DC activity in the depth of an experimental epileptic focus

Diverse nature of interictal oscillations: EEG-based biomarkers in epilepsy

Interictal electroencephalogram (EEG) patterns, including high-frequency oscillations (HFOs), interictal spikes (ISs), and slow wave activities (SWAs), are defined as specific oscillations between seizure events. These interictal oscillations reflect specific dynamic changes in network excitability and play various roles in epilepsy. In this review, we briefly describe the electrographic characteristics of HFOs, ISs, and SWAs in the interictal state, and discuss the underlying cellular and network mechanisms. We also summarize representative evidence from experimental and clinical epilepsy to address their critical roles in ictogenesis and epileptogenesis, indicating their potential as electrophysiological biomarkers of epilepsy. Importantly, we put forwards some perspectives for further research in the field.

Active direct current (DC) shifts and "Red slow": two new concepts for seizure mechanisms and identification of the epileptogenic zone

An accurate identification of the epileptogenic zone is essential for patients with intractable epilepsy who are candidates to neurosurgery. EEG recordings can provide predictive biomarkers of the epileptogenic zone. Wide-band EEG makes it possible to record from infraslow (including DC shifts) to high frequency (HFO, over 300 Hz) oscillations for diagnostic purposes in patients with epilepsy. Although the presence of HFOs have been proposed to sign the epileptogenic zone, DC-like recordings demonstrate that DC shifts precede HFOs at seizure onset. This led to the proposal that "ictal active DC shifts" are causally related to seizure onset as opposed to "ictal passive DC shifts". Thus, active DC shifts may constitute predictive biomarkers of the epileptogenic zone in epilepsy. Since DC shift is commonly associated to a rise in extracellular potassium, potassium homeostasis regulated by Kir4.1 channels in astrocytes may play an key role at seizure onset. In addition, we hypothesize that, during the interictal period, the co-occurrence of slow events and interictal HFOs, so-called "Red slow", may also delineate an epileptogenic zone, even if a seizure would not be actually recorded.

Uncensored EEG: The role of DC potentials in neurobiology of the brain

Brain direct current (DC) potentials denote sustained shifts and slow deflections of cerebral potentials superimposed with conventional electroencephalography (EEG) waves and reflect alterations in the excitation level of the cerebral cortex and subcortical structures. Using galvanometers, such sustained displacement of the EEG baseline was recorded in the early days of EEG recordings. To stabilize the EEG baseline and eliminate artefacts, EEG was performed later by voltage amplifiers with high-pass filters that dismiss slow DC potentials. This left slow DC potential recordings as a neglected diagnostic source in the routine clinical setting over the last few decades. Brain DC waves may arise from physiological processes or pathological phenomena. Recordings of DC potentials are fundamental electro-clinical signatures of some neurological and psychological disorders and may serve as diagnostic, prognostic, and treatment monitoring tools. We here review the utility of both physiological and pathological brain DC potentials in different aspects of neurological and psychological disorders. This may enhance our understanding of the role of brain DC potentials and improve our fundamental clinical and research strategies for brain disorders.

Interictal Infraslow Activity in Stereoelectroencephalography: From Focus to Network

PURPOSE

Infraslow activity (ISA) occurring during the interictal state in focal epilepsy is largely unstudied. In this exploratory analysis, the authors aimed to characterize features of interictal ISA in a cohort of patients studied by stereoelectroencephography.

METHODS

The interictal stereoelectroencephography records for 15 consecutive adult patients were retrospectively analyzed, after application of both conventional (1.6-70 Hz) and infraslow (0.01-0.1 Hz) bandpass filters. Visual analysis was complemented by time-frequency analysis to quantify the change in ISA power over hours. Linear correlation coefficient (R) calculations were used to map interictal connectivity in the infraslow band.

RESULTS

Interictal ISA background fluctuations were present throughout the interictal state in all patients, manifesting as recurrent and stereotyped oscillations. These oscillations had an apparent modulatory effect on conventional-band activities and spikes ("spike-crested oscillations"). In the infraslow band, the correlations between electrode contacts were shown to have a stable structure over time.

