Brain Electrical Source Analysis Research Articles

Electric source localization adds evidence for task-specific CNVs.

This study was an attempt to replicate recent magnetoencephalographic (MEG) findings on human task-specific CNV sources (Basile et al., Electroencephalography and Clinical Neurophysiology 90, 1994, 157-165) by means of a spatio-temporal electric source localization method (Scherg and von Cramon, Electroencephalography and Clinical Neurophysiology 62, 1985, 32-44; Scherg and von Cramon, Electroencephalography and Clinical Neurophysiology 65, 1986, 344-360; Scherg and Berg, Brain Electric Source Analysis Handbook, Version 2). The previous MEG results showed CNV sources in the prefrontal cortex of the two hemispheres for two tasks used, namely visual pattern recognition and visual spatial recognition tasks. In the right hemisphere, the sources were more anterior and inferior for the spatial recognition task than for the pattern recognition task. In the present study we obtained CNVs in five subjects during two tasks identical to the MEG study. The elicited electric potentials were modeled with four spatio-temporal dipoles for each task, three of which accounted for the visual evoked response and one that accounted for the CNV. For all subjects the dipole explaining the CNV was always localized in the frontal region of the head, however, the dipole obtained during the visual spatial recognition task was more anterior than the one obtained during the pattern recognition task. Thus, task-specific CNV sources were again observed, although the stable model consisted of only one dipole located close to the midline instead of one dipole in each hemisphere. This was a major difference in the CNV sources between the previous MEG and the present electric source analysis results. We discuss the possible basis for the difference between the two methods used to study slow brain activity that is believed to originate from extended cortical patches.

Dipolar generators of the early scalp somatosensory evoked potentials to tibial nerve stimulation in human subjects

We performed the brain electrical source analysis (BESA) of the early scalp somatosensory evoked potentials (SEPs) to tibial nerve stimulation. A four-dipole model could well explain the scalp SEP distribution. One dipole showing a subcortical location was activated at the latency of the lemniscal P30. The three remaining dipoles were located near the sagittal fissure in the hemisphere contralateral to the stimulation. Two of these dipoles showed a biphasic activity and contributed to the signal evoked in the N37-P40 latency range. Also in the N50-P50 latency range two different source activities were involved in SEP building. This finding suggests that one of two possible N50 subcomponents represents the negative counterpart of the N50/P50 dipolar field and the other is originated by a radial source.

A reinterpretation of the effect of stimulus rate on early cortical median nerve SEPs by brain electrical source analysis (BESA)

A reinterpretation of the effect of stimulus rate on early cortical median nerve SEPs by brain electrical source analysis (BESA)

Dipole source analysis of P300 component of the auditory evoked potential: a methodological advance?

A dipole model of the P300 component of the auditory evoked potential was developed with the aim of separating the overlapping P300 subcomponents and assessing the reliability of P300 dipole parameters compared with other methods such as principal component analysis (PCA). P300 was recorded within an auditory oddball paradigm in 57 healthy subjects. Dipole source analysis was performed with Brain Electrical Source Analysis (BESA). With a temporo-basal and a temporo-superior dipole per hemisphere, most of the variance of scalp-recorded P300 activity can be explained. When dipole source analysis is used, the test-retest reliability of P300-amplitude measurements is enhanced as compared with PCA or single channel analysis. The P300 activity of the temporo-basal dipoles and that of the temporo-superior dipoles differ in several respects (e.g. latency, correlations with age) and thus appear to reflect functionally different neuronal processes. In spite of problems in the interpretation of dipole locations, it is concluded that dipole source analysis enhances both the reliability and physiological validity of P300 parameters.

The pathophysiology of giant SEPs in cortical myoclonus: a scalp topography and dipolar source modelling study

