Time–frequency analysis of single pulse electrical stimulation to assist delineation of epileptogenic cortex

Tuning face perception with electrical stimulation of the fusiform gyrus

The fusiform gyrus (FG) is an important node in the face processing network, but knowledge of its causal role in face perception is currently limited. Recent work demonstrated that high frequency stimulation applied to the FG distorts the perception of faces in human subjects (Parvizi et al. []: J Neurosci 32:14915-14920). However, the timing of this process in the FG relative to stimulus onset and the spatial extent of FG's role in face perception are unknown. Here, we investigate the causal role of the FG in face perception by applying precise, event-related electrical stimulation (ES) to higher order visual areas including the FG in six human subjects undergoing intracranial monitoring for epilepsy. We compared the effects of single brief (100 μs) electrical pulses to the FG and non-face-selective visual areas on the speed and accuracy of detecting distorted faces. Brief ES applied to face-selective sites did not affect accuracy but significantly increased the reaction time (RT) of detecting face distortions. Importantly, RT was altered only when ES was applied 100ms after visual onset and in face-selective but not place-selective sites. Furthermore, ES applied to face-selective areas decreased the amplitude of visual evoked potentials and high gamma power over this time window. Together, these results suggest that ES of face-selective regions within a critical time window induces a delay in face perception. These findings support a temporally and spatially specific causal role of face-selective areas and signify an important link between electrophysiology and behavior in face perception. Hum Brain Mapp 38:2830-2842, 2017. © 2017 Wiley Periodicals, Inc.

Evoked versus spontaneous high frequency oscillations in the chronic electrocorticogram in focal epilepsy

OBJECTIVE

Spontaneous high frequency oscillations (HFOs; ripples 80-250Hz, fast ripples (FRs) 250-500Hz) are biomarkers for epileptogenic tissue in focal epilepsy. Single pulse electrical stimulation (SPES) can evoke HFOs. We hypothesized that stimulation distinguishes pathological from physiological ripples and compared the occurrence of evoked and spontaneous HFOs within the seizure onset zone (SOZ) and eloquent functional areas.

METHODS

Ten patients underwent SPES during 2048Hz electrocorticography (ECoG). Evoked HFOs in time-frequency plots and spontaneous HFOs were visually analyzed. We compared electrodes with evoked and spontaneous HFOs for: percentages in the SOZ, sensitivity and specificity for the SOZ, percentages in functional areas outside the SOZ.

RESULTS

Two patients without spontaneous FRs showed evoked FRs in the SOZ. Percentages of evoked and spontaneous HFOs in the SOZ were similar (ripples 32:33%, p=0.77; FRs 43:48%, p=0.63), but evoked HFOs had generally a lower specificity (ripples 45:69%, p=0.02; FRs 83:92%, p=0.04) and higher sensitivity (ripples 85:70%, p=0.27; FRs 52:37%, p=0.05). More electrodes with evoked than spontaneous ripples were found in functional (54:30%, p=0.03) and 'silent' areas (57:27%, p=0.01) outside the SOZ.

CONCLUSIONS

SPES can elicit SOZ-specific FRs in patients without spontaneous FRs, but activates ripples in all areas.

SIGNIFICANCE

SPES is an alternative for waiting for spontaneous HFOs, but does not warrant exclusively pathological ripples.

High frequency spectral changes induced by single-pulse electric stimulation: Comparison between physiologic and pathologic networks

OBJECTIVE

To investigate functional coupling between brain networks using spectral changes induced by single-pulse electric stimulation (SPES).

METHOD

We analyzed 20 patients with focal epilepsy, implanted with depth electrodes. SPES was applied to each pair of adjacent contacts, and responses were recorded from all other contacts. The mean response amplitude value was quantified in three time-periods after stimulation (10-60, 60-255, 255-500ms) for three frequency-ranges (Gamma, Ripples, Fast-Ripples), and compared to baseline. A total of 30,755 responses were analyzed, taking into consideration three dichotomous pairs: stimulating in primary sensory areas (S1-V1) vs. outside them, to test the interaction in physiologic networks; stimulating in seizure onset zone (SOZ) vs. non-SOZ, to test pathologic interactions; recording in default mode network (DMN) vs. non-DMN.

RESULTS

Overall, we observed an early excitation (10-60ms) and a delayed inhibition (60-500ms). More specifically, in the delayed period, stimulation in S1-V1 produced a higher gamma-inhibition in the DMN, while stimulation in the SOZ induced a higher inhibition in the epilepsy-related higher frequencies (Ripples and Fast-Ripples).

CONCLUSION

Physiologic and pathologic interactions can be assessed using spectral changes induced by SPES.

SIGNIFICANCE

This is a promising method for connectivity studies in patients with drug-resistant focal epilepsy.

