PURPOSE:Reports of direct current shifts at the onset of scalp-recorded seizures prompted us to inspect depth-recorded seizures for the presence of similar slow potential shifts at the onset of the seizure to determine whether slow potential (SP) shifts actually occur at the onset of depth-recorded seizures and if these shifts can facilitate localization of the seizure focus.METHODS:With the low frequency filter "opened" (LLF=0.1 Hz, HLF=70 Hz, 3 dB/octave), 32 seizures recorded with hippocampal depth and subdural electrodes were visually inspected to identify an SP shift at the onset of the seizure. A seizure was considered as having an SP shift when the slow potential waveform was > 1.5 sec in duration and > 100 microV in amplitude. Seizures were obtained from 5 subjects; 4 underwent epilepsy surgery (3=Engel I, 1=Engel II) and one received VNS. SP shift duration, peak voltage and polarity were measured for each seizure. The ability to identify seizures based on SP shift configuration was also evaluated.RESULTS:In 84% of the seizures, ictal onset was associated with a localized SP shift. Shift duration ranged from 1.5 sec to 11.5 sec (96% > 2 sec, 62% > 5 sec). The maximum shift ranged from 139 microV to 2305 microV (mean = 1123 microV, SD = 660 microV). In all the seizures, polarity was positive at the point of maximum shift. By visually examining the SP shift, seizures could be identified as originating from the same focus or from different foci.CONCLUSIONS:The onset of depth-recorded seizures appears to be commonly associated with a localized positive SP shift. An SP shift at the onset of depth-recorded seizures is likely to be a useful visual aid for localizing electrographic seizure onset.
Amyotrophic lateral sclerosis is the most common form of motor neuron disease. The diagnosis is based on clinical and electromyography criteria. The primary role of imaging in amyotrophic lateral sclerosis is to exclude other causes such as cervical degenerative disk disease, Chiari malformation or multiple sclerosis. Imaging is also helpful in atypical cases of the disease.
Various kinds of neuromuscular manifestations are known with the recreational drugs. We report an interesting case of extensive myositis and fasciitis of thigh following an injection of a solution of Buprenorphine. The inflammatory process affected the sciatic and obturator nerve as well.
EMG artifact produced by a VNS stimulator is described. A patient with a VNS stimulator underwent an EMG study for suspected ALS. Artifacts that appeared similar to positive sharp waves or fibrillations were noted that could produce a false clinical diagnosis. These VNS-EMG artifacts matched well with the VNS generator's set parameters. We conclude that EMG findings must be interpreted with caution in patients with VNS implants and also that EMG may have a possible monitoring value for VNS activity.
The effect of oral administration of sodium valproate in normal subjects was evaluated using whole-scalp magnetoencephalography, with results compared to the effect of sodium valproate in photosensitive children. Neuromagnetic responses to 10 Hz equiluminant red-green and red-blue flicker were measured before and after 5 days of sodium valproate administration. For the red-green flicker, relative power spectra at the stimulus frequency (10 Hz) were attenuated with medication in most brain regions. However, for the red-blue flicker, the 10-Hz power in the occipital region was enhanced with medication, while it was reduced in other regions. These results qualitatively resembled those in photosensitive children. The present findings suggest that (1) combinational chromatic sensitivity can be a critical factor for cortical excitability, that (2) the effect of sodium valproate is qualitatively similar in normal and photosensitive subjects, and that (3) the effect of sodium valproate on cortical excitability is not simply to suppress the stimulus-synchronized occipital activity, but rather to inhibit the spread of cortical activity from the occipital region to other regions.
