Extracting the Invisible: Mesial Temporal Source Detection in Simultaneous EEG and SEEG Recordings.

Epileptic source detection relies mainly on visual expertise of scalp EEG signals, but it is recognised that epileptic discharges can escape to this expertise due to a deep localization of the brain sources that induce a very low, even negative, signal to noise ratio. In this methodological study, we aimed to investigate the feasibility of extracting deep mesial temporal sources that were invisible in scalp EEG signals using blind source separation (BSS) methods (infomax ICA, extended infomax ICA, and JADE) combined with a statistical measure (kurtosis). We estimated the effect of different methodological and physiological parameters that could alter or improve the extraction. Using nine well-defined mesial epileptic networks (1949 spikes) obtained from seven patients and simultaneous EEG-SEEG recordings, the first independent component extracted from the scalp EEG signals was validated in mean from 46 to 80% according to the different parameters. The three BSS methods equally performed (no significant difference) and no influence of the number of scalp electrodes used was found. At the opposite, the number and amplitude of spikes included in the averaging before the extraction modified the performance. Anyway, despite their invisibility in scalp EEG signals, this study demonstrates that deep source extraction is feasible under certain conditions and with the use of common signal analysis toolboxes. This finding confirms the crucial need to continue the signal analysis of scalp EEG recordings which contains subcortical signals that escape to expert visual analysis but could be found by signal processing.

Fast periodic visual stimulation to highlight the relationship between human intracerebral recordings and scalp electroencephalography.

Despite being of primary importance for fundamental research and clinical studies, the relationship between local neural population activity and scalp electroencephalography (EEG) in humans remains largely unknown. Here we report simultaneous scalp and intracerebral EEG responses to face stimuli in a unique epileptic patient implanted with 27 intracerebral recording contacts in the right occipitotemporal cortex. The patient was shown images of faces appearing at a frequency of 6 Hz, which elicits neural responses at this exact frequency. Response quantification at this frequency allowed to objectively relate the neural activity measured inside and outside the brain. The patient exhibited typical 6 Hz responses on the scalp at the right occipitotemporal sites. Moreover, there was a clear spatial correspondence between these scalp responses and intracerebral signals in the right lateral inferior occipital gyrus, both in amplitude and in phase. Nevertheless, the signal measured on the scalp and inside the brain at nearby locations showed a 10-fold difference in amplitude due to electrical insulation from the head. To further quantify the relationship between the scalp and intracerebral recordings, we used an approach correlating time-varying signals at the stimulation frequency across scalp and intracerebral channels. This analysis revealed a focused and right-lateralized correspondence between the scalp and intracerebral recordings that were specific to the face stimulation is more broadly distributed in various control situations. These results demonstrate the interest of a frequency tagging approach in characterizing the electrical propagation from brain sources to scalp EEG sensors and in identifying the cortical sources of brain functions from these recordings.

Respective Contribution of Ictal and Inter-ictal Electrical Source Imaging to Epileptogenic Zone Localization.

Interictal electrical source imaging (ESI) encompasses a risk of false localization due to complex relationships between irritative and epileptogenic networks. This study aimed to compare the localizing value of ESI derived from ictal and inter-ictal EEG discharges and to evaluate the localizing value of ESI according to three different subgroups: MRI lesion, presumed etiology and morphology of ictal EEG pattern. We prospectively analyzed 54 of 78 enrolled patients undergoing pre-surgical investigation for refractory epilepsy. Ictal and inter-ictal ESI results were interpreted blinded to- and subsequently compared with stereoelectroencephalography as a reference method. Anatomical concordance was assessed at a sub-lobar level. Sensitivity and specificity of ictal, inter-ictal and ictal plus inter-ictal ESI were calculated and compared according to the different subgroups. Inter-ictal and ictal ESI sensitivity (84% and 75% respectively) and specificity (38% and 50% respectively) were not statistically different. Regarding the sensitivity, ictal ESI was never higher than inter-ictal ESI. Regarding the specificity, ictal ESI was higher than inter-ictal ESI in malformations of cortical development (MCD) (60% vs. 43%) and in MRI positive patients (49% vs. 30%). Within the ictal ESI analysis, we showed a higher specificity for ictal spikes (59%) and rhythmic discharges > 13 Hz (50%) than rhythmic discharges < 13 Hz (37%) and (ii) for MCD (60%) than in other etiologies (29%). This prospective study demonstrates the relevance of a combined interpretation of distinct inter-ictal and ictal analysis. Inter-ictal analysis gave the highest sensitivity. Ictal analysis gave the highest specificity especially in patients with MCD or a lesion on MRI.

