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

Lack of Habituation in Migraine Patients Based on High-Density EEG Analysis Using the Steady State of Visual Evoked Potential

Migraine is a periodic disorder in which a patient experiences changes in the morphological and functional brain, leading to the abnormal processing of repeated external stimuli in the inter-ictal phase, known as the habituation deficit. This is a significant feature clinically of migraine in both two types with aura or without aura and plays an essential role in studying pathophysiological differences between these two groups. Several studies indicated that the reason for migraine aura is cortical spreading depression (CSD) but did not clarify its impact on migraine without aura and lack of habituation. In this study, 22 migraine patients (MWA, N = 13), (MWoA, N = 9), and healthy controls (HC, N = 19) were the participants. Participants were exposed to the steady state of visual evoked potentials also known as (SSVEP), which are the signals for a natural response to the visual motivation at four Hz or six Hz for 2 s followed by the inter-stimulus interval that varies between 1 and 1.5 s. The order of the temporal frequencies was randomized, and each temporal frequency was shown 100 times. We recorded from 128 customized electrode locations using high-density electroencephalography (HD-EEG) and measured amplitude and habituation for the N1-P1 and P1-N2 from the first to the sixth blocks of 100 sweep features in patients and healthy controls. Using the entropy, a decrease in amplitude and SSVEP N1-P1 habituation between the first and the sixth block appeared in both MWA and MWoA (p = 0.0001, Slope = -0.4643), (p = 0.065, Slope = 0.1483), respectively, compared to HC. For SSVEP P1-N2 between the first and sixth block, it is varied in both MWA (p = 0.0029, Slope = -0.3597) and MWoA (p = 0.027, Slope = 0.2010) compared to HC. Therefore, migraine patients appear amplitude decrease and habituation deficit but with different rates between MWA, and MWoA compared to HCs. Our findings suggest this disparity between MWoA and MWA in the lack of habituation and amplitude decrease in the inter-ictal phase has a close relationship with CSD. In light of the fact that CSD manifests during the inter-ictal phase of migraine with aura, which is when migraine seizures are most likely to occur, multiple researchers have lately reached this conclusion. This investigation led us to the conclusion that CSD during the inter-ictal phase and migraine without aura are associated. In other words, even if previous research has not demonstrated it, CSD is the main contributor to both types of migraine (those with and without aura).

Therapeutic implications of cortical spreading depression models in migraine

Migraine is among the most common and disabling neurological diseases in the world. Cortical spreading depression (CSD) is a wave of near-complete depolarization of neurons and glial cells that slowly propagates along the cortex creating the perception of aura. Evidence suggests that CSD can trigger migraine headache. Experimental models of CSD have been considered highly translational as they recapitulate migraine-related phenomena and have been validated for screening migraine therapeutics. Here we outline the essential components of validated experimental models of CSD and provide a comprehensive review of potential modulators and targets against CSD. We further focus on novel interventions that have been recently shown to suppress CSD susceptibility that may lead to therapeutic targets in migraine.

Spreading depression as a preclinical model of migraine

Spreading depression (SD) is a slowly propagating wave of near-complete depolarization of neurons and glial cells across the cortex. SD is thought to contribute to the underlying pathophysiology of migraine aura, and possibly also an intrinsic brain activity causing migraine headache. Experimental models of SD have recapitulated multiple migraine-related phenomena and are considered highly translational. In this review, we summarize conventional and novel methods to trigger SD, with specific focus on optogenetic methods. We outline physiological triggers that might affect SD susceptibility, review a multitude of physiological, biochemical, and behavioral consequences of SD, and elaborate their relevance to migraine pathophysiology. The possibility of constructing a recurrent episodic or chronic migraine model using SD is also discussed.

Clinical neurophysiology of migraine with aura

BACKGROUND

The purpose of this review is to provide a comprehensive overview of the findings of clinical electrophysiology studies aimed to investigate changes in information processing of migraine with aura patients.