CONCLUSIONS

Infraslow activity exists as a fundamental component of wideband cortical dynamics in focal epilepsy, with features suggestive of scale-free (1/f) dynamics: evidence of phase-amplitude coupling and functional connectivity in the infraslow band. Rather than viewed as a focal paroxysmal activity, interictal ISA may be better understood as a network process, although this requires further study.

Intracranially recorded ictal direct current shifts may precede high frequency oscillations in human epilepsy

OBJECTIVE

We assessed the temporal-spatial characteristics of ictal direct current (DC) shifts (or infraslow activity) and high frequency oscillations (HFOs) in 16 patients with intractable focal epilepsy.

METHODS

The underlying etiology consisted of cortical dysplasia, glioma, hippocampal sclerosis, and low-grade neuroepithelial tumor in nine, four, two, and one patients, respectively. The median number of analyzed seizure events was 8.0 per patient (range: 2-10). Chronic electrocorticographic recording was performed with (1) a band-pass filter of 0.016-600Hz (or 0.016-300Hz) and a sampling rate of 2000Hz (or 1000Hz).

RESULTS

Ictal DC shifts and a sustained form of ictal HFOs were observed in 75.0% and 50.0% of the patients, and 71.3% and 46.3% of the analyzed seizures. Visual assessment revealed that the onset of ictal DC shifts preceded that of ictal HFOs with statistical significance in 5/7 patients. The spatial extent of ictal DC shifts or HFOs was smaller than that of the conventionally defined seizure onset zone in 9/12 patients.

CONCLUSION

Both ictal DC shifts and HFOs might represent the core of tissue generating seizures.

SIGNIFICANCE

The early occurrence of ictal DC shifts warrants further studies to determine the role of glia (possibly mediating ictal DC shifts) in seizure generation.

Interictal infraslow activity in patients with epilepsy

OBJECTIVE

To evaluate if interictal infraslow activity (ISA), as obtained from a conventional EEG system, can contribute information about the epileptogenic process.

METHODS

The entire long-term intracranial monitoring sessions of 12 consecutive patients were evaluated on an XLTEK system for ISA. Three additional patients had long-term scalp recordings.

RESULTS

In intracranial as well as scalp recordings, the ISA background was consistently higher in the waking state than during sleep. From this background emerged intermittently focal changes, which could achieve in intracranial recordings millivolt amplitudes, while they remained in the microvolt range in scalp recordings. Although they were mainly contiguous between adjacent channels, this was not necessarily the case and intermittent build-up could be seen distant from the epileptogenic zone or radiographic lesion.

CONCLUSIONS

Interictal ISA can be detected in routine intracranial and scalp recordings, without the need for DC amplifiers, and can provide additional information.

SIGNIFICANCE

Since ISA is a separate element of the electromagnetic spectrum, apparently non-neuronal in origin, its assessment should be included not only in the pre-surgical evaluation of epilepsy patients but also in patients with other neurologic disorders and normal volunteers.

Increased discharge threshold after an interictal spike in human focal epilepsy

It is commonly assumed that interictal spikes (ISs) in focal epilepsies set off a period of inhibition that transiently reduces tissue excitability. Post-spike inhibition was described in experimental models but was never demonstrated in the human epileptic cortex. In the present study post-spike excitability was retrospectively evaluated on intracerebral stereo-electroencephalographic recordings performed in the epileptogenic cortex of five patients suffering from drug-resistant focal epilepsy secondary to Taylor-type neocortical dysplasias. Patients typically presented with highly periodic interictal spiking activity at 2.33 +/- 0.87 Hz (mean +/- SD) in the dysplastic region. During the stereo-electroencephalographic procedure, low-frequency stimulation at 1 Hz was systematically performed for diagnostic purposes to identify the epileptogenic zone. The probability of evoking an IS during the interspike period in response to 1-Hz stimuli delivered close to the ictal-onset zone was examined. Stimuli that occurred early after a spontaneous IS (within 70% of the inter-IS period) had a very low probability of generating a further IS. On the contrary, stimuli delivered during the late inter-IS period had the highest probability of evoking a further IS. The generation of stimulus-evoked ISs is occluded for several hundred milliseconds after the occurrence of a preceding spike discharge. As previously shown in animal models, these findings suggest that, during focal, periodic interictal spiking, human neocortical excitability is phasically controlled by post-spike inhibition.