The inferior occipital gyrus is a major cortical source of the face-evoked N170: Evidence from simultaneous scalp and intracerebral human recordings.

The sudden onset of a face image leads to a prominent face-selective response in human scalp electroencephalographic (EEG) recordings, peaking 170 ms after stimulus onset at occipito-temporal (OT) scalp sites: the N170 (or M170 in magnetoencephalography). According to a widely held view, the main cortical source of the N170 lies in the fusiform gyrus (FG), whereas the posteriorly located inferior occipital gyrus (IOG) would rather generate earlier face-selective responses. Here, we report neural responses to upright and inverted faces recorded in a unique patient using multicontact intracerebral electrodes implanted in the right IOG and in the OT sulcus above the right lateral FG (LFG). Simultaneous EEG recordings on the scalp identified the N170 over the right OT scalp region. The latency and amplitude of this scalp N170 were correlated at the single-trial level with the N170 recorded in the lateral IOG, close to the scalp lateral occipital surface. In addition, a positive component maximal around the latency of the N170 (a P170) was prominent above the internal LFG, whereas this region typically generates an N170 (or "N200") over its external/ventral surface. This suggests that electrophysiological responses in the LFG manifest as an equivalent dipole oriented mostly along the vertical axis with likely minimal projection to the lateral OT scalp region. Altogether, these observations provide evidence that the IOG is a major cortical generator of the face-selective scalp N170, qualifying the potential contribution of the FG and questioning a strict serial spatiotemporal organization of the human cortical face network.

Optimal use of EEG recordings to target active brain areas with transcranial electrical stimulation.

To demonstrate causal relationships between brain and behavior, investigators would like to guide brain stimulation using measurements of neural activity. Particularly promising in this context are electroencephalography (EEG) and transcranial electrical stimulation (TES), as they are linked by a reciprocity principle which, despite being known for decades, has not led to a formalism for relating EEG recordings to optimal stimulation parameters. Here we derive a closed-form expression for the TES configuration that optimally stimulates (i.e., targets) the sources of recorded EEG, without making assumptions about source location or distribution. We also derive a duality between TES targeting and EEG source localization, and demonstrate that in cases where source localization fails, so does the proposed targeting. Numerical simulations with multiple head models confirm these theoretical predictions and quantify the achieved stimulation in terms of focality and intensity. We show that constraining the stimulation currents automatically selects optimal montages that involve only a few (4-7) electrodes, with only incremental loss in performance when targeting focal activations. The proposed technique allows brain scientists and clinicians to rationally target the sources of observed EEG and thus overcomes a major obstacle to the realization of individualized or closed-loop brain stimulation.

Discrimination of a medial functional module within the temporal lobe using an effective connectivity model: A CCEP study.

The temporal lobe is classically divided in two functional systems: the ventral visual pathway and the medial temporal memory system. However, their functional separation has been challenged by studies suggesting that the medial temporal lobe could be best understood as an extension of the hierarchically organized ventral visual pathway. Our purpose was to investigate (i) whether cerebral regions within the temporal lobe could be grouped into distinct functional assemblies, and (ii) which regions were central within these functional assemblies. We studied low intensity and low frequency electrical stimulations (0.5 mA, 1 Hz, 4 ms) performed during sixteen pre-surgical intracerebral EEG investigations in patients with medically intractable temporal or temporo-occipital lobe epilepsies. Eleven regions of interest were delineated per anatomical landmarks such as gyri and sulci. Effective connectivity based on electrophysiological feature (amplitude) of cortico-cortical evoked potentials (CCEPs) was evaluated and subjected to graph metrics. The amplitudes discriminated one medial module where the hippocampus could act as a signal amplifier. Mean amplitudes of CCEPs in regions of the temporal lobe showed a generalized Pareto distribution of probability suggesting neural synchronies to be self-organized critically. Our description of effective interactions within the temporal lobe provides a regional electrophysiological model of effective connectivity which is discussed in the context of the current hypothesis of pattern completion.

