Electrical Stimulation Mapping of the Brain: Basic Principles and Emerging Alternatives

Can the knight capture the queen? The role of supramarginal gyrus in chess rule-retrieval as evidenced by a novel combined awake brain mapping and fMRI protocol

Brain tumours represent a burden for society, not only due to the risks they entail but also because of the possibility of losing relevant cognitive functions for the patient's life after their resection. In the present study, we report how we monitored chess performance through a multimodal Electrical Stimulation Mapping (ESM) - functional Magnetic Resonance Imaging (fMRI) combined protocol. The ESM was performed under a left parietal lobe tumour resection surgery on a patient that expressed the desire to preserve his chess playing ability post-operative. We designed an ad-hoc protocol to evaluate processes involved in chess performance that could be potentially affected by the tumour location: (i) visual search, (ii) rule-retrieval, and (iii) anticipation of checkmate. The fMRI study reported functional regions for chess performance, some of them proximal to the lesion in the left parietal lobe. The most relevant result was a positive eloquent point encountered in the vicinity of the left supramarginal gyrus while performing the rule-retrieval task in the ESM. This functional region was convergent with the activations observed in the pre-operative fMRI study for this condition. The behavioural assessment comparison revealed post-operative an increase in reaction time in some tasks but correctness in performance was maintained. Finally, the patient maintained the ability to play chess after the surgery. Our results provide a plausible protocol for future interventions and suggest a role of the left supramarginal gyrus in chess cognitive operations for the case presented.

Timing and location of speech errors induced by direct cortical stimulation

Cortical regions supporting speech production are commonly established using neuroimaging techniques in both research and clinical settings. However, for neurosurgical purposes, structural function is routinely mapped peri-operatively using direct electrocortical stimulation. While this method is the gold standard for identification of eloquent cortical regions to preserve in neurosurgical patients, there is lack of specificity of the actual underlying cognitive processes being interrupted. To address this, we propose mapping the temporal dynamics of speech arrest across peri-sylvian cortices by quantifying the latency between stimulation and speech deficits. In doing so, we are able to substantiate hypotheses about distinct region-specific functional roles (e.g. planning versus motor execution). In this retrospective observational study, we analysed 20 patients (12 female; age range 14-43) with refractory epilepsy who underwent continuous extra-operative intracranial EEG monitoring of an automatic speech task during clinical bedside language mapping. Latency to speech arrest was calculated as time from stimulation onset to speech arrest onset, controlling for individual speech rate. Most instances of motor-based arrest (87.5% of 96 instances) were in sensorimotor cortex with mid-range latencies to speech arrest with a distributional peak at 0.47 s. Speech arrest occurred in numerous regions, with relatively short latencies in supramarginal gyrus (0.46 s), superior temporal gyrus (0.51 s) and middle temporal gyrus (0.54 s), followed by relatively long latencies in sensorimotor cortex (0.72 s) and especially long latencies in inferior frontal gyrus (0.95 s). Non-parametric testing for speech arrest revealed that region predicted latency; latencies in supramarginal gyrus and in superior temporal gyrus were shorter than in sensorimotor cortex and in inferior frontal gyrus. Sensorimotor cortex is primarily responsible for motor-based arrest. Latencies to speech arrest in supramarginal gyrus and superior temporal gyrus (and to a lesser extent middle temporal gyrus) align with latencies to motor-based arrest in sensorimotor cortex. This pattern of relatively quick cessation of speech suggests that stimulating these regions interferes with the outgoing motor execution. In contrast, the latencies to speech arrest in inferior frontal gyrus and in ventral regions of sensorimotor cortex were significantly longer than those in temporoparietal regions. Longer latencies in the more frontal areas (including inferior frontal gyrus and ventral areas of precentral gyrus and postcentral gyrus) suggest that stimulating these areas interrupts a higher-level speech production process involved in planning. These results implicate the ventral specialization of sensorimotor cortex (including both precentral and postcentral gyri) for speech planning above and beyond motor execution.

Functional Mapping of Movement and Speech Using Task-Based Electrophysiological Changes in Stereoelectroencephalography

Introduction

Stereoelectroencephalography (sEEG) has become the predominant method for intracranial seizure localization. When imaging, semiology, and scalp EEG are not in full agreement or definitively localizing, implanted sEEG recordings are used to test candidate seizure onset zones (SOZs). Discovered SOZs may then be targeted for resection, laser ablation, or neurostimulation. If a SOZ is eloquent, resection and ablation are both contraindicated, so identifying functional representation is crucial for therapeutic decision making.

Objective

We present a novel functional brain mapping technique that utilizes task-based electrophysiological changes in sEEG during behavioral tasks and test this in pediatric and adult patients.

