A single trial technique has been used for the analysis of neuromagnetic signals. This method, applied to evoked magnetoencephalograms, permitted us to evaluate the dynamics of brain responses under repetitive somatosensory stimulation. Two different phenomena have been investigated: synchronized spontaneous activity (SSA), elicited by bursts of stimuli at a given frequency; and somatosensory evoked fields (SEF), the transient response to a single stimulus. A transient period of adaptation of the SSA response to the particular stimulation paradigm is reported at specific stimulation frequencies; no significant decrease of the response amplitude even after 300 repetitions is observed. The amplitudes of single SEFs also appear to be constant. Possible physiological implications of the SSA response parameters studied (amplitude, frequency and decay time), and a preliminary discussion of the relationship between SEF and SSA, are presented.

MEG recordings visualized non-invasively a dynamic anterior-posterior activation in the pyramidal cell population of the human primary somatosensory cortex (S1) after posterior tibial nerve stimulation. Somatosensory evoked fields (SEFs) were recorded over the foot area in response to right posterior tibial nerve stimulation at the ankle in six normal subjects. A newly developed MEG vector beamformer technique applied to the SEFs revealed two distinct sources in the mesial wall of the left hemisphere around the primary P37m response typically separated by 1.3 cm. The first source was located in area 3b and oriented toward the contralateral hemisphere. The second source was assumed to be in an area near the marginal sulcus and the source orientation was directed posteriorly. The first source began to be active during the initial slope of the P37m. The second source was active after the P37m peak and the signal intensities of the first and second sources were equal at a mean latency of 2.6 ms after the peak of P37m. Then the first source became inactive and the second source was dominant after about 5 ms post-P37m peak. These findings suggest that a single peaked posterior tibial nerve P37m consists of partially overlapping two subcomponents generated in area 3b and an area near the marginal sulcus.

In the study of the neural basis of sensorimotor transformations, it has become clear that the brain does not always wait to sense external events and afterward select the appropriate responses. If there are predictable regularities in the environment, the brain begins to anticipate the timing of instructional cues and the signals to execute a response, revealing an internal representation of the sequential behavioral states of the task being performed. To investigate neural mechanisms that could represent the sequential states of a task, we recorded neural activity from two oculomotor structures implicated in behavioral timing--the supplementary eye fields (SEF) and the lateral intraparietal area (LIP)--while rhesus monkeys performed a memory-guided saccade task. The neurons of the SEF were found to collectively encode the progression of the task with individual neurons predicting and/or detecting states or transitions between states. LIP neurons, while also encoding information about the current temporal interval, were limited with respect to SEF neurons in two ways. First, LIP neurons tended to be active when the monkey was planning a saccade but not in the precue or intertrial intervals, whereas SEF neurons tended to have activity modulation in all intervals. Second, the LIP neurons were more likely to be spatially tuned than SEF neurons. SEF neurons also show anticipatory activity. The state-selective and anticipatory responses of SEF neurons support two complementary models of behavioral timing, state dependent and accumulator models, and suggest that each model describes a contribution SEF makes to timing at different temporal resolutions.

OBJECTIVE

We sought to clarify the presence of a temporal window of integration (TWI) in the somatosensory modality by manipulating the inter-stimulus interval (ISI).

METHODS

We recorded cortical activity following the last of a train of electric pulses (stimulus offset) applied to the left hand in nine healthy volunteers using magnetoencephalography (MEG). Somatosensory evoked magnetic fields (SEFs) were elicited by the offset of a train of pulses 3s in total duration with four ISIs (25, 50, 75, and 100 ms).

RESULTS

Results show that (i) off-M100 was clearly elicited by the ISI-25 and 50 ms conditions but not ISI-75 and 100 ms conditions, and (ii) the generator for off-M100 was mainly located in the contralateral primary and secondary somatosensory cortex (SI and SII).

CONCLUSIONS

The upper limit of the TWI in the somatosensory modality is between 50 and 75 ms, and the contralateral SI and SII play an important role in integrating temporal information.

CONCLUSIONS

The present study clarifies the presence of the TWI in the somatosensory modality.

