Epileptiform propagation patterns mediated by NMDA and non-NMDA receptors in rat neocortex

Preferential superficial cortical layer activation during seizure propagation

OBJECTIVE

Focal cortical seizures travel long distances from the onset zone, but the long-distance propagation pathways are uncertain. In vitro and in vivo imaging techniques have investigated the local spread of seizures but did not elucidate long-distance spread. Furthermore, classical studies in slices suggested seizure spread locally along deep cortical layers, whereas more recent in vivo imaging studies posit a role for superficial cortical layers in local spread.

METHODS

We imaged seizure-activated neurons using activity reporter mice and measured local field potentials (LFPs) using microelectrode arrays to map cortical seizure propagation in awake mice.

RESULTS

Frontal lobe onset seizures activate more neurons in superficial layers 2-3 than deep layers 5-6 throughout the cortex. LFP recordings demonstrate that seizures spread faster through the superficial than deep layers over long cortical distances of 3.5 mm. We also show that monosynaptically connected long-distance neurons are in the seizure circuit.

SIGNIFICANCE

We propose that long-distance cortical seizure spread occurs preferentially via synaptically connected superficial cortical neurons.

Comparison of Serum Selenium, Homocysteine, Zinc, and Vitamin D Levels in Febrile Children with and without Febrile Seizures: A Prospective Single-Center Study

OBJECTIVE

Febrile seizure is a complication that makes physicians and families uneasy when detected in children with a high fevers. This study aimed to compare children with febrile seizures and children without seizures in blood selenium, zinc, homocysteine, vitamin D, vitamin B12, and magnesium levels.

MATERIALS AND METHODS

The study group included sixty-one children between the ages of 1-5 who came to the pediatric emergency department with febrile seizure. The control group had 61 children with fever without seizure, who were compatible with the study group in age, sex, and elapsed time since the onset of fever. Blood samples were taken from the patients during their admission. Selenium, zinc, vitamin D, homocysteine, vitamin B12, and magnesium levels were measured, and the data of the two groups were compared. Additionally, patients in the study group had two subgroups, simple and complex febrile seizures, and their parameters were compared.

RESULTS

Selenium, zinc, vitamin D, and vitamin B12 levels were significantly lower in the study group than in the control group (p < 0.001), and there was no significant difference in homocysteine (p = 0.990) and magnesium levels (p = 0.787) between the two groups. Moreover, no significant difference was found between those with simple and complex febrile seizures in selenium, vitamin D, homocysteine, vitamin B12, and magnesium levels.

CONCLUSIONS

Elevated levels of selenium, zinc, vitamin D, and vitamin B12 in the blood of children with fevers help to prevent febrile seizures.

TMS-EEG signatures of glutamatergic neurotransmission in human cortex

Neuronal activity in the brain reflects an excitation-inhibition balance that is regulated predominantly by glutamatergic and GABAergic neurotransmission, and often disturbed in neuropsychiatric disorders. Here, we tested the effects of a single oral dose of two anti-glutamatergic drugs (dextromethorphan, an NMDA receptor antagonist; perampanel, an AMPA receptor antagonist) and an L-type voltage-gated calcium channel blocker (nimodipine) on transcranial magnetic stimulation (TMS)-evoked electroencephalographic (EEG) potentials (TEPs) and TMS-induced oscillations (TIOs) in 16 healthy adults in a pseudorandomized, double-blinded, placebo-controlled crossover design. Single-pulse TMS was delivered to the hand area of left primary motor cortex. Dextromethorphan increased the amplitude of the N45 TEP, while it had no effect on TIOs. Perampanel reduced the amplitude of the P60 TEP in the non-stimulated hemisphere, and increased TIOs in the beta-frequency band in the stimulated sensorimotor cortex, and in the alpha-frequency band in midline parietal channels. Nimodipine and placebo had no effect on TEPs and TIOs. The TEP results extend previous pharmaco-TMS-EEG studies by demonstrating that the N45 is regulated by a balance of GABAAergic inhibition and NMDA receptor-mediated glutamatergic excitation. In contrast, AMPA receptor-mediated glutamatergic neurotransmission contributes to propagated activity reflected in the P60 potential and midline parietal induced oscillations. This pharmacological characterization of TMS-EEG responses will be informative for interpreting TMS-EEG abnormalities in neuropsychiatric disorders with pathological excitation-inhibition balance.