Methods

sEEG was recorded in twenty patients with epilepsy, aged 6-39 (12 female, 18 of 20 patients < 21 years old), who underwent implanted monitoring to identify seizure onset. Each performed 1) visually cued simple repetitive movements of the hand, foot, or tongue while electromyography was recorded, and 2) simple picture naming or verb generation speech tasks while audio was recorded. Broadband changes in the power spectrum of the sEEG were compared between behavior and rest.

Results

Electrophysiological functional mapping of movement and/or speech areas was completed in all 20 patients. Eloquent representation was identified in both cortex and white matter, and generally corresponded to classically described functional anatomic organization as well as other clinical mapping results. Robust maps of brain activity were identified in healthy brain, regions of developmental or acquired structural abnormality, and SOZs.

Conclusion

Task based electrophysiological mapping using broadband changes in the sEEG signal reliably identifies movement and speech representation in pediatric and adult epilepsy patients.

How to assess the accuracy of volume conduction models? A validation study with stereotactic EEG data

Introduction

Volume conduction models of the human head are used in various neuroscience fields, such as for source reconstruction in EEG and MEG, and for modeling the effects of brain stimulation. Numerous studies have quantified the accuracy and sensitivity of volume conduction models by analyzing the effects of the geometrical and electrical features of the head model, the sensor model, the source model, and the numerical method. Most studies are based on simulations as it is hard to obtain sufficiently detailed measurements to compare to models. The recording of stereotactic EEG during electric stimulation mapping provides an opportunity for such empirical validation.

Methods

In the study presented here, we used the potential distribution of volume-conducted artifacts that are due to cortical stimulation to evaluate the accuracy of finite element method (FEM) volume conduction models. We adopted a widely used strategy for numerical comparison, i.e., we fixed the geometrical description of the head model and the mathematical method to perform simulations, and we gradually altered the head models, by increasing the level of detail of the conductivity profile. We compared the simulated potentials at different levels of refinement with the measured potentials in three epilepsy patients.

Results

Our results show that increasing the level of detail of the volume conduction head model only marginally improves the accuracy of the simulated potentials when compared to in-vivo sEEG measurements. The mismatch between measured and simulated potentials is, throughout all patients and models, maximally 40 microvolts (i.e., 10% relative error) in 80% of the stimulation-recording combination pairs and it is modulated by the distance between recording and stimulating electrodes.

Discussion

Our study suggests that commonly used strategies used to validate volume conduction models based solely on simulations might give an overly optimistic idea about volume conduction model accuracy. We recommend more empirical validations to be performed to identify those factors in volume conduction models that have the highest impact on the accuracy of simulated potentials. We share the dataset to allow researchers to further investigate the mismatch between measurements and FEM models and to contribute to improving volume conduction models.

Timing and location of speech errors induced by direct cortical stimulation

Cortical regions supporting speech production are commonly established using neuroimaging techniques in both research and clinical settings. However, for neurosurgical purposes, structural function is routinely mapped peri-operatively using direct electrocortical stimulation. While this method is the gold standard for identification of eloquent cortical regions to preserve in neurosurgical patients, there is lack of specificity of the actual underlying cognitive processes being interrupted. To address this, we propose mapping the temporal dynamics of speech arrest across peri-sylvian cortices by quantifying the latency between stimulation and speech deficits. In doing so, we are able to substantiate hypotheses about distinct region-specific functional roles (e.g., planning versus motor execution). In this retrospective observational study, we analyzed 20 patients (12 female; age range 14-43) with refractory epilepsy who underwent continuous extra-operative intracranial EEG monitoring of an automatic speech task during clinical bedside language mapping. Latency to speech arrest was calculated as time from stimulation onset to speech arrest onset, controlling for individual speech rate. Most instances of motor-based arrest (87.5% of 96 instances) were in sensorimotor cortex with mid-range latencies to speech arrest with a distributional peak at 0.47 seconds. Speech arrest occurred in numerous regions, with relatively short latencies in supramarginal gyrus (0.46 seconds), superior temporal gyrus (0.51 seconds), and middle temporal gyrus (0.54 seconds), followed by relatively long latencies in sensorimotor cortex (0.72 seconds) and especially long latencies in inferior frontal gyrus (0.95 seconds). Nonparametric testing for speech arrest revealed that region predicted latency; latencies in supramarginal gyrus and in superior temporal gyrus were shorter than in sensorimotor cortex and in inferior frontal gyrus. Sensorimotor cortex is primarily responsible for motor-based arrest. Latencies to speech arrest in supramarginal gyrus and superior temporal gyrus (and to a lesser extent middle temporal gyrus) align with latencies to motor-based arrest in sensorimotor cortex. This pattern of relatively quick cessation of speech suggests that stimulating these regions interferes with the outgoing motor execution. In contrast, the latencies to speech arrest in inferior frontal gyrus and in ventral regions of sensorimotor cortex were significantly longer than those in temporoparietal regions. Longer latencies in the more frontal areas (including inferior frontal gyrus and ventral areas of precentral gyrus and postcentral gyrus) suggest that stimulating these areas interrupts a higher-level speech production process involved in planning. These results implicate the ventral specialization of sensorimotor cortex (including both precentral and postcentral gyri) for speech planning above and beyond motor execution.

