Liset Menendez De La Prida

Gilles Huberfeld
email: gilles.huberfeld@sorbonne-universite.fr

+34915854359 Gilles

Inhibition and oscillations in the human brain tissue in vitro

Keywords: Human brain slices, Theta, Gamma, Ripples, Interictal, GABA

Local microcircuits support oscillations generated in postoperative

Brain oscillations as biomarkers of microcircuit function and dysfunction

The human brain activity spectrum ranges from very slow to very fast oscillations in association with different cognitive demands and physiological stages. Oscillations <1Hz are characteristic of slow-wave sleep and associated with up and down membrane potential fluctuations [START_REF] Steriade | Natural waking and sleep states: a view from inside neocortical neurons[END_REF]. While they involve thalamo-cortical circuits [START_REF] Crunelli | The slow (<1 Hz) rhythm of non-REM sleep: a dialogue between three cardinal oscillators[END_REF], intra-cortical networks contribute by organizing activity across deep and superficial layers in a state-dependent manner [START_REF] Cash | The human K-complex represents an isolated cortical down-state[END_REF][START_REF] Csercsa | Laminar analysis of slow wave activity in humans[END_REF][START_REF] Hangya | Complex propagation patterns characterize human cortical activity during slow-wave sleep[END_REF]. In turn, slow oscillation propagates brain-wide to play roles in regulating synaptic homeostasis [START_REF] Massimini | The sleep slow oscillation as a traveling wave[END_REF]. Intermediate neocortical oscillations from theta (4-8 Hz) to alpha (7)(8)(9)(10)(11)(12) frequencies are dynamically associated to specific cognitive demands [START_REF] Canolty | High gamma power is phase-locked to theta oscillations in human neocortex[END_REF][START_REF] Raghavachari | Gating of human theta oscillations by a working memory task[END_REF]. In the human hippocampus, short theta bouts organize neuronal spike timing during spatial and visual navigation as well as memory traces [START_REF] Bohbot | Low-frequency theta oscillations in the human hippocampus during real-world and virtual navigation[END_REF][START_REF] Jutras | Oscillatory activity in the monkey hippocampus during visual exploration and memory formation[END_REF][START_REF] Rutishauser | Human memory strength is predicted by theta-frequency phase-locking of single neurons[END_REF]. Perceptual processing associated with neocortical rhythms in the alpha-beta band (12-30 Hz) are associated with rhythmic inhibition and phase-locked firing [START_REF] Haegens | Top-down controlled alpha band activity in somatosensory areas determines behavioral performance in a discrimination task[END_REF], a mechanism shared with gamma (40-80 Hz) rhythms [START_REF] Jacobs | Brain oscillations control timing of singleneuron activity in humans[END_REF][START_REF] Van Quyen | Large-scale microelectrode recordings of high-frequency gamma oscillations in human cortex during sleep[END_REF]. Faster oscillations (>150 Hz) are locally restrained in the hippocampus and neocortical regions serving memory consolidation processes [START_REF] Kucewicz | High frequency oscillations are associated with cognitive processing in human recognition memory[END_REF][START_REF] Zhang | Electrophysiological mechanisms of human memory consolidation[END_REF].

Critically, these cognitive operations require coordinated activity across brain regions, believed to be reflected as a hierarchical and dynamical oscillatory coupling [START_REF] Clemens | Temporal coupling of parahippocampal ripples, sleep spindles and slow oscillations in humans[END_REF][START_REF] Fell | Human memory formation is accompanied by rhinal-hippocampal coupling and decoupling[END_REF][START_REF] Lakatos | An oscillatory hierarchy controlling neuronal excitability and stimulus processing in the auditory cortex[END_REF][START_REF] Staresina | Hierarchical nesting of slow oscillations, spindles and ripples in the human hippocampus during sleep[END_REF]. Theta-gamma interaction coordinates local microcircuits across neocortical territories [START_REF] Canolty | High gamma power is phase-locked to theta oscillations in human neocortex[END_REF]. Top-down and bottom-up processing implicates coordination between alpha-beta and gamma rhythms across prefrontal, parietal and occipital cortices [START_REF] Buschman | Top-down versus bottom-up control of attention in the prefrontal and posterior parietal cortices[END_REF][START_REF] Michalareas | Alpha-Beta and Gamma Rhythms Subserve Feedback and Feedforward Influences among Human Visual Cortical Areas[END_REF]. Different types of oscillatory coupling from phase-amplitude modulation to period concatenation may result from inter-laminar and inter-regional specific connections [START_REF] Kramer | Rhythm generation through period concatenation in rat somatosensory cortex[END_REF][START_REF] Palva | Phase synchrony among neuronal oscillations in the human cortex[END_REF]. Not surprising, dynamical patterns of oscillatory coupling predicts cognitive operations better than power spectral features alone [START_REF] Jensen | Cross-frequency coupling between neuronal oscillations[END_REF][START_REF] Bohbot | Low-frequency theta oscillations in the human hippocampus during real-world and virtual navigation[END_REF][START_REF] Jutras | Oscillatory activity in the monkey hippocampus during visual exploration and memory formation[END_REF][START_REF] Rutishauser | Human memory strength is predicted by theta-frequency phase-locking of single neurons[END_REF].

Oscillations recorded with intracranial electrodes predominantly reflect post-synaptic activity and transmembrane currents from local generators, with a strong contribution of inhibitory potentials, whereas scalp electroencephalography and subdural electrocorticography are biased by volume conducted activity from superficial cortical layers and large amplitude potentials (Buzsáki et al., 2012;[START_REF] Teleńczuk | Local field potentials primarily reflect inhibitory neuron activity in human and monkey cortex[END_REF]. Given the close relationship between oscillations and microcircuit function, they can be considered as biomarkers of neuropathological processes of brain diseases [START_REF] Buzsáki | Hippocampal sharp wave-ripple: A cognitive biomarker for episodic memory and planning[END_REF][START_REF] Palop | Network abnormalities and interneuron dysfunction in Alzheimer disease[END_REF][START_REF] Shuman | Theta Rhythmopathy as a Cause of Cognitive Disability in TLE[END_REF]. Similar to the normal brain, the epileptic brain may be considered an oscillatory system itself [START_REF] Baud | Multi-day rhythms modulate seizure risk in epilepsy[END_REF][START_REF] De La Prida | A Lunatic Dance: Circadian and Multidien Structures of Seizure Timing[END_REF]. The more we learn about the mechanisms underlying brain rhythms, the more we know about how deviations from the physiological function would transform the oscillatory spectrum. In temporal lobe epilepsy (TLE), a disease affecting the hippocampus and parahippocampal regions specifically, ripples (150-250 Hz) invade the fast ripple band (>250 Hz; [START_REF] Jefferys | Mechanisms of physiological and epileptic HFO generation[END_REF][START_REF] Worrell | High-frequency oscillations in human temporal lobe: simultaneous microwire and clinical macroelectrode recordings[END_REF]). These pathological high-frequency oscillations (HFOs) are associated with epileptogenic areas and seizure onset zones and linked with hippocampal sclerosis [START_REF] Staba | Increased Fast ripple to ripple Ratios Correlate with Reduced Hippocampal Volumes and Neuron Loss in Temporal Lobe Epilepsy Patients[END_REF][START_REF] Zijlmans | Highfrequency oscillations as a new biomarker in epilepsy[END_REF]. Other oscillatory bands, including theta and gamma, are equally impaired in the epileptic hippocampus and slow theta-gamma modulation supporting episodic memory encoding is deficient in the human TLE [START_REF] Lega | The gamma band effect for episodic memory encoding is absent in epileptogenic hippocampi[END_REF]. In experimental models of TLE, spatiotemporal features of theta, gamma and ripple oscillations are associated with a range of cognitive co-morbidities [START_REF] Lopez-Pigozzi | Altered Oscillatory Dynamics of CA1 Parvalbumin Basket Cells during Theta-Gamma Rhythmopathies of Temporal Lobe Epilepsy[END_REF][START_REF] Shuman | Theta Rhythmopathy as a Cause of Cognitive Disability in TLE[END_REF][START_REF] Valero | Mechanisms for Selective Single-Cell Reactivation during Offline Sharp-Wave Ripples and Their Distortion by Fast Ripples[END_REF].

Oscillations are particularly useful to track for changes of brain activity such as those accompanying seizures. Seizures are highly dynamic heterogenous phenomena at the cellular and network levels. They may result either from an excess of excitatory processes, from a deficiency/breakout of inhibitory control and/or from post-inhibitory rebounds. When the ictal discharge propagates brain-wide, different wavefront dynamics distinguish between the onset zone and surrounding territories based on the amount of cross-frequency interactions between slow and fast oscillations [START_REF] Alvarado-Rojas | Different mechanisms of ripple-like oscillations in the human epileptic subiculum[END_REF][START_REF] Weiss | Ictal high frequency oscillations distinguish two types of seizure territories in humans[END_REF]. The interictal state between seizures is characterized by the ability of the epileptic brain to generate short lasting hypersynchronized discharges accompanied by pathological HFOs. The hallmark of interictal discharges is their briefness, limited to 80-100 milliseconds, resulting in a large sharp wave typically called 'spike' that can be volume conducted to distant regions. They can also synaptically propagate following long-range connections brain-wide to affect memory function [START_REF] Gelinas | Interictal epileptiform discharges induce hippocampal-cortical coupling in temporal lobe epilepsy[END_REF]. Interictal discharges are followed by a slow component, or 'wave', lasting up to hundreds of milliseconds. The combination of both events, known as the spike-wave complex, can remain isolated or recur at variable frequencies from 3 to 10 Hz. In focal epilepsies, spike and wave complexes are generated within the irritative zone generally overlapping the seizure onset zone [START_REF] Rosenow | Presurgical evaluation of epilepsy[END_REF]. They are therefore considered as good markers of epileptogenic activity but have a poor localizing value. In contrast, patterns of fast oscillations characteristic of local epileptogenic microcircuits are gaining support as new biomarkers of epileptogenic regions [START_REF] Jobst | Is it time to replace epileptic spikes with fast ripples?[END_REF] although they are not obtained by scalp recordings.

