Quantitative analysis of depth spiking in relation to seizure foci in patients with temporal lobe epilepsy

Interictal discharges in the human brain are travelling waves arising from an epileptogenic source

While seizure activity may be electrographically widespread, increasing evidence has suggested that ictal discharges may in fact represent travelling waves propagated from a focal seizure source. Interictal epileptiform discharges (IEDs) are an electrographic manifestation of excessive hypersynchronization of cortical activity that occur between seizures and are considered a marker of potentially epileptogenic tissue. The precise relationship between brain regions demonstrating IEDs and those involved in seizure onset, however, remains poorly understood. Here, we hypothesize that IEDs likewise reflect the receipt of travelling waves propagated from the same regions which give rise to seizures. Forty patients from our institution who underwent invasive monitoring for epilepsy, proceeded to surgery and had at least one year of follow-up were included in our study. Interictal epileptiform discharges were detected using custom software, validated by a clinical epileptologist. We show that IEDs reach electrodes in sequences with a consistent temporal ordering, and this ordering matches the timing of receipt of ictal discharges, suggesting that both types of discharges spread as travelling waves. We use a novel approach for localization of ictal discharges, in which time differences of discharge receipt at nearby electrodes are used to compute source location; similar algorithms have been used in acoustics and geophysics. We find that interictal discharges co-localize with ictal discharges. Moreover, interictal discharges tend to localize to the resection territory in patients with good surgical outcome and outside of the resection territory in patients with poor outcome. The seizure source may originate at, and also travel to, spatially distinct IED foci. Our data provide evidence that interictal discharges may represent travelling waves of pathological activity that are similar to their ictal counterparts, and that both ictal and interictal discharges emerge from common epileptogenic brain regions. Our findings have important clinical implications, as they suggest that seizure source localizations may be derived from interictal discharges, which are much more frequent than seizures.

Quantitative spatio-temporal characterization of epileptic spikes using high density EEG: Differences between NREM sleep and REM sleep

In this study, we applied high-density EEG recordings (HD-EEG) to quantitatively characterize the fine-grained spatiotemporal distribution of inter-ictal epileptiform discharges (IEDs) across different sleep stages. We quantified differences in spatial extent and duration of IEDs at the scalp and cortical levels using HD-EEG source-localization, during non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep, in six medication-refractory focal epilepsy patients during epilepsy monitoring unit admission. Statistical analyses were performed at single subject level and group level across different sleep stages for duration and distribution of IEDs. Tests were corrected for multiple comparisons across all channels and time points. Compared to NREM sleep, IEDs during REM sleep were of significantly shorter duration and spatially more restricted. Compared to NREM sleep, IEDs location in REM sleep also showed a higher concordance with electrographic ictal onset zone from scalp EEG recording. This study supports the localizing value of REM IEDs over NREM IEDs and suggests that HD-EEG may be of clinical utility in epilepsy surgery work-up.

Interictal spike connectivity in human epileptic neocortex

OBJECTIVE

Interictal spikes are a biomarker of epilepsy, yet their precise roles are poorly understood. Using long-term neocortical recordings from epileptic patients, we investigated the spatial-temporal propagation patterns of interictal spiking.

METHODS

Interictal spikes were detected in 10 epileptic patients. Short time direct directed transfer function was used to map the spatial-temporal patterns of interictal spike onset and propagation across different cortical topographies.

RESULTS

Each patient had unique interictal spike propagation pattern that was highly consistent across times, regardless of the frequency band. High spiking brain regions were often not spike onset regions. We observed frequent spike propagations to shorter distances and that the central sulcus forms a strong barrier to spike propagation. Spike onset and seizure onset seemed to be distinct networks in most cases.

CONCLUSIONS

Patients in epilepsy have distinct and unique network of causal propagation pattern which are very consistent revealing the underlying epileptic network. Although spike are epileptic biomarkers, spike origin and seizure onset seems to be distinct in most cases.

SIGNIFICANCE

Understanding patterns of interictal spike propagation could lead to the identification patient-specific epileptic networks amenable to surgical or other treatments.

Effect of subthalamic nucleus stimulation on penicillin induced focal motor seizures in primate

BACKGROUND

Drug-resistant motor epilepsies are particularly incapacitating for the patients. In a primate model of focal motor seizures induced by intracortical injection of penicillin, we recently showed that seizures propagated from the motor cortex towards the basal ganglia.

OBJECTIVE

Using the same animal model here, we hypothesized that disruption of subthalamic nucleus (STN) activity by chronic high frequency stimulation (HFS) could modify pathological excessive cortical synchronisation occurring during focal motor seizures, and therefore could reduce seizure activity.

METHODS

Two monkeys were chronically implanted with one electrode positioned into the STN. In each experiment, seizures were induced during 6 hours by injecting penicillin into the motor cortex. During stimulation sessions, HFS-STN was applied at the beginning of penicillin injection.

RESULTS

Our results indicate that HFS-STN improved focal motor seizures by delaying the occurrence of the first seizure, by decreasing the number of seizures by 47% and therefore the total time spent seizing by 53% compared to control. These results argue for a therapeutic use of HFS-STN in motor seizures because they were obtained in a very severe primate model of motor status similar to that seen in human. Furthermore, HFS-STN was much more efficient than direct cortical HFS of the epileptic focus, which we already tested in the same primate model.

CONCLUSIONS

The present study suggests that HFS-STN could be used as an experimental therapy when other therapeutic strategies are not possible or have failed in humans suffering from motor epilepsy but the present study still warrants controlled studies in humans.

A novel spatiotemporal analysis of peri-ictal spiking to probe the relation of spikes and seizures in epilepsy

The relation between epileptic spikes and seizures is an important but still unresolved question in epilepsy research. Preclinical and clinical studies have produced inconclusive results on the causality or even on the existence of such a relation. We set to investigate this relation taking in consideration seizure severity and spatial extent of spike rate. We developed a novel automated spike detection algorithm based on morphological filtering techniques and then tested the hypothesis that there is a pre-ictal increase and post-ictal decrease of the spatial extent of spike rate. Peri-ictal (around seizures) spikes were detected from intracranial EEG recordings in 5 patients with temporal lobe epilepsy. The 94 recorded seizures were classified into two classes, based on the percentage of brain sites having higher or lower rate of spikes in the pre-ictal compared to post-ictal periods, with a classification accuracy of 87.4%. This seizure classification showed that seizures with increased pre-ictal spike rate and spatial extent compared to the post-ictal period were mostly (83%) clinical seizures, whereas no such statistically significant (α = 0.05) increase was observed peri-ictally in 93% of sub-clinical seizures. These consistent across patients results show the existence of a causal relation between spikes and clinical seizures, and imply resetting of the preceding spiking process by clinical seizures.