Layer- and Cell-Specific Recruitment Dynamics during Epileptic Seizures In Vivo

OBJECTIVE

To investigate the network dynamics mechanisms underlying differential initiation of epileptic interictal spikes and seizures.

METHODS

We performed combined in vivo 2-photon calcium imaging from different targeted neuronal subpopulations and extracellular electrophysiological recordings during 4-aminopyridine-induced neocortical spikes and seizures.

RESULTS

Both spikes and seizures were associated with intense synchronized activation of excitatory layer 2/3 pyramidal neurons (PNs) and to a lesser degree layer 4 neurons, as well as inhibitory parvalbumin-expressing interneurons (INs). In sharp contrast, layer 5 PNs and somatostatin-expressing INs were gradually and asynchronously recruited into the ictal activity during the course of seizures. Within layer 2/3, the main difference between onset of spikes and seizures lay in the relative recruitment dynamics of excitatory PNs compared to parvalbumin- and somatostatin-expressing inhibitory INs. Whereas spikes exhibited balanced recruitment of PNs and parvalbumin-expressing INs, during seizures IN responses were reduced and less synchronized than in layer 2/3 PNs. Similar imbalance was not observed in layers 4 or 5 of the neocortex. Machine learning-based algorithms we developed were able to distinguish spikes from seizures based solely on activation dynamics of layer 2/3 PNs at discharge onset.

INTERPRETATION

During onset of seizures, the recruitment dynamics markedly differed between neuronal subpopulations, with rapid synchronous recruitment of layer 2/3 PNs, layer 4 neurons, and parvalbumin-expressing INs and gradual asynchronous recruitment of layer 5 PNs and somatostatin-expressing INs. Seizures initiated in layer 2/3 due to a dynamic mismatch between local PNs and inhibitory INs, and only later spread to layer 5 by gradually and asynchronously recruiting PNs in this layer. ANN NEUROL 2020;87:97-115.

Sustained consumption of cocoa-based dark chocolate enhances seizure-like events in the mouse hippocampus

While the consumption of caffeine and cocoa has been associated with a variety of health benefits to humans, some authors have proposed that excessive caffeine intake may increase the frequency of epileptic seizures in humans and reduce the efficiency of antiepileptic drugs. Little is known, however, about the proconvulsant potential of the sustained, excessive intake of cocoa on hippocampal neural circuits. Using the mouse as an experimental model, we examined the effects of the chronic consumption of food enriched in cocoa-based dark chocolate on motor and mood-related behaviours as well as on the excitability properties of hippocampal neurons. Cocoa food enrichment did not affect body weights or mood-related behaviours but rather promoted general locomotion and improved motor coordination. However, ex vivo electrophysiological analysis revealed a significant enhancement in seizure-like population spike bursting at the neurogenic dentate gyrus, which was paralleled by a significant reduction in the levels of GABA-α1 receptors thus suggesting that an excessive dietary intake of cocoa-enriched food might alter some of the synaptic elements involved in epileptogenesis. These data invite further multidisciplinary research aiming to elucidate the potential deleterious effects of chocolate abuse on behaviour and brain hyperexcitability.

Mild systemic inflammation and moderate hypoxia transiently alter neuronal excitability in mouse somatosensory cortex

During the perinatal period, the brain is highly vulnerable to hypoxia and inflammation, which often cause white matter injury and long-term neuronal dysfunction such as motor and cognitive deficits or epileptic seizures. We studied the effects of moderate hypoxia (HYPO), mild systemic inflammation (INFL), or the combination of both (HYPO+INFL) in mouse somatosensory cortex induced during the first postnatal week on network activity and compared it to activity in SHAM control animals. By performing in vitro electrophysiological recordings with multi-electrode arrays from slices prepared directly after injury (P8-10), one week after injury (P13-16), or in young adults (P28-30), we investigated how the neocortical network developed following these insults. No significant difference was observed between the four groups in an extracellular solution close to physiological conditions. In extracellular 8mM potassium solution, slices from the HYPO, INFL, and HYPO+INFL group were more excitable than SHAM at P8-10 and P13-16. In these two age groups, the number and frequency of spontaneous epileptiform events were significantly increased compared to SHAM. The frequency of epileptiform events was significantly reduced by the NMDA antagonist D-APV in HYPO, INFL, and HYPO+INFL, but not in SHAM, indicating a contribution of NMDA receptors to this pathophysiological activity. In addition, the AMPA/kainate receptor antagonist CNQX suppressed the remaining epileptiform activity. Electrical stimulation evoked prominent epileptiform activity in slices from HYPO, INFL and HYPO+INFL animals. Stimulation threshold to elicit epileptiform events was lower in these groups than in SHAM. Evoked events spread over larger areas and lasted longer in treated animals than in SHAM. In addition, the evoked epileptiform activity was reduced in the older (P28-30) group indicating that cortical dysfunction induced by hypoxia and inflammation was transient and compensated during early development.