Secretory end-feet (or SEF) systems are present in Limnodrilus and Stylodrilus but are less highly organized than those of polychaetes. SEF contain secretory vesicles and abundant mitochondria. Typical neurosecretory terminals are not found within the brain although "neurosecretory" perikarya are present in all four species studied. In Limnodrilus, Stylodrilus and Enchytraeus extracerebral cells, of probable neurosecretory function, are invested by the pericapsular epithelium. Characteristically such cells bear several cilia. In these species and in Stylaria a pair of sensory cell groups is located anteriorly within the brain. These cells are ciliated but lack associated supporting cells.

The morphology of somatosensory evoked fields (SEF) following electrical separate stimulation of left and right median nerve in 22 healthy volunteers was explored. The analysis of morphological properties of individual SEFs has been performed, in order to quantify an inter-hemispheric correlation coefficient. Despite a high inter-subject variability of the SEF morphologies, a high intra-subject inter-hemispheric shape correlation occurs in the post-stimulus epoch of 100 ms from the stimulus onset. Accordingly, a normative value for the parameter describing the SEF inter-hemispheric correlation coefficient has been calculated. The present work establishes quantitative and normative descriptions of the shape similarity of SEFs from the two hands in the left and right hemispheres. This provides a database for a new parameter, which might be useful in detecting alterations in the primary sensory cortical activation due to possible unilateral alterations of sensory inflow because of either peripheral or central deficits. The described procedure has been successfully applied to a small number of patients affected by unilateral cerebrovascular lesion.

The pericentral primary sensorimotor cortices generate the "mu rhythm" with a distinct spectral signature exhibiting two peaks, generated predominantly anterior (20 Hz) or posterior (10 Hz) to the central sulcus; it defines a "background" network state upon which somatosensory inputs will impinge. We used the high spatiotemporal resolution of magnetoencephalography to analyze the perturbation dynamics of these cortical rhythms in response to a series of paired electric median nerve stimuli: single trials were sorted off-line according to increasing power of the 10- or 20-Hz rebounds which occurred 300-600 ms after the first stimulus; using subaverages formed from the upper and lower 20% of this distribution, we analyzed somatosensory evoked fields (SEF) and power modifications caused by the second stimulus in the pair. We report three key findings: (1) the power level of rhythm rebounds triggered by the first stimulus predicted the rebound strength after the second stimulus applied 600 ms later; yet, it was uncorrelated across the 2.4-s interval separating subsequent stimulus pairs. (2) Conventional averaging camouflages substantial trial-to-trial variations of rhythm dynamics including, for example, even non-occurrences of rhythm rebounds. (3) For six of the seven subjects, the background rhythm power did not affect any SEF component; for the subject with the strongest rhythms only intracortically generated deflections (peaking after the thalamocortical input component N20m) varied as function of pre-stimulus 10- or 20-Hz power. Thus, the perturbation dynamics of the pericentral mu rhythm exhibits a significant intertrial variance, which becomes effective mainly at a time scale larger than 600 ms.

Everyday life often necessitates dissociation between our directed attention and the intention to direct our gaze. Accordingly, the differential role of visual and motor related areas in the one or the other process is an issue of an ongoing debate. Here we used functional magnetic resonance imaging to elaborate a differentiation between visuospatial attention and the intention for a horizontal saccade in these cortical areas. Subjects fixated a central target, while they directed their attention to a colored cue in the left or right visual field. Regardless of its location, the color of the cue instructed the direction of the upcoming saccade (intention). The attention to the peripheral cue and the intention to perform the saccade were thus either directed to the same side or to opposite sides. A random effects analysis of the imaging data showed that activation of the early visual cortex and the motion sensitive complex was biased by attention to the contralateral cue, whereas activity of the color sensitive complex was modulated by the stimulus instructing a contraversive saccade. The posterior parietal cortex and the proper supplementary eye field (SEF) were most strongly activated in case of spatially congruent attention and intention. In contrast, activity of the pre-SEF and the frontal eye field was enhanced by spatially divergent attention and intention. The results presented here advance our understanding of how the human brain processes spatial information. Noteworthy, the visuomotor related areas show a subtle cortical separation for visual related attention and saccade related intention.

OBJECTIVE

To elucidate the temporal profile of interactions between sensory information from both hands in the somatosensory cortex.

METHODS

Somatosensory evoked fields (SEFs), generated by stimulation applied to the right index finger after a preceding stimulation to the left index finger, were recorded using a whole head-type magnetoencephalography (MEG). The paired electrical stimuli were applied with a stimulation onset asynchrony (SOA) of 50, 100, 200, 300, or 400 ms.

