Estimation of fMRI responses related to epileptic discharges using Bayesian hierarchical modeling.

Simultaneous electroencephalography-functional MRI (EEG-fMRI) is a unique and noninvasive method for epilepsy presurgical evaluation. When selecting voxels by null-hypothesis tests, the conventional analysis may overestimate fMRI response amplitudes related to interictal epileptic discharges (IEDs), especially when IEDs are rare. We aimed to estimate fMRI response amplitudes represented by blood oxygen level dependent (BOLD) percentage changes related to IEDs using a hierarchical model. It involves the local and distributed hemodynamic response homogeneity to regularize estimations. Bayesian inference was applied to fit the model. Eighty-two epilepsy patients who underwent EEG-fMRI and subsequent surgery were included in this study. A conventional voxel-wise general linear model was compared to the hierarchical model on estimated fMRI response amplitudes and on the concordance between the highest response cluster and the surgical cavity. The voxel-wise model overestimated fMRI responses compared to the hierarchical model, evidenced by a practically and statistically significant difference between the estimated BOLD percentage changes. Only the hierarchical model differentiated brief and long-lasting IEDs with significantly different BOLD percentage changes. Overall, the hierarchical model outperformed the voxel-wise model on presurgical evaluation, measured by higher prediction performance. When compared with a previous study, the hierarchical model showed higher performance metric values, but the same or lower sensitivity. Our results demonstrated the capability of the hierarchical model of providing more physiologically reasonable and more accurate estimations of fMRI response amplitudes induced by IEDs. To enhance the sensitivity of EEG-fMRI for presurgical evaluation, it may be necessary to incorporate more appropriate spatial priors and bespoke decision strategies.

FUS-mediated blood-brain barrier disruption for delivering anti-Aß antibodies in 5XFAD Alzheimer's disease mice.

PURPOSE: Amyloid-ß (Aß) peptides, the main component of amyloid plaques found in the Alzheimer's disease (AD) brain, are implicated in its pathogenesis, and are considered a key target in AD therapeutics. We herein propose a reliable strategy for non-invasively delivering a specific anti-Aß antibody in a mouse model of AD by microbubbles-enhanced Focused Ultrasound (FUS)-mediated Blood-brain barrier disruption (BBBD), using a simple single stage MR-compatible positioning device. METHODS: The initial experimental work involved wild-type mice and was devoted to selecting the sonication protocol for efficient and safe BBBD. Pulsed FUS was applied using a single-element FUS transducer of 1 MHz (80 mm radius of curvature and 50 mm diameter). The success and extent of BBBD were assessed by Evans Blue extravasation and brain damage by hematoxylin and eosin staining. 5XFAD mice were divided into different subgroups; control (n = 1), FUS + MBs alone (n = 5), antibody alone (n = 5), and FUS + antibody combined (n = 10). The changes in antibody deposition among groups were determined by immunohistochemistry. RESULTS: It was confirmed that the antibody could not normally enter the brain parenchyma. A single treatment with MBs-enhanced pulsed FUS using the optimized protocol (1 MHz, 0.5 MPa in-situ pressure, 10 ms bursts, 1% duty factor, 100 s duration) transiently disrupted the BBB allowing for non-invasive antibody delivery to amyloid plaques within the sonicated brain regions. This was consistently reproduced in ten mice. CONCLUSION: These preliminary findings should be confirmed by longer-term studies examining the antibody effects on plaque clearance and cognitive benefit to hold promise for developing disease-modifying anti-Aß therapeutics for clinical use.

Equipment Setup and Artifact Removal for Simultaneous Electroencephalogram and Functional Magnetic Resonance Imaging for Clinical Review in Epilepsy.

Simultaneous electroencephalogram and functional magnetic resonance imaging (EEG-fMRI) is a unique combined technique that provides synergy in the understanding and localization of seizure onset in epilepsy. However, reported experimental protocols for EEG-fMRI recordings fail to address details about conducting such procedures on epilepsy patients. In addition, these protocols are limited solely to research settings. To fill the gap between patient monitoring in an epilepsy monitoring unit (EMU) and conducting research with an epilepsy patient, we introduce a unique EEG-fMRI recording protocol of epilepsy during the interictal period. The use of an MR conditional electrode set, which can also be used in the EMU for a simultaneous scalp EEG and video recording, allows an easy transition of EEG recordings from the EMU to the scanning room for concurrent EEG-fMRI recordings. Details on the recording procedures using this specific MR conditional electrode set are provided. In addition, the study explains step-by-step EEG processing procedures to remove the imaging artifacts, which can then be used for clinical review. This experimental protocol promotes an amendment to the conventional EEG-fMRI recording for enhanced applicability in both clinical (i.e., EMU) and research settings. Furthermore, this protocol provides the potential to expand this modality to postictal EEG-fMRI recordings in the clinical setting.