Spatiotemporal dynamics of high-K+-induced epileptiform discharges in hippocampal slice and the effects of valproate

The epileptic seizure is a dynamic process involving a rapid transition from normal activity to a state of hypersynchronous neuronal discharges. Here we investigated the network properties of epileptiform discharges in hippocampal slices in the presence of high K(+) concentration (8.5 mmol/L) in the bath, and the effects of the anti-epileptic drug valproate (VPA) on epileptiform discharges, using a microelectrode array. We demonstrated that epileptiform discharges were predominantly initiated from the stratum pyramidale layer of CA3a-b and propagated bi-directionally to CA1 and CA3c. Disconnection of CA3 from CA1 abolished the discharges in CA1 without disrupting the initiation of discharges in CA3. Further pharmacological experiments showed that VPA at a clinically relevant concentration (100 μmol/L) suppressed the propagation speed but not the rate or duration of high-K(+)-induced discharges. Our findings suggest that pacemakers exist in the CA3a-b region for the generation of epileptiform discharges in the hippocampus. VPA reduces the conduction of such discharges in the network by reducing the propagation speed.

Making generalizations about seizure propagation

The Source of Afterdischarge Activity in Neocortical Tonic–Clonic Epilepsy. Trevelyan AJ, Baldeweg T, van Drongelen W, Yuste R, Whittington M. J Neurosci 2007;27(49):13513–13519. Tonic–clonic seizures represent a common pattern of epileptic discharges, yet the relationship between the various phases of the seizure remains obscure. Here we contrast propagation of the ictal wavefront with the propagation of individual discharges in the clonic phase of the event. In an in vitro model of tonic–clonic epilepsy, the after discharges (clonic phase) propagate with relative uniform speed and are independent of the speed of the ictal wavefront (tonic phase). For slowly propagating ictal wave fronts, the source of the afterdischarges, relative to a given recording electrode, switched as the wavefront passed by, indicating that afterdischarges are seeded from wavefront itself. In tissue that has experienced repeated ictal events, the wavefront generalizes rapidly, and the afterdischarges in this case show a different “flip-flop” pattern, with frequent switches in their direction of propagation. This same flip-flop pattern is also seen in subdural EEG recordings in patients suffering intractable focal seizures caused by cortical dysplasias. Thus, in both slowly and rapidly generalizing ictal events, there is not a single source of afterdischarge activity: rather, the source is continuously changing. Our data suggest a complex view of seizures in which the ictal event and its constituent discharges originate from distinct locations.

Glutamate and dopamine receptors contribute to the lateral spread of epileptiform discharges in rat neocortical slices

PURPOSE

The effects of AMPA-type glutamate receptor as well as dopamine D1 and D2 receptors on the lateral propagation of epileptiform field potentials (EFP) were studied across adjacent areas of rat neocortical tissues.

METHODS

Epileptiform burst discharges were induced by superfusion of Mg(2+)-free artificial cerebrospinal fluid. Simultaneous field potential recordings of EFP were obtained from four microelectrodes placed 2-3 mm apart across coronal slices in the third layer of the neocortex. The effects of AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), dopamine D1 receptor agonist SKF 81297, and dopamine D2 receptor agonist quinpirole on lateral propagation of burst discharges were investigated.

RESULTS

CNQX, applied focally between recording sites, blocked rapid propagation across treated areas and resulted in the emergence of spatially separate, independent pacemakers. Focal application of SKF 81297 between recording sites increased the repetition rate of EFP, but reduced the amplitude as well as the duration of epileptic discharges. However, addition of SKF 81297 to the bath medium abolished EFP. Both local and systemic applications of quinpirole irreversibly enhanced repetition rate of epileptiform burst discharges.