Understanding basic mechanisms of human brain oscillations and their relationship with disease-specific rhythmopathies and epileptic forms of activity will help to improve functional diagnostic tools and to better identify shared anatomical, biochemical and physiological correlates of such rhythmopathies and epileptic discharges, ignored so far.

Studying oscillations in human brain slices

Surgical treatment of refractory epilepsies and brain tumors offers a unique opportunity to study the human brain ex vivo. Patients undergoing surgery exhibit a range of clinical background and pathologies allowing for testing disease-specific aspects of microcircuit organization using state-of-the-art in vitro techniques. In some cases, the resected tissue also includes nonpathological normal-like regions surrounding a non-epileptic lesion or located on the way to deep surgical targets, permitting examination of human microcircuit function in a context considered as physiological. The morphological normality may however not exclude that some tissues may be remodeled by epileptogenesis, specifically around gliomas known to produce in vivo pathological activities from remote areas [START_REF] Mittal | Intracranial EEG analysis in tumor-related epilepsy: Evidence of distant epileptic abnormalities[END_REF]. After sectioning, brain slices from 300 to 500 µm are stabilized in artificial cerebrospinal fluid (ACSF) at different flow rates in either submerged or interface chambers for subsequent electrophysiological recordings. Some reviews cover methodological aspects of classical human brain slice approaches [START_REF] Jones | Human brain slices for epilepsy research: Pitfalls, solutions and future challenges[END_REF][START_REF] Kohling | Methodological approaches to exploring epileptic disorders in the human brain in vitro[END_REF]. More recently, organotypic human brain cultures have allowed for cell-type specific genetic interventions and more sophisticated functional experiments such as optogenetics, chemogenetics and multi-cellular imaging [START_REF] Andersson | Optogenetic control of human neurons in organotypic brain cultures[END_REF][START_REF] Eugène | An organotypic brain slice preparation from adult patients with temporal lobe epilepsy[END_REF][START_REF] Duigou | Imaging pathological activities of human brain tissue in organotypic culture[END_REF][START_REF] Parker | A Human Glioblastoma Organotypic Slice Culture Model for Study of Tumor Cell Migration and Patient-specific Effects of Anti-Invasive Drugs[END_REF][START_REF] Schwarz | Human Cerebrospinal fluid promotes long-term neuronal viability and network function in human neocortical organotypic brain slice cultures[END_REF]. New stem cell technologies now allow generation of brain organoids from human pluripotent cells [START_REF] Lancaster | Generation of cerebral organoids from human pluripotent stem cells[END_REF]. These emerging technologies will soon add to the arsenal of in vitro methodologies that can be brought to studying human tissue at an unprecedented scale.

A range of brain oscillations can be modeled in vitro using different ACSF composition and reagents (Table). Using different concentration of potassium (K + ), calcium (Ca 2+ ) and magnesium (Mg 2+ ) allows for recording self-organized oscillations in the local field potentials (LFP). The physiological or pathological nature of these oscillations will depend on how the different ionic composition affects excitability and synaptic transmission [START_REF] Aivar | Extracellular calcium controls the expression of two different forms of ripple-like hippocampal oscillations[END_REF][START_REF] Lux | Ionic changes and alterations in the size of the extracellular space during epileptic activity[END_REF]. For instance, reducing Ca 2+ and Mg 2+ have a profound impact on synaptic transmission in slices from healthy rodents, causing pathological-like activities to emerge [START_REF] Heinemann | Low-Ca(2+)induced epileptiform activity in rat hippocampal slices[END_REF][START_REF] Mody | Low extracellular magnesium induces epileptiform activity and spreading depression in rat hippocampal slices[END_REF]. Other factors such as the flow rate and interface conditions can affect the physiological level of oxygenation and pH required for a normal microcircuit operation [START_REF] Hájos | Establishing a physiological environment for visualized in vitro brain slice recordings by increasing oxygen supply and modifying aCSF content[END_REF][START_REF] Huchzermeyer | Gamma oscillations and spontaneous network activity in the hippocampus are highly sensitive to decreases in pO2 and concomitant changes in mitochondrial redox state[END_REF]. In slices prepared from different patients, and even within a given patient, variability observed under the same recording conditions may reflect different degrees of the underlying pathology in the resected block, together with technical irregularities (Menendez de la [START_REF] Menendez De La Prida | Origin of the synchronized network activity in the rabbit developing hippocampus[END_REF]. Therefore, it is advisable to use standard ACSF solutions that have been proven to operate at the physiological regime in order to extract meaningful conclusions regarding the pathophysiological mechanisms.

A full range of oscillations recorded intraoperatively can be mimicked in slices (Fig. 1). Slow oscillations are typically recorded under standard ACSF in interface conditions and highflow rates both in fresh and organotypic slices (Table ). Similar to rodent, activity from deep cortical layers variably contributes to trigger propagating waves in human neocortical slices [START_REF] Florez | In Vitro Recordings of Human Neocortical Oscillations[END_REF][START_REF] Sanchez-Vives | Cellular and network mechanisms of rhythmic recurrent activity in neocortex[END_REF]. In many reports (Florez et al., 2013;[START_REF] Kohling | Spontaneous sharp waves in human neocortical slices excised from epileptic patients[END_REF], they typically consist on recurrent spontaneous sharp-waves associated with composite synaptic currents that rhythmically reverse at around -70 mV, the expected value of GABAa receptors. These events are blocked by GABAa antagonists like bicuculline or picrotoxin but also by antagonists of glutamatergic AMPA receptors such as CNQX [START_REF] Graebenitz | Interictal-like network activity and receptor expression in the epileptic human lateral amygdala[END_REF][START_REF] Kohling | Spontaneous sharp waves in human neocortical slices excised from epileptic patients[END_REF]. Since spontaneous slow oscillations were not reported to systematically correlate neither with etiology nor with histopathological findings (Florez et al., 2013;[START_REF] Kohling | Spontaneous sharp waves in human neocortical slices excised from epileptic patients[END_REF] they could be considered a basic operational mode of local microcircuits. Importantly, inhibitory potentials in resected human tissues are variable with distinct reversal potentials in pyramidal cells. In slices from epileptogenic territories, a subpopulation of pyramidal cells may be driven by depolarizing effects of GABAa receptors and reestablishing normal intracellular chloride concentration pharmacologically blocks recurring rhythmic events [START_REF] Huberfeld | Perturbed chloride homeostasis and GABAergic signaling in human temporal lobe epilepsy[END_REF]Pallud et al., 2014b). In human hippocampal slices, sharp-waves recur at similar slower frequencies and involve similar inhibitory and excitatory potentials than slow cortical waves but they are more typically considered interictal-like events given their association with altered GABAa-mediated inhibition [START_REF] Alvarado-Rojas | Different mechanisms of ripple-like oscillations in the human epileptic subiculum[END_REF][START_REF] Huberfeld | Perturbed chloride homeostasis and GABAergic signaling in human temporal lobe epilepsy[END_REF]. Thus, it may be hypothesized that in epileptogenic hyperexcitable regions or when chloride dysregulation affects GABAergic signaling towards depolarizing responses, slow oscillations could be transformed into recurrent interictal-like events of shorter duration and higher amplitude [START_REF] Florez | In Vitro Recordings of Human Neocortical Oscillations[END_REF][START_REF] Huberfeld | Perturbed chloride homeostasis and GABAergic signaling in human temporal lobe epilepsy[END_REF].

Similar to healthy rodent slices, human neocortical slices bathed with kainate (50-100 nM) and/or carbachol (50 nM) can generate spontaneous intermediate oscillations in the theta band (4-8 Hz) (Florez et al., 2015) (Table). These extracellular oscillations are associated with rhythmic intracellular inhibitory potentials and/or currents sensitive to antagonists of muscarinic receptors, consistent with their induction by cholinergic activation. Similar to slow oscillations, they are typically initiated in deep neocortical layers. Unlike rodent slices, carbachol alone increases multi-unit firing in superficial layers of human neocortical slices but remains subthreshold for the generation of oscillatory activity from deep layers [START_REF] Florez | In Vitro Recordings of Human Neocortical Oscillations[END_REF]. Interestingly, manipulations that induced theta activity also results in coupled oscillations in the beta (12-30 Hz) and gamma band . In human neocortical slices, gamma oscillations emerge with kainate alone independently in superficial and deep layers [START_REF] Florez | In Vitro Recordings of Human Neocortical Oscillations[END_REF]. As far as we know, there is no report of theta/gamma oscillations induced in human hippocampal slices.

High-frequency oscillations in the form of ripples associated to sharp-waves are more typically seen in human hippocampal slices (Alvarado-Rojas et al., 2015) (Table). Hippocampal ripples recorded in the subiculum of TLE patients correlate with intrinsic bursts and rhythmic inhibitory and excitatory potentials at the single-cell level [START_REF] Alvarado-Rojas | Different mechanisms of ripple-like oscillations in the human epileptic subiculum[END_REF]. These events are blocked by antagonists of glutamatergic and GABAergic receptors, supporting their microcircuit origin [START_REF] Cohen | On the origin of interictal activity in human temporal lobe epilepsy in vitro[END_REF]. In rodents, both the spectral components and cellular correlates of physiological ripples appear similar to some interictal-like ripples recorded in human hippocampal slices [START_REF] Ellender | Priming of hippocampal population bursts by individual perisomatic-targeting interneurons[END_REF][START_REF] Schlingloff | Mechanisms of sharp wave initiation and ripple generation[END_REF][START_REF] Valero | Determinants of different deep and superficial CA1 pyramidal cell dynamics during sharp-wave ripples[END_REF]. In the rodent sclerotic hippocampus, fast ripples and HFOs coexist with physiological-like ripples both in vivo and in vitro [START_REF] Foffani | Reduced spike-timing reliability correlates with the emergence of fast ripples in the rat epileptic hippocampus[END_REF][START_REF] Valero | Mechanisms for Selective Single-Cell Reactivation during Offline Sharp-Wave Ripples and Their Distortion by Fast Ripples[END_REF]. Therefore, it is possible that similar circuits can generate both types of events but separation based on spectral features remain challenging (Menendez de la Prida et al., 2015).

