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Background: The purpose of this study is to clarify the source distribution patterns of magnetoencephalographic spikes correlated with postsurgical seizure-free outcome in pediatric patients with focal cortical dysplasia.Patients and Methods: Thirty-two patients with pathologically confirmed focal cortical dysplasia were divided into seizure-free and seizure-persistent groups according to their surgical outcomes based on Engel classification. In each patient, presurgical magnetoencephalography was reviewed. Dipole sources of magnetoencephalographic spikes were calculated according to a single dipole model. We obtained the following quantitative indices for evaluating dipole distribution: maximum distance over all pairs of dipoles, standard deviation of the distances between each dipole and the mean coordinate of all dipoles, average nearest neighbor distance, the rate of dipoles located within 10, 20, and 30 mm from the mean coordinate, and the rate of dipoles included in the resection. These indices were compared between the two patient groups.Results: Average nearest neighbor distance was significantly smaller in the seizure-free group than in the seizure-persistent group (P = 0.008). The rates of dipoles located within 10, 20, and 30 mm from the mean coordinate were significantly higher in the seizure-free group (P = 0.001, 0.001, 0.005, respectively). The maximum distance, standard deviation, and resection rate of dipoles did not show a significant difference between the two groups.Conclusions: A spatially restricted dipole distribution of magnetoencephalographic spikes is correlated with postsurgical seizure-free outcomes in patients with focal cortical dysplasia. The distribution can be assessed by quantitative indices that are clinically useful in the presurgical evaluation of these patients. [ABSTRACT FROM AUTHOR]

Objective We aimed to validate the usefulness of gradient magnetic-field topography (GMFT) for analysis of ictal magnetoencephalography (MEG) in patients with neocortical epilepsy. Methods We identified 13 patients presenting with an ictal event during preoperative MEG. We applied equivalent current dipole (ECD) estimation and GMFT to detect and localize the ictal MEG onset, and compared these methods with the ictal onset zone (IOZ) derived from chronic intracranial electroencephalography. The surgical resection areas and outcomes were also evaluated. Results GMFT detected and localized the ictal MEG onset in all patients, whereas ECD estimation showed localized ECDs in only 2. The delineation of GMFT was concordant with the IOZ at the gyral-unit level in 10 of 12 patients (83.3%). The detectability and precision of delineation of ictal MEG activity by GMFT were significantly superior to those of ECD ( p < 0.05 and p < 0.01, respectively). Complete resection of the IOZ in the concordant group provided seizure freedom in 3 patients, whereas seizures remained in 9 patients who had incomplete resections. Conclusions Because of its higher spatial resolution, GMFT of ictal MEG is superior to conventional ECD estimation in patients with neocortical epilepsy. Significance Ictal MEG study is a useful tool to estimate the seizure onset in patients with neocortical epilepsy. [ABSTRACT FROM AUTHOR]

Purpose : We evaluated whether magnetoencephalography (MEG), in addition to surgery, was valuable for the diagnosis and management of epileptic syndromes in patients with neocortical epilepsy (NE). Methods : We studied MEG in 73 patients (29 females; aged 1–26 years; mean 10.3 years) for the clinical diagnosis of epilepsy and for preoperative evaluation. MEG data were recorded by 204-channel whole head gradiometers with a 600 Hz sampling rate. MEG spike sources were localized on magnetic resonance images (MRI) using a single dipole model to project equivalent current dipoles. Results : MEG localized an epileptic focus with single clustered dipoles in 24 (33%) of 73 NE patients: 16 (25%) of 64 symptomatic localization-related epilepsy (SLRE) patients and eight (89%) of nine idiopathic localization-related epilepsy (ILRE) patients. MEG provided advantageous information in 12 (50%) of 24 patients with clustered dipoles and confirmed the diagnosis in the remaining 12 (50%). Furthermore, the use of MEG resulted in changes to surgical treatments in nine (38%) patients and in medical management in eight (33%). MEG confirmed the diagnosis in eight (16%) of 49 patients with scattered dipoles. MRI identified a single lesion (28 patients, 38%), multiple lesions (5, 7%), and no lesion (40, 55%). MRI provided confirming information in 19 of 28 patients with a single lesion and 18 of them required surgical resections. MRI did not provide any supportive information in 54 (74%) patients with a single (9), multiple (5) and no lesion (40). Conclusion : Our study shows that MEG provides fundamental information to aid the choice of diagnostic and therapeutic procedures including changes in medication in addition to surgical treatments for NE. [ABSTRACT FROM AUTHOR]