In-vivo measurements of human brain tissue conductivity using focal electrical current injection through intracerebral multicontact electrodes.

In-vivo measurements of human brain tissue conductivity at body temperature were conducted using focal electrical currents injected through intracerebral multicontact electrodes. A total of 1,421 measurements in 15 epileptic patients (age: 28 ± 10) using a radiofrequency generator (50 kHz current injection) were analyzed. Each contact pair was classified as being from healthy (gray matter, n = 696; white matter, n = 530) or pathological (epileptogenic zone, n = 195) tissue using neuroimaging analysis of the local tissue environment and intracerebral EEG recordings. Brain tissue conductivities were obtained using numerical simulations based on conductivity estimates that accounted for the current flow in the local brain volume around the contact pairs (a cube with a side length of 13 mm). Conductivity values were 0.26 S/m for gray matter and 0.17 S/m for white matter. Healthy gray and white matter had statistically different median impedances (P < 0.0001). White matter conductivity was found to be homogeneous as normality tests did not find evidence of multiple subgroups. Gray matter had lower conductivity in healthy tissue than in the epileptogenic zone (0.26 vs. 0.29 S/m; P = 0.012), even when the epileptogenic zone was not visible in the magnetic resonance image (MRI) (P = 0.005). The present in-vivo conductivity values could serve to create more accurate volume conduction models and could help to refine the identification of relevant intracerebral contacts, especially when located within the epileptogenic zone of an MRI-invisible lesion. Hum Brain Mapp 38:974-986, 2017. © 2016 Wiley Periodicals, Inc.

Catching the invisible: mesial temporal source contribution to simultaneous EEG and SEEG recordings.

Mesial temporal sources are presumed to escape detection in scalp electroencephalographic recordings. This is attributed to the deep localization and infolded geometry of mesial temporal structures that leads to a cancellation of electrical potentials, and to the blurring effect of the superimposed neocortical background activity. In this study, we analyzed simultaneous scalp and intracerebral electroencephalographic recordings to delineate the contribution of mesial temporal sources to scalp electroencephalogram. Interictal intracerebral spike networks were classified in three distinct categories: solely mesial, mesial as well as neocortical, and solely neocortical. The highest and earliest intracerebral spikes generated by the leader source of each network were marked and the corresponding simultaneous intracerebral and scalp electroencephalograms were averaged and then characterized both in terms of amplitude and spatial distribution. In seven drug-resistant epileptic patients, 21 interictal intracerebral networks were identified: nine mesial, five mesial plus neocortical and seven neocortical. Averaged scalp spikes arising respectively from mesial, mesial plus neocortical and neocortical networks had a 7.1 (n = 1,949), 36.1 (n = 628) and 10 (n = 1,471) µV average amplitude. Their scalp electroencephalogram electrical field presented a negativity in the ipsilateral anterior and basal temporal electrodes in all networks and a significant positivity in the fronto-centro-parietal electrodes solely in the mesial plus neocortical and neocortical networks. Topographic consistency test proved the consistency of these different scalp electroencephalogram maps and hierarchical clustering clearly differentiated them. In our study, we have thus shown for the first time that mesial temporal sources (1) cannot be spontaneously visible (mean signal-to-noise ratio -2.1 dB) on the scalp at the single trial level and (2) contribute to scalp electroencephalogram despite their curved geometry and deep localization.

Intracerebral electrical stimulation of a face-selective area in the right inferior occipital cortex impairs individual face discrimination.

During intracerebral stimulation of the right inferior occipital cortex, a patient with refractory epilepsy was transiently impaired at discriminating two simultaneously presented photographs of unfamiliar faces. The critical electrode contact was located in the most posterior face-selective brain area of the human brain (right "occipital face area", rOFA) as shown both by low- (ERP) and high-frequency (gamma) electrophysiological responses as well as a face localizer in fMRI. At this electrode contact, periodic visual presentation of 6 different faces by second evoked a larger electrophysiological periodic response at 6 Hz than when the same face identity was repeated at the same rate. This intracerebral EEG repetition suppression effect was markedly reduced when face stimuli were presented upside-down, a manipulation that impairs individual face discrimination. These findings provide original evidence for a causal relationship between the face-selective right inferior occipital cortex and individual face discrimination, independently of long-term memory representations. More generally, they support the functional value of electrophysiological repetition suppression effects, indicating that these effects can be used as an index of a necessary neural representation of the changing stimulus property.

