Epileptiform activity can be initiated in various neocortical layers: an optical imaging study

Kainate receptors have different modulatory effect in seizure-like events and slow rhythmic activity in entorhinal cortex ex vivo

In the neocortex, neurons form functional networks, the members of which exhibit a variable degree of synchronization. Slow rhythmic activity may be regarded as a balanced interplay of excitatory and inhibitory neuronal network activity, which is essential in learning and memory consolidation. On the other hand, seizures may be considered as hypersynchronized network states occurring in epileptic diseases. The brain slice method and multi-electrode array (MEA) systems offer a good opportunity for the modelling of cortical spontaneous activities by examining their initiation and propagation. Our main goals were to characterise and compare spontaneous activities developing in different conditions and cortical network states. The role of kainate receptors in these processes was also tested. According to our results, there are demonstrable dissimilarities between slow rhythmic activities vs. seizure-like events developing in the rat entorhinal cortex ex vivo in normal vs. epileptic conditions. Propagation velocity, time scale, activity pattern and pharmacological sensitivity are all different. Kainate receptors play a role in network activity in entorhinal cortex, they are capable to prolong the duration of the events of epileptiform activity. Their regulatory effect is more prominent under epileptic than under normal conditions.

Reliable and Elastic Propagation of Cortical Seizures In Vivo

Mapping the fine-scale neural activity that underlies epilepsy is key to identifying potential control targets of this frequently intractable disease. Yet, the detailed in vivo dynamics of seizure progression in cortical microcircuits remain poorly understood. We combine fast (30-Hz) two-photon calcium imaging with local field potential (LFP) recordings to map, cell by cell, the spread of locally induced (4-AP or picrotoxin) seizures in anesthetized and awake mice. Using single-layer and microprism-assisted multilayer imaging in different cortical areas, we uncover reliable recruitment of local neural populations within and across cortical layers, and we find layer-specific temporal delays, suggesting an initial supra-granular invasion followed by deep-layer recruitment during lateral seizure spread. Intriguingly, despite consistent progression pathways, successive seizures show pronounced temporal variability that critically depends on GABAergic inhibition. We propose an epilepsy circuit model resembling an elastic meshwork, wherein ictal progression faithfully follows preexistent pathways but varies flexibly in time, depending on the local inhibitory restraint.

Long-range and short-range tumor-stroma networks synergistically contribute to tumor-associated epilepsy

Epileptic seizures are frequently caused by brain tumors. Traditional anti-epileptic treatments do not acquire satisfactory responses. Preoperative and postoperative seizures seriously influence the quality of life of patients. Thus, tumor-associated epilepsy (TAE) is an important subject of the current research. The delineation of the etiology of epileptogenesis in patients with primary brain tumor may help to find the novel and effective drug targets for treating this disease. In this review, we describe the current status of treatment of TAE. More importantly, we focus on the factors that are involved in the functional connectivity between tumors and stromal cells. We propose that there exist two modes, namely, long-range and short-range modes, which likely trigger neuronal hyperexcitation and subsequent epileptic seizures. The long-range mode is referred to as factors released by tumors including glutamate and GABA, binding to the corresponding receptor on the cellular membrane and causing neuronal hyperactivity, while the short-range mode is considered to involve direct intracellular communication between tumor cells and stromas. Gap junctions and tunneling nanotube network are involved in cellular interconnections. Future investigations focused on those two modes may find a potential novel therapeutic target for treating TAE.

Emergence of dominant initiation sites for interictal spikes in rat neocortex

Neuronal populations with unbalanced inhibition can generate interictal spikes (ISs), where each IS starts from a small initiation site and then spreads activation across a larger area. We used in vivo voltage-sensitive dye imaging to map the initiation site of ISs in rat visual cortex disinhibited by epidural application of bicuculline methiodide. Immediately after the application of bicuculline, the IS initiation sites were widely distributed over the entire disinhibited area. After ∼ 10 min, a small number of sites became "dominant" and initiated the majority of the ISs throughout the course of imaging. Such domination also occurred in cortical slices, which lack long-range connections between the cortex and subcortical structures. This domination of IS initiation sites may allow timing-related plasticity mechanisms to provide a spatial organization where connections projecting outward from the dominant initiation site become strengthened. Understanding the spatiotemporal organization of IS initiation sites may contribute to our understanding of epileptogenesis in its very early stages, because a dominant IS initiation site with strengthened outward connectivity may ultimately develop into a seizure focus.

Human brain slices for epilepsy research: Pitfalls, solutions and future challenges

Increasingly, neuroscientists are taking the opportunity to use live human tissue obtained from elective neurosurgical procedures for electrophysiological studies in vitro. Access to this valuable resource permits unique studies into the network dynamics that contribute to the generation of pathological electrical activity in the human epileptic brain. Whilst this approach has provided insights into the mechanistic features of electrophysiological patterns associated with human epilepsy, it is not without technical and methodological challenges. This review outlines the main difficulties associated with working with epileptic human brain slices from the point of collection, through the stages of preparation, storage and recording. Moreover, it outlines the limitations, in terms of the nature of epileptic activity that can be observed in such tissue, in particular, the rarity of spontaneous ictal discharges, we discuss manipulations that can be utilised to induce such activity. In addition to discussing conventional electrophysiological techniques that are routinely employed in epileptic human brain slices, we review how imaging and multielectrode array recordings could provide novel insights into the network dynamics of human epileptogenesis. Acute studies in human brain slices are ultimately limited by the lifetime of the tissue so overcoming this issue provides increased opportunity for information gain. We review the literature with respect to organotypic culture techniques that may hold the key to prolonging the viability of this material. A combination of long-term culture techniques, viral transduction approaches and electrophysiology in human brain slices promotes the possibility of large scale monitoring and manipulation of neuronal activity in epileptic microcircuits.

