Somatosensory evoked potentials/fields--exploration of brain function

We have summarized the history of electroencephalography(EEG) since 1875, when a paper by Richard Caton was published describing the first EEG recordings in animals. Somatosensory evoked potentials (SEPs) were recorded by George Dawson in 1951. Thereafter, SEPs were developed for clinical use with other evoked potentials such as auditory evoked potentials(VEPs). To understand evoked potentials, related mechanism of induction of far-fields-potentials(FFP) following stimulation of the median nerve has been discussed. SEPs consisted of P9, N9, N10, P11, N11, N13, P13, P14, N18, N20 and P20/P22. Scalp recorded P9 FFP arises from the distal portion of the branchial plexus as reflected by N9 stationary negative potential recorded over the stimulated arm. Cervical N11 and N13 arise from the root entry zone and dorsal horn, respectively. Scalp recorded P13, P14 and N18 FFP originate from the brainstem. In this communication, magnetoencephalography(MEG) and results of one of our recent studies on somatosensory evoked fields(SEFs) are also discussed. One of the important features of MEG is that magnetic signals detected outside the head arise mainly from cortical currents tangential to the skull. Since the net postsynaptic current follows the orientation of cortical pyramidal cells, the MEG signals mainly reflect activity of the fissural cortex, whereas radial current may remain undetected. In our study, we demonstrated SEFs elicited by compression and decompression of a subject's glabrous skin by a human operator. Their dipoles were tangentially oriented from the frontal lobe to parietal lobe.     

Cortical somatosensory evoked potentials. II. Effects of excision of somatosensory or motor cortex in humans and monkeys

1. To clarify the generators of human short-latency somatosensory evoked potentials (SEPs) thought to arise in sensorimotor cortex, we studied the effects on SEPs of surgical excision of somatosensory or motor cortex in humans and monkeys. 2. Normal median nerve SEPs (P20-N30, N20-P30, and P25-N35) were recorded from the cortical surface of a patient (G13) undergoing a cortical excision for relief of focal seizures. All SEPs were abolished both acutely and chronically after excision of the hand area of somatosensory cortex. Similarly, excision of the hand area of somatosensory cortex abolished corresponding SEPs (P10-N20, N10-P20, and P12-N25) in monkeys. Excision of the crown of monkey somatosensory cortex abolished P12-N25 while leaving P10-N20 and N10-P20 relatively unaffected. 3. After excision of the hand area of motor cortex, all SEPs were present when recorded from the cortical surface of a patient (W1) undergoing a cortical excision for relief of focal seizures. Similarly, all SEPs were present in monkeys after excision of the hand area of motor cortex. 4. Although all SEPs were present after excision of motor cortex in monkeys, variable changes were observed in SEPs after the excisions. However, these changes were not larger than the changes observed after excision of parietal cortex posterior to somatosensory cortex. We concluded that the changes were not specific to motor cortex excision. 5. These results support two major conclusions. 1) Median nerve SEPs recorded from sensorimotor cortex are produced by generators in two adjacent regions of somatosensory cortex: a tangentially oriented generator in area 3b, which produces P20-N30 (human) and P10-N20 (monkey) [recorded anterior to the central sulcus (CS)] and N20-P30 (human) and N10-P20 (monkey) posterior to the CS; and a radially oriented generator in area 1, which produces P25-N35 (human) and P12-N25 (monkey) recorded from the postcentral gyrus near the CS. 2) Motor cortex makes little or no contribution to these potentials.     

Neuromagnetic investigation of somatotopy of human hand somatosensory cortex

Summary In order to investigate functional topography of human hand somatosensory cortex we recorded somatosensory evoked fields (SEFs) on MEG during the first 40 ms after stimulation of median nerve, ulnar nerve, and the 5 digits. We applied dipole modeling to determine the three-dimensional cortial representations of different peripheral receptive fields. Median nerve and ulnar nerve SEFs exhibited the previously described N20 and P30 components with a magnetic field pattern emerging from the head superior and re-entering the head inferior for the N20 component; the magnetic field pattern of the P30 component was of reversed orientation. Reversals of field direction were oriented along the anterior-posterior axis. SEFs during digit stimulation showed analogous N22 and P32 components and similar magnetic field patterns. Reversals of field direction showed a shift from lateral inferior to medial superior for thumb to little finger. Dipole modeling yielded good fits at these peak latencies accounting for an average of 83% of the data variance. The cortical digit representations were arranged in an orderly somatotopic way from lateral inferior to medial superior in the sequence thumb, index finger, middle finger, ring finger, and little finger. Median nerve cortical representation was lateral inferior to that of ulnar nerve. Isofield maps and dipole locations for these components are consistent with neuronal activity in the posterior bank of central fissure corresponding to area 3b. We conclude that SEFs recorded on MEG in conjunction with source localization techniques are useful to investigate functional topography of human hand somatosensory cortex non-invasively.     