RESULTS

The mean SEF intensities in the primary somatosensory area (SI) of five subjects, which were evoked approximately 40 ms after the latter of the paired stimuli, were not significantly smaller than that evoked in the control condition when only the right finger was stimulated. In contrast, SEFs in the secondary somatosensory area (SII), generated approximately 100 ms after the stimuli, were suppressed when the paired stimuli were applied at an SOA of 100 ms (P<0.05, t test). In addition, SEFs at approximately 150 ms after the stimuli were significantly suppressed at SOAs of 50, 100 (P<0.05), 200, and 300 ms (P<0.1).

CONCLUSIONS

Within a time window of approximately 300 ms, sensory information from the left finger significantly affected the SEFs generated by sensory inputs from the right finger. This time window may be required for the integration of sensory input.

We investigated the somatosensory evoked cortical magnetic field (SEF) components corresponding to the somatosensory evoked potential (SEP) components between 20 and 30 ms after median nerve stimulation. SEP and SEF were simultaneously recorded after right median nerve stimulation in seven healthy subjects. Twenty single-sweep epochs of SEF and SEP were selected, in which the first SEF component at 20 ms, 1M, and the second component at 30 ms, 2M, were identifiable. The selected epochs were re-averaged at the peaks of 1M and 2M as the triggering periods (zero ms). The width of the deflection, the temporal dispersion (TD), of SEP components, P20 and N30 (Fz-A2), N20, cP25 and cN30 (C3-Fz), N20 and pP30 (P3-A2), and N20 and P30 (P3-Fz), were compared between three averaging conditions. The N20/P20 components showed significantly smaller TDs when the epochs were averaged at the 1M peak (one-way factorial ANOVA, P<0.02) than those of the control, but averaging at the 1M peak did not decrease the TD of N30/P30. On averaging at the 2M peak, the TDs of N30/P30 components recorded from Fz-A2 and P3-Fz were smaller than those of the control. Neither the averaging at the 1M peak nor that at the 2M peak decreased the TD of the cP25 and cN30 components. Source analysis showed that the equivalent current dipoles (ECDs) for both 1M and 2M were located around the central sulcus, possibly in the primary somatosensory cortex (SI). We confirmed that the 1M and 2M temporally linked with N20/P20 and N30/P30, respectively. The difference of TD of N20/P20 and N30/P30 indicated that the neural pathways to the responses to N20/P20 and N30/P30 might be independent.

OBJECTIVE

To assess the clinical value of magnetoencephalography (MEG) in localising the primary hand motor area and evaluating cortical distortion of the sensorimotor cortices in patients with intracerebral tumour.

METHODS

10 normal volunteers (controls) and 14 patients with an intracerebral tumour located around the central region were studied. Somatosensory evoked magnetic fields (SEFs) following median nerve stimulation, and movement related cerebral magnetic fields (MRCFs) following index finger extension, were measured in all subjects and analysed by the equivalent current dipole (ECD) method to ascertain the neuronal sources of the primary sensory and motor components (N20m and MF, respectively). These ECD locations were defined as the primary hand sensory and motor areas and the positional relations between these two functional areas in controls and patients were investigated.

RESULTS

The standard range of ECD locations of MF to N20m was determined in controls. In 11 of the 14 patients, MRCFs could identify the primary motor hand area. ECD locations of MF were significantly closer to the N20m in the medial-lateral direction in patients than in controls. In patients with a tumour located below the sensorimotor hand area, relative ECD locations of MF to N20m moved anteriorly over the standard range determined in the control subjects. These MEG findings correlated well with radiological tumour locations. The mean estimated ECD strength of MF was significantly lower in patients than in controls.

CONCLUSIONS

MRCF was useful in localising the primary motor hand area in patients with intracerebral tumour. The relative ECD locations of MF to N20m describe the anatomical distortion of the sensorimotor cortex.