Interictal-and ictal-like discharges can be recorded spontaneously in slices from epileptic animals and in humans but they may also be generated pharmacologically in normal tissues, especially after blockade of GABAa receptors, a condition which however elicits almost exclusively interictal discharges (Li, 1959) (Table). Interictal-like events are associated with a paroxysmal depolarization shift composed of a depolarizing envelope surmounted by bursts of action potential underlying extracellular ripples, fast ripples and pathological HFOs [START_REF] Ayala | Genesis of epileptic interictal spikes. New knowledge of cortical feedback systems suggests a neurophysiological explanation of brief paroxysms[END_REF]Jefferys et al., 2012). Both the induction by GABAa antagonists and the cellular signature led to a proposed mechanism involving a defective GABAergic inhibition and an excess in glutamatergic excitation [START_REF] Prince | Mechanisms of interictal epileptogenesis[END_REF]) that could be summarized as a synchronized giant excitatory synaptic potential [START_REF] Johnston | Giant synaptic potential hypothesis for epileptiform activity[END_REF]. The secondary slow wave component rather represent inhibitory mechanisms particularly involving GABAb transmission [START_REF] Cohen | Emergence of disinhibition-induced synchrony in the CA3 region of the guinea pig hippocampus in vitro[END_REF][START_REF] De La Prida | Threshold behavior in the initiation of hippocampal population bursts[END_REF][START_REF] Destexhe | Spike-and-wave oscillations based on the properties of GABAB receptors[END_REF]. In fact, depending on the pharmacological method for inducing interictal-like discharges in vitro, different mechanisms have been described [START_REF] Rutecki | Epileptiform activity in the hippocampus produced by tetraethylammonium[END_REF][START_REF] Rutecki | 4-Aminopyridine produces epileptiform activity in hippocampus and enhances synaptic excitation and inhibition[END_REF][START_REF] Rutecki | Epileptiform activity induced by changes in extracellular potassium in hippocampus[END_REF]. Altered GABAergic potentials in some human hippocampal pyramidal cells support pro-epileptogenic changes of GABAa-mediated inhibition caused by alterations of potassiumchloride cotransporter KCC2 and the sodium-potassium chloride cotransporter NKCC1 [START_REF] Huberfeld | Perturbed chloride homeostasis and GABAergic signaling in human temporal lobe epilepsy[END_REF]. Consistently, rhythmic sharp-wave ripples and fast ripples are sensitive to bumetanide in the human epileptic hippocampus [START_REF] Huberfeld | Perturbed chloride homeostasis and GABAergic signaling in human temporal lobe epilepsy[END_REF]. In epileptogenic territories from both neocortical and hippocampal human slices, fast oscillations are also sensitive to gap junction blockers [START_REF] Simon | Gap junction networks can generate both ripple-like and fast ripple-like oscillations[END_REF]. Thus, as in the human brain in vivo, the postoperative tissue in vitro exhibits physiologicaland pathological-like oscillations coexisting with epileptiform activities. Presumably, the same microcircuits responsible for the basic oscillatory modes should operate during pathological forms of activity [START_REF] Beenhakker | Neurons that fire together also conspire together: is normal sleep circuitry hijacked to generate epilepsy?[END_REF][START_REF] Paz | Microcircuits and their interactions in epilepsy: is the focus out of focus?[END_REF] but these two conceptual frameworks have been traditionally separated in clinical research. There is no reason to think the brain operates independent mechanisms for normal and abnormal microcircuit function. Instead, by understanding how neuropathological territories are pushed away to operate in their physiological regime we will be able to better embrace the complexity of the human brain in health and disease [START_REF] Scott | WONOEP APPRAISAL: The many facets of epilepsy networks[END_REF]. The study of basic neurophysiological mechanisms underlying brain disorders may rather benefit from such an integrated view.

Microcircuit correlates of network oscillations in human brain slices: what is normal and what is not?

What are the basic microcircuit mechanisms underlying oscillations in the human brain in vitro? How do they deviate from physiological-like behavior? In vitro investigation of human brain tissue may help to understand basic principles of microcircuit operations and their alterations in disease. Given the difficulties in evaluating normal-like brain tissue, we need to identify what is physiological and what is not in terms of different oscillatory patterns by comparison with in vivo human recordings and animal data. In human brain slices prepared from resected tissue, an additional influence of disease-specific neuropathology should be always considered although pathological-like oscillations, such as epileptic activities, may be associated with no obvious neuropathological defect. Thus, the critical point is to adopt a phenomenological approach to understand what is normal and what is not at the single-cell and microcircuit level in vitro and to identify potentially cell-type and connectivity specific drivers of physiological-like activities.

Similar to human intracranial recordings, slow neocortical oscillations in vitro are associated with fluctuating levels of firing rate across layers. Intracellular data support interpretation of synaptic influences during these fluctuating states [START_REF] Florez | In Vitro Recordings of Human Neocortical Oscillations[END_REF][START_REF] Kohling | Current-source-density profiles associated with sharp waves in human epileptic neocortical tissue[END_REF]. During up states in vivo, large sinks recorded at superficial layers II-III are consistent with inward depolarizing currents flowing into the dendrites of superficial and deep pyramidal cells [START_REF] Csercsa | Laminar analysis of slow wave activity in humans[END_REF][START_REF] Staba | Sleep states differentiate single neuron activity recorded from human epileptic hippocampus, entorhinal cortex, and subiculum[END_REF]. These sinks are followed by prominent sources accompanied by neuronal silence consistent with hyperpolarized inhibitory currents [START_REF] Cash | The human K-complex represents an isolated cortical down-state[END_REF][START_REF] Csercsa | Laminar analysis of slow wave activity in humans[END_REF]. Interplay between GABAa-and GABAb-mediated inhibition plays complementary roles in balancing up and down states [START_REF] Mann | Distinct roles of GABA(A) and GABA(B) receptors in balancing and terminating persistent cortical activity[END_REF]. While GABAa-mediated inhibition is involved in controlling neuronal firing precisely during up states, the slow dynamics of GABAb receptor currents acts to prolong membrane potential hyperpolarization and to modulate dendritic electrogenesis [START_REF] Pérez-Garci | The GABAB1b isoform mediates long-lasting inhibition of dendritic Ca2+ spikes in layer 5 somatosensory pyramidal neurons[END_REF]. Dendrites of human pyramidal cells, especially from layer V, exhibit distinctive integrative features and enhanced input compartmentalization, a property that could change across regions [START_REF] Beaulieu-Laroche | Enhanced Dendritic Compartmentalization in Human Cortical Neurons[END_REF][START_REF] Eyal | Unique membrane properties and enhanced signal processing in human neocortical neurons[END_REF][START_REF] Kim | Three Types of Cortical Layer 5 Neurons That Differ in Brain-wide Connectivity and Function[END_REF]. A specialized human GABAergic cell from layer I can target the apical dendrites of layer III pyramidal neurons to inhibit back propagating action potentials [START_REF] Boldog | Transcriptomic and morphophysiological evidence for a specialized human cortical GABAergic cell type[END_REF]. Thus, disparities regarding the role of deep and superficial layers in the generation of slow oscillations across species and between preparations (in vivo, in vitro, anesthesia) may reflect regional differences in the cytoarchitectonic organization of cortical areas [START_REF] Amzica | Electrophysiological correlates of sleep delta waves[END_REF][START_REF] Chauvette | Origin of active states in local neocortical networks during slow sleep oscillation[END_REF][START_REF] Halgren | Superficial Slow Rhythms Integrate Cortical Processing in Humans[END_REF][START_REF] Sakata | Laminar structure of spontaneous and sensory-evoked population activity in auditory cortex[END_REF]. In general, appropriate firing modulation during up/down states across layers is indicative of healthy territories whereas their absence may reflect microcircuit alterations [START_REF] Csercsa | Laminar analysis of slow wave activity in humans[END_REF]. Thus, independently on the origin of slow oscillations, coherent modulation of cell-type specific firing can be considered a physiological-like treat.

Laminar interaction between deep and superficial layers appear critical in hierarchically organizing low and high frequency oscillations both in human and rodent neocortices. Human slices bathed with standard ACSF generate different degree of multi-unit firing during up states but not necessarily genuine LFP oscillations. Manipulations that increase intrinsic excitability and synaptic transmission, such as higher ionic composition of K + (4-6 mM) and Ca 2+ (2-4 mM) or low concentration of kainate (50-100 nM) may help eliciting bonna fide waves in the gamma range. The amplitude of these oscillations couples to the phase of slow wave activity that modulates neuronal firing [START_REF] Beltramo | Layer-specific excitatory circuits differentially control recurrent network dynamics in the neocortex[END_REF][START_REF] Florez | In Vitro Recordings of Human Neocortical Oscillations[END_REF][START_REF] Halgren | Superficial Slow Rhythms Integrate Cortical Processing in Humans[END_REF][START_REF] Ulbert | Laminar analysis of human neocortical interictal spike generation and propagation: current source density and multiunit analysis in vivo[END_REF]. Thus, elevated phase-amplitude coupling likely correlate with escalating levels of excitability in in vitro human data [START_REF] Alvarado-Rojas | Different mechanisms of ripple-like oscillations in the human epileptic subiculum[END_REF][START_REF] Mcginn | Phase-amplitude coupling and interlaminar synchrony are correlated in human neocortex[END_REF], which in pathological conditions would tend to reflect exacerbated synaptic activity and excessive neuronal firing [START_REF] Tort | Theta-associated high-frequency oscillations (110-160Hz) in the hippocampus and neocortex[END_REF].