Electrical, molecular and behavioral effects of interictal spiking in the rat

OBJECTIVE

Epilepsy is a disease characterized by chronic seizures, but is associated with significant comorbidities between seizures including cognitive impairments, hyperactivity, and depression. To study this interictal state, we characterized the electrical, molecular, and behavior effects of chronic, neocortical interictal spiking in rats.

METHODS

A single injection of tetanus toxin into somatosensory cortex generated chronic interictal spiking measured by long-term video EEG monitoring and was correlated with motor activity. The cortical pattern of biomarker activation and the effects of blocking MAPK signaling on interictal spiking and behavior were determined.

RESULTS

Interictal spiking in this model increases in frequency, size, and becomes repetitive over time, but is rarely associated with seizures. Interictal spiking was sufficient to produce the same molecular and cellular pattern of layer 2/3-specific CREB activation and plasticity gene induction as is seen in the human interictal state. Increasing spike frequency was associated with hyperactivity, demonstrated by increased ambulatory activity and preferential circling toward the spiking hemisphere. Loud noises induced epileptic discharges, identical to spontaneous discharges. Treatment with a selective MAPK inhibitor prevented layer 2/3 CREB activation, reduced the frequency of epileptic discharges, and normalized behavioral abnormalities, but had no effect on seizures induced by electrical kindling.

INTERPRETATION

These results provide insights into the development of interictal epileptic spiking, their relationship to behavior, and suggest that interictal and ictal activities utilize distinct molecular pathways. This model, that parallels recent observations in humans, will be useful to develop therapeutics against interictal spiking and its behavioral comorbidities.

Heterogeneous neuronal firing patterns during interictal epileptiform discharges in the human cortex

Epileptic cortex is characterized by paroxysmal electrical discharges. Analysis of these interictal discharges typically manifests as spike-wave complexes on electroencephalography, and plays a critical role in diagnosing and treating epilepsy. Despite their fundamental importance, little is known about the neurophysiological mechanisms generating these events in human focal epilepsy. Using three different systems of microelectrodes, we recorded local field potentials and single-unit action potentials during interictal discharges in patients with medically intractable focal epilepsy undergoing diagnostic workup for localization of seizure foci. We studied 336 single units in 20 patients. Ten different cortical areas and the hippocampus, including regions both inside and outside the seizure focus, were sampled. In three of these patients, high density microelectrode arrays simultaneously recorded between 43 and 166 single units from a small (4 mm x 4 mm) patch of cortex. We examined how the firing rates of individual neurons changed during interictal discharges by determining whether the firing rate during the event was the same, above or below a median baseline firing rate estimated from interictal discharge-free periods (Kruskal-Wallis one-way analysis, P<0.05). Only 48% of the recorded units showed such a modulation in firing rate within 500 ms of the discharge. Units modulated during the discharge exhibited significantly higher baseline firing and bursting rates than unmodulated units. As expected, many units (27% of the modulated population) showed an increase in firing rate during the fast segment of the discharge (+ or - 35 ms from the peak of the discharge), while 50% showed a decrease during the slow wave. Notably, in direct contrast to predictions based on models of a pure paroxysmal depolarizing shift, 7.7% of modulated units recorded in or near the seizure focus showed a decrease in activity well ahead (0-300 ms) of the discharge onset, while 12.2% of units increased in activity in this period. No such pre-discharge changes were seen in regions well outside the seizure focus. In many recordings there was also a decrease in broadband field potential activity during this same pre-discharge period. The different patterns of interictal discharge-modulated firing were classified into more than 15 different categories. This heterogeneity in single unit activity was present within small cortical regions as well as inside and outside the seizure onset zone, suggesting that interictal epileptiform activity in patients with epilepsy is not a simple paroxysm of hypersynchronous excitatory activity, but rather represents an interplay of multiple distinct neuronal types within complex neuronal networks.

An animal model to study the clinical significance of interictal spiking

Interictal spikes (IIS) are paroxysmal discharges commonly observed in patients with epilepsy which represent an abnormally-synchronized population of hyperexcitable neurons firing as an aggregate. Due to conflicting studies on the clinical significance of IIS, research focusing on IIS has been sparse. However, recent attention on IIS has increased for patients undergoing surgery for intractable epilepsy as a means to identify epileptic foci for surgical resection. There is growing evidence that IIS are not asymptomatic as has been commonly accepted. Other than epilepsy, IIS have been associated with a wide range of behavioral and psychiatric disorders, including attention deficit disorder, anxiety disorders and psychoses. For these reasons, a well-characterized animal model of interictal spiking which accurately mimics the human phenomenon would be a valuable tool to gain, insights both into the pathophysiology of epilepsy as well as a broad variety of human neuropsychiatric diseases. Here, we review the literature on the clinical significance of IIS in humans and on animal models where IIS has been observed. We then demonstrate the utility of using tetanus toxin to generate a reproducible pattem of progressive IIS for future studies into their clinical significance.

Interictal EEG spikes identify the region of electrographic seizure onset in some, but not all, pediatric epilepsy patients

PURPOSE

The role of sharps and spikes, interictal epileptiform discharges (IEDs), in guiding epilepsy surgery in children remains controversial, particularly with intracranial electroencephalography (IEEG). Although ictal recording is the mainstay of localizing epileptic networks for surgical resection, current practice dictates removing regions generating frequent IEDs if they are near the ictal onset zone. Indeed, past studies suggest an inconsistent relationship between IED and seizure-onset location, although these studies were based upon relatively short EEG epochs.

METHODS

We employ a previously validated, computerized spike detector to measure and localize IED activity over prolonged, representative segments of IEEG recorded from 19 children with intractable, mostly extratemporal lobe epilepsy. Approximately 8 h of IEEG, randomly selected 30-min segments of continuous interictal IEEG per patient, were analyzed over all intracranial electrode contacts.

RESULTS

When spike frequency was averaged over the 16-time segments, electrodes with the highest mean spike frequency were found to be within the seizure-onset region in 11 of 19 patients. There was significant variability between individual 30-min segments in these patients, indicating that large statistical samples of interictal activity were required for improved localization. Low-voltage fast EEG at seizure onset was the only clinical factor predicting IED localization to the seizure-onset region.