Electrographic waveform structure predicts laminar focus location in a model of temporal lobe seizures in vitro

Temporal lobe epilepsy is the most common form of partial-onset epilepsy and accounts for the majority of adult epilepsy cases in most countries. A critical role for the hippocampus (and to some extent amygdala) in the pathology of these epilepsies is clear, with selective removal of these regions almost as effective as temporal lobectomy in reducing subsequent seizure risk. However, there is debate about whether hippocampus is 'victim' or 'perpetrator': The structure is ideally placed to 'broadcast' epileptiform activity to a great many other brain regions, but removal often leaves epileptiform events still occurring in cortex, particularly in adjacent areas, and recruitment of the hippocampus into seizure-like activity has been shown to be difficult in clinically-relevant models. Using a very simple model of acute epileptiform activity with known, single primary pathology (GABAA Receptor partial blockade), we track the onset and propagation of epileptiform events in hippocampus, parahippocampal areas and neocortex. In this model the hippocampus acts as a potential seizure focus for the majority of observed events. Events with hippocampal focus were far more readily propagated throughout parahippocampal areas and into neocortex than vice versa. The electrographic signature of events of hippocampal origin was significantly different to those of primary neocortical origin - a consequence of differential laminar activation. These data confirm the critical role of the hippocampus in epileptiform activity generation in the temporal lobe and suggest the morphology of non-invasive electrical recording of neocortical interictal events may be useful in confirming this role.

Coalescence of deep and superficial epileptic foci into larger discharge units in adult rat neocortex

Epilepsy is a disease of neuronal hyper-synchrony that can involve both neocortical and hippocampal brain regions. While much is known about the network properties of the hippocampus little is known of how epileptic neocortical hyper-synchrony develops. We aimed at characterizing the properties of epileptic discharges of a neocortical epileptic focus. We established a multi-electrode-array method to record the spatial patterns of epileptiform potentials in acute adult rat brain slices evoked by 4-Aminopyridine in the absence of magnesium. Locations of discharges mapped to two anatomical regions over the somatosensory cortex and over the lateral convexity separated by a gap at a location matching the dysgranular zone. Focal epileptiform discharges were recorded in superficial and deep neocortical layers but over superficial layers, they exhibited larger surface areas. They were often independent even when closely spaced to one another but they became progressively coupled resulting in larger zones of coherent discharge. The gradual coupling of multiple, independent, closely spaced, spatially restricted, focal discharges between deep and superficial neocortical layers represents a possible mechanism of the development of an epileptogenic zone.

Optical triggered seizures using a caged 4-Aminopyridine

Animal models of epilepsy are critical not only for understanding the fundamental mechanism of epilepsy but also for testing the efficacy of new antiepileptic drugs and novel therapeutic interventions. Photorelease of caged molecules is widely used in biological research to control pharmacologic events with high spatio-temporal resolution. We developed a technique for in vivo optical triggering of neocortical seizures using a novel caged compound based on ruthenium photochemistry (RuBi-4AP). Epileptiform events in mouse cortex were induced with blue light in both whole brain and focal illumination. Multi-electrode array recording and optical techniques were used to characterize the propagation of these epileptic events, including interictal spikes, polyspikes, and ictal discharges. These results demonstrate a novel optically-triggered seizure model, with high spatio-temporal control, that could have widespread application in the investigation of ictal onset, propagation and to develop novel light-based therapeutic interventions.

Monitoring Population Membrane Potential Signals from Neocortex

Voltage-sensitive dyes (VSDs) and optical imaging are useful tools for studying spatiotemporal patterns of population neuronal activity in cortex. Because fast VSDs respond to membrane potential changes with microsecond temporal resolution, these are better suited than calcium indicators for recording rapid neural signals. Here we describe methods for using a 464 element photodiode array and fast VSDs to record signals ranging from large scale network activity in brain slices and in vivo mammalian preparations with sensitivity comparable to local field potential (LFP) recordings. With careful control of dye bleaching and phototoxicity, long recording times can be achieved. Absorption dyes have less photo-toxicity than fluorescent dyes. In brain slices, the total recording time in each slice can be 1,000-2,000 s, which can be divided into hundreds of short recording trials over several hours. In intact brains when fluorescent dyes are used, reduced light intensity can also increase recording time. In this chapter, we will discuss technical details for the methods to achieve reliable VSD imaging with high sensitivity and long recording time.

Regulation of epileptiform discharges in rat neocortex by HCN channels

Hyperpolarization-activated, cyclic nucleotide-gated, nonspecific cation (HCN) channels have a well-characterized role in regulation of cellular excitability and network activity. The role of these channels in control of epileptiform discharges is less thoroughly understood. This is especially pertinent given the altered HCN channel expression in epilepsy. We hypothesized that inhibition of HCN channels would enhance bicuculline-induced epileptiform discharges. Whole cell recordings were obtained from layer (L)2/3 and L5 pyramidal neurons and L1 and L5 GABAergic interneurons. In the presence of bicuculline (10 μM), HCN channel inhibition with ZD 7288 (20 μM) significantly increased the magnitude (defined as area) of evoked epileptiform events in both L2/3 and L5 neurons. We recorded activity associated with epileptiform discharges in L1 and L5 interneurons to test the hypothesis that HCN channels regulate excitatory synaptic inputs differently in interneurons versus pyramidal neurons. HCN channel inhibition increased the magnitude of epileptiform events in both L1 and L5 interneurons. The increased magnitude of epileptiform events in both pyramidal cells and interneurons was due to an increase in network activity, since holding cells at depolarized potentials under voltage-clamp conditions to minimize HCN channel opening did not prevent enhancement in the presence of ZD 7288. In neurons recorded with ZD 7288-containing pipettes, bath application of the noninactivating inward cationic current (Ih) antagonist still produced increases in epileptiform responses. These results show that epileptiform discharges in disinhibited rat neocortex are modulated by HCN channels.

Dynamic neurovascular coupling and uncoupling during ictal onset, propagation, and termination revealed by simultaneous in vivo optical imaging of neural activity and local blood volume

Traditional models of ictal propagation involve the concept of an initiation site and a progressive outward march of activation. The process of neurovascular coupling, whereby the brain supplies oxygenated blood to metabolically active neurons presumably results in a similar outward cascade of hyperemia. However, ictal neurovascular coupling has never been assessed in vivo using simultaneous measurements of membrane potential change and hyperemia with wide spatial sampling. In an acute rat ictal model, using simultaneous intrinsic optical signal (IOS) and voltage-sensitive dye (VSD) imaging of cerebral blood volume and membrane potential changes, we demonstrate that seizures consist of multiple dynamic multidirectional waves of membrane potential change with variable onset sites that spread through a widespread network. Local blood volume evolves on a much slower spatiotemporal scale. At seizure onset, the VSD waves extend beyond the IOS signal. During evolution, spatial correlation with hemodynamic signal only exists briefly at the maximal spread of the VSD signal. At termination, the IOS signal extends spatially and temporally beyond the VSD waves. Hence, vascular reactivity evolves in a separate but parallel fashion to membrane potential changes resulting in a mechanism of neurovascular coupling and uncoupling, which is as dynamic as the seizure itself.