Appropriately regulated gamma oscillations are associated with the integrity of GABAergic mechanisms [START_REF] Vida | Shunting inhibition improves robustness of gamma oscillations in hippocampal interneuron networks by homogenizing firing rates[END_REF]. Two main mechanisms support gamma oscillations: the interneuron-interneuron gamma model (ING) and the pyramidal cell-interneuron gamma (PING) that emerges from recurrent connections between glutamatergic and GABAergic cells [START_REF] Buzsáki | Mechanisms of gamma oscillations[END_REF]) (Fig. 2A). Depolarizing tonic inputs resulting in interneuronal firing may suffice to entrain an interconnected network of interneurons at the temporal scale of the intrinsic time constant of GABAa-mediated inhibitory currents. In PING models, activity of pyramidal cells drives recurrent networks to oscillate. Thus, the organization of local microcircuits may determine the ability of different neocortical territories to generate gamma oscillations in vitro [START_REF] Buzsáki | Mechanisms of gamma oscillations[END_REF][START_REF] Vida | Shunting inhibition improves robustness of gamma oscillations in hippocampal interneuron networks by homogenizing firing rates[END_REF]. Kainate receptor agonists activate directly interneurons while cholinergic agonists may require additional depolarization of the whole neuronal population [START_REF] Fisahn | Distinct roles for the kainate receptor subunits GluR5 and GluR6 in kainate-induced hippocampal gamma oscillations[END_REF][START_REF] Florez | In Vitro Recordings of Human Neocortical Oscillations[END_REF]. In human slices, cholinergic drive may act directly over upper layer latespiking neurogliaform cells and onto Martinotti cells to facilitate local and lateral inhibition [START_REF] Obermayer | Lateral inhibition by Martinotti interneurons is facilitated by cholinergic inputs in human and mouse neocortex[END_REF][START_REF] Poorthuis | Rapid Neuromodulation of Layer 1 Interneurons in Human Neocortex[END_REF]. Neurogliaform cells are a major source of slow extrasynaptic GABAa-mediated postsynaptic responses [START_REF] Szabadics | Different transmitter transients underlie presynaptic cell type specificity of GABAA,slow and GABAA,fast[END_REF][START_REF] Tamás | Identified sources and targets of slow inhibition in the neocortex[END_REF] and GABAb-mediated pre-and postsynaptic inhibition [START_REF] Craig | The emerging role of GABAB receptors as regulators of network dynamics: fast actions from a "slow" receptor?[END_REF][START_REF] Tamás | Identified sources and targets of slow inhibition in the neocortex[END_REF]. Predominance of gamma activity in upper cortical layers of the human neocortex can also reflect variabilities in the number of calretinin-or reelin-immunopositive interneurons in this zone [START_REF] Gabbott | Calretinin neurons in human medial prefrontal cortex (areas 24a,b,c, 32', and 25)[END_REF][START_REF] Varga | Molecular and Electrophysiological Characterization of GABAergic Interneurons Expressing the Transcription Factor COUP-TFII in the Adult Human Temporal Cortex[END_REF]. Calretinin+ cells and/or VIP-expressing interneurons preferentially targeting other local GABAergic cells would provide additional substrates for ING mechanisms (del Río and DeFelipe, 1997).

In general, physiological-like high-beta and gamma oscillations also reflect fast sequences of excitatory and inhibitory potentials at the fast gamma band (Fig. 2A). In this oscillatory PING regime, the delay between excitation and inhibition is critical to determine phase response dynamics [START_REF] Akao | Relationship between the mechanisms of gamma rhythm generation and the magnitude of the macroscopic phase response function in a population of excitatory and inhibitory modified quadratic integrate-and-fire neurons[END_REF][START_REF] Fuchs | Recruitment of parvalbumin-positive interneurons determines hippocampal function and associated behavior[END_REF]. In superficial layers of human neocortex, individual action potentials from pyramidal cells can initiate complex network events caused by the efficient recruitment of GABAergic interneurons [START_REF] Molnár | Complex Events Initiated by Individual Spikes in the Human Cerebral Cortex[END_REF][START_REF] Szegedi | High-Precision Fast-Spiking Basket Cell Discharges during Complex Events in the Human Neocortex[END_REF]. Human calretinin+ interneurons co-expressing the transcription factor COUP-TFII are able to activate inhibitory potentials in the dendrites of reciprocally connected pyramidal cells [START_REF] Varga | Molecular and Electrophysiological Characterization of GABAergic Interneurons Expressing the Transcription Factor COUP-TFII in the Adult Human Temporal Cortex[END_REF]. Perisomatic inhibition initiated synchronously by individual parvalbumin+ basket cells in subsets of pyramidal cells has proved effective for gamma [START_REF] Bartos | Rapid signaling at inhibitory synapses in a dentate gyrus interneuron network[END_REF]. Thus, different human interneuronal subtypes may play specific roles in generating and maintaining neocortical gamma oscillations. Subtle regional differences in the organization of local GABAergic and glutamatergic microcircuits may operate in the diseased human brain [START_REF] Alonso-Nanclares | Microanatomy of the dysplastic neocortex from epileptic patients[END_REF][START_REF] Defelipe | Microzonal decreases in the immunostaining for non-NMDA ionotropic excitatory amino acid receptor subunits GluR 2/3 and GluR 5/6/7 in the human epileptogenic neocortex[END_REF][START_REF] Defelipe | Selective changes in the microorganization of the human epileptogenic neocortex revealed by parvalbumin immunoreactivity[END_REF][START_REF] Foehring | Correlation of physiologically and morphologically identified neuronal types in human association cortex in vitro[END_REF][START_REF] Menendez De La Prida | Electrophysiological properties of interneurons from intraoperative spiking areas of epileptic human temporal neocortex[END_REF].

A repertoire of fast events including spindles (7-14 Hz), gamma bouts and ripples could naturally couple to slow neocortical oscillations [START_REF] Csercsa | Laminar analysis of slow wave activity in humans[END_REF], but depending on the excitability level and the integrity of GABAergic mechanisms these events could reflect abnormal synchronous firing in epileptogenic territories [START_REF] Alvarado-Rojas | Different mechanisms of ripple-like oscillations in the human epileptic subiculum[END_REF][START_REF] Kohling | Current-source-density profiles associated with sharp waves in human epileptic neocortical tissue[END_REF]. Epileptiform activities are typically characterized by large amplitude and brief events accompanied by high-gamma oscillations (90-150 Hz), ripples and HFOs (>250 Hz; [START_REF] Jiruska | Epileptic highfrequency network activity in a model of non-lesional temporal lobe epilepsy[END_REF], which in most cases are considered surrogates for neuronal bursting [START_REF] Mcginn | Phase-amplitude coupling and interlaminar synchrony are correlated in human neocortex[END_REF][START_REF] Ray | Neural correlates of highgamma oscillations (60-200 Hz) in macaque local field potentials and their potential implications in electrocorticography[END_REF][START_REF] Tort | Theta-associated high-frequency oscillations (110-160Hz) in the hippocampus and neocortex[END_REF]. Paired recordings in human neocortical slices showed that axoaxonic parvalbumin+ interneurons can initiate depolarizing synaptic events due the absence of KCC2 in the axon initial segment [START_REF] Szabadics | Excitatory effect of GABAergic axo-axonic cells in cortical microcircuits[END_REF]. Some groups have indeed suggested that population events recorded in the cortex could in fact correspond to physiological activities [START_REF] Tóth | Hyperexcitability of the network contributes to synchronization processes in the human epileptic neocortex[END_REF], initiated by normal interneuronal firing located apart from the surgical target zone [START_REF] Szegedi | Plasticity in Single Axon Glutamatergic Connection to GABAergic Interneurons Regulates Complex Events in the Human Neocortex[END_REF]. Thus, together with pathological GABAa-mediated perisomatic depolarization this may yield to concatenated series of complex synaptic events across neighboring interneurons and pyramidal cells that invade the fast ripple band [START_REF] Molnár | Complex Events Initiated by Individual Spikes in the Human Cerebral Cortex[END_REF]. Channelopathies affecting intrinsic excitability and synaptic integration can also contribute to distort the high-frequency spectrum [START_REF] Simeone | Loss of the Kv1.1 potassium channel promotes pathologic sharp waves and high frequency oscillations in in vitro hippocampal slices[END_REF]. It has to be stressed however that these tissues are mainly obtained from peritumoral cortices, in which the epileptogenic processes develop at considerable distance of the tumor macroscopical limits. This has been shown in vivo during chronic [START_REF] Mittal | Intracranial EEG analysis in tumor-related epilepsy: Evidence of distant epileptic abnormalities[END_REF] and peroperative cortical recordings [START_REF] Feyissa | High-frequency oscillations in awake patients undergoing brain tumor-related epilepsy surgery[END_REF] of areas infiltrated by sparse tumor cells (Pallud et al., 2014b[START_REF] Pallud | Diffuse low-grade oligodendrogliomas extend beyond MRI-defined abnormalities[END_REF].

In summary, local microcircuits in the human brain are tuned to generate oscillations in the gamma and ripple band, whereas inter-laminar and inter-regional connectivity modulate local fast oscillations by slower frequency activity in the beta, theta and at <1 Hz frequency band (Fig. 1B). Understanding how activity is organized in upper and lower neocortical layers and within different structures such as in the hippocampus-entorhinal system requires better knowledge of regional differences on cell-type specificity and connectivity and how neuropathological changes do affect microcircuit function (Fig. 1C). In general, a physiological regime of oscillations is reflected as an adequate balance between slow and fast oscillations keeping neuronal firing and bursting at a relatively low synchronization level. Sharp-wave events have moderate amplitudes in a range of few hundreds millivolts when measured locally with microelectrodes. Intermediate rhythms are well tuned by brain-wide slow frequency rhythms so that the depth of slow-frequency modulation of individual neurons is strong. In the diseased human brain, alterations of gamma activity and the higher frequency band are conspicuous, suggesting a shift towards high-frequency modulatory effects and pathological HFOs.