MAIN BODY

Abnormalities in alpha rhythm power and symmetry, the presence of slowing, and increased information flow in a wide range of frequency bands often characterize the spontaneous EEG activity of MA. Higher grand-average cortical response amplitudes, an increased interhemispheric response asymmetry, and lack of amplitude habituation were less consistently demonstrated in response to any kind of sensory stimulation in MA patients. Studies with single-pulse and repetitive transcranial magnetic stimulation (TMS) have reported abnormal cortical responsivity manifesting as greater motor evoked potential (MEP) amplitude, lower threshold for phosphenes production, and paradoxical effects in response to both depressing or enhancing repetitive TMS methodologies. Studies of the trigeminal system in MA are sparse and the few available showed lack of blink reflex habituation and abnormal findings on SFEMG reflecting subclinical, probably inherited, dysfunctions of neuromuscular transmission. The limited studies that were able to investigate patients during the aura revealed suppression of evoked potentials, desynchronization in extrastriate areas and in the temporal lobe, and large variations in direct current potentials with magnetoelectroencephalography. Contrary to what has been observed in the most common forms of migraine, patients with familial hemiplegic migraine show greater habituation in response to visual and trigeminal stimuli, as well as a higher motor threshold and a lower MEP amplitude than healthy subjects.

CONCLUSION

Since most of the electrophysiological abnormalities mentioned above were more frequently present and had a greater amplitude in migraine with aura than in migraine without aura, neurophysiological techniques have been shown to be of great help in the search for the pathophysiological basis of migraine aura.

Cortical Spreading Depression in Migraine

Cortical spreading depression (CSD) is associated with a dramatic failure of brain ion homeostasis as well as efflux of excitatory amino acids from nerve cells and increased energy metabolism. There is strong clinical and experimental evidence to suggest that CSD is involved in the mechanism of migraine. This paper will, based on the experience related to the detection of CSD in humans, discuss pitfalls and possible strategies for detection of CSD in man. Development of reliable methods for detection of CSD in humans will determine the extent to which the large body of experimental findings from animal studies of CSD can be applied to the investigation and treatment of human brain disease. The paper is based on the experience that has been gained from two decades of studies of CSD in relation to clinical neurological diseases.

Critical review evaluating the pig as a model for human nutritional physiology

The present review examines the pig as a model for physiological studies in human subjects related to nutrient sensing, appetite regulation, gut barrier function, intestinal microbiota and nutritional neuroscience. The nutrient-sensing mechanisms regarding acids (sour), carbohydrates (sweet), glutamic acid (umami) and fatty acids are conserved between humans and pigs. In contrast, pigs show limited perception of high-intensity sweeteners and NaCl and sense a wider array of amino acids than humans. Differences on bitter taste may reflect the adaptation to ecosystems. In relation to appetite regulation, plasma concentrations of cholecystokinin and glucagon-like peptide-1 are similar in pigs and humans, while peptide YY in pigs is ten to twenty times higher and ghrelin two to five times lower than in humans. Pigs are an excellent model for human studies for vagal nerve function related to the hormonal regulation of food intake. Similarly, the study of gut barrier functions reveals conserved defence mechanisms between the two species particularly in functional permeability. However, human data are scant for some of the defence systems and nutritional programming. The pig model has been valuable for studying the changes in human microbiota following nutritional interventions. In particular, the use of human flora-associated pigs is a useful model for infants, but the long-term stability of the implanted human microbiota in pigs remains to be investigated. The similarity of the pig and human brain anatomy and development is paradigmatic. Brain explorations and therapies described in pig, when compared with available human data, highlight their value in nutritional neuroscience, particularly regarding functional neuroimaging techniques.

Auditory evoked fields measured noninvasively with small-animal MEG reveal rapid repetition suppression in the guinea pig

In animal models, single-neuron response properties such as stimulus-specific adaptation have been described as possible precursors to mismatch negativity, a human brain response to stimulus change. In the present study, we attempted to bridge the gap between human and animal studies by characterising responses to changes in the frequency of repeated tone series in the anesthetised guinea pig using small-animal magnetoencephalography (MEG). We showed that 1) auditory evoked fields (AEFs) qualitatively similar to those observed in human MEG studies can be detected noninvasively in rodents using small-animal MEG; 2) guinea pig AEF amplitudes reduce rapidly with tone repetition, and this AEF reduction is largely complete by the second tone in a repeated series; and 3) differences between responses to the first (deviant) and later (standard) tones after a frequency transition resemble those previously observed in awake humans using a similar stimulus paradigm.

