Computer recognition of generalized spike-wave discharges.

Twelve 12h daytime telemetered EEGs were read for generalized spike-wave (S-W) bursts by a computer and independently by three experienced electroencephalographers. The computer system was a hybrid of analog devices (tape-recorder, voltage summator, and high-pass filters) and a multipurpose laboratory digital device (PDP-12). The computer was programmed to identify only the location on the record and the duration of the generalized S-W paroxysms. From 1,462 discharges identified by both the readers and the computer, a consensus list of 609 was derived by selecting only those bursts identified by all three readers. The computer recognized 516 (85%) of those bursts on the consensus list; the computer's accuracy, as compared with the consensus, rose to 92% when portions of the EEG containing sleep were discounted. The computer also recognized as generalized S-W paroxysms 15 high-voltage transients, which none of the three readers selected. Ten of these, however, occurred during sleep. When the computer results were added to those of the three readers to permit a four-way inter-reader comparison, the computer was found to have had about as many isolated decisions as any of the three readers. In the number of bursts recognized, the computer tended to be about as conservative as the reader under whose guidance it was originally programmed. The computer's accuracy in measuring burst duration was not evaluated statistically.     

EEG-based motor imagery analysis using weighted wavelet transform features

In this study, an electroencephalogram (EEG) analysis system for single-trial classification of motor imagery (MI) data is proposed. Feature extraction in brain-computer interface (BCI) work is an important task that significantly affects the success of brain signal classification. The continuous wavelet transform (CWT) is applied together with Student's two-sample t-statistics for 2D time-scale feature extraction, where features are extracted from EEG signals recorded from subjects performing left and right MI. First, we utilize the CWT to construct a 2D time-scale feature, which yields a highly redundant representation of EEG signals in the time-frequency domain, from which we can obtain precise localization of event-related brain desynchronization and synchronization (ERD and ERS) components. We then weight the 2D time-scale feature with Student's two-sample t-statistics, representing a time-scale plot of discriminant information between left and right MI. These important characteristics, including precise localization and significant discriminative ability, substantially enhance the classification of mental tasks. Finally, a correlation coefficient is used to classify the MI data. Due to its simplicity, it will enable the performance of our proposed method to be clearly demonstrated. Compared to a conventional 2D time-frequency feature and three well-known time-frequency approaches, the experimental results show that the proposed method provides reliable 2D time-scale features for BCI classification.     

Fixed and random effect analysis of multi-subject fMRI data using wavelet transform

We propose a new method to estimate the random effect variance in group analysis of fMRI data. In the first level of analysis, general linear model (GLM) is used to estimate a parameter map ("effect") for each subject. After applying discrete wavelet transform to the "effect" maps, noise is reduced through a vertical energy thresholding (VET). The fixed effect component in each coefficient is derived by averaging the wavelet coefficients along all subjects. Then, the wavelet coefficients containing significant random effect are identified by their higher sample variance along the subjects. Wavelet coefficients containing random effect component in each subject are used to reconstruct the random effect maps through an inverse wavelet transform. Random effect variance is obtained from random effect maps for use in random effect analysis. The proposed method and other methods like GLM group analysis and variance ratio smoothing are applied to both experimental and artificial fMRI data. ROC curves, obtained from the simulated data, show improved group activation detection compared to existing random effect analysis methods. For the experimental data, the proposed method shows its high sensitivity by detecting multiple activation regions, namely visual cortex, cuneus, precuneus, thalamus, and cerebellum. From these regions, precuneus and cerebellum are not detected by majority of the previously published methods.     

An algorithm for real-time detection of spike-wave discharges in rodents

The automatic real-time detection of spike-wave discharges (SWDs), the electroencephalographic hallmark of absence seizures, would provide a complementary tool for rapid interference with electrical deep brain stimulation in both patients and animal models. This paper describes a real-time detection algorithm for SWDs based on continuous wavelet analyses in rodents. It has been implemented in a commercially available data acquisition system and its performance experimentally verified. ECoG recordings lasting 5-8h from rats (n=8) of the WAG/Rij strain were analyzed using the real-time SWD detection system. The results indicate that the algorithm is able to detect SWDs within 1s with 100% sensitivity and with a precision of 96.6% for the number of SWDs. Similar results are achieved for 24-h ECoG recordings of two rats. The dependence of accuracy and speed of detection on program settings and attributes of ECoG are discussed. It is concluded that the wavelet based real-time detecting algorithm is well suited for automatic, real-time detection of SWDs in rodents.     