Single pulse electrical stimulation to probe functional and pathological connectivity in epilepsy

In the last decade, single pulse electrical stimulation (SPES) has been used as an investigational tool in the field of epilepsy surgery. Direct cortical stimulation applied at a frequency of ∼1Hz can probe cortico-cortical connections by averaging electrocorticogram time-lock to the stimuli (2×20-30 trials). These evoked potentials that emanate from adjacent and remote cortices have been termed cortico-cortical evoked potentials (CCEPs). Although limited to patients undergoing invasive presurgical evaluations with intracranial electrodes, CCEP provides a novel way to explore inter-areal connectivity in vivo in the living human brain to probe functional brain networks such as language and cognitive motor networks. In addition to its impact on systems neuroscience, this method, in combination with 50Hz electrical cortical stimulation, could contribute clinically to map the functional brain systems by tracking the cortico-cortical connections among the functional cortical regions in each individual patient. This approach may help identify the normal cortico-cortical network within pathology as well as reveal connections that might arise from neural plasticity. Because of its high practicality, it has been recently applied for intraoperative monitoring of the functional brain networks for patients with brain tumor. With regard to epilepsy, SPES has been used for the two major purposes, one to probe cortical excitability of the focus, namely, epileptogenicity, and the other to probe seizure networks. Both early (i.e., CCEP) and delayed responses, and probably their high frequency oscillation counterparts, are regarded as a surrogate marker of epileptogenicity. With regards to its impact on the human brain connectivity map, worldwide collaboration is warranted to establish the standardized CCEP connectivity map as a solid reference for non-invasive connectome researches.

Cortico-cortical and motor evoked potentials to single and paired-pulse stimuli: An exploratory transcranial magnetic and intracranial electric brain stimulation study

BACKGROUND

Paired-pulse (PP) paradigms are commonly employed to assess in vivo cortical excitability using transcranial magnetic stimulation (TMS) to stimulate the primary motor cortex and modulate the induced motor evoked potential (MEP). Single-pulse cortical direct electrical stimulation (DES) during intracerebral EEG monitoring allows the investigation of brain connectivity by eliciting cortico-cortical evoked potentials (CCEPs). However, PP paradigm using intracerebral DES has rarely been reported and has never been previously compared with TMS.

OBJECTIVE

The work was intended (i) to verify that the well-established modulations of MEPs following PP TMS remain similar using DES in the motor cortex, and (ii) to evaluate if a similar pattern could be observed in distant cortico-cortical connections through modulations of CCEP.

METHODS

Three patients undergoing intracerebral EEG monitoring with electrodes implanted in the central region were studied. Single-pulse DES (1-3 mA, 1 ms, 0.2 Hz) and PP DES using six interstimulus intervals (5, 15, 30, 50, 100, and 200 ms) in the motor cortex with concomitant recording of CCEPs and MEPs in contralateral muscles were performed. Finally, a navigated PP TMS session targeted the intracranial stimulation site to record TMS-induced MEPs in two patients.

RESULTS

MEP modulations elicited by PP intracerebral DES proved similar among the three patients and to those obtained by PP TMS. CCEP modulations elicited by PP intracerebral DES usually showed a pattern comparable to that of MEP, although a different pattern could be observed occasionally.

CONCLUSION

PP intracerebral DES seems to involve excitatory and inhibitory mechanisms similar to PP TMS and allows the recording of intracortical inhibition and facilitation modulation on cortico-cortical connections. Hum Brain Mapp 37:3767-3778, 2016. © 2016 Wiley Periodicals, Inc.

Current and Emerging Surgical Therapies for Severe Pediatric Epilepsies

The use of epilepsy surgery in various medically resistant epilepsies is well established. For patients with intractable pediatric epilepsy, the role of intracranial electrodes, resective surgery, hemispherectomy, corpus callosotomy, neurostimulation, and multiple subpial transections continues to be very effective in select cases. Newer treatment and diagnostic methods include laser thermal ablation, minimally invasive surgeries, stereo electroencephalography, electrocorticography, and other emerging techniques. This article will review the established and emerging surgical therapies for severe pediatric epilepsies, their respective indications and overall efficacy.

Interictal high-frequency oscillations generated by seizure onset and eloquent areas may be differentially coupled with different slow waves

OBJECTIVE

High-frequency oscillations (HFOs) can be spontaneously generated by seizure-onset and functionally-important areas. We determined if consideration of the spectral frequency bands of coupled slow-waves could distinguish between epileptogenic and physiological HFOs.

METHODS

We studied a consecutive series of 13 children with focal epilepsy who underwent extraoperative electrocorticography. We measured the occurrence rate of HFOs during slow-wave sleep at each electrode site. We subsequently determined the performance of HFO rate for localization of seizure-onset sites and undesirable detection of nonepileptic sensorimotor-visual sites defined by neurostimulation. We likewise determined the predictive performance of modulation index: MI(XHz)&(YHz), reflecting the strength of coupling between amplitude of HFOsXHz and phase of slow-waveYHz. The predictive accuracy was quantified using the area under the curve (AUC) on receiver-operating characteristics analysis.

RESULTS

Increase in HFO rate localized seizure-onset sites (AUC⩾0.72; p<0.001), but also undesirably detected nonepileptic sensorimotor-visual sites (AUC⩾0.58; p<0.001). Increase in MI(HFOs)&(3-4Hz) also detected both seizure-onset (AUC⩾0.74; p<0.001) and nonepileptic sensorimotor-visual sites (AUC⩾0.59; p<0.001). Increase in subtraction-MIHFOs [defined as subtraction of MI(HFOs)&(0.5-1Hz) from MI(HFOs)&(3-4Hz)] localized seizure-onset sites (AUC⩾0.71; p<0.001), but rather avoided detection of nonepileptic sensorimotor-visual sites (AUC⩽0.42; p<0.001).

CONCLUSION

Our data suggest that epileptogenic HFOs may be coupled with slow-wave3-4Hz more preferentially than slow-wave0.5-1Hz, whereas physiologic HFOs with slow-wave0.5-1Hz more preferentially than slow-wave3-4Hz during slow-wave sleep.