Pathological forms of activity in the human brain in vitro

From the sections above it seems clear that looking at the spatiotemporal oscillatory patterns in vitro may help to inform on potential pathophysiological alterations underlying human brain diseases. However, recordings from human epileptic postoperative tissues provide evidence that the diseased human brain in vitro is able to generate other pathological forms of activities (Fig. 1). High-amplitude sharp-wave interictal-like activities have been observed in tissues obtained from patients undergoing surgery for pharmacoresistant temporal lobe epilepsy (frequently associated to hippocampal sclerosis) [START_REF] Cohen | On the origin of interictal activity in human temporal lobe epilepsy in vitro[END_REF][START_REF] Gabriel | Stimulus and potassiuminduced epileptiform activity in the human dentate gyrus from patients with and without hippocampal sclerosis[END_REF][START_REF] Huberfeld | Perturbed chloride homeostasis and GABAergic signaling in human temporal lobe epilepsy[END_REF][START_REF] Isokawa | Extracellular slow negative transient in the dentate gyrus of human epileptic hippocampus in vitro[END_REF][START_REF] Kivi | Effects of barium on stimulus-induced rises of [K+]o in human epileptic non-sclerotic and sclerotic hippocampal area CA1[END_REF][START_REF] Kohling | Current-source-density profiles associated with sharp waves in human epileptic neocortical tissue[END_REF][START_REF] Kohling | Spontaneous sharp waves in human neocortical slices excised from epileptic patients[END_REF][START_REF] Prince | Human epileptic neurons studied in vitro[END_REF][START_REF] Roopun | A nonsynaptic mechanism underlying interictal discharges in human epileptic neocortex[END_REF][START_REF] Schwartzkroin | Spontaneous rhythmic synchronous activity in epileptic human and normal monkey temporal lobe[END_REF][START_REF] Schwartzkroin | Intracellular study of human epileptic cortex: in vitro maintenance of epileptiform activity?[END_REF][START_REF] Schwartzkroin | Epileptogenesis in mammalian cortex studied in vitro[END_REF][START_REF] Schwartzkroin | Studies of human and monkey "epileptic" neocortex in the in vitro slice preparation[END_REF][START_REF] Wittner | The epileptic human hippocampal cornu ammonis 2 region generates spontaneous interictal-like activity in vitro[END_REF][START_REF] Wozny | Comment on "On the origin of interictal activity in human temporal lobe epilepsy in vitro[END_REF], peritumoral cortices in low and high grade gliomas [START_REF] Dossi | Pannexin-1 channels contribute to seizure generation in human epileptic brain tissue and in a mouse model of epilepsy[END_REF]Pallud et al., 2014;[START_REF] Tóth | Hyperexcitability of the network contributes to synchronization processes in the human epileptic neocortex[END_REF], dysembryoplastic neuroepithelial tumors [START_REF] Dossi | Pannexin-1 channels contribute to seizure generation in human epileptic brain tissue and in a mouse model of epilepsy[END_REF] and focal cortical dysplasias [START_REF] Dossi | Pannexin-1 channels contribute to seizure generation in human epileptic brain tissue and in a mouse model of epilepsy[END_REF][START_REF] Dossi | Multi-electrode Array Recordings of Human Epileptic Postoperative Cortical Tissue[END_REF]. These spontaneous events may be recorded from slices when neuronal viability is optimized by procedures improving oxygen availability, such as interface condition or high flow rates (Table ). In these tissues, GABAa receptors antagonists rather control interictal activities and cellular recordings show that interneurons fire before the onset of the event, suggesting that these activities are driven by interneurons that depolarize pyramidal cells with defective chloride regulation [START_REF] Cohen | On the origin of interictal activity in human temporal lobe epilepsy in vitro[END_REF][START_REF] Huberfeld | Perturbed chloride homeostasis and GABAergic signaling in human temporal lobe epilepsy[END_REF]Pallud et al., 2014). In some cases such as recordings in the hippocampal spared CA2 region, excitatory glutamatergic signals seem preeminent [START_REF] Wittner | The epileptic human hippocampal cornu ammonis 2 region generates spontaneous interictal-like activity in vitro[END_REF] and some authors have questioned the epileptic nature of the recorded field potentials given similarities with rodent physiology [START_REF] Alvarado-Rojas | Different mechanisms of ripple-like oscillations in the human epileptic subiculum[END_REF][START_REF] Tóth | Hyperexcitability of the network contributes to synchronization processes in the human epileptic neocortex[END_REF]. The difficulties in assessing indubitable control human tissue make difficult to address directly this question. Interestingly, spontaneous interictal-like discharges are less conspicuously reported from slices obtained from animal models of epilepsy. In a mouse model of hippocampal epilepsy, hyperpolarizing post-synaptic currents are recorded in pyramidal cells during discharges but no clear depolarizing effects of GABA were demonstrated (Le [START_REF] Duigou | Epileptiform activities in slices of hippocampus from mice after intra-hippocampal injection of kainic acid[END_REF]. The complexity and variability of neuronal behavior during interictal discharges is supported by human cellular recordings in vivo in which neurons can either fire before, during the event or remain silent [START_REF] Keller | Heterogeneous neuronal firing patterns during interictal epileptiform discharges in the human cortex[END_REF][START_REF] Truccolo | Single-neuron dynamics in human focal epilepsy[END_REF].

Transition from the interictal state to seizures has been studied extensively in vivo in patients to try to identify seizure anticipation markers [START_REF] Kuhlmann | Seizure prediction -ready for a new era[END_REF]. Mathematical descriptions of seizures suggest their dynamics can be reduced to few basic principles that scale invariantly from neurons to microcircuits and brain-wide networks [START_REF] Jirsa | On the nature of seizure dynamics[END_REF]. These basic principles can be summarized as the existence of coupled slow-fast dynamical processes that govern seizure initiation and termination. At any level (from neurons to networks), a slow variable may shift the system away from the equilibrium resulting in fast reverberating activity (oscillations) that somehow couples back to a slow process responsible for seizure offset. Slowly fluctuating processes leading to a seizure are responsible of pushing neurons away from the resting membrane potential towards the action potential threshold. Several mechanisms from DC shifts to buildup activity have been shown to precede seizures [START_REF] Huberfeld | Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy[END_REF][START_REF] Sedigh-Sarvestani | Rapid eye movement sleep and hippocampal theta oscillations precede seizure onset in the tetanus toxin model of temporal lobe epilepsy[END_REF][START_REF] Vanhatalo | Very slow EEG responses lateralize temporal lobe seizures: an evaluation of non-invasive DC-EEG[END_REF][START_REF] Voipio | Millivolt-scale DC shifts in the human scalp EEG: evidence for a nonneuronal generator[END_REF]. These mounting slow shifts result in accumulating depolarizations across different cell-types due to the integrative properties of membranes and synaptic potentials reverberating through local microcircuit interactions together with changes of the local microenvironment [START_REF] Antonio | In vitro seizure like events and changes in ionic concentration[END_REF][START_REF] Avoli | GABAA-mediated inhibition and in vitro epileptogenesis in the human neocortex[END_REF][START_REF] Avoli | Seizure-like discharges induced by lowering [Mg2+]o in the human epileptogenic neocortex maintained in vitro[END_REF].

In epileptic human slices exhibiting spontaneous interictal discharges, seizure-like events are only recorded when elicited by a pharmacological increase of excitability using modified ACSF (Table ) [START_REF] Avoli | Epileptiform synchronization in the human dysplastic cortex[END_REF][START_REF] D'antuono | GABAA receptor-dependent synchronization leads to ictogenesis in the human dysplastic cortex[END_REF][START_REF] Dossi | Pannexin-1 channels contribute to seizure generation in human epileptic brain tissue and in a mouse model of epilepsy[END_REF][START_REF] Gabriel | Stimulus and potassiuminduced epileptiform activity in the human dentate gyrus from patients with and without hippocampal sclerosis[END_REF][START_REF] Huberfeld | Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy[END_REF][START_REF] Jandova | Carbamazepine-resistance in the epileptic dentate gyrus of human hippocampal slices[END_REF]Pallud et al., 2014). The main characteristic of this period is the emergence of recurrent high amplitude events, similar to that preceding seizures and therefore named preictal discharges [START_REF] Huberfeld | Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy[END_REF]. In vitro, preictal discharges propagate faster than interictal events and are more synchronous and widespread while in vivo they are rather localized to the seizure onset area. Both inter and preictal discharges are associated with a HFO signature centered on 250 Hz in the epileptic human subiculum [START_REF] Alvarado-Rojas | Different mechanisms of ripple-like oscillations in the human epileptic subiculum[END_REF][START_REF] Huberfeld | Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy[END_REF] and the peritumoral cortex (Pallud et al., 2014) while in dysplastic tissues, fast ripples are only evident in preictal discharges (Blauwblomme and Huberfeld; unpublished observations). At a cellular scale, preictal discharges are rather initiated by pyramidal cells, widely synchronizing surrounding neurons through AMPA glutamatergic signals.

While interictal and preictal discharges are transient events, epileptic seizures are complex dynamic events with variable oscillatory components [START_REF] Baud | Multi-day rhythms modulate seizure risk in epilepsy[END_REF][START_REF] Eissa | Cross-scale effects of neural interactions during human neocortical seizure activity[END_REF][START_REF] Jirsa | On the nature of seizure dynamics[END_REF][START_REF] Wagner | Microscale spatiotemporal dynamics during neocortical propagation of human focal seizures[END_REF]. The oscillatory pattern at seizure onset may reflect the neuronal and signaling behavior responsible for seizure generation. In TLE, two main seizure onset patterns have been described in both human and animals, in vivo and in vitro. In the hypersynchronous (HYP) pattern, the seizure is initiated by recurring bursts of activity at 3-15 Hz with dynamic modulation, while the low voltage fast activity (LVFA) pattern consists in fast discharges progressively emerging from HFO bursts [START_REF] Bragin | Analysis of seizure onset on the basis of wideband EEG recordings[END_REF][START_REF] Gotman | Frequency of the electroencephalographic discharge in seizures of focal and widespread onset in intracerebral recordings[END_REF][START_REF] Spencer | Morphological patterns of seizures recorded intracranially[END_REF]. The two patterns may be not so clearly distinguished since: a) series of preictal discharges may be followed by a fast activity onset [START_REF] Weiss | Ictal onset patterns of local field potentials, high frequency oscillations, and unit activity in human mesial temporal lobe epilepsy[END_REF]; b) the fast-low voltage activity may be initiated by a single transient large field potential and c) dissection of preictal discharges versus seizure onset hypersynchronous discharges may be misleading. In neocortical focal epilepsy, different oscillatory seizure-onset patterns may interact at multiple scales [START_REF] Eissa | Cross-scale effects of neural interactions during human neocortical seizure activity[END_REF][START_REF] Wagner | Microscale spatiotemporal dynamics during neocortical propagation of human focal seizures[END_REF] and be affected by the underlying pathology (Perucca et al., 2014). We would therefore propose to distinguish clearly 1-LFVA onset seizures by events in which this fastoscillatory desynchronized component initiates the seizure, whether synchronized preictal discharges or a slow defection precedes this discharge or not from 2-HYP seizures that lack such tonic fast behavior and are launched by highly synchronized slowly oscillating bursts of discharge. These wo seizure pattern would clearly differentiate according to their field potential signature and to the neuronal synchronizing pattern which may reflect distinct neuronal behaviors.