Right hemispheric dominance of visual phenomena evoked by intracerebral stimulation of the human visual cortex.

Electrical brain stimulation can provide important information about the functional organization of the human visual cortex. Here, we report the visual phenomena evoked by a large number (562) of intracerebral electrical stimulations performed at low-intensity with depth electrodes implanted in the occipito-parieto-temporal cortex of 22 epileptic patients. Focal electrical stimulation evoked primarily visual hallucinations with various complexities: simple (spot or blob), intermediary (geometric forms), or complex meaningful shapes (faces); visual illusions and impairments of visual recognition were more rarely observed. With the exception of the most posterior cortical sites, the probability of evoking a visual phenomenon was significantly higher in the right than the left hemisphere. Intermediary and complex hallucinations, illusions, and visual recognition impairments were almost exclusively evoked by stimulation in the right hemisphere. The probability of evoking a visual phenomenon decreased substantially from the occipital pole to the most anterior sites of the temporal lobe, and this decrease was more pronounced in the left hemisphere. The greater sensitivity of the right occipito-parieto-temporal regions to intracerebral electrical stimulation to evoke visual phenomena supports a predominant role of right hemispheric visual areas from perception to recognition of visual forms, regardless of visuospatial and attentional factors.

Electrical source imaging in cortical malformation-related epilepsy: a prospective EEG-SEEG concordance study.

OBJECTIVE: Delineation of the epileptogenic zone (EZ) in refractory epilepsy related to malformations of cortical development (MCDs) often requires intracranial electroencephalography (EEG) recordings, especially in cases of negative magnetic resonance imaging (MRI) or discordant MRI and video-EEG findings. It is therefore crucial to promote the development of noninvasive methods such as electrical source imaging (ESI). We aimed to (1) analyze the localization concordance of ESI derived from interictal discharges and EZ estimated by stereo-EEG (SEEG); (2) compare the concordance of ESI, MRI, and electroclinical correlations (ECCs) with SEEG-EZ; and (3) assess ESI added value in the EZ localization. METHODS: We prospectively analyzed 28 consecutive patients undergoing presurgical investigation for MCD-related refractory epilepsy in 2009-2012. ESI derived from 64-channel scalp EEG was interpreted with blinding to, and subsequently compared with, SEEG-estimated EZ. Anatomic concordance of ESI with SEEG-EZ was compared with that of video-EEG and MRI. We further assessed ESI added value to ECC and MRI. RESULTS: Twelve patients (43%) had temporal and 16 (57%) had extratemporal epilepsy. MRI was negative in 11 (39%) and revealed a cortical malformation in 17 (61%). ESI was fully concordant with the EZ in 10 (36%) and partly concordant in 15 (53%). ECC presented a full and partial concordance with EZ in 11% and 82% of cases, respectively, and MRI in 11% and 46%, respectively. Of 11 patients with negative MRI, ESI was fully concordant with the EZ in 7 (64%) and partly concordant in 4 (36%). ESI correctly confirmed restricted or added localizations to ECC and MRI in 12 (43%) of 28 patients and in 8 (73%) of 11 patients with negative MRI. SIGNIFICANCE: ESI contributes to estimating the EZ in MCD-related epilepsy. The added value of ESI to ECC is particularly high in patients with MCD and negative MRI, who represent the most challenging cases for epilepsy surgery. A PowerPoint slide summarizing this article is available for download in the Supporting Information section here.

Simultaneous subdural and scalp EEG correlates of frontal lobe epileptic sources.