Rapid Passive Gamma Mapping as an Adjunct to Electrical Stimulation Mapping for Functional Localization in Resection of Primary Brain Neoplasms

OBJECTIVE

We examined the utility of passive high gamma mapping (HGM) as an adjunct to conventional awake brain mapping during glioma resection. We compared functional and survival outcomes before and after implementing intraoperative HGM.

METHODS

This was a retrospective cohort study of 75 patients who underwent a first-time, awake craniotomy for glioma resection. Patients were stratified by whether their operation occurred before or after the implementation of a U.S. Food and Drug Administration-approved high-gamma mapping tool in July 2017.

RESULTS

The preimplementation and postimplementation cohorts included 28 and 47 patients, respectively. Median intraoperative time (261 vs. 261 minutes, P = 0.250) and extent of resection (97.14% vs. 98.19%, P = 0.481) were comparable between cohorts. Median Karnofsky performance status at initial follow-up was similar between cohorts (P = 0.650). Multivariable Cox regression models demonstrated an adjusted hazard ratio for overall survival of 0.10 (95% confidence interval: 0.02-0.43, P = 0.002) for the postimplementation cohort relative to the preimplementation cohort. Progression-free survival adjusted for insular involvement showed an adjusted hazard ratio of 1.00 (95% confidence interval: 0.49-2.06, P = 0.999) following HGM implementation. Falling short of statistical significance, prevalence of intraoperative seizures and/or afterdischarges decreased after HGM implementation as well (12.7% vs. 25%, P = 0.150).

CONCLUSIONS

Our results tentatively indicate that passive HGM is a safe and potentially useful adjunct to electrical stimulation mapping for awake cortical mapping, conferring at least comparable functional and survival outcomes with a nonsignificant lower rate of intraoperative epileptiform events. Considering the limitations of our study design and patient cohort, further investigation is needed to better identify optimal use cases for HGM.

Human brain mapping using co-registered fUS, fMRI and ESM during awake brain surgeries: A proof-of-concept study

Accurate, depth-resolved functional imaging is key in both understanding and treatment of the human brain. A new sonography-based imaging technique named functional Ultrasound (fUS) uniquely combines high sensitivity with submillimeter-subsecond spatiotemporal resolution available in large fields-of-view. In this proof-of-concept study we show that: (A) fUS reveals the same eloquent regions as found by fMRI while concomitantly visualizing in-vivo microvascular morphology underlying these functional hemodynamics and (B) fUS-based functional maps are confirmed by Electrocortical Stimulation Mapping (ESM), the current gold-standard in awake neurosurgical practice. This unique cross-modality experiment was performed using motor, visual and language-related functional tasks in patients undergoing awake brain tumor resection. The current work serves as an important milestone towards further maturity of fUS as well as a novel avenue to increase our understanding of hemodynamics-based functional brain imaging.

Intraoperative Augmented Reality Fiber Tractography for Primary Motor Area Glioma Resection

The implementation of intraoperative augmented reality fiber tractography (iAR-FT) into the surgical workflow for high-grade supratentorial gliomas has been shown to be effective and safe in maximizing the extent of resection and progression-free survival through the surgeon's enhanced 3-dimensional awareness of the spatial localization of fiber tracts.1-3 Primary motor area tumors present special challenges due to the high eloquence of the precentral gyrus and risk of postoperative onset or worsening of motor deficits, as well as limited postoperative plasticity.4 Although essential, electrical stimulation mapping (ESM) techniques have a number of limitations with respect to primary motor pathways, including a higher risk of intraoperative stimulation-evoked seizures, a risk of false negatives in the presence of preoperative deficits, a nonnegligible risk of permanent deterioration even in the presence of negative stimulation maps, and, most importantly, limited spatial resolution.4-8 The rationale for integrating ESM and iAR-FT is to compensate for the limitations of the former in terms of morphologic and spatial representation of fiber tracts. The benefits of coupling iAR-FT with ESM techniques allow for continuous integrated anatomical-functional feedback during surgery. In Video 1 we describe the key technical aspects and benefits of iAR-FT-assisted surgery for maximal safe gross total resection of a primary motor area grade IV astrocytoma.