CONCLUSIONS

The results indicate the prerequisite of AMPA synaptic transmission for synchronized lateral propagation of Mg(2+)-free ACSF-induced epileptic activity and the modulatory effects of dopamine D1 and D2 receptors on both EFP initiation and propagation in epileptic tissues.

Modular propagation of epileptiform activity: evidence for an inhibitory veto in neocortex

What regulates the spread of activity through cortical circuits? We present here data indicating a pivotal role for a vetoing inhibition restraining modules of pyramidal neurons. We combined fast calcium imaging of network activity with whole-cell recordings to examine epileptiform propagation in mouse neocortical slices. Epileptiform activity was induced by washing Mg2+ ions out of the slice. Pyramidal cells receive barrages of inhibitory inputs in advance of the epileptiform wave. The inhibitory barrages are effectively nullified at low doses of picrotoxin (2.5-5 microM). When present, however, these inhibitory barrages occlude an intense excitatory synaptic drive that would normally exceed action potential threshold by approximately a factor of 10. Despite this level of excitation, the inhibitory barrages suppress firing, thereby limiting further neuronal recruitment to the ictal event. Pyramidal neurons are recruited to the epileptiform event once the inhibitory restraint fails and are recruited in spatially clustered populations (150-250 microm diameter). The recruitment of the cells within a given module is virtually simultaneous, and thus epileptiform events progress in intermittent (0.5-1 Hz) steps across the cortical network. We propose that the interneurons that supply the vetoing inhibition define these modular circuit territories.

Cellular and molecular mechanisms of epilepsy in the human brain

Animal models have provided invaluable data for identifying the pathogenesis of epileptic disorders. Clearly, the relevance of these experimental findings would be strengthened by the demonstration that similar fundamental mechanisms are at work in the human epileptic brain. Epilepsy surgery has indeed opened the possibility to directly study the functional properties of human brain tissue in vitro, and to analyze the mechanisms underlying seizures and epileptogenesis. Here, we summarize the findings obtained over the last 40 years from electrophysiological, histochemical and molecular experiments made with the human brain tissue. In particular, this review will focus on (i) the synaptic and non-synaptic properties of neocortical neurons along with their ability to produce synchronous activity; (ii) the anatomical and functional alterations that characterize limbic structures in patients presenting with mesial temporal lobe epilepsy; (iii) the issue of antiepileptic drug action and resistance; and (iv) the pathophysiology of seizure genesis in Taylor's type focal cortical dysplasia. Finally, we will address some of the problems that are inherent to this type of experimental approach, in particular the lack of proper controls and possible strategies to obviate this limitation.

Persistent Synchronized Bursting Activity in Cortical Tissues With Low Magnesium Concentration: A Modeling Study

We explore the mechanism of synchronized bursting activity with frequency of ∼10 Hz that appears in cortical tissues at low extracellular magnesium concentration [Mg2+]o. We hypothesize that this activity is persistent, namely coexists with the quiescent state and depends on slow N-methyl-d-aspartate (NMDA) conductances. To explore this hypothesis, we construct and investigate a conductance-based model of excitatory cortical networks. Population bursting activity can persist for physiological values of the NMDA decay time constant (∼100 ms). Neurons are synchronized at the time scale of bursts but not of single spikes. A reduced model of a cell coupled to itself can encompass most of this highly synchronized network behavior and is analyzed using the fast-slow method. Synchronized bursts appear for intermediate values of the NMDA conductance gNMDA if NMDA conductances are not too fast. Regular spiking activity appears for larger gNMDA. If the single cell is a conditional burster, persistent synchronized bursts become more robust. Weakly synchronized states appear for zero AMPA conductance gAMPA. Enhancing gAMPA increases both synchrony and the number of spikes within bursts and decreases the bursting frequency. Too strong gAMPA, however, prevents the activity because it enhances neuronal intrinsic adaptation. When [Mg2+]o is increased, higher gNMDA values are needed to maintain bursting activity. Bursting frequency decreases with [Mg2+]o, and the network is silent with physiological [Mg2+]o. Inhibition weakly decreases the bursting frequency if inhibitory cells receive enough NMDA-mediated excitation. This study explains the importance of conditional bursters in layer V in supporting epileptiform activity at low [Mg2+]o.

