Ferromagnetic high-permeability alloy alone can provide sufficient low-frequency and eddy-current shieldings for biomagnetic measurements

Invariance in current dipole moment density across brain structures and species: physiological constraint for neuroimaging

Although anatomical constraints have been shown to be effective for MEG and EEG inverse solutions, there are still no effective physiological constraints. Strength of the current generator is normally described by the moment of an equivalent current dipole Q. This value is quite variable since it depends on size of active tissue. In contrast, the current dipole moment density q, defined as Q per surface area of active cortex, is independent of size of active tissue. Here we studied whether the value of q has a maximum in physiological conditions across brain structures and species. We determined the value due to the primary neuronal current (q primary) alone, correcting for distortions due to measurement conditions and secondary current sources at boundaries separating regions of differing electrical conductivities. The values were in the same range for turtle cerebellum (0.56-1.48 nAm/mm(2)), guinea pig hippocampus (0.30-1.34 nAm/mm(2)), and swine neocortex (0.18-1.63 nAm/mm(2)), rat neocortex (~2.2 nAm/mm(2)), monkey neocortex (~0.40 nAm/mm(2)) and human neocortex (0.16-0.77 nAm/mm(2)). Thus, there appears to be a maximum value across the brain structures and species (1-2 nAm/mm(2)). The empirical values closely matched the theoretical values obtained with our independently validated neural network model (1.6-2.8 nAm/mm(2) for initial spike and 0.7-3.1 nAm/mm(2) for burst), indicating that the apparent invariance is not coincidental. Our model study shows that a single maximum value may exist across a wide range of brain structures and species, varying in neuron density, due to fundamental electrical properties of neurons. The maximum value of q primary may serve as an effective physiological constraint for MEG/EEG inverse solutions.

Can magnetoencephalography track the afferent information flow along white matter thalamo-cortical fibers?

White matter thalamo-cortical fibers allow the communication of distant brain regions by carrying neuronal signals. Mapping non-invasively the information flow within white matter fibers is regarded so far as impossible. We investigated here whether information flow propagating along thalamo-cortical fibers can be detected using magnetoencephalography (MEG). Somatosensory evoked fields (SEFs) were recorded from healthy subjects and a patient with a unilateral, prenatally acquired, white matter lesion, which had induced the development of an abnormal trajectory of thalamo-cortical fibers. Equivalent current dipole (ECD) was used to model sources of SEFs. ECD at ~15 ms after stimulus onset was located within or close to the contralateral thalamus at the proximity of a hemodynamic response detected during a similar fMRI experiment. At the M20 peak latency, ECD was localized within the hand area of the contralateral primary somatosensory cortex (Brodmann area 3b (BA3b)). In healthy subjects, ECD changed dynamically position from thalamus to BA3b following a curved path, which was partially overlapping the thalamo-cortical fibers reconstructed by tractography. In the patient, ECD followed a similar path only in the intact hemisphere. In the affected hemisphere, the dipole trajectory circumnavigated the extended lesion on its way to the preserved primary somatosensory cortex--similar to the trajectory findings. Evidence from different methodological approaches converges on the conclusion that MEG can track the afferent information flow along thalamo-cortical fibers and in contrast to the traditional view can localize under presuppositions deep thalamic sources.

Transhemispheric depolarizations persist in the intracerebral hemorrhage swine brain following corpus callosal transection

