Dipole Source Localization Research Articles

Dipole source localization of ictal epileptiform activity in estimation of epileptogenic zone

Localization analysis of epileptogenic zone was performed by dipole tracing method with scalp–skull–brain head model (SSB-DT) in patients with intractable epilepsy. Eight temporal lobe epilepsy (TLE) and one frontal lobe epilepsy were studied. EEG was recorded by digital electroencephalograph with 21 scalp electrodes. Generator source of the peak of ictal epileptiform activity (IEA) was estimated by SSB-DT. All the patients underwent electrocorticography (ECoG) and resection of epileptogenic zone. Surgical outcome was good in every patient. Locations of generator source of the peak of IEA estimated by SSB-DT were consistent with locations of epileptogenic zone determined by ECoG and neuroimagings, such as magnetic resonance imaging (MRI), positron emission tomography (PET) and single photon emission computed tomography (SPECT). In TLE, medial and lateral temporal epileptogenic zone could be discriminated, which could not be determined by visual interpretation of scalp EEG. In medial TLE, as an advance of seizure, generator source of IEA either stayed in the medial temporal lobe, or propagated to the lateral temporal lobe. Locations of epileptogenic zone could be estimated by SSB-DT not invasively. Electrophysiological event in the brain during an advance of seizure attack could also be observed. Application of SSB-DT to IEA was useful in the medical treatment of epilepsy, especially in surgical planning for patients with medically intractable epilepsy.

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.

Different patterns of dipole source localization in gelastic seizure with or without a sense of mirth

Dipole source localization corresponding to interictal spikes were estimated using EEG dipole tracing with a realistic three-shell head model in three patients with cryptogenic gelastic epilepsy. The dipole sources in two patients, whose gelastic seizures were accompanied by a subjective feeling of mirth, were estimated in the right or left medio-basal temporal regions. In the other patient, with gelastic seizures without a sense of mirth, the dipole sources were localized in the right frontal region corresponding to the anterior cingulate. The results suggest that the neural activities in hippocampal regions are involved with the generation of gelastic seizures with a sense of mirth and those in the cingulate might be associated with the motor act of laughter.

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.

Generators of visual evoked potentials for faces and eyes in the human brain as determined by dipole localization.

Human visual evoked potentials were recorded during presentation of photos of human and animal faces and various face features. Negative waves with approximate peak latencies of 165 msec (N170) were bilaterally recorded from the occipito-temporal regions. Mean peak latencies of the N170 were shorter for faces than eyes only. Analyses of amplitudes of evoked potentials indicated that the N170 elicited by faces reflected activity of a specific neural system which was insensitive to detailed differences among individual faces regardless of species, and consequently suggest that this system might function to detect existence of faces in general. On the other hand, the mean amplitude of the N170 elicited by human eyes was significantly larger than those by animal eyes. These differences in response latencies and amplitudes of the N170 suggest existence of at least 2 different visual evoked potentials with similar latencies (i.e., N170) which are sensitive to faces in general and human eyes, respectively. Dipole source localization analysis indicated that dipoles for the N170 elicited by eyes were located in the posterior inferior temporal gyrus, and those for faces, located initially in the same region, but moved toward the fusiform and lingual gyri at the late phase of the N170. The results indicated that information processing of faces and eyes separated at least as early as the latency of the N170 at the posterior inferior temporal gyrus as well as the fusiform and lingual gyri, and might provide neurophysiological and anatomical bases to an initial structural encoding stage of human faces.

Improving lesion conductivity estimate by means of EEG source localization sensitivity to model parameter.

