Autoregressive modeling of the EEG in systemic kainic acid-induced epileptogenesis

Gene replacement therapy for Lafora disease in the Epm2a -/- mouse model

Lafora disease is a rare and fatal form of progressive myoclonic epilepsy typically occurring early in adolescence. Common symptoms include seizures, dementia, and a progressive neurological decline leading to death within 5-15 years from onset. The disease results from mutations transmitted with autosomal recessive inheritance in the EPM2A gene, encoding laforin, a dual-specificity phosphatase, or the EPM2B gene, encoding malin, an E3-ubiquitin ligase. Laforin has glucan phosphatase activity, is an adapter of enzymes involved in glycogen metabolism, is involved in endoplasmic reticulum-stress and protein clearance, and acts as a tumor suppressor protein. Laforin and malin work together in a complex to control glycogen synthesis and prevent the toxicity produced by misfolded proteins via the ubiquitin-proteasome system. Disruptions in either protein can lead to alterations in this complex, leading to the formation of Lafora bodies that contain abnormal, insoluble, and hyperphosphorylated forms of glycogen called polyglucosans. We used the Epm2a -/- knock-out mouse model of Lafora disease to apply a gene replacement therapy by administering intracerebroventricular injections of a recombinant adeno-associated virus carrying the human EPM2A gene. We evaluated the effects of this treatment by means of neuropathological studies, behavioral tests, video-electroencephalography recording, and proteomic/phosphoproteomic analysis. Gene therapy with recombinant adeno-associated virus containing the EPM2A gene ameliorated neurological and histopathological alterations, reduced epileptic activity and neuronal hyperexcitability, and decreased the formation of Lafora bodies. Differential quantitative proteomics and phosphoproteomics revealed beneficial changes in various molecular pathways altered in Lafora disease. Improvements were observed for up to nine months following a single intracerebroventricular injection. In conclusion, gene replacement therapy with human EPM2A gene in the Epm2a -/- knock-out mice shows promise as a potential treatment for Lafora disease.

High-Frequency Oscillations in the Scalp EEG of Intensive Care Unit Patients With Altered Level of Consciousness

PURPOSE

In comatose patients, distinguishing between nonconvulsive status epilepticus and diffuse structural or metabolic encephalopathies is often challenging. Both conditions can generate periodic discharges on EEG with similar morphology and periodicity. We investigated the occurrence of high-frequency oscillations-potential biomarkers of epileptogenesis-on scalp EEG of comatose patients with periodic discharges in the EEG.

METHODS

Fifteen patients were included. Patients were divided into three groups, according to underlying etiology: Group 1, seizure related; group 2, structural; group 3, nonstructural. EEG recordings were compared with respect to the presence and rates of gamma (30-80 Hz) and ripples (80-250 Hz).

RESULTS

Patients were 23 to 106 years old (median, 68 years); 60% were female. 206 channels were eligible for analysis (median, 15 channels/patient). Overall, 43% of channels showed gamma, and 24% had ripples. Group 2 showed the highest proportion of channels with gamma (47%), followed by group 1 (38%) and group 3 (36%). Mean gamma rates were higher in group 2 (4.65 gamma/min/channel) than in group 1 (1.52) and group 3 (1.44) (P < 0.001). Group 2 showed the highest proportion of channels with ripples (29.2%), followed by group 1 (15%) and group 3 (24.2%). Mean ripple rates were higher in group 2 (5.09 ripple/min/channel) than in group 1 (0.96) and group 3 (0.83) (P < 0.001).

CONCLUSIONS

Fast oscillations, including high-frequency oscillations, can be detected in scalp EEG of patients with altered consciousness. High rates of fast activity may suggest an underlying structural brain lesion. Future studies are needed to determine whether fast oscillations in the setting of acute/subacute brain lesions are a biomarker of subsequent development of human epilepsy.