Spontaneous episodes of spreading depression (SD) originating in multiple sources adjacent to a focal intracerebral hemorrhage (ICH) propagate into brain regions away from the lesion site soon after injury onset. Although these transient depolarizations have not been established in the opposite hemisphere of the swine ICH model, we have reported a diminishing of sensory responsiveness in this homotopic brain region following induction of a unilateral hemorrhage lesion. This study examined whether transient depolarizations exist in this distant brain region contralateral to the ICH site. Electrocorticographic (ECoG) recordings of brain activity were collected bilaterally from the primary somatosensory (SI) cortices of the swine brain prior to and immediately after an intracerebral injection of collagenase or saline or the insertion of the infusion pipette into the SI cortex of the right hemisphere. Transient depolarizations were present in both hemispheres of all the experimental groups. The earliest negative DC potential shifts were observed in the injured SI cortex within the first hour after collagenase injection, as compared to T = 3 h in the saline-injected group and T = 4 h in the infusion pipette only group. In contrast, transient depolarizations were first detected in the left SI cortex contralateral to the lesioned hemisphere within 2 h after collagenase infusion, by T = 4 h after saline infusion and by T = 3 h in the pipette only group. Propagating waves of negative DC potential shifts continued in both brain hemispheres, particularly in the ICH group, throughout the 11-h recording period. This novel finding of recurrent depolarizing waves in the hemisphere contralateral to the injury site prompted us to examine whether corpus callosal connections may play a role in this transhemispheric phenomenon. In a separate group of animals, the corpus callosum was transected prior to acquiring DC potential recordings and collagenase injection. The onset pattern of negative DC shifts in the callosotomized + collagenase-injected group was similar to the collagenase group with an intact corpus callosum. Initial generation of SD in the callosotomized + collagenase-injected group occurred by T = 1 h in the ICH injured right hemisphere and T = 2 h in the contralateral hemisphere. These transient depolarizations also persisted throughout 11-h recording period indicating that the corpus callosal transection did not hinder these remote propagating waves of depolarization. The presence of SD in the SI cortices of both hemispheres in all experimental groups of this study suggests that a focal mechanical or hemorrhagic injury increases the susceptibility of distant ipsilateral and contralateral brain regions to depolarizing perturbations. The mechanism for these transient depolarizations in the contralateral hemisphere apparently does not involve transhemispheric propagation along corpus callosal fibers.

Origins of the somatic N20 and high-frequency oscillations evoked by trigeminal stimulation in the piglets

OBJECTIVE

In humans, the somatic evoked potentials (SEPs) and magnetic fields (SEFs) elicited by peripheral nerve stimulation contain high-frequency oscillations (HFOs) around 600 Hz superimposed on the initial cortical response N20. Responses elicited by snout stimulation in the swine also contain similar HFOs during the rising phase of the porcine N20. This study examined the generators of the N20 and HFOs in the swine.

METHODS

We recorded intracortical SEPs and multi-unit activities in the sulcal area of the primary somatosensory cortex (SI) simultaneously with SEFs. The laminar profiles of the potential and current-source-density (CSD) were analyzed.

RESULTS

The CSD analysis revealed that the N20 was produced by two dipolar generators, both directed toward the cortical surface. After the arrival of the initial thalamocortical volley in layer IV, the sink of the first generator shifted toward shallower layers II-III with a velocity of 0.109+/-0.038 m/s (mean+/-SD). The sink of the second generator moved to layer V. The initial thalamocortical axonal component of the HFO was produced by repolarizing current with the sink in layer IV. The CSD laminar profile of the postsynaptic component was very similar to the profile of intracortical N20. The current sink within each cycle of HFO propagated upward with a velocity of 0.633+/-0.189 m/s, indicating backpropagation.

CONCLUSIONS

We propose that the N20 is generated by two sets of excitatory neurons which also produce the HFOs. Although the loci of synaptic inputs are unknown, these neurons appear to fire initially in the soma and produce backpropagating spikes toward distal apical dendrites.

SIGNIFICANCE

These conclusions relate the N20 to the HFO and provide a new explanation of how the current underlying the N20 is invariantly directed toward superficial layers across species.

Acute changes in cortical excitability in the cortex contralateral to focal intracerebral hemorrhage in the swine

Injury to the cerebral cortex results in functional deficits not only within the vicinity of the lesion but also in remote brain regions sharing neuronal connections with the injured site. To understand the electrophysiological basis of this phenomenon, we evaluated the effects of a focal intracerebral hemorrhage (ICH) on cortical excitability in a remote, functionally connected brain region. Cortical excitability was assessed by measuring the somatic evoked potential (SEP) elicited by electrical stimulation of the swine snout, which is somatotopically represented in the rostrum area of the primary somatosensory (SI) cortex. The SEP was measured on the SI cortex ipsilateral to the site of ICH and on the contralateral SI cortex during the acute period (< or =11 h) after collagenase-induced ICH. The ICH rapidly attenuated the SEP on the ipsilateral cortex as we reported earlier. Interestingly, the ICH also attenuated the SEP on the contralateral SI cortex. Evoked potentials in the contralateral SI cortex showed a gradual decrease in amplitude during this acute period of ICH. We then investigated whether the interhemispheric connections shared by the contralateral SI and the lesion cortex were responsible for the diminished evoked potentials in the uninjured hemisphere after ICH. A separate group of animals underwent corpus callosal transection prior to electrocorticography (ECoG) recordings and ICH injury. Within hours of hemorrhagic injury, a gradual but marked increase in evoked potential amplitude was observed in the homotopic SI cortex of callosotomized animals as compared to pre-injection recordings. The enhancement suggests that there are additional effects of ICH on remote areas functionally connected to the site of injury. Functional deficits were present in both SI cortices within the first several hours of a unilateral injury indicating that the cessation of brain activity in the lesioned SI is mirrored in the contralateral hemisphere. This electrophysiological depression in the uninjured SI cortex is mediated in part by the interhemispheric connections of the corpus callosum.