Dipolar sources of the early scalp somatosensory evoked potentials to upper limb stimulation Effect of increasing stimulus rates

Brain electrical source analysis (BESA) of the scalp electroencephalographic activity is well adapted to distinguish neighbouring cerebral generators precisely. Therefore, we performed dipolar source modelling in scalp medium nerve somatosensory evoked potentials (SEPs) recorded at 1.5-Hz stimulation rate, where all the early components should be identifiable. We built a four-dipole model, which was issued from the grand average, and applied it also to recordings from single individuals. Our model 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 potential and, with opposite polarity, the P24/N24 response. The second perirolandic dipole showed an initial peak of activity slightly earlier than that of the N20/P20 dipolar source and, later, it was active at the same latency as the central P22 potential. Lastly, the third perirolandic dipole exaplaining the fronto-central N30 potential scalp distribution was constantly more posterior than the first one. In order to evaluate the effect of an increasing repetition frequency on the activity of SEP dipolar sources, we applied the model built from 1.5-Hz SEPs to traces recorded at 3-Hz and 10-Hz repetition rates. We found that the 10-Hz stimulus frequency reduced selectively the later of the two activity phases of the first perirolandic dipole. The decrement in strength of this dipolar source can be explained if we assume that: (a) the later activity of the first perirolandic dipole can represent the inhibitory phase of a “primary response”; (b) two different clusters of cells generate the opposite activities of the tangential perirolandic dipole. An additional finding in our model was that two different perirolandic dipoles contribute to the centro-parietal N20 potential generation. Received: 5 August 1997 / Accepted: 26 November 1997     

Cortical somatosensory evoked potentials. I. Recordings in the monkey Macaca fascicularis

1. The anatomic generators of somatosensory evoked potentials (SEPs) to median nerve stimulation in the 10- to 30-ms latency range were investigated in monkeys (Macaca fascicularis) by means of cortical-surface and laminar recordings. 2. Three groups of SEPs evoked by stimulation of the contralateral median nerve were recorded from the hand representation area of sensorimotor cortex: P10-N20, recorded anterior to the central sulcus (CS); N10-P20, recorded posterior to the CS; and P12-N25, recorded near the CS. These potentials were similar in morphology and surface distribution whether the animal was awake or anesthetized. 3. P10-N20 exhibited a polarity inversion to N10-P20 across the CS, both in cortical-surface recordings and in laminar recordings within cortex and white matter of motor and somatosensory cortex. In contrast, P10-N20 and N10-P20 did not exhibit polarity inversion in recordings from the surface and white matter of the crowns of motor and somatosensory cortex, respectively. These results strongly suggest that these potentials are produced by a tangential generator located in the posterior wall of the CS, primarily in area 3b of somatosensory cortex. 4. P12-N25 was largest over the hand area of somatosensory cortex and showed polarity inversion across the crown of somatosensory cortex but not across the crown of motor cortex or across the walls of the CS, suggesting that P12-N25 is due to a radially oriented generator located in areas 1 and 2 of somatosensory cortex. 5. P10-N20 and P12-N25 are thought to be equivalent to the "primary evoked response" recorded from somatosensory cortex of other mammals. 6. These results are very similar to those obtained in human cortical-surface recordings and demonstrate that the monkey P10-N20, N10-P20, and P12-N25 potentials correspond to the human P20-N30, N20-P30, and P25-N35 potentials, respectively. The only appreciable difference in human and monkey SEPs is that the monkey P12-N25 appears to be generated in areas 1 and 2, whereas the human P25-N35 appears to be generated only in area 1. 7. There was no evidence of locally generated activity in areas 3a and 4.     

Synchronized spikes of thalamocortical axonal terminals and cortical neurons are detectable outside the pig brain with MEG.