Animal studies have suggested that both neuronal behavior and signaling may differ according to the seizure onset pattern. Although performed in normal animal by administration of convulsant, blockade of GABAa receptors was associated with a HYP onset while kainic acid injection rather determined LVFA pattern [START_REF] Bragin | The cause of the imbalance in the neuronal network leading to seizure activity can be predicted by the electrographic pattern of the seizure onset[END_REF]. These data question the involvement of GABAergic interneurons versus pyramidal cells in the generation of seizures according to the oscillatory pattern and may possibly indicate different contribution by intra-laminar and interregional microcircuits. In vitro, in models of seizures induced by various convulsants, most seizures have a LVFA onset, eventually preceded by preictal spikes (leading to confusions in naming ictal patterns). In published cases, a strong interneuronal discharge is recorded at seizure onset [START_REF] Fujiwara-Tsukamoto | Distinct types of ionic modulation of GABA actions in pyramidal cells and interneurons during electrical induction of hippocampal seizure-like network activity[END_REF][START_REF] Gnatkovsky | Fast activity at seizure onset is mediated by inhibitory circuits in the entorhinal cortex in vitro[END_REF][START_REF] Ziburkus | Interneuron and Pyramidal Cell Interplay During In Vitro Seizure-Like Events[END_REF]. In human data, microelectrode recordings are required to identify individual neuronal behavior [START_REF] Lambrecq | Single-unit activities during the transition to seizures in deep mesial structures[END_REF][START_REF] Merricks | Single unit action potentials in humans and the effect of seizure activity[END_REF][START_REF] Truccolo | Single-neuron dynamics in human focal epilepsy[END_REF][START_REF] Weiss | Ictal onset patterns of local field potentials, high frequency oscillations, and unit activity in human mesial temporal lobe epilepsy[END_REF]. Seizure events are far from recruiting all neurons and during the preictal period, single cell firing may increase, decrease or not change independently of the underlying pathology [START_REF] Lambrecq | Single-unit activities during the transition to seizures in deep mesial structures[END_REF][START_REF] Truccolo | Neuronal ensemble synchrony during human focal seizures[END_REF][START_REF] Truccolo | Single-neuron dynamics in human focal epilepsy[END_REF]. However, seizure initiation as well as progression in the neocortex as a wavefront is sustained by robust neuronal firing with local recruitment/synchronization of limited neuronal populations [START_REF] Merricks | Single unit action potentials in humans and the effect of seizure activity[END_REF][START_REF] Schevon | Evidence of an inhibitory restraint of seizure activity in humans[END_REF][START_REF] Weiss | Ictal high frequency oscillations distinguish two types of seizure territories in humans[END_REF] and failing inhibitory processes [START_REF] Schevon | Evidence of an inhibitory restraint of seizure activity in humans[END_REF][START_REF] Weiss | Ictal high frequency oscillations distinguish two types of seizure territories in humans[END_REF]. In a recent study, the preictal period, the onset and the beginning of LVFA seizures were all associated with robust interneuronal firing, largely exceeding that of presumed pyramidal cells, both in the hippocampus, the amygdala and the entorhinal cortex [START_REF] Elahian | Low-voltage fast seizures in humans begin with increased interneuron firing[END_REF]. These data claim for a clear involvement of interneurons in the generation of ictal activities but caution should be taken due to difficulties in unit isolation [START_REF] Merricks | Single unit action potentials in humans and the effect of seizure activity[END_REF]. Spike sorting techniques may soon allow for reliable differentiation of interneurons vs pyramidal cells behavior during a seizure in vivo by improving sorting approaches [START_REF] Niediek | Reliable Analysis of Single-Unit Recordings from the Human Brain under Noisy Conditions: Tracking Neurons over Hours[END_REF]. The causality of interneuronal firing on seizure initiation per se cannot be assessed by observational recordings. GABAergic interneurons may inhibit other neurons but may also depolarize them in case of chloride dysregulation and contribute to local increase in extracellular potassium with additional excitatory effects. This question can be hardly addressed by in vivo recordings and requires complementary in vitro approaches using for example optogenetics [START_REF] Andersson | Optogenetic control of human neurons in organotypic brain cultures[END_REF].

Inhibitory vs excitatory GABAergic signaling in epileptic tissues: where and when?

Epilepsy-associated changes of GABAergic mechanisms have major impact in controlling oscillations and neuronal activity. Recordings from human postoperative epileptic tissues have greatly contributed to highlight depolarizing and possibly excitatory effects of GABA as a key epileptogenic mechanism. In all cases in which spontaneous interictal activities were recorded from human epileptic tissues, either from the subiculum or the neocortex, GABAa receptors blockade suppressed these discharges [START_REF] Cohen | On the origin of interictal activity in human temporal lobe epilepsy in vitro[END_REF][START_REF] D'antuono | GABAA receptor-dependent synchronization leads to ictogenesis in the human dysplastic cortex[END_REF][START_REF] Huberfeld | Perturbed chloride homeostasis and GABAergic signaling in human temporal lobe epilepsy[END_REF][START_REF] Kohling | Spontaneous sharp waves in human neocortical slices excised from epileptic patients[END_REF]Pallud et al., 2014;[START_REF] Roopun | A nonsynaptic mechanism underlying interictal discharges in human epileptic neocortex[END_REF][START_REF] Schwartzkroin | Spontaneous rhythmic synchronous activity in epileptic human and normal monkey temporal lobe[END_REF][START_REF] Tóth | Hyperexcitability of the network contributes to synchronization processes in the human epileptic neocortex[END_REF][START_REF] Wozny | Comment on "On the origin of interictal activity in human temporal lobe epilepsy in vitro[END_REF]. When individual pyramidal cells were recorded, both depolarizing and hyperpolarizing post synaptic inputs were observed during interictal field potentials [START_REF] Cohen | On the origin of interictal activity in human temporal lobe epilepsy in vitro[END_REF][START_REF] Huberfeld | Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy[END_REF][START_REF] Huberfeld | Perturbed chloride homeostasis and GABAergic signaling in human temporal lobe epilepsy[END_REF][START_REF] Kohling | Current-source-density profiles associated with sharp waves in human epileptic neocortical tissue[END_REF][START_REF] Kohling | Spontaneous sharp waves in human neocortical slices excised from epileptic patients[END_REF][START_REF] Prince | Human epileptic neurons studied in vitro[END_REF][START_REF] Roopun | A nonsynaptic mechanism underlying interictal discharges in human epileptic neocortex[END_REF][START_REF] Schwartzkroin | Spontaneous rhythmic synchronous activity in epileptic human and normal monkey temporal lobe[END_REF][START_REF] Schwartzkroin | Intracellular study of human epileptic cortex: in vitro maintenance of epileptiform activity?[END_REF][START_REF] Schwartzkroin | Epileptogenesis in mammalian cortex studied in vitro[END_REF][START_REF] Wozny | Comment on "On the origin of interictal activity in human temporal lobe epilepsy in vitro[END_REF], suggesting that during these events a subset of pyramidal cells varying from 20 to 65 % may be rather depolarized by GABA [START_REF] Huberfeld | Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy[END_REF][START_REF] Huberfeld | Perturbed chloride homeostasis and GABAergic signaling in human temporal lobe epilepsy[END_REF]. Recorded interneurons fired, with variable strength, at the onset of these discharges [START_REF] Cohen | On the origin of interictal activity in human temporal lobe epilepsy in vitro[END_REF][START_REF] Huberfeld | Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy[END_REF][START_REF] Huberfeld | Perturbed chloride homeostasis and GABAergic signaling in human temporal lobe epilepsy[END_REF]Pallud et al., 2014;[START_REF] Roopun | A nonsynaptic mechanism underlying interictal discharges in human epileptic neocortex[END_REF] and hyperpolarization of parvalbumin interneurons by the µ opioid agonist DAGO suppressed interictal discharges (Pallud et al., 2014) emphasizing the hypothesis of a network activity triggered by interneurons. In vivo investigations of interneuronal firing dynamic is only available in an animal model of epilepsy (pilocarpine) but clearly showed by calcium imaging that "GABAergic neurons are the main participants in interictal spikes in CA1" [START_REF] Muldoon | GABAergic inhibition shapes interictal dynamics in awake epileptic mice[END_REF]. Depolarizing responses to GABA are related to chloride dysregulation in mature neurons, although, according to chloride, neuronal maturation is a very early process [START_REF] Dzhala | NKCC1 transporter facilitates seizures in the developing brain[END_REF]. Such dysregulation results mainly from an increased expression of the chloride cotransporter NKCC1, loading neurons with chloride, and from a down regulation or decreased efficiency of its counterpart KCC2, responsible for chloride outload.