Glial Gap Junction Pathology in the Spinal Cord of the 5xFAD Mouse Model of Early-Onset Alzheimer's Disease.

Gap junctions (GJs) are specialized transmembrane channels assembled by two hemi-channels of six connexin (Cx) proteins that facilitate neuroglial crosstalk in the central nervous system (CNS). Previous studies confirmed the crucial role of glial GJs in neurodegenerative disorders with dementia or motor dysfunction including Alzheimer's disease (AD). The aim of this study was to examine the alterations in astrocyte and related oligodendrocyte GJs in association with Aß plaques in the spinal cord of the 5xFAD mouse model of AD. Our analysis revealed abundant Aß plaque deposition, activated microglia, and astrogliosis in 12-month-old (12M) 5xFAD mice, with significant impairment of motor performance starting from 3-months (3M) of age. Additionally, 12M 5xFAD mice displayed increased immunoreactivity of astroglial Cx43 and Cx30 surrounding Aß plaques and higher protein levels, indicating upregulated astrocyte-to-astrocyte GJ connectivity. In addition, they demonstrated increased numbers of mature CC1-positive and precursor oligodendrocytes (OPCs) with higher immunoreactivity of Cx47-positive GJs in individual cells. Moreover, total Cx47 protein levels were significantly elevated in 12M 5xFAD, reflecting increased oligodendrocyte-to-oligodendrocyte Cx47-Cx47 GJ connectivity. In contrast, we observed a marked reduction in Cx32 protein levels in 12M 5xFAD spinal cords compared with controls, while qRT-PCR analysis revealed a significant upregulation in Cx32 mRNA levels. Finally, myelin deficits were found focally in the areas occupied by Aß plaques, whereas axons themselves remained preserved. Overall, our data provide novel insights into the altered glial GJ expression in the spinal cord of the 5xFAD model of AD and the implicated role of GJ pathology in neurodegeneration. Further investigation to understand the functional consequences of these extensive alterations in oligodendrocyte-astrocyte (O/A) GJ connectivity is warranted.

Clinical Yield of Electromagnetic Source Imaging and Hemodynamic Responses in Epilepsy: Validation With Intracerebral Data.

OBJECTIVES: Accurate delineation of the seizure-onset zone (SOZ) in focal drug-resistant epilepsy often requires stereo-electroencephalography (SEEG) recordings. We aimed at: (1) proposing a truly objective and quantitative comparison between electro-encephalography/magnetoencephalography (EEG/MEG) source-imaging (EMSI), EEG/functional MRI (EEG/fMRI) responses for similar spikes with primary-irritative zone (PIZ) and SOZ defined by SEEG and (2) evaluating the value of EMSI and EEG/fMRI to predict postsurgical outcome. METHODS: We identified patients with drug-resistant epilepsy who underwent EEG/MEG, EEG/fMRI, and subsequent SEEG at the Epilepsy Service from the Montreal Neurological Institute and Hospital. We quantified multimodal concordance within the SEEG channel-space, as spatial overlap with PIZ/SOZ and distances to the Spike-onset, Spike-maximum-amplitude and Seizure-core intracerebral channels, by applying a new methodology consisting of converting EMSI results into SEEG electrical potentials (EMSIe-SEEG) and projecting the most significant fMRI response on the SEEG channels (fMRIp-SEEG). Spatial overlaps with PIZ/SOZ (AUCPIZ, AUCSOZ) were assessed by using the area under the receiver operating characteristic curve (AUC). Here, AUC represents the probability that a randomly picked active contact exhibited higher amplitude when located inside the spatial reference than outside. RESULTS: Seventeen patients were included. Mean spatial overlaps with the primary-irritative zone and seizure-onset zone were 0.71 and 0.65 for EMSIe-SEEG, and 0.57 and 0.62 for fMRIp-SEEG. Good EMSIe-SEEG  spatial overlap with the primary-irritative zone was associated with smaller distance from the maximum EMSIe-SEEG contact to the Spike-maximum-amplitude channel (median distance 14 mm). Conversely, good fMRIp-SEEG spatial overlap with the seizure-onset zone was associated with smaller distances from the maximum  fMRIp-SEEG contact to the Spike-onset and Seizure-core channels (median distances 10 mm and 5mm respectively). Surgical outcomes were correctly predicted by EEG/MEG in 12/15 (80%) patients and EEG/fMRI in 6/11(54%) patients. CONCLUSIONS: Using a unique quantitative approach estimating EMSI and fMRI results in the reference SEEG channel-space, EEG/MEG and EEG/fMRI accurately localized the seizure-onset zone as well as the primary-irritative zone. Precisely, EEG/MEG more accurately localized the primary-irritative zone, whereas EEG/fMRI was more sensitive to the seizure-onset zone. Both neuro-imaging techniques provide complementary localization that can help guiding SEEG implantation and selecting good candidates for surgery.