Somatosensory evoked potential (SEP) recordings in patients suffering from cortical myoclonus (CM) are characterised by evidence of abnormally enhanced scalp components. Our aim was to verify whether enhanced activity in giant SEPs arises from the same generators as in healthy subjects. We used the brain electrical source analysis (BESA) to compare scalp SEP generators of healthy subjects to those calculated in 3 patients with CM of varying causes. Firstly, we built a 4-dipole model explaining scalp distribution of early SEPs in normal subjects and then applied it to traces recorded from CM patients. Our model, issued from the right median nerve grand average and applied also to recordings from single individuals, included a dipole at the base of the skull and three other perirolandic dipoles. The first of the latter dipoles was tangentially oriented and was active at the same latencies as the N20/P20 potentials and, with opposite polarity, the P24/N24 responses; the second dipole explained the central P22 distribution and the third had a peak of activity corresponding to the N30 component. When we applied our 4-dipole model to CM recordings, the first perirolandic dipole had a third peak of activity in all patients at the same latency as a parietal negativity and a frontal positivity, both following giant P24/N24 components; on the other hand, in one patient the second perirolandic dipole showed a later activation corresponding to a high central negativity, following a giant P22 response. We suggest that only the initial giant SEPs correspond to physiological potentials evoked in healthy subjects The occurrence of late giant SEPs could be explained by hyperpolarization, following the postsynaptic excitatory potentials responsible for the early giant components.

Activity in posterior parietal cortex following somatosensory stimulation in man: magnetoencephalographic study using spatio-temporal source analysis.

We investigated the activation of posterior parietal cortex (PPC) to somatosensory stimulation in humans to determine its fundamental role as a somatosensory associated area using magnetoencephalography (MEG). We studied somatosensory evoked magnetic fields (SEF) after stimulation of median nerve, posterior tibial nerve and lip, and analyzed them by the single dipole model and also by the multidipole model using brain electric source analysis (BESA) system. In single source model analysis, the dipole at the peak latency of short-latency components following each site stimulation were located in the corresponding receptive fields in the primary somatosensory cortex (SI) contralateral to the stimulation. The dipole at the peak latency of the middle latency components were located in bilateral upper bank of Sylvian fissure (SII), By contrast, in the five-dipole model of BESA, the equivalent current dipoles (ECDs) of the middle-latency SEF after stimulation of median nerve and posterior tibial nerve were identified in the contralateral SI and in the bilateral SII and PPC, while all activities of middle-latency SEF after lip stimulation appeared to be restricted in the contralateral SI and bilateral SII. Around 80 msec in latency, the ECD location in PPC after median nerve stimulation was, on the average, 2.4 cm posterior, 2.9 cm medial and 2.6 cm superior to the hand area in SI. The ECD in PPC after posterior tibial nerve stimulation was also located posterior to the foot area in SI, but it was close to the SI area of foot, their distance being approximately 1.3 cm. ECD in PPC was almost equally demonstrated in each hemisphere. These findings suggested that the somatosensory associated cortex in PPC represented somatotopic organization in parallel with 'homunculus' in SI, but the hand area was much wider than the foot area. It was not clear whether the lip area in PPC was absent or was too close to be separated from the SI.

Processing of faces in the human brain: A multi-task MEG study

A dipole model of the P300 component of the auditory evoked potential was developed with the aim of separating the overlapping P300 subcomponents and assessing the reliability of P300 dipole parameters compared with other methods such as principal component analysis (PCA). P300 was recorded within an auditory oddball paradigm in 57 healthy subjects. Dipole source analysis was performed with Brain Electrical Source Analysis (BESA). With a temporo-basal and a temporo-superior dipole per hemisphere, most of the variance of scalp-recorded P300 activity can be explained. When dipole source analysis is used, the test-retest reliability of P300-amplitude measurements is enhanced as compared with PCA or single channel analysis. The P300 activity of the temporo-basal dipoles and that of the temporo-superior dipoles differ in several respects (e.g. latency, correlations with age) and thus appear to reflect functionally different neuronal processes. In spite of problems in the interpretation of dipole locations, it is concluded that dipole source analysis enhances both the reliability and physiological validity of P300 parameters.

Scalp topography and dipolar source modelling of potentials evoked by CO 2 laser stimulation of the hand

CO 2 laser evoked potentials to hand stimulation recorded using a scalp 19-channel montage in 11 normal subjects consistently showed early N1/P1 dipolar field distribution peaking at a mean latency of 159 ms. The N1 negativity was distributed in the temporoparietal region contralateral to stimulation and the P1 positivity in the frontal region. The N1/P1 response was followed by 3 distinct components: (1) N2a reaching its maximal amplitude at the vertex and ipsilaterally to the stimulated hand, (2) N2b mostly distributed in the frontal region, and (3) P2 with a mid-central topography. Brain electrical source analysis showed that this sequence was explained, with a residual variance below 5%, by a model including two dipoles in the upper bank of the Sylvian fissure of each hemisphere, a frontal dipole close to the midline, and two anterior medial temporal dipoles, thus suggesting a sequential activation of the two second somatosensory areas, anterior cingulate gyrus and the amygdalar nuclei or the hippocampal formations, respectively. This model fitted well with the scalp field topography of grand average responses to stimulation of left and right hand obtained across all subjects as well as when applied to individual data. Our findings suggest that the second somatosensory area contralateral to the stimulation is the first involved in the building of pain-related responses, followed by ipsilateral second somatosensory area and limbic areas receiving noxious inputs from the periphery.