Abstract: Objective: To determine whether quantitative modification of a standardised low-resolution brain electromagnetic tomography (sLORETA-qm) could be used as a reliable tool for quantitative analysis of magnetoencephalography (MEG) for analysis of the interictal epileptic spike. To verify the performance of sLORETA-qm, magnetic source location and quantity were compared with the equivalent current dipole (ECD) method. Methods: A total of 50 sources from 10 patients with epilepsy were obtained. Analyses were performed after the MEG data were 3–70Hz band-pass filtered. Time points for analysis were selected referring to waveform patterns and the isofield contour map. With the same spherical model, source estimation was conducted with two methods of analysis: ECD and sLORETA-qm. Distance from the centre of the spherical model and intensities were compared between the methods. Results: There were no significant differences between the methods in the distance from the spherical model (paired t-test, p =0.8761). Source intensities between the methods were strongly correlated (Spearman’s Rho=0.9803, p <0.001). Conclusions: sLORETA-qm was closely correlated with ECD concerning point source location and quantity in analysis of the interictal epileptic spike. Significance: sLORETA-qm is a reliable quantifiable method without arbitrariness for analysis of the interictal epileptic spike. [Copyright &y& Elsevier]

Abstract: We report a method of inferring the dipole location using iterative nonlinear least square optimization based on Levenberg–Marquardt algorithm, wherein, we use different sets of pseudo-random numbers as initial parameter values. The method has been applied to (i) the simulated data representing the calculated magnetic field distribution produced by a point dipole placed at a known position, (ii) the experimental data from SQUID based measurements of the magnetic field distribution produced by a source coil carrying current, and (iii) the actual experimentally measured magnetocardiograms of human subjects using a SQUID based system. [Copyright &y& Elsevier]

Summary: We analysed the association between focal cortical dysplasia (FCD) visible in MRI and the location of equivalent current dipole (ECD) of single interictal scalp EEG spikes (IIS) in 11 epilepsy patients. We calculated several indicators of distance of ECDs to the FCD border. The results confirm some previous studies suggesting that the epileptogenic zone associated to the location of ECDs extends beyond the FCD visible in MRI. The analysis suggests the ECDs to be in a shell parallel to part of the FCD surface. [ABSTRACT FROM AUTHOR]

Summary: Purpose: To compare and validate the gradient magnetic-field topography (GMFT) method of current source localization for understanding epileptic zones against equivalent current dipole (ECD) and intracranial video-EEG (IVEEG) data in patients with intractable neocortical epilepsy. Methods: We used retrospective data from eight patients to determine GMFT at onset (O) and peak (P) of interictal magnetoencephalography (MEG) spikes and mapped GMFT(O) and GMFT(P) locations and distributions using 12 zones in unilateral hemisphere. We compared GMFT with ECD, ictal onset zones (IOZ) and interictal zones on IVEEG, and seizure outcomes. Results: We projected GMFT(Os) and (Ps) for all spikes on volume-rendered brain surfaces. We localized ECDs for 6–61% of spikes (mean, 28.4%). GMFT(Ps) (mean, 10.3 zones) extended over more zones than GMFT(Os) (6.3 zones) for each spike (p <0.01). The ECD distributions (2.3 zones) were almost equal to the zones of IOZ and surgical areas. GMFT(O) localizations distributed much more extensively than IOZs and surgical areas in three patients with residual seizures comparing with those in five seizure-free patiens. Conclusion: We validate the potential of GMFT to study the distribution of MEG spikes. GMFT has an advantage in analyzing the cortical activity and propagation from MEG spikes in neocortical epilepsy. [Copyright &y& Elsevier]