New interinstitutional, multimodal presurgical evaluation protocol associated with improved seizure freedom for poorly defined cases of focal epilepsy in children.

OBJECTIVE: In an attempt to improve postsurgical seizure outcomes for poorly defined cases (PDCs) of pediatric focal epilepsy (i.e., those that are not visible or well defined on 3T MRI), the authors modified their presurgical evaluation strategy. Instead of relying on concordance between video-electroencephalography and 3T MRI and using functional imaging and intracranial recording in select cases, the authors systematically used a multimodal, 3-tiered investigation protocol that also involved new collaborations between their hospital, the Montreal Children's Hospital, and the Montreal Neurological Institute. In this study, the authors examined how their new strategy has impacted postsurgical outcomes. They hypothesized that it would improve postsurgical seizure outcomes, with the added benefit of identifying a subset of tests contributing the most. METHODS: Chart review was performed for children with PDCs who underwent resection following the new strategy (i.e., new protocol [NP]), and for the same number who underwent treatment previously (i.e., preprotocol [PP]); ≥ 1-year follow-up was required for inclusion. Well-defined, multifocal, and diffuse hemispheric cases were excluded. Preoperative demographics and clinical characteristics, resection volumes, and pathology, as well as seizure outcomes (Engel class Ia vs > Ia) at 1 year postsurgery and last follow-up were reviewed. RESULTS: Twenty-two consecutive NP patients were compared with 22 PP patients. There was no difference between the two groups for resection volumes, pathology, or preoperative characteristics, except that the NP group underwent more presurgical evaluation tests (p < 0.001). At 1 year postsurgery, 20 of 22 NP patients and 10 of 22 PP patients were seizure free (OR 11.81, 95% CI 2.00-69.68; p = 0.006). Magnetoencephalography and PET/MRI were associated with improved postsurgical seizure outcomes, but both were highly correlated with the protocol group (i.e., independent test effects could not be demonstrated). CONCLUSIONS: A new presurgical evaluation strategy for children with PDCs of focal epilepsy led to improved postsurgical seizure freedom. No individual presurgical evaluation test was independently associated with improved outcome, suggesting that it may be the combined systematic protocol and new interinstitutional collaborations that makes the difference rather than any individual test.

Association of EEG-fMRI Responses and Outcome After Epilepsy Surgery.

OBJECTIVE: To assess the utility of EEG-fMRI for epilepsy surgery, we evaluated surgical outcome in relation to the resection of the most significant EEG-fMRI response. METHODS: Patients with post-operative neuroimaging and follow-up of at least one year were included. In EEG-fMRI responses, we defined as "primary" the cluster with the highest absolute t-value located in the cortex, and evaluated three levels of confidence for the results. The threshold for low confidence was t ≥ 3.1 (p < 0.005); the one for medium confidence corresponded to correction for multiple comparisons with a false discovery rate of 0.05; and a result reached high confidence when the primary cluster was much more significant than the next highest cluster. Concordance with the resection was determined by comparison to post-operative neuroimaging. RESULTS: We evaluated 106 epilepsy surgeries in 84 patients. An increasing association between concordance and surgical outcome with higher levels of confidence was demonstrated. If the peak response was not resected, the surgical outcome was likely to be poor: for the high confidence level, no patient had a good outcome; for the medium and low levels, only 18% and 28% had a good outcome. The positive predictive value remained low for all confidence levels, indicating that removing the maximum cluster did not ensure seizure freedom. CONCLUSION: Resection of the primary EEG-fMRI cluster, especially in high confidence cases, is necessary to obtain a good outcome, but not sufficient. CLASSIFICATION OF EVIDENCE: This study provided Class II evidence that failure to resect the primary EEG-fMRI cluster is associated with poorer epilepsy surgery outcomes.