We show that it is feasible to monitor the synchronized population spikes of the thalamocortical axonal terminals and cortical neurons outside the brain using high-resolution magnetoencephalography (MEG). Electrical stimulation of the snout elicited somatic-evoked magnetic fields (SEFs) above the primary somatosensory cortex (SI) of the piglet. The SEFs contained high-frequency oscillations (HFOs) around 600 Hz similar in many respects to the noninvasively measured HFOs from humans with MEG and electroencephalography (EEG). These HFOs were highly correlated with those in simultaneously measured intracortical somatic-evoked potentials (SEPs) in the snout projection area in SI. Both HFOs in SEFs and SEPs consisted of an initial component insensitive to cortically injected kynurenic acid (Kyna, 20 mM), a nonspecific antagonist of glutamatergic receptors, and a subsequent Kyna-sensitive component. The former was localized in cortical layer IV, indicating that it was due to spikes produced by the specific thalamocortical axonal terminals, whereas the latter was initially localized in layer IV and subsequently in the superficial and deeper layers. These results suggest that it may be possible to study properties of the thalamocortical and cortical spike activities in humans with MEG.     

Human cortical potentials evoked by stimulation of the median nerve. II. Cytoarchitectonic areas generating long-latency activity

1. The anatomic generators of human median nerve somatosensory evoked potentials (SEPs) in the 40 to 250-ms latency range were investigated in 54 patients by means of cortical-surface and transcortical recordings obtained during neurosurgery. 2. Contralateral stimulation evoked three groups of SEPs recorded from the hand representation area of sensorimotor cortex: P45-N80-P180, recorded anterior to the central sulcus (CS) and maximal on the precentral gyrus; N45-P80-N180, recorded posterior to the CS and maximal on the postcentral gyrus; and P50-N90-P190, recorded near and on either side of the CS. 3. P45-N80-P180 inverted in polarity to N45-P80-N180 across the CS but was similar in polarity from the cortical surface and white matter in transcortical recordings. These spatial distributions were similar to those of the short-latency P20-N30 and N20-P30 potentials described in the preceding paper, suggesting that these long-latency potentials are generated in area 3b of somatosensory cortex. 4. P50-N90-P190 was largest over the anterior one-half of somatosensory cortex and did not show polarity inversion across the CS. This spatial distribution was similar to that of the short-latency P25-N35 potentials described in the preceding paper and, together with our and Goldring et al. 1970; Stohr and Goldring 1969 transcortical recordings, suggest that these long-latency potentials are generated in area 1 of somatosensory cortex. 5. SEPs of apparently local origin were recorded from several regions of sensorimotor cortex to stimulation of the ipsilateral median nerve. Surface and transcortical recordings suggest that the ipsilateral potentials are generated not in area 3b, but rather in other regions of sensorimotor cortex perhaps including areas 4, 1, 2, and 7. This spatial distribution suggests that the ipsilateral potentials are generated by transcallosal input from the contralateral hemisphere. 6. Recordings from the periSylvian region were characterized by P100 and N100, recorded above and below the Sylvian sulcus (SS) respectively. This distribution suggests a tangential generator located in the upper wall of the SS in the second somatosensory area (SII). In addition, N125 and P200, recorded near and on either side of the SS, suggest a radial generator in a portion of SII located in surface cortex above the SS. 7. In comparison with the short-latency SEPs described in the preceding paper, the long-latency potentials were more variable and were more affected by intraoperative conditions.     

Human cortical potentials evoked by stimulation of the median nerve. II. Cytoarchitectonic areas generating short-latency activity