Aberrant Connectivity During Pilocarpine-Induced Status Epilepticus

Status epilepticus (SE) is a common, life-threatening neurological disorder that may lead to permanent brain damage. In rodent models, SE is an acute phase of seizures that could be reproduced by injecting with pilocarpine and then induce chronic temporal lobe epilepsy (TLE) seizures. However, how SE disrupts brain activity, especially communications among brain regions, is still unclear. In this study, we aimed to identify the characteristic abnormalities of network connections among the frontal cortex, hippocampus and thalamus during the SE episodes in a pilocarpine model with functional and effective connectivity measurements. We showed that the coherence connectivity among these regions increased significantly during the SE episodes in almost all frequency bands (except the alpha band) and that the frequency band with enhanced connections was specific to different stages of SE episodes. Moreover, with the effective analysis, we revealed a closed neural circuit of bidirectional effective interactions between the frontal regions and the hippocampus and thalamus in both ictal and post-ictal stages, implying aberrant enhancement of communication across these brain regions during the SE episodes. Furthermore, an effective connection from the hippocampus to the thalamus was detected in the delta band during the pre-ictal stage, which shifted in an inverse direction during the ictal stage in the theta band and in the theta, alpha, beta and low-gamma bands during the post-ictal stage. This specificity of the effective connection between the hippocampus and thalamus illustrated that the hippocampal structure is critical for the initiation of SE discharges, while the thalamus is important for the propagation of SE discharges. Overall, our results demonstrated enhanced interaction among the frontal cortex, hippocampus and thalamus during the SE episodes and suggested the modes of information flow across these structures for the initiation and propagation of SE discharges. These findings may reveal an underlying mechanism of aberrant network communication during pilocarpine-induced SE discharges and deepen our knowledge of TLE seizures.

Effects of Diazepam and Ketamine on Pilocarpine-Induced Status Epilepticus in Mice

Status epilepticus (SE) is a life-threatening condition needing immediate care to prevent brain damage. SE with electrographic and behavioral features similar to those seen in humans is reproduced in rodents by i.p. pilocarpine injection, and can be terminated by diazepam and ketamine treatment but only behaviourally, not electrographically. Little is known on the behavioral and EEG effects induced by a delayed administration of ketamine (25 mg/kg) after diazepam (10 mg/kg) or vice versa. Therefore, we analysed behavior and EEG activity recorded from the mouse hippocampal CA3 region before, during SE and after anticonvulsant treatments. In the first group (n = 4), diazepam was administered one hour before ketamine whereas in the second group (n = 4) ketamine was administered one hour before diazepam. The EEG SE did not disappear after each of the two treatments but progressed within 4 h to a pattern of interictal discharges. However, diazepam administration before ketamine significantly shortened the time of behavioral recovery compared to when ketamine was administered before diazepam (p < 0.05). The two protocols were also associated to distinct EEG changes in gamma and high frequency oscillations. In conclusion, although diazepam and ketamine are not effective in stopping EEG SE, diazepam administration one hour before ketamine shortens behavioral recovery in pilocarpine-treated mice.

Prediction of epilepsy seizure from multi-channel electroencephalogram by effective connectivity analysis using Granger causality and directed transfer function methods

Epilepsy is a chronic disorder, which causes strange perceptions, muscle spasms, sometimes seizures, and loss of awareness, associated with abnormal neuronal activity in the brain. The goal of this study is to investigate how effective connectivity (EC) changes effect on unexpected seizures prediction, as this will authorize the patients to play it safe and avoid risk. We approve the hypothesis that EC variables near seizure change significantly so seizure can be predicted in accordance with this variation. We introduce two time-variant coefficients based on standard deviation of EC on Freiburg EEG dataset by using directed transfer function and Granger causality methods and compare index changes over the course of time in five different frequency bands. Comparison of the multivariate and bivariate analysis of factors is implemented in this investigation. The performance based on the suggested methods shows the seizure occurrence period is approximately 50 min that is expected onset stated in, the maximum value of sensitivity approaching ~ 80%, and 0.33 FP/h is the false prediction rate. The findings revealed that greater accuracy and sensitivity are obtained by the designed system in comparison with the results of other works in the same condition. Even though these results still are not sufficient for clinical applications. Based on the conclusions, it can generally be observed that the greater results by DTF method are in the gamma and beta frequency bands.