Abstract: We examined memory performance and cortical source localization of old/new effects in a source memory task in obsessive–compulsive disorder (OCD) patients by employing an equivalent current dipole (ECD) model using EEG and a realistic head model. Event-related potentials (ERPs) were recorded while 14 OCD patients and 14 age-, sex-, handedness-, and educational level-matched healthy control subjects performed recognition tasks for spoken words (items) or for the voice of the speaker of spoken words (sources). In the item memory task, both groups showed ERP old/new effects at 300–700 ms. In the source memory task, the controls showed ERP old/new effects at 400–700 ms, whereas the OCD patients did not. Compared with the controls, the OCD patients showed significantly lower source accuracy and prolonged reaction times to the old words with accurate voice judgments. There were no differences between the OCD and control groups with regard to the locations of the ERP generators elicited by source correct and correct rejection conditions. The OCD patients showed significantly altered hemispheric asymmetry of ECD power in the frontal lobe during source memory retrieval, compared with the controls. These results indicate that OCD patients have preserved item memory about content, but impaired source memory about context. [Copyright &y& Elsevier]

Abstract: We recorded auditory evoked magnetic fields (AEFs) to monaural 400Hz tone bursts and investigated spatio-temporal features of the N100m current sources in the both hemispheres during the time before the N100m reaches at the peak strength and 5ms after the peak. A hemispheric asymmetry was evaluated as the asymmetry index based on the ratio of N100m peak dipole strength between right and left hemispheres for either ear stimulation. The results of asymmetry indices showed right-hemispheric dominance for left ear stimulation but no hemispheric dominance for right ear stimulation. The current sources for N100m in both hemispheres in response to monaural 400Hz stimulation moved toward anterolateral direction along the long axis of the Heschl gyri during the time before it reaches the peak strength; the ipsilateral N100m sources were located slightly posterior to the contralateral N100m ones. The onset and peak latencies of the right hemispheric N100m in response to right ear stimulation are shorter than those of the left hemispheric N100m to left ear stimulation. The traveling distance of the right hemispheric N100m sources following right ear stimulation was longer than that for the left hemispheric ones following left ear stimulation. These results suggest the right-dominant hemispheric asymmetry in pure tone processing. [Copyright &y& Elsevier]

Abstract: Event-related synthetic aperture magnetometry (erSAM) is a beamforming approach that allows measurement of instantaneous, time-locked source activity. Using erSAM, MEG data can be transformed into three-dimensional functional images and source locations of epileptiform discharges can be estimated. Both erSAM and the equivalent current dipole model (ECD) were used to analyze interictal spikes in three children with intractable partial epilepsy. We acquired MEG data using a whole-head 151-channel system (VSM MedTech Ltd, Coquitlam, Canada). We visually identified MEG spikes and applied markers at the peak of each spike. erSAM images with a spatial sampling resolution of 2 mm were computed for the whole head utilizing three different parameters: a single time point corresponding to ECD best fit utilizing band pass filters of a) 3–70 Hz and b) 20–70 Hz and c) mean power over a 50 ms time window of +25 to −25 ms from the ECD best fit with band pass filter 20–70 Hz. All three methods of erSAM analysis yielded concordant source localization results with the ECD in all three patients. The mean difference in source locations between erSAM and ECD ranged from 0.01 to 1.3 cm. erSAM presents an alternative accurate approach for source localization of epileptiform discharges. [Copyright &y& Elsevier]