OBJECTIVE: To assess the visibility and detectability in scalp electroencephalography (EEG) of cortical sources in frontal lobe epilepsy (FLE) as to their localization, and the extent and amplitude of activation. METHODS: We analyzed the simultaneous subdural and scalp interictal EEG recordings of 14 patients with refractory frontal lobe epilepsy (FLE) associated with focal cortical dysplasia. Subdural spike types were identified and averaged for source localization and detection of their scalp EEG correlates. Both raw and averaged scalp EEG segments were reviewed for spikes, blinded to subdural segments. We further analyzed the correlation of spike-to-background amplitude ratios in subdural and scalp EEG. RESULTS: We identified 36 spike types in subdural EEG, corresponding to 29 distinct sources. Four of 29 sources were visible by visual evaluation of scalp EEG and six additional sources were detectable after averaging: four in the medial frontal, two in the dorsolateral gyri, two in the depth of dorsolateral sulci, and two in the basal frontal region. Cortical sources generating scalp-detectable spikes presented a median of 6 cm(2) of activated cortical convexity surface and a subdural spike-to-background-amplitude ratio >8. These sources were associated with a higher number of activated subdural grid contacts and a higher subdural spike-to-background amplitude ratio than sources generating non-scalp-detectable spikes. SIGNIFICANCE: Not only dorsolateral but also basal and medial sources can be detectable in FLE. This is the first in vivo demonstration derived from simultaneous subdural and scalp EEG recordings of the complementary significance of extensive source activation and higher subdural spike-to-background amplitude ratio in the detection of cortical sources in FLE.

Intracranial evaluation of the epileptogenic zone in regional infrasylvian polymicrogyria.

PURPOSE: To define the relationship between the epileptogenic zone and the polymicrogyric area using intracranial electroencephalography (EEG) recordings in patients with structural epilepsy associated with regional infrasylvian polymicrogyria (PMG). METHODS: We retrospectively reviewed the medical charts, scalp, and intracranial video-EEG recordings, neuroimaging findings, and neuropsychological evaluations of four patients with refractory temporal lobe epilepsy related to PMG who consequently underwent resective surgery. KEY FINDINGS: High-resolution magnetic resonance imaging (MRI) revealed temporal lobe PMG in all cases, accompanied by hippocampal malrotation and closed lip schizencephaly in 3/4 cases, respectively. In intracranial recordings, interictal spike activity was localized within the PMG in only 2/4 and within the amygdala, hippocampus, and entorhinal cortex in all cases. In the first patient, two epileptogenic networks coexisted: the prevailing network initially involved the mesial temporal structures with spread to the anterior PMG; the secondary network successively involved the anterior part of the PMG and later the mesial temporal structures. In the second patient, the epileptogenic network was limited to the mesial temporal structures, fully sparing the PMG. In the third patient, the epileptogenic network first involved the mesial temporal structures and later the PMG. Conversely, in the last case, part of the PMG harbored an epileptogenic network that propagated to the mesial temporal structures. Consistent with these findings a favorable outcome (Engel class I in three of four patients; Engel class II in one of four) at last follow-up was obtained by a resection involving parts of the PMG cortex in three of four and anteromesial temporal lobe structures in another three of four cases. SIGNIFICANCE: Infrasylvian PMG displays a heterogeneous epileptogenicity and is occasionally and partially involved in the epileptogenic zone that commonly includes the mesial temporal structures. Our results highlight the intricate interrelations between the MRI-detectable lesion and the epileptogenic zone as delineated by intracranial recordings. Seizure freedom can be accomplished as a result of a meticulous intracranial study guiding a tailored resection that may spare part of the PMG.

A Bayesian approach for simultaneous spike/LFP separation and spike sorting.

Objective.The aim of this paper is to present a novel method for simultaneous spike waveforms extraction and sorting from the raw recorded signal. The objective is twofold: on the one hand, to enhance spike sorting performance by extracting the spike waveforms of each spike and, on the other hand, to improve the analysis of the multi-scale relationships between spikes and local field potentials (LFP) by offering an accurate separation of these two components constitutive of the raw micro recordings.Approach.The method, based on a Bayesian approach, is fully automated and provides a mean spike shape for each cluster, but also an estimate for each singular spike waveform, as well as the LFP signal cleaned of spiking activity.Main results.The performance of the algorithm is evaluated on simulated and real data, for which both the clustering and spike removal aspects are analyzed. Clustering performance significantly increases when compared to state-of-the-art methods, taking benefit from the separation of the spikes from the LFP handled by our model. Our method also performs better in removing the spikes from the LFP when compared to previously proposed methodologies, especially in the high frequency bands. The method is finally applied on real data (ClinicalTrials.gov Identifier: NCT02877576) and confirm the results obtained on benchmark signals.Significance.By separating more efficiently the spikes from the LFP background, our method allows both a better spike sorting and a more accurate estimate of the LFP, facilitating further analysis such as spike-LFP relationships.