NMDA receptor-mediated epileptiform persistent activity requires calcium release from intracellular stores in prefrontal neurons

Various normal and pathological forms of synchronized population activity are generated by recurrent excitation among pyramidal neurons in the neocortex. However, the intracellular signaling mechanisms underlying this activity remain poorly understood. In this study, we have examined the cellular properties of synchronized epileptiform activity in the prefrontal cortex with particular emphasis on a potential role of intracellular calcium stores. We find that the zero-magnesium-induced synchronized activity is blocked by inhibition of sarco-endoplasmic reticulum Ca(2+)-ATPases, phospholipase C (PLC), the inositol 1,4,5-trisphosphate (IP3) receptor, and the ryanodine receptor. This same activity is, however, not affected by application of metabotropic glutamatergic receptor (mGluR) agonists, nor by introduction of an mGluR antagonist. These results suggest that persistent synchronized activity in vitro is dependent upon calcium release from internal calcium stores through the activation of PLC-IP3 receptor pathway. Our findings also raise the possibility that intracellular calcium release may be involved in the generation of pathologic synchronized activity in epilepsy in vivo and in physiological forms of synchronized cortical activity.

Anisotropic functional connections between the auditory cortex and area 18a in rat cerebral slices

We developed a new method to visualize the myeloarchitecture in fresh slices, and investigated the properties of the functional neural connections around the boundary between the primary auditory cortex (area 41) and area 18a in rat cerebral slices. A fresh slice illuminated by near-vertical light was observed with a CCD camera. The translucent images of the slice showed contrast patterns very similar to myeloarchitecture. The boundary between these areas was identified by the well-developed layer IV/V in area 41 but not in area 18a. Antidromic/presynaptic components of the field potentials stimulated and recorded across the areal boundary showed symmetric distribution, while the postsynaptic field potentials in the direction from area 41 to 18a were more prominent than those in the opposite direction in layer II/III. In contrast, the dominant direction of propagation of postsynaptic potentials was from area 18a to 41 in layer V. In the presence of 1 microM bicuculline, an inhibitor of GABA(A) receptors, the polysynaptic activities propagating from area 18a into 41 via layer V were elicited by stimulation of area 18a. The propagation measured by Ca(2+) imaging or field potential recordings was potentiated after both areas 18a and 41 were alternately stimulated several times.

Early-onset cobalamin C/D deficiency: epilepsy and electroencephalographic features

PURPOSE

To describe epilepsy and EEG findings in the early-onset cobalamin (Cbl) C/D deficiency, an inborn error of intracellular Cbl metabolism characterized by high plasma levels of methylmalonic acid, homocystine, and homocysteine.

METHODS

Type and frequency of seizures were studied in 10 patients (six boys and four girls) who underwent waking and sleep EEG.

RESULTS

Half of patients had seizures in the first year of life (either concurrent with the other symptoms of disease or some months after the onset of disease); seizures occurred after 2 years in the other half of patients. Convulsive status epilepticus was the initial manifestation in three patients. During the follow-up, nine patients had seizures (mainly partial) despite specific treatment for Cbl C/D deficiency and antiepileptic drugs. Focal or multifocal epileptiform abnormalities during waking EEG that increased during sleep EEG were recorded in the majority of patients. Plasma levels of homocystine and homocysteine were constantly higher than normal, despite therapy institution.

CONCLUSIONS

Epilepsy and EEG abnormalities are prominent features in the early-onset type of combined methylmalonic aciduria and homocystinuria due to Cbl C/D deficiency, possibly related to the pathologically and persistently high levels of homocysteine, experimentally proven to induce seizures. Plasma amino acids evaluation and urinary acid organic analysis should be performed in any infant showing seizures associated with feeding difficulties and failure to thrive, at onset during the first year of life, as well as in any child with convulsive status epilepticus and a history of psychomotor developmental delay of unknown origin.