The role of neuroimaging in the discovery of processing stages a review

In this contribution we show how neuroimaging methods can augment behavioural methods to discover processing stages. Event Related Brain Potentials (ERPs), Brain Electrical Source Analysis (BESA) and regional changes in cerebral blood flow (rCBF) do not necessarily require behavioural responses. With the aid of rCBF we are able to discover several cortical and subcortical brain systems (processors) active in selective attention and memory search tasks. BESA describes cortical activity with high temporal resolution in terms of a limited number of neural generators within these brain systems. The combination of behavioural methods and neuroimaging provides a picture of the functional architecture of the brain. The review is organized around three processors: the Visual, Cognitive and Manual Motor Processors.

Electric source localization of the auditory P300 agrees with magnetic source localization

The event-related cortical potential elicited in the context of auditory target detection tasks includes the N1, P2 and P3 components. The aim of the present study was to identify the sources of these scalp-recorded components using an electrical multiple dipole model. Nine healthy adults volunteered for the study. An auditory oddball paradigm was used. Stimuli (18% target and 82% non-target tones) were delivered through ear-phones and subjects were required to silently count the targets. Event-related potentials (ERPs) to these stimuli were recorded by 30 electrodes placed on the scalp. The identification of the sources of the ERP was attempted using the brain electric source analysis (BESA) program. The instantaneous source locations of N1, P2 and P3 reported in magnetoencephalographic (MEG) literature were used as initial starting locations for the spatio-temporal multiple dipole modeling of the EEG data. First the auditory long latency responses were modeled separately. Bilateral superior temporal plane sources with almost vertical orientations explained the first 250 msec window of the non-target tone recording including Nl P2 complex. This agrees with MEG source localization of N1 m P2 m . Two slightly deeper dipoles in superior temporal gyri and bilateral dipoles in hippocampi or parahippocampal areas explained P3 (analysis window 250–600 msec). The final model explained the complete epoch of 600 msec with 6 dipoles and the residual variances of individual models ranged from 3.83% to 7.77%. The concordance between MEG and BESA source localization results supports the notion of generators in temporal lobes for the Nl P2 complex and generators in temporal and hippocampal areas for the P3 component.

Brain electrical source analysis of laser evoked potentials in response to painful trigeminal nerve stimulation

Cerebral generators of long latency brain potentials in response to painful heat stimuli were identified from potential distributions in 31 EEG leads, using the brain electrical source analysis (BESA) programme in the multiple spatio-temporal dipole mode. Data were taken from a study with 10 young healthy male subjects who participated in 3 identical sessions, 1 week apart, with 4 blocks of 40 stimuli (randomized intensities above mean pain threshold). Brief infrared laser heat pulses were applied to the right temple; laser evoked brain potentials (LEPs) were averaged over 40 stimuli per block. BESA was applied to the grand mean maps averaged over the 10 subjects, 3 sessions and 4 stimulus blocks per session, as well as to the individual maps. In all cases 4 generators could consistently be identified by BESA, which were able to explain up to 98.8% of the total variance in scalp distributions at certain time intervals: dipole I with a maximum activity at 106.3 msec in the contralateral somatosensory trigeminal cortex, 19.0 mm beneath the surface; dipole II with a maximum activity at 112.1 msec at the corresponding ipsilateral area at a depth of 13.6 mm; dipole III with a maximum activity at 130.4 msec in the frontal cortex; dipole IV with 2 relative maximum activities at 150.6 and 220.5 msec, localized centrally under the vertex at a depth of 33.1 mm, which described both the late vertex negativity and the consecutive positivity. BESA applied to the individual LEP maps of each individual and session yielded again 4 major generators with sites, strengths and orientations comparable to those of the grand mean evaluations. The standard deviation (S.D.) of site coordinates within subjects was less than 3 mm for dipoles I, II and IV (5 mm for dipole III). The between-subject standard deviation was considerably larger (15 mm), which was attributed to individual differences in head geometry, size and anatomy. Dipoles I and II are assumed to be generators in secondary somatosensory areas of the trigeminal nerve system with bilateral representation, though significantly stronger in the contralateral site. Dipole III in the frontal cortex may be related to attention and arousal processes, as well as to motor cortical initiation for eye movements and muscle effects. The central dipole IV describing all late activity between 150 and 220 msec is probably a representative of perceptual activation and cognitive information processing; it was located in deep midline brain structure, e.g., the cingular gyrus.