Cortical up state activity is enhanced after seizures: a quantitative analysis

In the neocortex, neurons participate in epochs of elevated activity, or Up states, during periods of quiescent wakefulness, slow-wave sleep, and general anesthesia. The regulation of firing during and between Up states is of great interest because it can reflect the underlying connectivity and excitability of neurons within the network. Automated analysis of the onset and characteristics of Up state firing across different experiments and conditions requires a robust and accurate method for Up state detection. Using measurements of membrane potential mean and variance calculated from whole-cell recordings of neurons from control and postseizure tissue, the authors have developed such a method. This quantitative and automated method is independent of cell- or condition-dependent variability in underlying noise or tonic firing activity. Using this approach, the authors show that Up state frequency and firing rates are significantly increased in layer 2/3 neocortical neurons 24 hours after chemoconvulsant-induced seizure. Down states in postseizure tissue show greater membrane-potential variance characterized by increased synaptic activity. Previously, the authors have found that postseizure increase in excitability is linked to a gain-of-function in BK channels, and blocking BK channels in vitro and in vivo can decrease excitability and eliminate seizures. Thus, the authors also assessed the effect of BK-channel antagonists on Up state properties in control and postseizure neurons. These data establish a robust and broadly applicable algorithm for Up state detection and analysis, provide a quantitative description of how prior seizures increase spontaneous firing activity in cortical networks, and show how BK-channel antagonists reduce this abnormal activity.

Aberrant neuronal avalanches in cortical tissue removed from juvenile epilepsy patients

Some forms of epilepsy may arise as a result of pathologic interactions among neurons. Many forms of collective activity have been identified, including waves, spirals, oscillations, synchrony, and neuronal avalanches. All these emergent activity patterns have been hypothesized to show pathologic signatures associated with epilepsy. Here, the authors used 60-channel multielectrode arrays to record neuronal avalanches in cortical tissue removed from juvenile epilepsy patients. For comparison, they also recorded activity in rat cortical slices. The authors found that some human tissue removed from epilepsy patients exhibited prolonged periods of hyperactivity not seen in rat slices. In addition, they found a positive correlation between the branching parameter, a measure of network gain, and firing rate in human slices during periods of hyperactivity. This relationship was not present in rat slices. The authors suggest that this positive correlation between the branching parameter and the firing rate is part of a positive feedback loop and may contribute to some forms of epilepsy. These results also indicate that neuronal avalanches are abnormally regulated in slices removed from pediatric epilepsy patients.

Neocortical seizure foci localization by means of a directed transfer function method

PURPOSE

Determination of the origin of extratemporal neocortical onset seizures is often challenging due to the rapid speed at which they propagate throughout the cortex. Typically, these patients are poor surgical candidates and many times experience recurrences of seizure activity following resection of the assumed seizure focus.

METHODS

We applied a causal measurement technique--the directed transfer function (DTF)--in an effort to determine the cortical location responsible for the propagation of the seizure activity. Intracranial seizure recordings were obtained from a group of 11 pediatric patients with medically intractable neocortical-onset epilepsy. Time windows were selected from the recordings following onset of the ictal activity. The DTF was applied to the selected time windows, and the frequency-specific statistically significant source activity arising from each cortical recording site was quantified. The DTF-estimated source activity was then compared with the seizure-onset zone(s) identified by the epileptologists.

RESULTS

In an analysis of the 11 pediatric patients, the DTF was shown to identify estimated ictal sources that were highly correlated with the clinically identified foci. In addition, it was observed that in the patients with multiple ictal foci, the topography of the casual source activity from the analyzed seizures was associated with the separate clinically identified seizure-onset zones.

DISCUSSION

Although localization of neocortical-onset seizures is typically challenging, the causal measures employed in this study-namely the directed transfer function-identified generators of the ictal activity that were highly correlated with the cortical regions identified as the seizure-onset zones by the epileptologists. This technique could prove useful in the identification of seizure-specific propagation pathways in the presurgical evaluation of patients with epilepsy.

Endogenous nitric oxide is a key promoting factor for initiation of seizure-like events in hippocampal and entorhinal cortex slices

Nitric oxide (NO) modulates synaptic transmission, and its level is elevated during epileptic activity in animal models of epilepsy. However, the role of NO for development and maintenance of epileptic activity is controversial. We studied this aspect in rat organotypic hippocampal slice cultures and acute hippocampal-entorhinal cortex slices from wild-type and neuronal NO synthase (nNOS) knock-out mice combining electrophysiological and fluorescence imaging techniques. Slice cultures contained nNOS-positive neurons and an elaborated network of nNOS-positive fibers. Lowering of extracellular Mg(2+) concentration led to development of epileptiform activity and increased NO formation as revealed by NO-selective probes, 4-amino-5-methylamino-2',7'-difluorofluorescein and 1,2-diaminoanthraquinone sulfate. NO deprivation by NOS inhibitors and NO scavengers caused depression of both EPSCs and IPSCs and prevented initiation of seizure-like events (SLEs) in 75% of slice cultures and 100% of hippocampal-entorhinal cortex slices. This effect was independent of the guanylyl cyclase/cGMP pathway. Suppression of SLE initiation in acute slices from mice was achieved by both the broad-spectrum NOS inhibitor N-methyl-L-arginine acetate and the nNOS-selective inhibitor 7-nitroindazole, whereas inhibition of inducible NOS by aminoguanidine was ineffective, suggesting that nNOS activity was crucial for SLE initiation. Additional evidence was obtained from knock-out animals because SLEs developed in a significantly lower percentage of slices from nNOS(-/-) mice and showed different characteristics, such as prolongation of onset latency and higher variability of SLE intervals. We conclude that enhancement of synaptic transmission by NO under epileptic conditions represents a positive feedback mechanism for the initiation of seizure-like events.

Identification of epileptogenic foci from causal analysis of ECoG interictal spike activity

OBJECTIVE

In patients with intractable epilepsy, the use of interictal spikes as surrogate markers of the epileptogenic cortex has generated significant interest. Previous studies have suggested that the cortical generators of the interictal spikes are correlated with the epileptogenic cortex as identified from the ictal recordings. We hypothesize that causal analysis of the functional brain networks during interictal spikes are correlated with the clinically-defined epileptogenic zone.

METHODS

We employed a time-varying causality measure, the adaptive directed transfer function (ADTF), to identify the cortical sources of the interictal spike activity in eight patients with medically intractable neocortical-onset epilepsy. The results were then compared to the foci identified by the epileptologists.