Identification and functional characterization of the trigeminal ventral cervical reflex pathway in the swine

OBJECTIVE

Trigeminal motor reflexes are clinically useful for diagnosing brain stem lesions. We identified and functionally characterized the neuronal circuit of a new variant of the trigemino-cervical reflex involving the ventral cervical muscles in the swine.

METHODS

The distribution and functional properties of the field potentials of the trigeminal reflex elicited by electrical stimulations of the snout were determined with electrodes in different regions of the brain. The generator of the reflex was determined from scanning evoked magnetic fields over the brainstem. The neuronal circuit was determined with electrodes placed along the trigeminal reflex pathway.

RESULTS

The reflex produced a large positive potential in the brain (up to 200 microV in the primary somatosensory cortex) with a latency of about 14ms. Its amplitude declined rapidly with stimulation rate above 0.5Hz and was abolished by a muscle relaxant. The accompanying magnetic field was produced by a generator ventral to the pons. This generator was found to be the ventral cervical muscles that were activated by a circuit involving the brainstem trigeminal nuclei and cervical motor nuclei.

CONCLUSIONS

It is not known whether this new variant of the trigeminal cervical reflex is species-specific or it also exists in humans. It should be possible to test this possibility in humans, since it produces strong characteristic electrical potentials and magnetic fields in the swine. If it exists in humans, then it may be useful as a clinical tool for testing the integrity of the brainstem and C1 and C2 since the circuit is now identified and characterized in the animal model.

Physiological origins of evoked magnetic fields and extracellular field potentials produced by guinea-pig CA3 hippocampal slices

This study examined whether evoked magnetic fields and intra- and extracellular potentials from longitudinal CA3 slices of guinea-pig can be interpreted within a single theoretical framework that incorporates ligand- and voltage-sensitive conductances in the dendrites and soma of the pyramidal cells. The 1991 CA3 mathematical model of R. D. Traub is modified to take into account the asymmetric branching patterns of the apical and basal dendrites of the pyramidal cells. The revised model accounts for the magnitude and waveform of the bi- and triphasic magnetic fields evoked by somatic and apical stimulations, respectively, in the slice in the absence of fast inhibition (blocked by 0.1 mM picrotoxin). The revised model also accounts for selective effects of 4-aminopyridine (4-AP) and tetraethylammonium (TEA), which block the potassium channels of A and C type, respectively, on the slow wave of the magnetic fields. Furthermore, the model correctly predicts the laminar profiles of field potential as well as intracellular potentials in the pyramidal cells produced by two classes of cells - those directly activated and those indirectly (synaptically) activated by the applied external stimulus. The intracellular potentials in this validated model reveal that the spikes and slow waves of the magnetic fields are generated in or near the soma and apical dendrites, respectively. These results demonstrate that a single theoretical framework couched within the modern concepts of cellular physiology provides a unified account of magnetic fields outside the slice, extracellular potentials within the slice and intracellular potentials of the pyramidal cells for CA3.