Single neuron responses underlying face recognition in the human midfusiform face-selective cortex.

Faces are critical for social interactions and their recognition constitutes one of the most important and challenging functions of the human brain. While neurons responding selectively to faces have been recorded for decades in the monkey brain, face-selective neural activations have been reported with neuroimaging primarily in the human midfusiform gyrus. Yet, the cellular mechanisms producing selective responses to faces in this hominoid neuroanatomical structure remain unknown. Here we report single neuron recordings performed in 5 human subjects (1 male, 4 females) implanted with intracerebral microelectrodes in the face-selective midfusiform gyrus, while they viewed pictures of familiar and unknown faces and places. We observed similar responses to faces and places at the single cell level, but a significantly higher number of neurons responding to faces, thus offering a mechanistic account for the face-selective activations observed in this region. Although individual neurons did not respond preferentially to familiar faces, a population level analysis could consistently determine whether or not the faces (but not the places) were familiar, only about 50 ms after the initial recognition of the stimuli as faces. These results provide insights into the neural mechanisms of face processing in the human brain.

Intracerebral Correlates of Scalp EEG Ictal Discharges Based on Simultaneous Stereo-EEG Recordings.

BACKGROUND AND OBJECTIVES: It remains unknown to what extent ictal scalp EEG can accurately predict the localization of the intracerebral seizure onset in presurgical evaluation of drug-resistant epilepsies. In this study, we aimed to define homogeneous ictal scalp EEG profiles (based on their first ictal abnormality) and assess their localizing value using simultaneously recorded scalp EEG and stereo-EEG. METHODS: We retrospectively included consecutive patients with drug-resistant focal epilepsy who had simultaneous stereo-EEG and scalp EEG recordings of at least 1 seizure in the epileptology unit in Nancy, France. We analyzed 1 seizure per patient and used hierarchical cluster analysis to group similar seizure profiles on scalp EEG and then performed a descriptive analysis of their intracerebral correlates. RESULTS: We enrolled 129 patients in this study. The hierarchical cluster analysis showed 6 profiles on scalp EEG first modification. None were specific to a single intracerebral localization. The "normal EEG" and "blurred EEG" clusters (early muscle artifacts) comprised only 5 patients each and corresponded to no preferential intracerebral localization. The "temporal discharge" cluster (n = 46) was characterized by theta or delta discharges on ipsilateral anterior temporal scalp electrodes and corresponded to a preferential mesial temporal intracerebral localization. The "posterior discharge" cluster (n = 42) was characterized by posterior ipsilateral or contralateral rhythmic alpha discharges or slow waves on scalp and corresponded to a preferential temporal localization. However, this profile was the statistically most frequent scalp EEG correlate of occipital and parietal seizures. The "diffuse suppression" cluster (n = 9) was characterized by a bilateral and diffuse background activity suppression on scalp and corresponded to mesial, and particularly insulo-opercular, localization. Finally, the "frontal discharge" cluster (n = 22) was characterized by bilateral frontal rhythmic fast activity or preictal spike on scalp and corresponded to preferential ventrodorsal frontal intracerebral localizations. DISCUSSION: The hierarchical cluster analysis identified 6 seizure profiles regarding the first abnormality on scalp EEG. None of them were specific of a single intracerebral localization. Nevertheless, the strong relationships between the "temporal," "frontal," "diffuse suppression," and "posterior" profiles and intracerebral discharge localizations may contribute to hierarchize hypotheses derived from ictal scalp EEG analysis regarding intracerebral seizure onset.

Transcranial Electrical Stimulation generates electric fields in deep human brain structures.