We have developed an analysis toolbox called NUTMEG (Neurodynamic Utility Toolbox for Magnetoencephalography) for reconstructing the spatiotemporal dynamics of neural activations and overlaying them onto structural MR images. The toolbox runs under MATLAB in conjunction with SPM2 and can be used with the Linux/UNIX, Mac OS X, and even Windows platforms. Currently, evoked magnetic field data from 4-D Neuroimaging, CTF, and KIT systems can be imported to the toolbox for analysis. NUTMEG uses an eigenspace vector beamforming algorithm to generate a tomographic reconstruction of spatiotemporal magnetic source activity over selected time intervals and spatial regions. The MEG coordinate frame is coregistered with an anatomical MR image using fiducial locations and, optionally, head shape information. This allows the reconstruction to be superimposed onto an MRI to provide a convenient visual correspondence to neuroanatomy. Navigating through the MR volume automatically updates the displayed time series of activation for the selected voxel. Animations can also be generated to view the evolution of neural activity over time. Since NUTMEG displays activations using SPM2's engine, certain SPM functions such as brain rendering and spatial normalization may be applied as well. Finally, as a MATLAB package, the end user can easily add customized functions. Source code is available at http://bil.ucsf.edu/ and distributed under a BSD-style license.
SAM(g2) is an automated analysis that transforms the MEG data into a functional image of spike-like activity, giving the source waveforms for those locations. Since the source waveforms estimated by SAM have higher signal-to-noise ratio (SNR) than does the raw MEG data, it is possible to automatically mark the location and timing of each spike for comparisons with dipole fit procedures. Both SAM(g2) and equivalent current dipole (ECD) fits were used to analyze MEG interictal spike recordings in 10 patients with cortical dysplasias and medial temporal lobe epilepsy. The ECD fit locations obtained by manual spike classification and latency marking were compared with those found by automated SAM(g2) procedures. When the SNR of interictal activity was high (compared to the background) with a clear single focus, there was excellent agreement between the ECD cluster location and the SAM(g2) maximum. However, when the SNR of spikes was low, manual single ECD location scatter was larger than SAM(g2) reconstructions. When multiple independent interictal spike loci were present, there was some disagreement between SAM(g2) and ECD scatter in the cases of low SNR spikes. When SAM(g2) indicated multiple coupled spike loci, the residual variance for the dipole fit was high and its scatter unacceptably large--even for multiple dipole models. This study demonstrates that SAM(g2) is equivalent to ECD fit for localizing interictal spikes when there is a single locus and good SNR. Further studies are required to validate cases in which there are multiple spike loci or poor SNR.
A whole-head 64-channel SQUID magnetometer of SNS (Superconductor/ Normal metal / Superconductor) junctions has been constructed to be operated in a superconducting magnetic shield of Bi(Pb)SrCaCuOx demonstrating the sensitivity of 5 femto Tesla in our laboratory only 10 meter far from both an elevator and power transformer banks. Signal-to-noise ratio of data in neuromagnetic measurements is excellent because a SQUID of SNS junctions has a smaller telegraph noise than that of SIS tunnel junctions and because shielding factors of the superconducting magnetic shield does not reduce even at as low frequencies as 0.05 Hz unlike that of Permalloy shield room. A movie is made to compare the evoked current dipole in the MEG data with the MRI images.
It is difficult to localize the somatosensory cortex in children under 6 years using magnetoencephalography (MEG) with conventional median nerve stimulation. One main reason is that MEG data recorded from small children have poor signal-to-noise ratio due to their small heads and short hands. To find a better approach, this study investigated neuromagnetic activities following finger stimulation using spatially filtered MEG. Four healthy children have been studied with a whole cortex MEG system. Electric stimulation was applied to the thumb and the middle fingers with two Digital Rings. Two trials were recorded for each hand. Focal increases of spectral power were localized using spectrogram and synthetic aperture magnetometry (SAM). A clear response at a latency of 21 ms was identified in 3 children (3/4). Dipole modeling localized the somatosensory cortex in 2 children (2/4). SAM successfully localized the somatosensory cortex in 4 children (4/4). Interestingly, the functional region estimated for the thumb was significantly larger than that of the middle finger (p < 0.01). However, it is possible that this effect is due to a larger source amplitude rather than larger source volume. Our results demonstrated that the developed approach could map the somatosensory cortex in children ranging in age from 3 to 6. To our knowledge, this is the first report using non-invasive methods to provide quantitative data indicating that the functional area of the thumb is larger than that of the middle finger in small children.