Recurring episodes of spreading depression are spontaneously elicited by an intracerebral hemorrhage in the swine

Intracranial bleeding damages the surrounding tissue in a complex fashion that involves contamination by blood-borne products and loss of ionic homeostasis. We used electrophysiological techniques to examine the functional changes in the developing intracerebral bleed and in surrounding regions using an in vivo swine model. Intracerebral hemorrhage (ICH) was induced by collagenase injection into the primary somatosensory cortex (SI). Somatic evoked potential (SEP) elicited by electrical stimulation of the contralateral snout as well as changes in DC-coupled potential were monitored in the SI from the time of collagenase injection in order to measure the effects of ICH. The SEP decreased in amplitude within minutes of the intracerebral injection. Its short-latency component was abolished within the first hour after collagenase injection without any sign of recovery for the duration of the experiment. As the SEP started decreasing in amplitude, we observed spontaneous, recurring episodes of cortical spreading depression (SD) as early as 20 min post-injection. The timing of SDs in SI is consistent with our interpretation that SDs were initially generated at multiple sites adjacent to the lesion core and propagated into the surrounding area. With time, SD became less frequent near the injection site, shifting to more distant electrodes in the surrounding area. Our results indicate that ICH leads to the reduction in SEP amplitude and induces spontaneous episodes of SD. Loss of ionic homeostasis is most likely the physiological basis for the SEP change and for the induction of SD. Recurring SD spontaneously generated in experimental ICH needs further study in humans with ICH.

Roles of calcium- and voltage-sensitive potassium currents in the generation of neuromagnetic signals and field potentials in a CA3 longitudinal slice of the guinea-pig

OBJECTIVES

Roles of calcium- and voltage-sensitive potassium currents in generation of neuromagnetic signals and field potentials were evaluated using the longitudinal CA3 slice preparation of the guinea-pig.

METHODS

Their roles were evaluated by using selective channel blockers (tetraethyl-ammonium (TEA) and 4-aminopyridine (4AP)) and measuring their effects on the two types of signals and intracellular potentials. Fast gamma-aminobutyric acid type A inhibition was blocked with picrotoxin.

RESULTS

Stimulation of the apical dendrites with an array of extracellular bipolar electrodes produced triphasic evoked magnetic fields with a spike and a slow wave typical of the slices. The evoked potentials in the apical and basal areas of the pyramidal cells closely resembled the magnetic field waveforms. Blockade of the potassium currents with TEA and 4AP had only subtle effects on the initial spike, but dramatically altered the slow wave. They also induced long-lasting spontaneous burst discharges synchronized across the slice. The results could be interpreted in terms of their known pre- and postsynaptic effects. Their post-synaptic effects were confirmed with intracellular recordings.

CONCLUSION

Our results are consistent with a hypothesis that the calcium- and voltage-sensitive potassium currents, especially the A and C currents, play important roles in shaping the slow wave of the neuromagnetic and field potential signals produced by the mammalian hippocampus.

Roles of a potassium afterhyperpolarization current in generating neuromagnetic fields and field potentials in longitudinal CA3 slices of the guinea-pig

OBJECTIVES

Roles of a calcium-dependent potassium conductance of slow afterhyperpolarization (AHP) type (gK(AHP)) in generating magnetoencephalographic (MEG) signals were studied in hippocampal longitudinal CA3 slices of the guinea pig.

METHODS

The roles of gK(AHP) were experimentally inferred from effects of its blocker, carbamylcholine-chloride (carbachol, CCh), on MEG signals. The MEG signals were compared with extracellular field potentials and intracellular potentials of the pyramidal cells in the slice.

RESULTS

CCh profoundly affected MEG waveforms. CCh reduced the initial spike of the evoked MEG signals independently of stimulation of the cell layer and apical dendrites. The slow wave of the evoked MEG signals was reduced by the somatic stimulation, but was enhanced by the apical stimulation. Elevated extracellular calcium and bath-applied tetraethylammonium (TEA) enhanced the CCh effects. CCh also increased spontaneous MEG signals. These effects on MEG and field potentials could be interpreted on the basis of synaptic and intracellular effects of CCh.

CONCLUSIONS

Our results indicate that abnormality in this subtle calcium-dependent potassium channel may profoundly influence MEG and EEG signals.