Subject-Specific Automatic Reconstruction of White Matter Tracts

MRI-based tractography is still underexploited and unsuited for routine use in brain tumor surgery due to heterogeneity of methods and functional-anatomical definitions and above all, the lack of a turn-key system. Standardization of methods is therefore desirable, whereby an objective and reliable approach is a prerequisite before the results of any automated procedure can subsequently be validated and used in neurosurgical practice. In this work, we evaluated these preliminary but necessary steps in healthy volunteers. Specifically, we evaluated the robustness and reliability (i.e., test-retest reproducibility) of tractography results of six clinically relevant white matter tracts by using healthy volunteer data (N = 136) from the Human Connectome Project consortium. A deep learning convolutional network-based approach was used for individualized segmentation of regions of interest, combined with an evidence-based tractography protocol and appropriate post-tractography filtering. Robustness was evaluated by estimating the consistency of tractography probability maps, i.e., averaged tractograms in normalized space, through the use of a hold-out cross-validation approach. No major outliers were found, indicating a high robustness of the tractography results. Reliability was evaluated at the individual level. First by examining the overlap of tractograms that resulted from repeatedly processed identical MRI scans (N = 10, 10 iterations) to establish an upper limit of reliability of the pipeline. Second, by examining the overlap for subjects that were scanned twice at different time points (N = 40). Both analyses indicated high reliability, with the second analysis showing a reliability near the upper limit. The robust and reliable subject-specific generation of white matter tracts in healthy subjects holds promise for future validation of our pipeline in a clinical population and subsequent implementation in brain tumor surgery.

Can we improve electrocorticography using a circular grid array in brain tumor surgery?

Intraoperative electrocorticography (iECoG) is used as an adjunct to localize the epileptogenic zone during surgical resection of brain tumors in patients with focal epilepsies. It also enables monitoring of after-discharges and seizures with EEG during functional brain mapping with electrical stimulation. When seizures or after-discharges are present, they complicate accurate interpretation of the mapping strategy to outline the brain's eloquent function and can affect the surgical procedure. Recurrent seizures during surgery requires urgent treatment and, when occurring during awake craniotomy, often leads to premature termination of brain mapping due to post-ictal confusion or sedation from acute rescue therapy. There are mixed results in studies on efficacy with iECoG in patients with epilepsy and brain tumors influencing survival and functional outcomes following surgery. Commercially available electrode arrays have inherent limitations. These could be improved with customization potentially leading to greater precision in safe and maximal resection of brain tumors. Few studies have assessed customized electrode grid designs as an alternative to commercially available products. Higher density electrode grids with intercontact distances less than 1 cm improve spatial delineation of electrophysiologic sources, including epileptiform activity, electrographic seizures, and afterdischarges on iECoG during functional brain mapping. In response to the shortcomings of current iECoG grid technologies, we designed and developed a novel higher-density hollow circular electrode grid array. The 360-degree iECoG monitoring capability allows continuous EEG recording during surgical intervention through the aperture with and without electrical stimulation mapping. Compared with linear strip electrodes that are commonly used for iECoG during surgery, the circular grid demonstrates significant benefits in brain tumor surgery. This includes quicker recovery of post-operative motor deficits (2.4 days versus 9 days, p = 0.05), more extensive tumor resection (92.0% versus 77.6%, p = 0.003), lesser reduction in Karnofsky Performance scale postoperatively (-2 versus -11.6, p = 0.007), and more sensitivity to recording afterdischarges. In this narrative review, we discuss the advantages and disadvantages of commercially available recording devices in the operating room and focus on the usefulness of the higher-density circular grid.

Associated factors with stimulation induced seizures and the relevance with surgical outcomes

OBJECTIVE

To analyze the associated factors with stimulation-induced seizures (SIS) and the relevant factors in predicting surgical outcomes.

METHODS

We analyzed 80 consecutive epilepsy patients explored by stereo-electroencephalography with routine electrical stimulation mapping (ESM). If seizures induced by ESM, patients were classified as SIS-positive (SIS-P); otherwise, SIS-negative (SIS-N). Patients received radical surgery were further classified as favorable (Engel I) and unfavorable (Engel II-IV) groups.

RESULTS

Of the 80 patients included, we identified 44 (55.0%) and 36(45.0%) patients in the SIS-P and SIS-N groups, respectively. Multivariate analysis revealed that the seizure onset pattern (SOP) of preceding repetitive epileptiform discharges following LVFA (PRED→LVFA) (OR 3.319, 95% CI 1.200-9.183, P = 0.021) and pathology of focal cortical dysplasia (FCD) type II (OR 3.943, 95% CI 1.093-14.226, P = 0.036) were independent factors influencing whether the electrical stimulation can induce a seizure. Among the patients received radical surgery, there were 55 and 15 patients in the favorable and unfavorable groups separately. Multivariate analysis revealed that the SOP of PRED→LVFA induced seizures by stimulation (OR 11.409, 95% CI 1.182-110.161, P = 0.035) and bilateral implantation (OR 0.048, 95% CI 0.005-0.497, P = 0.011) were independent factors affecting surgical outcomes. The previous epilepsy surgery had a trend to be a negative factor with SIS (OR 0.156, 95% CI 0.028-0.880, P = 0.035) and surgical outcomes (OR 0.253, 95% CI 0.053-1.219, P = 0.087).