RESULTS

In all eight patients, the majority of the ADTF-calculated source activity was observed within the clinically-defined SOZs. Furthermore, in three of the five patients with two separate epileptogenic foci, the calculated source activity was correlated with both cortical sites.

CONCLUSIONS

The ADTF method identified the cortical sources of the interictal spike activity as originating from the same cortical locations as the recorded ictal activity.

SIGNIFICANCE

Evaluation of the sources of the cortical networks obtained during interictal spikes may provide information as to the generators underlying the ictal activity.

The role of optical spectroscopy in epilepsy surgery in children

OBJECT

Surgery is an important therapeutic modality for pediatric patients with intractable epilepsy. However, existing imaging and diagnostic technologies such as MR imaging and electrocochleography (ECoG) do not always effectively delineate the true resection margin of an epileptic cortical lesion because of limitations in their sensitivity. Optical spectroscopic techniques such as fluorescence and diffuse reflectance spectroscopy provide a nondestructive means of gauging the physiological features of the brain in vivo, including hemodynamics and metabolism. In this study, the authors investigate the feasibility of using combined fluorescence and diffuse reflectance spectroscopy to assist epilepsy surgery in children.

METHODS

In vivo static fluorescence and diffuse reflectance spectra were acquired from the brain in children undergoing epilepsy surgery. Spectral measurements were obtained using a portable spectroscopic system in conjunction with a fiber optic probe. The optical investigations were conducted at the normal and abnormal cortex as defined by intraoperative ECoG and preoperative imaging studies. Biopsy samples were taken from the investigated sites located within the zone of resection. The optical spectra were classified into multiple subsets in accordance with the ECoG and histological study results. The authors used statistical comparisons between 2 given data subsets to identify unique spectral features. Empirical discrimination algorithms were developed using the identified spectral features to determine if the objective of the study was achieved.

RESULTS

Fifteen pediatric patients were enrolled in this pilot study. Elevated diffuse reflectance signals between 500 and 600 nm and/or between 650 and 850 nm were observed commonly in the investigated sites with abnormal ECoG and/or histological features in 10 patients. The appearance of a fluorescent peak at 400 nm was observed in both normal and abnormal cortex of 5 patients. These spectral alterations were attributed to changes in morphological and/or biochemical characteristics of the epileptic cortex. The sensitivities and specificities of the empirical discrimination algorithms, which were constructed using the identified spectral features, were all > 90%.

CONCLUSIONS

The results of this study demonstrate the feasibility of using static fluorescence and diffuse reflectance spectroscopy to differentiate normal from abnormal cortex on the basis of intraoperative assessment of ECoG and histological features. It is therefore possible to use fluorescence and diffuse reflectance spectroscopy as an aid in epilepsy surgery.

Cortical activation mapping of epileptiform activity derived from interictal ECoG spikes

PURPOSE

To develop and evaluate a new cortical activation mapping (CAM) method to obtain the neuronal activation sequences from the cortical potential distributions.

METHODS

Interictal electrocorticogram (ECoG) recordings were analyzed for eight pediatric epilepsy patients to find the cortical activation maps, which were compared with the patients' seizure-onset zones identified from ictal ECoG recordings. Various relations between the local activation time and cortical potential were assumed. The most effective relation was determined by accessing their capability to predict the seizure-onset zone. Computer simulations using a moving dipole source model were also conducted to test the present approach in imaging the propagated cortical activity.

RESULTS

In both clinical data analysis and computer simulations, the maximal amplitude proved to be the most effective criterion with which to determine the local cortical activation time. The present method successfully predicted the seizure-onset zone in seven of eight patients by the CAM analysis of ECoG-recorded interictal spikes (IISs). For patients with multiple seizure foci, each focus can be revealed by analyzing IISs with different spatial patterns.

CONCLUSIONS

The time difference between spike peaks of the interictal events in the leading channel and other channels can be effectively defined as the local cortical activation time. The cortical activation mapping method based on this time latency can be used to predict the seizure-onset zones, suggesting that the present CAM method is useful to assist the presurgical evaluation for the epilepsy patients.

Neurovascular coupling and oximetry during epileptic events

Epilepsy is an abnormal brain state in which a large population of neurons is synchronously active, causing an enormous increase in metabolic demand. Recent investigations using highresolution imaging techniques, such as optical recording of intrinsic signals and voltagesensitive dyes, as well as measurements with oxygen-sensitive electrodes have elucidated the spatiotemporal relationship between neuronal activity, cerebral blood volume, and oximetry in vivo. A focal decrease in tissue oxygenation and a focal increase in deoxygenated hemoglobin occurs following both interictal and ictal events. This "epileptic dip" in oxygenation can persist for the duration of an ictal event, suggesting that cerebral blood flow is inadequate to meet metabolic demand. A rapid focal increase in cerebral blood flow and cerebral blood volume also accompanies epileptic events; however, this increase in perfusion soon (>2 s) spreads to a larger area of the cortex than the excitatory change in membrane potential. Investigations in humans during neurosurgical operations have confirmed the laboratory data derived from animal studies. These data not only have clinical implications for the interpretation of noninvasive imaging studies such as positron emission tomography, single-photon emission tomography, and functional magnetic resonance imaging but also provide a mechanism for the cognitive decline in patients with chronic epilepsy.

Bidirectional multisite seizure propagation in the intact isolated hippocampus: the multifocality of the seizure "focus"

Localizing the seizure focus is difficult and frequently, multiple sites are found. This reflects our poor understanding of the fundamental mechanisms of seizure generation and propagation. We used multisite electrophysiological recordings in two seizure models and voltage-sensitive dye imaging, to spatiotemporally characterize the initiation and propagation of seizures in an intact epileptogenic brain region, the isolated hippocampus. In low-magnesium perfusate, seizures always originated in the temporal region, and propagated along the septotemporal axis to the septal region. After the seizure spread across the hippocampus, the bursts within a seizure became bidirectional, with different propagation patterns at different frequencies. When the intact hippocampus was separated along the septotemporal axis, independent bidirectional activity was observed in the two halves, and region-specific cuts to the tissue reveal that the CA3 region is critical for seizure generation and propagation. In a second seizure model, using focal tetanic stimulation of the septal and temporal CA3 region, seizures always originated at the stimulated site with bidirectionality later developing at different frequencies, as noted in the low magnesium model, behavior compatible with coupled neuronal network oscillators. These data provide novel insights into the dynamic multifocality of seizure onset and propagation, revealing that the current concept of a single seizure "focus" is complex.