A depolarization shift of chloride dependent GABAergic currents due to increased intraneuronal concentration is well documented in various types of human epileptic tissues [START_REF] Alvarado-Rojas | Different mechanisms of ripple-like oscillations in the human epileptic subiculum[END_REF][START_REF] Cohen | On the origin of interictal activity in human temporal lobe epilepsy in vitro[END_REF][START_REF] Huberfeld | Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy[END_REF][START_REF] Huberfeld | Perturbed chloride homeostasis and GABAergic signaling in human temporal lobe epilepsy[END_REF]Pallud et al., 2014b). KCC2 has been shown to be down-regulated in the human epileptogenic hippocampus, subiculum or temporal neocortex [START_REF] Huberfeld | Perturbed chloride homeostasis and GABAergic signaling in human temporal lobe epilepsy[END_REF][START_REF] Muñoz | Cation-Chloride Cotransporters and GABA-ergic Innervation in the Human Epileptic Hippocampus[END_REF][START_REF] Palma | Anomalous levels of Cl-transporters in the hippocampal subiculum from temporal lobe epilepsy patients make GABA excitatory[END_REF], from patients with tumor related epilepsies [START_REF] Conti | Anomalous levels of Cl-transporters cause a decrease of GABAergic inhibition in human peritumoral epileptic cortex[END_REF]Pallud et al., 2014), focal cortical dysplasias [START_REF] Aronica | Differential expression patterns of chloride transporters, Na+-K+-2Cl\textminus-cotransporter and K+-Cl\textminus-cotransporter, in epilepsy-associated malformations of cortical development[END_REF][START_REF] Jansen | Impaired maturation of cortical GABA(A) receptor expression in pediatric epilepsy[END_REF][START_REF] Munakata | Altered distribution of KCC2 in cortical dysplasia in patients with intractable epilepsy[END_REF][START_REF] Shimizu-Okabe | KCC2 was downregulated in small neurons localized in epileptogenic human focal cortical dysplasia[END_REF][START_REF] Talos | Altered inhibition in tuberous sclerosis and type IIb cortical dysplasia[END_REF] and tuberous sclerosis [START_REF] Talos | Altered inhibition in tuberous sclerosis and type IIb cortical dysplasia[END_REF]. A single study has reported so far an upregulation in the epileptic temporal lobe [START_REF] Karlócai | Enhanced expression of potassium-chloride cotransporter KCC2 in human temporal lobe epilepsy[END_REF]. NKCC1 expression, although difficult to study due to lack of specific antibody and to expression of both neuron and astrocytes in the central nervous system, has been reported to be up-regulated in temporal lobe epilepsies [START_REF] Huberfeld | Perturbed chloride homeostasis and GABAergic signaling in human temporal lobe epilepsy[END_REF][START_REF] Muñoz | Cation-Chloride Cotransporters and GABA-ergic Innervation in the Human Epileptic Hippocampus[END_REF][START_REF] Palma | Anomalous levels of Cl-transporters in the hippocampal subiculum from temporal lobe epilepsy patients make GABA excitatory[END_REF], peritumoral neocortex [START_REF] Conti | Anomalous levels of Cl-transporters cause a decrease of GABAergic inhibition in human peritumoral epileptic cortex[END_REF]Pallud et al., 2014), focal cortical dysplasias [START_REF] Aronica | Differential expression patterns of chloride transporters, Na+-K+-2Cl\textminus-cotransporter and K+-Cl\textminus-cotransporter, in epilepsy-associated malformations of cortical development[END_REF][START_REF] Jansen | Impaired maturation of cortical GABA(A) receptor expression in pediatric epilepsy[END_REF][START_REF] Sen | Increased NKCC1 expression in refractory human epilepsy[END_REF][START_REF] Shimizu-Okabe | KCC2 was downregulated in small neurons localized in epileptogenic human focal cortical dysplasia[END_REF][START_REF] Talos | Altered inhibition in tuberous sclerosis and type IIb cortical dysplasia[END_REF], and tubers [START_REF] Talos | Altered inhibition in tuberous sclerosis and type IIb cortical dysplasia[END_REF]. Blockade of NKCC1, reestablishing normal chloride concentrations, by the diuretic bumetanide has also been shown to block interictal activities [START_REF] Huberfeld | Perturbed chloride homeostasis and GABAergic signaling in human temporal lobe epilepsy[END_REF]Pallud et al., 2014;[START_REF] Talos | Altered inhibition in tuberous sclerosis and type IIb cortical dysplasia[END_REF].

Different methodological concerns question data on intracellular chloride disruption. Population events recorded in human slices may be related to injuries due to the slicing procedure [START_REF] Dzhala | Traumatic Alterations in GABA Signaling Disrupt Hippocampal Network Activity in the Developing Brain[END_REF], disrupting chloride regulation and artificially determining excitatory responses to GABA. These activities are however never recorded from normal animal brain slices, nor from human control tissues obtained by the resection of non-epileptic neocortex, surrounding brain lesions or on the way to access them [START_REF] Dossi | Pannexin-1 channels contribute to seizure generation in human epileptic brain tissue and in a mouse model of epilepsy[END_REF]Pallud et al., 2014b), although they may be involved in seizure propagation [START_REF] Wang | Depolarized GABAergic Signaling in Subicular Microcircuits Mediates Generalized Seizure in Temporal Lobe Epilepsy[END_REF]. Depolarizing effects of GABA as well as epileptic activities are recorded deep within slices, while the trauma affecting chloride regulation is reported at about 50 µm beneath the slice surface. When resected tissue is sliced, epileptiform activities are located consistently in the block, suggesting that the defect is spatially constrained and unrelated to the slicing procedure. Up to 90% of the patients with low grade gliomas have seizures at surgery (Pallud et al., 2014) and epileptic activities are recorded in the areas surrounding glioma limits both in vivo and in vitro [START_REF] Mittal | Intracranial EEG analysis in tumor-related epilepsy: Evidence of distant epileptic abnormalities[END_REF]Pallud et al., 2014). Moreover, when tissues are exposed to proconvulsant ACSF, the only slices producing ictal-like events are those obtained from epileptic patients in which spontaneous interictal activities were recorded [START_REF] Dossi | Pannexin-1 channels contribute to seizure generation in human epileptic brain tissue and in a mouse model of epilepsy[END_REF][START_REF] Huberfeld | Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy[END_REF]. Nevertheless, dual cortical or hippocampal in vivo/in vitro recordings in the same patients at comparable spatial scale should definitively resolve these issues.

Transition from interictal state, characterized by recurring interictal discharges (which can be considered as markers of epileptogenic tissues) to seizures is never spontaneous in vitro, highlighting that global fluctuations of brain excitability, whether endogenous or exogenous, are mandatory to trigger seizures [START_REF] Huberfeld | The Paradox of the Paroxysm: Can Seizure Precipitants Help Explain Human Ictogenesis?[END_REF]. While the seizure prediction field unraveled numerous mathematical concepts and tools to anticipate seizure occurrence [START_REF] Jirsa | On the nature of seizure dynamics[END_REF][START_REF] Lamberink | Individualised prediction model of seizure recurrence and long-term outcomes after withdrawal of antiepileptic drugs in seizure-free patients: a systematic review and individual participant data meta-analysis[END_REF], no clear biological phenomenon or oscillatory pattern has been identified so far and interictal basal discharges show various unpredictable dynamic patterns possibly reflecting different underlying microcircuits across regions and etiologies (de [START_REF] De Curtis | Initiation, Propagation, and Termination of Partial (Focal) Seizures[END_REF]. In vitro human tissues studies have contributed to emphasize a key oscillatory pattern, specific of the ictogenesis process: preictal discharges accompanied by HFOs [START_REF] Huberfeld | Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy[END_REF]Pallud et al., 2014b). Most seizures initiate from recurring preictal discharges but these population events are progressively generated earlier during the transition period. In vitro, preictal discharges are initiated by firing from some pyramidal cells able to synchronize local neurons, giving raise to high amplitude events [START_REF] Huberfeld | Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy[END_REF]. During the transition period, preictal discharges progressively grow in amplitude while synchronizing increasing number of cells via an NMDA-dependent synaptic plasticity during periods lasting tens of minutes in vitro [START_REF] Huberfeld | Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy[END_REF]. Once fully blown-up preictal discharges mainly recruit local cells by AMPA signals. It should be emphasized that they are not specific markers of epilepsy since human non-epileptic tissues may generate them by increasing excitability through ACSF modifications. However, in these non-epileptic tissues, they fail to trigger seizures, suggesting some potential collaborating mechanisms are required for ictogenesis, including chloride dysregulation and depolarizing effects of GABA represented by interictal discharges and resulting from epileptogenic remodeling [START_REF] Huberfeld | Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy[END_REF].

