Dynamics underlying rhythmic and non-rhythmic variants of abnormal, waking delta activity

Estimated EEG functional connectivity and aperiodic component induced by vagal nerve stimulation in patients with drug-resistant epilepsy

Background

Vagal nerve stimulation (VNS) improves seizure frequency and quality of life in patients with drug-resistant epilepsy (DRE), although the exact mechanism is not fully understood. Previous studies have evaluated the effect of VNS on functional connectivity using the phase lag index (PLI), but none has analyzed its effect on EEG aperiodic parameters (offset and exponent), which are highly conserved and related to physiological functions.

Objective

This study aimed to evaluate the effect of VNS on PLI and aperiodic parameters and infer whether these changes correlate with clinical responses in subjects with DRE.

Materials and methods

PLI, exponent, and offset were derived for each epoch (and each frequency band for PLI), on scalp-derived 64-channel EEG traces of 10 subjects with DRE, recorded before and 1 year after VNS. PLI, exponent, and offset were compared before and after VNS for each patient on a global basis, individual scalp regions, and channels and separately in responders and non-responders. A correlation analysis was performed between global changes in PLI and aperiodic parameters and clinical response.

Results

PLI (global and regional) decreased after VNS for gamma and delta bands and increased for an alpha band in responders, but it was not modified in non-responders. Aperiodic parameters after VNS showed an opposite trend in responders vs. non-responders: both were reduced in responders after VNS, but they were increased in non-responders. Changes in aperiodic parameters correlated with the clinical response.

Conclusion

This study explored the action of VNS therapy from a new perspective and identified EEG aperiodic parameters as a new and promising method to analyze the efficacy of neuromodulation.

Increased delta power as a scalp marker of epileptic activity: a simultaneous scalp and intracranial electroencephalography study

BACKGROUND AND PURPOSE

The purpose was to evaluate whether intracranial interictal epileptiform discharges (IEDs) that are not visible on the scalp are associated with changes in the frequency spectrum on scalp electroencephalograms (EEGs).

METHODS

Simultaneous scalp high-density EEG and intracranial EEG recordings were recorded in nine patients undergoing pre-surgical invasive recordings for pharmaco-resistant temporal lobe epilepsy. Epochs with hippocampal IED visible on intracranial EEG (ic-IED) but not on scalp EEG were selected, as well as control epochs without ic-IED. Welch's power spectral density was computed for each scalp electrode and for each subject; the power spectral density was further averaged across the canonical frequency bands and compared between the two conditions with and without ic-IED. For each patient the peak frequency in the delta band (the significantly strongest frequency band in all patients) was determined during periods of ic-IED. The five electrodes showing strongest power at the peak frequency were also determined.

RESULTS

It was found that intracranial IEDs are associated with an increase in delta power on scalp EEGs, in particular at a frequency ≥1.4 Hz. Electrodes showing slow frequency power changes associated with IEDs were consistent with the hemispheric lateralization of IEDs. Electrodes with maximum power of slow activity were not limited to temporal regions but also involved frontal (bilateral or unilateral) regions.

CONCLUSIONS

In patients with a clinical picture suggestive of temporal lobe epilepsy, the presence of delta slowing ≥1.4 Hz in anterior temporal regions can represent a scalp marker of hippocampal IEDs. To our best knowledge this is the first study that demonstrates the co-occurrence of ic-IED and increased delta power.

Diagnostic and prognostic value of EEG in prodromal dementia with Lewy bodies

OBJECTIVE

Early biomarkers for dementia with Lewy bodies (DLB) are lacking. To determine whether EEG differentiates the prodromal phase of DLB from other causes of mild cognitive impairment (MCI) and whether EEG is predictive for time to conversion from MCI to DLB, we compared EEGs and clinical follow-up of patients with MCI due to DLB with those of patients with MCI due to Alzheimer disease (MCI-AD).

