Practical constraints on estimation of source extent with MEG beamformers

We aimed to determine practical constraints on the estimation of the spatial extent of neuronal activation using MEG beamformers. Correct estimation of spatial extent is a pre-requisite for accurate models of electrical activity, allows one to estimate current density, and enables non-invasive monitoring of functional recovery following stroke. The output of an MEG beamformer is maximum when the correct source model is used, so that the spatial extent of a source can in principal be determined through evaluation of different source models with the beamformer. Here, we simulated 275-channel MEG data using sources of varying spatial extents that followed the cortical geometry. These data were subsequently used to estimate the spatial extent of generic disc elements without knowledge of the underlying surface, and we compared these results to estimates based on cortical surface geometry (with and without error in surface location). We found that disc-shaped source models are too simplistic, particularly for areas with high curvature. For areas with low curvature spatial extent was underestimated, although on average there was a linear relationship between the true and estimated extent. In contrast, cortical surface models gave accurate predictions of spatial extent. However, adding small errors (>2 mm) to the estimated location of the cortical surface abolished this relationship between true and estimated extent, implying that accurate co-registration is needed with such models. Our results show that models exploiting surface information are necessary in order to model spatial extent and in turn current density, but in order to render such models applicable in practical situations, the accuracy of the cortical surface model itself needs to improve.     

The role of GABAergic modulation in motor function related neuronal network activity

At rest, the primary motor cortex (M1) exhibits spontaneous neuronal network oscillations in the beta (15-30 Hz) frequency range, mediated by inhibitory interneuron drive via GABA-A receptors. However, questions remain regarding the neuropharmacological basis of movement related oscillatory phenomena, such as movement related beta desynchronisation (MRBD), post-movement beta rebound (PMBR) and movement related gamma synchronisation (MRGS). To address this, we used magnetoencephalography (MEG) to study the movement related oscillatory changes in M1 cortex of eight healthy participants, following administration of the GABA-A modulator diazepam. Results demonstrate that, contrary to initial hypotheses, neither MRGS nor PMBR appear to be GABA-A dependent, whilst the MRBD is facilitated by increased GABAergic drive. These data demonstrate that while movement-related beta changes appear to be dependent upon spontaneous beta oscillations, they occur independently of one other. Crucially, MRBD is a GABA-A mediated process, offering a possible mechanism by which motor function may be modulated. However, in contrast, the transient increase in synchronous power observed in PMBR and MRGS appears to be generated by a non-GABA-A receptor mediated process; the elucidation of which may offer important insights into motor processes.     

Dissociation of summation and peak latencies in visual processing: An MEG study on stimulus eccentricity

Research on the temporal characteristics of visual processing, as measured with critical flicker fusion or the latency of visual evoked potential (VEP), shows controversial results if different eccentricities of visual stimuli are compared. To clarify this question, a direct measure of cortical activity with magnetoencephalography (MEG) was applied to examine the neuronal summation latency and peak latency for both near and far peripheral stimuli. Consistent with cortical magnification, the peak amplitude for less eccentric stimuli was larger than that for more eccentric stimuli. More importantly, the current data also demonstrated longer cortical summation latency and peak latency for more eccentric visual stimuli, but only the summation latency difference between near and far stimuli correlated with the peak amplitude difference between near and far stimuli. These results suggest dissociable mechanisms of summation latency and peak latency with respect to their contributions to the stimulus eccentricity effect, and provide potential explanations for controversial results in previous studies.     