EEG-based source localization techniques use scalp-potential data to estimate the location of underlying neural activity. EEG source location reconstruction requires the assumption of a source model and the assumption of a conductive head model. Brain lesions can present conductivity values that are dramatically different from those of surrounding normal tissues and have to be included in head models for accurate neural source reconstruction. It is therefore necessary to analyze subjects' anatomic images (using MRI or computed tomography) to identify lesion type and to assign the appropriate conductivity value. Source localization accuracy may be influenced by uncertainties in tissue conductivity assignment during head model construction. The authors present a sensitivity study quantifying the effect of uncertainty in brain lesion conductivity assignment on EEG dipole source localization. They adopted an eccentric-spheres head model in which an eccentric bubble approximated the effects of actual brain lesions. After simulating EEG signal measurement in 64 different pathologic situations, an inverse dipole fitting procedure was carried out, assuming an incorrect lesion conductivity assignment ranging from a half to twice the real value. Incorrect lesion conductivity assignment led to markedly wrong source reconstruction for highly conductive lesions like liquid-filled ones (localization errors as much as 1.7 cm). Conversely, low sensitivity to uncertainties in conductivity assignment was found for lesions with low conductivity like calcified tumors. The authors propose a method based on residual error analysis to improve the lesion conductivity estimate. This procedure can identify lesion tissue conductivity with only a few percent error and guarantees source localization errors less than 5 mm.

Presurgical evaluation of epilepsy.

An overview of the following six cortical zones that have been defined in the presurgical evaluation of candidates for epilepsy surgery is given: the symptomatogenic zone; the irritative zone; the seizure onset zone; the epileptogenic lesion; the epileptogenic zone; and the eloquent cortex. The stepwise historical evolution of these different zones is described. The current diagnostic techniques used in the definition of these cortical zones, such as video-EEG monitoring, MRI and ictal single photon emission computed tomography, are discussed. Established diagnostic tests are set apart from procedures that should still be regarded as experimental, such as magnetoencephalography, dipole source localization and spike-triggered functional MRI. Possible future developments that might lead to a more direct definition of the epileptogenic zone are presented.

The influence of electrode location errors on EEG dipole source localization with a realistic head model

Objectives: Inaccurate information about the electrode locations on the scalp will introduce errors in electroencephalogram dipole source localization results. The present study uses computer simulations to evaluate such errors in a realistic head model and in the context of noise. Methods: A realistic head model was constructed from magnetic resonance imaging scans and 29 electrodes placed on the head according to the 10–20 International System. Twenty sets of electrode displacements, with a mean value of 5 mm, were generated and 200 single dipoles evenly located in the brain were used as test sources. The boundary element method was employed for the froward calculation and dipole fitting was carried out at different noise levels. Results: For a noise-free signal, the source localization error due to electrode misplacement is about 5 mm, whereas it is about 2 mm for normal noisy signals. Conclusions: For realistic head models, dipole estimation error due to electrode misplacement is negligible compared with errors caused by noise.

No evidence for dependence of early cortical auditory processing on dopamine D 2-receptor modulation: a whole-head magnetoencephalographic study

Magnetoencephalography (MEG) was used to determine the effect of neuroleptic challenge on brain responses in healthy subjects. In a double-blind, randomized, placebo-controlled, cross-over design study, the dopamine D 2 receptor antagonist haloperidol (2 mg) was given orally to 12 healthy volunteers. The middle-latency auditory evoked magnetic fields (MAEF) were recorded 3 h after administration of haloperidol or placebo with a whole-head 122-channel MEG. Haloperidol did not significantly affect MAEF responses. The dipole moments and source locations of the responses were not significantly influenced by haloperidol. These results suggest that dopamine D 2 receptors are not involved in the early phases of auditory cortical processing.