Pitfalls in the dipolar model for the neocortical EEG sources

For about six decades, primary current sources of the electroencephalogram (EEG) have been assumed dipolar in nature. In this study, we used electrophysiological recordings from anesthetized Wistar rats undergoing repeated whisker deflections to revise the biophysical foundations of the EEG dipolar model. In a first experiment, we performed three-dimensional recordings of extracellular potentials from a large portion of the barrel field to estimate intracortical multipolar moments generated either by single spiking neurons (i.e., pyramidal cells, PC; spiny stellate cells, SS) or by populations of them while experiencing synchronized postsynaptic potentials. As expected, backpropagating spikes along PC dendrites caused dipolar field components larger in the direction perpendicular to the cortical surface (49.7 ± 22.0 nA·mm). In agreement with the fact that SS cells have “close-field” configurations, their dipolar moment at any direction was negligible. Surprisingly, monopolar field components were detectable both at the level of single units (i.e., −11.7 ± 3.4 nA for PC) and at the mesoscopic level of mixed neuronal populations receiving extended synaptic inputs within either a cortical column (−0.44 ± 0.20 μA) or a 2.5-m3-voxel volume (−3.32 ± 1.20 μA). To evaluate the relationship between the macroscopically defined EEG equivalent dipole and the mesoscopic intracortical multipolar moments, we performed concurrent recordings of high-resolution skull EEG and laminar local field potentials. From this second experiment, we estimated the time-varying EEG equivalent dipole for the entire barrel field using either a multiple dipole fitting or a distributed type of EEG inverse solution. We demonstrated that mesoscopic multipolar components are altogether absorbed by any equivalent dipole in both types of inverse solutions. We conclude that the primary current sources of the EEG in the neocortex of rodents are not precisely represented by a single equivalent dipole and that the existence of monopolar components must be also considered at the mesoscopic level.

Slow brain activity (ISA/DC) detected by MEG

Infraslow activity (ISA), direct coupled (DC), and direct current (DC) are the terms used to describe brain activity that occurs in frequencies below 0.1 Hz. Infraslow activity amplitude increase is also associated with epilepsy, traumatic brain injuries, strokes, tumors, and migraines and has been studied since the early 90s at the Henry Ford Hospital MEG Laboratory. We have used a DC-based magnetoencephalography (MEG) system to validate and characterize the ISA from animal models of cortical spreading depression thought to be the underlying mechanism of migraine and other cortical spreading depression-like events seen during ischemia, anoxia, and epilepsy. Magnetoencephalography characterizes these slow shifts easier than electroencephalography because there is no attenuation of these signals by the skull. In the current study, we report on ISA MEG signals of 12 patients with epilepsy in the preictal and postictal states. In the minutes just before the onset of a seizure, large-amplitude ISA MEG waveforms were detected, signaling the onset of the seizure. It is suggested that MEG assessment of ISA, in addition to activity in the conventional frequency band, can at times be useful in the lateralization of epileptic seizures.

Is the cerebral cortex hyperexcitable or hyperresponsive in migraine?

Although migraineurs appear in general to be hypersensitive to external stimuli, they maybe also have increased daytime sleepiness and complain of fatigue. Neurophysiological studies between attacks have shown that for a number of different sensory modalities the migrainous brain is characterised by a lack of habituation of evoked responses. Whether this is due to increased cortical hyperexcitability, possibly due to decreased inhibition, or to an abnormal responsivity of the cortex due a decreased preactivation level remains disputed. Studies using transcranial magnetic stimulation in particular have yielded contradictory results. We will review here the available data on cortical excitability obtained with different methodological approaches in patients over the migraine cycle. We will show that these data congruently indicate that the sensory cortices of migraineurs react excessively to repetitive, but not to single, stimuli and that the controversy above hyper- versus hypo-excitability is merely a semantic misunderstanding. Describing the migrainous brain as 'hyperresponsive' would fit most of the available data. Deciphering the precise cellular and molecular underpinnings of this hyperresponsivity remains a challenge for future research. We propose, as a working hypothesis, that a thalamo-cortical dysrhythmia might be the culprit.

