Equivalent Current Dipole Source Localization Research Articles

Effects of head modeling errors on the spatial frequency representation of MEG

Objectives. We aim to investigate the effects of head model inaccuracies on signal and source reconstruction accuracies for various sensor array distances to the head. This allows for the assessment of the importance of head modeling for next-generation magnetoencephalography (MEG) sensors, optically-pumped magnetometers (OPM). Approach. A 1-shell boundary element method (BEM) spherical head model with 642 vertices of radius 9 cm and conductivity of 0.33 S m−1 was defined. The vertices were then randomly perturbed radially up to 2%, 4%, 6%, 8% and 10% of the radius. For each head perturbation case, the forward signal was calculated for dipolar sources located at 2 cm, 4 cm, 6 cm and 8 cm from the origin (center of the sphere), and for a 324 sensor array located at 10 cm to 15 cm from the origin. Equivalent current dipole (ECD) source localization was performed for each of these forward signals. The signal for each perturbed spherical head case was then analyzed in the spatial frequency domain, and the signal and ECD errors were quantified relative to the unperturbed case. Main results. In the noiseless and high signal-to-noise ratio (SNR) case of approximately ≥6 dB, inaccuracies in our spherical BEM head conductor models lead to increased signal and ECD inaccuracies when sensor arrays are placed closer to the head. This is true especially in the case of deep and superficial sources. In the noisy case however, the higher SNR for closer sensor arrays allows for an improved ECD fit and outweighs the effects of head geometry inaccuracies. Significance. OPMs may be placed directly on the head, as opposed to the more commonly used superconducting quantum interference device sensors which must be placed a few centimeters away from the head. OPMs thus allow for signals of higher spatial resolution to be captured, resulting in potentially more accurate source localizations. Our results suggest that an increased emphasis on accurate head modeling for OPMs may be necessary to fully realize its improved source localization potential.

Equivalent current dipole source localization of deceptive response during crime-relevant sentence processing

Equivalent current dipole source localization of deceptive response during crime-relevant sentence processing

Nonlinear Coupling between Cortical Oscillations and Muscle Activity during Isotonic Wrist Flexion.

Coupling between cortical oscillations and muscle activity facilitates neuronal communication during motor control. The linear part of this coupling, known as corticomuscular coherence, has received substantial attention, even though neuronal communication underlying motor control has been demonstrated to be highly nonlinear. A full assessment of corticomuscular coupling, including the nonlinear part, is essential to understand the neuronal communication within the sensorimotor system. In this study, we applied the recently developed n:m coherence method to assess nonlinear corticomuscular coupling during isotonic wrist flexion. The n:m coherence is a generalized metric for quantifying nonlinear cross-frequency coupling as well as linear iso-frequency coupling. By using independent component analysis (ICA) and equivalent current dipole source localization, we identify four sensorimotor related brain areas based on the locations of the dipoles, i.e., the contralateral primary sensorimotor areas, supplementary motor area (SMA), prefrontal area (PFA) and posterior parietal cortex (PPC). For all these areas, linear coupling between electroencephalogram (EEG) and electromyogram (EMG) is present with peaks in the beta band (15–35 Hz), while nonlinear coupling is detected with both integer (1:2, 1:3, 1:4) and non-integer (2:3) harmonics. Significant differences between brain areas is shown in linear coupling with stronger coherence for the primary sensorimotor areas and motor association cortices (SMA, PFA) compared to the sensory association area (PPC); but not for the nonlinear coupling. Moreover, the detected nonlinear coupling is similar to previously reported nonlinear coupling of cortical activity to somatosensory stimuli. We suggest that the descending motor pathways mainly contribute to linear corticomuscular coupling, while nonlinear coupling likely originates from sensory feedback.