Ictal propagation of high frequency activity is recapitulated in interictal recordings: effective connectivity of epileptogenic networks recorded with intracranial EEG

Seizures are increasingly understood to arise from epileptogenic networks across which ictal activity is propagated and sustained. In patients undergoing invasive monitoring for epilepsy surgery, high frequency oscillations have been observed within the seizure onset zone during both ictal and interictal intervals. We hypothesized that the patterns by which high frequency activity is propagated would help elucidate epileptogenic networks and thereby identify network nodes relevant for surgical planning. Intracranial EEG recordings were analyzed with a multivariate autoregressive modeling technique (short-time direct directed transfer function--SdDTF), based on the concept of Granger causality, to estimate the directionality and intensity of propagation of high frequency activity (70-175 Hz) during ictal and interictal recordings. These analyses revealed prominent divergence and convergence of high frequency activity propagation at sites identified by epileptologists as part of the ictal onset zone. In contrast, relatively little propagation of this activity was observed among the other analyzed sites. This pattern was observed in both subdural and depth electrode recordings of patients with focal ictal onset, but not in patients with a widely distributed ictal onset. In patients with focal ictal onsets, the patterns of propagation recorded during pre-ictal (up to 5 min immediately preceding ictal onset) and interictal (more than 24h before and after seizures) intervals were very similar to those recorded during seizures. The ability to characterize epileptogenic networks from interictal recordings could have important clinical implications for epilepsy surgery planning by reducing the need for prolonged invasive monitoring to record spontaneous seizures.

ABNORMAL INTERICTAL GAMMA ACTIVITY MAY MANIFEST A SEIZURE ONSET ZONE IN TEMPORAL LOBE EPILEPSY

Even though recent studies have suggested that seizures do not occur suddenly and that before a seizure there is a period with an increased probability of seizure occurrence, neurophysiological mechanisms of interictal and pre-seizure states are unknown. The ability of mathematical methods to provide much more sensitive tools for the detection of subtle changes in the electrical activity of the brain gives promise that electrophysiological markers of enhanced seizure susceptibility can be found even during interictal periods when EEG of epilepsy patients often looks 'normal'. Previously, we demonstrated in animals that hippocampal and neocortical gamma-band rhythms (30–100 Hz) intensify long before seizures caused by systemic infusion of kainic acid. Other studies in recent years have also drawn attention to the fast activity (>30 Hz) as a possible marker of epileptogenic tissue. The current study quantified gamma-band activity during interictal periods and seizures in intracranial EEG (iEEG) in 5 patients implanted with subdural grids/intracranial electrodes during their pre-surgical evaluation. In all our patients, we found distinctive (abnormal) bursts of gamma activity with a 3 to 100 fold increase in power at gamma frequencies with respect to selected by clinicians, quiescent, artifact-free, 7–20 min "normal" background (interictal) iEEG epochs 1 to 14 hours prior to seizures. Increases in gamma activity were largest in those channels which later displayed the most intensive electrographic seizure discharges. Moreover, location of gamma-band bursts correlated (with high specificity, 96.4% and sensitivity, 83.8%) with seizure onset zone (SOZ) determined by clinicians. Spatial localization of interictal gamma rhythms within SOZ suggests that the persistent presence of abnormally intensified gamma rhythms in the EEG may be an important tool for focus localization and possibly a determinant of epileptogenesis.

Accurate epileptogenic focus localization through time-variant functional connectivity analysis of intracranial electroencephalographic signals