The brain is hyperexcitable in migraine

Migraine is a very common disorder occurring in 20% of women and 6% of men. Central neuronal hyperexcitability is proposed to be the putative basis for the physiological disturbances in migraine. Since there are no consistent structural disturbances in migraine, physiological and psychophysical studies have provided insight into the underlying mechanisms. This is a review of the neurophysiological studies which have provided an insight to migraine pathogenesis supporting the theory of hyperexcitability.

New insights into brain dysfunction in migraine

Headache is one of the most common presenting symptoms to a physician's office. The majority of headaches are in the category known as primary headaches, where there are no structural disturbances. Secondary headaches are uncommon and usually occur in less than 10% of patients. The mechanisms of secondary headaches are usually due to the underlying pathology. These are usually evident on neuroimaging or laboratory testing. This review will focus mainly on mechanisms of primary headache (i.e., migraine and cluster).

Cortical spreading depression and peri-infarct depolarization in acutely injured human cerebral cortex

Electrocorticographic (ECoG) activity was recorded for up to 129 h from 12 acutely brain-injured human patients using six platinum electrodes placed near foci of damaged cortical tissue. The method probes ECoG activity in the immediate vicinity of the injured cortex and in adjacent supposedly healthy tissue. Six out of twelve patients displayed a total of 73 spontaneous episodes of spreading depression of the ECoG. Of the remaining 6 patients 1 displayed an episode of synchronous depression of ECoG during surgery. Using the same electrodes we also measured the slow potential changes (SPC) (0.005-0.05 Hz) to test the hypothesis that the ECoG depressions were identical to Leao's cortical spreading depression (CSD), and to be able to record peri-infarct depolarisations (PIDs) in electrically 'silent' cortical tissue. Changes in the SPC indicate depolarization of brain tissue. For the analysis, the SPCs were enhanced by calculating the time integral of the ECoG signal. Spreading ECoG depressions were accompanied at every single recording site by stereotyped SPCs, which spread across the cortical mantle at 3.3 (0.41-10) mm/min (median, range), i.e. at the same speed of spread as the depression of the ECoG activity. The amplitude of the SPCs was 0.06-3 mV. In 4 out of 6 patients the ECoG recovered spontaneously. In 2 patients we subsequently recorded recurrent SPCs, but without recovery of the initial ECoG background activity until 2-5 h later. This represents the first direct recording of PIDs in acutely injured human brain. Evidence from this and our previous study of 14 brain-injured patients suggests that CSDs in acute brain disorders occur at higher incidence in patients <30 years (83%) than above (33%). CSD was recorded in 4 out of 5 traumatic brain injury patients, and in 2 out of 7 patients with spontaneous haemorrhages. We conclude that the spreading ECoG depressions recorded in patients are identical to CSDs recorded in animal experiments. We furthermore provide direct electrophysiological evidence for the existence of PIDs and hence a penumbra in the human brain. We hypothesize that the depolarization events might contribute to tissue damage in acute disorders in the human brain.

Cortical Hyperexcitability in Migraine Patients Before and After Sodium Valproate Treatment

DC-magnetoencephalography (DC-MEG)waveforms arising during migraine aura were used to determine the effectiveness of prophylactic medication therapy on neuronal hyperexcitability. Nine patients were prescribed valproate (Depakote) for migraine prophylaxis. MEG scans were recorded during visual stimulation before commencing medication and again after 30 days of daily use of valproate. Cortical brain activity was recorded during stimulation with a black-and-white circular checkerboard pattern alternating at 8 Hz and were analyzed with MR-FOCUSS. Large-amplitude DC-MEG signals, imaged to extended areas of occipital cortex, were seen before therapy. After 30 days of prophylactic treatment, reduced DC-MEG shifts in the occipital cortex and reduced incidence of migraine attacks were observed. Using visual stimulation, the authors demonstrated the hyperexcitability of widespread regions throughout occipital cortex in migraine patients, explaining the susceptibility for triggering spreading cortical depression and migraine aura. This study confirms that MEG can noninvasively determine the status of neuronal excitability before and after therapy. This finding may be helpful in determining which prophylactic medications will be most effective in reducing hyperexcitability in particular patients.