SIGNIFICANCE

Further studies in larger samples are warranted to determine if consideration of the spectral frequency bands of slow-waves coupled with HFOs can positively contribute to presurgical evaluation of patients with focal epilepsy.

Single and paired-pulse electrical stimulation during invasive EEG recordings

Invasive EEG recordings are frequently required during the presurgical exploration of patients with drug-resistant focal epilepsy in order to clarify the epileptic zone location. Intracranial direct electrical stimulations (DES) induce EEG and/or clinical responses that participate in this evaluation. Clinical DES protocols (1Hz and/or 50Hz) trigger massive cortical activation that can elicit seizures, after-discharges or complex clinical signs. In contrast, low-energy (<1Hz) protocols activate more localized cortical regions using single-pulse electrical stimulations (SPES). SPES can elicit two main types of responses. Cortico-cortical evoked potentials (CCEPs) correspond to highly consistent early responses, appearing before 100ms after stimulation, with fixed latency; they are considered physiological and assess the effective connectivity between the recorded regions. Late responses appear after 100ms; they are rare, inconsistent with variable latency and are suggestive of an underlying epileptogenic cortex. Paired-pulse stimulation paradigm associates a conditioning and a test stimulation to induce intracortical inhibition or facilitation by modifying the response amplitude. Largely used in transcranial magnetic stimulation, it has rarely been applied to CCEP although the mechanisms put in place seem highly similar. Low frequency intracerebral stimulations allow analysing brain connectivity and cortical excitability with a high temporal and spatial resolution. The development of new stimulation protocols and the combination with imaging or statistical techniques recently offered promising results.

Invasive Evaluations for Epilepsy Surgery: A Review of the Literature

Invasive evaluations play important roles in identifying epileptogenic zones and functional areas in patients with intractable focal epilepsy. This article reviews the usefulness, methods, and limitations of invasive evaluations for epilepsy surgery. Invasive evaluations include various types of intracranial electrodes such as stereotactically implanted intracranial depth electrodes (stereo-EEG), chronic subdural electrodes, and intraoperative electrocorticography. Scalp EEG is distorted by the skull, meninges, and skin. On the other hand, intracranial electrodes provide spatial information with higher resolution than scalp electrodes, thereby enabling further delineation of epileptogenic zones and mapping of functional areas with electrical stimulation. In addition, intracranial electrodes record a wide frequency range of electrical activity, which is not possible with scalp electrodes. The very slow potentials in ictal recordings, known as ictal direct current (DC) shifts and ictal/interictal high frequency oscillations, such as ripples (100–200 Hz) and fast ripples (200–500 Hz), have been correlated with the ictal onset zone and are a sensitive and specific marker for epileptogenicity. Furthermore, several studies reported that the electrical stimulation of epileptogenic zones elicited enhanced cortical evoked potentials, abnormal delayed or repetitive responses, and fast ripples. These responses may assist in the delineation of the epileptogenic cortex as a potential new marker. There are definite risks of complications associated with the use of intracranial electrodes. However, when an invasive evaluation is selected based on careful consideration of the risks and benefits, it provides useful information for establishing a strategy for epilepsy surgery.

High-frequency oscillations in epilepsy and surgical outcome. A meta-analysis

High frequency oscillations (HFOs) are estimated as a potential marker for epileptogenicity. Current research strives for valid evidence that these HFOs could aid the delineation of the to-be resected area in patients with refractory epilepsy and improve surgical outcomes. In the present meta-analysis, we evaluated the relation between resection of regions from which HFOs can be detected and outcome after epilepsy surgery. We conducted a systematic review of all studies that related the resection of HFO-generating areas to postsurgical outcome. We related the outcome (seizure freedom) to resection ratio, that is, the ratio between the number of channels on which HFOs were detected and, among these, the number of channels that were inside the resected area. We compared the resection ratio between seizure free and not seizure free patients. In total, 11 studies were included. In 10 studies, ripples (80-200 Hz) were analyzed, and in 7 studies, fast ripples (>200 Hz) were studied. We found comparable differences (dif) and largely overlapping confidence intervals (CI) in resection ratios between outcome groups for ripples (dif = 0.18; CI: 0.10-0.27) and fast ripples (dif = 0.17; CI: 0.01-0.33). Subgroup analysis showed that automated detection (dif = 0.22; CI: 0.03-0.41) was comparable to visual detection (dif = 0.17; CI: 0.08-0.27). Considering frequency of HFOs (dif = 0.24; CI: 0.09-0.38) was related more strongly to outcome than considering each electrode that was showing HFOs (dif = 0.15; CI = 0.03-0.27). The effect sizes found in the meta-analysis are small but significant. Automated detection and application of a detection threshold in order to detect channels with a frequent occurrence of HFOs is important to yield a marker that could be useful in presurgical evaluation. In order to compare studies with different methodological approaches, detailed and standardized reporting is warranted.

Single Pulse Electrical Stimulation to identify epileptogenic cortex: Clinical information obtained from early evoked responses

OBJECTIVE

Single Pulse Electrical Stimulation (SPES) probes epileptogenic cortex during electrocorticography. Two SPES responses are described: pathological delayed responses (DR, >100 ms) associated with the seizure onset zone (SOZ) and physiological early responses (ER, <100 ms) that map cortical connectivity. We analyzed properties of ERs, including frequencies >80 Hz, in the SOZ and seizure propagation areas.