CONCLUSIONS

Our data suggest that automated IED detection over multiple representative samples of IEEG may be of utility in planning epilepsy surgery for children with intractable epilepsy. Further research is required to better determine which patients may benefit from this technique a priori.

Interictal spikes on intracranial recording: behavior, physiology, and implications

PURPOSE

The physiological, pathological, and clinical meaning of interictal spikes (IISs) remains controversial. We systematically analyzed the frequency, occurrence, and distribution of IISs recorded from multiple intracranial electrodes in 34 refractory epileptic patients with respect to seizures and antiepileptic drug (AED) changes.

METHODS

Continuous spike counts from all recorded contacts of all implanted electrodes, and also separately for the subset of contacts involved at seizure onset, were tabulated for every hour of every day of recording, and expressed as spikes per hour in six preselected, 24-h intervals (defined to exclude seizures): (1) on medications; (2) prepreseizure; (3) preseizure; (4) postseizure; (5) off meds; and (6) resumed meds. Mean spike rates were analyzed for differences between designated 24-h intervals.

RESULTS

Spike rate in all recorded contacts consistently and significantly decreased after AED withdrawal, despite variability in initial spike rate, diurnal occurrence, seizure character/number/localization of onset, and type(s) of AED continued or withdrawn (p < 0.0001). A significant increase in spike rate was noted in the 24 h after seizures of medial temporal origin, in the medial temporal lobe contacts; neocortical onset seizures did not show any increase.

CONCLUSIONS

These observations confirm and extend previous reports, suggesting a general effect of AED withdrawal, and a more specific effect of medial temporal lobe seizures, on IIS rate. AED mechanisms and efficacy might be demonstrated by quantifying IIS with changes in AEDs. Furthermore, variability in IIS rate after seizures distinguishes localization of seizure onset in medial temporal versus neocortical locations.

A method to identify reproducible subsets of co-activated structures during interictal spikes. Application to intracerebral EEG in temporal lobe epilepsy

OBJECTIVE

We present a novel quantitative method to statistically analyze the distribution of multichannel intracerebral interictal spikes (multi-IIS) in stereoelectroencephalographic (SEEG) recordings. The method automatically extracts groups of brain structures conjointly and frequently involved in the generation of interictal activity. These groups are referred to as 'subsets of co-activated structures' (SCAS). We applied the method to long duration interictal recordings in patients with mesial temporal lobe epilepsy (MTLE) and analyzed the reproducibility of subsets of structures involved in the generation of multi-IIS for each patient and among patients.

METHODS

Fifteen patients underwent long-term intracerebral EEG recording (SEEG technique) using depth electrodes. A 1 h period of continuous interictal EEG recording was selected for each patient with precautions regarding the time after anesthesia pre-SEEG, the temporal distance with respect to seizures, the vigilance state of the patient, and the anti-epileptic drug withdrawal. A research of SCAS was conducted on each recording using the developed method that includes 3 steps: (i) automatic detection of monochannel intracerebral interictal spikes (mono-IIS), (ii) formation of multi-IIS using a temporal sliding window, and (iii) extraction of SCAS. In the third step, statistical tests are used to evaluate the frequency of multi-IIS as well as their significance (with respect to the 'random distribution of mono-IIS' case).

RESULTS

In each patient, several thousands of multi-IIS (mean+/-SD, 3322+/-2190) were formed and several SCAS (mean+/-SD, 3.80+/-1.47) were automatically extracted. Results show that reproducible subsets of brain structures are involved in the generation of interictal activity. Although SCAS were found to be variable from one patient to another, some invariant information was pointed up. In all patients, multi-IIS distribute over two distinct groups of structures: mesial structures (15/15) and lateral structures (7/15). Moreover, two particular structures, the internal temporal pole and the temporo-basal cortex, may be conjointly involved with either the first or the second group. Finally, some extracted SCAS seem to match well-defined anatomo-functional circuits of the temporal lobe.

CONCLUSIONS AND SIGNIFICANCE

During interictal activity in MTLE, similar subsets of temporal lobe structures are involved in the generation of spikes. This paper brings statistical evidence for the existence of these subsets and presents a method to automatically extract them from SEEG recordings. Interictal activity is spatially organized in the temporal lobe and preferentially involves two functional systems of the temporal lobe (either mesial or lateral).

Toward a neurodynamical understanding of ictogenesis

Although considerable information on cellular and network mechanisms of epilepsy exists, it is still not understood why, how, and when the transition from interictal to ictal state takes place. The authors review their work on nonlinear EEG analysis and provide consistent evidences that dynamical changes in the neural activity allows the characterization of a preictal state several minutes before seizure onset. This new neurodynamical approach of ictogenesis opens new perspectives for studying the basic mechanisms in epilepsy as well as for possible therapeutic interventions.

Epileptic seizure prediction and control

Epileptic seizures are manifestations of epilepsy, a serious brain dynamical disorder second only to strokes. Of the world's approximately 50 million people with epilepsy, fully 1/3 have seizures that are not controlled by anti-convulsant medication. The field of seizure prediction, in which engineering technologies are used to decode brain signals and search for precursors of impending epileptic seizures, holds great promise to elucidate the dynamical mechanisms underlying the disorder, as well as to enable implantable devices to intervene in time to treat epilepsy. There is currently an explosion of interest in this field in academic centers and medical industry with clinical trials underway to test potential prediction and intervention methodology and devices for Food and Drug Administration (FDA) approval. This invited paper presents an overview of the application of signal processing methodologies based upon the theory of nonlinear dynamics to the problem of seizure prediction. Broader application of these developments to a variety of systems requiring monitoring, forecasting and control is a natural outgrowth of this field.

Single neuron burst firing in the human hippocampus during sleep

Although there are numerous non-primate studies of the single neuron correlates of sleep-related hippocampal EEG patterns, very limited hippocampal neuronal data are available for correlation with human sleep. We recorded human hippocampal single neuron activity in subjects implanted with depth electrodes required for medical diagnosis and quantitatively evaluated discharge activity from each neuron during episodes of wakefulness (Aw), combined stage 3 and 4 slow-wave sleep (SWS), and rapid eye movement (REM) sleep. The mean firing rate of the population of single neurons was significantly higher during SWS and Aw compared with REM sleep (p = 0.002; p < 0.0001). In addition, burst firing was significantly greater during SWS compared with Aw (p = 0.001) and REM sleep (p < 0.0001). The synchronized state of SWS and associated high-frequency burst discharge found in human hippocampus may subserve functions similar to those reported in non-primate hippocampus that require burst firing to induce synaptic modifications in hippocampal circuitry and in hippocampal projections to neocortical targets that participate in memory consolidation.