CONCLUSION

ESM is a highly valuable method for localizing the seizure onset zone. The SOP of PRED→LVFA and FCD type II were associated with elicitation of SIS by ESM, whereas a previous epilepsy surgery showed a negative association. Furthermore, the SOP of PRED→LVFA together with SIS in the same patient predicted favorable surgical outcomes, whereas bilateral electrode implantation predicted unfavorable outcomes.

Understanding the neural bases of bodily self-consciousness: recent achievements and main challenges

The last two decades have seen a surge of interest in the mechanisms underpinning bodily self-consciousness (BSC). Studies showed that BSC relies on several bodily experiences (i.e., self-location, body ownership, agency, first-person perspective) and multisensory integration. The aim of this literature review is to summarize new insights and novel developments into the understanding of the neural bases of BSC, such as the contribution of the interoceptive signals to the neural mechanisms of BSC, and the overlap with the neural bases of conscious experience in general and of higher-level forms of self (i.e., the cognitive self). We also identify the main challenges and propose future perspectives that need to be conducted to progress into the understanding of the neural mechanisms of BSC. In particular, we point the lack of crosstalk and cross-fertilization between subdisciplines of integrative neuroscience to better understand BSC, especially the lack of research in animal models to decipher the neural networks and systems of neurotransmitters underpinning BSC. We highlight the need for more causal evidence that specific brain areas are instrumental in generating BSC and the need for studies tapping into interindividual differences in the phenomenal experience of BSC and their underlying mechanisms.

Modeling the Impact of Electrode/Tissue Geometry on Electrical Stimulation in Stereo-EEG

PURPOSE

Electrical stimulation through depth electrodes is used to map function and seizure onset during stereoelectroencephalography in patients undergoing evaluation for epilepsy surgery. Factors such as electrode design, location, and orientation are expected to impact effects of electrical stimulation.

METHODS

We developed a steady-state finite element model of brain tissue including five layers (skull through white matter) and an implanted electrode to explore the impact of electrode design and placement on the activation of brain tissue by electrical stimulation. We calculated electric potentials, current densities, and volume of tissue activated ( Volact ) in response to constant current bipolar stimulation. We modeled two depth electrode designs (3.5- and 4.43-mm intercontact spacing) and varied electrode location and orientation.

RESULTS

The electrode with greater intercontact spacing produced 8% to 23% larger Volact (1% to 16% considering only gray matter). Vertical displacement of the electrodes by half intercontact space increased Volact for upward displacement (6% to 83% for all brain tissue or -5% to 96% gray matter only) and decreased Volact (1% to 16% or 24% to 49% gray matter only) for downward displacement. Rotating the electrode in the tissue by 30° to 60° with respect to the vertical axis increased Volact by 30% to 90% (20%-48% gray matter only).

CONCLUSIONS

Location and orientation of depth electrodes with respect to surrounding brain tissue have a large impact on the amount of tissue activated during electrical stimulation mapping in stereoelectroencephalography. Electrode design has an impact, although modest for commonly used designs. Individualization of stimulation intensity at each location remains critical, especially for avoiding false-negative results.

BOLD fMRI and DTI fiber tracking for preoperative mapping of eloquent cerebral regions in brain tumor patients: impact on surgical approach and outcome

PURPOSE

Task-based BOLD fMRI and DTI-fiber tracking have become part of the routine presurgical work-up of brain tumor patients in many institutions. However, their potential impact on both surgical treatment and neurologic outcome remains unclear, in despite of the high costs and complex implementation.

METHODS

We retrospectively investigated whether performing fMRI and DTI-ft preoperatively substantially impacted surgical planning and patient outcome in a series of brain tumor patients. We assessed (i) the quality of fMRI and DTI-ft results, by using a scale of 0-2 (0 = failed mapping; 1 = intermediate confidence; 2 = good confidence), (ii) whether functional planning substantially contributed to defining the surgical strategy to be undertaken (i.e., no surgery, biopsy, or resection, with or without ESM), the surgical entry point and extent of resection, and (iii) the incidence of neurological deficits post-operatively.

RESULTS

Twenty-seven patients constituted the study population. The mean confidence rating was 1.9/2 for fMRI localization of the eloquent cortex and lateralization of the language function and 1.7/2 for DTI-ft results. Treatment strategy was altered in 33% (9/27) of cases. Surgical entry point was modified in 8% (2/25) of cases. The extent of resection was modified in 40% (10/25). One patient (1/25, 4%) developed one new functional deficit post-operatively.

CONCLUSION

Functional MR mapping - which must not be considered an alternative to ESM - has a critical role preoperatively, potentially modifying treatment strategy or increasing the neurosurgeons' confidence in the surgical approach hypothesized based on conventional imaging.

