Cancellation of EEG and MEG signals generated by extended and distributed sources.

Extracranial patterns of scalp potentials and magnetic fields, as measured with electro- and magnetoencephalography (EEG, MEG), are spatially widespread even when the underlying source in the brain is focal. Therefore, loss in signal magnitude due to cancellation is expected when multiple brain regions are simultaneously active. We characterized these cancellation effects in EEG and MEG using a forward model with sources constrained on an anatomically accurate reconstruction of the cortical surface. Prominent cancellation was found for both EEG and MEG in the case of multiple randomly distributed source dipoles, even when the number of simultaneous dipoles was small. Substantial cancellation occurred also for locally extended patches of simulated activity, when the patches extended to opposite walls of sulci and gyri. For large patches, a difference between EEG and MEG cancellation was seen, presumably due to selective cancellation of tangentially vs. radially oriented sources. Cancellation effects can be of importance when electrophysiological data are related to hemodynamic measures. Furthermore, the selective cancellation may be used to explain some observed differences between EEG and MEG in terms of focal vs. widespread cortical activity.

Sensitivity of MEG and EEG to source orientation.

An important difference between magnetoencephalography (MEG) and electroencephalography (EEG) is that MEG is insensitive to radially oriented sources. We quantified computationally the dependency of MEG and EEG on the source orientation using a forward model with realistic tissue boundaries. Similar to the simpler case of a spherical head model, in which MEG cannot see radial sources at all, for most cortical locations there was a source orientation to which MEG was insensitive. The median value for the ratio of the signal magnitude for the source orientation of the lowest and the highest sensitivity was 0.06 for MEG and 0.63 for EEG. The difference in the sensitivity to the source orientation is expected to contribute to systematic differences in the signal-to-noise ratio between MEG and EEG.

Evaluation of smoothing in an iterative lp-norm minimization algorithm for surface-based source localization of MEG.

The imaging of neural sources of magnetoencephalographic data based on distributed source models requires additional constraints on the source distribution in order to overcome ill-posedness and obtain a plausible solution. The minimum l(p) norm (0 < p < or = 1) constraint is known to be appropriate for reconstructing focal sources distributed in several regions. A well-known recursive method for solving the l(p)-norm minimization problem, for example, is the focal underdetermined system solver (FOCUSS). However, this iterative algorithm tends to give spurious sources when the noise level is high. In this study, we present an algorithm to incorporate a smoothing technique into the FOCUSS algorithm and test different smoothing kernels in a surface-based cortical source space. Simulations with cortical source patches assumed in auditory areas show that the incorporation of the smoothing procedure improves the performance of the FOCUSS algorithm, and that using the geodesic distance for constructing a smoothing kernel is a better choice than using the Euclidean one, particularly when employing a cortical source space. We also apply these methods to a real data set obtained from an auditory experiment and illustrate their applicability to realistic data by presenting the reconstructed source images localized in the superior temporal gyrus.

Air mattress sensor system with balancing tube for unconstrained measurement of respiration and heart beat movements.

The cardio-respiratory signal is a fundamental vital sign used for assessment of a patient's status. Additionally, the cardio-respiratory signal provides a great deal of information to healthcare providers wishing to monitor healthy individuals. The air mattress sensor system allows the measurement of the respiration and heart beat movements without the use of a harness or sensor on the subject's body, which eliminates the difficulties these pose for long term measurements. In order to increase the sensitivity, a differential measurement technique between two air cells was used. The concept of a balancing tube between two air cells is suggested in order to increase the robustness against postural changes during the measurements. With this balancing tube, the meaningful frequency range could be selected using a pneumatic method. A mathematical model was constructed and validation experiments were performed for step and sinusoidal input signals. This technique was applied to measurements of respiration and heart beat movements in the supine posture on the bed, which showed potential for applications in sleep analysis, unconstrained healthcare monitoring and neonate monitoring.

Cortico-cortical modulation induced by 1-Hz repetitive transcranial magnetic stimulation of the temporal cortex.

BACKGROUND AND PURPOSE: Repetitive transcranial magnetic stimulation (rTMS) has potential as a noninvasive neuromodulation treatment method for various neuropsychiatric disorders, and repeated sessions of rTMS are more likely to enhance the therapeutic efficacy. This study investigated neurophysiologic and spatiodynamic changes induced by repeated 1-Hz rTMS of the temporal cortex using transcranial magnetic stimulation (TMS) indices and fluorodeoxyglucose positron emission tomography (FDG-PET). METHODS: Twenty-seven healthy subjects underwent daily 1-Hz active or sham rTMS of the right temporal cortex for 5 consecutive days. TMS indices of motor cortical excitability were measured in both hemispheres daily before and after each rTMS session, and 2 weeks after the last stimulation. FDG-PET was performed at baseline and after the 5 days of rTMS sessions. RESULTS: All subjects tolerated all of the sessions well, with only three of them (11.1%) reporting mild transient side effects (i.e., headache, tinnitus, or local irritation). One-Hz rTMS decreased motor evoked potential amplitudes and delayed cortical silent periods in the stimulated hemisphere. Statistical parametric mapping of FDG-PET data revealed a focal reduction of glucose metabolism in the stimulated temporal area and an increase in the bilateral precentral, ipsilateral superior and middle frontal, prefrontal and cingulate gyri. CONCLUSIONS: Repeated rTMS sessions for 5 consecutive days were tolerated in all subjects, with only occasional minor side effects. Focal 1-Hz rTMS of the temporal cortex induces cortico-cortical modulation with widespread functional changes in brain neural networks via long-range neural connections.

Gamma phase locking modulated by phonological contrast during auditory comprehension in reading disability.

Children with specific reading impairment may have subtle deficits in speech perception related to difficulties in phonological processing. The aim of this study was to examine brain oscillatory activity related to phonological processing in the context of auditory sentence comprehension using magnetoencephalography to better understand these deficits. Good and poor readers, 16-18 years of age, were tested on speech perception of sentence-terminal incongruent words that were phonologically manipulated to be similar or dissimilar to corresponding congruent target words. Functional coupling between regions was measured using phase-locking values (PLVs). Gamma-band (30-45 Hz) PLV between auditory cortex and superior temporal sulcus in the right hemisphere was differentially modulated in the two groups by the degree of phonological contrast between the congruent and the incongruent target words in the latency range associated with semantic processing. Specifically, the PLV was larger in the phonologically similar than in the phonologically dissimilar condition in the good readers. This pattern was reversed in the poor readers, whose lower PLV in the phonologically similar condition may be indicative of the impaired phonological coding abilities of the group, and consequent vulnerability under perceptually demanding conditions. Overall, the results support the role of gamma oscillations in spoken language processing.

Regularized FOCUSS algorithm for EEG/MEG source imaging.

In this paper, we derived a generalized version of the regularized FOCUSS algorithm which was derived in [3]. It allows general forms of noise covariance and reduces depth effect when imaging focal neural sources from electroencephalography (EEG) / magnetoencephalography (MEG) data. We compared a depth-weighted regularized algorithm with FOCUSS and a regularized FOCUSS through simulation study. The suggested algorithm gave sparser and less spurious solutions than the others.