It is of historical interest that our latest conference, Biomag 2004 (14 International Conference on Biomagnetism), is taking place in Boston. There are two historical events to consider. First, because the initial biomagnetic SQUID measurements were made about 3 km away in Cambridge (in 1969), this Boston area seems to be called "the birthplace of biomagnetism" [Science, 1989], so in a sense we are returning to the birthplace. Second, the first of these conferences on Biomagnetism took place in Cambridge (in 1976), so at this 14th conference we have come full circle, returning to the Boston area where these conferences began. I will here summarize both of these seminal events. But first I'll say something about the biomag period before the 1969 "birth".
Somatosensory evoked cortical activity is well investigated in both fMRI and MEG/EEG. Investigation with functional magnetic resonance spectroscopic (fMRS) imaging is relatively new and provides a means to image the metabolic activity of the brain. We for the first time combined fMRS, fMRI, MEG and EEG. This provides information about the metabolic, hemodynamic and electrical activity of the brain and also the verification of one imaging modality with the other. The dipolar source localized from the EEG/MEG data is in the vicinity of the fMRI activation site and also in the same area where lactate consumption is high as measured with fMRS imaging.
An approach is presented for representing spatially extended cortical activity using a basis function expansion. The bases are designed to represent patches on the cortical surface. The basis function expansion coefficients are estimated for each patch by scanning modified linearly constrained minimum variance (LCMV) spatial filters over the entire surface. Next, a generalized likelihood ratio test (GLRT) is performed to detect patches with significant activity. In the last step, an image of the activity within each patch is reconstructed using a minimum norm solution to a local inverse problem. We show that the basis function representation enables the LCMV approach to identify patches of coherent activity that are missed by the conventional LCMV method and has potential for extended source detection and localization.
A number of MEG/EEG studies have shown modulation of endogenous sensorimotor (mu and beta) rhythms during the observation of hand movements. These modulations are similar to patterns that occur during execution of movement and it has been hypothesised that the neural substrates of these rhythms may play a role in action representation and understanding the actions of others. In this experiment we wished to determine whether similar responses would be obtained during the observation of oro-facial movements. Neuromagnetic recordings (151 channels, CTF Systems) were obtained from six healthy subjects while they (1) observed a video of an experimenter making oro-facial movements (2) imitated the same movements and (3) observed hand movements. Source scanning using synthetic aperture magnetometry (SAM) was used to find changes in source power between these active conditions compared to pre-stimulus control conditions where no movement occurred. SAM images were created with 5 mm resolution in the beta (15-35 Hz) and mu (8-15 Hz) bands and showed source power decreases over parietal, occipital and sensorimotor areas. Time-frequency analysis of virtual SAM sensors from sensorimotor areas showed event-related desynchronisation of mu and beta bands following the onset of movement in all three conditions. These data demonstrate comparable activations of visuomotor mechanisms during observation or imitation of mouth movements and during observation of hand movements. These results support the notion that sensorimotor mechanisms play a role in achieving a representation of the oro-facial gestures of others.
We discuss the optimal arrangement of detectors for a 52-channel magnetocardiogram (MCG) system measuring tangential components of the cardiac magnetic fields. Nowadays, most MCG instruments are designed to cover the whole heart area to maximize the information available from the myocardial magnetic field in a simultaneous measurement. In such a system, detectors should be spread over a sufficiently wide area. However, an increased diameter of the cooling dewar will result in more heat-loss and higher production and maintenance costs. Therefore, we reviewed the spatial sampling theory to determine the proper interval between detectors, and we decided on the number of channels to cover the whole heart area. In order to fit the detector array on the cylindrical dewar economically, we removed the detectors at the corners of the square array. Through simulations using the confidence region method, we verified that our design of the detector array is enough to obtain adequate information from the heart. Simulations also suggested that tangential-component MCG measurement can localize deep current dipoles better than normal-component measurement with the same confidence volume; therefore, we conclude that measurement of the tangential component is more suitable to an MCG system than the normal component.