Inhomogeneous propagation of cortical spreading depression—detection by electro- and magnetoencephalography in rats

Spreading depression (SD) propagates in cortical regions that are different in their morphological and functional characteristics. We tested whether the propagation pattern of spreading depression was different between parts of the cortex. In six adult rats, we recorded the ECoG by a 4 x 4 electrode array that covered parts of the frontal, parietal cortex and the cingulate cortex. Simultaneously a 16-channel magnetoencephalogram was recorded to characterize the development and direction of intracortical ion movements accompanying this phenomenon. Spreading depression was initiated by occipital application of 0.3 molar KCl solution. Depolarization was observed, at first, at lateral cortical regions and then at medial cortical regions. Thereafter, the propagation velocity increased in medial cortical regions and was faster than in lateral regions. Negative potential shifts were detected by all electrodes, but the depolarization reached a maximum over lateral and caudal cortical regions. The recorded magnetic fields indicated the same orientation of currents underlying these fields, which was perpendicular to the wave front and points away from the depolarization region. Overall, the data indicated that propagation patterns of spreading depression differed between parts of the cortex and, thus, propagation was inhomogeneous. This propagation was accompanied by strong currents parallel to the cortical surface.

Spatio-temporal Imaging of Cortical Desynchronization in Migraine Visual Aura: A Magnetoencephalography Case Study

OBJECTIVE

To determine cortical oscillatory changes involved in migraine visual aura using magnetoencephalography (MEG).

BACKGROUND

Visual aura in the form of scintillating scotoma precedes migraine in many cases. The involvement of cortical spreading depression within striate and extra-striate cortical areas is implicated in the generation of the disturbance, but the details of its progression, the effects on cortical oscillations, and the mechanisms of aura generation are unclear.

METHODS

We used MEG to directly image changes in cortical oscillatory power during an episode of scintillating scotoma in a patient who experiences aura without subsequent migraine headache. Using the synthetic aperture magnetometry method of MEG source imaging, focal changes in cortical oscillatory power were observed over a 20-minute period and visualized in coregistration with the patient's magnetic resonance image.

RESULTS

Alpha band desynchronization in both the left extra-striate and temporal cortex persisted for the duration of reported visual disturbance, terminating abruptly upon disappearance of scintillations. Gamma frequency desynchronization in the left temporal lobe continued for 8 to 10 minutes following the reported end of aura.

CONCLUSIONS

Observations implicate the extra-striate and temporal cortex in migraine visual aura and suggest involvement of alpha desynchronization in generation of phosphenes and gamma desynchronization in sustained inhibition of visual function.

MEG and EEG in epilepsy

Both EEG and magnetoencephalogram (MEG), with a time resolution of 1 ms or less, provide unique neurophysiologic data not obtainable by other neuroimaging techniques. MEG has now emerged as a mature clinical technology. While both EEG and MEG can be performed with more than 100 channels, MEG recordings with 100 to 300 channels are more easily done because of the time needed to apply a large number of EEG electrodes. EEG has the advantage of the long-term video EEG recordings, which facilitates extensive temporal sampling across all periods of the sleep/wake cycle. MEG and EEG seem to complement each other for the detection of interictal epileptiform discharges, because some spikes can be recorded only on MEG but not on EEG and vice versa. Most studies indicate that MEG seems to be more sensitive for neocortical spike sources. Both EEG and MEG source localizations show excellent agreement with invasive electrical recordings, clarify the spatial relationship between the irritative zone and structural lesions, and finally, attribute epileptic activity to lobar subcompartments in temporal lobe and to a lesser extent in extratemporal epilepsies. In temporal lobe epilepsy, EEG and MEG can differentiate between patients with mesial, lateral, and diffuse seizure onsets. MEG selectively detects tangential sources. EEG measures both radial and tangential activity, although the radial components dominate the EEG signals at the scalp. Thus, while EEG provides more comprehensive information, it is more complicated to model due to considerable influences of the shape and conductivity of the volume conductor. Dipole localization techniques favor MEG due to the higher accuracy of MEG source localization compared to EEG when using the standard spherical head shape model. However, if special care is taken to address the above issues and enhance the EEG, the localization accuracy of EEG and MEG actually are comparable, although these surface EEG analytic techniques are not typically approved for clinical use in the United States. MEG dipole analysis is approved for clinical use and thus gives information that otherwise usually requires invasive intracranial EEG monitoring. There are only a few dozen whole head MEG units in operation in the world. While EEG is available in every hospital, specialized EEG laboratories capable of source localization techniques are nearly as scarce as MEG facilities. The combined use of whole-head MEG systems and multichannel EEG in conjunction with advanced source modeling techniques is an area of active development and will allow a better noninvasive characterization of the irritative zone in presurgical epilepsy evaluation. Finally, additional information on epilepsy may be gathered by either MEG or EEG analysis of data beyond the usual bandwidths used in clinical practice, namely by analysis of activity at high frequencies and near-DC activity.

