Identification of Early Hippocampal Dynamics during Recognition Memory with Independent Component Analysis

The hippocampus is generally considered to have relatively late involvement in recognition memory, its main electrophysiological signature being between 400 and 800 ms after stimulus onset. However, most electrophysiological studies have analyzed the hippocampus as a single responsive area, selecting only a single-site signal exhibiting the strongest effect in terms of amplitude. These classical approaches may not capture all the dynamics of this structure, hindering the contribution of other hippocampal sources that are not located in the vicinity of the selected site. We combined intracerebral electroencephalogram recordings from epileptic patients with independent component analysis during a recognition memory task involving the recognition of old and new images. We identified two sources with different responses emerging from the hippocampus: a fast one (maximal amplitude at ∼250 ms) that could not be directly identified from raw recordings and a latter one, peaking at ∼400 ms. The former component presented different amplitudes between old and new items in 6 out of 10 patients. The latter component had different delays for each condition, with a faster activation (∼290 ms after stimulus onset) for recognized items. We hypothesize that both sources represent two steps of hippocampal recognition memory, the faster reflecting the input from other structures and the latter the hippocampal internal processing. Recognized images evoking early activations would facilitate neural computation in the hippocampus, accelerating memory retrieval of complementary information. Overall, our results suggest that the hippocampal activity is composed of several sources with an early activation related to recognition memory.

Organization of the epileptogenic zone and signal analysis at seizure onset in patients with drug-resistant epilepsy due to focal cortical dysplasia with mTOR pathway gene mutations-An SEEG study

Epilepsy surgery in genetic drug-resistant epilepsy is a debated subject as more histological and molecular data are available. We retrospectively collected data from focal drug-resistant epilepsy patients that underwent stereoelectroencephalography (SEEG) invasive recordings. Patients with nonlesional brain imaging or in whom a first epilepsy surgery failed to control seizures were selected. We computed and displayed the intracranial ictal onset activity pattern on structural imaging. Patients underwent epilepsy gene panel testing, next generation sequencing-NGS. Of 113 patients, 13 underwent genetic testing, and in 6 patients, a mechanistic target of rapamycin pathway gene germline mutation (mTOR) was identified. Brain imaging was nonlesional except for one patient in whom two abnormalities suggestive of focal cortical dysplasia (FCD) were found. Patients underwent tailored brain surgery based on SEEG data, tissue analysis revealed FCD and postsurgical outcome was favorable. Our findings are similar to previous case series suggesting that epilepsy surgery can be a treatment option in patients with mTOR pathway mutation. In patients with mTOR pathway mutation, the postsurgical outcome is favorable if complete resection of the epileptogenic zone is performed. Electrophysiological seizure onset patterns in FCDs associated with mTOR pathway mutations display low-voltage fast activity as previously described.

Cingulate Cortex: Anatomy, Structural and Functional Connectivity

SUMMARY

The cingulate cortex is a paired brain region located on the medial wall of each hemisphere. This review explores the anatomy as well as the structural and functional connectivity of the cingulate cortex underlying essential roles this region plays in emotion, autonomic, cognitive, motor control, visual-spatial processing, and memory.

Functional Enhancement and Characterization of an Electrophysiological Mapping Electrode Probe with Carbonic, Directional Macrocontacts

Electrophysiological mapping (EM) using acute electrode probes is a common procedure performed during functional neurosurgery. Due to their constructive specificities, the EM probes are lagging in innovative enhancements. This work addressed complementing a clinically employed EM probe with carbonic and circumferentially segmented macrocontacts that are operable both for neurophysiological sensing ("recording") of local field potentials (LFP) and for test stimulation. This paper illustrates in-depth the development that is based on the direct writing of functional materials. The unconventional fabrication processes were optimized on planar geometry and then transferred to the cylindrically thin probe body. We report and discuss the constructive concept and architecture of the probe, characteristics of the electrochemical interface deduced from voltammetry and chronopotentiometry, and the results of in vitro and in vivo recording and pulse stimulation tests. Two- and three-directional macrocontacts were added on probes having shanks of 550 and 770 μm diameters and 10-23 cm lengths. The graphitic material presents a ~2.7 V wide, almost symmetric water electrolysis window, and an ultra-capacitive charge transfer. When tested with clinically relevant 150 μs biphasic current pulses, the interfacial polarization stayed safely away from the water window for pulse amplitudes up to 9 mA (135 μC/cm2). The in vivo experiments on adult rat models confirmed the high-quality sensing of LFPs. Additionally, the in vivo-prevailing increase in the electrode impedance and overpotential are discussed and modeled by an ionic mobility-reducing spongiform structure; this restricted diffusion model gives new applicative insight into the in vivo-uprisen stimulation overpotential.

Studying memory processes at different levels with simultaneous depth and surface EEG recordings

Investigating cognitive brain functions using non-invasive electrophysiology can be challenging due to the particularities of the task-related EEG activity, the depth of the activated brain areas, and the extent of the networks involved. Stereoelectroencephalographic (SEEG) investigations in patients with drug-resistant epilepsy offer an extraordinary opportunity to validate information derived from non-invasive recordings at macro-scales. The SEEG approach can provide brain activity with high spatial specificity during tasks that target specific cognitive processes (e.g., memory). Full validation is possible only when performing simultaneous scalp SEEG recordings, which allows recording signals in the exact same brain state. This is the approach we have taken in 12 subjects performing a visual memory task that requires the recognition of previously viewed objects. The intracranial signals on 965 contact pairs have been compared to 391 simultaneously recorded scalp signals at a regional and whole-brain level, using multivariate pattern analysis. The results show that the task conditions are best captured by intracranial sensors, despite the limited spatial coverage of SEEG electrodes, compared to the whole-brain non-invasive recordings. Applying beamformer source reconstruction or independent component analysis does not result in an improvement of the multivariate task decoding performance using surface sensor data. By analyzing a joint scalp and SEEG dataset, we investigated whether the two types of signals carry complementary information that might improve the machine-learning classifier performance. This joint analysis revealed that the results are driven by the modality exhibiting best individual performance, namely SEEG.