Sequence-sensitive subcomponents of P300: Topographical analyses and dipole source localization

Sequence-sensitive subcomponents of P300: Topographical analyses and dipole source localization

Sequence‐sensitive subcomponents of P300: Topographical analyses and dipole source localization

P300 amplitude and reaction time (RT) are strongly affected by the sequence of events preceding the eliciting stimulus. Sommer, Leuthold and Soetens (1999) found that robust sequential effects in P300 amplitude could be dissociated from more variable sequential effects in RTs. However, global changes in P300 amplitude and topography gave rise to the suggestion that sequential effects are specific for a subcomponent of P300 that is separate from and anterior to the classical parietal P300. Here, confirming evidence for dissociable subcomponents of P300 is reported from two experiments. Independent component analysis separated a centrally distributed sequence-sensitive subcomponent from a more parietal subcomponent. Subsequent dipole source analysis indicated a deep mesial source for the sequence-sensitive subcomponent. Overlap with reafferent somatosensory activity appears to be responsible for an apparent lateralization of this component towards the hemisphere ipsilateral to the responding hand.

Simultaneous 32-channel-EEG based ERP and fMRI: Spatial correlation of dipole source localization and fMRI activations

Simultaneous 32-channel-EEG based ERP and fMRI: Spatial correlation of dipole source localization and fMRI activations

Intra-subject reproducibility of laser evoked potential dipole source locations, estimated using a realistic head model

Intra-subject reproducibility of laser evoked potential dipole source locations, estimated using a realistic head model

Multiple frequency steady-state evoked magnetic field mapping of digit representation in primary somatosensory cortex

Magnetic source imaging of multiple frequency steady-state somatosensory evoked responses was examined using a 151-channel magnetoencephalography (MEG) system and a dual-channel electrical stimulator. Somatotopy of digit representation was studied in healthy subjects and effects of injury-related cortical plasticity in patients with unilateral transections of the median or the ulnar nerve. Dipole source locations exhibited somatotopic order with overlap between neighboring digits. In two of three nerve injury patients evidence for cortical reorganization was found. The location of sources related to digits neighboring deafferented digits was changed and their dipole moments were enlarged by comparsion with the sources related to contralateral homologue control digits. As a basis for magnetic source imaging, the recording of multiple frequency somatosensory steady-state evoked responses may be a viable and time saving alternative to the recording of transient evoked responses.

Mathematical and computer modelling of the human brain with reference to cortical magnification and dipole source localisation in the visual cortex

Mathematical and computer modelling of the human brain with reference to cortical magnification and dipole source localisation in the visual cortex

The development of auditory evoked dipole source activity from childhood to adulthood

Objective: Multi-channel recordings show that observed developmental changes of late auditory evoked potentials (LAEP) depend on the location of the scalp electrode. These findings suggest that different LAEP generators have a distinct developmental course. The goal of this study was to investigate the maturational process of cortical LAEP generators. Methods: Eighty-seven healthy children and adolescents with normal hearing, ages 5–16 years, and 21 adults, ages 20–30 years, participated in the study. Pure tone LAEP were recorded from 21 derivations. Dipole source analysis was performed by means of brain electric source analysis (BESA). Peak latencies and amplitudes of dipole source activity were estimated. Results: While the number, location, and direction of dipole sources were similar in children and adults, the course of their activity differed greatly. The latencies shortened and the amplitudes decreased during development. In adolescence a new component appeared in the activity of the tangential dipole, which reflects the generators in the supra-temporal plane. The variability of parameters was greater in children than in adults. Conclusions: Since the dipole source activity of LAEP in childhood differs considerably from that in adulthood, dipole source analysis could be a useful tool for studying both normal and disturbed maturation of the auditory perceptual function.

Influence of measurement noise and electrode mislocalisation on EEG dipole-source localisation.