Cortico-cortical phase synchrony in auditory mismatch processing

Previous studies have shown a reduced MMN in patients with lesions in the temporal or frontal lobes, suggesting a temporal-frontal involvement in change detection. However, how the temporal lobe interacts with other brain areas in responding to unexpected deviant stimuli remains unclear. This study aimed to evaluate the functional connectivity between cerebral regions by measuring the phase synchrony of magnetoencephalographic (MEG) signals elicited by regular simple tones and their duration-deviants in an oddball paradigm. We measured MEG responses to deviant (1000-Hz frequency, 50-ms duration, probability of 15%) and standard (1000-Hz frequency, 100-ms duration) sounds in 10 healthy adults. By using the Morlet wavelet-based analysis, relative phase synchronization values of 4-40 Hz MEG responses at 150-300 ms after stimulus onset were calculated with respect to a reference channel from the temporal region. Phase synchronization was clearly identified between the temporal and ipsilateral frontal region in the auditory evoked responses. This temporal-frontal synchronization was significantly larger in deviants-elicited than standards-elicited activation at 4-25 Hz in the left hemisphere (p < 0.05), and at 4-8 Hz in the right hemisphere (p < 0.01). Also, temporal-temporal and temporal-parietal phase synchronies were found in deviants-evoked 4-8 Hz responses.The present results suggest an involvement of temporal-temporal, temporal-frontal, and temporal-parietal neuronal network in detecting auditory change. Phase synchronization analysis may provide a useful window to further understanding of the cerebral reactivity during the processing of auditory deviants.     

A comparison of stimulus synchronous activity in the primary motor cortices of athletes and non-athletes

In this study, we measured primary motor cortex (MI) activity during a reaction time task to examine the appearance of MI activity that synchronized with the stimulus presentation (stimulus synchronous MI activity, SSMA). Because brain activity was expected to be enhanced by the repetitive/extensive activation, we hypothesized that the SSMA would be more clearly observable in athletes who were trained to perform reactive movements than in non-athletes. MI activity was measured in ten athletes and ten non-athletes by magnetoencephalography. The tasks were a simple reaction task and a Go/Nogo reaction task in which the subjects were asked to abduct their right index fingers in response to a visual stimulus. The Go/Nogo reaction time task was adopted to confirm the presence of the SSMA, because the MI activity in response to a Nogo stimulus did not overlap with the MI activity that was synchronous with the execution of the movement. The results show that the SSMA was clearly apparent in the athlete group (9/10). In the non-athlete group, however, only three subjects showed the SSMA (3/10). Moreover, the MI activity of the athletes tended to be larger than that of the non-athletes, even though the athletes did not specifically practice these index finger movements during their daily training. We concluded that long-term physical training promotes MI activity and the effects of reactive task repetition were more clearly apparent in the MI activity of the athletes.     

Usefulness of MEG magnetometer for spike detection in patients with mesial temporal epileptic focus

The present study investigated the sensitivity of magnetoencephalography (MEG) for spikes depending on sensor type in patients with mesial temporal epileptic focus. We recorded MEG in 6 patients with mesial temporal epileptic focus using two sensor types (magnetometer and gradiometer) simultaneously. The number of spikes detected and the corresponding equivalent current dipole (ECD) parameters (distance from the coordinated head center (radius), and dipole moment) were evaluated with respect to sensor type. Among 426 MEG 'consensus spikes' determined by 3 reviewers, 378 spikes satisfied the predetermined criteria for source localization. Comparing ECD parameters, spikes detected by magnetometer alone displayed a smaller radius and larger dipole moment than those detected by gradiometer alone. Spikes estimated in the mesial temporal area were more frequently detected by magnetometer alone (38.5%) than by gradiometer alone (11.5%), whereas spikes in the lateral temporal area were detected less by magnetometer alone (3.7%) than by gradiometer alone (53.9%). The present results suggest that a magnetometer is advantageous for spike detection in patients with mesial temporal epileptic focus. This also implies the higher sensitivity of magnetometer for deep sources.     

The perception of pain in others suppresses somatosensory oscillations: a magnetoencephalography study

Accumulating evidence demonstrates that similar neural circuits are activated during the first-hand experience of pain and the observation of pain in others. However, most functional MRI studies did not detect signal change in the primary somatosensory cortex during pain empathy. To test if the perception of pain in others involves the primary somatosensory cortex, neuromagnetic oscillatory activity was recorded from the primary somatosensory cortex in 16 participants while they observed static pictures depicting body parts in painful and non-painful situations. The left median nerve was stimulated at the wrist, and the poststimulus rebounds of the approximately 10-Hz somatosensory cortical oscillations were quantified. Compared to the baseline condition, the level of the approximately 10-Hz oscillations was suppressed during both of the observational situations, indicating the activation of the primary somatosensory cortex. Importantly, watching painful compared to non-painful situations suppressed somatosensory oscillations to a significant stronger degree. In addition, the suppression caused by perceiving others in the painful relative to the non-painful situations correlated with the perspective taking subscale of the interpersonal reaction index. These results, consistent with the mirror-neuron system, demonstrate that the perception of pain in others modulates neural activity in primary somatosensory cortex and supports the idea that the perception of pain in others elicits subtle somatosensory activity that may be difficult to detect by fMRI techniques.     