Musicogenic seizures in temporal lobe epilepsy: Case reports based on ictal source localization analysis

Musicogenic epilepsy is a rare form of reflex epilepsy in which seizures are provoked by music. Different musicogenic stimuli have been identified: pleasant/unpleasant music or specific musical patterns. Several etiologies have been uncovered, such as focal cortical dysplasia, autoimmune encephalitis, tumors, or unspecific gliosis. In this article, we report two patients with musicogenic seizures. The first patient was diagnosed with structural temporal lobe epilepsy. Her seizures were elicited by music that she liked. Interictal and ictal video-electroencephalography (video-EEG) and signal analysis using independent component analysis revealed the right temporal lobe seizure onset extending over the neocortical regions. The patient underwent right temporal lobectomy (including the amygdala, the head, and the body of the hippocampus) and faced an Engel IA outcome 3 years post-surgery. The second patient was diagnosed with autoimmune temporal lobe epilepsy (GAD-65 antibodies). Her seizures were triggered by contemporary hit radio songs without any personal emotional significance. Interictal and ictal video-electroencephalography (video-EEG) and independent component analysis highlighted the left temporal lobe seizure onset extending over the neocortical regions. Intravenous immunoglobulin therapy was initiated, and the patient became seizure-free at 1 year. In conclusion, musicogenic seizures may be elicited by various auditory stimuli, the presence or absence of an emotional component offering an additional clue for the underlying network pathophysiology. Furthermore, in such cases, the use of independent component analysis of the scalp EEG signals proves useful in revealing the location of the seizure generator, and our findings point toward the temporal lobe, both mesial and neocortical regions.

A comparison of uni- and multi-variate methods for identifying brain networks activated by cognitive tasks using intracranial EEG

Cognitive tasks are commonly used to identify brain networks involved in the underlying cognitive process. However, inferring the brain networks from intracranial EEG data presents several challenges related to the sparse spatial sampling of the brain and the high variability of the EEG trace due to concurrent brain processes. In this manuscript, we use a well-known facial emotion recognition task to compare three different ways of analyzing the contrasts between task conditions: permutation cluster tests, machine learning (ML) classifiers, and a searchlight implementation of multivariate pattern analysis (MVPA) for intracranial sparse data recorded from 13 patients undergoing presurgical evaluation for drug-resistant epilepsy. Using all three methods, we aim at highlighting the brain structures with significant contrast between conditions. In the absence of ground truth, we use the scientific literature to validate our results. The comparison of the three methods’ results shows moderate agreement, measured by the Jaccard coefficient, between the permutation cluster tests and the machine learning [0.33 and 0.52 for the left (LH) and right (RH) hemispheres], and 0.44 and 0.37 for the LH and RH between the permutation cluster tests and MVPA. The agreement between ML and MVPA is higher: 0.65 for the LH and 0.62 for the RH. To put these results in context, we performed a brief review of the literature and we discuss how each brain structure’s involvement in the facial emotion recognition task.

Magnetoencephalography can reveal deep brain network activities linked to memory processes

Recording from deep neural structures such as hippocampus noninvasively and yet with high temporal resolution remains a major challenge for human neuroscience. Although it has been proposed that deep neuronal activity might be recordable during cognitive tasks using magnetoencephalography (MEG), this remains to be demonstrated as the contribution of deep structures to MEG recordings may be too small to be detected or might be eclipsed by the activity of large‐scale neocortical networks. In the present study, we disentangled mesial activity and large‐scale networks from the MEG signals thanks to blind source separation (BSS). We then validated the MEG BSS components using intracerebral EEG signals recorded simultaneously in patients during their presurgical evaluation of epilepsy. In the MEG signals obtained during a memory task involving the recognition of old and new images, we identified with BSS a putative mesial component, which was present in all patients and all control subjects. The time course of the component selectively correlated with stereo‐electroencephalography signals recorded from hippocampus and rhinal cortex, thus confirming its mesial origin. This finding complements previous studies with epileptic activity and opens new possibilities for using MEG to study deep brain structures in cognition and in brain disorders.

Imaging the effective networks associated with cortical function through intracranial high-frequency stimulation

Direct electrical stimulation (DES) is considered to be the gold standard for mapping cortical function. A careful mapping of the eloquent cortex is key to successful resective or ablative surgeries, with a minimal postoperative deficit, for treatment of drug‐resistant epilepsy. There is accumulating evidence suggesting that not only local, but also remote activations play an equally important role in evoking clinical effects. By introducing a new intracranial stimulation paradigm and signal analysis methodology allowing to disambiguate EEG responses from stimulation artifacts we highlight the spatial extent of the networks associated with clinical effects. Our study includes 26 patients that underwent stereoelectroencephalographic investigations for drug‐resistant epilepsy, having 337 depth electrodes with 4,351 contacts sampling most brain structures. The routine high‐frequency electrical stimulation protocol for eloquent cortex mapping was altered in a subtle way, by alternating the polarity of the biphasic pulses in a train, causing the splitting the spectral lines of the artifactual components, exposing the underlying tissue response. By performing a frequency‐domain analysis of the EEG responses during DES we were able to capture remote activations and highlight the effect's network. By using standard intersubject averaging and a fine granularity HCP‐MMP parcellation, we were able to create local and distant connectivity maps for 614 stimulations evoking specific clinical effects. The clinical value of such maps is not only for a better understanding of the extent of the effects' networks guiding the invasive exploration, but also for understanding the spatial patterns of seizure propagation given the timeline of the seizure semiology.

A method to assess the default EEG macrostate and its reactivity to stimulation

OBJECTIVE

The default mode network (DMN) is deactivated by stimulation. We aimed to assess the DMN reactivity impairment by routine EEG recordings in stroke patients with impaired consciousness.