A hemodynamic network involving the insula, the cingulate, and the basal forebrain correlates with EEG synchronization phases of sleep instability.

The cyclic alternating pattern (CAP) encompasses the pseudoperiodic appearance of synchronized brain waves and rhythms and is considered a regulator of the nonrapid eye movement (NREM) sleep vigilance level, reflecting sleep instability. To determine the brain regions responsible for this phenomenon, we scored and analyzed sleep functional magnetic resonance imaging data acquired with simultaneous electroencephalography (EEG-fMRI). Group analysis revealed a set of brain areas showing statistically significant blood oxygen-level dependent signal correlated positively with the synchronization phase of the CAP, most prominent being the insula, the middle cingulate gyrus, and the basal forebrain. These areas may form a network acting as a synchronization pacemaker, controlling the level of NREM sleep vigilance and the sleeper's arousability.

Spatiotemporal propagation patterns of generalized ictal spikes in childhood absence epilepsy.

OBJECTIVE: This work investigates the spatial distribution in time of generalized ictal spikes in the typical absences of childhood absence epilepsy (CAE). METHODS: We studied twelve children with CAE, who had more than two typical absences during their routine video-EEG. Seizures were identified, and ictal spikes were marked over the maximum electronegative peak, clustered, waveform-averaged and spatiotemporaly analyzed in 2D electrode space. RESULTS: Consistency of spatiotemporal patterns of ictal spikes was high between the absences of the same child, but low between children. Three main discharge patterns were identified: of anterio-posterior propagation, of posterio-anterior propagation and confined to the frontal/prefrontal regions. In 4 patients, the propagation patterns transformed during the seizure into either a lateralized diminished or a non-lateralized reverse direction form. Most spikes originated fronto-temporaly, all maximized over the frontal/prefrontal electrodes and mostly decayed prefrontaly. In 4 patients, lateralized propagation patterns were identified. CONCLUSIONS: Ictal spike propagation patterns suggest that epileptogenic CAE networks are personalized, interconnect distal areas in the brain - not the entire cortex - with a tendency to generate bilateral symmetrical discharges, sometimes unsuccessfully. The transformation of propagation patterns during the seizure indicates the existence of dynamic interplay within epileptogenic networks. SIGNIFICANCE: Our results support the revised concept of ictogenesis of ILAE definition in genetic (also known as idiopathic) generalized epilepsies. Understanding the focal features in CAE avoids misdiagnosis as focal epilepsy and inappropriate treatment.

Connectivity Measures in EEG Microstructural Sleep Elements.

During Non-Rapid Eye Movement sleep (NREM) the brain is relatively disconnected from the environment, while connectedness between brain areas is also decreased. Evidence indicates, that these dynamic connectivity changes are delivered by microstructural elements of sleep: short periods of environmental stimuli evaluation followed by sleep promoting procedures. The connectivity patterns of the latter, among other aspects of sleep microstructure, are still to be fully elucidated. We suggest here a methodology for the assessment and investigation of the connectivity patterns of EEG microstructural elements, such as sleep spindles. The methodology combines techniques in the preprocessing, estimation, error assessing and visualization of results levels in order to allow the detailed examination of the connectivity aspects (levels and directionality of information flow) over frequency and time with notable resolution, while dealing with the volume conduction and EEG reference assessment. The high temporal and frequency resolution of the methodology will allow the association between the microelements and the dynamically forming networks that characterize them, and consequently possibly reveal aspects of the EEG microstructure. The proposed methodology is initially tested on artificially generated signals for proof of concept and subsequently applied to real EEG recordings via a custom built MATLAB-based tool developed for such studies. Preliminary results from 843 fast sleep spindles recorded in whole night sleep of 5 healthy volunteers indicate a prevailing pattern of interactions between centroparietal and frontal regions. We demonstrate hereby, an opening to our knowledge attempt to estimate the scalp EEG connectivity that characterizes fast sleep spindles via an "EEG-element connectivity" methodology we propose. The application of the latter, via a computational tool we developed suggests it is able to investigate the connectivity patterns related to the occurrence of EEG microstructural elements. Network characterization of specified physiological or pathological EEG microstructural elements can potentially be of great importance in the understanding, identification, and prediction of health and disease.