Relations between slow extracellular potential changes, glial potassium buffering, and electrolyte and cellular volume changes during neuronal hyperactivity in cat brain

The aim of this investigation is to estimate the contribution of spatial glial K+ buffer currents to extracellular K+ homeostasis during enhanced neuronal activity. Neuronal hyperactivity was induced by electrical stimulation of the cortical surface or the ventrobasal thalamic nuclei of cats (5-50 Hz, 0.1-0.2 ms, two to three times threshold stimulation intensity, 5-20 s). The accompanying slow field potential changes were recorded simultaneously across the grey matter with vertical assemblies of eight micropipettes glued 300 microns apart. Using the Poisson equation, the amplitudes of the underlying current sources and sinks were calculated. The current source densities depended on the depth of recording, frequency, strength, and duration of the stimulation. Current sinks, corresponding to a removal of 0.1-0.5 mmoles of monovalent cations per liter of brain tissue and second from the extracellular space, were observed in middle cortical layers, whereas sources appeared at superficial and deeper sites. These sinks and sources might represent K+ moved across glial membranes by spatial buffer currents. The consequences of glial buffer currents of this magnitude were investigated with model calculations. It turned out that measurements of electrolyte and volume changes of the extracellular space (Dietzel et al. Exp. Brain Res. 40:432-439, 1980; Exp. Brain Res. 46:73-84, 1982) could only partially be explained by spatial buffer currents of this magnitude. Comparison of the calculated values with intracellular measurements in neurons and glial cells (Coles et al. Ann. N.Y. Acad. Sci. 481:303-317, 1986; Ballanyi et al. J. Physiol. 382:159-174, 1987) suggests that spatial buffering combines with an approximately equimolar KCl transport and, depending on the preparation, also K+/Na+-exchange across glial membranes.

Laminar interactions during neocortical epileptogenesis

Interactions among laminar subpopulations of cat striate cortical neurons were assessed during the evolution of discrete and temporary epileptic foci, which were induced by selective microinjection of penicillin into different cortical layers. Field potentials and multiunit cellular discharges, evoked by selective visual field stimulation, were recorded simultaneously from 3 layers by multibarreled glass microelectrodes. Laminar response profiles at distinct stages of epileptogenesis were characterized for foci induced in superficial pyramidal, middle stellate, and deep pyramidal layers. Layer 4 was verified to be the most susceptible to epileptogenesis. Penicillin's action within this stellate layer appeared to be sufficient for epileptogenesis and was supportative of, if not necessary for, the development of foci originating in pyramidal cell layers. These findings could not be fully appreciated by monitoring only spontaneous interictal spike potentials. Of the two types of neuronal discharge routinely observed, early latency bursting was principally a characteristic of layer 4 stellate populations, whereas longer-latency bursts comparable to paroxysmal depolarization shifts were recorded equally well from both stellate and pyramidal layers. Epileptiform alterations in both field potential and unit responses were quickly evident in cortical laminae having known anatomic connections with the layer where the focus was induced: e.g. in layers 2-3 with layer 4 foci, in layers 5-6 with layers 2-3 foci, and in layer 4 with layers 5-6 foci. The spread of epileptogenesis was slower between laminae where pathways are purported to be less well developed, and appeared to be principally dependent upon the diffusion of penicillin.