Electroencephalographic (EEG) recordings were performed on five New Zealand White rabbits anesthetized by using four intramuscular drug combinations: 1) ketamine (30 mg/kg) and midazolam (0.2 mg/kg), 2) ketamine (30 mg/kg), midazolam (0.2 mg/kg), and xylazine (3 mg/kg), 3) Telazol (20 mg/kg), and 4) Telazol (20 mg/kg) and xylazine (3 mg/kg). All four combinations were administered randomly to each rabbit. To evaluate anesthesia depth, response to toe pinch and various measurements from the recordings were assessed before and after injection. For each EEG recorded epoch, b/d ratios and the spectral edge frequencies (SEF) at 80% and 95% were measured. Results show that after injection of combinations without xylazine, b/d ratios and SEFs decreased only slightly; concurrently the withdrawal reflex remained present. Adding xylazine decreased the b/d ratios (p, 0.001) and the SEFs at 80% (p, 0.001) and 95% (p, 0.001). No withdrawal reflex was observed for 30 min after injection of ketamine-midazolam-xylazine and for 60 min after administration of Telazol-xylazine. Therefore, EEGs may be used to evaluate depth of anesthesia when using injectable drug combinations in rabbits.Abstract>

We present a benchmarking protocol for quantitatively comparing emerging on-scalp magnetoencephalography (MEG) sensor technologies to their counterparts in state-of-the-art MEG systems.

As a means of validation, we compare a high-critical-temperature superconducting quantum interference device (high Tc SQUID) with the low- Tc SQUIDs of an Elekta Neuromag TRIUX system in MEG recordings of auditory and somatosensory evoked fields (SEFs) on one human subject.

We measure the expected signal gain for the auditory-evoked fields (deeper sources) and notice some unfamiliar features in the on-scalp sensor-based recordings of SEFs (shallower sources).

The experimental results serve as a proof of principle for the benchmarking protocol. This approach is straightforward, general to various on-scalp MEG sensors, and convenient to use on human subjects. The unexpected features in the SEFs suggest on-scalp MEG sensors may reveal information about neuromagnetic sources that is otherwise difficult to extract from state-of-the-art MEG recordings.

As the first systematically established on-scalp MEG benchmarking protocol, magnetic sensor developers can employ this method to prove the utility of their technology in MEG recordings. Further exploration of the SEFs with on-scalp MEG sensors may reveal unique information about their sources.

Single-unit recording in monkeys and functional imaging of the human frontal lobe indicate that the supplementary eye field (SEF) and the frontal eye field (FEF) are involved in ocular decision making. To test whether these structures have distinct roles in decision making, single-neuron activity was recorded from each structure while monkeys executed an ocular go/nogo task. The task rule is to pursue a moving target if it intersects a visible square or "go zone." We found that most SEF neurons showed differential go/nogo activity during the delay period, before the target intersected the go zone (delay period), whereas most FEF neurons did so after target intersection, during the period in which the movement was executed (movement period). Choice probability (CP) for SEF neurons was high in the delay period but decreased in the movement period, whereas for FEF neurons it was low in the delay period and increased in the movement period. Directional selectivity of SEF neurons was low throughout the trial, whereas that of FEF neurons was highest in the delay period, decreasing later in the trial. Increasing task difficulty led to later discrimination between go and nogo in both structures and lower CP in the SEF, but it did not affect CP in the FEF. The results suggest that the SEF interprets the task rule early but is less involved in executing the motor decision than is the FEF and that these two areas collaborate dynamically to execute ocular decisions.

Frequency organization in the human primary somatosensory cortex (SI) was re-examined. Somatosensory evoked magnetic fields (SEFs) were measured with a 37 channel first-order axial gradiometer system. Sensory stimuli comprising a 20ms vibration at frequencies of 50, 100, 200 and 400 Hz were delivered to the volar surface of the tip of the right index finger. Using a single dipole model, the sources of the first main peak (M50) of SEFs were estimated. All of the sources were located in the 3b area. There were no statistically significant differences between the locations of the dipoles evoked by different frequency stimulations. These results support our previous study using a 122 channel whole head planar gradiometer system that systematic frequency organization at the hand area of SI cortex may not be present in humans.

This paper presents a three-dimensional finite element model of skeletal muscle and its validation incorporating inital tissue strains. A constitutive relation was determined by using a convex free strain energy function (SEF) where active and passive response contributions were obtained fitting experimental data from the rat tibialis anterior (TA) muscle. The passive and active finite strains response was modelled within the framework of continuum mechanics by a quasi-incompressible transversely isotropic material formulation. Magnetic resonance images (MRI) were obtained to reconstruct the external geometry of the TA. This geometry includes initial strains also taken into account in the numerical model. The numerical results show excellent agreement with the experimental results when comparing reaction force-extension curves both in passive and active tests. The proposed constitutive model for the muscle is implemented in a subroutine in the commercial finite element software package ABAQUS.