Semi-automatic sleep EEG scoring based on the hypnospectrogram.

BACKGROUND: Sleep EEG organization is revealed by sleep scoring, a time-consuming process based on strictly defined visual criteria. NEW METHOD: We explore the possibility of sleep scoring using the whole-night time-frequency analysis, termed hypnospectrogram, with a computer-assisted K-means clustering method. RESULTS: Hypnograms were derived from 10 whole-night sleep EEG recordings using either standard visual scoring under the Rechtshaffen and Kales criteria or semi-automated analysis of the hypnospectrogram derived from a single EEG electrode. We measured substantial agreement between the two approaches with Cohen's kappa considering all 7 stages at 0.61. COMPARISON WITH EXISTING METHODS: A number of existing automated procedures have reached the level of human inter-rater agreement using the standard criteria. However, our approach offers the scorer the opportunity to exploit the information-rich graphic representation of the whole night sleep upon which the automated method works. CONCLUSION: This work suggests that the hypnospectrogram can be used as an objective graphical rep-resentation of sleep architecture upon which sleep scoring can be performed with computer-assisted methods.

One-class classification of temporal EEG patterns for K-complex extraction.

The purpose of this study was to detect one of the constituent brain waveforms in electroencephalography (EEG), the K-complex (KC). The role and significance of the KC include its engagement in information processing, sleep protection, and memory consolidation [1]. The method applies a two-step methodology in which first all the candidate KC waves are extracted based on fundamental morphological features imitating visual criteria. Subsequently each candidate wave is classified as KC or outlier according to its similarity to a set of different patterns (clusters) of annotated KCs. The different clusters are constructed by applying graph partitioning on the training set based on spectral clustering and exhibit temporal similarities in both signal and frequency content. The method was applied in whole-night sleep activity recorded using multiple EEG electrodes. Cross-validation was performed against visual scoring of singular generalized KCs during all sleep cycles and showed high sensitivity in KC detection.

Spatiotemporal profiles of focal and generalised spikes in childhood absence epilepsy.

The EEG in childhood absence epilepsy (CAE) may contain focal and generalised spike-wave discharges (SWDs) with focal, mainly frontal, "lead-in". The term "frontal absence" has been used to imply fast, secondary, 3-Hz generalisation from occult frontal foci with potential impact on clinical EEG interpretation and syndrome classification. The aim of this study was to investigate the relationship between focal and generalised SWDs. We studied five children with CAE and examined a sufficient number of focal ("interictal") and generalised SWDs in order to obtain reliable analysis. All generalised SWDs with focal lead-in were "decomposed" into their "pre-generalisation" focal and "generalised" constituents, which were studied separately. Two types of focal SWD ("interictal" and "pre-generalisation") and generalised SWD were visually clustered into groups, waveform-averaged, and plotted in the 2D-electrode space. Spatiotemporal analysis demonstrated a variety (mean: 4.2 per child; SD: 2.12) of mainly frontal and occipital locations for pre-generalisation focal SWDs with propagation along the longitudinal axis in either direction and across homologous sites. Interictal focal SWDs demonstrated similar spatiotemporal characteristics. In contrast, the topography and propagation patterns of the first generalised spike of the SWD showed less variability (mean: 2.5 per child; SD: 2.07), mainly involved the fronto-temporal/temporal areas, and correlated poorly (<10%) with that of the pre-generalisation focal SWD. Our findings suggest that the process of generalised epileptogenesis in genetic epilepsies with electrographic "frontal absences" is far more complex than that proposed by the model for occult frontal focus with fast secondary generalisation. (Published with Supplemental data).

An intra-K-complex oscillation with independent and labile frequency and topography in NREM sleep.