METHODS

We compared 37 patients with MCI who developed DLB during follow-up or had an abnormal ¹²³I-PF-CIT SPECT scan (MCI-DLB) with 67 age-matched patients with MCI-AD. EEGs were assessed visually with a score of increasing abnormality (range 1-5). We performed fast Fourier transform to analyze the power spectrum. With survival analyses, EEG characteristics were related to time to progression to dementia.

RESULTS

The visual EEG score was higher in MCI-DLB (score >2 in 60%) compared to MCI-AD (score >2 in 8%, p < 0.001). We found frontal intermittent delta activity in 22% of MCI-DLB, not in MCI-AD. Patients with MCI-DLB had a lower peak frequency (7.5 [6.0-9.9] Hz vs 8.8 [6.8-10.2] in MCI-AD, p < 0.001) and more slow-wave activity. Several individual EEG measures showed good performance to discriminate MCI-DLB from MCI-AD (areas under the curve up to 0.94). In MCI-DLB, high visual EEG score, diffuse abnormalities, and low α2 power were related to time to progression to dementia (hazard ratios 4.1, 9.9, 5.1, respectively).

CONCLUSIONS

Profound EEG abnormalities are already present in the prodromal stage of DLB and have diagnostic and prognostic value.

CLASSIFICATION OF EVIDENCE

This study provides Class III evidence that EEG abnormalities are more common in MCI-DLB than MCI-AD.

Generalized and Lateralized Rhythmic Patterns

The relationship between generalized and lateralized rhythmic delta activity (RDA) and seizures is more ambiguous than the relationship between periodic discharges and seizures. Although frontally predominant generalized RDA is not associated with seizures, occipitally predominant RDA may be associated with the absence of seizures. Lateralized RDA seems to be more strongly associated with the presence of seizure activity. Appropriate recognition of generalized RDA and lateralized rhythmic delta activity may be confounded by benign etiologies of RDA, such as phi rhythm, slow alpha variant, subclinical rhythmic electrographic discharges of adults, or hyperventilation-induced high-amplitude rhythmic slowing. Angelman syndrome and NMDA-receptor antibody encephalitis can also produce morphologically distinct patterns of RDA.

Longitudinal association of delta activity at sleep onset with cognitive and affective function in community-dwelling older adults

OBJECTIVE

This investigation sought to determine whether delta activity at sleep onset (DASO) in the sleep electroencephalography of older adults represents normal variation or is associated with clinical pathology. To this end, we examined its longitudinal associations with cognitive and affective function in older adults without dementia.

METHODS

Participants were 153 community-dwelling older adults without dementia. We evaluated polysomnography (PSG), cognitive performance, and affective function at four time points: baseline, 12, 24, and 36 months. All participants completed PSG and measures of global cognition, delayed verbal memory, information processing speed, attention, inhibition, verbal naming, visuospatial ability, and measures of anxiety and depression. DASO was defined as sequences of rhythmic anterior delta activity on PSG in the transition from awake to sleep during the baseline assessment (Figure ).

RESULTS

At the baseline, 83 women and 70 men, mean age 71.3 ± 0.6 years participated and 19.6% of participants exhibited DASO. Age, years of education, gender, and body mass index did not differ according to DASO status. Linear mixed modeling showed that the presence of DASO was actually associated with lower levels of anxiety and depression. Further, participants with DASO, versus those without DASO, exhibited a trend towards better cognitive performance over time, although none of these associations reached statistical significance.

CONCLUSIONS

Whereas DASO was associated with better affective function, no significant association was found between DASO and cognitive change over time. These longitudinal findings support the view that the presence of DASO in healthy older adults represents normal variation rather than pathological aging. Copyright © 2016 John Wiley & Sons, Ltd.