BACKGROUND: Transcranial electrical stimulation (TES) efficiency is related to the electric field (EF) magnitude delivered on the target. Very few studies (n = 4) have estimated the in-vivo intracerebral electric fields in humans. They have relied mainly on electrocorticographic recordings, which require a craniotomy impacting EF distribution, and did not investigate deep brain structures. OBJECTIVE: To measure the electric field in deep brain structures during TES in humans in-vivo. Additionally, to investigate the effects of TES frequencies, intensities, and montages on the intracerebral EF. METHODS: Simultaneous bipolar transcranial alternating current stimulation and intracerebral recordings (SEEG) were performed in 8 drug-resistant epileptic patients. TES was applied using small high-definition (HD) electrodes. Seven frequencies, two intensities and 15 montages were applied on one, six and one patients, respectively. RESULTS: At 1 mA intensity, we found mean EF magnitudes of 0.21, 0.17 and 0.07 V·m-1 in the amygdala, hippocampus, and cingulate gyrus, respectively. An average of 0.14 ± 0.07 V·m-1 was measured in these deep brain structures. Mean EF magnitudes in these structures at 1Hz were 11% higher than at 300Hz (+0.03 V·m-1). The EF was correlated with the TES intensities. The TES montages that yielded the maximum EF in the amygdalae were T7-T8 and in the cingulate gyri were C3-FT10 and T7-C4. CONCLUSION: TES at low intensities and with small HD electrodes can generate an EF in deep brain structures, irrespective of stimulation frequency. EF magnitude is correlated to the stimulation intensity and depends upon the stimulation montage.

From perception to recognition memory: time course and lateralization of neural substrates of word and abstract picture processing.

Through study of clinical cases with brain lesions as well as neuroimaging studies of cognitive processing of words and pictures, it has been established that material-specific hemispheric specialization exists. It remains however unclear whether such specialization holds true for all processes involved in complex tasks, such as recognition memory. To investigate neural signatures of transition from perception to recognition, according to type of material (words or abstract pictures), high-resolution scalp ERPs were recorded in adult humans engaged either in categorization or in memory recognition tasks within the same experimental setup. Several steps in the process from perception to recognition were identified. Source localization showed that the early stage of perception processing (N170) takes place in the fusiform gyrus and is lateralized according to the nature of stimuli (left side for words and right side for pictures). Late stages of processing (N400/P600) corresponding to recognition are material independent and involve anterior medial-temporal and ventral prefrontal structures bilaterally. A crucial transitional process between perception (N170) and recognition (N400/P600) is reflected by the N270, an often overlooked component, which occurs in anterior rhinal cortices and shows material-specific hemispheric lateralization.

Source localization of ictal epileptic activity investigated by high resolution EEG and validated by SEEG.

High resolution electroencephalography (HR-EEG) combined with source localization methods has mainly been used to study interictal spikes and there have been few studies comparing source localization of scalp ictal patterns with depth EEG. To address this issue, 10 patients with four different scalp ictal patterns (ictal spikes, rhythmic activity, paroxysmal fast activity, obscured) were investigated by both HR-EEG and stereoelectroencephalography (SEEG). Sixty-four scalp-EEG sensors and a sampling rate of 1kHz were used to record scalp ictal patterns. Five different source models (moving dipole, rotating dipole, MUSIC, LORETA, and sLORETA) were used in order to perform source localization. Seven to 10 intracerebral electrodes were implanted during SEEG investigations. For each source model, the concordance between ictal source localization and epileptogenic zone defined by SEEG was assessed. Results were considered to agree if they localized in the same sublobar area as defined by a trained epileptologist. Across the study population, the best concordance between source localization methods and SEEG (9/10) was obtained with equivalent current dipole modeling. MUSIC and LORETA had a concordance of 7/10 whereas sLORETA had a concordance of only 5/10. Four of our patients classified into different groups (ictal spikes, paroxysmal fast activity, obscured) had complete concordance between source localization methods and SEEG. A high signal to noise ratio, a short time window of analysis (<1s) and bandpass filtering around the frequency of rhythmic activity allowed improvement of the source localization results. A high level of agreement between source localization methods and SEEG can be obtained for ictal spike patterns and for scalp-EEG paroxysmal fact activities whereas scalp rhythmic discharges can be accurately localized but originated from seizure propagation network.