Interictal epileptiform discharges do not change before seizures during sleep

PURPOSE

Whether interictal epileptiform discharges (IEDs) increase, decrease, or are unchanged before epileptic seizures has implications for the pathophysiology of epilepsy. Prior studies relating IEDs and seizures have not demonstrated a change in IEDs before seizures. However, they have not controlled for changes in the depth of sleep. Our objective was to test the hypothesis that IEDs are related to seizures during sleep while adjusting for log delta power (LDP), a continuous measure of sleep depth.

METHODS

Twenty-two seizures during sleep were identified in 16 subjects with epilepsy admitted for presurgical monitoring. The IEDs that occurred in the hour of sleep before each seizure were used to test the relation between IEDs and seizure occurrence. Sleep depth was measured by LDP (quantity of 1- to 4-Hz activity in 30-s epochs), and records were scored visually for sleep staging and for IEDs. Multivariate logistic regression analyses were applied.

RESULTS

Adjusting for LDP, number of seizures before the current seizure, quartile of the night, and total number of IEDs that occurred during the night, IED did not increase or decrease before seizures (p > 0.1). The rate of IEDs increased directly with LDP (p=0.0001), as shown in prior work.

CONCLUSIONS

IEDs are not activated or suppressed before seizures during sleep, suggesting that different pathophysiologic processes underlie these two phenomena. These results corroborate prior studies, while providing a more advanced analysis by adjusting for sleep depth and applying multivariate logistic regression analyses.

Characterizing neurodynamic changes before seizures

The study of dynamic changes in neural activity preceding epileptic seizure allows the characterization of a preictal state several minutes before seizure onset. This opens up new perspectives for studying the mechanisms of epileptogenesis as well as for possible therapeutic interventions, which represent a major breakthrough. In this review the authors present and discuss the results from their group in this domain using nonlinear analysis of brain signals, as well as the limitations of this topic and current questions.

Temporal epileptogenesis: localizing value of scalp and subdural interictal and ictal EEG data

PURPOSE

To determine the value of scalp epileptiform EEG data and subdural interictal spikes in localizing temporal epileptogenesis among patients requiring invasive recordings. For this delineation, we related such factors to site of subdural seizure origin in 27 consecutive patients.

METHODS

Patients with temporal lobe epilepsy whose non-invasive lateralizing data were inconclusive and therefore required subdural electroencephalography were studied. All patients had (a) 24-h scalp telemetered EEGs, (b) adequate bitemporal subdural placements with an inferomesial line extending from a posterior burr hole anteriorly to <2.5 cm from anterior uncus and a lateral line reaching within 2.5 cm of the temporal tip, and (c) > or =2 subdurally recorded seizures.

RESULTS

Three hundred one (96%) of 314 subdurally recorded clinical seizures involving all 27 patients arose from a discrete focus; 266 (85%) arose from mesial temporal regions, which was the origin of the majority of seizures in 24 (89%) patients. The majority of subdural seizures arose ipsilateral to the majority of scalp EEG spikes in 22 (81%) of 27, and most subdural seizures of 15 (75%) of 20 arose ipsilateral to scalp seizures. Lateralization of interictal subdural spikes correlated with that of subdural seizures in 74-92% of patients, depending on the method of spike compilation: for example, most subdural seizures arose from the same lobe of most consistent principal temporal spikes in 92% of patients. These indices of epileptogenesis also appeared more commonly on the side of effective (> or =90% improvement) temporal lobectomy than contralaterally in the following proportions: most consistent principal subdural spikes, 86% of patients ipsilateral vs. 9% contralateral; scalp-recorded clinical seizures, 55% vs. 18%; scalp EEG spikes, 45% vs. 9%.

CONCLUSIONS

Even among patients whose scalp data are sufficiently complex to require invasive recording for clarification, lateralization of temporal scalp interictal and ictal epileptiform activity and subdural interictal spikes should be included when assessing the side of temporal epileptogenesis.

Interictal spikes in focal epileptogenesis

Interictal electroencephalography (EEG) potentials in focal epilepsies are sustained by synchronous paroxysmal membrane depolarization generated by assemblies of hyperexcitable neurons. It is currently believed that interictal spiking sets a condition that preludes to the onset of an ictal discharge. Such an assumption is based on little experimental evidence. Human pre-surgical studies and recordings in chronic and acute models of focal epilepsy showed that: (i) interictal spikes (IS) and ictal discharges are generated by different populations of neuron through different cellular and network mechanisms; (ii) the cortical region that generates IS (irritative area) does not coincide with the ictal-onset area; (iii) IS frequency does not increase before a seizure and is enhanced just after an ictal event; (iv) spike suppression is found to herald ictal discharges; and (v) enhancement of interictal spiking suppresses ictal events. Several experimental evidences indicate that the highly synchronous cellular discharge associated with an IS is generated by a multitude of mechanisms involving synaptic and non-synaptic communication between neurons. The synchronized neuronal discharge associated with a single IS induces and is followed by a profound and prolonged refractory period sustained by inhibitory potentials and by activity-dependent changes in the ionic composition of the extracellular space. Post-spike depression may be responsible for pacing interictal spiking periodicity commonly observed in both animal models and human focal epilepsies. It is proposed that the strong after-inhibition produced by IS protects against the occurrence of ictal discharges by maintaining a low level of excitation in a general condition of hyperexcitability determined by the primary epileptogenic dysfunction.

Multifold features determine linear equation for automatic spike detection applying neural nin interictal ECoG

OBJECTIVE

A 3-layer detection procedure was designed including preselection applying TEMPLAS software, feature extraction and artificial neural networks to determine a fast, precise and highly selective spike algorithm.

METHODS

Ten intraoperative ECoG recordings of patients with temporal lobe epilepsy were computer-assisted and evaluated by 3 experts upon preselected events. For each event, 19 features were extracted, normalized and fed into a two-layer and 3-layer feedforward, back-propagate network. The weights of the 5 best individual two-layer networks of patients were averaged separately to derive a mean network, where weights were pruned, rounded off and the configuration approximated by a linear equation.