METHODS

We used data from 12 refractory epilepsy patients. SPES consisted of 10 pulses of 1 ms, 4-8 mA and 5s interval on adjacent electrodes pairs. Data were available at 2048 samples/s for six and 512 samples/s (22 bits) for eight patients and analyzed in the time-frequency (TF) and time-domain (TD).

RESULTS

Electrodes with ERs were stronger associated with SOZ than non-SOZ electrodes. ERs with frequency content >80 Hz exist and are specific for SOZ channels. ERs evoked by stimulation of seizure onset electrodes were associated with electrodes involved in seizure propagation.

CONCLUSION

Analysis of ERs can reveal aspects of pathology, manifested by association with seizure propagation and areas with high ER numbers that coincide with the SOZ.

SIGNIFICANCE

Not only DRs, but also ERs could have clinical value for mapping epileptogenic cortex and help to unravel aspects of the epileptic network.

Gap Junctions as Common Cause of High-Frequency Oscillations and Epileptic Seizures in a Computational Cascade of Neuronal Mass and Compartmental Modeling

High frequency oscillations (HFO) appear to be a promising marker for delineating the seizure onset zone (SOZ) in patients with localization related epilepsy. It remains, however, a purely observational phenomenon and no common mechanism has been proposed to relate HFOs and seizure generation. In this work we show that a cascade of two computational models, one on detailed compartmental scale and a second one on neural mass scale can explain both the autonomous generation of HFOs and the presence of epileptic seizures as emergent properties. To this end we introduce axonal–axonal gap junctions on a microscopic level and explore their impact on the higher level neural mass model (NMM). We show that the addition of gap junctions can generate HFOs and simultaneously shift the operational point of the NMM from a steady state network into bistable behavior that can autonomously generate epileptic seizures. The epileptic properties of the system, or the probability to generate epileptic type of activity, increases gradually with the increase of the density of axonal–axonal gap junctions. We further demonstrate that ad hoc HFO detectors used in previous studies are applicable to our simulated data.

Automated Seizure Onset Zone Approximation Based on Nonharmonic High-Frequency Oscillations in Human Interictal Intracranial EEGs

A novel automated algorithm is proposed to approximate the seizure onset zone (SOZ), while providing reproducible output. The SOZ, a surrogate marker for the epileptogenic zone (EZ), was approximated from intracranial electroencephalograms (iEEG) of nine people with temporal lobe epilepsy (TLE), using three methods: (1) Total ripple length (TRL): Manually segmented high-frequency oscillations, (2) Rippleness (R): Area under the curve (AUC) of the autocorrelation functions envelope, and (3) Autoregressive model residual variation (ARR, novel algorithm): Time-variation of residuals from autoregressive models of iEEG windows. TRL, R, and ARR results were compared in terms of separability, using Kolmogorov–Smirnov tests, and performance, using receiver operating characteristic (ROC) curves, to the gold standard for SOZ delineation: visual observation of ictal video-iEEGs. TRL, R, and ARR can distinguish signals from iEEG channels located within the SOZ from those outside it (p < 0.01). The ROC AUC was 0.82 for ARR, while it was 0.79 for TRL, and 0.64 for R. ARR outperforms TRL and R, and may be applied to identify channels in the SOZ automatically in interictal iEEGs of people with TLE. ARR, interpreted as evidence for nonharmonicity of high-frequency EEG components, could provide a new way to delineate the EZ, thus contributing to presurgical workup.

Fast Activity Evoked by Intracranial 50 Hz Electrical Stimulation as a Marker of the Epileptogenic Zone

Epilepsy is a disease characterized by aberrant connections between brain areas. The altered activity patterns generated by epileptic networks can be analyzed with intracerebral electrodes during pre-surgical stereo-electroencephalographic (EEG) monitoring in patients candidate to epilepsy surgery. The responses to high frequency stimulation (HFS) at 50 Hz performed for diagnostic purposes during SEEG were analyzed with a new algorithm, to evaluate signal parameters that are masked to visual inspection and to define the boundaries of the epileptogenic network. The analysis was focused on 60-80 Hz activity that represented the largest frequency component evoked by HFS. The distribution of HFS-evoked fast activity across all (up to 162) recording contacts allowed to define different clusters of contacts that retrospectively correlated to the epileptogenic zone identified by the clinicians on the basis of traditional visual analysis. The study demonstrates that computer-assisted analysis of HFS-evoked activities may contribute to the definition of the epileptogenic network on intracranial recordings performed in a pre-surgical setting.

Defining regions of interest using cross-frequency coupling in extratemporal lobe epilepsy patients

OBJECTIVE

Clinicians identify seizure onset zones (SOZs) for resection in an attempt to localize the epileptogenic zone (EZ), which is the cortical tissue that is indispensible for seizure generation. An automated system is proposed to objectively localize this EZ by identifying regions of interest (ROIs).