BACKGROUND

Halothane and propofol depress the central nervous system, and this is partly manifested by a decrease in electroencephalographic (EEG) activity. Little work has been performed to determine the differences between these anesthetics with regard to their effects on evoked EEG activity. We examined the effects of halothane and propofol on EEG responses to electrical stimulation of the reticular formation.

METHODS

Rats (n= 12) were anesthetized with either halothane or propofol, and EEG responses were recorded before and after electrical stimulation of the reticular formation. Two anesthetic concentrations were used (0.8 and 1.2 times the amount needed to prevent gross, purposeful movement in response to supramaximal noxious stimulation), and both anesthetics were studied in each rat using a cross-over design.

RESULTS

Electrical stimulation in the reticular formation increased the spectral edge (SEF) and median edge (MEF) frequencies by approximately 1-2 Hz during halothane anesthesia at low and high concentrations. During propofol anesthesia, MEF increased at the low propofol infusion rate, but SEF was unaffected. At the high propofol infusion rate, SEF and MEF decreased following electrical stimulation in the reticular formation.

CONCLUSIONS

At immobilizing concentrations, propofol produces a larger decrease than halothane in EEG responses to reticular formation stimulation, consistent with propofol having a more profound depressant effect on cortical and subcortical structures.

A child-customized magnetoencephalography system was used to investigate somatosensory evoked field (SEF) in 3- to 4-year-old children. Three stimulus conditions were used in which the children received tactile-only stimulation to their left index finger or visuotactile stimulation. In the two visuotactile conditions, the children received tactile stimulation to their finger while they watched a video of tactile stimulation applied either to someone else's finger (the finger-touch condition) or to someone else's toe (the toe-touch condition). The latencies and source strengths of equivalent current dipoles (ECDs) over contralateral (right) somatosensory cortex were analyzed. In the preschoolers who provided valid ECDs, the stimulus conditions induced an early-latency ECD occurring between 60 and 68 ms mainly with an anterior direction. We further identified a middle-latency ECD between 97 and 104 ms, which predominantly had a posterior direction. Finally, initial evidence was found for a late-latency ECD at about 139-151 ms again more often with an anterior direction. Differences were found in the source strengths of the middle-latency ECDs among the stimulus conditions. For the paired comparisons that could be formed, ECD source strength was more pronounced in the finger-touch condition than in the tactile-only and the toe-touch conditions. Although more research is necessary to expand the data set, this suggests that visual information modulated preschool SEF. The finding that ECD source strength was higher when seen and felt touch occurred to the same body part, as compared to a different body part, might further indicate that connectivity between visual and tactile information is indexed in preschool somatosensory cortical activity, already in a somatotopic way.

Somatosensory evoked fields (SEFs) to repetitive tactile stimuli were recorded from eight dyslexic and eight normal-reading adults. Three successive stimuli, produced by diaphragms driven by compressed air, were delivered to thumb, index finger and thumb in sequence, with stimulus-onset asynchronies (SOAs) of 100 and 200 ms in different runs. Both hands were stimulated alternatingly with an intertrain interval of 1 s, and the responses were recorded with a whole-scalp neuromagnetometer. Whereas the primary somatosensory cortex responses to the first stimuli of the trains did not differ between dyslexics and controls, responses to the second stimuli (and the ratios of second to first responses) were significantly smaller in dyslexic than in control subjects in the right hemisphere (differences 41 and 28% for response amplitudes at the 100 and 200 ms SOAs). The results agree with the proposed pansensory nature of temporal processing deficits in dyslexia, specifically demonstrating abnormal response recovery in the right somatosensory cortex.

OBJECTIVE

We investigate the synaptic factor for the recovery function of evoked responses using a repetitive stimulation technique.

METHODS

Somatosensory evoked cortical magnetic field (SEF) was recorded following stimulation of the median nerve using single to 6-train stimulation in 8 healthy subjects. The SEF responses after each stimulus in the train stimulation were extracted by subtraction of the waveforms.