Epilepsy is a neurological disorder characterized by seizures, i.e. abnormal synchronous activity of neurons in the brain. During a focal seizure, the abnormal synchronous activity starts in a specific brain region and rapidly propagates to neighboring regions. Intracranial ElectroEncephaloGraphy (IEEG) is the recording of brain activity at a high temporal resolution through electrodes placed within different brain regions. Intracranial electrodes are used to access structures deep within the brain and to reveal brain activity that cannot be observed with scalp EEG recordings. In order to identify the pattern of propagation across brain areas, a connectivity measure named the Adapted Directed Transfer Function (ADTF) has been developed. This measure reveals connections between different regions by exploiting statistical dependencies within multichannel recordings. The ADTF can be derived from the coefficients of a time-variant multivariate autoregressive (TVAR) model fitted to the data. In this paper the applicability to locate the epileptogenic focus by time-variant connectivity analysis of seizure onsets based on the ADTF is shown. Furthermore, different normalizations of the ADTF (the integrated ADTF, the masked ADTF and the full frequency ADTF) are compared to investigate whether one is more suitable to describe the spreading of epileptic activity during an epileptic seizure. We quantified the performance of different connectivity measures during simulations of an epileptic seizure onset. The full frequency ADTF outperforms the integrated ADTF and masked ADTF. Accordingly, we applied this full frequency ADTF to 4 seizure onset and 29 subclinical seizure IEEG recordings of a patient with refractory epilepsy. Hereby, we showed that connectivity patterns derived from IEEG recordings can provide useful information about seizure propagation and may improve the accuracy of the pre-surgical evaluation in patients with refractory epilepsy.

Potential role of self-induced EEG fast oscillations in predisposition to seizures in meditators

Meditation is a mental exercise practiced widely as an antistress measure and in the belief that it possesses remedial efficacy for a number of medical ailments, especially neurological disorders. Further, there is a general belief that meditation is an absolutely safe practice devoid of any harmful effects. However, with the advent of neuroimaging techniques, the possibility of adverse effects has been raised in recent times. One such issue that has been debated is the potential epileptogenic versus antiepileptic influence exerted by meditation. This brief article attempts to study the potential role of meditation-induced EEG fast oscillations in the predisposition to seizures in meditation practitioners with epilepsy.

Cannabinoid receptor activation reverses kainate-induced synchronized population burst firing in rat hippocampus

Cannabinoids have been shown to possess anticonvulsant properties in whole animal models of epilepsy. The present investigation sought to examine the effects of cannabinoid receptor activation on kainic acid (KA)-induced epileptiform neuronal excitability. Under urethane anesthesia, acute KA treatment (10 mg kg⁻¹, i.p.) entrained the spiking mode of simultaneously recorded neurons from random firing to synchronous bursting (% change in burst rate). Injection of the high-affinity cannabinoid agonist (-)-11-hydroxy-8-tetrahydrocannabinol-dimethyl-heptyl (HU210, 100 μg kg⁻¹, i.p.) following KA markedly reduced the burst frequency (% decrease in burst frequency) and reversed synchronized firing patterns back to baseline levels. Pre-treatment with the central cannabinoid receptor (CB1) antagonist N-piperidino-5-(4-clorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazole-carboxamide (rimonabant, SR141716A 3 mg kg⁻¹, i.p.) completely prevented the actions of HU210. The present results indicate that cannabinoids exert their antiepileptic effects by impeding pathological synchronization of neuronal networks in the hippocampus.

Synchronized gamma oscillations (30-50 Hz) in the amygdalo-hippocampal network in relation with seizure propagation and severity

In this study, we demonstrate that gamma oscillations (30-50 Hz) recorded in the local field potentials (LFP) of the hippocampus are a marker of temporal lobe seizure propagation and that the level of LFP synchrony in the amygdalo-hippocampal network, during these oscillations, is related to the severity of seizures. Sprague-Dawley rats were given a single systemic dose of kainic acid (KA; 6 mg/kg, i.p.) and local field potential activity (1-475 Hz) of the dorsal hippocampus, the amygdala and the neocortex was recorded. Of 135 ictal discharges, 55 (40.7%) involved both limbic structures. We demonstrated that 78.2% of seizures involving both the hippocampus and amygdala showed hippocampal gamma oscillations. Seizure duration was also significantly correlated with the frequency of hippocampal gamma oscillations (r2=0.31, p<0.01) and LFP synchrony in the amygdalo-hippocampal network (r2=0.21, p<0.05). These results suggest that gamma oscillations in the amygdalo-hippocampal network could facilitate long-range synchrony and participate in the propagation of seizures.