Abstract: MEG phantoms with saline water, as a practical realization of the Sarvas model, have been used to evaluate the accuracy of magnetic field measurement from MEG systems as well as to confirm the validity of algorithms for magnetic source analysis. But there are several concerns to be addressed before they will be accepted as “the evaluation standard.” The concerns are that mechanical accuracy is hard to achieve, linear current drive is also hard to achieve and that electrode quality degrades easily. On the other hand, “dry phantom” proposed by Ilmoniemi et al., which is an isosceles-triangle coil with an infinitely small base, is free from these concerns and is a promising candidate for the standard. To realize a practical standard with this model, the base should be finite. So, we calculated the magnetic field from a finite-small base isosceles-triangle coil in comparison with the magnetic field of the ideal Sarvas model. Calculations were done for 80-mm legs with 10-mm base triangle and for 80-mm legs with 5-mm base triangle, which results in maximum 2% error and maximum 0.2% error from the ideal Sarvas model, respectively. These results suggest that the isosceles-triangle coil with a 5-mm base is a good selection for the standard phantom. [Copyright &y& Elsevier]

Abstract: We examined the non-linear impedance of a saline water phantom using a prototype with 150-mm diameter sphere and with 5-mm dipole electrodes located at 20 mm from the center of the sphere. We found that the electrical impedance is a series of 0.84-μF capacitor and 250-Ω resistor, which requires unpractical source impedance and voltage, 1.9 MΩ and 760 V respectively, to drive the phantom without distortion. Also observed was a non-linearity due to the electrolysis, which can be avoided by constraining the frequency to be more than 1 kHz, in which frequency human brain signal is not properly emulated. Due to these difficulties, the phantom with saline water is not a good candidate for “the evaluation standard”. [Copyright &y& Elsevier]

Abstract: Fitting an equivalent current dipole (ECD) to magnetoencephalography (MEG) data is a widely used method for analyzing MEG data, especially in clinical applications. However, as is well known, the iterative fitting algorithms routinely employed can converge on a local minima far from the best fit. While a host of fitting algorithms have been proposed to remedy this problem, the simple exhaustive search optimization algorithm has not been considered in the literature — possibly because it is assumed to be too slow for routine use. Taking advantage of the speed of modern computers, it is demonstrated that using exhaustive search to fit the parameters of a single ECD to 151 MEG channels yields both robust and reproducible fits within reasonable computation times. On a high end personal computer, with a 2 mm grid spacing over the head, exhaustive search can fit 450 ECD''s per hour with only an additional 5 min spent on calculating the lead fields. As the algorithm is fully automatic, it is very useful when the goal is to detect ECD''s in a large dataset. Also, an interpreter can be provided with parameters from the ECD that fits for all latencies after stimulus, allowing a more informed decision of which latency to use for further analysis. [Copyright &y& Elsevier]

Abstract: Few have reported the activities of auditory cortices to high-frequency sounds over 5000 Hz; though several studies have reported that bone-conducted ultrasounds (BCUs) are perceived even by those who are profoundly sensorineural deaf. Brain magnetic fields evoked by air- and bone-conducted sounds with frequency variations in an audible to ultrasonic range were recorded to clarify differences of perception mechanisms between (i) low- and high-frequency sounds, (ii) air- and bone-conducted sounds, and (iii) BCU and 12-kHz air-conducted sound, which had almost same pitch and loudness as BCU. Auditory-evoked magnetic fields evoked by air-conducted audible sounds (ACs; 500–16,000 Hz), bone-conducted audible sounds (BCs; 500–16,000 Hz), and a BCU (30,000 Hz) were measured and following results were obtained; (1) for all stimuli, substantial N1m responses were observed and equivalent current dipoles (ECDs) were localized in the Heschl''s gyrus or the planum temporale. (2) Generally, the latencies increased and the ECD moments decreased as the frequency increased. (3) ACs and BCs produced by the same frequency sounds showed no differences in N1m latency and ECD moment below 8000 Hz; on the other hand, BCs showed larger latencies and smaller ECD moments than ACs over 12,000 Hz. (4) BCUs showed a larger latency and smaller ECD moments, and more posterior and lateral location than all ACs and BCs. Especially, BCU was different from 12-kHz AC in ECD moment, latency, and source location. (5) With all kinds of stimuli, the contralateral stimuli showed faster and larger N1m than the ipsilateral stimuli. These results suggest that air- and bone-conducted sounds are processed similarly below 8000 Hz, but high-frequency bone-conducted sounds over 12,000 Hz are processed differently. Considering the previous reports, the differences in the perception mechanisms possibly exists in the inner ear. [Copyright &y& Elsevier]