Recent developments in oximetry and perfusion-based mapping techniques and their role in the surgical treatment of neocortical epilepsy

Detailed understanding of neurovascular coupling during epilepsy is critical for the interpretation of various perfusion-based imaging techniques, such as positron emission tomography, single-photon-emission computed tomography, and functional magnetic resonance imaging, which are used to guide surgical therapy. We used high-resolution intrinsic signal- and voltage-sensitive dye imaging, as well as oxygen-sensitive electrodes, to map the precise spatiotemporal relationship between excitatory and inhibitory neuronal activity, cerebral blood volume, and oximetry during epilepsy. We observed a rapid focal decrease in tissue oxygenation and an increase in deoxygenated hemoglobin in association with both interictal and ictal events. This "epileptic dip" in oxygenation lasts several seconds following both interictal and ictal events, implying that for a period, cerebral blood flow is inadequate to meet metabolic demand. We also observed a rapid focal increase in cerebral blood volume that soon spread to adjacent nonepileptic gyri. Likewise, a diffuse decrease in deoxygenated hemoglobin, related to the blood oxygen level-dependent signal recorded with functional magnetic resonance imaging, spread to adjacent gyri and was poorly localized.

Initiation and Propagation of Neuronal Coactivation in the Developing Hippocampus

Correlated neuronal activity is ubiquitous in developing nervous systems, where it may introduce spatiotemporal coherence and contribute to the organization of functional circuits. In this report, we used voltage-sensitive dyes and optical imaging to examine the spatiotemporal pattern of a spontaneous network activity, giant depolarizing potentials (GDPs), in rat hippocampal slices during the first postnatal week. The propagation pattern of the GDP is closely correlated to the anatomical organization of the network. In the hilus, where mossy cells and interneurons are not organized in layers, GDPs propagate at the same velocity in all directions. In CA3 and CA1, the activation is synchronous along the axis of the pyramidal cells' dendritic tree. The velocity of wave propagation is significantly different in three hippocampal subfields: it is slowest in the hilus, faster in CA3, and fastest in CA1. The velocity of horizontal propagation (along the axis of the pyramidal layer) has a large variation from trial to trial, suggesting that the horizontal velocity is determined to some extent by dynamic network factors. Imaging revealed that each GDP event is initiated from a small focus. The location of the initiation focus differs from event to event. All together, our data suggest that GDP is a propagating excitation wave, initiated from a small site, and propagating to the whole hippocampus. The spatiotemporal patterns of the wave in CA3 and CA1 areas show better synchrony along the pyramidal cell dendritic trees and progressive activation along the axis of the pyramidal cell layer.

Epileptiform activity preferentially arises outside tumor invasion zone in glioma xenotransplants

Seizures occur commonly with brain tumors. The underlying mechanisms are not understood. We analyzed network and cellular excitability changes in tumor-invaded and sham neocortical tissue in vitro using a rat glioblastoma model. Rat C6 glioma cells were transplanted into rat neocortex, yielding diffusely invading gliomas resembling human glioblastomas. We hypothesized that network excitability would increase in regions neighboring the tumor, and that initiation of epileptic discharges might be correlated to a higher density of intrinsically bursting neurones. Voltage-sensitive dye imaging revealed epileptic activity to be initiated in paratumoral zones (1-2 mm from main tumor mass), in contrast to control tissue, where epileptic foci appeared randomly throughout the neocortex. Neuronal firing patterns revealed significantly more intrinsically bursting neurones within these initiation zones than in regions directly adjacent to the tumor or in control tissue. We conclude that gliomas are associated with a higher density of intrinsically bursting neurones, and that these may preferentially initiate epileptiform events.

Optical imaging of epileptiform activity in experimentally induced cortical malformations

Electrophysiological studies of human cortical dysplasia and rodent models revealed widespread hyperexcitability in the malformation itself as well as in its vicinity. We here analyzed the initiation of paroxysmal epileptiform activity using optical imaging of neuronal activity in rats with cortical malformations induced by neonatal freeze lesions. Brain slice preparations were incubated with the voltage-sensitive dye RH795 and neuronal activity was monitored using a fast-imaging photodiode array combined with standard field potential recordings. Spontaneous paroxysmal epileptiform activity emerged in all slices from animals with cortical malformations and sham-operated controls 20-40 min after omission of extracellular Mg(2+). Following electrophysiological and optical recordings, slices were histochemically processed. Using this approach, the present study demonstrated that in animals with freeze-lesion-induced focal cortical malformations, paroxysmal epileptiform activity always emerged from the dysplastic cortex and then spread to adjacent areas through superficial layers. This distribution of initiation sites was significantly different to sham-operated controls in which epileptogenic foci were located in various cytoarchitectonic areas. The present study indicates that following global changes in excitability, the dysplastic cortex itself is the main initiation site of paroxysmal epileptiform activity in animals with focal cortical malformations.

Laminar analysis of human neocortical interictal spike generation and propagation: current source density and multiunit analysis in vivo

Multicontact microelectrodes were chronically implanted in epilepsy patients undergoing subdural grid implantation for seizure localization. Current source density and multiple unit activity of interictal spikes (IISs) were sampled every approximately 150 microm in a line traversing all layers of a cortical column. Our data suggest that interictal epileptiform events in humans are initiated by large postsynaptic depolarizations, consistent with the hypothesis that human IISs correspond to animal paroxysmal depolarization shifts. Furthermore, the cortical layer where the initial depolarization occurs may differ according to whether the IIS is locally generated or propagated from a distant location, and among the propagated IISs, whether the IIS is in the direct path of propagation or on the periphery of that path.