METHODS

Binocular light flashes were delivered at 1 Hz in 1-minute epochs, following a 1-minute baseline (PRE). The EEG was decomposed in a series of binary oscillatory macrostates by topographic spectral clustering. The most deactivated macrostate was labeled the default EEG macrostate (DEM). Its reactivity (DER) was quantified as the decrease in DEM occurrence probability during stimulation. A normalized DER index (DERI) was calculated as DER/PRE. The measures were compared between 14 healthy controls and 32 comatose patients under EEG monitoring following an acute stroke.

RESULTS

The DEM was mapped to the posterior DMN hubs. In the patients, these DEM source dipoles were 3-4 times less frequent and were associated with an increased theta activity. Even in a reduced 6-channel montage, a DER below 6.26% corresponding to a DERI below 0.25 could discriminate the patients with sensitivity and specificity well above 80%.

CONCLUSION

The method detected the DMN impairment in post-stroke coma patients.

SIGNIFICANCE

The DEM and its reactivity to stimulation could be useful to monitor the DMN function at bedside.

Network of autoscopic hallucinations elicited by intracerebral stimulations of periventricular nodular heterotopia: An SEEG study

Periventricular nodular heterotopias (PVNH) are areas of neurons abnormally located in the white matter that might be involved in physiological cortical functions. Autoscopic hallucinations are changes in self-consciousness determined by a mismatch in integration of multiple sensory inputs. Our goal is to highlight the brain network involved in generation of autoscopic hallucination elicited by electrical stimulation of a PVNH in a drug resistant epilepsy patient. Our patient was explored using stereo-electroencephalography with electrodes covering the right posterior temporal PVNH and the adjacent cortex. Direct electrical high frequency stimulation of the PVNH elicited autoscopic hallucinations mainly involving the face and upper trunk. We then used multiple modalities to determine brain connectivity: single pulse electrical stimulation of the PVNH and stimulation-evoked potentials were used to highlight resting state effective connectivity. High-frequency stimulation using alternating polarity pulses enabled us to identify the network involved, time-locked to the clinical effect and to map symptom-related effective connectivity. Functional connectivity using a non-linear regression method was used to determine dependencies between different cortical regions following the stimulation. Finally, structural connectivity was highlighted using deterministic fiber tracking. Multi-modal connectivity analysis identified a network involving the PVNH, occipital and temporal neocortex, fusiform gyrus and parietal cortex.

Extracting seizure onset from surface EEG with independent component analysis: Insights from simultaneous scalp and intracerebral EEG

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Independent component analysis (ICA) is able to identify seizure generators.

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Simultaneous long-term scalp-SEEG allows validation of the ICA results.

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Ability to record seizure onset patterns on scalp depends on generator depth.

The success of stereoelectroencephalographic (SEEG) investigations depends crucially on the hypotheses on the putative location of the seizure onset zone. This information is derived from non-invasive data either based on visual analysis or advanced source localization algorithms. While source localization applied to interictal spikes recorded on scalp is the classical method, it does not provide unequivocal information regarding the seizure onset zone. Raw ictal activity contains a mixture of signals originating from several regions of the brain as well as EMG artifacts, hampering direct input to the source localization algorithms. We therefore introduce a methodology that disentangles the various sources contributing to the scalp ictal activity using independent component analysis and uses equivalent current dipole localization as putative locus of ictal sources. We validated the results of our analysis pipeline by performing long-term simultaneous scalp – intracerebral (SEEG) recordings in 14 patients and analyzing the wavelet coherence between the independent component encoding the ictal discharge and the SEEG signals in 8 patients passing the inclusion criteria. Our results show that invasively recorded ictal onset patterns, including low-voltage fast activity, can be captured by the independent component analysis of scalp EEG. The visibility of the ictal activity strongly depends on the depth of the sources. The equivalent current dipole localization can point to the seizure onset zone (SOZ) with an accuracy that can be as high as 10 mm for superficially located sources, that gradually decreases for deeper seizure generators, averaging at 47 mm in the 8 analyzed patients. Independent component analysis is therefore shown to have a promising SOZ localizing value, indicating whether the seizure onset zone is neocortical, and its approximate location, or located in mesial structures. That may contribute to a better crafting of the hypotheses used as basis of the stereo-EEG implantations.

Accuracy and Safety of Customized Stereotactic Fixtures for Stereoelectroencephalography in Pediatric Patients

Stereoelectroencephalography (SEEG) in children with intractable epilepsy presents particular challenges. Their thin and partially ossified cranium, specifically in the temporal area, is prone to fracture while attaching stereotactic systems to the head or stabilizing the head in robot's field of action. Postponing SEEG in this special population of patients can have serious consequences, reducing their chances of becoming seizure-free and impacting their social and cognitive development. This study demonstrates the safety and accuracy offered by a frameless personalized 3D printed stereotactic implantation system for SEEG investigations in children under 4 years of age. SEEG was carried out in a 3-year-old patient with drug-resistant focal epilepsy, based on a right temporal-perisylvian epileptogenic zone hypothesis. Fifteen intracerebral electrodes were placed using a StarFix patient-customized stereotactic fixture. The median lateral entry point localization error of the electrodes was 0.90 mm, median lateral target point localization error was 1.86 mm, median target depth error was 0.83 mm, and median target point localization error was 1.96 mm. There were no perioperative complications. SEEG data led to a tailored right temporal-insular-opercular resection, with resulting seizure freedom (Engel IA). In conclusion, patient-customized stereotactic fixtures are a safe and accurate option for SEEG exploration in young children.