Delta Activity at Sleep Onset and Cognitive Performance in Community-Dwelling Older Adults

STUDY OBJECTIVES

Frontal intermittent rhythmic delta activity (FIRDA) has long been considered to be an abnormal variant in the electroencephalogram (EEG) among older adults. Prior work also indicates a predominance of slow wave EEG activity among patients with dementia. However, instability of state control occurring with aging generally and among many neurodegenerative diseases raises the possibility that FIRDA might represent the intrusion of sleep related elements of the EEG into the waking state. We examined delta activity at sleep onset (DASO) in community-dwelling, older adults without dementia, and examined whether this activity is related to poorer cognitive performance.

METHODS

153 community-dwelling, older adults without dementia underwent overnight polysomnography and measures of global cognition, delayed verbal memory, information processing speed, attention, inhibition, verbal naming, and visuospatial ability. Delta activity during sleep/wake transitions (scored either as Waking or N1) was analyzed visually.

RESULTS

Participants were 83 women and 70 men, mean age 71.3 ± 0.6 y. DASO was present in 30 participants (19.6%). Age, years of education, sex, and body mass index did not differ between DASO (+) and (-) groups. Multiple regression analyses indicated faster reading of the Stroop color words in DASO (+) subjects (P = 0.007). None of the other cognitive domains differed between the two groups.

CONCLUSIONS

DASO was relatively common in our sample of community-dwelling, older adults without dementia. DASO was not associated with poorer performance on any cognitive domain. Instead, individuals with DASO demonstrated better performance on a simple reading task. Although these findings suggest that an abnormal EEG activity may represent normal variation, our work underscores the importance of distinguishing DASO from FIRDA when examining sleep in older adults.

COMMENTARY

A commentary on this article appears in this issue on page 725.

Neural network modeling of EEG patterns in encephalopathy

The EEG is an accessible tool for detecting encephalopathy, which usually manifests as delirium and sometimes as coma. Several disturbances have been described in the EEG of patients with encephalopathy, including diffuse slowing and periodic discharges. The pathophysiology of these EEG alterations, however, is poorly understood. This article shows that simulating activity of large populations of neurons, using neural mass models and neural network analysis, may increase our understanding of EEG disturbances in encephalopathy. We provide a brief introduction on the concepts of neural mass modeling and graph theoretical network analysis, and insights from this approach in previous work on neurologic disease, with a focus on encephalopathy. Finally, we speculate how anatomically coupled neural mass modeling combined with network analysis could provide new insights in pathophysiology of encephalopathy.

Phase-locking of epileptic spikes to ongoing delta oscillations in non-convulsive status epilepticus

The EEG of patients in non-convulsive status epilepticus (NCSE) often displays delta oscillations or generalized spike-wave discharges. In some patients, these delta oscillations coexist with intermittent epileptic spikes. In this study we verify the prediction of a computational model of the thalamo-cortical system that these spikes are phase-locked to the delta oscillations. We subsequently describe the physiological mechanism underlying this observation as suggested by the model. It is suggested that the spikes reflect inhibitory stochastic fluctuations in the input to thalamo-cortical relay neurons and phase-locking is a consequence of differential excitability of relay neurons over the delta cycle. Further analysis shows that the observed phase-locking can be regarded as a stochastic precursor of generalized spike-wave discharges. This study thus provides an explanation of intermittent spikes during delta oscillations in NCSE and might be generalized to other encephathologies in which delta activity can be observed.

Local polymorphic delta activity in cortical lesions causes global decreases in functional connectivity