Abstract: Objective: The purpose of the present study was to determine the number of the equivalent dipole sources corresponding to the scalp EEG using the information criterion method based on the instantaneous-state modeling. Methods: A three-concentric-spheres head model was used to represent the head volume conductor. The Powell algorithm was used to solve the inverse problem of estimating the equivalent dipoles from the scalp EEG. The information criterion with different penalty functions was used to determine the dipole number. Computer simulations were conducted to evaluate effects of various parameters on the estimation of dipole number. Results: The present results suggest that the present method is able to estimate the number of equivalent current dipoles (ECDs) from instantaneous scalp EEG measurements, and that increase in the electrode number can improve the accuracy of estimation of the ECD number. For two ECDs, the best performance of estimation with 20% white noise were 85%, 92% and 94%, when 64, 128 and 256 electrodes are used, respectively. When there are 3 ECDs, the present results suggest that using 256 electrodes gave up to 82% estimation accuracy. The present simulation results also indicate that the accuracies of identification are similar when the minimum distance between dipoles is either 1 or 2cm, which was used in the simulation. It was also found that the different penalty functions used in the information criterion method could have substantial influence on the estimation accuracy. Conclusions: The present method can estimate the number of ECDs from instantaneous scalp EEG distribution for up to three dipoles. Significance: The successful estimation of the number of ECDs will play an important role in expanding the applicability of dipole source localization to multiple sources. [Copyright &y& Elsevier]

Abstract: Several studies have reported that high-frequency sounds of more than 20,000 Hz can be perceived by the profoundly sensorineural deaf subjects, as well as normal-hearing, through bone conduction. Although the mechanisms of how bone-conducted ultrasound (BCU) is perceived remain unclear, several characteristics that differ from air-conducted audible sounds are reported; for example, the pitch of BCU is independent of the frequency. In this study, the effects of BCU frequency on the activities of auditory cortex were investigated by magnetoencephalography (MEG). Auditory-evoked magnetic fields evoked by four kinds of BCU stimuli, 22,000, 27,000, 32,000, and 37,000 Hz tone bursts, were measured, and equivalent current dipoles (ECDs) of N1m were estimated. No significant differences were observed among the four kinds of BCU stimuli, not only in pitch, but also in N1m latency, ECD moment, and ECD location; whereas obvious differences were observed between the air-conducted sounds and BCUs. These results suggest that the mechanisms for BCU perception are different from those for air-conducted audible sounds. [Copyright &y& Elsevier]

Magnetic fields were recorded from human subjects during the performance of a 1-back recognition short-term memory task with pictorial stimuli using a whole-head neuromagnetometer. In the memory condition the subjects were required to respond to the same stimuli and memorize the presented pictures for a short period of time, in control condition to respond to all stimuli and not memorize the picture. Magnetic event-related fields (MEFs) were computed and analyzed to reveal information about the encoding process of pictorial stimuli in short-term memory. Based on comparative MEFs analysis between conditions and behavioral response times, the encoding process for pictorial stimuli in this particular memory task was estimated at an interval with peaking activity around 500–550 ms from the stimulus presentation onset. Equivalent current dipoles (ECDs) were fitted to localize the brain sources related to visual information processing and the encoding process. A dipole located in the left anterior medial temporal lobe was estimated as an important source underlying encoding activity. [Copyright &y& Elsevier]