METHODS

Intracranial electroencephalogram recordings were obtained from seven patients presenting with extratemporal lobe epilepsy and the interaction between neuronal rhythms in the form of phase-amplitude coupling was investigated. Modulation of the amplitude of high frequency oscillations (HFOs) by the phase of low frequency oscillations was measured by computing the modulation index (MI). Delta- (0.5-4 Hz) and theta- (4-8 Hz) modulation of HFOs (30-450 Hz) were examined across the channels of a 64-electrode subdural grid. Surrogate analysis was performed and false discovery rates were computed to determine the significance of the modulation observed. Mean MI values were subjected to eigenvalue decomposition (EVD) and channels defining the ROIs were selected based on the components of the eigenvector corresponding to the largest eigenvalue. ROIs were compared to the SOZs identified by two independent neurologists. Global coherence values were also computed.

MAIN RESULTS

MI was found to capture the seizure in time for six of seven patients and identified ROIs in all seven. Patients were found to have a poorer post-surgical outcome when the number of EVD-selected channels that were not resected increased. Moreover, in patients who experienced a seizure-free outcome (i.e., Engel Class I) all EVD-selected channels were found to be within the resected tissue or immediately adjacent to it. In these Engel Class I patients, delta-modulated HFOs were found to identify more of the channels in the resected tissue compared to theta-modulated HFOs. However, for the Engel Class IV patient, the delta-modulated HFOs did not identify any of the channels in the resected tissue suggesting that the resected tissue was not appropriate, which was also suggested by the Engel Class IV outcome. A sensitivity of 75.4% and a false positive rate of 15.6% were achieved using delta-modulated HFOs in an Engel Class I patient.

SIGNIFICANCE

LFO-modulated HFOs can be used to identify ROIs in extratemporal lobe patients. Moreover, delta-modulated HFOs may provide more accurate localization of the EZ. These ROIs may result in better surgical outcomes when used to compliment the SOZs identified by clinicians for resection.

Epilepsy: new advances

Epilepsy affects 65 million people worldwide and entails a major burden in seizure-related disability, mortality, comorbidities, stigma, and costs. In the past decade, important advances have been made in the understanding of the pathophysiological mechanisms of the disease and factors affecting its prognosis. These advances have translated into new conceptual and operational definitions of epilepsy in addition to revised criteria and terminology for its diagnosis and classification. Although the number of available antiepileptic drugs has increased substantially during the past 20 years, about a third of patients remain resistant to medical treatment. Despite improved effectiveness of surgical procedures, with more than half of operated patients achieving long-term freedom from seizures, epilepsy surgery is still done in a small subset of drug-resistant patients. The lives of most people with epilepsy continue to be adversely affected by gaps in knowledge, diagnosis, treatment, advocacy, education, legislation, and research. Concerted actions to address these challenges are urgently needed.

Proceedings of the Fifth International Workshop on Advances in Electrocorticography

The Fifth International Workshop on Advances in Electrocorticography convened in San Diego, CA, on November 7-8, 2013. Advancements in methodology, implementation, and commercialization across both research and in the interval year since the last workshop were the focus of the gathering. Electrocorticography (ECoG) is now firmly established as a preferred signal source for advanced research in functional, cognitive, and neuroprosthetic domains. Published output in ECoG fields has increased tenfold in the past decade. These proceedings attempt to summarize the state of the art.

Electrical stimulation for cortical mapping reduces the density of high frequency oscillations

BACKGROUND

High frequency oscillations (HFOs, 80-500 Hz) are EEG biomarkers for epileptogenic areas. HFOs are also indicators of disease activity as HFO rates increase after reduction of antiepileptic medication. Electrical stimulation (ES) can be used for diagnostic purposes as well as therapy in patients with refractory epilepsy. This study investigates the occurrence and changes of HFOs during ES in patients with refractory epilepsy.

OBJECTIVE

Analysis of the effects of ES using intracranial ES on the occurrence of epileptic HFOs.

METHODS

Patients underwent ES for diagnostic purposes. Ripples (80-200 Hz) and fast ripples (200-500 Hz) were visually marked in a baseline EEG segment prior to ES, after each period of ES as well as after the end of ES. In patients in whom ES triggered a seizure a pre- and postictal segment was marked. Rates of HFOs were compared for the different time periods using a Spearman's correlation and Wilcoxon rank sum test (p<0.05).

RESULTS

12 patients with 911 EEG channels were analyzed. Ripple (r=-0.42, p<0.001) as well as fast ripple (r=-0.21, p<0.001) rates decreased significantly over the course of stimulation. This phenomenon was not focal over the seizure onset or neighboring contacts but even observed over distant contacts.

CONCLUSIONS

ES resulted in a gradual decrease of HFO-Rates over time. The decrease of HFOs was not limited to SOZ areas. If HFOs are considered as markers of disease activity the reduction in HFO-rates as a result of intracranial ES has to be interpreted as a reduction of disease activity.

Stimulus-evoked potentials contribute to map the epileptogenic zone during stereo-EEG presurgical monitoring

Presurgical monitoring with intracerebral electrodes in patients with drug-resistant focal epilepsy represents a standard invasive procedure to localize the sites of seizures origin, defined as the epileptogenic zone (EZ). During presurgical evaluation, intracerebral single-pulse electrical stimulation (SPES) is performed to define the boundaries of eloquent areas and to evoke seizure-associated symptoms. Extensive intracranial exploration and stimulation generate a large dataset on brain connectivity that can be used to improve EZ detection and to understand the organization of the human epileptic brain. We developed a protocol to analyse field responses evoked by intracranial stimulation. Intracerebral recordings were performed with 105-162 recording sites positioned in fronto-temporal regions in 12 patients with pharmacoresistant focal epilepsy. Recording sites were used for bipolar SPES at 1 Hz. Reproducible early and late phases (<60 ms and 60-500 ms from stimulus artefact, respectively) were identified on averaged evoked responses. Phase 1 and 2 responses recorded at all and each recording sites were plotted on a 3D brain reconstructions. Based on connectivity properties, electrode contacts were primarily identified as receivers, mainly activators or bidirectional. We used connectivity patterns to construct networks and applied cluster partitioning to study the proprieties between potentials evoked/stimulated in different regions. We demonstrate that bidirectional connectivity during phase 1 is a prevalent feature that characterize contacts included in the EZ. This study shows that the application of an analytical protocol on intracerebral stimulus-evoked recordings provides useful information that may contribute to EZ detection and to the management of surgical-remediable epilepsies.