Seizure generation mechanisms have been studied in human epileptic tissues in vitro and fit with animal data reporting a leading role of interneurons and GABAergic input [START_REF] Curtis | GABAergic networks jump-start focal seizures[END_REF]. Most seizure-like events induced in human epileptic by increasing excitability and/or neurotransmitter release trough low magnesium, low calcium, high potassium concentrations, alkalization or 4-aminopyridine application [START_REF] Avoli | Epileptiform discharges in the human dysplastic neocortex: in vitro physiology and pharmacology[END_REF][START_REF] Avoli | GABAA-mediated inhibition and in vitro epileptogenesis in the human neocortex[END_REF][START_REF] Avoli | Pharmacology and electrophysiology of a synchronous GABA-mediated potential in the human neocortex[END_REF][START_REF] Avoli | Epileptiform activity induced by low extracellular magnesium in the human cortex maintained in vitro[END_REF][START_REF] Avoli | 4-Aminopyridine induces a long-lasting depolarizing GABA-ergic potential in human neocortical and hippocampal neurons maintained in vitro[END_REF][START_REF] Avoli | Seizure-like discharges induced by lowering [Mg2+]o in the human epileptogenic neocortex maintained in vitro[END_REF][START_REF] D'antuono | GABAA receptor-dependent synchronization leads to ictogenesis in the human dysplastic cortex[END_REF][START_REF] Dossi | Pannexin-1 channels contribute to seizure generation in human epileptic brain tissue and in a mouse model of epilepsy[END_REF][START_REF] Gigout | Effects of gap junction blockers on human neocortical synchronization[END_REF][START_REF] Huberfeld | Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy[END_REF][START_REF] Hwa | Excitatory synaptic transmission mediated by NMDA and non-NMDA receptors in the superficial/middle layers of the epileptogenic human neocortex maintained in vitro[END_REF][START_REF] Klaft | Adenosine A1 receptor-mediated suppression of carbamazepine-resistant seizure-like events in human neocortical slices[END_REF][START_REF] Mattia | Seizure-like discharges recorded in human dysplastic neocortex maintained in vitro[END_REF], are preceded by preictal discharges firing and often start by a fast low voltage tonic discharge grafted on a preictal discharge. Animal data suggest that low magnesium condition would rather elicit HYP seizures while 4-AP rather induces LVFA seizures [START_REF] Avoli | Specific imbalance of excitatory/inhibitory signaling establishes seizure onset pattern in temporal lobe epilepsy[END_REF]. The same stimulus may however produce different seizure types in human tissues, with no clear effect of the underlying neuropathology [START_REF] Dossi | Pannexin-1 channels contribute to seizure generation in human epileptic brain tissue and in a mouse model of epilepsy[END_REF]. While preictal discharges are associated with depolarizing excitatory input, they are not blocked by GABAa receptors antagonists or NKCC1 inhibition [START_REF] Huberfeld | Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy[END_REF] suggesting that they mostly depend on pyramidal cell firing and glutamatergic signaling. At seizure onset, preictal discharges are also associated with transient increases in extracellular potassium concentration [START_REF] D'antuono | GABAA receptor-dependent synchronization leads to ictogenesis in the human dysplastic cortex[END_REF]. The source of such potassium increase is thought to be related to interneurons both directly (potassium being released during high-frequency firing) and indirectly by exacerbated GABA release (chloride being then extruded together with potassium by the cotransporter KCC2) [START_REF] Viitanen | The K+-Cl\textminus cotransporter KCC2 promotes GABAergic excitation in the mature rat hippocampus[END_REF]. Interneuronal firing appears to be mandatory in ictogenesis possibly operating as a synchronizing mechanism after inhibitory rebounds or tonically driving pyramidal cells, since the pre-synaptic µ opioid receptors agonist DAGO, which does not affect preictal discharges, blocks seizures [START_REF] Avoli | Network and pharmacological mechanisms leading to epileptiform synchronization in the limbic system in vitro[END_REF][START_REF] D'antuono | GABAA receptor-dependent synchronization leads to ictogenesis in the human dysplastic cortex[END_REF][START_REF] Gnatkovsky | Fast activity at seizure onset is mediated by inhibitory circuits in the entorhinal cortex in vitro[END_REF]. The seizure onset itself, during the tonic discharge, is associated with a strong increase of extracellular potassium [START_REF] D'antuono | GABAA receptor-dependent synchronization leads to ictogenesis in the human dysplastic cortex[END_REF]. However, direct evidence of strong interneuronal discharge together with the initial silence of pyramidal cells, largely documented in rodents, has not been shown in human tissues yet. Interestingly, the initial phase of the tonic discharge is associated in human pyramidal cells with a membrane depolarizing potential shift compatible with outward chloride currents [START_REF] Huberfeld | Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy[END_REF]. While AMPA and NMDA glutamatergic signaling contributes, the fact that GABAa receptors blockade, reduction of presynaptic GABA release by DAGO and reduction of chloride load by blocking NKCC1 all prevent seizures provide additional support to the leading role of GABAergic interneuronal activity in ictogenesis [START_REF] D'antuono | GABAA receptor-dependent synchronization leads to ictogenesis in the human dysplastic cortex[END_REF][START_REF] Huberfeld | Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy[END_REF]. Non synaptic mechanisms as gap junctions may further be involved and identified by HFOs in the > 80 Hz high frequency band [START_REF] Gigout | Effects of gap junction blockers on human neocortical synchronization[END_REF][START_REF] Roopun | A nonsynaptic mechanism underlying interictal discharges in human epileptic neocortex[END_REF][START_REF] Simon | Gap junction networks can generate both ripple-like and fast ripple-like oscillations[END_REF]. Seizures in human tissues have also been shown to be associated with a drop in extracellular calcium [START_REF] Avoli | GABAA-mediated inhibition and in vitro epileptogenesis in the human neocortex[END_REF][START_REF] Avoli | Seizure-like discharges induced by lowering [Mg2+]o in the human epileptogenic neocortex maintained in vitro[END_REF] known to be associated with HFOs [START_REF] Aivar | Extracellular calcium controls the expression of two different forms of ripple-like hippocampal oscillations[END_REF].

Studying pathological forms of activities in the human brain in vitro can connect with the specific mechanisms pushing the human microcircuits to transiently operate outside their physiological-like oscillatory regime. Seizures, preictal and interictal events can all represent hyperexcitable states emerging from the very same altered microcircuits responsible for physiological rhythms underlying cognitive operation. Not rarely, neurological diseases correlate with a range of cognitive co-morbidities that can be associated with rhythmopathies of the natural oscillatory patterns. By integrating these two different perspectives, we can possibly better identify common and separate mechanisms of brain operation in health and disease.

Conclusion

This review reflects two different perspectives on human brain neurophysiology in vitro. The fact that postoperative human brain tissue can be only available from the surgical treatment of epilepsies and tumors has traditionally focused research in understanding basic pathophysiological mechanisms. More recently, by exploiting innovative approaches for intraoperative electrocorticography and intracranial microelectrode recordings coupled with detailed anatomical examinations of brain samples, a better delineation of pathological and non-pathological regions has allowed examining aspects of microcircuit function not addressed before. New cell-types and mechanisms supporting specialized human functions are now identified and integrated into the existing knowledge accumulated from animal research. In many cases, these tissues are presumed to reflect normal-like regions away from the neuropathological territories or on the way to the surgical targets. There is however no human brain tissue to control for neurophysiological studies and we must rely in an adequate assessment of what is normal and what is not in comparison with existing in vivo human and animal data.

From data reviewed above, we can extract some major criteria to help identifying physiological-and pathological-like oscillations and activities in human brain slices. Slow oscillations (<1Hz) and intermediate theta oscillations of moderate amplitude and duration coupled to gamma oscillations can be considered a basic operational mode of human neocortical microcircuit. These oscillations can be induced in thick brain slices with standard ACSF supplemented by neuromodulatory agents such as cholinergic agonists such as carbachol. Using these approaches we can try understanding the contribution of different cell-types and regions to the generation of these rhythms and their distortion by the underlying neuropathology. In this regime, synchronization between neurons and across layers correlates with the degree of phaseamplitude coupling and neuronal excitability. The higher the firing rate the stronger the slow-fast modulation and the higher the contribution at the pathological HFO spectrum.

In the human hippocampus, the slow-fast coupled oscillatory regime is more typically associated with fast oscillations in the ripple band accompanying sharp-waves of moderate amplitude. Physiological-like ripples are associated with rhythmic inhibitory and excitatory potentials in a proportion of cells and with relatively low levels of excitability. Any sign of spectral contribution at the fast ripple band, or excessive bursting activity in pyramidal cells, accompanied by large amplitude sharp-waves and HFOs is indicative of a pathological condition. Further studies of the non-sclerotic hippocampus may help in trying understanding how physiological-and pathological-like forms of ripples are generated and coexists.

When normal excitability, connectivity and GABAergic transmission is compromised, either pharmacologically or due to the underlying neuropathology, physiological-like oscillations transform into pathological forms of activity in the form of HFOs, interictal-like and ictal-like activity patterns. A major mechanism in epileptic human tissue is an altered equilibrium for intracellular chloride that renders GABAa-mediated inhibition depolarizing and channelopathies affecting intrinsic excitability. It is important to bear in mind that pathological activities can be induced in healthy territories when the microcircuit is pushed away from the physiological regime of operation. In vivo recordings at similar scales that in vitro analysis may contribute to distinguish between physiological and pathological activities, to better delineate local oscillatory pattern, obviously different in human compared to rodents, to determine cellular behaviors underlying rhythms and finally to raise rhythmopathies as specific components of neurological diseases.

We propose that by integrating cell-type and microcircuit specific studies of physiologicaland pathological-like oscillations with the mechanisms of ictogenesis in the human brain in vitro we should advance the clinical neuroscience field beyond our current limits for a better understanding of brain function in health and disease.

  Figure caption.

Figure 1 .

 1 Figure 1. Various oscillatory patterns are generated by the human cortex both in vivo and in vitro.A, Electrocorticographic (ECOG) recording of frontal cortex surrounding a glioma using a combined macro (3 mm -peripheral contacts) and micro (1 mm central contacts) electrodes (sampling rate: 20 000 Hz) in awake non-anesthetized condition to perform functional mapping. Filtering the raw signal unravels fast oscillations (100-200 Hz) coupled to slow recurring waves (1-2 Hz) and interacting with intermediate rhythms(7)(8)(9)(10)(11)(12)(13)(14)(15). The time frequency map for frequencies >50 Hz is shown at bottom. B, In vitro multielectrode array (MEA) recordings of a temporal lobe cortical slice from an epileptic patient (sampling rate: 10 000 Hz) showing recurring slow waves (1-2 Hz) of interictal-like discharges associated to fast oscillations (100-150 Hz).

Figure 2 .

 2 Figure 2. Physiological-like oscillations and associated pathological mechanisms. A, Local microcircuits support generation of gamma rhythms (40-80 Hz) by pyramidal-interneurons (PING) and interneuron-interneuron (ING) interactions. Ripples (>150 Hz) involve similar interactions brought about by recurrent pyramidal connections. B, Inter-laminar and inter-regional connections in interaction with subcortical structures facilitate modulation of fast local oscillations by slower (<1 Hz) and intermediate rhythms, such as theta (4-12 Hz). C, Physiological-and pathological forms of oscillations can be interpreted as extremes of a continuum that characterize different activity features.

[START_REF] Alvarado-Rojas | Different mechanisms of ripple-like oscillations in the human epileptic subiculum[END_REF][START_REF] Dossi | Pannexin-1 channels contribute to seizure generation in human epileptic brain tissue and in a mouse model of epilepsy[END_REF][START_REF] Florez | In Vitro Recordings of Human Neocortical Oscillations[END_REF][START_REF] Huberfeld | Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy[END_REF][START_REF] Duigou | Imaging pathological activities of human brain tissue in organotypic culture[END_REF] Pallud et al., 2014;[START_REF] Roopun | A nonsynaptic mechanism underlying interictal discharges in human epileptic neocortex[END_REF] Wittner et al., 2009)

Acknowledgements:

We thank the Spanish Ministerio de Economía y Competitividad (BFU2015-66887-R) and the Fundación Tatiana Perez de Guzman el Bueno for supporting grants to LMP.