Decoding Silent Speech in Japanese from Single Trial EEGS: Preliminary Results

We propose a new scheme for speaker-dependent silent speech recognition systems (SSRSs) using both single-trial electroencephalograms (EEGs) scalp-recorded and speech signals measured during overtly and covertly speaking “janken” and “season” in Japanese. This scheme consists of two phases. The learning phase specifies a Kalman filter using spectrograms of the speech signals and independent components (ICs), whose equivalent current dipole source localization (ECDL) solutions were located mainly at the Broca’s area, of the EEGs during the actual speech. In case of the “season” task, the speech signals were transformed into vowel and consonant sequences, and these relationships were learned by hidden Markov model (HMM) with Gaussian mixture densities. The decoding phase predicts spectrograms for the silent “janken” and “season” using the Kalman filter with the EEGs during the silent speech. For the silent “season”, the predicted spectrograms were inputted to the HMM, and which “season” was silently spoken was determined by the maximal log-likelihood among each HMM. Our preliminary results as training steps are as follows: the silent “jankens” were correctly discriminated; the silent “season”-HMMs worked well, suggesting that this scheme might be applied to the discrimination between all the pairs of the hiraganas

MEG-based detection and localization of perilesional dysfunction in chronic stroke.

Post-stroke impairment is associated not only with structural lesions, but also with dysfunction in surviving perilesional tissue. Previous studies using equivalent current dipole source localization of MEG/EEG signals have demonstrated a preponderance of slow-wave activity localized to perilesional areas. Recent studies have also demonstrated the utility of nonlinear analyses such as multiscale entropy (MSE) for quantifying neuronal dysfunction in a wide range of pathologies. The current study utilized beamformer-based reconstruction of signals in source space to compare spectral and nonlinear measures of electrical activity in perilesional and healthy cortices. Data were collected from chronic stroke patients and healthy controls, both young and elderly. We assessed relative power in the delta (1–4 Hz), theta (4–7 Hz), alpha (8–12 Hz) and beta (15–30 Hz) frequency bands, and also measured the nonlinear complexity of electrical activity using MSE. Perilesional tissue exhibited a general slowing of the power spectrum (increased delta/theta, decreased beta) as well as a reduction in MSE. All measures tested were similarly sensitive to changes in the posterior perilesional regions, but anterior perilesional dysfunction was detected better by MSE and beta power. The findings also suggest that MSE is specifically sensitive to electrophysiological dysfunction in perilesional tissue, while spectral measures were additionally affected by an increase in rolandic beta power with advanced age. Furthermore, perilesional electrophysiological abnormalities in the left hemisphere were correlated with the degree of language task-induced activation in the right hemisphere. Finally, we demonstrate that single subject spectral and nonlinear analyses can identify dysfunctional perilesional regions within individual patients that may be ideal targets for interventions with noninvasive brain stimulation.

Evaluating dipolar source localization feasibility from intracerebral SEEG recordings

Stereo-electroencephalography (SEEG) is considered as the golden standard for exploring targeted structures during pre-surgical evaluation in drug-resistant partial epilepsy. The depth electrodes, inserted in the brain, consist of several collinear measuring contacts (sensors). Clinical routine analysis of SEEG signals is performed on bipolar montage, providing a focal view of the explored structures, thus eliminating activities of distant sources that propagate through the brain volume. We propose in this paper to exploit the common reference SEEG signals. In this case, the volume propagation information is preserved and electrical source localization (ESL) approaches can be proposed. Current ESL approaches used to localize and estimate the activity of the neural generators are mainly based on surface EEG/MEG signals, but very few studies exist on real SEEG recordings, and the case of equivalent current dipole source localization has not been explored yet in this context. In this study, we investigate the influence of volume conduction model, spatial configuration of SEEG sensors and level of noise on the ESL accuracy, using a realistic simulation setup. Localizations on real SEEG signals recorded during intracerebral electrical stimulations (ICS, known sources) as well as on epileptic interictal spikes are carried out. Our results show that, under certain conditions, a straightforward approach based on an equivalent current dipole model for the source and on simple analytical volume conduction models yields sufficiently precise solutions (below 10mm) of the localization problem. Thus, electrical source imaging using SEEG signals is a promising tool for distant brain source investigation and might be used as a complement to routine visual interpretations.

Categorical-Data-Based BCI with Motor Imagery Using Equivalent Current Dipole Source Localization

Categorical-Data-Based BCI with Motor Imagery Using Equivalent Current Dipole Source Localization

Magnetoencephalography recording and analysis.