Cingulate cortex function and multi-modal connectivity mapped using intracranial stimulation

The cingulate cortex is part of the limbic system. Its function and connectivity are organized in a rostro-caudal and ventral-dorsal manner which was addressed by various other studies using rather coarse cortical parcellations. In this study, we aim at describing its function and connectivity using invasive recordings from patients explored for focal drug-resistant epilepsy. We included patients that underwent stereo-electroencephalographic recordings using intracranial electrodes in the University Emergency Hospital Bucharest between 2012 and 2019. We reviewed all high frequency stimulations (50 ​Hz) performed for functional mapping of the cingulate cortex. We used two methods to characterize brain connectivity. Effective connectivity was inferred based on the analysis of cortico-cortical potentials (CCEPs) evoked by single pulse electrical stimulation (SPES) (15 ​s inter-pulse interval). Functional connectivity was estimated using the non-linear regression method applied to 60 ​s spontaneous electrical brain signal intervals. The effective (stimulation-evoked) and functional (non-evoked) connectivity analyses highlight brain networks in a different way. While non-evoked connectivity evidences areas having related activity, often in close proximity to each other, evoked connectivity highlights spatially extended networks. To highlight in a comprehensive way the cingulate cortex's network, we have performed a bi-modal connectivity analysis that combines the resting-state broadband h² non-linear correlation with cortico-cortical evoked potentials. We co-registered the patient's anatomy with the fsaverage FreeSurfer template to perform the automatic labeling based on HCP-MMP parcellation. At a group level, connectivity was estimated by averaging responses over stimulated/recorded or recorded sites in each pair of parcels. Finally, for multiple regions that evoked a clinical response during high frequency stimulation, we combined the connectivity of individual pairs using maximum intensity projection. Connectivity was assessed by applying SPES on 2094 contact pairs and recording CCEPs on 3580 contacts out of 8582 contacts of 660 electrodes implanted in 47 patients. Clinical responses elicited by high frequency stimulations in 107 sites (pairs of contacts) located in the cingulate cortex were divided in 10 groups: affective, motor behavior, motor elementary, versive, speech, vestibular, autonomic, somatosensory, visual and changes in body perception. Anterior cingulate cortex was shown to be connected to the mesial temporal, orbitofrontal and prefrontal cortex. In the middle cingulate cortex, we located affective, motor behavior in the anterior region, and elementary motor and somatosensory in the posterior part. This region is connected to the prefrontal, premotor and primary motor network. Finally, the posterior cingulate was shown to be connected with the visual areas, mesial and lateral parietal and temporal cortex.

Unsupervised Detection of High-Frequency Oscillations Using Time-Frequency Maps and Computer Vision

High-frequency oscillations >80 Hz (HFOs) have unique features distinguishing them from spikes and artifactual components that can be well-evidenced in the time-frequency representations. We introduce an unsupervised HFO detector that uses computer-vision algorithms to detect HFO landmarks on two-dimensional (2D) time-frequency maps. To validate the detector, we introduce an analytical model of the HFO based on a sinewave having a Gaussian envelope, for which analytical equations in time-frequency space can be derived, allowing us to establish a direct correspondence between common HFO detection criteria in the time domain with the ones in the frequency domain, used by the computer-vision detection algorithm. The detector identifies potential HFO events on the time-frequency representation, which are classified as true HFOs if criteria regarding the HFO's frequency, amplitude, and duration are met. The detector is validated on simulated HFOs according to the analytical model, in the presence of noise, with different signal-to-noise ratios (SNRs) ranging from −9 to 0 dB. The detector's sensitivity was 0.64 at an SNR of −9 dB, 0.98 at −6 dB, and >0.99 at −3 dB and 0 dB, while its positive prediction value was >0.95, regardless of the SNR. Using the same simulation dataset, our detector is benchmarked against four previously published HFO detectors. The F-measure, a combined metric that takes into account both sensitivity and positive prediction value, was used to compare detection algorithms. Our detector surpassed the other detectors at −6, −3, and 0 dB and had the second best F-score at −9 dB SNR after the MNI detector (0.77 vs. 0.83). The ability to detect HFOs in clinical recordings has been tested on a set of 36 intracranial electroencephalogram (EEG) channels in six patients, with 89% of the detections being validated by two independent reviewers. The results demonstrate that the unsupervised detection of HFOs based on their 2D features in time-frequency maps is feasible and has a performance comparable or better than the most used HFO detectors.

Illusory own body perceptions mapped in the cingulate cortex-An intracranial stimulation study

Body awareness is the result of sensory integration in the posterior parietal cortex; however, other brain structures are part of this process. Our goal is to determine how the cingulate cortex is involved in the representation of our body. We retrospectively selected patients with drug-resistant epilepsy, explored by stereo-electroencephalography, that had the cingulate cortex sampled outside the epileptogenic zone. The clinical effects of high-frequency electrical stimulation were reviewed and only those sites that elicited changes related to body perception were included. Connectivity of the cingulate cortex and other cortical structures was assessed using the h2 coefficient, following a nonlinear regression analysis of the broadband EEG signal. Poststimulation changes in connectivity were compared between two sets of stimulations eliciting or not eliciting symptoms related to body awareness (interest and control groups). We included 17 stimulations from 12 patients that reported different types of body perception changes such as sensation of being pushed toward right/left/up, one limb becoming heavier/lighter, illusory sensation of movement, sensation of pressure, sensation of floating or detachment of one hemi-body. High-frequency stimulation in the cingulate cortex (1 anterior, 15 middle, 1 posterior part) elicits body perception changes, associated with a decreased connectivity of the dominant posterior insula and increased coupling between other structures, located particularly in the nondominant hemisphere.