1. The anatomic generators of human median nerve somatosensory evoked potentials (SEPs) in the 40 to 250-ms latency range were investigated in 54 patients by means of cortical-surface and transcortical recordings obtained during neurosurgery. 2. Contralateral stimulation evoked three groups of SEPs recorded from the hand representation area of sensorimotor cortex: P45-N80-P180, recorded anterior to the central sulcus (CS) and maximal on the precentral gyrus; N45-P80-N180, recorded posterior to the CS and maximal on the postcentral gyrus; and P50-N90-P190, recorded near and on either side of the CS. 3. P45-N80-P180 inverted in polarity to N45-P80-N180 across the CS but was similar in polarity from the cortical surface and white matter in transcortical recordings. These spatial distributions were similar to those of the short-latency P20-N30 and N20-P30 potentials described in the preceding paper, suggesting that these long-latency potentials are generated in area 3b of somatosensory cortex. 4. P50-N90-P190 was largest over the anterior one-half of somatosensory cortex and did not show polarity inversion across the CS. This spatial distribution was similar to that of the short-latency P25-N35 potentials described in the preceding paper and, together with our and Goldring et al. 1970; Stohr and Goldring 1969 transcortical recordings, suggest that these long-latency potentials are generated in area 1 of somatosensory cortex. 5. SEPs of apparently local origin were recorded from several regions of sensorimotor cortex to stimulation of the ipsilateral median nerve. Surface and transcortical recordings suggest that the ipsilateral potentials are generated not in area 3b, but rather in other regions of sensorimotor cortex perhaps including areas 4, 1, 2, and 7. This spatial distribution suggests that the ipsilateral potentials are generated by transcallosal input from the contralateral hemisphere. 6. Recordings from the periSylvian region were characterized by P100 and N100, recorded above and below the Sylvian sulcus (SS) respectively. This distribution suggests a tangential generator located in the upper wall of the SS in the second somatosensory area (SII). In addition, N125 and P200, recorded near and on either side of the SS, suggest a radial generator in a portion of SII located in surface cortex above the SS. 7. In comparison with the short-latency SEPs described in the preceding paper, the long-latency potentials were more variable and were more affected by intraoperative conditions.(ABSTRACT TRUNCATED AT 400 WORDS)     

The Existence of Two Sources in Rolandic Epilepsy: Confirmation with High Resolution EEG, MEG and fMRI

In benign rolandic epilepsy seizure semiology suggests that the epileptic focus resides in the lower sensorimotor cortex. Previous studies involving dipole modeling based on 32 channel EEG have confirmed this localization. These studies have also suggested that two distinct dipole sources are required to adequately describe the typical interictal spikes. Since in benign epilepsy invasive validation is prohibited, this study tries to further establish these results using a multi-modal approach, involving 32 channel EEG, high resolution 84 channel EEG, 151 channel MEG and fMRI. From one patient interictal spikes were recorded and analyzed using the MUSIC algorithm in a realistic volume conductor model. In an fMRI experiment the same patient performed voluntary tongue movements, thus mimicking a typical seizure. Results show that EEG, MEG and fMRI localization converge on the same area in the lower part of the sensorimotor cortex, and that high resolution EEG clearly reveals two distinct sources, one in the post- and one in the pre-central cortex.     

Cortical and subcortical SEPs following posterior tibial nerve stimulation

Summary Intra-operative cortical and subcortical SEPs from the cerebral convexity and from the inter-hemispheric fissure were recorded following posterior tibial nerve (PTN) stimulation. Cortical and subcortical SEPs from the cerebral convexity after contra-lateral PTN stimulation consisted of N38 and P46, and their polarity reversed when the ipsi-lateral site was stimulated. On the other hand, cortical SEPs from the inter-hemispheric fissure always showed P38 and N46, whether the right or the left PTN was stimulated. Cortical and subcortical SEPs from the inter-hemispheric fissure showed clear cut polarity reversals. These findings provide good evidence for the existence of a tangential dipole oriented perpendicular to the inter-hemispheric fissure in the foot sensory area of the primary sensory cortex. SEPs recorded from the superficial part of the inter-hemispheric fissure showed smaller amplitudes and longer latencies than those of SEPs from the deeper regions. These findings suggest the existence of another dipole responsible for the generation of SEPs after PTN stimulation.     

EEG versus MEG localization accuracy: Theory and experiment

Summary We first review the theoretical and computer modelling studies concerning localization accuracy of EEG and MEG, both separately and together; the source is here a dipole. The results show that, of the three causes of localization errors, noise and head modelling errors have about the same effect on EEG and MEG localization accuracies, while the results for measurement placement errors are inconclusive. Thus, these results to date show no significant superiority of MEG over EEG localization accuracy. Secondly, we review the experimental findings, where there are again localization accuracy studies of EEG and MEG both separately and together. The most significant EEG-only study was due to dipoles implanted in the heads of patients, and produced an average localization error of 20 mm. Various MEG-only studies gave an average error of 2–3 mm in saline spheres and 4–8 mm in saline-filled skulls. In the one study where EEG and MEG localization were directly compared in the same actual head, again using dipoles implanted in patients, the average EEG and MEG errors of localization were 10 and 8 mm respectively. The MEG error was later confirmed by a similar (but MEG-only) experiment in another study, using a more elaborate MEG system. In summary, both theory and experiment suggests that the MEG offers no significant advantage over the EEG in the task of localizing a dipole source. The main use of the MEG, therefore, should be based on the proven feature that the MEG signal from a radial source is highly suppressed, allowing it to complement the EEG in selecting between competing source configurations. A secondary useful feature is that it handles source modelling errors differently than does the EEG, allowing it to help clarify non-dipolar extended sources.This work was supported by grants RO1NS26433, RO1NS19558 and RO1NS22703 from the National Institutes of Health.     