Magnetoencephalography (MEG) non-invasively measures the magnetic field generated due to the excitatory postsynaptic electrical activity of the apical dendritic pyramidal cells. Such a tiny magnetic field is measured with the help of the biomagnetometer sensors coupled with the Super Conducting Quantum Interference Device (SQUID) inside the magnetically shielded room (MSR). The subjects are usually screened for the presence of ferromagnetic materials, and then the head position indicator coils, electroencephalography (EEG) electrodes (if measured simultaneously), and fiducials are digitized using a 3D digitizer, which aids in movement correction and also in transferring the MEG data from the head coordinates to the device and voxel coordinates, thereby enabling more accurate co-registration and localization. MEG data pre-processing involves filtering the data for environmental and subject interferences, artefact identification, and rejection. Magnetic resonance Imaging (MRI) is processed for correction and identifying fiducials. After choosing and computing for the appropriate head models (spherical or realistic; boundary/finite element model), the interictal/ictal epileptiform discharges are selected and modeled by an appropriate source modeling technique (clinically and commonly used — single equivalent current dipole — ECD model). The equivalent current dipole (ECD) source localization of the modeled interictal epileptiform discharge (IED) is considered physiologically valid or acceptable based on waveform morphology, isofield pattern, and dipole parameters (localization, dipole moment, confidence volume, goodness of fit). Thus, MEG source localization can aid clinicians in sublobar localization, lateralization, and grid placement, by evoking the irritative/seizure onset zone. It also accurately localizes the eloquent cortex-like visual, language areas. MEG also aids in diagnosing and delineating multiple novel findings in other neuropsychiatric disorders, including Alzheimer's disease, Parkinsonism, Traumatic brain injury, autistic disorders, and so oon.

Spatiotemporal Human Brain Activities on Recalling Names of Body Parts

The authors measure electroencephalograms (EEGs) from a subject looking at line drawings of body parts and recalling their names silently. The equivalent current dipole source localization (ECDL) method is applied to the event related potentials (ERPs): summed EEGs. As the dominant language area of the subject is considered to be in the right hemisphere in the previous research study, ECDs are localized to the right middle temporal gyrus: the angular gyrus. Then ECDs are localized to the right fusiform gyrus, the right middle temporal pole (TEP), and the right inferior temporal white matter (TWM). ECDs are located in the ventral pathway. The areas are related to the integrated process of visual recognition of pictures and the recalling of words. Some of these areas are also related to image recognition and word generation.

Spatiotemporal Analysis of Brain Activity During Understanding Honorific Expressions

This paper discusses brain activity during the understanding of sentences from the perspective of Systemic Functional Linguistics. We focus on ideational meaning (propositional meaning in an ordinary sense) and interpersonal meaning (as is typically seen in honorific expressions). The present study is an experimental exploration of the spatiotemporal pathways of neuronal activation. Japanese sentences containing and not containing honorific expressions are compared in electroencephalography experiments. In these experiments, the sentences without honorific expressions have ideationalmeaning, but those with honorific expressions have both ideational and interpersonal meanings. Through the use of the equivalent current dipole source localization method, the spatiotemporal processes of activation of the brain are analyzed. There is a single pathway during the understanding of the sentences without honorific expressions; this pathway is mainly observed in the left hemisphere. On the other hand, there are three pathways in the case of the sentences with honorific expressions, two of which are observed in the right hemisphere. The remaining pathway is the same as the aforementioned single pathway. This fact strongly suggests that the common pathway is concerned with processing ideational meaning. The other two pathways observed during understanding of the sentences with honorific expressions are considered to be related to processing interpersonal meaning.

Association between equivalent current dipole source localization and focal cortical dysplasia in epilepsy patients

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.