Functional mapping and effective connectivity of the human operculum

The operculum, defined as the cortex adjacent to the insula, is a large structure encompassing three lobes, with a recognized role in a variety of neurologic and psychiatric conditions. Its complex functions include sensory, motor, autonomic and cognitive processing. In humans, these are extended with the addition of language. These functions are implemented by highly specialized neuronal populations and their widespread connections, which our study aims at mapping in detail. We studied a group of 31 patients that were explored with intracranial electrodes during the pre-surgical workup for drug-resistant epilepsy. We have selected the subset of contacts implanted in non-epileptogenic opercular cortex and we analyzed the neurophysiological and behavioral responses to direct electrical stimulation. The functional mapping was performed by applying 1 Hz and 50 Hz electrical stimulation on 252 contact pairs and recording the threshold for evoking clinical effects. The effective connectivity was assessed using cortico-cortical evoked potentials elicited by single-pulse electrical stimulation in a subset of 19 patients. The locations of the effects grouped in twelve distinct semiological classes were analyzed. The most frequent effects evoked by stimulation of the frontal operculum were language related (29%). The Rolandic area produced most often oropharyngeal symptoms (47%), the parietal operculum produced somatosensory effects (67%), while the temporal evoked auditory (58%) semiology. The connectivity pattern was complex, with these structures having widespread ipsilateral and contralateral projections. The local connections between the opercular subregions and with the insula, as well as with more distant areas like the cingulate gyrus, were distinguished by strength and between-subjects consistency. In conclusion, we demonstrate specific opercular functionality, distinct from the one of the insular cortex. The study is complemented by a literature review on the opercular functional connectome in human and non-human primates.

Occipital Nerve Stimulation Attenuates Neuronal Firing Response to Mechanical Stimuli in the Ventral Posteromedial Thalamus of a Rodent Model of Chronic Migraine

BACKGROUND

Chronic migraine (CM) is a highly debilitating disease, and many patients remain refractory to medicinal therapy. Given the convergent nature of neuronal networks in the ventral posteromedial nucleus (VPM) and the evidence of sensitization of pain circuitry in this disease, we hypothesize CM rats will have increased VPM neuronal firing, which can be attenuated using occipital nerve stimulation (ONS).

OBJECTIVE

To determine whether VPM firing frequency differs between CM and sham rats, and whether ONS significantly alters firing rates during the application of mechanical stimuli.

METHODS

Fourteen male Sprague-Dawley rats were infused with inflammatory media once daily through an epidural cannula for 2 wk to induce a CM state. Sham animals (n = 6) underwent cannula surgery but received no inflammatory media. ONS electrodes were implanted bilaterally and single-unit recordings were performed in the VPM of anesthetized rats during mechanical stimulation of the face and forepaw in the presence and absence of ONS.

RESULTS

CM rats had significantly higher neuronal firing rates (P < .001) and bursting activity (P < .01) in response to mechanical stimuli when compared to shams. ONS significantly reduced neuronal firing in the VPM of CM rats during the application of 0.8 g (P = .04), 4.0 g (P = .04), and 15.0 g (P = .02) Von Frey filaments. ONS reduced bursting activity in CM rats during the 4.0 and 15 g filaments (P < .05). No significant changes in bursting activity or firing frequency were noted in sham animals during ONS.

CONCLUSION

We demonstrate that neuronal spike frequencies and bursting activity in the VPM are increased in an animal model of CM compared to shams. Our results suggest that the mechanism of ONS may involve attenuation of neurons in the VPM of CM rats during the application of mechanical stimuli.

DBStar: An Open-Source Tool Kit for Imaging Analysis with Patient-Customized Deep Brain Stimulation Platforms

BACKGROUND/OBJECTIVES

To create an open-source method for reconstructing microelectrode recording (MER) and deep brain stimulation (DBS) electrode coordinates along multiple parallel trajectories with patient-specific DBS implantation platforms to facilitate DBS research.

METHODS

We combined the surgical geometry (extracted from WayPoint Planner), pre-/intra-/postoperative computed tomography (CT) and/or magnetic resonance (MR) images, and integrated them into the Analysis of Functional NeuroImages (AFNI) neuroimaging analysis environment using functions written in Python. Electrode coordinates were calculated from image-based electrode surfaces and recording trajectory depth values. Coordinates were translated into appropriate trajectories, and were tested for proximity to patient-specific or atlas-based anatomical structures. Final DBS electrode coordinates for 3 patient populations (ventral intermediate nucleus [VIM], subthalamic nucleus [STN], and globus pallidus pars interna [GPi]) were calculated. For STN cases, MER site coordinates were then analyzed to see whether they were inside or outside the STN.

RESULTS

Final DBS electrode coordinates were described for VIM, STN, and GPi patient populations. 115/169 (68%) STN MER sites were within 1 mm of the STN in AFNI's Talairach and Tournoux (TT) atlas.

CONCLUSIONS

DBStar is a robust tool kit for understanding the anatomical location and context of electrode locations, and can easily be used for imaging, behavioral, or electrophysiological analyses.

How to record high-frequency oscillations in epilepsy: A practical guideline

OBJECTIVE

Technology for localizing epileptogenic brain regions plays a central role in surgical planning. Recent improvements in acquisition and electrode technology have revealed that high-frequency oscillations (HFOs) within the 80-500 Hz frequency range provide the neurophysiologist with new information about the extent of the epileptogenic tissue in addition to ictal and interictal lower frequency events. Nevertheless, two decades after their discovery there remain questions about HFOs as biomarkers of epileptogenic brain and there use in clinical practice.

METHODS

In this review, we provide practical, technical guidance for epileptologists and clinical researchers on recording, evaluation, and interpretation of ripples, fast ripples, and very high-frequency oscillations.

RESULTS

We emphasize the importance of low noise recording to minimize artifacts. HFO analysis, either visual or with automatic detection methods, of high fidelity recordings can still be challenging because of various artifacts including muscle, movement, and filtering. Magnetoencephalography and intracranial electroencephalography (iEEG) recordings are subject to the same artifacts.

SIGNIFICANCE

High-frequency oscillations are promising new biomarkers in epilepsy. This review provides interested researchers and clinicians with a review of current state of the art of recording and identification and potential challenges to clinical translation.