Inter-hemispheric asymmetry of cerebral flow velocities during generalized spike-wave discharges

The aim of this study was to verify the symmetry of cerebral blood flow changes during the generalized spike-wave discharges of typical absence seizures. A recording of mean flow velocity in the left and right middle cerebral arteries and of electroencephalographic activity was performed simultaneously in two subjects with multiple daily absence seizures. A total of 12 generalized spike-wave discharges were recorded. Mean flow velocities showed a significant increase during the discharges with respect to baseline. The increase of flow velocity started simultaneously or a few seconds before the discharges. In 91.6 % of all recordings, the percentage increase of mean flow velocity was significantly higher in the left than in the right side (7.03 % ± 3.3 vs 5.14 % ± 3.3; p < 0.1). The extent of the following decrease of flow velocity was also significantly greater in the left than in the right side (-16.91 % ± 8.1 vs -14.07 % ± 8.3; p < 0.01). These findings show an inter-hemispheric asymmetry in cerebral blood flow during generalized spike-wave discharges in two patients with absence seizures. Transcranial Doppler ultrasonography promises to be an interesting approach to detect rapid changes in cerebral hemispheric activity not otherwise recognizable. Received: 14 November 2001, Received in revised form: 18 February 2002, Accepted: 5 March 2002     

Circadian rhythm of regular spike-wave discharges in childhood absence epilepsy

Four girls with childhood absence epilepsy with several seizures every day were investigated using an ambulatory cassette EEG. Recordings were started at about 6 pm, and were run continuously for about 22 hours. We studied only the regular and symmetrical 3 Hz spike-wave discharges of at least 5 seconds duration, which are quite similar to or identical with those found in the clinical seizures. Regular spike-wave discharges occurred frequently during wakefulness in 2 cases and during sleep in the other 2 cases: in the latter, they occurred rarely during wakefulness. During wakefulness, we did not find a special time zone in which regular spike-wave discharges were facilitated; during nocturnal sleep, however, they were concentrated in the last third. The rate of regular spike-wave discharges per hour was the highest during stage 1, low during stages 2 and REM, and zero during stage 3 + 4. Average duration of regular spike-wave discharges was the longest during wakefulness in most cases, and shortest during stage 2 in all the cases.     

Measuring clusters of spontaneous spike-wave discharges in absence epileptic rats

Spike-wave discharges (SWDs) characterizing absence epilepsy appear in closely packed aggregated sequences, which gave rise to the name "pyknolepsy" for this disease. In WAG/Rij rats, genetically prone to absence epilepsy, spontaneous SWDs seem to occur in clusters as well. Here, we aimed to quantify the seizures' clusters. SWDs sequences were extracted from long-term (complete estrous cycle) EEG recordings of adult female WAG/Rij rats. Spectral characteristics and half-decay time of autocorrelation functions (AC-tau) were calculated for time series of i(SWD) (proportion of time occupied by spike-wave activity), measured for subsequent periods. The clusters were characterized by means of AC-tau calculated for time series of i(SWD). The absence seizures were indeed clustered in a minute range. The clustering had a non-periodical character, since no significant and consistent periodicity was found in the minute range. AC-tau correlated positively with propensity of SWDs: i.e. the aggravation of absence epilepsy led to longer sequences of paroxysms and thus to a less random distribution. AC-tau was not sensitive to various phases of the estrous cycle, but was larger in the dark than in the light periods. We suggest that AC-tau can be used to quantify aggregation of epileptic events in the search for physiological basis of its temporal clustering.     

Spontaneous generalized spike-wave discharges in the electrocorticograms of albino rats

Generalized epileptiform discharges occur spontaneously in the electrocorticograms of some albino laboratory rats. These discharges occur during periods of quiet wakefulness, are composed of 6–10/sec spike-wave complexes, are accompanied by locomotor arrest and often by mild clonic facial movements. The occurrence of these discharges is unrelated to surgical procedures used to create chronic recording preparations. Similar discharges and behavior have been induced in controls by injections of pentylenetetrazol.     