Multichannel derived median nerve SEP compared to EEG in patients with vascular cerebral lesions.

In order to compare the sensitivity of multichannel derived median nerve SEP with EEG in vascular cerebral lesions we examined 22 normals and 23 patients. SEP components within the first 50 ms could be divided into main waveform patterns: (1) a W-shaped parietal pattern consisting of N20, P25, N35 and P45 in most cases. (2) a frontal pattern with P20 and N30 as well as possibly detectible N24, P28, P33, N40 and P50. (3) a central P22. Two younger normals showed a V-shaped parietal pattern with N20 and P35, a frontal pattern with P20 and N36, and central P22 with a remarkably long latency. All components could be analysed sufficiently by means of three representative electrode positions (stimulation right/left): P3/P4, C3/C4, and F3/F4, which reduces the expense of recording and analysing considerably. 21 patients (91.3%) showed abnormal results in SEP, whereas 14 patients (60.9%) in EEG. A three channel electrode array can increase the usefulness of SEP and detect cerebral dysfunctions in cerebral lesions in spite of normal EEG under routine examination conditions. Analysis of multichannel derived SEPs during treatment and recovery after stroke and search for the prognostic value in the acute stage of the disease should be done in future.     

A method for combining MEG and EEG to determine the sources

A three-step method is presented which combines an MEG and EEG map over the head to solve the inverse problem (to determine the sources). This method uses the feature that the MEG does not see a radial source, but only a tangential source, while the EEG sees both. A first test is also made of the method, using computer simulation, and the results presented. The purpose of the test is to see if the method is valid with noisy MEG and EEG data, and when some modelling errors are present; a single dipole source was used in a spherical head. It was found that the method works well when the RMS noise at each map location is 5% of the maximum MEG and EEG (readily attained in practice), but breaks down when the noise is 10% (quite noisy data). The modelling errors involved grid size, head radius and distance to the MEG coil, and were studied only through the first step of the method; with errors in a reasonable range, this limited test again worked well.     

Representation of bioelectric current sources using Whitney elements in the finite element method

Bioelectric current sources of magneto- and electroencephalograms (MEG, EEG) are usually modelled with discrete delta-function type current dipoles, despite the fact that the currents in the brain are naturally continuous throughout the neuronal tissue. In this study, we represent bioelectric current sources in terms of Whitney-type elements in the finite element method (FEM) using a tetrahedral mesh. The aim is to study how well the Whitney elements can reproduce the potential and magnetic field patterns generated by a point current dipole in a homogeneous conducting sphere. The electric potential is solved for a unit sphere model with isotropic conductivity and magnetic fields are calculated for points located on a cap outside the sphere. The computed potential and magnetic field are compared with analytical solutions for a current dipole. Relative difference measures between the FEM and analytical solutions are less than 1%, suggesting that Whitney elements as bioelectric current sources are able to produce the same potential and magnetic field patterns as the point dipole sources.     

Sensitivity of EEG and MEG to the N1 and P2 Auditory Evoked Responses Modulated by Spectral Complexity of Sounds