Presurgical Evaluation and Epilepsy Surgery in MRI Negative Resistant Epilepsy of Childhood with Good Outcome

Magnetic resonance imaging (MRI)-negative epilepsy may be successfully solved with a multidisciplinary approach using invasive recordings, image and signal analysis. The whole methodology used by the epilepsy surgery team is systematically described based on an resistant epilepsy case with all steps and rationale of choosing different investigation methods from surface electroencephalography (EEG) to invasive recordings. Due to negative MRI and non-concordant ictal surface EEG with clinical semiology, the patient was investigated with stereo- EEG (SEEG), aiming to delimitate epileptogenic and eloquent cerebral areas. Implantation strategy, seizures recordings, stimulation, resection planning using quantitative EEG analysis, and the surgery plan are presented. The patient has been seizure-free for 14 months so far, with improved behavior and daily life quality. Post-operative examination revealed focal cortical dysplasia type II B.

Intrusive Thoughts Elicited by Direct Electrical Stimulation during Stereo-Electroencephalography

Cortical direct electrical stimulation (DES) is a method of brain mapping used during invasive presurgical evaluation of patients with intractable epilepsy. Intellectual auras like intrusive thoughts, also known as forced thinking (FT), have been reported during frontal seizures. However, there are few reports on FT obtained during DES in frontal cortex. We report three cases in which we obtained intrusive thoughts while stimulating the dorsolateral prefrontal cortex and the white matter in the prefrontal region. In order to highlight the effective connectivity that might explain this clinical response, we have analyzed cortico-cortical potentials evoked by single pulse electrical stimulation.

Stimulation artifact correction method for estimation of early cortico-cortical evoked potentials

BACKGROUND

Effective connectivity can be explored using direct electrical stimulations in patients suffering from drug-resistant focal epilepsies and investigated with intracranial electrodes. Responses to brief electrical pulses mimic the physiological propagation of signals and manifest as cortico-cortical evoked potentials (CCEP). The first CCEP component is believed to reflect direct connectivity with the stimulated region but the stimulation artifact, a sharp deflection occurring during a few milliseconds, frequently contaminates it.

NEW METHOD

In order to recover the characteristics of early CCEP responses, we developed an artifact correction method based on electrical modeling of the electrode-tissue interface. The biophysically motivated artifact templates are then regressed out of the recorded data as in any classical template-matching removal artifact methods.

RESULTS

Our approach is able to make the distinction between the physiological responses time-locked to the stimulation pulses and the non-physiological component. We tested the correction on simulated CCEP data in order to quantify its efficiency for different stimulation and recording parameters. We demonstrated the efficiency of the new correction method on simulations of single trial recordings for early responses contaminated with the stimulation artifact. The results highlight the importance of sampling frequency for an accurate analysis of CCEP. We then applied the approach to experimental data.

COMPARISON WITH EXISTING METHOD

The model-based template removal was compared to a correction based on the subtraction of the averaged artifact.

CONCLUSIONS

This new correction method of stimulation artifact will enable investigators to better analyze early CCEP components and infer direct effective connectivity in future CCEP studies.

Multi-parametric analysis assists in STN localization in Parkinson's patients

BACKGROUND

Initial subthalamic nucleus (STN) localization is based on MRI and an anatomical atlas and then refined intraoperatively using electrophysiological mapping with microelectrode recordings (IOA - intraoperative multi-unit activity) during deep brain stimulation (DBS) in Parkinson's disease (PD). IOA is time consuming and subjective. The purpose of this study was to assess the value of high frequency multi-unit background activity (MUA, frequency >500Hz), and local field potentials (LFP, frequency 5-500Hz) in detection of the STN borders.

METHODS

This was a retrospective, single center study. 18 leads in ten PD patients that underwent STN DBS surgery were evaluated. IOA, MUA and LFP have been compared in detection of the STN. IOA using single train spikes analysis have been used as a gold standard.

RESULTS

Both LFP in beta range (20-35Hz) and MUA increased as the microelectrode entered the STN and their increase correlated with dorsal/ventral STN borders. The differences (mean±sd) were: between IOA and MUA of the dorsal/ventral border 0.20±0.76/0.28±0.30mm; between IOA and LFP of the dorsal/ventral border 0.08±0.94/0.05±0.53mm. Using Bland-Altman statistics, only 2/36 (5.6%) differences between IOA and MUA and also 2/36 differences between IOA and LFP (one for the dorsal border and one for the ventral border) were out of ±1.96 SD line of measurement differences. Correlation between dorsal border/ventral border positions obtained by IOA and MUA was 0.86, p<0.000005/0.97, p<10(-11); by IOA and LFP was 0.78, p<0.00015/0.88, p<0.000001.

CONCLUSIONS

Both MUA and LFP are characteristically elevated in the STN compared to neighboring structures. They may provide fast, real-time, objective and reliable markers of STN borders.

A personalized stereotactic fixture for implantation of depth electrodes in stereoelectroencephalography

BACKGROUND

The stereoelectroencephalographic (SEEG) implantation procedures still represent a challenge due to the intrinsic complexity of the method and the number of depth electrodes required.

OBJECTIVES

We aim at designing and evaluating the accuracy of a custom stereotactic fixture based on the StarFix™ technology (FHC Inc., Bowdoin, ME) that significantly simplifies and optimizes the implantation of depth electrodes used in presurgical evaluation of patients with drug-resistant epilepsy.

METHODS

Fiducial markers that also serve as anchors for the fixture are implanted into the patient's skull prior to surgery. A 3D fixture model is designed within the surgical planning software, with the planned trajectories incorporated in its design, aligned with the patient's anatomy. The stereotactic fixture is built using 3D laser sintering technology based on the computer-generated model. Bilateral rectangular grids of guide holes orthogonal to the midsagittal plane and centered on the midcommissural point are incorporated in the fixture design, allowing a wide selection of orthogonal trajectories. Up to two additional grids can be accommodated for targeting structures where oblique trajectories are required. The frame has no adjustable parts, this feature reducing the risk of inaccurate coordinate settings while simultaneously reducing procedure time significantly.