Increasing evidence from neuroimaging and modeling studies suggests that local lesions can give rise to global network changes in the human brain. These changes are often attributed to the disconnection of the lesioned areas. However, damaged brain areas may still be active, although the activity is altered. Here, we hypothesize that empirically observed global decreases in functional connectivity in patients with brain lesions can be explained by specific alterations of local neural activity that are the result of damaged tissue. We simulated local polymorphic delta activity (PDA), which typically characterizes EEG/MEG recordings of patients with cerebral lesions, in a realistic model of human brain activity. 78 neural masses were coupled according to the human structural brain network. Lesions were created by altering the parameters of individual neural masses in order to create PDA (i.e. simulating acute focal brain damage); combining this PDA with weakening of structural connections (i.e. simulating brain tumors), and fully deleting structural connections (i.e. simulating a full resection). Not only structural disconnection but also PDA in itself caused a global decrease in functional connectivity, similar to the observed alterations in MEG recordings of patients with PDA due to brain lesions. Interestingly, connectivity between regions that were not lesioned directly also changed. The impact of PDA depended on the network characteristics of the lesioned region in the structural connectome. This study shows for the first time that locally disturbed neural activity, i.e. PDA, may explain altered functional connectivity between remote areas, even when structural connections are unaffected. We suggest that focal brain lesions and the corresponding altered neural activity should be considered in the framework of the full functionally interacting brain network, implying that the impact of lesions reaches far beyond focal damage.

Intermittent rhythmic delta activity patterns

Intermittent rhythmic delta activity is a typical EEG pattern that was originally described by W.A. Cobb in 1945 (J Neurol Neurosurg Psychiatr 1945;8:65-78). It may be classified into three distinct forms according to the main cortical region involved on the EEG: frontal (FIRDA), temporal (TIRDA), and occipital (OIRDA) intermittent delta activity. This article is a review of the main aspects of these patterns, with a special focus on EEG features and problems that may be encountered during interpretation of these patterns. In contrast to FIRDA and OIRDA, TIRDA is highly indicative of ipsilateral pathology. OIRDA and TIRDA are highly correlated with epilepsy, whereas FIRDA is a rather nonspecific EEG pattern.

The significance of frontal intermittent rhythmic delta activity in children

OBJECTIVE

To determine the correlation between frontal intermittent rhythmic delta activity (FIRDA) and the clinical and radiological correlates in children.

METHODS

Retrospective review of the EEG and imaging studies of 37 children with documented FIRDA.

RESULTS

FIRDA was associated with multiple neurological conditions and not necessarily with midline lesions. Patients with abnormal neurological exam had a longer FIRDA duration (average 9.5 seconds) compared to children with no reported abnormal examination (average of 6.5 seconds). FIRDA ranged from 2 to 2.5 Hz (n = 15), 3 Hz (n = 17) and from 1.5 to 3 Hz (n = 5) and there was a significant association between the duration of FIRDA and abnormal laboratory tests (p. < 0.05, Student's T test).

CONCLUSION

FIRDA was not correlated with midline brain lesions in children. FIRDA may be a non specific oscillation of an unhealthy pediatric brain with or without seizures.

Nonlinear dynamical analysis of the neonatal EEG time series: the relationship between neurodevelopment and complexity

OBJECTIVE

To investigate the relationship between the complexity of sleep EEG time series and neurodevelopment for premature or full-term neonates.

METHODS

Nonlinear dynamical analysis of neonatal sleep EEG time series is used to compute the correlation dimension D2 which is an index of the complexity of the dynamics of the developing brain. The dimensional complexity is estimated using Theiler's modification of the Grassberger-Procaccia algorithm for two different values of Theiler's w parameter. The hypothesis that neonatal EEG data during sleep contains nonlinear features is verified by means of surrogate data testing.

RESULTS

The dimensional complexity of the neonatal EEG increases with neurodevelopment and brain maturation. There is furthermore a statistically significant difference between the dimensional complexity of the EEG for neonates born prematurely when compared to full-term neonates at the same postmenstrual age (PMA). The neonatal EEG time series data used in this study proved to contain nonlinear features where the 'null hypothesis' of surrogate data testing is rejected with p<<0.0001.

CONCLUSIONS

A relationship between neurodevelopment and brain maturation and the complexity of the dynamics of the brain as measured by the dimensional complexity of the sleep EEG time series has been established. In particular, the dimensional complexity tends to increase with neurodevelopment and maturation as indicated by their PMA and birth status (premature or full-term). In particular, the brain dynamics of neonates born prematurely is less complex than the brain dynamics of neonates born full-term even at the same PMA. We attribute this to differences in the neurodevelopment between these two cohorts. We propose that the dimensional complexity can be used as an index for quantifying neurodevelopment.