Synthetic Aperture Magnetometry (SAM) measures changes in task-related power using pseudo-t values which are affected by changes in both signal and noise. Detecting significant signal power changes between two separate experimental conditions should not be done directly due to possible fluctuation in the noise as well as the response. This study proposes a method to estimate the noise within a single condition, which is then used to test the null hypothesis of no difference between the conditions. The noise estimation is based on a split-half resampling technique. For each resampling, the data of a given condition is divided into two halves. The difference of the pseudo-t volumes between the pair of the datasets is calculated. After multiple resamplings, the confidence limits of the differences within this single condition are computed for a given p-value so that one can test the null hypotheses that the second condition is within the same distribution as the first. The limits are calculated using a bootstrap technique to correct for any bias in the estimated threshold. Power changes between the two conditions are considered significantly different if the difference of the pseudo-t value is larger than expected within conditions. To demonstrate the effectiveness of the technique, the proposed method was applied to MEG responses to two distinct visual stimuli recorded from a single subject. Major differences of brain activity between the two conditions were found in the occipital region. These results were validated using four pairs of split-half datasets, generated from either the odd or even trials in each condition. The method of split-half resampling should therefore be useful for localizing significant differences in brain activity between conditions within individual subjects.
Both an EEG P50 sensory gating deficit and abnormalities of the temporal lobe structure are considered characteristic of schizophrenia. The standard P50 sensory gating measure does not foster differential assessment of left- and right-hemisphere contributions, but its analogous MEG M50 component may be used to measure gating of distinct auditory source dipoles localizing to left- and right-hemisphere primary auditory cortex. The present study sought to determine how sensory gating ratio may relate to cortical thickness at the site of the auditory dipole localization. A standard auditory paired-click paradigm was used during MEG for patients (n=22) and normal controls (n=11). Sensory gating ratios were determined by measuring the strength of the 50 ms response to the second click divided by that of the first click (S2/S1). Cortical thickness was assessed by two reliable raters using 3D sMRI. Results showed that: (1) patients had a P50 and left M50 sensory gating deficit relative to controls; (2) cortex in both hemispheres was thicker in the control group; (3) in schizophrenia, poorer left-hemisphere M50 sensory gating correlated with thinner left-hemisphere auditory cortical thickness; and (4) poorer right-hemisphere M50 auditory sensory gating ratio correlated with thinner right-hemisphere auditory cortical thickness in patients. The MEG-assessed hemisphere-specific auditory sensory gating ratio may be driven by this structural abnormality in auditory cortex.
Since the high costs of common large array SQUID system may hinder widespread application of fetal magnetoencephalography (fMEG) and magnetocardiography (fMCG), we intended to investigate a small non-commercial 3-channel SQUID system. The system comprises 3 axial first order gradiometers with 7 cm base length, 2 cm diameter and 2x2 windings of niobium wire, dc-SQUIDs (UJ-111), and current locked mode SQUID electronics that form an equal length triangle (22.5 mm). The system is mounted in a Cryostat BFH-7 model 16 with 5 mm "warm"-"cold" distance. System noise is about 10 fT/Hz1/2. The fMEG and fMCG were recorded between 29 - 40 weeks of gestation after sonographic localization of the fetal head and heart using a 31-channel biomagnetometer (Philips) and the 3-channel-system, both in the same magnetically shielded room. The fMEG was recorded continuously over 500 sec (500 auditory stimuli, 100 dB SPL, 500 Hz, 50 ms, ISI 0.8-1.2/1.6-2.4 sec, trigger channel, maternal ECG lead, sampling rate 1 kHz). The fMCG was recorded over a period of 5 minutes after dewar readjustment. The detection rates of cortical auditory evoked responses (CAER) reached 100 % for both systems. Cross confirmation of the components was difficult and may have uncovered false positive component detection. The fMCG was characterized by a systematic increase in SNR under application of the smaller device. The small size array provides a profitable alternative for the fetal applications.
