in all participants. The auditory MEG measures were also found to be manipulable, exhibiting sensitivity to tone frequency. Furthermore, there appeared to be a predictable sensitivity of evoked components to development, with latencies decreasing with age. Examination of resting-state data showed characteristic oscillatory activity. Finally, phantom data showed that dipole sources could be localized with an error less than 0.5 cm.Conclusions:Artemis 123 allows efficient recording of high-quality whole-head MEG in infants four years and younger. Future work will involve examining the feasibility of obtaining somatosensory and visual recordings in similar-age children as well as obtaining recordings from younger infants. Thus, the Artemis 123 offers the promise of detecting earlier diagnostic signatures in such neurodevelopmental disorders.

Effect of the number of pins and inter-pin distance on somatosensory evoked magnetic fields following mechanical tactile stimulation

Magnetoencephalography (MEG) recordings were collected to investigate the effect of the number of mechanical pins and inter-pin distance on somatosensory evoked magnetic fields (SEFs) following mechanical stimulation (MS). We used a 306-ch whole-head MEG system. SEFs were elicited through tactile stimuli with 1-, 2-, 3-, 4- and 8-pins using healthy participants. Tactile stimuli were applied to the tip of the right index finger. SEF following electrical stimulation of the index finger was recorded in order to compare the activity in the primary somatosensory cortex (S1) following MS. Prominent SEFs were recorded from the contralateral hemisphere approximately 54ms (P50m) and 125ms (P100m) after MS regardless of the number of pins. Equivalent current dipoles were located in the S1. The source activities for P50m and P100m significantly increased in tandem with the number of pins for MS. However, the increased ratios for the source activities according to the increase in the number of pins were significantly smaller than that induced by electrical stimulation, and when the number of the pins doubled from 1-pin to 2-pins, from 2-pins to 4-pins, and from 4-pins to 8-pins, S1 activities increased by only 130%. Additionally, source activities significantly increased when the inter-pin distance increased from 2.4 to 7.2mm. The number of stimulated receptors was considered to have increased with an increase in the inter-pin distance as well as an increase in the number of pins. These findings clarified the effect of the number of pins and inter-pin distance for MS on SEFs.

Frequency diversity of posterior oscillatory activity in human revealed by spatial filtered MEG

The posterior EEG alpha rhythm is a distinctive feature of the normal brain in the waking state, consisting of oscillations within the 8-15 Hz frequency range over posterior cortical regions. This activity appears in resting, eyes-closed condition and is typically suppressed by eyes-opening. Other physiological rhythms in the alpha band, in particular the Rolandic mu rhythm, are proposed to include a fast component in the beta range. In this study we used spatial filtering techniques and permutation analysis to explore cortical source-power changes related to the magnetoencephalography (MEG) counterpart of the posterior alpha rhythm. We also aimed at determining a possible implication of components outside the alpha frequency range in the posterior rhythm reactivity to eye closure. We recorded resting brain activity using a whole-head MEG system in fifteen normal subjects. We applied an eyes-open/eyes-closed paradigm. A significant increase in alpha oscillations after eyes closing, representing the posterior alpha rhythm, was observed bilaterally in the occipital and parietal cortex, including the calcarine fissure and the parieto-occipital sulcus. We also found significant increase in beta (15-30 Hz) and low gamma (30-60 Hz) oscillations. This fast components and the classical alpha rhythm had similar topographic distribution in posterior brain regions, although with different strength and spatial extension. These features were highest for alpha synchronized oscillations, intermediate for beta, and lowest for gamma activity. These results suggest that, like the Rolandic mu rhythm, the MEG posterior dominant rhythm may be impure, with a mixture of predominant alpha oscillations and high-frequency components.