Functional imaging of the exposed brain

When the brain is exposed, such as after a craniotomy in neurosurgical procedures, we are provided with the unique opportunity for real-time imaging of brain functionality. Real-time functional maps of the exposed brain are vital to ensuring safe and effective navigation during these neurosurgical procedures. However, current neurosurgical practice has yet to fully harness this potential as it pre-dominantly relies on inherently limited techniques such as electrical stimulation to provide functional feedback to guide surgical decision-making. A wealth of especially experimental imaging techniques show unique potential to improve intra-operative decision-making and neurosurgical safety, and as an added bonus, improve our fundamental neuroscientific understanding of human brain function. In this review we compare and contrast close to twenty candidate imaging techniques based on their underlying biological substrate, technical characteristics and ability to meet clinical constraints such as compatibility with surgical workflow. Our review gives insight into the interplay between technical parameters such sampling method, data rate and a technique's real-time imaging potential in the operating room. By the end of the review, the reader will understand why new, real-time volumetric imaging techniques such as functional Ultrasound (fUS) and functional Photoacoustic Computed Tomography (fPACT) hold great clinical potential for procedures in especially highly eloquent areas, despite the higher data rates involved. Finally, we will highlight the neuroscientific perspective on the exposed brain. While different neurosurgical procedures ask for different functional maps to navigate surgical territories, neuroscience potentially benefits from all these maps. In the surgical context we can uniquely combine healthy volunteer studies, lesion studies and even reversible lesion studies in in the same individual. Ultimately, individual cases will build a greater understanding of human brain function in general, which in turn will improve neurosurgeons' future navigational efforts.

Stimulation-Evoked Effective Connectivity (SEEC): An in-vivo approach for defining mesoscale corticocortical connectivity

BACKGROUND

Cortical electrical stimulation is a versatile technique for examining the structure and function of cortical regions and for implementing novel therapies. While electrical stimulation has been used to examine the local spread of neural activity, it may also enable longitudinal examination of mesoscale interregional connectivity.

NEW METHOD

Here, we sought to use intracortical microstimulation (ICMS) in conjunction with recordings of multi-unit action potentials to assess the mesoscale effective connectivity within sensorimotor cortex. Neural recordings were made from multielectrode arrays placed into sensory, motor, and premotor regions during surgical experiments in three squirrel monkeys. During each recording, single-pulse ICMS was repeatably delivered to a single region. Mesoscale effective connectivity was calculated from ICMS-evoked changes in multi-unit firing.

RESULTS

Multi-unit action potentials were able to be detected on the order of 1 ms after each ICMS pulse. Across sensorimotor regions, short-latency (< 2.5 ms) ICMS-evoked neural activity strongly correlated with known anatomical connections. Additionally, ICMS-evoked responses remained stable across the experimental period, despite small changes in electrode locations and anesthetic state.

COMPARISON WITH EXISTING METHODS

Previous imaging studies investigating cross-regional responses to stimulation are limited to utilizing indirect hemodynamic responses and thus lack the temporal specificity of ICMS-evoked responses.

CONCLUSIONS

These results show that monitoring ICMS-evoked neural activity, in a technique we refer to as Stimulation-Evoked Effective Connectivity (SEEC), is a viable way to longitudinally assess effective connectivity, enabling studies comparing the time course of connectivity changes with the time course of changes in behavioral function.

Concordance of Lateralization Index for Brain Asymmetry Applied to Identify a Reliable Language Task

How can we determine which language task is relevant for examining functional hemispheric asymmetry? A problem in measuring brain asymmetry using functional magnetic resonance imaging lies in the uncertain reliability of the computed index regarding the “true” asymmetry degree. Strictly speaking, the results from the Wada test or direct cortical stimulation cannot be an exact “ground truth”, specifically for the degree of asymmetry. Therefore, we developed a method to evaluate task performance using reproducibility independent of the phenomenon of functional lateralization. Kendall’s coefficient of concordance (W) was used as the statistical measure. The underlying idea was that although various algorithms to compute the lateralization index show considerably different index values for the same data, a superior language task would reproduce similar individual ranking sequences across the algorithms; the high reproducibility of rankings across various index types would indicate a reliable task to investigate functional asymmetry regardless of index computation algorithms. Consequently, we found specificity for brain locations; a verb-generation task demonstrated the highest concordance across index types along with sufficiently high index values in the inferior frontal gyrus, whereas a narration–listening task demonstrated the highest concordance in the posterior temporo-parietal junction area.