Analysis of MEG signals of spreading cortical depression with propagation constrained to a rectangular cortical strip. I. Lissencephalic rabbit model

Magnetic fields arising from the rabbit cortex during spreading cortical depression (SCD) were measured in order to study the currents in the neocortex during SCD. SCD was constrained to propagate in a rectangular cortical strip perpendicular to the midline. This simplified in vivo cortical preparation enabled us to correlate magnetoencephalographic (MEG) signals to their underlying currents within the cortical strip. The propagation of SCD was monitored with an array of electrodes placed along the strip. The propagation speed for SCD in the lissencephalic rabbit brain was 3. 5+/-0.3 mm/min (mean+/-S.E.M., n=14). Slow, quasi-dc, MEG signals were observed as the SCD entered into the longitudinal fissure. The currents giving rise to the MEG signals were perpendicular to the cortical surface and directed from the surface to deeper layers of the cortex. A distributed dipolar source model was used to relate the data to the underlying cortical current. The moment of the single equivalent current dipole source was 38+/-9 nA-m (n=17). This study clarified the nature of the cortical currents during SCD in a lissencephalic in vivo preparation.

Analysis of MEG signals of spreading cortical depression with propagation constrained to a rectangular cortical strip. II. Gyrencephalic swine model

Currents produced during spreading cortical depression (SCD) in a gyrencephalic species (swine) were studied with magnetoencephalography (MEG) and electrocorticography (ECoG). SCD, initiated using electrical stimulation of the cortex, was constrained to propagate within a rectangular cortical strip in order to simplify the interpretation of the underlying currents. The ECoG signals monitored along the strip revealed that SCD propagated from an initiation site on the gyrus at a rate of 7.9+/-3.2 mm/min (n=23), entered the deep coronal sulcus and in most cases emerged from the other side of the sulcus, continuing to propagate across the next gyrus at a rate of 5.9+/-2.7 mm/min (n=22). The apparent propagation velocity within the sulcus was reduced to 1.7+/-0.8 mm/min (n=21). Strong MEG signals were observed as SCD entered the sulcus. The direction of magnetic field was opposite for SCD's on opposite banks of the sulcus. The currents were directed from a superficial layer to deeper layers of the cortex. The characteristics of SCD and associated MEG patterns from a gyrencephalic species may be similar to those in human patients during migraine aura.

Comparison of MEG and EEG on the basis of somatic evoked responses elicited by stimulation of the snout in the juvenile swine

OBJECTIVE

Some basic characteristics of magnetoencephalographic (MEG) and electroencephalographic (EEG) signals were studied by comparing somatic evoked fields (SEFs) and potentials (SEPs) elicited by electrical stimulations of different areas of the snout in piglets.

METHODS

SEFs were measured with and without an intact skull, whereas SEPs were measured on the skull and cortex (Electrocorticograms - ECoG) and within the cortex of the same animal.

RESULTS

The SEFs above the skull and dura were very similar to each other in temporal waveform and spatial topography, indicating small effects of the skull. They both revealed very similar somatotopic projections of the snout. The SEPs on the skull and cortex were, in contrast, clearly different in their amplitudes as well as temporal and spatial morphologies, indicating significant effects of the skull. However, an early component of the SEP on the skull revealed a somatotopic representation of the snout, indicating that EEG can be also useful for inferring cortical projection areas. Discrepancies in their maps were due to predominance of the potentials produced by currents in the gyral cortex. The projection sites inferred from SEFs were quite accurate in comparison to those inferred from ECoGs and intracortical SEPs.

CONCLUSION

The similarities and differences clearly point out the complementary nature of MEG and EEG.

Experimental analysis of distortion of magnetoencephalography signals by the skull

OBJECTIVES

Magnetoencephalography (MEG) signals are, on theoretical grounds, thought to be relatively undistorted by the skull in contrast to electroencephalographic (EEG) signals. This assumption was experimentally tested in an animal preparation with a brain similar to the human brain in many respects.

METHODS

Possible skull effects on MEG were evaluated directly using an in vivo porcine preparation, by measuring the somatic evoked magnetic field (SEF) above the skull with and without the skull under, otherwise, the same condition.

RESULTS

The SEF was virtually undistorted by the skull with no obvious visible change in its waveform and amplitude under these two conditions. However, there was some small, but significant attenuation when the skull was removed, the distortion being greater for deeper sources.

CONCLUSION

Our results are consistent with a theoretical expectation that the skull should be virtually 'transparent' to the magnetic fields for shallow sources, but less so for fields generated by deeper sources.