Functional connectivity of insular efferences

OBJECTIVES

The aim of our study was to explore the functional connectivity between the insula and other cortical regions, in human, using cortico-cortical evoked potentials (CCEPs)

EXPERIMENTAL DESIGN

We performed intra-cerebral electrical stimulation in eleven patients with refractory epilepsy investigated with depth electrodes, including 39 targeting the insula. Electrical stimulation consisted of two series of 20 pulses of 1-ms duration, 0.2-Hz frequency, and 1-mA intensity delivered at each of the 39 insular bipoles. Rates of connectivity were reported whenever a noninsular cortical region was tested by at least ten stimulating/recording electrode pairs in three or more patients

RESULTS

Significant CCEPs were elicited in 193 of the 578 (33%) tested connections, with an average latency of 33 ± 5 ms. The highest connectivity rates were observed with the nearby perisylvian structures (59%), followed by the pericentral cortex (38%), the temporal neocortex (28%), the lateral parietal cortex (26%), the orbitofrontal cortex (25%), the mesial temporal structures (24%), the dorsolateral frontal cortex (15%), the temporal pole (14%), and the mesial parietal cortex (11%). No connectivity was detected in the mesial frontal cortex or cingulate gyrus. The pattern of connectivity also differed between the five insular gyri, with greater connectivity rate for the posterior short gyrus (49%), than for the middle short (29%), and two long gyri (28 and 33%)

CONCLUSION

The human insula is characterized by a rich and complex connectivity that varies as a function of the insular gyrus and appears to partly differ from the efferences described in nonhuman primates.

Electrophysiological biomarkers of epilepsy

In patients being evaluated for epilepsy and in animal models of epilepsy, electrophysiological recordings are carried to capture seizures to determine the existence of epilepsy. Electroencephalography recordings from the scalp, or sometimes directly from the brain, are also used to locate brain areas where seizure begins, and in surgical treatment help plan the area for resection. As seizures are unpredictable and can occur infrequently, ictal recordings are not ideal in terms of time, cost, or risk when, for example, determining the efficacy of existing or new anti-seizure drugs, evaluating potential anti-epileptogenic interventions, or for prolonged intracerebral electrode studies. Thus, there is a need to identify and validate other electrophysiological biomarkers of epilepsy that could be used to diagnose, treat, cure, and prevent epilepsy. Electroencephalography recordings in the epileptic brain contain other interictal electrophysiological disturbances that can occur more frequently than seizures, such as interictal spikes (IIS) and sharp waves, and from invasive studies using wide bandwidth recording and small diameter electrodes, the discovery of pathological high-frequency oscillations (HFOs) and microseizures. Of IIS, HFOs, and microseizures, a significant amount of recent research has focused on HFOs in the pathophysiology of epilepsy. Results from studies in animals with epilepsy and presurgical patients have consistently found a strong association between HFOs and epileptogenic brain tissue that suggest HFOs could be a potential biomarker of epileptogenicity and epileptogenesis. Here, we discuss several aspects of HFOs, as well as IIS and microseizures, and the evidence that supports their role as biomarkers of epilepsy.

The role of high-frequency oscillations in epilepsy surgery planning

BACKGROUND

Epilepsy is a serious brain disorder characterized by recurrent unprovoked seizures. Approximately two-thirds of seizures can be controlled with antiepileptic medications (Kwan 2000). For some of the others, surgery can completely eliminate or significantly reduce the occurrence of disabling seizures. Localization of epileptogenic areas for resective surgery is far from perfect, and new tools are being investigated to more accurately localize the epileptogenic zone (the zone of the brain where the seizures begin) and improve the likelihood of freedom from postsurgical seizures. Recordings of pathological high-frequency oscillations (HFOs) may be one such tool.

OBJECTIVES

To assess the ability of HFOs to improve the outcomes of epilepsy surgery by helping to identify more accurately the epileptogenic areas of the brain.

SEARCH METHODS

We searched the Cochrane Epilepsy Group Specialized Register (15 April 2013), the Cochrane Central Register of Controlled Trials (CENTRAL) in The Cochrane Library (2013, Issue 3), MEDLINE (Ovid) (1946 to 15 April 2013), CINAHL (EBSCOhost) (15 April 2013), Web of Knowledge (Thomson Reuters) (15 April 2013), www.clinicaltrials.gov (15 April 2013), and the World Health Organization International Clinical Trials Registry Platform (15 April 2013).

SELECTION CRITERIA

We included studies that provided information on the outcomes of epilepsy surgery at at least six months and which used high-frequency oscillations in making decisions about epilepsy surgery.