On the evaluation of information flow in multivariate systems by the directed transfer function

The directed transfer function (DTF) has been proposed as a measure of information flow between the components of multivariate time series. In this paper, we discuss the interpretation of the DTF and compare it with other measures for directed relationships. In particular, we show that the DTF does not indicate multivariate or bivariate Granger causality, but that it is closely related to the concept of impulse response function and can be viewed as a spectral measure for the total causal influence from one component to another. Furthermore, we investigate the statistical properties of the DTF and establish a simple significance level for testing for the null hypothesis of no information flow.

The use of radiotelemetry to evaluate electrographic seizures in rats with kainate-induced epilepsy

Temporal lobe epilepsy in humans is a chronic condition with a highly variable temporal evolution. Animal models of this disorder have been developed to recapitulate many of the characteristics seen in humans with temporal lobe epilepsy. These animal models generate chronic spontaneous electrographic and motor seizures with a progressive increase in frequency over many months. In order to understand the underlying cellular and molecular mechanisms driving epileptogenesis, a practical means for accurately assessing seizure progression over this extended time period must be devised. In this report, we describe the use of a three-channel radiotelemetry system to record spontaneous electrographic interictal "spikes" and seizure activity from the cortical surface and the two hippocampi. This approach has allowed continuous recording before, during, and several months after kainate-induced status epilepticus. The important advantages of this approach are the potential for long-term continuous electrographic recording with comparatively unrestricted behavior; the disadvantages include increased cost, surgical difficulty and lower frequency-response in the recordings.

Efficient application of Internet databases for new signal processing methods

This paper highlights the ways in which Internet databases may be efficiently used to foster the application of progress in biomedical sciences via data sharing and new algorithms. Employing the Internet to accelerate the pace of interdisciplinary research has significant potential, yet as with all new technologies, the first applications often cause more disappointment than positive outcomes. We discuss examples of solutions to the basic issues: (1) finding the relevant datasets (in portals connected via the Inter-neuro infrastructure), (2) reading the particular format in which the data was stored (using the SignalML language for metadescription of time series), (3) choosing the right method for the data analysis (we provide a brief review of the methods used for the analysis of EEGs, and discuss two of them in detail: Directed Transfer Function and Matching Pursuit), and (4) sharing the software for chosen methods of analysis (via repositories such as the eeg.pl thematic portal).

Granger causality and information flow in multivariate processes

The multivariate versus bivariate measures of Granger causality were considered. Granger causality in the application to multivariate physiological time series has the meaning of the information flow between channels. It was shown by means of simulations and by the example of experimental electroencephalogram signals that bivariate estimates of directionality in case of mutually interdependent channels give erroneous results, therefore multivariate measures such as directed transfer function should be used for determination of the information flow.

Determination of EEG activity propagation: pair-wise versus multichannel estimate

Performance of different estimators describing propagation of electroencephalogram (EEG) activity, namely: Granger causality, directed transfer function (DTF), direct DTF (dDTF), short-time DTF (SDTF), bivariate coherence, and partial directed coherence are compared by means of simulations and on the examples of experimental signals. In particular, the differences between pair-wise and multichannel estimates are studied. The results show unequivocally that in most cases, the pair-wise estimates are incorrect and a complete set of signals involved in a given process has to be used to obtain the correct pattern of EEG flows. Different performance of multivariate estimators of propagation depending on their normalization is discussed. Advantages of multivariate autoregressive model are pointed out.

Measurement of cortical and hippocampal epileptiform activity in freely moving rats by means of implantable radiotelemetry

Implanted radiotelemetry has been used for the measurement of cortical electroencephalogram (EEG), locomotor activity, body temperature and cardiovascular parameters. This technique allows high quality data acquisition from freely moving animals with no complications of externalised apparatus. This paper focuses on the methodology for short and long-term monitoring of epileptiform activity by simultaneous cortical EEG, hippocampal (HC) EEG and electromyogram (EMG) in rats. The circadian rhythm of temperature (CRT) was monitored after surgery to estimate the need for post surgical recovery of animals. Different placements of EMG electrodes were assessed in order to minimise artefacts and increase sensitivity. The occurrence of epileptiform ictal and interictal activity following an acute injection of either 40 mg/kg pentylenetetrazole (PTZ) or 13.8 mg/kg kainic acid (KA) was investigated. The occurrence of spontaneous seizures was also monitored 5-8 weeks after administration of KA. The present study demonstrated a sensitive method for monitoring cortical EEG, hippocampal EEG and EMG short and long-term by implantable radiotelemetry in freely moving rats.