Measurement noise in the electro-encephalogram (EEG) and inaccurate information about the locations of the EEG electrodes on the head induce localisation errors in the results of EEG dipole source analysis. These errors are studied by performing dipole source localisation for simulated electrode potentials in a spherical head model, for a range of different dipole locations and for two different numbers (27 and 148) of electrodes. Dipole source localisation is performed by iteratively minimising the residual energy (RE), using the simplex algorithm. The ratio of the dipole localisation error (cm) to the noise level (%) of Gaussian measurement noise amounts to 0.15 cm/% and 0.047 cm/% for the 27 and 148 electrode configurations, respectively, for a radial dipole with 40% eccentricity The localisation error due to noise can be reduced by taking into account multiple time instants of the measured potentials. In the case of random displacements of the EEG electrodes, the ratio of dipole localisation errors to electrode location errors amounts to 0.78 cm-1 cm and 0.27 cm-1 cm for the 27 and 148 electrode configurations, respectively. It is concluded that it is important to reduce the measurement noise, and particularly the electrode mislocalisation, as the influence of the latter is not reduced by taking into account multiple time instants.

An inverse EEG problem solving environment and its applications to EEG source localization

BioPSE is a scientific programming environment that allows the interactive construction, debugging, and steering of large-scale scientific computations. BioPSE can be envisioned as a “computational workbench,” in which a scientist can design and modify simulations interactively via a dataflow programming model. As opposed to the typical “off-line” simulation mode (in which the scientist manually sets input parameters, computes results, visualizes the results via a separate visualization package, and then starts again at the beginning), BIoPSE “closes the loop” and allows interactive steering of the design, computation, and visualization phases of a simulation. A snapshot of our source localization algorithm running within the BioPSE environment is shown in Fig. . We demonstrate the application of BioPSE to the construction of an EEG cost field for FEM a realistic head model, and single dipole source localization using the downhill simplex method.

EEG dipole source localization using artificial neural networks

Localization of focal electrical activity in the brain using dipole source analysis of the electroencephalogram (EEG), is usually performed by iteratively determining the location and orientation of the dipole source, until optimal correspondence is reached between the dipole source and the measured potential distribution on the head. In this paper, we investigate the use of feed-forward layered artificial neural networks (ANNs) to replace the iterative localization procedure, in order to decrease the calculation time. The localization accuracy of the ANN approach is studied within spherical and realistic head models. Additionally, we investigate the robustness of both the iterative and the ANN approach by observing the influence on the localization error of both noise in the scalp potentials and scalp electrode mislocalizations. Finally, after choosing the ANN structure and size that provides a good trade-off between low localization errors and short computation times, we compare the calculation times involved with both the iterative and ANN methods. An average localization error of about 3.5 mm is obtained for both spherical and realistic head models. Moreover, the ANN localization approach appears to be robust to noise and electrode mislocations. In comparison with the iterative localization, the ANN provides a major speed-up of dipole source localization. We conclude that an artificial neural network is a very suitable alternative for iterative dipole source localization in applications where large numbers of dipole localizations have to be performed, provided that an increase of the localization errors by a few millimetres is acceptable.

Dipole source localization of interictal epileptiform activity in temporal lobe epilepsy with medial temporal lesion

Dipole sources of interictal epileptiform activities recorded by conventional electroencephalogram (EEG) were estimated using the dipole tracing method. Four cases of temporal lobe epilepsy with medial temporal lesions were studied. Two patients with hippocampal sclerosis, one patient with granulation in the hippocampus and one patient with cavernous angioma were involved in the study. Interictal epileptiform activities were classified into two patterns according to the topography of spikes. They were widespread spikes over the parasagittal electrodes (parasagittal spikes) and restricted spikes at the temporal electrodes (temporal spikes). Dipole sources of parasagittal spikes were localized in the medio-basal temporal lobe with vertically orientated vector moment. Dipole sources of temporal spikes were localized in the medio-basal temporal lobe with horizontally orientated vector moment. Locations of dipoles and directions of vector moments were consistent with topography and polarity of spikes. The difference in the two patterns of interictal epileptiform activities was derived from the difference in the direction of the vector moment of dipole sources. There was no difference in the location of dipole sources. Both the dipole sources and the lesions were localized in the same medio-basal temporal lobe. Dipole tracing was very useful in localizing the dipole sources of interictal epileptiform activities and in understanding the neurophysiological background.