Penicillin-induced epileptic foci in the motor cortex: vertical inhibition

Neuronal mechanisms responsible for a vertical restriction of focal seizure activity in the motor cortex were analysed. For this purpose intracellular recording from neurones in superficial (50-300 microns below cortical surface), middle (300-800 microns) and deep cortical layers (800-1300 microns) was performed. As a model of foci of various vertical extensions the spread of seizure activity from superficial to deeper cortical laminae following epicortical penicillin application was used. The appearance of characteristic epileptiform potentials in the surface record with a focus restricted to upper cortical laminae was accompanied (i) in superficial neurones by the development of paroxysmal depolarization shifts (PDS), (ii) in middle neurones by depolarization often followed by hyperpolarization, and (iii) in deep neurones by a sequence of membrane potential changes. The latter consisted of an initial depolarization, an early hyperpolarization, an intermediate depolarization, a late hyperpolarization and a final depolarization. The hyperpolarizing components led to complete suppression of action potentials (vertical inhibition). The early hyperpolarization and the first part of the late hyperpolarization were reduced in amplitude when the intracellular chloride activity was elevated. The intermediate depolarization was replaced by PDS with the enlargement of the epileptic focus into the cortex. The actual effect of the vertical inhibition may in part be responsible for the variability in epileptic motor phenomena coinciding with epileptiform potentials in the surface EEG.

Pattern of intracortical potential distribution during focal interictal epileptiform discharges (FIED) and its relation to spinal field potentials in the rat

The pattern of intracortical potential distribution during focal interictal epileptiform discharges (FIED) was analysed with respect to the occurrence of descending neuronal activity to the spinal cord recorded as spinal field potentials (SFPs). The experiments were performed in rats. Epileptiform activity was elicited by application of penicillin to the motor cortex. The spread of active penicillin was limited by penicillinase in part of the experiments. (1) When penicillinase was applied 10--20 sec before penicillin to the cortical surface typical FIED appeared in the epicortical lead. During well-established focal activity they were accompanied by negative field potentials at a depth of 300 micrometers and 600 micrometers and by positive field potentials in deeper records. This pattern of intracortical potential distribution was not associated with characteristic SFPs. (2) When penicillinase was applied simultaneously with penicillin, the fully developed epicortical FIED were accompanied by negative intracortical field potentials which in this case reached a depth of 900 micrometers. In the layers below predominantly positive potential fluctuations occurred. This pattern of intracortical potential distribution was associated with characteristic SFPs. (3) Intracortical application of penicillin at a depth of 800--900 micrometers led to negative field potentials of large amplitude in all intracortical records, with the concomitant epicortical potentials being positive in polarity. In this case SFPs occurred throughout the interictal activity. Since seizure activity can be restricted to only a few cortical laminae, descending neuronal activity to the spinal cord need not be correlated with definite epicortical potentials. A prerequisite for cortical output is intracortical activity reflected negative potentials at a depth of approx. 900 micrometers.

Autorhythmicity of spontaneous interictal spike discharge at hippocampal penicillin foci

Penicillin-induced epileptogenic foci in the cat hippocampus show a marked tendency for brief but periodic seizure discharges known as 'interictal spikes' (IS). Here, each IS is shown to be followed by a marked elevation and subsequent slow fall-off of the focal seizure threshold. The time constant of this process approximates the spontaneous inter-IS interval and these two parameters appear to vary in concert. The timing of the IS train is always reset by interjected ISs but not by stimuli that are subthreshold for the IS. In sum, this modulation of focal excitability does not appear to be imposed by local or projected rhythmic activity other than that initiated by the IS itself. The firing patterns of the majority of observed hippocampal single units in the vicinity of the focus show a prolonged suppression of spontaneous firing for from 2 to 10 sec or more after each IS, independent of whether the IS was spontaneous or elicited. A smaller number of units show delayed, intense activation following each IS. Both of these forms of response appear to originate from large cells in and near the pyramidal cell body layer. Assuming that these single unit data represent a sampling of pyramidal cell discharge, then the prevalence of a prolonged post-IS pause suggests that the rhythmicity of spontaneous penicillin foci derives from an inhibitory phasing of the population based paroxysmal activity. The periodic spontaneous IS discharge can be viewed, therefore, as a locally regulated, autorhythmic process impressed upon the activity of the neuronal population by the development of a functional suppression of unit activity following each IS.