Impulse response function based on multivariate AR model can differentiate focal hemisphere in temporal lobe epilepsy

The purpose of this study is to propose and investigate a new approach for discriminating between focal and non-focal hemispheres in intractable temporal lobe epilepsy, based on applying multivariate time series analysis to the discharge-free background brain activity observed in nocturnal electrocorticogram (ECoG) time series. Five unilateral focal patients and one bilateral focal patient were studied. In order to detect the location of epileptic foci, linear multivariate autoregressive (MAR) models were fitted to the ECoG data; as a new approach for the purpose of summarizing these models in a single relevant parameter, the behavior of the corresponding impulse response functions was studied and described by an attenuation coefficient. In the majority of unilateral focal patients, the averaged attenuation coefficient was found to be almost always significantly larger in the focal hemisphere, as compared to the non-focal hemisphere. Also the amplitude of the fluctuations of the attenuation coefficient was significantly larger in the focal hemisphere. Moreover, in one patient showing a typical regular sleep cycle, the attenuation coefficient in the focal hemisphere tended to be larger during REM sleep and smaller during Non-REM sleep. In the bilateral focal patient, no statistically significant distinction between the hemispheres was found. This study provides encouraging results for new investigations of brain dynamics by multivariate parametric modeling. It opens up the possibility of relating diseases like epilepsy to the properties of inconspicuous background brain dynamics, without the need to record and analyze epileptic seizures or other evidently pathological waveforms.

Mental Tasks Induce Gamma EEG with Reduced Responsiveness in Primary Generalized Epilepsies

PURPOSE

We previously revealed an interictal increase in intensity of EEG rhythms during quiescent mental activity in the 30- to 100-Hz frequency (gamma) range in primary generalized epilepsy (PGE). We have evidence that there is induction of gamma EEG in normal subjects in response to controlled mental activity. Here we test whether mental tasks further augment interictal gamma oscillations in people with PGE.

METHODS

We recorded interictal EEG from patients with PGE and partial epilepsy and compared EEG power spectral responses (increases over resting) during mental tasks.

RESULTS

In partial epilepsy, mental tasks (except for alternating checkerboard visual stimulation) induced 1.5- to 2.5-fold increases in power of gamma EEG. In generalized epilepsy, generalized increases of 1.5-fold in gamma EEG were induced by only two mental tasks (reading and subtraction), and enhancement of 1- to 1.5-fold in the remaining six (checkerboard, expectancy, music, learning, recalling, and a video).

CONCLUSIONS

Gamma EEG is less responsive to mental activation in PGE than in partial epilepsy, confirming an abnormality in gamma mechanisms in PGE. Our findings also provide a possible mechanistic link between mental activity and seizures in reading- and arithmetic-induced seizures.

Fluorocitrate-mediated astroglial dysfunction causes seizures

A role for astroglia in epileptogenesis has been hypothesised but is not established. Low doses of fluorocitrate specifically and reversibly disrupt astroglial metabolism by blocking aconitase, an enzyme integral to the tricarboxylic acid cycle. We used cerebral cortex injections of fluorocitrate, at a dose that we demonstrated to inhibit astroglial metabolism selectively, to determine whether astroglial disturbances lead to seizures. Rats were halothane-anesthetized, and 0.8 nmol of sodium fluorocitrate was injected into the cerebral cortex. Extradural electroencephalogram (EEG) electrodes were implanted, after which the anesthesia was ceased and the animals were observed. In all experiments, 14 of 15 fluorocitrate-treated animals exhibited epileptiform EEG discharges, with some animals exhibiting convulsive seizures. Discharges commenced as early as 30 min postfluorocitrate injection. Intraperitoneal octanol, but not halothane by inhalation, given to test the possible participation of gap junctions in EEG discharge generation, blocked or delayed the occurrence of discharges after fluorocitrate. These results indicate that focal cerebrocortical astroglial dysfunction leads to focal epileptiform discharges and sometimes to convulsive seizures and that the process possibly depends on effects mediated by gap junctions.