Cortical spreading depression in the feline brain following sustained and transient stimuli studied using diffusion-weighted imaging

Cortical spreading depression (CSD) was induced by transient (10 min) applications of KCl in agar upon the cortical surface of alpha-chloralose anaesthetised cats. Its features were compared with CSD resulting from sustained applications of crystalline KCl through a mapping of the apparent diffusion coefficient (ADC) using diffusion-weighted echo planar imaging (DWI) over a poststimulus period of 60-100 min. Individual CSD events were computationally detected with the aid of Savitzky-Golay smoothing applied to critically sampled data derived from regions of interest (ROIs) made up of 2 x 2 pixel matrices. The latter were consistently placed at three selected sites on the suprasylvian gyrus (SG) and six sites on the marginal gyrus (MG). The CSD events thus detected were then quantitatively characterised for each ROI using the original time series. Both stimuli consistently elicited similar spreading patterns of initial, primary CSD events that propagated over the SG and marginal MG and were restricted to the hemispheres on which the stimuli were applied. There followed secondary events over smaller extents of cortical surface. Sustained stimuli elicited primary and secondary CSD events with similar amplitudes of ADC deflection that were distributed around a single mean. The ADC deflections were also conserved in peak amplitude throughout the course of their propagation. The initial primary event showed a poststimulus latency of 1.1 +/- 0.1 min. Successive secondary events followed at longer, but uniform, time intervals of around 10 min. Primary and secondary CSDs showed significantly different velocities of conduction (3.32 +/- 0.43 mm min(-1) vs. 2.11 +/- 0.21 mm min(-1), respectively; n = 5) across the cerebral hemisphere. In contrast, transient stimuli produced significantly fewer numbers of CSD events (3.8 +/- 0.5 events per animal, n = 5) than did sustained stimuli (7.4 +/- 0.5 events per animal, mean +/- S.E.M., n = 5, P = 0.002). The peak ADC deflection of their primary CSD events declined by approximately 30 % as they propagated from their initiation site to the interhemispheric boundary. The primary CSD event following a transient stimulus showed a latency of 1.4 +/- 0.1 min. It was followed by successive and smaller secondary ADC deflections that were separated by progressively longer time intervals. Conduction velocities of secondary events were similar to those of primary events. Conduction velocities of both primary and secondary events were slower than their counterparts following a sustained stimulus. ADC changes associated with CSD thus persist at times well after stimulus withdrawal and vary markedly with the nature of the initiating stimulus even in brain regions remote from the stimulus site.

Diffusion-weighted MRI used to detect in vivo modulation of cortical spreading depression: comparison of sumatriptan and tonabersat