Hilbert transform assisted complex wavelet transform for neuroelectric signal analysis

In this work, we present a new approach for shift invariant complex wavelet analysis of neuroelectric signals. A key idea is to preprocess the signal with the Hilbert transformer to yield an analytic signal, which is then wavelet transformed using the linear phase complex scaling and wavelet filters. In different scales, the total energy of the wavelet transform coefficients is shift invariant. The decimated analytic wavelet coefficients suffer no aliasing effects, which are predominant in conventional wavelet analysis. We show the usefulness of the present method in multi-scale analysis of the neuroelectric signal waveforms.     

Time-frequency spectral analysis of TMS-evoked EEG oscillations by means of Hilbert-Huang transform

A single pulse of Transcranial Magnetic Stimulation (TMS) generates electroencephalogram (EEG) oscillations that are thought to reflect intrinsic properties of the stimulated cortical area and its fast interactions with other cortical areas. Thus, a tool to decompose TMS-evoked oscillations in the time-frequency domain on a millisecond timescale and on a broadband frequency range may help to understand information transfer across cortical oscillators. Some recent studies have employed algorithms based on the Wavelet Transform (WT) to study TMS-evoked EEG oscillations in healthy and pathological conditions. However, these methods do not allow to describe TMS-evoked EEG oscillations with high resolution in time and frequency domains simultaneously. Here, we first develop an algorithm based on Hilbert-Huang Transform (HHT) to compute statistically significant time-frequency spectra of TMS-evoked EEG oscillations on a single trial basis. Then, we compared the performances of the HHT-based algorithm with the WT-based one by applying both of them to a set of simulated signals. Finally, we applied both algorithms to real TMS-evoked potentials recorded in healthy or schizophrenic subjects. We found that the HHT-based algorithm outperforms the WT-based one in detecting the time onset of TMS-evoked oscillations in the classical EEG bands. These results suggest that the HHT-based algorithm may be used to study the communication between different cortical oscillators on a fine time scale.     

Some peculiarities of time-frequency dynamics of spike-wave discharges in humans and rats.

OBJECTIVE: Time-frequency dynamics of spike-wave discharges (SWDs) were investigated in patients with absence seizures and in WAG/Rij rats, a genetic model of absence epilepsy using a specially developed wavelet transform. METHODS: Two types of SWDs were analyzed in both species: the most frequently occurring discharges (of minimal 3.6-4.0 s or more) and shorter ones recorded from various cortical regions. RESULTS: The more prolonged discharges had two phases: during the initial part (from tenth of seconds to 1 s) of the seizure the frequency decreased quickly from 5 to 3.5 Hz in patients and from about 15 to 10 Hz in rats. A slower frequency decrease with periodical fluctuations was observed in both species during the second part of the discharge: the frequency decreased towards the end of the discharge to 3 Hz in patients and to 6-7 Hz in rats. The frequency of the short discharges decreased fast during the whole discharge: from 5 to 2-2.5 Hz and from about 15 to 5 Hz in patients and rats, respectively. CONCLUSIONS: Comparison of data obtained in patients with typical absence epilepsy and WAG/Rij rats with genetic absence epilepsy revealed that the time-frequency dynamics of SWDs had similar properties but in a different frequency range. SIGNIFICANCE: The study of time-frequency dynamics using this specially developed wavelet transform revealed two different types of SWDs, which most likely represent different dynamics in the cortico-thalamo-cortical loop during shorter and more prolonged discharges.     

Effects of evoked spike-wave discharges upon short term memory in patients with epilepsy

Memory for digits was examined during EEG examination in five female children suffering from light sensitive epilepsy. Sequences of random digits equal to the patients' digit span, span-less-one and span-less-two were presented, followed by a 2 sec. interval, after which recall of the digits was required. The 2 sec. interval could be filled in any of four possible ways by: a burst of photic stimulation which evoked a paroxysm of generalised spike-wave in the EEG; a burst of slow (3 Hz) photic stimulation having no discernible effect upon the EEG; a burst of fast (30 Hz) stimulation, again having no observable effect; or a period of silence with no photic stimulation. It was found that recall of digits series of all three lengths was impaired if a paroxysm of spike-wave had been generated between the end of presentation and the beginning of recall. There were no significant differences in correctness of recall when the slow, fast and no-stimulation photic conditions were compared. It was found that the last two digits in the digit span were more vulnerable to recall failure than were the first two. On the basis of recent work in human experimental psychology, the initial and final digit pairs appear to be retrieved from functionally different storage systems. It is tentatively suggested therefore, that since different parts of the digit span are dislocated to different extents by spike-wave activity, they may depend upon different neural substrates.     