Synchronization of kainate-induced epileptic activity via GABAergic inhibition in the superfused rat hippocampus in vivo

We studied cellular mechanisms of synchronization of epileptiform activity induced by kainic acid in a novel preparation of superfused rat hippocampus in vivo. Under urethane anesthesia, kainate induced epileptic population spikes occurring at 30-40 Hz. Pyramidal cells fired exclusively during population spikes with an average probability of 0.34 on rebound of rhythmic GABA(A)-mediated inhibitory postsynaptic events. Excitatory synaptic events contributed little to seizure activity. Rhythmic epileptiform activity was suppressed by blocking GABA(A) receptors and was slowed by barbiturates. Thus, GABAergic inhibition is instrumental in synchronizing kainate-induced epileptiform rhythmic activity in the gamma frequency band in the intact hippocampus in vivo.

Determination of information flow direction among brain structures by a modified directed transfer function (dDTF) method

A modification of directed transfer function-direct DTF-is proposed for the analysis of direct information transfer among brain structures on the basis of local field potentials (LFP). Comparison of results obtained by the analysis of simulated and experimental data with a new dDTF and DTF method is shown. A new measure to estimate direct causal relations between signals is defined. The present results demonstrate the effectiveness of the new dDTF method and indicate that the dDTF method can be used to obtain the reliable patterns of connections between various brain structures.

Persistent abnormality detected in the non-ictal electroencephalogram in primary generalised epilepsy

OBJECTIVES

Gamma oscillations (30-100 Hz gamma electroencephalographic (EEG) activity) correlate with high frequency synchronous rhythmic bursting in assemblies of cerebral neurons participating in aspects of consciousness. Previous studies in a kainic acid animal model of epilepsy revealed increased intensity of gamma rhythms in background EEG preceding epileptiform discharges, leading the authors to test for intensified gamma EEG in humans with epilepsy.

METHODS

64 channel cortical EEG were recorded from 10 people with primary generalised epilepsy, 11 with partial epilepsy, and 20 controls during a quiescent mental state. Using standard methods of EEG analysis the strength of EEG rhythms (fast Fourier transformation) was quantified and the strengths of rhythms in the patient groups compared with with controls by unpaired t test at 1 Hz intervals from 1 Hz to 100 Hz.

RESULTS

In patients with generalised epilepsy, there was a threefold to sevenfold increase in power of gamma EEG between 30 Hz and 100 Hz (p<0.01). Analysis of three unmedicated patients with primary generalised epilepsies revealed an additional 10-fold narrow band increase of power around 35 Hz-40 Hz (p<0.0001). There were no corresponding changes in patients with partial epilepsy.

CONCLUSIONS

Increased gamma EEG is probably a marker of the underlying ion channel or neurotransmitter receptor dysfunction in primary generalised epilepsies and may also be a pathophysiological prerequisite for the development of seizures. The finding provides a new diagnostic approach and also links the pathophysiology of generalised epilepsies to emerging concepts of neuronal correlates of consciousness.

Epileptiform spikes desynchronize and diminish fast (gamma) activity of the brain. An "anti-binding" mechanism?