Spreading cortical depolarization and depression of electroencephalographic activity (SD) may underlie the aura and spreading neurovascular events of migraine. Cortical depolarization may also precipitate the progressive development of cerebral pathology following ischemia. However, data on SD in the human brain are sparse, most likely reflecting the technical difficulties involved in performing such clinical studies. We have previously shown that the transient cerebral water disturbances during SD can be quantitatively investigated in the gyrencephalic brain using repetitive diffusion-weighted magnetic resonance imaging (DWI). To investigate whether DWI could detect modulation of the spatiotemporal properties of SD in vivo, the effects of the antimigraine drug sumatriptan (0.3 mg/kg iv) and the novel anticonvulsant tonabersat (10 mg/kg ip) were evaluated in the cat brain. Supporting previous findings, sumatriptan did not affect the numbers of events (range, 4-8), the duration of SD activity (39.8 +/- 4.4 min, mean +/- SEM), and event velocity (2.2 +/- 0.4 mm min(-1)); tonabersat significantly reduced SD event initiation (range, 0-3) and duration (13.2 +/- 5.0 min) and increased primary event velocity (5.4 +/- 0.7 mm min(-1)). However, both drugs significantly decreased, by >50%, the spatial extent of the first KCl-evoked SD event, and sumatriptan significantly increased event propagation across the suprasylvian sulcus (5.5 +/- 0.6 vs 2.4 +/- 0.4 events in controls). These results demonstrate (1) the feasibility of using DWI to evaluate therapeutic effects on SD, and (2) that sumatriptan may directly modulate the spatial distribution of SD activity in the gyrencephalic brain.

Cortical spreading depression and migraine: new insights from imaging?

The possibility that spreading depression (SD) of cortical activity, a phenomenon observed in all vertebrates, causes the aura of migraine remains an open question in spite of nearly half a century of investigation. SD is also thought to be associated with the progressive neuronal injury observed during cerebral ischaemia. Thus, the ability to detect and investigate SD in humans might prove clinically significant. Animal studies of cortical spreading depression (CSD) have benefited greatly from the advent of relatively non-invasive imaging techniques. The use of these new imaging techniques for clinical studies will accelerate progress in this area of neurobiology.

Pathophysiology of migraine headache

The underlying mechanism of migraine and pain has been unraveled recently with the advent of neuroimaging. In this article mechanism of migraine aura and the pain of migraine are discussed. In addition, interictal studies demonstrating hyperexcitability in migraine are reviewed.

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.

Cortical potential distributions and information processing

The use of sets of spatiotemporal cortical potential distributions (CPDs) as the basis for cognitive information processing results in a very large space of cognitive elements with natural metrics. Results obtained from current source density (CSD) analysis suggest that in the CPD picture, action potentials may make only a relatively minor contribution to the brain's code. In order to establish if two CPDs are close, we consider standard metrics in spaces of continuous functions, and these may be employed to ascertain if two stimuli will be identified as the same. The correspondence between the electrical activity within brain regions, including not only action potentials but all postsynaptic potentials (PSPs), and CPDs is considered. We examine the possibility of using the CSD approach to find potential distributions using the descriptive approach in which precise sets of times are ascribed to the occurrence of action potentials and PSPs. Using metrics in the multidimensional space of paths of collections of point processes, we show that closeness of CPDs is implied by closeness of sets of spike times and PSP times if a certain metric is used but not others. We also set forth a dynamical model consisting of a system of reaction-dif fusion equations for ionic concentrations coupled with nerve membrane potential equations and active transport systems. Making the approximation of a descriptive approach, the correspondence between sets of spike times and PSP times and CPDs is obtained as with the CSD method. However, since it is not possible to ascribe precise times to the occurrence of PSPs and action potentials, the descriptive approach cannot be used to describe the configuration of electrical activity in cortical regions accurately. We also discuss how the CPD framework relates to the binding problem and submillisecond timing.

Haemodynamic correlates of penumbral depolarization following focal cerebral ischaemia

Transient ischaemic depolarizations (IDs) are thought to play a key role in the pathogenesis of focal cerebral ischaemia. Most transient IDs are akin to spreading depression (SD), although a negative DC shift is not observed in half the cases. The other IDs may represent transient anoxic depolarizations. Using cortical DC and blood flow recordings, following middle cerebral artery occlusion in rats, we show here that: (i) these later depolarizations do indeed represent transient anoxic depolarizations; (ii) SD-like IDs, DC and haemodynamic parameters are similar to those of SDs when blood flow remains close to base line and; (iii) when blood flow decreases, the hyperaemia associated with SD-like IDs is largely reduced and there is an increasing proportion of cortical sites which fail to display a DC shift. These data demonstrate the coexistence of two mechanisms of IDs, and yield new information as to the flow-dependence of DC and haemodynamic correlates of SD-like IDs, the pathophysiological significance of which remains to be determined.

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.