NREM sleep is characterized by K-complexes (KCs), over the negative phase of which we identified brief activity in the theta range. We recorded high resolution EEG of whole-night sleep from seven healthy volunteers and visually identified 2nd and 3rd stage NREM spontaneous KCs. We identified three major categories: (1) KCs without intra-KC-activity (iKCa), (2) KCs with non-oscillatory iKCa, and (3) KCs with oscillatory iKCa. The latter group of KCs with intra-KC-oscillation (iKCo), was clustered according to the duration of the iKCo. iKCa was observed in most KCs (1150/1522, 75%). iKCos with 2, 3, and 4 waves were observed in 52% (786/1522) of KCs in respective rates of 49% (386/786), 44%, and 7%. Successive waves of iKCos showed on average a shift of their maximal amplitude in the anterio-posterior axis, while the average amplitude of the slow KC showed no spatial shift in time. The iKCo spatial shift was accompanied by transient increases in instantaneous frequency from the theta band toward the alpha band, followed by decreases to upper theta. The study shows that the KC is most often concurrently accompanied by an independent brief iKCo exhibiting topographical relocation of amplitude maxima with every consecutive peak and transient increases in frequency. The iKCo features are potentially reflecting arousing processes taking place during the KC.

Spindle power is not affected after spontaneous K-complexes during human NREM sleep.

K-complexes and sleep spindles often grouped together characterize the second stage of NREM sleep and interest has been raised on a possible interaction of their underlying mechanisms. The reported inhibition of spindles power for about 15 seconds following evoked K-complexes has implications on their role in arousal. Our objective was to assess this inhibition following spontaneous K-complexes. We used time-frequency analysis of spontaneous K-complexes selected from whole-night EEG recordings of normal subjects. Our results show that spindles are most often observed at the positive phase following the peak of a spontaneous KC (70%). At latencies of 1-3 s following the peak of the K-complex, spindles almost disappear. Compared to long-term effects described for evoked KCs, sleep spindle power is not affected by spontaneous KCs for latencies of 5-15 s. Observation of the recurrence rate of sporadic spindles suggests that the reduction of power at 1-3 s most likely reflects a refractory period of spindles lasting for 1-2 s, rather than an effect of KCs. These results suggest that the mechanisms underlying spontaneous KCs do not affect spindle power as in the case of evoked KCs.

Multifocal spatiotemporal distribution of interictal spikes in Panayiotopoulos syndrome.

OBJECTIVE: To investigate the spatiotemporal course of interictal spikes in Panayiotopoulos syndrome (PS), and in particular whether seemingly independent extra-occipital spikes are truly autonomous or secondary, triggered by occipital spikes. METHODS: Seven children with the most representative interictal spike patterns on visual analysis were studied. Five had a single focus (occipital in two, suggestive of posterior to anterior spike propagation in two, and frontal) and two had two foci over the posterior and the anterior areas independently. Spikes were marked, clustered and waveform - averaged, and mapped on electrode space. RESULTS: The patterns of spatial and temporal dynamics of the interictal spikes were not stereotypical for any brain area, including the occipital lobe. Some of the anterior and the posterior spikes remained focal or showed little spread, but others appeared to propagate to the opposite direction (occipital to frontal and vice versa). CONCLUSIONS: In PS all cerebral locations are able to spontaneously and independently generate and propagate interictal spikes, indicating that PS is a multifocal epileptic syndrome. SIGNIFICANCE: Confirmation of the multifocal character of PS improves clinical diagnosis and challenges our current taxonomic concepts by expanding the anatomical boundaries of a distinct focal epilepsy phenotype from lobar to system.

The hypnospectrogram: an EEG power spectrum based means to concurrently overview the macroscopic and microscopic architecture of human sleep.

This study introduces a complementary tool for the description and evaluation of human sleep. The nocturnal sleep electroencephalographic (EEG) time-frequency analysis (TFA) plot (hypnospectrogram for short) is hereby proposed as a means to visualize both the macroscopic and the microscopic architecture of human sleep. It provides the ability to concurrently visually inspect the coarse sleep architecture, that is, the time-course of non-rapid eye movement (NREM) and REM stages, along with finer sleep elements such as slow and fast spindles, NREM delta distribution, REM alpha and beta, microarousals (MAs), and NREM cyclic alternating patterns (CAPs). Furthermore, the hypnospectrogram has the potential to provide visual quality of sleep (QoS) evaluation, as well as reveal the dominant rhythms and their transitions for every cerebral locus - as represented at the electrode space - during the night.