A critical epileptic area in the cat's cortex and its relation to the cortical columns

The critical size of active penicillin foci in cat's parietal cortex was determined in two different ways: (a) by gradually enlarging the area of drug application from 0.78 mm2 to 3.14 mm2; (b) by reducing the area of the active penicillin focus, using subpial incisions. Results indicate that a critical area of 0.7 mm2 is necessary. The possibility that in the parietal cortex a single functional column may be the basic generator unit for inter-ictal spikes is discussed. It is concluded that: (a) the size of the critical area is related to the cross-sectional area of a single functional column. This column is not yet determined by parallel anatomical findings; (b) inactivation of a given epileptic focus in the cat's cortex may be achieved by cutting layer III of the focus.

The penicillin focus. I. Distribution of potential at the cortical surface

The potential field of a penicillin focus of controlled size was recorded from a rectangular array of 12 electrodes occupying a 4 X 6 mm area on the exposed anterior sigmoid gyrus of the cat. The array was made with 2 mm interelectrode spacing of 0.4 mm I.D. glass capillary tubes filled with Ringer's in agar, excepting one containing penicillin to create the focus. Early in the development of the focus a negative spike appears at the penicillin electrode reaching amplitudes of as high as 3,000 muV while all of the other electrodes showed no synchronous activity of more than 100 muV. Within the first 15 min spike activity becomes visible at about 100 muV at all the electrodes and after 30 min, waveforms of all activity become quite stable. At the penicillin-containing electrode, at intervals of 2--10 sec each, a negative sharp wave of up to 4 mV occurs, having a shorter rise time than fall time. A smaller positive spike of about 30 msec duration which showed marked variations in amplitude often preceded the stable negative sharp wave. During the rise of the negativity at the penicillin electrode, practically all the surrounding electrodes showed a predominantly positive spike. This was occasionally followed by negativity during the falling phase at the penicillin electrode. Displays of potential surfaces interpolating the average values at the 12 recording points on the cortex at 4 msec intervals demonstrate a relationship between the field of the sharp wave at the penicillin focus which is less than 2 mm in diameter and that of the surrounding electrode which indicates that symmetrically located synaptic inhibitory processes are strongly activated in areas adjacent to the small simultaneous excitation at the penicillin electrode. Degrees of attenuation of interictal spikes that take place between cortex and scalp are estimated from the measured potential distributions. Explanation is offered for the reported apparent discrepancies between the findings at the cortex and the scalp EEG.

Temporo-spatial propagation of epileptoform after-discharges in the isolated cat suprasylvian gyrus

1. Epileptiform after-discharges (EADSs) induced by electrical stimulation of the isolated suprasylvian gyrus were studied in cats with chronically implanted electrodes. 2. In a given region and at a certain time after stimulation, the following events took place: (a) a slow radial spread of the zone of maximal depolarization, from the cortical surface downward, as evidenced by a laminar study; (b) a massive cellular discharge preceded by a period during which few unit activities were detected, followed by bursts of spike activity timed with the surface-positive waves of the ECoG; (c) a surface-negative DC shift with maximal amplitude around 1000 mum below the surface; (d) the occurrence of a synchronizing focus from which the paraoxysmal waves propagated to the whole gyrus. 3. All these phenomena spread across the surface of the gyrus with a velocity (7-20 mm/min) similar to that of focal seizures in man.

DC potentials of temporal lobe seizures in the monkey

In 8 monkeys, made epileptic by alum or penicillin injection into temporal lobe structures, 40 seizures were studied by both DC cortical potential and subcortical EEG recordings. Eighteen seizures of lateral temporal origin had an abrupt negative DC potential shift of 0.5 to 2.0 mV in and around the focus. The frontal, parietal and occipital cortices did not develop DC potential changes, perhaps due to the limited propagation of the neocortical seizures. Twenty-two seizures of medial temporal origin showed a negative shift of the anterior, inferior or lateral temporal cortex in 85% of seizures. The other 15% had a positive or no shift. In hippocampal seizures, a positive displacement was sometimes seen prior to the main negative shift in the lateral temporal cortex. The remote cortex developed only a minimal positive shift in 30% of the mediotemporal seizures. A marked negative shift in the frontocentral cortex was the first sign of impending generalization, which may result from a series of chain reactions with seizure propagation, involving more and more structures of the brain. Registration of DC potentials in temporal lobe seizures may give insight into the nature of abnormal EEG activities and to some extent into the origin of seizures.