Acoustic complexity of a stimulus has been shown to modulate the electromagnetic N1 (latency ∼110 ms) and P2 (latency 190 ms) auditory evoked responses. We compared the relative sensitivity of electroencephalography (EEG) and magnetoencephalography (MEG) to these neural correlates of sensation. Simultaneous EEG and MEG were recorded while participants listened to three variants of a piano tone. The piano stimuli differed in their number of harmonics: the fundamental frequency (f ₀ ), only, or f ₀ and the first two or eight harmonics. The root mean square (RMS) of the amplitude of P2 but not N1 increased with spectral complexity of the piano tones in EEG and MEG. The RMS increase for P2 was more prominent in EEG than MEG, suggesting important radial sources contributing to the P2 only in EEG. Source analysis revealing contributions from radial and tangential sources was conducted to test this hypothesis. Source waveforms revealed a significant increase in the P2 radial source amplitude in EEG with increased spectral complexity of piano tones. The P2 of the tangential source waveforms also increased in amplitude with increased spectral complexity in EEG and MEG. The P2␣auditory evoked response is thus represented by both tangential (gyri) and radial (sulci) activities. The radial contribution is expressed preferentially in EEG, highlighting the importance of combining EEG with MEG where complex source configurations are suspected.     

Genetic influence demonstrated for MEG‐recorded somatosensory evoked responses

We tested for a genetic influence on magnetoencephalogram (MEG)‐recorded somatosensory evoked fields (SEFs) in 20 monozygotic (MZ) and 14 dizygotic (DZ) twin pairs. Previous electroencephalogram (EEG) studies that demonstrated a genetic contribution to evoked responses generally focused on characteristics of representative brain potentials. Here we demonstrate significantly smaller amplitude differences within MZ compared to DZ twin pairs for the complete SEF time series (across left and right hand SEFs: 0.37 vs. 0.60 pT² and 0.28 vs. 0.39 pT² for primary [SI] and secondary [SII] sensory cortex activation) and higher MZ than DZ wave shape correlations (.71 vs. .44 and .52 vs. .35 for SI and SII activation). Our findings indicate a genetic influence on MEG‐recorded evoked brain activity and also confirm our recent conclusion ( van 't Ent, van Soelen, Stam, De Geus, & Boomsma, 2009 ) that higher MZ resemblance for EEG amplitudes is not trivially reflecting greater MZ concordance in intervening biological tissues.     

Precision of dipole localization in a spherical volume conductor: A comparison of referential EEG,magnetoencephalography and scalp current density methods

Summary In this study, we determined the influence of dipole orientation, dipole location, and number of recording sites on the precision of dipole localization in a spherical volume conductor. We compared localization from referential EEG (R-EEG), scalp current density EEG (SCD-EEG) and magnetoencephalography (MEG). Dipole orientation had a small influence on the precision of dipole localization for R-EEG and SCD-EEG. Dipole location relative to the recording sites, dipole depth, and number of recording channels strongly influenced the precision of dipole localization. Assuming equal signal to noise conditions for each recording method, MEG and SCD-EEG had a similar precision for dipole localization of a single tangential dipole source and both methods were more precise than R-EEG. However, SCD-EEG was inferior to MEG for distinguishing a single tangential current source from a pair of deeper radial current sources. In summary, these results suggest that the MEG will be most useful for localization of multiple simultaneous dipole sources.     

Paradoxical scalp lateralization of the P100 cognitive somatic potential in humans: a magnetic field study

In somatic selective attention, electrical brain mapping disclosed a P100 cognitive electrogenesis with a scalp field paradoxically lateralized ipsilaterally to the target finger stimulus. We used 64 sensors magnetoencephalography (MEG) and source localization software in six normal humans to identify the P100 neural generator. Calculated dipole sources for P100 were iteratively compared with the recorded MEG data. The equivalent dipole at somatic P100 latency was located in the parietal lobe contralateral to the finger stimulus and had an oblique positive gradient pointing towards the ipsilateral side, thus explaining the paradoxical positivity in electrical brain mapping. It is suggested that the somatic P100 is generated in parietal area 7b which indeed appears to be specialized for cognitive processing within the somatic modality rather than for multimodal association.     

Sensitivity of MEG and EEG to Source Orientation

An important difference between magnetoencephalography (MEG) and electroencephalography (EEG) is that MEG is insensitive to radially oriented sources. We quantified computationally the dependency of MEG and EEG on the source orientation using a forward model with realistic tissue boundaries. Similar to the simpler case of a spherical head model, in which MEG cannot see radial sources at all, for most cortical locations there was a source orientation to which MEG was insensitive. The median value for the ratio of the signal magnitude for the source orientation of the lowest and the highest sensitivity was 0.06 for MEG and 0.63 for EEG. The difference in the sensitivity to the source orientation is expected to contribute to systematic differences in the signal-to-noise ratio between MEG and EEG.     

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 30-80ms) 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 1mm. 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.