SIGNIFICANCE

The dimensional complexity as measured by the correlation dimension of the sleep EEG time series may potentially be a useful measure for quantifying neurodevelopment in neonates. Future work is directed at the analysis of other EEG channels to understand the relationship between complexity in different regions of the brain and maturation and neurodevelopment, along with the utility of complexity to relate to neurodevelopment at older ages as measured by the Bayley score.

The nonlinear theory of schizophrenia

OBJECTIVE

Nonlinear properties exist within the brain across a hierarchy of scales and within a variety of critical neural processes. Only a few studies of brain activity in schizophrenia, however, have used nonlinear methods. This review paper evaluates the contribution of the nonlinear sciences towards understanding schizophrenia.

METHOD

Applications of nonlinear methods to the study of schizophrenia symptoms and to healthy and schizophrenia functional neuroscience data are reviewed. The main flaws of nonlinear algorithms and recent methods to correct these are also appraised.

RESULTS

Initial research methods utilized in the study of nonlinearity in schizophrenia have fundamental methodological limitations. In the last decade, many of these problems have been addressed, facilitating future progress. Research incorporating these improvements has been applied to normal electroencephalogram (EEG) data and to the symptoms of schizophrenia, but not systematically to brain imaging data collected from patients with schizophrenia.

CONCLUSION

There is strong statistical evidence for weak nonlinearity in normal EEG and in the fluctuations of the symptoms of schizophrenia. However, the contribution of nonlinear processes to brain dysfunction in schizophrenia is yet to be properly established or accurately quantified. Despite this, recent methodological advances suggest that a 'nonlinear theory' of schizophrenia may be helpful in understanding this disorder.

A unifying explanation of primary generalized seizures through nonlinear brain modeling and bifurcation analysis

The aim of this paper is to explain critical features of the human primary generalized epilepsies by investigating the dynamical bifurcations of a nonlinear model of the brain's mean field dynamics. The model treats the cortex as a medium for the propagation of waves of electrical activity, incorporating key physiological processes such as propagation delays, membrane physiology, and corticothalamic feedback. Previous analyses have demonstrated its descriptive validity in a wide range of healthy states and yielded specific predictions with regards to seizure phenomena. We show that mapping the structure of the nonlinear bifurcation set predicts a number of crucial dynamic processes, including the onset of periodic and chaotic dynamics as well as multistability. Quantitative study of electrophysiological data supports the validity of these predictions. Hence, we argue that the core electrophysiological and cognitive differences between tonic-clonic and absence seizures are predicted and interrelated by the global bifurcation diagram of the model's dynamics. The present study is the first to present a unifying explanation of these generalized seizures using the bifurcation analysis of a dynamical model of the brain.

BOLD-contrast functional MRI signal changes related to intermittent rhythmic delta activity in EEG during voluntary hyperventilation-simultaneous EEG and fMRI study

Differences in the blood oxygen level dependent (BOLD) signal changes were studied during voluntary hyperventilation (HV) between young healthy volunteer groups, (1) with intermittent rhythmic delta activity (IRDA) (N = 4) and (2) controls (N = 4) with only diffuse arrhythmic slowing in EEG (normal response). Subjects hyperventilated (3 min) during an 8-min functional MRI in a 1.5-T scanner, with simultaneous recording of EEG (successful with N = 3 in both groups) and physiological parameters. IRDA power and average BOLD signal intensities (of selected brain regions) were calculated. Hypocapnia showed a tendency to be slightly lighter in the controls than in the IRDA group. IRDA power increased during the last minute of HV and ended 10-15 s after HV. The BOLD signal decreased in white and gray matter after the onset of HV and returned to the baseline within 2 min after HV. The BOLD signal in gray matter decreased approximately 30% more in subjects with IRDA than in controls, during the first 2 min of HV. This difference disappeared (in three subjects out of four) during IRDA in EEG. BOLD signal changes seem to depict changes, which precede IRDA. IRDA due to HV in healthy volunteers represent a model with a clearly defined EEG pattern and an observable BOLD signal change.