RESULTS

In addition. when investigating latency histograms, a method for multi-channel artefact detection and elimination of too close intra-channel events could be found. Out of several training trails only the mean network and the linear equation were able to generalize. In comparison with the results of 19 publications, the developed solution and the estimated overall detection rates (spikes: 81%; non-spikes: 99.3%) were found to be of high quality. The processing time is short, and therefore, the method can be used to initiate other measurements.

CONCLUSION

The developed solution is a fast, precise and highly selective spike detection method.

Clinical challenges in invasive monitoring in epilepsy surgery

Invasive monitoring aids in selecting patients for epilepsy surgery. This article reviews the methods employed to obtain intracranial EEG, data interpretation, and problems that arise during intracranial investigations. The relative merits of depth, subdural, epidural, and foramen ovale electrodes are reviewed, and a strategy for their use is suggested. Characteristic interictal and ictal EEG findings are summarized, and the problems encountered in interpreting data are discussed.

Regional "rigidity" of background EEG activity in the epileptogenic zone

Spectral analysis of interictal background EEG activity recorded through foramen ovale (FO) electrodes during monitoring (mean 5.8 days per patient) was studied in 10 medically refractory complex partial seizure (CPS) patients who were candidates for epilepsy surgery. Data of the spectral analysis was plotted as compressed spectral array (CSA) with spectral edge frequency (SEF) markers. For each patient, time-varying fluctuations of the SEF markers were compared visually and by a computer-assisted method between two symmetrical FO electrode EEG channels recording from both mesiobasotemporal lobes (MTL). The amount of asymmetrical variations of the SEF markers ("rigidity" phenomenon) was first determined visually and then quantified by the computer-assisted method. These findings were correlated with the results of other clinical tests, including FO electrode-recorded seizure onset (FO ict), positron emission tomography with [18F]fluorodeoxyglucose (FDG-PET), and magnetic resonance imaging (MRI) to investigate whether the rigidity phenomenon could lateralize the primary epileptogenic zone. The rigid side had 80, 70, and 60% coincidence rates with the pathologic side indicated by FDG-PET, FO ict, and MRI, respectively, in a single test. We conclude that the rigidity phenomenon of FO electrode-recorded interictal background EEG activity is a valuable sign for lateralization of the primary epileptogenic zone in MTL epilepsy. The relative invariance of SEF may be caused by interictal deafferentation of epileptic neurons.

Suppression of interictal bursting in hippocampus unleashes seizures in entorhinal cortex: a proepileptic effect of lowering [K+]o and raising [Ca2+]o

The relation between interictal bursts (IIBs) and seizures in epilepsy is obscure. Results from some human and animal studies suggest that IIBs may actually suppress seizure activity. This appears particularly true in the zero magnesium in vitro seizure model. Here we provide new evidence in support of this and new insight into the mechanisms of seizure suppression in this model. Brain slices containing hippocampus and entorhinal cortex were bathed in zero magnesium medium. Electrographic seizures appeared, then were replaced by IIBs. Upon lowering [K+]o and raising [Ca2+]o the IIBs disappeared and the seizures reappeared. Repeated stimuli mimicking IIBs then suppressed seizures again. Selective knife cuts revealed that the IIBs originated in the hippocampus (area CA3) whereas the seizures originated in entorhinal cortex. These results confirm that IIBs suppress seizures in the zero magnesium model. They also show that an important aspect of the interaction between IIBs and seizures in this model is the anatomical segregation of their respective sites of origin. This may apply in other models and in human epilepsy as well. Finally, these results illustrate that one consequence of the anatomical segregation and mutual interaction of IIBs and seizures is that influences which are locally pro- or antiepileptic can have opposite effects in a broader region.

Assessment of a computer program to detect epileptiform spikes

This study compares an automated spike detection program to a group of 6 electroencephalographers. Since group members varied in experience, an expertise factor was devised to weight their scoring. EEGers underscored epileptiform events on 6 records in a manner analogous to the computer's storage of EEG segments. A summation of expertise factors was determined for every event. This sum was normalized and interpreted as a probability the event would be called a spike by a given EEGer. The performance of each scorer and of the computer at different amplitude thresholds was analyzed based on this probability. Higher rated scorers identified more subtle events. Lowering the threshold of the computer program produced a comparable increase in sensitivity. The increase in total events detected by the computer was linear over the range studied. While the proportion of false positive detections increased with lowering threshold, our readers have not found a moderate number of these distracting. We conclude that the computer system, while not as specific as an EEGer, can be as sensitive and can be a reliable screening editor for large amounts of monitoring data. On balance it is more effective than an EEGer for this limited purpose.

Comparison of bitemporal and unitemporal epilepsy defined by depth electroencephalography

Of 166 consecutive patients studied with depth electroencephalography (EEG), 87 had seizures arising from one temporal lobe and 23 had seizures arising independently from each temporal lobe. We retrospectively reviewed and compared those patients with unitemporal and those with bitemporal seizures. There was no statistically significant differences between the two groups in terms of age at onset of seizures, duration of epilepsy, localization of scalp EEG abnormalities, surgical results, or pathological findings. The bitemporal group, however, had significantly fewer patients with a history of febrile seizures (p less than 0.025). Two patients with bitemporal seizures were later found to have extratemporal lesions, and one an extrahippocampal temporal lesion, on magnetic resonance scans; one patient's extratemporal lesion was resected and all habitual seizures stopped. These findings suggest that a single pathophysiological process accounts for unitemporal and some bitemporal epilepsy, but that independent onset of seizures from each temporal lobe detected by depth EEG may also indicate extratemporal foci.

Does interictal spiking change prior to seizures?

We studied 10 patients with intractable epilepsy being evaluated for epilepsy surgery for preictal changes in spiking. All patients were implanted with intracranial electrodes and underwent continuous EEG/audiovisual monitoring. Interictal spikes were detected and recorded continuously by a dedicated computerized system. Edited spikes were counted during 0-5, 5-10, and 0-60 min epochs before each seizure, during epochs of unvarying state of arousal (awake or sleep stage II). When comparing by repeated measures, 1-way ANOVA, total spiking (in all recording channels) did not differ among the different preictal epochs (0-5, 5-10, 0-60 min) in 45 seizures (F = 0.88, P = 0.40, using the Geisser-Greenhouse adjustment--GGA). Likewise, no significant differences were obtained during those same epochs when comparing spiking originating from the channel of seizure onset in 5 patients with 28 seizures of localized onset (F = 1.19, P = 0.38 using the GGA). Our findings indicate that in patients with intractable epilepsy, no changes in spiking occur in the 5 min prior to seizures, when compared to more distant preictal epochs.