RESULTS

We have used the fixture for the implantation of depth electrodes for presurgical evaluation of 4 patients with drug-resistant focal epilepsy, with nearly 2-fold reduction in the duration of the implantation procedure. We have obtained a high accuracy with a submillimetric mean positioning error of 0.68 mm for the anchor bolts placed at the trajectory entry point and 1.64 mm at target.

CONCLUSIONS

The custom stereotactic fixture design greatly simplifies the planning procedure and significantly reduces the time in the operating room, while maintaining a high accuracy.

Effect of intraoperative subthalamic nucleus DBS on human single-unit activity in the ipsilateral and contralateral subthalamic nucleus

Object

Insight may be gained into the physiological mechanisms of deep brain stimulation (DBS) by analyzing local and contralateral subthalamic nucleus (STN) single-unit activity during activation of previously placed DBS electrodes. Special techniques are required to perform such analysis due to the presence of a large stimulus artifact. The purpose of this study was to determine the effects of DBS stimulation on single unit activity acquired from patients undergoing new or revised DBS placements.

Methods

Subthalamic nucleus single unit activity was acquired from awake patients during activation of a previously implanted STN DBS electrode. Stimulation was contralateral to the recording site in 4 cases and ipsilateral in 3. Data were acquired at stimulation frequencies of 30, 60, and 130 Hz and with other stimulation parameters at clinically effective settings. Cells were included if they showed kinesthetic activity before and after the stimulation paradigm and if their action potential morphology was maintained throughout the experiment. Analysis of single-unit activity acquired before, during, and after stimulation was performed employing a time-domain algorithm to overcome the stimulus artifact.

Results

Both ipsilateral and contralateral acute stimulation resulted in reversible STN firing rate suppression. The degree of suppression became greater as stimulus frequency increased and was significant at 60 Hz (t-test, p < 0.05) and 130 Hz (p < 0.01). Suppression with ipsilateral 130-Hz stimulation ranged between 52.8% and 99.8%, whereas with similar contralateral STN stimulation, the range was lower (1.9%–50.3%). Return to baseline activity levels typically occurred within seconds after stimulation ended.

Conclusions

Stimulation of the STN at clinically effective frequencies has an acute suppressive rather than an excitatory effect on STN single-unit activity. The effect is bilateral, even though the degree of suppression is greater on the ipsilateral than the contralateral STN. The authors' algorithm helps reveal this effect in human patients.

Localization of the subthalamic nucleus in Parkinson disease using multiunit activity

BACKGROUND

Refinement of the subthalamic nucleus (STN) coordinates using intraoperative microelectrode recordings (MER) is routinely performed during deep brain stimulation (DBS) surgeries in Parkinson disease (PD). The commonly used criteria for electrophysiological localization of the STN are qualitative. The goal of this study was to validate quantitative STN detection algorithm (QD) derived from the multi-unit activity in a prospective setting.

METHODS

Ten PD patients underwent STN DBS surgery. The MUA was obtained by removing large spikes close to microelectrode using wavelet method and integrating the 500-2000Hz band in the power spectral density. The qualitative intraoperative mapping of the STN using MER (IOM) versus QD was compared using Bland-Altman and Pearson's correlation analysis.

RESULTS

The clinical efficacy was confirmed in all subjects. The mean difference between IOM and QD of the dorsal/ventral border was 0.31±0.84/0.44±0.47mm. Using Bland-Altman statistic, only 2/36 (5.6%) differences (one for the dorsal border and one for the ventral border) were out of ±2 sd line of measurement differences. Correlation between dorsal border/ventral border positions obtained by IOM and QD was 0.79, p<0.0001/0.91, p<0.0001.

CONCLUSION

Both methods are in reasonable agreement and are strongly correlated. The QD gives objective coordinates of the STN borders at high precision and may be more accurate than IOM. Prospective blinded comparative studies where the DBS leads will be placed using either QD or IOM are warranted.

Modulation of Visual Responses in Macaque Frontal Eye Field during Covert Tracking of Invisible Targets

The purpose of this study was to investigate the interaction between internal representations of invisible moving targets and visual responses of neurons in frontal eye fields (FEFs). Monkeys were trained to make saccades to the extrapolated position of a target that was temporarily rendered invisible for variable durations as if it had passed behind an occluder. Flashed, task-irrelevant visual probe stimuli were used to study the visual responsiveness of FEF neurons during this task. Probes were flashed at various times and locations during the occlusion interval. Net changes in neuronal activity were obtained by comparing the activity on trials with probes with randomly interleaved trials without any probe. Most neurons showed an increase in firing rate in response to the probe, but some showed a decrease. Both types of responses were enhanced when the invisible target moved toward the receptive field (RF) as compared with trials on which the target moved away from the RF. Some neurons showed a spatial shift in the visual response during the occlusion interval. For cells that were excited by the probe, the shift tended to be correlated with the direction of motion of the target, whereas for cells that were inhibited the shift tended to be in the opposite direction. These results suggest that the role of FEF in predicting invisible target motion includes a sensory/perceptual component.

A flashing line can warp your mind

Keeping pace with a constantly changing world requires the ability to make predictions about the future on a variety of timescales. A very basic example of this is the ability to predict the future location of a moving object in the brief time that it takes to perceive and respond to that object. In this issue of Neuron, experiments by Sundberg, Fallah, and Reynolds reveal a potential neural substrate for making short-range predictions about motion in visual area V4.