DATA COLLECTION AND ANALYSIS

The primary outcome of the review was the Engel Class Outcome System. Secondary outcomes were responder rate, International League Against Epilepsy (ILAE) epilepsy surgery outcome, frequency of adverse events from any source and quality of life outcomes. We intended to analyse outcomes via an aggregated data fixed-effect model meta-analysis.

MAIN RESULTS

Two studies met the inclusion criteria. Both studies were small non-randomised trials, with no control group and no blinding. The quality of evidence for all outcomes was very low. The combination of these two studies resulted in 11 participants who prospectively used ictal HFOs for epilepsy surgery decision making. Results of the postsurgical seizure freedom Engel class I to IV outcome were determined over a period of 12 to 38 months (average 23.4 months) and indicated that six participants had an Engel class I outcome (seizure freedom), two had class II (rare disabling seizures), three had class III (worthwhile improvement). No adverse effects were reported. Neither study compared surgical results guided by HFOs versus surgical results guided without HFOs.

AUTHORS' CONCLUSIONS

No reliable conclusions can be drawn regarding the efficacy of using HFOs in epilepsy surgery decision making at present.

Proceedings of the Fourth International Workshop on Advances in Electrocorticography

The Fourth International Workshop on Advances in Electrocorticography (ECoG) convened in New Orleans, LA, on October 11-12, 2012. The proceedings of the workshop serves as an accurate record of the most contemporary clinical and experimental work on brain surface recording and represents the insights of a unique multidisciplinary ensemble of expert clinicians and scientists. Presentations covered a broad range of topics, including innovations in passive functional mapping, increased understanding of pathologic high-frequency oscillations, evolving sensor technologies, a human trial of ECoG-driven brain-machine interface, as well as fresh insights into brain electrical stimulation.

Intrainsular functional connectivity in human

OBJECTIVES

The anatomical organization of the insular cortex is characterized by its rich and heterogeneous cytoarchitecture and its wide network of connections. However, only limited knowledge is available regarding the intrainsular connections subserving the complex integrative role of the insular cortex. The aim of this study was to analyze the functional connectivity within- and across-insular subregions, at both gyral and functional levels.

EXPERIMENTAL DESIGN

We performed intracerebral electrical stimulation in 10 patients with refractory epilepsy investigated with depth electrodes, 38 of which were inserted in the insula. Bipolar electrical stimulation, consisting of two series of 20 pulses of 1-ms duration, 0.2-Hz frequency, and 1-mA intensity, was delivered at each insular contact. For each stimulated insular anatomical region, we calculated a rate of connectivity, reflecting the proportion of other insular contacts, showing significant evoked potentials.

RESULTS

Statistically significant evoked potentials were recorded in 74% of tested connections, with an average latency of 26 ± 3 ms. All insular gyri were interconnected, except the anterior and posterior short gyri. Most connections were reciprocal, showing no clear anterior to posterior directionality. No connection was observed between the right and the left insula.

CONCLUSIONS

These findings point to specific features of human insula connectivity as compared to non-Human primates, and remain consistent with the complex integration role devoted to the human insula in many cognitive domains. Periodicals, Inc.

Cortico-cortical evoked potentials and stimulation-elicited gamma activity preferentially propagate from lower- to higher-order visual areas

OBJECTIVE

The lower-order visual cortex in the medial-occipital region is suggested to send feed-forward signals to the higher-order visual cortex including ventral-occipital-temporal and dorsal-occipital regions. We determined how stimulation-elicited cortical-signals propagate between lower- and higher-order visual cortices, and whether the magnitudes of stimulation-elicited cortical-signals recorded in the higher-order visual cortex differed from those recorded in the lower-order one.

METHODS

We studied 10 patients with focal epilepsy who underwent extraoperative electrocorticography recording. Trains of 1-Hz stimuli with an intensity of 3 mA were delivered to an electrode pair within the medial-occipital region; then, cortico-cortical evoked-potential (CCEP) and stimulation-elicited gamma-activity at 80-150 Hz were measured in the ventral-occipital-temporal and dorsal-occipital regions. Likewise, CCEP and stimulation-elicited gamma-activity, driven by stimuli within the higher-order visual cortex, were measured in the lower-order visual cortex.

RESULTS

CCEPs generated, via feed-forward propagations, in the higher-order visual cortex were significantly larger than those generated, via feed-back propagations, in the lower-order visual cortex. Stimulation of the lower-order visual cortex elicited augmentation of gamma-activity in the higher-order visual cortex after the preceding CCEP subsided.

CONCLUSION

The propagation manners of stimulation-elicited cortical-signals differ between feed-forward and feed-back directions in the human occipital lobe.

SIGNIFICANCE

: Such difference may need to be taken into consideration for future clinical application of CCEPs and stimulation-elicited gamma-augmentation in presurgical evaluation for epilepsy surgery.

Clinical significance and developmental changes of auditory-language-related gamma activity

OBJECTIVE

We determined the clinical impact and developmental changes of auditory-language-related augmentation of gamma activity at 50-120 Hz recorded on electrocorticography (ECoG).

METHODS

We analyzed data from 77 epileptic patients ranging 4-56 years in age. We determined the effects of seizure-onset zone, electrode location, and patient-age upon gamma-augmentation elicited by an auditory-naming task.