Spatiotemporal Brain Activity During Hiragana Word Recognition Task

The 19-channel Event-Related Potentials (ERPs) we recorded during recognition of hiragana (one type of Japanese phonetic characters) were simultaneously and independently presented as a word and a nonword to opposite eyes using a field-sequential stereoscopic 3D display with a liquid-crystal shutter, a word and a non-word were simultaneously and independently presented to the left (right) and the right (left) eyes, respectively. Each word consists of 3 hiragana characters. Three subjects were instructed to press a button when they understood the meaning of the visual stimuli after 3,000 ms poststimulus. Equivalent Current Dipole source Localization (ECDL) with 3 unconstrained ECDs was applied to the ERPs. In the case of right-handed subjects, the ECDs were localized to the Wernicke’s area at around 600 ms. In the case of left-handed subject, the ECD was localized to the Wernicke’s homologue. After that ECDs were then localized to the prefrontal area, the superior frontal gyrus, and the middle frontal gyrus. At around 800 ms, the ECDs were localized to the Broca’s area, then after that ECDs were relocalized to the the Wernicke’s area and to the Broca’s area.

Spatio-temporal Analysis of Brain Activities on Aperture Problem

The aperture problem is a motion perception through a small window involved ambiguity both in a speed and a direction of the motion. It has been analyzed in various experimental approaches with knowledge about the early motion process in visual pathways of the human brain. Some of the present authors had investigated correct answer rate of the moving orientation and analyzed brain activities with different experimental parameters, such as line speed, radius of the apertures, and length of a line. The present authors recorded electroencephalograms (EEGs) from a subject and estimated their sources and latencies in the brain using the equivalent current dipole source localization (ECDL) method. We compared localized ECDs for two different line speeds (Type 1: 10msec/pixel and Type 2: 20msec/pixel). At the latency of the appearance of aperture, ECDs were localized along the ventral pathway concern with the recognition of form. After appearance of the line, ECDs were localized along the dorsal pathway concern with the recognition of movement. In addition, after appearance of another aperture, ECDs were localized to the middle frontal gyrus and the inferior frontal gyrus.

Micro Robot Control by Use of Electroencephalograms from Right Frontal Area

In order to develop a brain machine interface, the authors have investigated the brain activity during human recognition of characters and symbols representing directional meaning. They have recorded electroencephalograms (EEGs) from subjects viewing four types of Kanji (Chinese characters being used currently in the Japanese language) and arrows that were presented on a CRT. Each of the four characters or symbols denoted direction for upward, downward, leftward and rightward, respectively. Subjects were asked to read the characters or symbols, silently. EEGs were averaged for each stimulus type and direction, and event related potentials (ERPs) were obtained. The equivalent current dipole source localization (ECDL) method has been applied to these ERPs. In both cases, equivalent current dipoles (ECDs) were localized to areas related to the working memory for spatial perception, such as the right upper or the right middle frontal areas. Taking into account these facts, the authors have investigated a single trial EEGs of the subject precisely after the latency at 400 ms, and it was determined effective sampling latencies for the discriminant analysis to four types of arrow: ↑, ↓, ←, and →. EEG data have been sampled at latency from 400 ms to 900 ms at 25 ms interval by the three channels in the right upper and the right middle frontal gyri. Results of the discriminant analysis for four type objective variates, presented discriminant rates were above 80%. By four type code of infrared rays according to the discrimination results from a PC, the authors have controlled a micro robot, the e-puck, with four orders: forward, rotate clockwise, rotate counterclockwise and stop.

Multiple-dipole analysis of visual event-related potentials during oddball paradigms

Visual event-related potentials (ERPs) during oddball paradigms with button pressing, silent counting and movement imagery tasks were recorded in right-handed subjects from 32 scalp electrodes. By incorporating the knowledge deduced from the task relevance of the P300 into equivalent current dipole source localization (ECDL), stable multiple ECD solutions with goodness-of-fit (GOF) of more than 90% and confidence limits (CLs) of less than 1 mm were estimated for all the three target P300s. The obtained frontal dipoles were discussed in terms of visual working memory, sustained attention and higher motor control in reference to the previous PET, fMRI and MEG studies. The distributed multiple ECDs may suggest that P300 should be interpreted as being the embodiment of the cortico-limbic-thalamic network accomplishing various cognitive functions.