Modulation of muscarinic facilitation of epileptiform discharges in immature rat neocortex

We examined the cholinergic effects on epileptiform discharge generation in immature (postnatal days 10-20) rat neocortex. Evoked and spontaneous field potentials were recorded from the deep layers of neocortical slices during GABA(A) receptor blockade by bicuculline methiodide (BMI, 50 microM). The anticholinesterase eserine (10 microM) as well as the ACh-analog carbamylcholine chloride (CCh, 25 microM) decreased the amplitude and duration of evoked field potentials and in parallel, increased significantly the rate of occurrence of spontaneous discharges. These effects were reversed by the muscarinic antagonist atropine (2.5 microM, n = 20), but not by the nicotinic receptor antagonist hexamethonium (50 microM, n = 3). The M1 subtype-selective muscarinic antagonist pirenzepine (1 microM, n = 12) blocked spontaneous discharges in 8/12 slices, while muscarinic antagonists of the M2 (AFDX 116 n = 4), M3 (4-DAMP n = 4) and M4 (gallamine n = 5, tropicamide n = 6) type, all at 1 microM, only reduced their frequency. CCh-induced spontaneous discharges were blocked by the combination of the glutamate receptor antagonists AP5 and CNQX (both at 10 microM; n = 11). Gap junction blockers abolished them (halothane, n = 7) or reduced their frequency by 65% (carbenoxolone, n = 8). Inhibiting Ca2+ release from intracellular stores by dantrolene (100 microM, n = 5) or thapsigargin (1 microM, n = 5) also depressed their frequencies by 55-65%. By contrast, their rates were not altered by perfusion with high Ca2+ (7 mM; n = 6) medium, a manipulation suppressing polysynaptic connections. These findings demonstrate that activation of muscarinic receptors, notably of the M1 type, in immature rat neocortex facilitates the generation of glutamatergic epileptiform discharges. These discharges are strongly inhibited by gap junction blockers, and are also partly mediated by the, presumably muscarinic receptor-dependent, mobilization of intracellular calcium.

Initiation of spontaneous epileptiform events in the rat neocortex in vivo

We used voltage-sensitive dye imaging to visualize the distribution of initiation sites of the spontaneous interictal-like spikes (sISs) in rat neocortex, in vivo, induced by bicuculline or picrotoxin over the exposed cortex. The initiation site was small (approximately 200 microm diam). On average each initiation site initiated 2.0 +/- 0.8 sISs (9 animals, 499 sISs, 251 sites). This is significantly different from that in neocortical slices, where each initiation site initiated 30-100 sISs. The initiation sites were not randomly distributed. The distance between two consecutive sites tended to be either <800 or >1200 microm, suggesting a temporal "suppression annulus" surrounding each initiation site. Within the annulus, the likelihood for initiating the next sIS was reduced. Suppression annulus did not have a noticeable change in the presence of GABA(b) antagonist, suggesting it did not depend on the GABA(b) inhibition. We also applied bicuculline locally to a spot of 800 x 800 microm(2) for approximately 45 min. During this period approximately 1000 sISs occurred within the spot. Bicuculline or picrotoxin was then applied to the entire craniotomy window. The pretreatment created an obvious cluster of initiation sites. Around this cluster, the suppression annulus became obvious in individual animals. Our results suggest that, in disinhibited cortex, epileptiform events were initiated from small sites. The initiation sites may cluster in an area with increased local activity. Surrounding each initiation site there may be a temporal suppression annulus.

Stimulus-induced patterns of bioelectric activity in human neocortical tissue recorded by a voltage sensitive dye

Stimulus-induced pattern of bioelectric activity in human neocortical tissue was investigated by use of the voltage sensitive dye RH795 and a fast optical recording system. During control conditions stimulation of layer I evoked activity predominantly in supragranular layers showing a spatial extent of up to 3000 microm along layer III. Stimulation in white matter evoked distinct activity in infragranular layers with a spatial extent of up to 3000 microm measured along layer V. The mean amplitude of optical signals close to the stimulated sites in layer I and white matter determined 25 ms following the stimulus, decreased by 50% at a lateral distance of approximately 900 microm and 1200 microm, respectively. Velocity of spread along the vertical stimulation axis reached 0.24 m/s in the supragranular layers (layers I to III) and then decreased to 0.09 m/s following layer I activation; stimulation of white matter induced a velocity of spread in layer V of 0.38 m/s, which slowed down to 0.12 m/s when passing the lower border of lamina IV. The horizontal velocities of spread determined from the stimulation site to a lateral distance of 500 microm reached 0.26-0.28 m/s and 0.28-0.35 m/s for layer I and white matter stimulation, respectively. At larger distances velocity of spread decreased. Increased excitability (Mg(2+)-free solution) had no significant effect on the spatio-temporal distribution of evoked activity as compared with control conditions. There were also no obvious differences between the results obtained in slices, which generated spontaneously sharp waves and those which were not spontaneously active. About 30% of the slices (n=7) displayed a greatly different response pattern, which seemed not to be related in a simple way to the stimulation as was the case in the majority of the investigated slices. The activity pattern of those slices appeared atypical in regard to their deviations of the vertical and horizontal extent of activity, to their reduced spatial extent of activity during increased excitability, to their layer-related distribution of activity, and to the appearance of afterdischarges.Concluding, in 30% of the human temporal lobe slices atypical activity pattern occurred which obviously reflect intrinsic epileptiform properties of the resected tissue. The majority of slices showed stereotyped activity pattern without evidence for increased excitability.

Optical analysis of acute spontaneous epileptiform discharges in the in vivo rat cerebral cortex

We examined the spatiotemporal patterns of spontaneous epileptiform activity observed in the in vivo rat cerebral cortex using an optical recording technique of detecting transmembrane voltage changes. The surface of the cerebral cortex was exposed under anesthesia and stained with a fluorescent voltage-sensitive dye, RH414. Acute spontaneous epileptiform discharges were induced by application of a GABA(A) receptor antagonist, bicuculline. Changes in the intensity of fluorescence were recorded from the cerebral cortex using a 464-channel optic fiber photodiode system. We succeeded in recording spontaneous epileptiform discharges, and constructed their initiation-site maps. We found that the initiation site was neither unique nor randomly located, but exhibited a multimodal distribution pattern. The incidence of epileptiform discharges was different between the initiation sites, and some sites showed dominance in the induction of spontaneous epileptiform discharges.