Objective: To study early cognitive processes and hemispheric differences in the primary auditory cortex during selective attention.Methods: We measured auditory evoked magnetic fields (AEFs) to 400 and 4000 Hz tone pips that were randomly presented at the right or left ear. Subjects paid attention to target stimuli during pitch (high or low) or laterality (left or right) discrimination tasks. In the control session, 400 or 4000 Hz tone alone was presented at the left or right ear. We calculated the location and strength of N100m dipole for 400 and 4000 Hz tones, based on the AEFs obtained from the hemisphere contralateral to the stimulated ear.Results: N100m amplitude increased in both hemispheres in pitch or laterality discriminating conditions. N100m latency also shortened during selective attention. The N100m dipole distance between 400 and 4000 Hz tones was enlarged, especially in the right auditory cortex during pitch discrimination task, but was unchanged during the laterality discrimination task.Conclusions: We conclude that these dynamic changes in the N100m dipole reflect short-term plastic changes in the primary auditory cortex, supporting early selection models.Significance: This work is the first to disclose short-term plastic changes during pitch discrimination in the human auditory cortex based on the analysis of magnetoencephalography. [Copyright &y& Elsevier]

The sequence of neural activation during a visual search task was investigated using magnetoencephalography and the source locations for the activations were analyzed using a single-dipole algorithm. Five components (M1–5) were detected at mean latencies of 110, 146, 196, 250 and 333 ms in both of two different stimulus conditions; a target popped out in one stimulus condition (pop-out), while it did not in the other condition (non-pop-out). Statistical analysis showed that the M3 amplitude was larger and the M5 latency was shorter in the pop-out condition than in the non-pop-out condition, while there was no difference in the other components between the conditions. Neural sources were localized in the calcarine sulcus (M1) and the posterior fusiform gyrus (M2) of the hemisphere contralateral to the stimuli, the intraparietal sulcus and the posterior superior temporal sulcus (M3) in either of the hemispheres, and the calcarine sulcus (M4) of the same hemisphere in which the early processing (M1 and M2) occurred. The criteria for source localization were not satisfied for M5. The results suggest that the processing for pop-out and non-pop-out stimuli share a common mechanism; after early feature processing in the occipital cortex (M1 and M2), visual information is processed in the parietal and temporal regions (M3) and then some of this information is fed back to the occipital cortex (M4). [Copyright &y& Elsevier]

The abnormality of mismatch negativity (MMN) in schizophrenia is thought to be associated with perceptional disturbance and cognitive dysfunction. The purpose of the present study was to investigate the change of the normal functional hemispheric lateralization in schizophrenia by employing the equivalent current dipole (ECD) model of auditory MMN with individual MRI and high-density electroencephalography (EEG). The MMNs resulting from auditory stimuli with passive oddball paradigm in a group of schizophrenics (n=15), and also a group of age-, sex-, and handedness-matched normal controls, were recorded by 128 channel EEG. The location and power of ECD sources at the peak point were calculated. Individual 3-D brain magnetic resonance images (MRI) were used for realistic head modeling and for source localization. For both groups, the MMN source was determined to be located in the superior temporal gyrus (STG). However, the normal functional hemispheric asymmetry of ECD power was significantly altered in the schizophrenics (χ2 test=16.13, p<0.001). Left MMN ECD power and the asymmetry coefficient (AC) were negatively correlated with the positive scores from Positive and Negative Syndrome Scale (PANSS) (r=−0.673, p=0.008), especially with the hallucinatory behavior subscale (r=−0.677, p=0.008). These findings support the deficits in preattentive automatic processing of auditory stimuli, especially in the left hemisphere, and indicate the correlation between positive symptoms, especially auditory hallucination, and left temporal lobe dysfunction in schizophrenia. [Copyright &y& Elsevier]

To analyze the temporal changes in localization of an equivalent current dipole (ECD) for the auditory N100m, we recorded auditory evoked magnetic fields (AEFs) to 400 Hz tone pips presented at the right or left ear. Using a single ECD model, the dipole location for the N100m sources was successively calculated from the AEFs obtained from the hemisphere contralateral to the stimulated ear. We found that the location of the N100m current sources moved dynamically in medio-lateral and postero-anterior directions before the N100m peak. This direction was parallel to the surface of the supratemporal cortex. We propose that the dynamic movement of the N100m dipole reflects spread of intracortical activation through horizontal fibers of pyramidal neurons in the auditory cortex, forming the isofrequency bands in humans. [Copyright &y& Elsevier]