On the cellular and network bases of epileptic seizures

The highly interconnected networks of the mammalian forebrain can generate a wide variety of synchronized activities, including those underlying epileptic seizures, which often appear as a transformation of otherwise normal brain rhythms. The cerebral cortex and hippocampus are particularly prone to the generation of the large, synchronized bursts of activity underlying many forms of seizures owing to strong recurrent excitatory connections, the presence of intrinsically burst-generating neurons, ephaptic interactions among closely spaced neurons, and synaptic plasticity. The simplest form of epileptiform activity in these structures is the interictal spike, a synchronized burst of action potentials generated by recurrent excitation, followed by a period of hyperpolarization, in a localized pool of pyramidal neurons. Seizures can also be generated in response to a loss of balance between excitatory and inhibitory influences and can take the form of either tonic depolarizations or repetitive, rhythmic burst discharges, either as clonic or spike-wave activity, again mediated both by intrinsic membrane properties and synaptic interactions. The interaction of the cerebral cortex and the thalamus, in conjunction with intrathalamic communication, can also generate spike waves similar to those occurring during human absence seizure discharges. Although epileptic syndromes and their causes are diverse, the cellular mechanisms of seizure generation appear to fall into only two categories: rhythmic or tonic "runaway" excitation or the synchronized and rhythmic interplay between excitatory and inhibitory neurons and membrane conductances.

NMDA receptor-dependent plasticity of granule cell spiking in the dentate gyrus of normal and epileptic rats

Because granule cells in the dentate gyrus provide a major synaptic input to pyramidal neurons in the CA3 region of the hippocampus, spike generation by granule cells is likely to have a significant role in hippocampal information processing. Granule cells normally fire in a single-spike mode even when inhibition is blocked and provide single-spike output to CA3 when afferent activity converging into the entorhinal cortex from neocortex, brainstem, and other limbic regions increases. The effects of enhancement of N-methyl-D-aspartate (NMDA) receptor-dependent excitatory synaptic transmission and reduction in gamma-aminobutyric acid-A (GABA(A)) receptor-dependent inhibition on spike generation were examined in granule cells of the dentate gyrus. In contrast to the single-spike mode observed in normal bathing conditions, perforant path stimulation in Mg(2+)-free bathing conditions evoked graded burst discharges in granule cells which increased in duration, amplitude, and number of spikes as a function of stimulus intensity. After burst discharges were evoked during transient exposure to bathing conditions that relieve the Mg(2+) block of the NMDA receptor, there was a marked increase in the NMDA receptor-dependent component of the EPSP, but no significant increase in the non-NMDA receptor-dependent component of the EPSP in normal bathing medium. Supramaximal perforant path stimulation still evoked only a single spike, but granule cell spike generation was immediately converted from a single-spike firing mode to a graded burst discharge mode when inhibition was then reduced. The induction of graded burst discharges in Mg(2+)-free conditions and the expression of burst discharges evoked in normal bathing medium with subsequent disinhibition were both blocked by DL-2-amino-4-phosphonovaleric acid (APV) and were therefore NMDA receptor dependent, in contrast to long-term potentiation (LTP) in the perforant path, which is induced by NMDA receptors and is also expressed by alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA) receptors. The graded burst discharge mode was also observed in granule cells when inhibition was reduced after a single epileptic afterdischarge, which enhances the NMDA receptor-dependent component of evoked synaptic response, and in the dentate gyrus reorganized by mossy fiber sprouting in kindled and kainic acid-treated rats. NMDA receptor-dependent plasticity of granule cell spike generation, which can be distinguished from LTP and induces long-term susceptibility to epileptic burst discharge under conditions of reduced inhibition, could modify information processing in the hippocampus and promote epileptic synchronization by increasing excitatory input into CA3.

Cellular and network mechanisms of rhythmic recurrent activity in neocortex

The neocortex generates periods of recurrent activity, such as the slow (0.1-0.5 Hz) oscillation during slow-wave sleep. Here we demonstrate that slices of ferret neocortex maintained in vitro generate this slow (< 1 Hz) rhythm when placed in a bathing medium that mimics the extracellular ionic composition in situ. This slow oscillation seems to be initiated in layer 5 as an excitatory interaction between pyramidal neurons and propagates through the neocortex. Our results demonstrate that the cerebral cortex generates an 'up' or depolarized state through recurrent excitation that is regulated by inhibitory networks, thereby allowing local cortical circuits to enter into temporarily activated and self-maintained excitatory states. The spontaneous generation and failure of this self-excited state may account for the generation of a subset of cortical rhythms during sleep.