RESULTS

Gamma-augmentation was less frequently elicited within seizure-onset sites compared to other sites. Regardless of age, gamma-augmentation most often involved the 80-100 Hz frequency band. Gamma-augmentation initially involved bilateral superior-temporal regions, followed by left-side dominant involvement in the middle-temporal, medial-temporal, inferior-frontal, dorsolateral-premotor, and medial-frontal regions and concluded with bilateral inferior-Rolandic involvement. Compared to younger patients, those older than 10 years had a larger proportion of left dorsolateral-premotor and right inferior-frontal sites showing gamma-augmentation. The incidence of a post-operative language deficit requiring speech therapy was predicted by the number of resected sites with gamma-augmentation in the superior-temporal, inferior-frontal, dorsolateral-premotor, and inferior-Rolandic regions of the left hemisphere assumed to contain essential language function (r(2) = 0.59; p = 0.001; odds ratio = 6.04 [95% confidence-interval: 2.26-16.15]).

CONCLUSIONS

Auditory-language-related gamma-augmentation can provide additional information useful to localize the primary language areas.

SIGNIFICANCE

These results derived from a large sample of patients support the utility of auditory-language-related gamma-augmentation in presurgical evaluation.

In vivo neuronal firing patterns during human epileptiform discharges replicated by electrical stimulation

OBJECTIVE

To describe neuronal firing patterns observed during human spontaneous interictal epileptiform discharges (IEDs) and responses to single pulse electrical stimulation (SPES).

METHODS

Activity of single neurons was recorded during IEDs and after SPES in 11 consecutive patients assessed with depth EEG electrodes and attached microelectrodes.

RESULTS

A total of 66 neurons were recorded during IEDs and 151 during SPES. We have found essentially similar patterns of neuronal firing during IEDs and after SPES, namely: (a) a burst of high frequency firing lasting less than 100 ms (in 39% and 25% of local neurons, respectively for IED and SPES); (b) a period of suppression in firing lasting around 100-1300 ms (in 19% and 14%, respectively); (c) a burst followed by suppression (in 10% and 12%, respectively); (d) no-change (in 32% and 50%, respectively).

CONCLUSIONS

The similarities in neuronal firing patterns associated with IEDs and SPES suggest that, although both phenomena are initiated differently, they result in the activation of a common cortical mechanism, probably initiated by brief synchronised burst firing in some cells followed by long inhibition.

SIGNIFICANCE

The findings provide direct in vivo human evidence to further comprehend the pathophysiology of human focal epilepsy.

CNTF-mediated preactivation of astrocytes attenuates neuronal damage and epileptiform activity in experimental epilepsy

Activated astrocytes display a broad spectrum of properties, ranging from neuroprotection to active contribution to demise of neural tissue. To investigate if activation of astrocytes by a single, defined stimulus enhances neuroprotective properties, we tested whether injection of ciliary neurotrophic factor (CNTF) can ameliorate epilepsy-related brain damage. Intrahippocampal CNTF injection in mice induced a rapid (within 2 days) and persistent (3 weeks) activation of astrocytes reflected by strong upregulation of glial fibrillary acidic protein (GFAP) mRNA synthesis and GFAP immunoreactivity. Moreover, CNTF signaling via phosphorylation and nuclear translocation of STAT3 (signal transducer and activator of transcription 3) was specifically activated in GFAP-positive astrocytes. CNTF-mediated activation of astrocytes 2 days prior to an epileptogenic intrahippocampal injection of kainate (KA) resulted in strongly reduced cell death in the hilus and CA3 region of the hippocampus, revealed by Fluoro-Jade B staining. Granule cell dispersion, the pathological widening of the granule cell layer, was also significantly reduced 16 days after KA injection. Importantly, intrahippocampal in vivo recordings 3 weeks after KA injection showed that the occurrence of high frequency oscillations (fast ripples, FR), a surrogate marker for epileptic activity, was significantly reduced in CNTF+KA-injected mice as compared to KA-injected animals. However, when CNTF was applied in the chronic epileptic phase at 3 weeks after KA injection, no reduction of FR activity was observed. In summary, our results indicate that the activation of astrocytes prior to an excitotoxic injury effectively reduces neuronal damage and the severity of epileptiform activity, whereas activation in the chronic phase is no longer protective.

Quantification of spontaneous and evoked HFO's in SEEG recording and prospective for pre-surgical diagnostics. Case study

High frequency oscillations (HFO) in stereo electroencephalographic (SEEG) signals have been recently the focus of attention as biomarkers that can have potential predictive power for the spatial location and possibly the timing of the onset of epileptic seizures. In this work we present a case study where we compare two quantitative paradigms for automated detection of biomarkers, one based on spontaneous SEEG recordings of HFOs and the other using activity induced by direct electrical stimulation (relative Phase Clustering Index algorithm). We compare the performance of these automated methods with manually detected HFO ripples by a trained EEG analyst and explore their potential diagnostic relevance. Intracranial recordings from patients undergoing pre-surgical evaluation are processed with a combination of morphological filtering and the analysis of the auto-correlation function. The results were compared to those obtained by visual inspection and to results from an active paradigm involving stimulation with 20 Hz trains of biphasic pulses. The quantity of HFOs, estimated automatically, or "rippleness", was found to correspond to the findings of a trained EEG analyst. The relative phase clustering index (rPCI) obtained using periodic stimulation appeared to be associated with the closeness to the seizure onset zone (SOZ) detected from ictal epochs. The HFO estimates were also indicative for the SOZ but with less specificity.