Developmental changes in spontaneous smooth muscle activity in the neonatal rat urinary bladder

Changes in spontaneous activity of the urinary bladder during postnatal development were examined in muscle strips from the base and dome of bladders from 1- to 5-wk-old rats. Activity was analyzed using fast Fourier transformation (FFT), nonlinear cross prediction, and the Shannon entropy test. Spontaneous activity was not detected in strips from 1- to 5-day-old rats but was observed in 50% of strips from 6- to 7-day-old rats and was prominent in strips from 2-wk-old animals. FFT analysis revealed one peak in activity, which was significantly faster in the bladder base (0.21 +/- 0.03 Hz) than in the dome (0.08 +/- 0.01 Hz). A second peak at approximately 0.5 Hz was detected at 3-5 wk of age. Atropine but not tetrodotoxin decreased the amplitude of spontaneous contractions, whereas carbachol, a muscarinic agonist, unmasked or stimulated spontaneous activity. These data suggest that slow rhythmic activity observed previously in neonatal whole bladders is generated by pacemaker cells in the bladder base or dome. The emergence of faster activity in bladders from older animals may reflect the development of multiple pacemaker sites, which would reduce coordination within the bladder wall and improve storage function in the mature bladder.

Clinical correlates of frontal intermittent rhythmic delta activity in children

Frontal intermittent rhythmic delta activity is an electrographic pattern of unclear origin. Previously thought to correlate with deep midline and infratentorial pathology, rather, it appears to be associated with encephalopathy states in adults. The significance of frontal intermittent rhythmic delta activity in children has not been studied. We analyzed the electrographic characteristics and clinical associations of pediatric frontal intermittent rhythmic delta activity. This pattern was rarely detected, occurring in 20 of 1500 electroencephalographic (EEG) studies. Patients' ages ranged between 1.5 and 17 years. Most patients were awake and showed no signs of acute encephalopathy when frontal intermittent rhythmic delta activity occurred. Half of the children were cognitively impaired, and half had a history of epilepsy. Epileptiform activity was present in 55% of the EEG recordings. However, frontal intermittent rhythmic delta activity was part of the epileptiform discharge in only a minority of cases. The duration of frontal intermittent rhythmic delta activity bursts was longer in the mentally retarded group. Most patients did not have structural brain pathology. None had deep midline or infratentorial lesions. In conclusion, frontal intermittent rhythmic delta activity is rare in children, is not associated with acute encephalopathy or with deep midline or infratentorial lesions, and tends to occur during wakefulness. The electrographic characteristics of frontal intermittent rhythmic delta activity appear to differ between cognitively normal and mentally retarded children.

Clinical and radiologic correlates of frontal intermittent rhythmic delta activity