Electrical source localization of human movement-related cortical potentials

Movement-related cortical potentials (MRCP) of healthy humans were analyzed with a brain electrical source analysis program for modelling equivalent electrical dipoles in order to suggest the generators of the MRCP. Both right and left voluntary self-paced index finger movements were performed by 14 subjects and MRCP were recorded on the scalp with 29 electrodes. A spatiotemporal three-dipole model was developed and it explained 94.1% of the right-hand data and 96.4% of the left-hand data. The models for right- and left-hand data were almost similar except for the different hemispheres. A midline dipole was most active during the preparatory period before the onset of voluntary movement and two contralaterally locating dipoles were most active during and immediately after the motor act. The present model did not support recently suggested bilateral activation of primary motor cortices preceding the voluntary movement. The locations, orientations and the time course of activation of the dipoles suggest the involvement of the supplementary motor area in preparation of the movement and the contralateral primary motor cortex during the execution of the movement and the contralateral primary sensory cortex following the movement.

Brain source imaging of focal and multifocal epileptiform EEG activity

Brain electric source analysis (BESA) of the scalp EEG has been used to identify multiple equivalent current sources in the brain during during interictal spikes and seizure onset. To obviate the need for fitting dipole sources to every EEG segment, a new method has been developed on the basis of multiple fixed dipoles, each designed to emphasize functional imaging of particular cortical areas. "FOCUS" can quickly display EEG in various montages including new "sources montages" which provide a high sensitivity for source currents near each dipole while largely suppressing contributions from other brain areas. By comparing this "source EEG" to routine digital EEG in patients with complex partial epilepsy, we have observed that "FOCUS" can more readily determine whether an epileptiform discharge is consistent with a discrete or multifocal generator, characterize likely cerebral source(s), differentiate between spikes and seizures of mesio-basal versus lateral temporal or frontal origin, and estimate the presence and direction of propagation from source potential timing differences. Improved non-invasive EEG evaluations of partial epilepsy will undoubtedly result from this advance.

Somatosensory evoked potentials and magnetic fields: separation of multiple source activities

Median nerve somatosensory evoked potentials (SEP) and magnetic fields (SEF) were recorded in two subjects with multichannel devices. Single-moving- and multiple-stationary-dipole models were compared with the brain electric source analysis program of Scherg. Subcortical sources, reflecting the afferent neural volley when entering the brainstem and leaving the thalamus, were found only in the SEP. The analysis of SEF and SEP revealed a minimum of four overlapping source activities in the region of the contralateral post- and precentral cortical projection areas. Two sources in the depth of the central sulcus could not be resolved unambiguously. The third, more superficial source, which probably reflects activation of area 1, was better defined in the source analysis of the SEP, because dipole orientation was close to radial. The fourth source was more posterior. Several problems observed in the analysis of the present MEG and EEG data suggest that the simultaneous measurement and analysis of multichannel EEG and MEG data will substantially increase spatio-temporal resolution.

Equivalent electrical source analysis of pain-related somatosensory evoked potentials elicited by a CO2 laser.