Invasive Epilepsy Monitoring: The Switch from Subdural Electrodes to Stereoelectroencephalography

Stereoelectroencephalography (SEEG) has experienced an explosion in use due to a shifting understanding of epileptic networks and wider application of minimally invasive epilepsy surgery techniques. Both subdural electrode (SDE) monitoring and SEEG serve important roles in defining the epileptogenic zone, limiting functional deficits, and formulating the most effective surgical plan. Strengths of SEEG include the ability to sample difficult to reach, deep structures of the brain without a craniotomy and without disrupting the dura. SEEG is complementary to minimally invasive epilepsy treatment options and may reduce the treatment gap in patients who are hesitant about craniotomy and surgical resection. Understanding the strengths and limitations of SDE monitoring and SEEG allows epileptologists to choose the best modality of invasive monitoring for each patient living with drug-resistant seizures.

Intracranial direct electrical mapping reveals the functional architecture of the human basal ganglia

The basal ganglia play a key role in integrating a variety of human behaviors through the cortico–basal ganglia–thalamo–cortical loops. Accordingly, basal ganglia disturbances are implicated in a broad range of debilitating neuropsychiatric disorders. Despite accumulating knowledge of the basal ganglia functional organization, the neural substrates and circuitry subserving functions have not been directly mapped in humans. By direct electrical stimulation of distinct basal ganglia regions in 35 refractory epilepsy patients undergoing stereoelectroencephalography recordings, we here offer currently the most complete overview of basal ganglia functional characterization, extending not only to the expected sensorimotor responses, but also to vestibular sensations, autonomic responses, cognitive and multimodal effects. Specifically, some locations identified responses weren’t predicted by the model derived from large-scale meta-analyses. Our work may mark an important step toward understanding the functional architecture of the human basal ganglia and provide mechanistic explanations of non-motor symptoms in brain circuit disorders.

Direct electrical stimulation of the basal ganglia using implanted SEEG electrodes produced a variety of motor and non-motor effects in human participants, providing insight into the functional architecture of this key brain region.

An fMRI dataset for whole-body somatotopic mapping in humans

The somatotopic representation of the body is a well-established organizational principle in the human brain. Classic invasive direct electrical stimulation for somatotopic mapping cannot be used to map the whole-body topographical representation of healthy individuals. Functional magnetic resonance imaging (fMRI) has become an indispensable tool for the noninvasive investigation of somatotopic organization of the human brain using voluntary movement tasks. Unfortunately, body movements during fMRI scanning often cause large head motion artifacts. Consequently, there remains a lack of publicly accessible fMRI datasets for whole-body somatotopic mapping. Here, we present public high-resolution fMRI data to map the somatotopic organization based on motor movements in a large cohort of healthy adults (N = 62). In contrast to previous studies that were mostly designed to distinguish few body representations, most body parts are considered, including toe, ankle, leg, finger, wrist, forearm, upper arm, jaw, lip, tongue, and eyes. Moreover, the fMRI data are denoised by combining spatial independent component analysis with manual identification to clean artifacts from head motion associated with body movements.

Automated intraoperative central sulcus localization and somatotopic mapping using median nerve stimulation

OBJECTIVE

Accurate identification of functional cortical regions is essential in neurological resection. The central sulcus (CS) is an important landmark that delineates functional cortical regions. Median nerve stimulation (MNS) is a standard procedure to identify the position of the CS intraoperatively. In this paper, we introduce an automated procedure that uses MNS to rapidly localize the CS and create functional somatotopic maps.

APPROACH

We recorded electrocorticographic signals from 13 patients who underwent MNS in the course of an awake craniotomy. We analyzed these signals to develop an automated procedure that determines the location of the CS and that also produces functional somatotopic maps.

MAIN RESULTS

The comparison between our automated method and visual inspection performed by the neurosurgeon shows that our procedure has a high sensitivity (89%) in identifying the CS. Further, we found substantial concordance between the functional somatotopic maps generated by our method and passive functional mapping (92% sensitivity).

SIGNIFICANCE

Our automated MNS-based method can rapidly localize the CS and create functional somatotopic maps without imposing additional burden on the clinical procedure. With additional development and validation, our method may lead to a diagnostic tool that guides neurosurgeon and reduces postoperative morbidity in patients undergoing resective brain surgery.

EEG Essentials

PURPOSE OF REVIEW

EEG is the best study for evaluating the electrophysiologic function of the brain. The relevance of EEG is based on an accurate interpretation of the recording. Understanding the neuroscientific basis for EEG is essential. The basis for recording and interpreting EEG is both brain site-specific and technique-dependent to detect and represent a complex series of waveforms. Separating normal from abnormal EEG lies at the foundation of essential interpretative skills.

RECENT FINDINGS

Seizures and epilepsy are the primary targets for clinical use of EEG in diagnosis, seizure classification, and management. Interictal epileptiform discharges on EEG support a clinical diagnosis of seizures, but only when an electrographic seizure is recorded is the diagnosis confirmed. New variations of normal waveforms, benign variants, and artifacts can mimic epileptiform patterns and are potential pitfalls for misinterpretation for inexperienced interpreters. A plethora of medical conditions involve nonepileptiform and epileptiform abnormalities on EEG along the continuum of people who appear healthy to those who are critically ill. Emerging trends in long-term EEG monitoring to diagnose, classify, quantify, and characterize patients with seizures have unveiled epilepsy syndromes in patients and expanded medical and surgical options for treatment. Advances in terminology and application of continuous EEG help unify neurologists in the diagnosis of nonconvulsive seizures and status epilepticus in patients with encephalopathy and prognosticate recovery from serious neurologic injury involving the brain.