Difference in Areas of the Brain for Fuzzy and Crisp Calculation

We recorded event-related potentials (ERPs) by electroencephalography (EEG) during fuzzy and crisp calculation. Questions art divided into 2 types. As type A, questions were presented as sentences. Questions of type B were presented as numerical calculation. In type A, the peak latency of the EEG was around 1100ms. In type B, the peak latency was around 650ms. In type A, from multiple equivalent current dipole source localization (ECDL) around the latency, it followed that sources during fuzzy calculation lie in the right hemisphere and that sources during crisp calculation lie in the left hemisphere. In type B, no significant difference was observed.

Multiple equivalent current dipole source localization of visual event-related potentials during oddball paradigm with motor response.

Event-related potentials (ERPs) during a visual oddball paradigm with button-pressing responses were recorded in 12 right-handed subjects from 32 scalp electrodes. The single equivalent current dipole (ECD) of the target C1 (weak occipito-parietal negativity from 30-80 ms) was consistently located at the primary visual cortex. From the 4-ECD localization of the target P1/N1 (temporally coincident frontal positivity and occipito-temporal negativity), it was suggested that this complex reflected activities from distributed sources along both dorsal occipito-parietal and ventral occipito-temporal areas. The stable multiple ECD solutions for the target P3b were chosen as those including the left primary motor and/or sensorimotor dipole and satisfying goodness-of-fit (GOF) of more than 98% and confidence limit (CL) of less than 1 mm. The obtained frontal dipoles were discussed in terms of visual working memory and sustained attention in reference to the previous PET, fMRI and MEG studies. The distributed multiple ECDs may suggest that P3 should be interpreted as being the embodiment of the cortico-limbic-thalamic network which involves Halgren and Marinkovic's emotional and behavioral model and Mesulam's attentional circuit.

Integrated approach of an artificial neural network and numerical analysis to multiple equivalent current dipole source localization.

The authors have developed a PC-based multichannel electroencephalogram (EEG) measurement and analysis system. This system enables us (1) to simultaneously record a maximum of 64 channels of EEG data, (2) to measure three-dimensional positions of the recording electrodes, (3) to rapidly and precisely localize equivalent current dipoles (ECDs) responsible for the EEG data, and (4) to superimpose the localization results on magnetic resonance images. A new neural network and numerical analysis (NNN) approach to ECD localization is described which integrates a feedforward artificial neural network (ANN) and a numerical optimization (Powell's hybrid) method. It was shown that the ANN method has the advantages of high-speed localization and noise robustness, because in this approach: (1) ECD parameters are immediately initialized from the recorded EEG data by the ANN and (2) ECD parameters are accurately refined by the hybrid method. Our multiple ECD localization method was applied to sensory evoked potentials and event-related potentials using the present system.

Magnetoencephalographic analysis of periodic lateralized epileptiform discharges (PLEDs)

Objective: To clarify the spatial and temporal distribution of the electroencephalographic (EEG) pattern termed ‘periodic lateralized epileptiform discharges’ (PLEDs), we performed magnetoencephalography (MEG) to analyze PLEDs in a patient with a right parietal metastasis associated with meningeal carcinomatosis.Methods: A 37-channel biomagnetometer was used to simultaneously record the EEG and MEG. Equivalent current dipole (ECD) source localization was calculated based on a single-dipole model and mapped onto a magnetic resonance image. Single-photon emission computed tomography with technetium-99-hexamethyl-propyleneamine oxime (HMPAO-SPECT study) was also performed during both presence and absence of PLEDs according to an EEG monitor.Results: By EEG the PLEDs, predominantly right-sided, consisted of a typical negative triphasic spike followed by a slow negative wave. By MEG the PLEDs had a sequence with 3 distinct components. ECDs in the 3 components were localized to the cortex around the lesion, although exact localization and dipole direction varied between components. HMPAO-SPECT demonstrated hypoperfusion of the lesion and adjacent cortex during both quiescence and appearance of PLEDs.Conclusion: Our results indicate that PLEDs originated from the hypoperfused cortex surrounding the lesion.

Accuracy of Multiple Equivalent Current Dipole Source Localization from EEG in Terms of Confidence Limits for Radial Distances

Accuracy of Multiple Equivalent Current Dipole Source Localization from EEG in Terms of Confidence Limits for Radial Distances