Human brain response to visual stimulus between lower/upper visual fields and cerebral hemispheres

We studied the human brain response to visual stimulation in which a square area was randomly presented in upper and lower visual fields (VFs). Seven normal volunteers carried out a contrast-based visual search task. Magnetic responses were detected in the bilateral parietal regions at 200–250 ms after stimulus onset. We compared the response latencies and strengths of the essential single sensor and root mean square (RMS) of the regions. The former evaluates the strength of neural activity with relatively high spatial resolution, while the latter evaluates the global neural activity. The single sensor and RMS latencies for the lower left VF were significantly longer than that for the upper left (paired t-test, P < 0.05). The strengths did not differ between the upper and lower left VFs. There was no significant difference in latency or strength between the upper right and lower right VFs. These findings suggest that only left VF has different response properties in the upper versus lower VF, and that both local and global extrastriate activities are responsible for this anisotropy.     

Neuronal network coherent with hand kinematics during fast repetitive hand movements

We quantified the coupling between magnetoencephalographic (MEG) cortical signals and the kinematics of fast repetitive voluntary hand movements monitored by a 3-axis accelerometer. Ten healthy right-handed adults performed self-paced flexion-extension movements of right-hand fingers at ~ 3 Hz with either touching the thumb during flexions (TOUCH) or not (noTOUCH). At the sensor level, we found in all subjects and conditions significant coherence at the movement frequency (F0) and its first harmonic (F1). Coherence values were significantly higher in TOUCH compared to noTOUCH. At the group level, dynamic imaging of coherent sources localized the main source of coherent activity at the left primary motor (M1) hand area, except at F0 TOUCH were the main source was localized at the left primary sensory (S1) hand area. Other coherent brain areas were also identified at right S1-M1 cortices (F0), left dorsolateral prefrontal cortex (F1), left posterior parietal cortex (F0 TOUCH and F1 noTOUCH) and left medial S1-M1 areas (TOUCH). This study highlights the prominent role of rhythmic neuronal activity phase-locked to movements for the encoding and the integration of key sensori-motor features of limb kinematics. This study also suggests that somatosensory afferences play a key role to sustain a high synchronization level between the neuronal activity in coherent brain areas and hand acceleration. Some coherent brain regions differed between F0 and F1 in both conditions, suggesting that distinct cortical areas are involved in different features of hand kinematics.     

Aberrant high-gamma oscillations in the somatosensory cortex of children with cerebral palsy: a meg study

Objective: Our study is to investigate somatosensory dysfunction in children with spastic cerebral palsy (CP) using magnetoencephalography (MEG) and synthetic aperture magnetometry (SAM). Methods: Six children with spastic CP and six age- and gender-matched typically developing children were studied using a 275-channel MEG system while their left and right index fingers were stimulated in random order. The latency and amplitude of somatosensory evoked magnetic fields were analyzed at sensor level. The patterns of high-gamma oscillations were investigated with SAM at source level. Results: In comparison to the children with typical development, the latency of the first response of somatosensory evoked magnetic fields (SEFs) in the children with spastic CP was significantly delayed (p < 0.05). High-gamma oscillations were identified in the somatosensory cortex in both children with CP and typical developing children. Interestingly, children with spastic CP had significantly higher incidence of ipsilateral activation in the somatosensory cortex following right and left finger stimulation, compared to typically developing children (p = 0.05). Conclusion: The results suggest that children with spastic CP have a measurable delay of SEFs and high-gamma oscillations. The high rates of ipsilateral cortical activation imply the impairments of functional lateralization in the developing brain. This is the first MEG study to demonstrate abnormal high-gamma oscillations of somatosensory cortices representing the finger in children with spastic CP.