The neocortico to mesio-basal limbic propagation of focal epileptic activity during the spike-wave complex

In order to localize epileptogenic electrophysiological sources, a multichannel MEG system was used in 3 patients with partial epilepsy during presurgical evaluation. MEG and EEG (including scalp, sphenoidal and intracranial foramen ovale electrodes) were recorded simultaneously during a period of intensive video-EEG monitoring in order to observe single spontaneous spikes. In addition to MRI, SPECT and PET investigations were performed. Electrical activity subsequent to the activity of the epileptic focus could be localized by the MEG after noise reduction using a temporal correlation technique. Simultaneous registration of the magnetic field and the electrical field showed that the source of the primary focal epileptic activity (first period during the total spike wave complex where a dipolar magnetic field pattern is found) is localized in neocortical lateral regions, whereas another focal epileptic activity in a later phase of propagation occurs in temporal mesial regions. In 1 patient (case 1) the primary focal epileptic activity was localized in the surrounding neocortical tissue of an angioma and the middle and inferior temporal gyrus. The second phase of propagation is localized in temporo-basal-mesial regions, including para- and hippocampal structures. The latest center of activity occurred in posterior parts of the gyrus cinguli. In 2 other patients, the primary focal epileptogenic activity was localized at the insula and also spread into temporal basal mesial regions. A multi-modal approach to research of focal epilepsy, combining metabolic, electrical potential, magnetoencephalographic and morphological data, recorded by non-invasive techniques, offers new perspectives for the detection of involved brain regions. The 3-D and time-resolved localization of focal epileptic activity, correlated with the individual anatomy of the human brain, may improve the determination of neuronal populations involved in the individual epileptogenic process, especially in the interaction between temporal or extratemporal neocortex and limbic system.

Structurally stable burst and synchronized firing in human amygdala neurons: auto- and cross-correlation analyses in temporal lobe epilepsy

Burst structure and synchronized firing of bursts were studied, in the interictal period, using auto- and cross-correlation analyses in human amygdala neurons in temporal lobe epilepsy patients diagnosed as having a unilateral limbic seizure focus in anterior hippocampus and/or amygdala. Satisfactory single unit recordings were obtained from chronically implanted microelectrodes in 51 amygdala neurons, and auto-correlation analysis identified 27 of 51 neurons where burst firings recurred with regular interspike interval structures (structurally stable burst: S-burst). This structural stability was characteristic only for a short burst, or at the beginning of a series of repetitive firings, involving 2-5 action potentials. In 'non-epileptic' amygdala neurons located contralateral to the seizure focus, the average duration of S-burst was 15 msec and the number of action potentials (spikes) in the S-burst was inversely related to the interspike intervals in the S-burst, suggesting that endogenous membrane characteristics of non-epileptic amygdala neurons determine the patterns of S-burst. In contrast, in the seizure focus amygdala ('epileptic'), the duration of the S-burst was prolonged among epileptic neurons, not because of the occurrence of more action potentials within the S-burst, but because of a prolonged interspike interval within the S-burst. Furthermore, there was no relationship between the interspike interval and the number of action potentials in the S-burst, suggesting that synaptic inputs and/or extracellular environmental factors may affect an intrinsic mechanism for generating stable S-burst in epileptic neurons. Cross-correlation analysis identified synchronized firings in epileptic neurons: when two epileptic neurons both exhibited S-bursts, when either epileptic neuron exhibited S-burst, but never when neither exhibited S-bursts. Conversely, non-epileptic neurons rarely fired synchronously; even though they showed S-bursts. The difference in the pattern of S-bursts between epileptic and non-epileptic amygdala neurons seems to be the degree of firing synchrony. Our results provide, for the first time, direct evidence that human epileptogenic amygdala neurons recorded in vivo have unique burst firing patterns and significant synchronous excitatory interactions, different from a burst pattern found in non-epileptogenic amygdala neurons during the interictal period.

Visual versus computer evaluation of thiopental-induced EEG changes in temporal lobe epilepsy

Thiopental-induced EEG beta activity was analyzed both visually and by computer in 33 patients with complex partial epilepsy. Studies were done in 16 patients with depth electrodes in limbic structures and 17 patients with scalp and sphenoidal electrodes. The percentage of drug-induced change in beta activity was quantified by computer using spectral analysis. The statistical significance of asymmetries between homologous sites in the amount of change was determined. The spatial distribution of significant asymmetries was used for localization and compared with the results of independent visual analysis of the thiopental EEG. Concordance between computer and visual evaluation occurred in 10 of 17 scalp/sphenoidal and 10 of 16 depth electrode tests. The accuracy of visual and computer localization was determined by comparing them with locus of itcal EEG onset, interictal spikes, and positron emission tomography. In scalp/sphenoidal studies, computer analysis indicating asymmetry appeared more likely to correlate with independent clinical criteria than visual analysis. In depth studies the reverse appeared to be true. Scalp/sphenoidal tests yielded positive results in 25-30% of patients whereas depth electrode tests were positive in 50-70% of patients. The results indicate that computer analysis of surface thiopental tests is an accurate and useful supplement to visual evaluation of these tests.

Spatio-temporal EEG measures and their application to human intracranially recorded epileptic seizures

We describe analysis methods which involve the computation of band-integrated, intrachannel spectral measures from the EEG. These measures require only very general assumptions, have a simple interpretation and achieve significant data reduction, while providing a quantitative assessment of significant EEG modification at seizure or onset or in the course of seizure evolution. In developing these measures, we experimented with various measures derived from spatio-temporal power spectra. Bandpass (8-30 Hz) integrated relative energy measures appear to be the most useful since they objectively measure paroxysmal modification. Two such measures are illustrated in this report: the band-integrated power z-score (BIPZ), which is a quantitative measure designed to allow significance testing, and the band-integrated power ratio (BIPR), which is a simply calculated qualitative measure that behaves very similarly to the BIPZ measure and may be useful for real-time data screening. In addition to developing these intrachannel measures, we have also developed methods of displaying these measures in a format appropriate to the analysis of interchannel relationships. In a general sense, the displays provide objective information regarding the spatio-temporal dynamics of the EEG in a condensed format which is still readily understandable to the electroencephalographer. This analysis was applied to 29 spontaneous, depth-recorded seizures in 14 patients with the aim of identifying the initial locus of seizure activity in each seizure episode. The initial locus was defined as that location (or set of locations) to first exhibit statistically significant values of the BIPZ measure. The method was very effective in this regard, as shown by correspondence between the conclusions of this automated analysis and conventional visual analysis.