Radial motion bias in macaque frontal eye field

The visual responsiveness and spatial tuning of frontal eye field (FEF) neurons were determined using a delayed memory saccade task. Neurons with visual responses were then tested for direction selectivity using moving random dot patterns centered in the visual receptive field. The preferred axis of motion showed a significant tendency to be aligned with the receptive-field location so as to favor motion toward or away from the center of gaze. Centrifugal (outward) motion was preferred over centripetal motion. Motion-sensitive neurons in FEF thus appear to have a direction bias at the population level. This bias may facilitate the detection or discrimination of expanding optic flow patterns. The direction bias is similar to that seen in visual area MT and in posterior parietal cortex, from which FEF receives afferent projections. The outward motion bias may explain asymmetries in saccades made to moving targets. A representation of optic flow in FEF might be useful for planning eye movements during navigation.

Effects of electrical microstimulation in monkey frontal eye field on saccades to remembered targets

Spatially selective delay activity in the frontal eye field (FEF) is hypothesized to be part of a mechanism for the transformation of visual signals into instructions for voluntary movements. To understand the linkage between FEF activity and eye movement planning, we recorded neuronal responses of FEF neurons while monkeys performed a memory-saccade task. We then electrically stimulated the same sites during the memory-delay epoch of the task. The stimulation currents used were subthreshold for evoking saccades during a gap-fixation task. Microstimulation resulted in changes in the spatial and temporal components of saccade parameters: an increase in latency, and a shift in amplitude and direction. We performed a vector analysis to determine the relative influence of the visual cue and electrical stimulus on the memory-saccade. In general, the memory-saccade was strongly weighted toward the visual cue direction, yet the electrical stimulus introduced a consistent bias away from the receptive/movement field of the stimulation site. The effects of sub-threshold stimulation were consistent with a combination of vector subtraction and averaging, but not with vector summation. Vector subtraction may play a role in spatial updating of movement plans for memory-guided saccades when eye position changes during the memory period.

Microstimulation of the dorsolateral prefrontal cortex biases saccade target selection

A long-standing issue concerning the executive function of the primate dorsolateral prefrontal cortex is how the activity of prefrontal neurons is linked to behavioral response selection. To establish a functional relationship between prefrontal memory fields and saccade target selection, we trained three macaque monkeys to make saccades to the remembered location of a visual cue in a delayed spatial match-to-sample saccade task. We electrically stimulated sites in the prefrontal cortex with subthreshold currents during the delay epoch while monkeys performed this task. Our results show that the artificially injected signal interacts with the neural activity responsible for target selection, biasing saccade choices either towards the receptive/movement field (RF/MF) or away from the RF/MF, depending on the stimulation site. These findings might reflect a functional link between prefrontal signals responsible for the selection bias by modulating the balance between excitation and inhibition in the competitive interactions underlying behavioral selection.

Modification of saccades evoked by stimulation of frontal eye field during invisible target tracking

We investigated the internal representation of invisible moving targets using electrical microstimulation in the prefrontal cortex. Monkeys were trained to make saccades to the extrapolated position of a small moving target that was rendered invisible during part of its trajectory. Although the target was invisible, involuntary saccades were evoked by electrical microstimulation of the frontal eye field. Stimulation was applied at different times relative to the disappearance of the target while the monkey fixated. When stimulation was applied immediately after target disappearance, electrically evoked saccades were biased toward the starting point of the target trajectory. When stimulation was applied later in the trial, evoked saccades were biased toward the end of the trajectory. The bias in evoked saccade direction changed continuously over time. The magnitude and statistical significance of the electrically evoked saccade deviation depended on the accuracy of the monkeys' voluntary saccades relative to the invisible target. The results suggest that covert tracking is accompanied by a continuously shifting saccade plan that moves along the target path.

Comparison of performance on memory-guided saccade and delayed spatial match-to-sample tasks in monkeys

To investigate the sources of spatial error in memory-guided saccades (MGS), we have trained monkeys on two different tasks: a MGS task and a delayed spatial match-to-sample (MTS) task. We first tested the effect of introducing a post-saccadic visual feedback on the accuracy of MGS. We found that visual feedback had a pronounced effect on the systematic saccade error, but less of an effect on the variable error. Visual feedback can improve the accuracy of saccadic eye movements over several days, while feedback removal can decrease accuracy in a reversible way. These effects also depend both on target eccentricity and the duration of the memory delay. To test whether saccade error is due to the accuracy of spatial memory storage or arises downstream from that memory, we measured behavioral performance on a spatial MTS task both before and after training with visual feedback. The results showed no significant difference in performance of the MTS task before and after feedback training despite significant changes in MGS accuracy. The results suggest that the accuracy of spatial memory is not the source of the systematic errors that accompany MGS.

Estimating invisible target speed from neuronal activity in monkey frontal eye field

Working memory involves transient storage of information and the ability to manipulate that information for short-range planning and prediction. The computational aspect of working memory can be probed using dynamic sensorimotor behavior requiring complex stimulus-response mappings. Such a transformation occurs when extrapolating the future location of a moving target that is rendered temporarily invisible. Estimating the trajectory of an invisible moving target requires encoding and storing several target features, including the direction and speed of motion. We trained monkeys to make saccades to the estimated position of invisible targets moving at various speeds. The activity of neurons in the frontal eye field (FEF) was consistently modulated according to the speed of target motion. A reconstruction algorithm showed that estimates of target speed based on FEF activity were similar to behavioral speed estimates. FEF may therefore be involved in updating an internal representation of target trajectory for predictive saccades.

On the gap effect for saccades evoked by electrical microstimulation of frontal eye fields in monkeys

To investigate the mechanisms of fixation disengagement and saccade initiation, we electrically stimulated the macaque frontal eye fields (FEF) while monkeys performed a visual fixation task. We tested the effect of introducing a temporal gap between fixation target offset and the onset of the electrical stimulus. We found that the duration of the gap had a pronounced effect on the probability of producing electrically evoked saccades at a given current level. The highest probability was found for gaps of 200 ms duration. There were also effects of gap duration on saccade latency and amplitude for most of the stimulation sites. The increase in saccade probability may be associated with lower current thresholds for evoking saccades.