Physiological bases of the synchronized population spikes and slow wave of the magnetic field generated by a guinea-pig longitudinal CA3 slice preparation

OBJECTIVE

The physiological bases of evoked magnetic fields were examined in a guinea-pig hippocampal slice preparation, motivated by new concepts in central nervous system (CNS) electrophysiology brought about by discoveries of active conductances in the dendrites and soma of neurons.

METHODS

Their origins were elucidated by comparing them with intracellular and extracellular field potentials.

RESULTS

With excitatory synaptic transmissions blocked, the magnetic signal elicited by an electrical stimulus applied to the pyramidal cell layer consisted of a spike and a depolarizing afterpotential-like waveform. With the excitatory synaptic transmissions intact, but with inhibitory synaptic transmissions blocked, the magnetic signal was bi- or triphasic depending on whether the cell layer or the apical dendrite area of the pyramidal cells was, respectively, depolarized. In both cases the signal consisted of a train of synchronized population spikes superimposed on a brief wave followed by a longer, slow wave. The spike train was correlated with synaptically mediated intracellular spikes. The underlying currents for the slow wave were directed from the apical to the basal side for both types of stimulation. It was most likely generated by depolarization of the apical dendrites, caused by recurrent excitatory synaptic activation.

CONCLUSIONS

This analysis illustrates how synaptic connections and intrinsic conductances in a disinhibited mammalian CNS structure can generate spikes and waves of the magnetic field and electrical potential.

Genesis of MEG signals in a mammalian CNS structure

Neuromagnetic signals of guinea pig hippocampal slices were characterized and compared with the extracellular field potential to elucidate the genesis of magnetoencephalographic signals in a mammalian CNS structure. The spatial distribution of magnetic evoked field (MEF) directed normal to bath surface was similar for transverse CA1, longitudinal CA1 and longitudinal CA3 slices in the presence of 0.1 mM picrotoxin (PTX) which blocks inhibitory synaptic transmission. Their MEFs were produced by currents along the longitudinal axis of the pyramidal cells. Comparisons of the MEF with the laminar potential profile revealed that the MEF was generated by intracellular longitudinal currents. The dipolar component of the intracellular currents was the dominant factor generating the MEF even at a distance of 2 mm from the slice. The MEF from a slice in Ringer's solution without PTX became similar in temporal waveform with time to the MEF in the presence of PTX, indicating the predominance of excitatory connections in generating the MEF and the existence of highly synchronous populations activities across the slice even in PTX-free Ringer's solution. The presence of such highly synchronous population activities underlying the MEFs was verified directly with field potentials recorded across the slice. A systematic variation of the stimulation site revealed a characteristic waveform for each site. The variation of the with stimulation site suggested the contribution of many factors, both synaptic and voltage-sensitive conductances, to the overall waveform of the MEF.

Single-epoch neuromagnetic signals during epileptiform activities in guinea pig longitudinal CA3 slices

Neuromagnetic fields with high signal-to-noise ratios can be measured above longitudinal CA3 slices of the guinea pig during single epochs of interictal- and ictal-like synchronized population activities. Technical refinements enabled us to reduce the number of epochs for clear responses from over 5000 needed in an earlier study to single epochs. Simultaneous recording of field potentials revealed that neuromagnetic fields reflect intracellular currents in the pyramidal cells. Intracellular currents could be estimated from the external magnetic fields during paroxysmal depolarization shifts, multiple bursts, and slowly varying potential shifts lasting several seconds.

Neuronal activities from a deep subcortical structure can be detected magnetically outside the brain in the porcine preparation

We assessed whether subcortical structures can generate magnetic fields detectable outside the brain by first measuring the somatic evoked magnetic fields (SEFs) from a decorticated porcine preparation and then from an intact preparation. Strong SEFs were detected a few millimeters above the corpus callosum after electrical stimulations of the snout. The waveforms consisted of a large spike ( < or = 6 pT) with a peak latency of 11-18 ms depending on the age of the animal, followed by a slow wave. The waveform and latency of the SEF spike were virtually identical to those of the field potential within the brain. The SEF topography indicated that the underlying generator of the spike was in a region contralateral to the stimulation and inferior to the thalamus. The subcortical SEF was strong enough to be detectable even above the intact brain, after the cortically generated SEF was removed by ablation of the primary cortical area. The results indicate that a structure deep in the brain can produce remarkably strong magnetic fields detectable outside the brain.