Epileptic seizure disorders. Developments in diagnosis and therapy

There has been considerable progress in various segments of epileptology over the past two to three decades. The diagnostic sector has benefited from more advanced and sophisticated EEG-related techniques. The advent of computerized tomography has expedited the clinical evaluation of epileptic patients and new high-technology methods have been introduced. A new type of diagnostic subdivision (based on age-determined epileptic conditions and certain epileptic syndromes) is of great practical significance because of its prognostic implications (distinction of basically benign and severe forms of epileptic seizure disorders). The therapeutic sector has been stimulated by the introduction of new antiepileptic medications and particularly by profound insights into metabolic and pharmacokinetic characteristics of anticonvulsants; this has resulted in the introduction of techniques for serum level determinations. There have been new developments in the field of neurosurgical treatment of epileptic seizure disorders.

Interictal epileptic activity during sleep: a stereo-EEG study in patients with partial epilepsy

Cerebral electrical activity was recorded through chronic stereotactically implanted electrodes in 19 epileptic patients suffering from different types of severe and medically refractory partial seizures and who were considered for surgical treatment. 213 brain sites, in all cerebral lobes, in neocortical as well as in archicortical structures, were explored. The behaviour of the interictal spiking across wakefulness and nocturnal physiological sleep was analysed, using automatic elaboration. (i) Spike rate is affected by the occurrence of sleep and by the passage from one sleep phase to another. The degree and direction of the phenomenon differ remarkably in the various patients and, in the same patient, in the different cerebral sites explored. Generally, interictal spiking increases at the beginning of sleep, reaches its maximum during the deep non-REM phases and returns to a level slightly lower than that in wakefulness during REM. (ii) The nocturnal spike rate is hardly influenced by spike location. In most cases, however, the variations recorded during sleep are more significant in the frontal regions than elsewhere. (iii) Spike rate across wakefulness and sleep is affected by the local level of epileptogenicity: spiking variations are less in the most epileptogenic cerebral zone (identified by the origin of the seizure discharges and by the disappearance of seizures following its surgical removal) than elsewhere. The physio-pathological meaning and the diagnostic value of these findings, and particularly of the peculiar stability or autonomy of the electrical epileptic activity of the most epileptogenic cerebral zone, is discussed.

Temporo-spatial patterns of pre-ictal spike activity in human temporal lobe epilepsy

The statistical properties of pre-ictal EEG spike activity in medial temporal lobe sites were analyzed in 6 patients with medically refractory complex partial seizures. A total of 24 1 h pre-ictal periods (2-6 periods per patient) were evaluated by quantifying the rate of occurrence of individual spatial patterns of spike activity derived from a subset (n = 6) of the recording channels. The channels chosen for analysis always included those medial temporal lobe sites which were documented to be most likely to initiate seizures, as well as their respective contralateral homologues. Each 1 h pre-ictal period was divided into 360 10 sec bins which were then visually classified into 1 of 64 possible spatial patterns of spike activity. These patterns, in turn, were grouped into 1 of 5 general spatial patterns and evaluated for trends across 3 20 min pre-ictal segments. Pooling these data across patients yielded the following results: (1) Focal patterns of spike activity tended to decline significantly in rate of occurrence several minutes prior to seizures, while the rate of independent contralateral patterns did not change. (2) The rate of occurrence of patterns of bilateral loosely coupled spike activity (involving focal and contralateral sites) increased significantly across the 20 min pre-ictal segments and was clearly augmented during the 20 min prior to seizures. These findings indicate that the degree of bilateral independence in medial temporal lobe spike activity tends to decrease several minutes prior to the localized onset of temporal lobe seizures; such changes may reflect the mechanisms responsible for the inter-ictal-ictal transition.

Unilateral connections between amygdala and hippocampus in man. A study of epileptic patients with depth electrodes

During stereotaxic explorations of patients suffering from intractable temporal or non-temporal epilepsies, isolated electrical shocks could be delivered through depth electrodes to either the amygdaloid nucleus or the hippocampus, recording of the elicited response being performed from hippocampus or amygdala, respectively, on the same side. All patients displayed hippocampal responses to amygdala stimulation, whatever the type of epilepsy diagnosed. On the other hand, amygdala responses to hippocampal single shocks were only observed in a majority of patients suffering temporal epilepsy, while such back to forth activation could seldom be assessed in a group of patients not clearly characterized as temporal epileptics. It is suggested that subtle changes in excitability of an (indirect) pathway from hippocampus to amygdala are revealed through this procedure of low intensity, short-lasting stimulation, as a correlate of the temporal epileptic process.

Machine detection of spike-wave activity in the EEG and its accuracy compared with visual interpretation

Machine detection of epileptiform activity in the EEG is useful in seizure monitoring because of its inherent consistency and the rapid data reduction it can provide. Devices based on a few detection criteria have lacked reliability of detection and those with more complex algorithms have sacrificed operating speed and portability. This paper describes a largely analog device which detects irregular as well as classic spike and wave activity. It is portable and it can process the accelerated playback of 24 h tape recorders as well as real-time EEG. It recognizes spikes by their shape and waves by their frequency. It makes inter-channel comparisons to identify trains of bilateral synchronous spikes, generalized waves, and coincidence of spikes and waves and furnishes a limited description of each event in terms of these characteristics. The device was tested against the judgment of 3 experienced and certified electroencephalographers in 18 h of EEG containing 769 bursts of spike-wave activity from 6 patients. It detected 96.5% of the consensus spike and wave activity (i.e., activity identified by all 3 electroencephalographers). Only 0.56% of the machine's detections were false positives (i.e., activity identified by none of the electroencephalographers), though the false positive rate was higher in the presence of chewing artifact. It measured burst duration with an average error of 0.43 sec/burst. While reader-machine agreement varied somewhat by patient, in general, the machine disagreed with the consensus no more than the readers disagreed with each other. In a second reading session after 6 months, the amount of activity identified by the readers changed by an amount ranging from 2.4% to 57% while the machine was consistent within a few tenths of 1%. Hence, this paper demonstrates that by implementing a multi-criteria detection algorithm in special purpose circuitry, a cost-effective solution to the problem of reliable machine detection of spike and wave activity can be obtained.