Electroencephalographic characterization of spike-wave discharges in cortex and thalamus in WAG/Rij rats

PURPOSE: The waveform of spontaneous spike-wave discharges (SWD) in the electroencephalogram (EEG) was delineated in the WAG/Rij rat model of absence epilepsy according to the definitions of clinical electroencephalography. We defined four elements in SWD based on the schema of Weir (1965): Spike 1 and 2, Positive Transient (PT), and Wave. The EEG patterns of generalized type I and local type II SWD in cortical and thalamic areas were analyzed. METHODS: EEGs were recorded in freely moving rats epidurally from different cortical regions and with deep electrodes from the specific and reticular thalamic nuclei. Grand average SWD waveforms were computed to assess spatiotemporal patterns of seizures. RESULTS: SWD I in the frontal cortex comprised of a large Spike 2 + Wave, and in the thalamus PT + Wave. Small transient spikes were associated with SWD I in the anterior-middle part of the cortex. SWD II were found in the occipital cortex as a sequence of (occasional) Spike 1 + PT + Wave. CONCLUSIONS: The EEG structure of SWD in WAG/Rij rats was comparable with that of epileptic patients, suggesting face validity of the WAG/Rij model. Fast transients spikes are an integrative part of SWD I. Time-amplitude linkage between cortical and thalamic counterparts of SWD I suggests a complex spatiotemporal organization of SWD I. The thalamus sustained SWD I, but not SWD II.     

Effects of neurosteroids on spike-wave discharges in the genetic epileptic WAG/Rij rat

Effects of i.p. administration of the neurosteroids, allopregnanolone and pregnenolone sulfate, were studied in WAG/Rij rats, a genetic model for generalized absence epilepsy. EEG recordings showed that allopregnanolone, a positive modulator of the GABA(A) receptor, in doses ranging from 5 to 20 mg/kg, increased dose-dependently the number- and total duration of spike-wave discharges. Pregnenolone sulfate, a positive modulator of NMDA receptors, also increased those parameters, though only at the highest dose used (100 mg/kg). Significant changes in spike-wave discharges occurred during the first hour post-injection and were not accompanied with behavioral alterations. The obtained data indicate that both these neurosteroids aggravate the spike-wave activity. This finding contrasts with the anti-convulsant effects of some neurosteroids and they point to a different pharmacological profile of epilepsy with convulsive or non-convulsive seizures.     

Effect of intracranial administration of ethosuximide in rats with spontaneous or pentylenetetrazol-induced spike-wave discharges

Purpose: Generalized absence seizures are characterized by bilateral spike‐wave discharges (SWDs), particularly in the frontoparietal cortical region. In WAG/Rij and GAERS rats with absence epilepsy, recent evidence indicates that SWDs arise first from the lateral somatosensory cortex (LSC), that is, the cortical focus theory. To further understand the cortical role in SWD generation, two epileptic rat models were assessed. Methods: Two models, Long‐Evans rats with spontaneous SWDs and Wistar rats with low‐dose pentylenetetrazol‐induced SWDs (20 mg/kg, i.p.), were administered intracortical or intrathalamic ethosuximide (ESM) or saline. Electroencephalographic recordings were analyzed before and after intracranial microinfusion to evaluate onset, frequency, and duration of SWDs. Key Findings: In both epileptic rat models, ESM in the LSC significantly reduced SWD number, shortened SWD duration, and delayed SWD onset compared to saline. By contrast, ESM in the medial somatosensory cortex had little effect compared to saline. Intrathalamic infusion of ESM only delayed SWD onset. Significance: These findings suggest that the LSC may be essential for the occurrence of SWDs. Our data support the cortical focus theory for the generation of absence seizures.     