SAM(g2) analysis, a combination of synthetic aperture magnetometry (SAM) and excess kurtosis (g2) method, is a novel epilepsy analysis procedure based on a spatial filtering technique. By producing a three-dimensional image of the g2 values and superimposing them onto a patient's MR images, this analysis can automatically estimate spike localization from raw MEG epilepsy signals including spikes. The aim of this study is to examine SAM(g2) analysis using MEG signals of an epileptic patient, whose clinical symptoms of colored elementary visual auras had ceased in accordance with the changes of the estimated localizations of the equivalent current dipoles (ECDs) of the interictal spikes. His visual auras were experienced in 1997, while they ceased in 1999 with effective medication. The patient provided written informed consent for the experimental procedures. The MEG signals were recorded in 1997 and 1999, and were analyzed using both ECD and SAM(g2) analyses. For the MEG signals of 1997, ECD analysis estimated most of the interictal spikes in the right fusiform and inferior temporal gyri, which subserve human color processing. SAM(g2) analysis also estimated them in the same areas. For those of 1999, both ECD and SAM(g2) analyses estimated them in the right transverse gyrus of Heschl. As well as ECD analysis, SAM(g2) analysis successfully estimated the changes of the localizations of the interictal spikes in accordance with the changes of the patient's clinical symptoms, indicating that SAM(g2) analysis is useful for detection of interictal spike localization in epileptic patients.
Channel count in modern MEG systems has been steadily increasing, but are more channels necessary? Assuming that the spatial sampling considerations are satisfied, this question can be answered by examining the MEG system's ability to localize and resolve brain sources. For the simple situation where only uncorrelated sensor noise is present, dipole localization accuracy monotonically increases with increasing number of channels, while for spatially correlated brain noise the accuracy increases only until the number of channels reaches 100 to 200. Beyond this limit the inter-channel separation is comparable to the brain noise correlation distance and increasing the channel count does not help. Contrary to the above dipole result, we show by simulations with up to several thousand channels, that if the data is analyzed by beamformers even in the presence of correlated brain noise, the two-source resolvability and single-source localization accuracy monotonically improve with increasing number of channels. We demonstrate such behavior for a 275 channels system, where we have inserted an artificial dipole into real measured brain noise and resampled the number of channels to 138. Beamformer analysis of the data shows markedly improved localization accuracy when the number of channels is increased from 138 to 275. This finding also signifies that the beamformer performance is not limited by system imperfections when the number of channels is as large as 275. To clarify these results, we illustrate analytically the mechanism of beamformer resolution dependence on the number of channels, using an example of a simple system containing two dipole sources, and uncorrelated sensor noise.
Previous psychophysical studies have reported a few hundred millisecond difference in the reaction time (RT) to luminance versus color motion in low speed condition. Electroencephalogram (EEG) studies have reported a small difference between initial responses to luminance and color motion, but a big difference comparable to that reported in psychophysical studies has not been observed. The present study aimed to investigate late responses in low speed condition in order to clarify the difference of RTs between luminance and color motion. In general, measurement of the late responses is difficult because the late responses are weaker than the initial responses. A previous EEG study of binocular rivalry has reported that binocular rivalry stimuli amplify late responses. Therefore, we used binocular rivalry stimuli to measure late responses. Visual evoked fields were recorded with a whole-head MEG system. A rivalry-related field (RRF) was obtained from the subtraction between the rivalry and a control condition. The RRF was measured between 400 to 550 ms after the stimulus onset for each motion. Results of source localizations of RRFs had similar positions for both the luminance and the color motion. No statistically significant difference between the latencies of the two RRFs was found.