Fast (20-100 Hz) rhythms of electrical activity of the brain have been suggested to be important for perception and cognition providing a mechanism for temporal binding of neural activities underlying mental representations. Also, fast rhythms often precede epileptiform discharges in patients and some experimental models. Generalized slow (2-3 Hz) spike activity after systemic kainic acid (KA) in the rat has been shown to be preceded by intense gamma activity. A relationship between the intensified gamma rhythms and the subsequent spike activity was studied during kainate-induced acute epileptogenesis. Power, multiple coherence and phase were analyzed at frequencies 1-100 Hz in the EEG recorded from the hippocampal-neocortical structures of the rat. Gamma rhythms, extremely intense and highly coherent at the onset of discharges, were followed by a slow rhythm of epileptiform spikes/sharp waves. During this spike activity and immediately afterwards, the gamma power and coherence were significantly decreased. These data show an antagonism between gamma rhythms and spike activity and ability of the latter to desynchronize and suppress the former. They are supportive to the hypothesis that epileptiform spike activity may result from the extreme activation of the "anti-binding" mechanism controlling temporal binding at high frequencies. It is suggested that when fast activity is abnormally intensified, "over-binding" with global synchrony of gamma rhythms can occur in the neural networks. It may lead to inadequate synaptic modifications. To prevent this process, epileptiform discharge develops as a protective mechanism suppressing fast activity. This proposal has implications for our understanding of temporal binding in the brain and how its excessive activation may precipitate the development of pathological states.

Temporal binding at gamma frequencies in the brain: paving the way to epilepsy?

Fast (beta-gamma band 20-100 Hz) rhythms of electrical activity of the brain have been suggested to play an important role in perception, cognition and consciousness providing temporal binding of neural activities and allowing the formation of mental representations. The recent advances in the concept of temporal binding and their relation to the theory of neural networks (connectionism) are reviewed here as well as some experimental results concerning the intensified gamma rhythms and epilepsy. The hippocampal-neocortical gamma rhythms are extremely intense and hypersynchronous at onset of the epileptiform discharges induced by systemic kainic acid in the rat. Those gamma rhythms are followed by a slow rhythm of epileptiform spikes/sharp waves or spike-wave complexes ('spike-wave' activity). During spike-wave activity, gamma synchronisation is significantly decreased. A novel unifying concept is proposed which relates the associative principle of neural networks to the mechanism of temporal binding at high frequencies. It suggests that for each memory stored in an associative network there is a corresponding quasi-stable state of synchronous oscillation at some frequency within the gamma band. It also suggests that excessive temporal binding ("over-binding") occurs at seizure onset when abnormally intensified and globally synchronous fast activity is often observed. "Over-binding" may cause the undesirable formation of false associations due to inadequate synaptic modifications. To prevent this process, spike-wave discharge develops as an extreme activation of the mechanism capable to desynchronise and eventually suppress fast activity and erase the spurious modes of activity associated with hypersynchronous gamma rhythms. Thus, spike-wave activity is suggested to be the "anti-binding" mechanism. This mechanism is also related to the spikes/sharp waves normally occurring in the brain mostly in sleep. It is qualitatively similar to the "unlearning" mechanism of Crick and Mitchison presumably associated with the PGO spikes of the REM sleep.

Kainic acid induces distinct types of epileptiform discharge with differential involvement of hippocampus and neocortex

Systemic administration of kainic acid (KA), an excitatory amino acid agonist, provides a model of epilepsy due to increased neural excitation. We examined discharges using multi-channel EEG recording and spectral analysis in rats implanted with neocortical and hippocampal electrodes after intravenous infusion of KA (10 mg/kg), until and including the first convulsive seizure. Gamma activity (30-80 Hz) increased in hippocampus from 3-9 min after KA administration. Two types of preconvulsive bilateral rhythmic discharges were observed, both consisting of generalised high voltage sharp waves at low frequencies (<10 Hz) mixed with fast oscillations (<20 Hz): (1) generalised non-convulsive discharges (GNCD) occurred in all animals and (2) spike-wave discharges (SW), predominantly localised in neocortex, occurred in 45% of animals. Convulsive seizure evolved out of a GNCD. Spectral profiles of epileptiform discharges were characterised by an increase in power of low (<10 Hz) and high (beta and gamma range, 20-80 Hz) frequencies which were differently expressed in neocortex and hippocampus. Thus, in this model of convulsive epilepsy caused by increased excitation, there is an early increase in gamma activity, a process that might contribute to synchronisation, and two distinct types of bilateral discharges, hippocampal-neocortical (GNCD) and preferentially neocortical (SW). Neocortical, not hippocampal, changes in EEG power correlated with development of convulsive behaviours.