Stephen E. Robinson

Medial prefrontal cortex generates frontal midline theta rhythm

Frontal midline theta rhythm (Fm theta) is a distinct theta activity of EEG in the frontal midline area that appears during concentrated performance of mental tasks in normal subjects and reflects focused attentional processing. To tomographically visualize the source current density distributions of Fm theta, we recorded Fm theta by using a 64-channel whole-head MEG system from four healthy subjects, and applied a new analysis method, synthetic aperture magnetometry (SAM), an adaptive beam forming method. Fm theta was observed in the MEG signals over the bilateral frontal regions. SAM analysis showed bilateral medial prefrontal cortices, including anterior cingulate cortex, as the source of Fm theta. This result suggests that focused attention is mainly related to medial prefrontal cortex.

Movement-Related Desynchronization of the Cerebral Cortex Studied with Spatially Filtered Magnetoencephalography

Event-related desynchronization (ERD) within the alpha and beta bands on unilateral index finger extension and hand grasping was investigated on six normal volunteers with magnetoencephalography (MEG). A novel spatial filtering technique for imaging cortical source power, synthetic aperture magnetometry (SAM), was employed for the tomographic demonstration of ERD. SAM source image results were transformed into statistical parametric images. On the same hand grasping task, a functional MRI (fMRI) study was conducted on two subjects and compared with the ERD result. When the MEG data were analyzed with the fast Fourier transformation, power attenuation within the alpha and beta bands was evident on the contralateral sensorimotor area just prior to movement onset. The tomographic distribution of ERD was clearly obtained with SAM statistical imaging analysis. The equivalent current dipole (ECD) for the signal-averaged motor field was localized to the hemisphere contralateral to the hand movement, roughly at the center of the region displaying beta-band ERD. The signal increase on fMRI roughly colocalized with the ERD on the contralateral sensorimotor area. In conclusion, with the novel spatial filtering technique for the brain magnetic field, SAM, cortical regions contributing to ERD on finger movement were successfully demonstrated in a tomographic manner. The relative colocalization of the contralateral SAM ERD with ECD as well as the fMRI activation suggests that SAM is a practically useful technique to extract event-related signals from brain noise.

Optimising experimental design for MEG beamformer imaging

In recent years, the use of beamformers for source localisation has significantly improved the spatial accuracy of magnetoencephalography. In this paper, we examine techniques by which to optimise experimental design, and ensure that the application of beamformers yields accurate results. We show that variation in the experimental duration, or variation in the bandwidth of a signal of interest, can significantly affect the accuracy of a beamformer reconstruction of source power. Specifically, power will usually be underestimated if covariance windows are made too short, or bandwidths too narrow. The accuracy of spatial localisation may also be reduced. We conclude that for optimum accuracy, experimenters should aim to collect as much data as possible, and use a bandwidth spanning the entire frequency distribution of the signal of interest. This minimises distortion to reconstructed source images, time courses and power estimation. In the case where experimental duration is short, and small covariance windows are therefore used, we show that accurate power estimation can be achieved by matrix regularisation. However, large amounts of regularisation cause a loss in the spatial resolution of the MEG beamformer, hence regularisation should be used carefully, particularly if multiple sources in close proximity are expected.

Improving permutation test power for group analysis of spatially filtered MEG data.

Non-parametric statistical methods, such as permutation, are flexible tools to analyze data when the population distribution is not known. With minimal assumptions and better statistical power compared to the parametric tests, permutation tests have recently been applied to the spatially filtered magnetoencephalography (MEG) data for group analysis. To perform permutation tests on neuroimaging data, an empirical maximal null distribution has to be found, which is free from any activated voxels, to determine the threshold to classify the voxels as active at a given probability level. An iterative procedure is used to determine the distribution by computing the null distribution, which is recomputed when a possible activated voxel is found within the current distributions. Besides the high computational costs associated with this approach, there is no guarantee that all activated voxels are excluded when constructing the maximal null distribution, which may reduce the statistical power. In this study, we propose a novel way to construct the maximal null distribution from the data of the resting period. The approach is tested on the MEG data from a somatosensory experiment, and demonstrated that the approach could improve the power of the permutation test while reducing the computational cost at the same time.