Comparison of spontaneous and evoked epileptiform activity in three in vitro epilepsy models

Rat neocortical slices express spontaneous epileptiform activity after incubation with GABA(A) receptor blocker bicuculline (BIC, 20 microM), with potassium channel blocker 4-aminopyridine (4-AP, 50 microM) or in Mg(2+)-free medium (LMG). Various parameters of spontaneous and evoked epileptiform discharges and their pharmacological sensitivity were analysed using extracellular field potential recordings in this comparative in vitro study. All types of convulsant solution induced spontaneous epileptiform activity, however, the analysed parameters showed that characteristics of epileptiform discharges are rather different in the three models. The longest duration of discharges was recorded in LMG, while the highest frequency of spontaneous events was detected in 4-AP. The epileptiform field responses elicited by electrical stimulation appeared in an all-or-none manner in BIC. On the contrary, in 4-AP and in LMG the amplitude of the responses increased gradually with increasing stimulation intensities. The NMDA receptor antagonist D,L-2-amino-5-phosphonovaleric acid (APV, 25 microM) abolished the LMG induced spontaneous epileptiform activity and significantly reduced the frequency of the epileptiform discharges in BIC and 4-AP. Blocking the AMPA type of glutamate transmission with 1-(aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466, 40 microM) rapidly abolished BIC-induced spontaneous epileptiform activity and caused a significant decrease in the frequency of 4-AP induced spontaneous epileptiform discharges. However, it had only a weak effect on the LMG-induced epileptiform activity. We conclude that the contribution of NMDA and AMPA types of glutamate receptors to the development and maintenance of epileptiform activity in cortical cell assemblies is different in the three models. There are significant alterations in contribution of NMDA and AMPA types of glutamate receptors to the above-mentioned processes in the different convulsants. In BIC the synchronisation is mainly due to the altered network properties, namely inhibition is reduced in the local circuits. Although inhibition is reduced in the local circuits, the AMPA receptor antagonist relatively easily blocked the synchronised excitation. In 4-AP, and especially in LMG, changes in the membrane characteristics of neurones play a crucial role in the increased excitability. In this case the AMPA antagonist was less effective.

Continuous potential display of ictal electrocorticography

The objective of this study was to determine whether the animation of electrical activity recorded on ictal electrocorticograms (ECoGs) can demonstrate the propagation of seizure discharges from the epileptogenic zone (EZ) to the surrounding cortical area. A computer program, continuous potential display (CPD), was designed to animate the color-coded potential changes in 5-msec intervals at each recorded site. This program was used to analyze 35 ictal ECoGs recorded by subdural grid electrodes from 11 subjects who underwent epilepsy surgery for intractable partial seizures. Continuous potential display demonstrated recurrent cycles of seizure propagation from the EZ to the surrounding cortical area even when seizure discharges appeared widespread on ECoG. Hence, the EZ could be mapped at any time during the seizure course. The EZ mapped by analyzing a small fraction of ECoG during widespread seizure discharges using CPD only overlapped 69 +/- 24% (mean +/- standard deviation) of the surgical area. The EZ mapped by CPD had 34 +/- 22% false positives and 35 +/- 27% false negatives. Animation of potential changes recorded by ictal ECoG can assist in studying the temporal and spatial patterns of seizure propagation and in mapping the EZ for surgical resection.

Voltage-sensitive dye imaging of population neuronal activity in cortical tissue

Voltage-sensitive dyes (VSDs) and optical imaging are useful for studying spatiotemporal patterns of population neuronal activity in cortical tissue. Using a photodiode array and absorption dyes we were able to detect neuronal activity in single trials before it could be detected by local field potential (LFP) recordings. Simultaneous electrical and optical recordings from the same tissue also showed that VSD and LFP signals have different waveforms during different activities, suggesting that they are sensitive to different aspects of the synchronization across the population. Noise, dye bleaching, phototoxicity and optical filter selection are important to the quality of the VSD signal and are discussed in this report. With optimized signal-to-noise ratio (S/N) and total recording time, we can optically monitor approximately 500 locations in an area of 1 mm(2) of cortical tissue with a sensitivity comparable to that of LFP electrodes. The total recording time and S/N of fluorescence and absorption dyes are also compared. At S/N of 8-10, absorption dye NK3630 allows a total recording time of 15-30 min, which can be divided into hundreds of 4-8 s recording trials over several hours, long enough for many kinds of experiments. In conclusion, the VSD method provides a reliable way for examining neuronal activity and pharmacological properties of synapses in brain slices.

Spatio-temporal patterns of neuronal activity: analysis of optical imaging data using geometric shape matching

Optical imaging of neuronal network activity yields information of spatial dynamics which generally is analyzed visually. The transient appearance of spatial activity patterns is difficult to gauge in a quantifiable manner, or may even altogether escape detection. Here, we employ geometric shape matching using Fréchet distances or straight skeletons to search for pre-selected patterns in optical imaging data with adjustable degrees of tolerance. Data were sampled from fluorescence changes of a voltage-sensitive dye recorded with a 464-photodiode array. Fluorescence was monitored in a neuronal network in vitro. Neuronal activity prompting fluorescence fluctuations consisted of spontaneous epileptiform discharges in neocortical slices from patients undergoing epilepsy surgery. The experiments show that: (a) spatial activity patterns can be detected in optical imaging data; (b) shapes such as "mini-foci" appear in close correlation to bioelectric discharges monitored with field potential electrodes in a reproducible manner; (c) Fréchet distances yield more conservative matches regarding rectangular, and less conservative hits with respect to radially symmetric shapes than the straight skeleton approach; and (d) tolerances of 0.03-0.1 are suited to detect faithful images of pre-selected shapes, whereas values >0.8 will report matches with any polygonal pattern. In conclusion, the methods reported here are suited to detect and analyze spatial, geometric dynamics in optical imaging data.

Spatiotemporal properties of an evoked population activity in rat sensory cortical slices

We have examined the spatiotemporal properties of ensemble activity, an evoked all-or-none polysynaptic activity in rat neocortical slices. Ensemble activity occurred in cortical slices bathed in normal artificial cerebrospinal fluid (ACSF) and was evoked by a single electrical shock either to the white matter or directly to the cortical tissue. This activity was seen in slices of somatosensory and auditory cortices; in other cortical areas we have not been able to evoke it. The activity developed 10 to 250 ms poststimulus and lasted 280 +/- 120 ms in local field potential (LFP) recordings. Voltage-sensitive dye imaging showed that this activity was an area of activation 0.8 +/- 0.4 mm wide that propagated slowly (11.4 +/- 6.2 mm/s, n = 60, 6 animals) in the horizontal direction. Due to this propagation, the actual duration in the whole tissue may be longer (approximately 400 ms) than that recorded by a single LFP electrode. Ensemble activity produced a low-amplitude optical signal (7-14% of the interictal-like spikes in the same tissue), suggesting a moderate net depolarization of the population. These were very different from hyperexcitable (epileptiform) events in the same tissue that had about 10 times the optical signal amplitude and propagated at 125 +/- 24 mm/s (n = 21, 6 animals). On a global spatial scale (approximately 0.8 mm wide in layers II-III) ensemble activity had a smooth waveform in voltage-sensitive dye signals (population transmembrane potential). On a local scale, field potential recordings showed large fluctuations with complex oscillations and substantial trial-to-trial variation. This suggests that oscillations in cortical circuits occurred only in small clusters of correlated neurons. Ensemble activity was sensitive to the excitation-inhibition balance of the local network. Antagonists of N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, and GABAa receptors, and muscarinic agonists and other modest manipulations such as increasing bath concentration of Mg(2+) to 2.5-4 mM (normally at 2 mM), or K(+) to 5-7 mM (normally 3 mM), all significantly reduced the probability of evoking the activity. The metabotropic glutamate receptor agonist, aminocyclopentane-1,3-dicarboxylic acid, blocked the activity at a low concentration (10-15 microM), while the antagonist (R,S)-alpha-methyl-4-carboxyphenylglycine had no effect even at high concentration (240 microM). Our data suggest that locally organized neuronal clusters may play a role in the organization of oscillatory activities in the gamma band and may participate in cortical integration/amplification occurring on a scale of approximately 1 mm x 300 ms.