The purpose of this study was to localize possible neural sources of pain-related cortical evoked potentials. A brain electrical source analysis was performed on late somatosensory evoked potential data (500-ms window was analyzed) elicited by short heat pulses produced by a CO2 laser. These stimuli activate pain and temperature pathways. The first, fairly small, negative response can be recorded on the scalp about 160 ms from the stimulus (N1 component). The major negativity on the scalp has its peak about 240 ms from the stimulus and is followed by a positivity (N2 and P2 components). A four-dipole model was developed using data following left-hand stimulation. Spatiotemporal source analysis suggested that the N1 component could be generated by contralateral activity in both primary and secondary somatosensory cortices and that N2 could be generated by bilateral activity mainly in secondary somatosensory cortices. A deep dipole in the midline was active during the time range of the negative potentials and seemed to be mostly responsible for the P2 component. This dipole was located too frontally to be thalamic, but it corresponded well to the location of the anterior cingulate gyrus. The model also yielded good fits for right-hand and left-foot stimulation data and, in addition, another set of left-hand data obtained with different electrode spacing in a different group of subjects (residual variances from 2.8% to 3.3%). The model explaining data sets from different body part stimulations varied very little, except with respect to the location of the dipole representing the activity of the primary somatosensory area.(ABSTRACT TRUNCATED AT 250 WORDS)

Source analysis of scalp-recorded movement-related electrical potentials

We used brain electric source analysis to study the sources generating the movement-related cortical potentials during the interval from 200 msec before to 200 msec after the movement onset. Dipole solutions were obtained for the peak of the negative slope (pNS′) and the frontal peak of the motor potential (fpMP) on scalp-recorded movement-related electrical potentials elicited by self-paced, repetitive unilateral finger movements in 10 normal volunteers. Two sources in homologous areas on each side of a spherical head model provided a satisfactory solution for the activity occurring at the instant of the pNS′ in all subjects. The fpMP was modeled by a contralateral source and a midline source in 6 subjects and by a single contralateral source in the remaining 4 subjects. The percentage of the residual variance, or goodness-of-fit, over the interval from −200 to 200 msec, using the solutions derived at pNS′ and fpMP, was low. The results support the hypothesis that the NS′ originates from the activity of bilateral generators in the sensorimotor cortex, and the motor potential arises from the combined activity of sources in the contralateral postcentral regions and the supplementary motor area.

Visual pattern comparison, working memory load, and event related potentials

This chapter reviews certain studies on somatosensory evoked field (SEF) dipolar source modeling to show how clinical and physiological SEP studies can benefit from this type of approach. Studies on dipolar source modeling of the early lower limb SEPs have offered a significant contribution in identifying the tibial nerve SEP generators. By using the brain electrical source analysis (BESA), two different dipoles contributing to the potential field in the N37–P40 latency range are shown. A tangential dipole oriented perpendicularly to the mesial hemispheric wall generated both the N37 potential and a subcomponent of the P40 potential located in the ipsilateral parietal region, whereas a radial dipole in the crown of the perirolandic cerebral cortex represented the source of the vertex subcomponent of the P40 SEP. In the presurgical assessment of patients with space occupying lesions around the sensorimotor cortex, the localization of the central fissure by brain magnetic resonance imaging (MRI) is made difficult by the displacement induced by the lesion. The chapter underlines the technical limitations that have to be considered in evaluating the results of dipolar analysis.

Use of prior knowledge in brain electromagnetic source analysis

Using multichannel measurements of EEG and/or MEG, macroscopic source activities can be estimated in the human brain using brain electric source analysis (BESA, Scherg 1990). If a discrete number of brain areas is active, functional brain images which depict the locations and orientations of equivalent dipole sources as well as the dynamics of the local macroscopic currents can be obtained from such data--in principle--without external knowledge. However, given a certain number of sources or 'neural masses' (Freeman 1975) which contribute to an event related response (ERP), it can be difficult to find the correct solution due to background noise in the data and distortions from the head model. Prior knowledge based on anatomy and physiology can be useful to constrain spatial or temporal parameters of the model and to define better cost functions for fitting locations and orientations. An analysis of the auditory evoked N100 complex and of the auditory mismatch negativity (MMN) is presented which illustrates the use of spatial constraints. Also, the use of a modified cost function is demonstrated which limits source currents in certain time intervals.

Clinical Neurophysiology and the Study of Human Functionality: Recent Developments and Future Applications

A variety of new technologies at different stages of development now appear on the horizon. It is unlikely that any of these methods will supplant the standard techniques now used; they are more likely to supplement current methods and address special situations. The author considers such techniques as magnetic resonance spectroscopy, muscle sounds and mechanics, optical methods for studying excitable tissue function, electromyographic kinesiology, dynamic posturography, topographic mapping of brain electrical activity, brain electrical source analysis, positron emission tomography, and transcranial cortical stimulation.