Analysis of EEG records in an epileptic patient using wavelet transform

About 1% of the people in the world suffer from epilepsy and 30% of epileptics are not helped by medication. Careful analyses of the electroencephalograph (EEG) records can provide valuable insight and improved understanding of the mechanisms causing epileptic disorders. Wavelet transform is particularly effective for representing various aspects of non-stationary signals such as trends, discontinuities, and repeated patterns where other signal processing approaches fail or are not as effective. In this research, discrete Daubechies and harmonic wavelets are investigated for analysis of epileptic EEG records. Wavelet transform is used to analyze and characterize epileptiform discharges in the form of 3-Hz spike and wave complex in patients with absence seizure. Through wavelet decomposition of the EEG records, transient features are accurately captured and localized in both time and frequency context. The capability of this mathematical microscope to analyze different scales of neural rhythms is shown to be a powerful tool for investigating small-scale oscillations of the brain signals. Wavelet analyses of EEGs obtained from a population of patients can potentially suggest the physiological processes undergoing in the brain in epilepsy onset. A better understanding of the dynamics of the human brain through EEG analysis can be obtained through further analysis of such EEG records.     

小波分析用于颅内电极记录发作起始脑电图信号的研究

目的 初步研究颅内电极记录脑电图发作起始时期的小波熵变化趋势,探索其用于脑电图自动检测的前景.方法 收集9例术后预后为Engel Ⅰ级的癫痫患者,回顾性分析颅内电极脑电图记录到的28次发作事件,比较发作起始区(SOZ)电极和非SOZ电极在发作前、发作起始、发作中的小波熵变化情况.结果 SOZ电极发作起始小波熵(0.326±0.250)较发作前背景(0.053±0.047)升高,发作中小波熵(0.138±0.097)较发作起始有所下降,差异有统计学意义(F=30.904,P=0.000);非SOZ电极的小波熵在不同状态下末见明显变化.进一步将时窗小波熵用于发作起始脑电活动的检测显示,时窗小波熵可以敏感地反映出发作起始电活动的时间及电极.结论 SOZ电极小波熵在发作起始时显著增高,可用于协助癫痫术前定位.     

Thalamic lesions in a genetic rat model of absence epilepsy: Dissociation between spike-wave discharges and sleep spindles

Recent findings have challenged the traditional view that the thalamus is the primary driving source of generalized spike-wave discharges (SWDs) characteristic for absence seizures, and indicate a leading role for the cortex instead. In light of this we investigated the effects of thalamic lesions on SWDs and sleep spindles in the WAG/Rij rat, a genetic model of absence epilepsy. EEG was recorded from neocortex and thalamus in freely moving rats, both before and after unilateral thalamic ibotenic acid lesions. Complete unilateral destruction of the reticular thalamic nucleus (RTN) combined with extensive destruction of the thalamocortical relay (TCR) nuclei, resulted in the bilateral abolishment of SWDs and ipsilateral abolishment of sleep spindles. A suppression of both types of thalamocortical oscillations was found when complete or extensive damage to the RTN was combined with minor to moderate damage to the TCR nuclei. Lesions that left the rostral pole of the RTN and part of the TCR nuclei intact, resulted in an ipsilateral suppression of sleep spindles, but a large increase of bilateral SWDs. These findings demonstrate that the thalamus in general and the RTN in particular are a prerequisite for both the typical bilateral 7-11 Hz SWDs and natural occurring sleep spindles in the WAG/Rij rat, but suggest that different intrathalamic subcircuits are involved in the two types of thalamocortical oscillations. Whereas the whole RTN appears to be critical for the generation of sleep spindles, the rostral pole of the RTN seems to be the most likely part that generates SWDs.     

Electrophysiological and pharmacological characteristics of two types of spike-wave discharges in WAG/Rij rats

Rats of the WAG/Rij strain are commonly seen as a genetic model for generalised absence epilepsy in man. Interestingly, generalised absence epilepsy shows, in addition to the fully generalised spike-wave discharges, a second type of spike-wave discharge, which lasts for a shorter time, has a lower frequency, and a lower incidence. The originally described distinction between the two types of spike-wave discharges was mainly based on the shape, polarity and duration of the discharges. In the present study other characteristics such as the spatial and temporal distribution of the spike and wave components of the two discharges and frequency spectra were found to differ between the two types. In addition, a reciprocal regulation of the two types of spike-wave discharges by drugs affecting the dopaminergic system (haloperidol and apomorphine) was observed. The results convincingly demonstrate the difference between the two phenomena and warrant the search for neurobiological mechanisms underlying both types of spike-wave discharges.