Role of NMDA, non-NMDA, and GABA receptors in signal propagation in the amygdala formation

Although the synaptic physiology of the amygdala has been studied with single neuron recordings, the properties of the networks between the various nuclei have resisted characterization because of the limitations of field recording in a neuronally diffuse structure. We addressed this issue in the rat amygdala complex in vitro by using a photodiode array coupled with a voltage-sensitive dye. Low-intensity single pulse stimulation of the lateral amygdala nucleus produced a complex multi-phasic potential. This signal propagated to the basolateral nucleus and the amygdalostriatal transition zone but not to the central nucleus. The local potential, which depended on both synaptic responses and activation of voltage-dependent ion channels, was reduced in amplitude by the non-N-methyl-D-aspartate (non-NMDA) glutamate receptor antagonist 6,7-dinitroquinoxaline (DNQX) and reduced to a lesser extent by the NMDA glutamate receptor antagonist D-2-amino-5-phosphonovaleric acid (D-APV). We next characterized the less complex signals that propagated to more distal regions with or without the addition of the GABA receptor antagonist bicuculline (BIC). BIC alone greatly increased the signal propagation and permitted activation of previously silent areas within the amygdala. DNQX blocked signal propagation to amygdala regions outside of La, even in the presence of BIC, whereas D-APV had minimal effects on these distal signals. These data represent several novel findings: the characterization of the multi-component potential near the site of stimulation, the gating of signal propagation within the amygdala by GABAergic inhibition, the critical role of non-NMDA receptor-mediated depolarization in signal propagation, and the lack of a role for NMDA receptors in maintaining propagation.

Endogenous acetylcholine facilitates epileptogenesis in immature rat neocortex

In the presence of the gamma-amino butyric acid-A (GABAA) antagonist bicuculline methiodide (50 microM), synchronous spontaneous and evoked potentials were recorded extracellularly from the deep layers of immature neocortex (postnatal days 10-31, P10-P31) in vitro. Addition of the anticholinesterase eserine (10 microM) depressed the amplitude (by 29.5+/-6.6%, n=13) and duration (by 26.3+/-4.7%, n=11) of the evoked field potentials in 13/19 slices (68%), and increased significantly the rates of occurrence of spontaneous epileptiform discharges or induced them in 9/19 slices (47%). All these effects were blocked by the muscarinic antagonist atropine (2.5 microM, n=3), suggesting that they were mediated by the activation of muscarinic receptors by endogenous acetylcholine. The cholinergic inhibitory effect is unlikely to terminate seizures, while the excitatory effect, could conceivably promote or aggravate their manifestation. In conclusion, these findings demonstrate that endogenous acetylcholine may contribute to epileptogenesis in immature neocortex.

Origin of synchronized oscillations induced by neocortical disinhibition in vivo

During disinhibition, the neocortex generates synchronous activities. Block of GABA(A) receptors in neocortex transforms cortical slow-wave oscillations into large-amplitude approximately 1 Hz discharges consisting of a negative spike or multiple negative spikes riding on a positive wave. Further block of GABA(B) receptors in neocortex slows the discharges to approximately 0.5 Hz and increments the number of negative spikes forming rhythmic approximately 10 Hz neocortical oscillations. Although the thalamus responds robustly to these neocortical discharges, these are unaffected by thalamic inactivation using tetrodotoxin. Thus, an important problem relates to the origin of these activities within the neocortex. Current source density analysis and intracellular recordings revealed that the first negative spike in a discharge corresponded to a current sink that reflected a paroxysmal depolarizing shift (PDS) and could originate in the lower layers or in the upper layers. Regardless of the origin (upper or lower layer), the initial current sink always spreads to the same site in upper layer V-IV. In contrast, the approximately 10 Hz oscillation that follows the initial negative spike corresponds to current sinks that always originate in the lower layers but do not spread to upper layer V-IV, jumping directly to the upper layers. Each current sink in the approximately 10 Hz oscillation reflects a small PDS and is followed by a current source that reflects the repolarization after each PDS.

Propagating activation during oscillations and evoked responses in neocortical slices

Population activity in the cortex is poorly understood. In this report we use voltage-sensitive dye imaging to examine the spatiotemporal patterns of a 7-10 Hz oscillation in neocortical slices from rat somatosensory areas. This oscillation appeared as a component of spontaneous epochs when the preparation was bathed in low [Mg] artificial CSF (ACSF) (Silva et al., 1991). Each epoch started with a synchronized spike, and 3-200 cycles of oscillation emerged afterward. Voltage-sensitive dye imaging revealed that the oscillations in the local field potential recordings were actually caused by a propagating population activation. This activation propagated in a relatively uniform size (not expanding). We call this confined, propagating activation a "dynamic ensemble." During each oscillation cycle, one (occasionally two) dynamic ensemble(s) appeared in the slice and was sustained for 60-200 msec. Dynamic ensembles propagated at approximately 30 mm/sec; the activity could propagate in both directions in cortical slices. The propagation consisted in part of "jumps," the locations of which were not fixed. Dynamic ensembles were distinguishable from the epileptiform spikes that occurred in low [Mg] ACSF. Population events similar to dynamic ensembles were also evoked under conditions of unaltered excitability (slice in normal ACSF) by electrical stimulation that activated a low density of neurons in a large area. Our data suggest that self-sustained, spatially confined, and propagating dynamic ensembles might be related to the epoch oscillations in somatosensory cortex seen in vivo (Nicolelis et al., 1995) and thus resemble one form of population activation in the neocortex.