Changes in intelligence following temporal lobectomy: relationship to EEG activity, seizure relief, and pathology

Pre- and posttemporal lobectomy measures of intelligence and memory in 36 patients with medically refractory complex partial seizures were compared with (1) various aspects of presurgical ictal and interictal EEG activity derived from surface and deep electrodes, (2) postlobectomy seizure relief, and (3) pathological findings in the resected lobe. With respect to interictal EEG data, bilaterally synchronous surface spikes (accompanied or unaccompanied by simultaneous deep spikes) and sharp waves were significantly correlated with lower prelobectomy intelligence scores and a drop in these scores following lobectomy. With respect to ictal EEG data, bilaterally synchronous and multifocal onsets were significantly correlated with a postlobectomy drop in intelligence scores. Patients with poor postlobectomy seizure relief tended to have lower presurgical intelligence scores and a drop in intelligence scores following lobectomy. The patients most likely to show a postlobectomy drop in intelligence were those demonstrating some combination of poor seizure relief, an absence of pathology in the resected specimen, or EEG signs indicative of poor seizure relief. Postlobectomy changes in intellectual status are therefore not necessarily exclusively attributable to the amount of postlobectomy seizure relief experienced by these patients, but might be due to a combination of factors.

Surface and deep EEG correlates of surgical outcome in temporal lobe epilepsy

Interictal and ictal EEG characteristics derived from limited surface montages and medial temporal lobe sites were compared with long-term seizure relief following anterior temporal lobectomy in 52 epileptics. Patients were classified into one of four surgical outcome groups, ranging from seizure free to no clinical improvement. For each patient, interictal records were analyzed according to deep and surface spike characteristics and background activity. Ictal records were analyzed according to the proportion of episodes initiated in a unilateral or bilaterally synchronous fashion, the proportion of surface or deep onsets, the variability of onset location, and the morphology of seizures onsets. Interictal EEG variables that correlates with surgical outcome included: (a) various types of bilaterally synchronous surface/deep spikes; (b) diffuse background slowing; (c) sharp waves; and (d) the presence of multiple independent deep spike patterns in the lobe chosen for resection. Relevant ictal EEG variables included: (a) episodes initiated in a bilaterally synchronous fashion; (b) variability in seizure onset location; (c) the proportion of precisely focal onsets from deep sites; (d) the proportion of surface onsets; and (e) the proportion of onsets from the side chosen for resection. Multivariate analysis of these data with linear, stepwise, discriminate analysis and adaptive, nonlinear, distribution-free pattern recognition demonstrated that: (a) both interictal and ictal EEG characteristics can independently predict surgical outcome at levels significantly better than chance; (b) ictal and interictal EEG data contain nonredundant information for making such predictions; and (c) nonlinear pattern recognition techniques are capable of deriving the most accurate rules for predicting the effects of surgery.

Correlation of criteria used for localizing epileptic foci in patients considered for surgical therapy of epilepsy

Criteria for anterior temporal lobectomy, performed on seven patients with partial complex seizures, were derived from a battery of fourteen presurgical tests. Seven tests were routine studies aimed at identifying a focus of epileptic excitability, while seven were designed to reveal areas of focal functional deficit. Conflicting information was frequently obtained from the tests of epileptic excitability, suggesting that it is probably inaccurate to view patients with partial complex seizures as having a single epileptogenic focus. Presurgical evaluation must therefore be aimed at identifying the focus most responsible for the patient's habitual seizures. Tests of focal functional deficit provided useful nonconflicting confirmatory information in each of the seven patients studied. The most reliable information was obtained from depth electrode implantation, and this procedure should be considered essential except when all evidence of surface-recorded epileptic excitability, including ictal onset, and evidence of focal functional deficit agree.

Sleep state and seizure foci related to depth spike activity in patients with temporal lobe epilepsy

Depth spike activity was evaluated from medial temporal lobe sites using computer spike recognition techniques in all-night sleep records derived from 10 patients with medically refractory complex partial seizures. Sleep stages were classified into 1 of 4 groups: wakefulness, REM sleep, light sleep and deep sleep. Some disturbance in the periodicity of the sleep cycle was noted in most patients, but the relative proportions of REM sleep, light sleep and deep sleep were close to that reported for normals. Depth spike activity was observed to be most frequent in a majority of sites during deep sleep in 6 patients and during light sleep in 3 patients. In 1 patient equal numbers of sites showed maximal activation during light sleep and deep sleep. In 4 patients, certain sites in the more epileptogenic lobe demonstrated a maximal rate during waking or REM sleep. All patients reported in this study were considered to be suitable for temporal lobectomy. In contrast to the results obtained from a previous study, the side with the site demonstrating maximal mean spike rate did not necessarily correspond to the side chosen for lobectomy. Significant correspondence across patients between the more epileptogenic lobe and maximal spike rate was not found during waking and was further reduced during light sleep and deep sleep. The correspondence was, however, significant during REM sleep and for the side containing the site demonstrating the smallest activation in mean spike rate during light sleep or deep sleep relative to waking. These results indicate that an analysis of sleep induced changes in depth spike activity can be useful in improving predictions concerning epileptogenicity. Quantification of other aspects of the interictal EEG, such as background activity, may further improve such predictions.

A topographical display of epileptiform transients based on a statistical approach

A method for the analysis of topographical relations of EEG epileptiform transients based on a statistical approach is described. The degree of time association of those transients recorded in pairs of derivations, or in 3 (triads), 4 (tetrads) or more derivations is estimated, using the chi2 statistic. Pairs of derivations which are found to be significantly associated are tabulated. This information is also graphically displayed on a computer drawing of the head (10-20 system). The same is done for triads, tetrads or higher order associations. The number of phase reversals between epileptiform transients found in significantly associated pairs of derivations is also computed. The resulting spatial displays enable a visualization of the location of the dominant epileptogenic area and of the spread of epileptiform activity over the head.

Computerized detection of rapid eye movements during paradoxical sleep

A technique for automatically analysing rapid eye movements in sleep EOG is described, in which time of occurrence, amplitude and duration of each REM are measured. This method is based on the pattern recognition algorithm that simulates visual analysis.