Frequency-dependent spatial distribution of human somatosensory evoked neuromagnetic fields

Using synthetic aperture magnetometry (SAM), we examined the spatial distribution of frequency changes in magnetoencephalography signal rhythms on individual magnetic resonance images following somatosensory stimulation. SAM is a novel statistical spatial filtering method that uses an adaptive beamformer. Electrical stimulation of the right median nerve demonstrated high-frequency event-related synchronization (ERS) in the 50-200-Hz range, consistently localized in the contralateral primary sensorimotor area in all subjects (n=7). Event-related desynchronization (ERD) was demonstrated in the 8-13, 13-25 and 25-50-Hz ranges bilaterally in the area surrounding the central sulcus. The differences in the spatial distribution as well as the frequency bands between ERS and ERD suggest that ERS and ERD reflect the responses of different cell assemblies rather than a frequency shift of the same cell assembly.

In vivo assessment of human brain oscillations during application of transcranial electric currents

Brain oscillations reflect pattern formation of cell assemblies’ activity, which is often disturbed in neurological and psychiatric diseases like depression, schizophrenia and stroke. In the neurobiological analysis and treatment of these conditions, transcranial electric currents applied to the brain proved beneficial. However, the direct effects of these currents on brain oscillations have remained an enigma because of the inability to record them simultaneously. Here we report a novel strategy that resolves this problem. We describe accurate reconstructed localization of dipolar sources and changes of brain oscillatory activity associated with motor actions in primary cortical brain regions undergoing transcranial electric stimulation. This new method allows for the first time direct measurement of the effects of non-invasive electrical brain stimulation on brain oscillatory activity and behavior.

Magnetoencephalographic recordings of visual evoked brain activity in the human fetus

We investigated the feasibility of recording visual evoked brain activity in the human fetus by use of non-invasive magnetoencephalography (MEG). Each recording lasted 6 min and consisted of a sequence of 180 flashes with 33 ms duration delivered 2 s apart over the maternal abdomen. Four of ten fetuses included showed a response; the ranges of amplitude and latency of peak response were 15-30 x 10(-15) Tesla and 180-390 ms, respectively. Six fetuses showed no discernible response. With improvement, this method could aid in the testing of fetal neurological status throughout pregnancy.

Fetal MEG redistribution by projection operators

The fetal magnetoencephalogram (fMEG) is measured in the presence of large interference from the maternal and fetal magnetocardiograms. This interference can be efficiently attenuated by orthogonal projection of the corresponding spatial vectors. However, the projection operators redistribute the fMEG signal among sensors. Although redistribution can be readily accounted for in the forward solution, visual interpretation of the fMEG signal topography is made difficult. We have devised a general, model-independent method for correction of the redistribution effect that utilizes the assumption that we know in which channels the fMEG should be negligible (such channels are distant from the known fetal head position). In a simplified case where the fMEG can be explained by equivalent current dipoles, the correction can also be obtained from fitting the dipoles to the fMEG signal. The corrected fMEG signal topography then corresponds to the dipole forward solution, but without orthogonal projection. We illustrate the redistribution correction on an example of experimentally measured flash evoked fMEG.

Theta rhythm increases in left superior temporal cortex during auditory hallucinations in schizophrenia : a case report

Auditory hallucinations (AH), the perception of sounds and voices in the absence of external stimuli, remain a serious problem for a large subgroup of patients with schizophrenia. Functional imaging of brain activity associated with AH is difficult, since the target event is involuntary and its timing cannot be predicted. Prior efforts to image the patterns of cortical activity during AH have yielded conflicting results. In this study, MEG was used to directly image the brain electrophysiological events associated with AH in schizophrenia. We observed an increase in theta rhythm, as sporadic bursts, in the left superior temporal area during the AH states, whereas there was steady theta band activity in the resting state. The present finding suggests strong association of the left superior temporal cortex with the experience of AH in this patient. This is consistent with the hypothesis that AH arises from areas of auditory cortex subserving receptive language processing.

Short-term serial magnetoencephalography recordings offetal auditory evoked responses.

The study objective was to determine whether short-term serial magnetoencephalographic (MEG) measurements would increase the odds in favor of obtaining fetal auditory evoked responses in normal fetuses. The recordings were performed in two phases using the newly developed 151-channel fetal MEG system, superconducting quantum interference device array for reproductive assessment. Ten pregnant subjects with gestational ages ranging from 30-35 weeks were recruited to participate. Daily recordings were performed over a minimum of 3 days during 1 week of gestation and repeated in the same subjects between 36 and 40 weeks gestation. In 80% of subjects, auditory evoked responses were detected at least once. In healthy fetuses, serial recordings over a short span of time increased the rate of detecting fetal evoked response.