RESULTS

An attenuation of the SEF components was recognized after the second of the stimuli, but there was no significant attenuation with the third or later stimulations. The root mean square (RMS) of the 1M (peak latency at 20 ms after stimulation) and 4M (70 ms) components were smaller than that of the single stimulation during the train stimulation, while the 2M (30 ms) and 3M (45 ms) components were not attenuated, but the 3M was facilitated at the fourth to sixth stimulation.

CONCLUSIONS

The synaptic factor was not responsible for the attenuation of the SEF components during repetitive stimulation in healthy subjects. The SEF change disclosed a functional difference among the SEF components during the train stimulation, especially among the later components.

OBJECTIVE

To improve the interpretability of figures containing an amplitude-integrated electroencephalogram (aEEG), we devised a color scale that allows us to incorporate spectral edge frequency (SEF) information into aEEG figures. Preliminary clinical assessment of this novel technique, which we call aEEG/SEF, was performed using neonatal and early infantile seizure data.

METHODS

We created aEEG, color density spectral array (DSA), and aEEG/SEF figures for focal seizures recorded in seven infants. Each seizure was paired with an interictal period from the same patient. After receiving instructions on how to interpret the figures, eight test reviewers examined each of the 72 figures displaying compressed data in aEEG, DSA, or aEEG/SEF form (12 seizures and 12 corresponding interictal periods) and attempted to identify each as a seizure or otherwise. They were not provided with any information regarding the original record.

RESULTS

The median number of correctly identified seizures, out of a total of 12, was 7 (58.3%) for aEEG figures, 8 (66.7%) for DSA figures and 10 (83.3%) for aEEG/SEF figures; the differences among these are statistically significant (p=0.011). All reviewers concluded that aEEG/SEF figures were the easiest to interpret.

CONCLUSIONS

The aEEG/SEF data presentation technique is a valid option in aEEG recordings of seizures.

OBJECTIVE

Anesthetics, including propofol, depress the electroencephalogram (EEG) and neuronal activity in the midbrain reticular formation (MRF). Because propofol has anesthetic effects in the spinal cord, we hypothesized that it would indirectly depress EEG and MRF neuronal responses to noxious stimulation in part by a spinal cord action.

METHODS

Six goats were anesthetized with isoflurane and the jugular veins and carotid arteries were isolated to permit cranial bypass and differential propofol delivery. A noxious mechanical stimulus was applied to the distal forelimb while recording bifrontal EEG and MRF single-unit activities. Propofol was separately administered to the cranial (0.08 +/- 0.06 mg/kg) and torso circulations (4 mg/kg) and the noxious stimulus applied at 1,5, 10, and 15 min after each injection.

METHODS

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METHODS

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METHODS

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RESULTS

Noxious stimulation decreased total power (TP) from 96 +/- 33, microV2/Hz to 38 +/- 20microV2/Hz, (mean +/- SD) and increased spectral edge frequency (SEF) from 10 +/- 3 Hz to 19 +/- 5 Hz (p<0.01). Propofol administered to the torso prevented stimulus-evoked changes in TP (121+/- 80 microV2/Hz, 121 +/- 74 microV2/Hz, 114 +/- 74 microV2/Hz at 1,5, and 10 min respectively, p<0.01 compared to control evoked response) and SEF (11 +/- 6Hz, 9 +/- 2Hz, 10 +/- 6Hz, and 12 +/- 5Hz at 1, 5, 10 and 15 min, respectively, p<0.001 compared to control evoked response). Propofol administered to the cranial circulation significantly blunted the EEG and MRF response, while torso-administered propofol had slight effects on MRF responses.

CONCLUSIONS

Propofol blunted the EEG response to noxious stimulation in part via a subcortical action.

We present a novel non-invasive technique for identification of the central sulcus on the brain surface by using three-dimensional magnetic resonance imaging (3D-MRI) in combination with somatosensory evoked magnetic fields (SEFs). The central sulcus was supposed anatomically on the brain surface by using 3D-MRI. On the other hand, the primary somatosensory area is determined by using SEF data with median nerve stimulation. Superimposition of the SEFs of the 25 ms response onto the brain surface MR images clearly demonstrated the dipole source located in the gyral fold just behind the supposed central sulcus in all subjects analyzed. Three-dimensional reconstruction of the brain surface image data facilitated visualizing the precise anatomical localization of the magnetic field activities from any angle and measuring the distance from the source to any point of interest. The potential clinical application of this technique is discussed.