To assess the clinical and radiologic correlates of frontal intermittent rhythmic delta activity (FIRDA), the authors reviewed the hospital charts of patients whose EEGs depicted this EEG finding, and recorded their past medical and neurologic history, the reason for hospital admission, and their neurologic status both on admission and during EEG recordings. Laboratory results on admission and concomitant to the EEG recording, computed tomography, or MRI findings during hospital admission were also reviewed. Sixty-eight patients were assessed. The gender ratio was 1:1; mean age was 56 years. Chronic disease occurred in 78% of patients, including hypertension (34%), diabetes (32%), and renal failure (18%). On admission, renal failure (n = 34) and hyperglycemia (n = 22) were most prominent. The majority of patients had at least one abnormal laboratory result. Thirty-eight of 51 patients in whom the level of consciousness was stated during EEG were described as awake. More than half of 58 patients whose EEG background activity was stipulated demonstrated diffuse slowing, mostly in the theta range. MRI was abnormal in 15 of 17 patients. Intrahemispheric lesions, particularly ischemic and hemorrhagic, were most common (n = 10), followed by basal ganglia lacunae (n = 4). Computed tomography was abnormal in 29 of 44 patients. Hemispheric pathology, diffuse or localized, occurred in 22 patients. Frontal intermittent rhythmic delta activity is associated with mild to moderate encephalopathy and is detected principally in awake patients. Most patients in this series had chronic systemic illness. Old ischemic structural brain lesions may predispose some patients to develop FIRDA during acute metabolic derangement, such as uremia and hyperglycemia. Frontal intermittent rhythmic delta activity was not associated with EEG epileptiform activity. Deep midline lesions, posterior fossa tumors, and hydrocephalus were not detected in this series of patients with FIRDA.

The role of TNF-alpha in amygdala kindled rats

In the present study, the interaction between epileptogenesis and the immune system were studied in a kindling model. First, the effects of a single administration of TNF-alpha (5.0 microg/kg, i.p.) on seizure and EEG activity were investigated in amygdala-kindled rats. TNF-alpha treated rats showed more prolonged epileptiformic discharges than control rats. TNF-alpha also induced a decrease in the power of delta band and an increase in theta and alpha activity. In addition, a marked increase in the power of beta and gamma band was observed. The EEG changes were most numerous in the frontal cortex and amygdala. All effects were registered 24 h after TNF-alpha administration. Finally, electrical stimulation enhanced the level of TNF-alpha in blood serum from 1.9 +/- 1.5 to 12.7 +/- 3.8 pg/ml and in brain tissue 56.8 +/- 6.0 to 109.2 +/- 6.0 pg/mg, as was determined via the ELISA method. It can be concluded that there is a mutual facilitative interaction of both epileptogenic and cytokine-derived mechanisms on this type of seizure. The changes in the power spectrum of the EEG after TNF-alpha might contribute to intensify thalamic-derived facilitation of epileptic discharge in cortical structures.

Non-linear EEG analysis in children with epilepsy and electrical status epilepticus during slow-wave sleep (ESES)

OBJECTIVE

The objective of this work was to study the non-linear aspects of electroencephalography (EEG) in children with epilepsy and electrical status epilepticus during slow-wave sleep (ESES).

METHODS

In this study, we recorded the sleep EEG in 5 subjects with ESES (4 males and one female, aged 6.5-10 years) who were also mentally retarded and affected by cerebral palsy (3 subjects) and hydrocephalus (two subjects). The signals were sampled at 128Hz and stored on hard disk. All the subsequent computational steps were performed on EEG epochs (4096 data points) selected from wakefulness and non-rapid eye movement (non-REM) (with ESES) or REM sleep. The dynamic properties of the EEG were assessed by means of the non-linear cross prediction (NLCP) test which uses 3 different 'model' time series in order to predict non-linearly the original data set (Pred, Ama and Tir). Pred is a measure of the predictability of the time series and Ama and Tir are measures of asymmetry, indicating non-linear structure. Moreover, the correlation dimension (D2) was estimated by means of the algorithm by for the epochs showing non-linear nature.

RESULTS

The NLCP test provided evidence of significant non-linear dynamics in all epochs of non-REM sleep, when ESES was evident. Only during this stage, the possible presence of low-dimensional chaos could also be suspected (average D2=4.02; range 3.16-6.21). EEG without ESES could not be distinguished from linearly filtered noise.

CONCLUSIONS

The results of the present study seem to indicate that subjects with ESES show a profound modification of their EEG dynamics with the occurrence, during sleep, of long periods characterized by non-linear dynamics and, probably, low-dimensional chaotic structure able to modify in a substantial way their brain functioning during sleep.