SUMMARY

After 100 years, EEG has retained a key role in the neurologist's toolkit as a safe, widely available, versatile, portable test of neurophysiology, and it is likely to remain at the forefront for patients with neurologic diseases. Interpreting EEG is based on qualitative review, and therefore, the accuracy of reporting is based on the interpreter's training, experience, and exposure to many new and older waveforms.

Effective treatment of refractory complex facial pain with motor cortex stimulation by spinal paddle electrodes using multimodal imaging

Background

Complex facial pain is a debilitating condition with varying etiologies that overall responds poorly to both medical and traditional surgical management. Cortical stimulation is a unique therapeutic intervention which can be effective for some types of complex facial pain syndromes (CFPS). However, the novel use of preoperative functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI) coupled with intraoperative stimulation mapping and phase reversal to improve the accuracy for placement of spinal paddle electrodes in motor cortex stimulation, to our knowledge, has not been reported in the literature.

Case presentation

Here, we present a unique case of a 56-year-old male who developed left-sided complex facial pain syndrome after a stroke refractory to medical management and peripheral nerve stimulation. He previously underwent microvascular decompression (MVD) with limited control of his left-sided facial pain. In order to treat this, the patient underwent motor cortex stimulation. The motor strip of the face and tongue was identified preoperatively with functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI). Intraoperatively, phase reversal was used to identify corticospinal tracts and stimulus mapping confirmed the location before the epidural placement of two spinal paddle electrodes. Postoperatively, the patient reported significant reduction in pain levels, burning dysesthesias, and intensity and frequency of symptoms. This trend continued, and the patient experienced equivalent levels of relief at 6 months.

Conclusions

This is a rare case report of successful motor cortex stimulation with the novel preoperative use of fMRI and DTI, coupled with intraoperative functional mapping, to successfully guide the placement of spinal paddle electrodes for the treatment of CFPS.

Awake brain surgery for language mapping in pediatric patients: a single-center experience

OBJECTIVE

The goal of this study was to evaluate the feasibility, benefit, and safety of awake brain surgery (ABS) and intraoperative language mapping in children and adolescents with structural epilepsies. Whereas ABS is an established method to monitor language function in adults intraoperatively, reports of ABS in children are scarce.

METHODS

A retrospective chart review of pediatric patients ≤ 18 years of age who underwent ABS and cortical language mapping for supratentorial tumors and nontumoral epileptogenic lesions between 2008 and 2019 was conducted. The authors evaluated the global intellectual and specific language performance by using detailed neuropsychological testing, the patient's intraoperative compliance, results of intraoperative language mapping assisted by electrocorticography (ECoG), and postsurgical language development and seizure outcomes. Descriptive statistics were used for this study, with a statistical significance of p < 0.05.

RESULTS

Eleven children (7 boys) with a median age of 13 years (range 10-18 years) underwent ABS for a lesion in close vicinity to cortical language areas as defined by structural and functional MRI (left hemisphere in 9 children, right hemisphere in 2). Patients were neurologically intact but experiencing seizures; these were refractory to therapy in 9 patients. Compliance during the awake phase was high in 10 patients and low in 1 patient. Cortical mapping identified eloquent language areas in 6/10 (60%) patients and was concordant in 3/8 (37.5%), discordant in 3/8 (37.5%), and unclear in 2/8 (25%) patients compared to preoperative functional MRI. Stimulation-induced seizures occurred in 2 patients and could be interrupted easily. ECoG revealed that afterdischarge potentials (ADP) were involved in 5/9 (56%) patients with speech disturbances during stimulation. None of these patients harbored postoperative language dysfunction. Gross-total resection was achieved in 10/11 (91%) patients, and all were seizure free after a median follow-up of 4.3 years. Neuropsychological testing using the Wechsler Intelligence Scale for Children and the verbal learning and memory test showed an overall nonsignificant trend toward an immediate postoperative deterioration followed by an improvement to above preoperative levels after 1 year.

CONCLUSIONS

ABS is a valuable technique in selected pediatric patients with lesions in language areas. An interdisciplinary approach, careful patient selection, extensive preoperative training of patients, and interpretation of intraoperative ADP are pivotal to a successful surgery.

Investigating the Precise Localization of the Grasping Action in the Mid-Cingulate Cortex and Future Directions

Objective

To prospectively study the cingulate cortex for the localization and role of the grasping action in humans during electrical stimulation of depth electrodes.

Methods

All the patients (n = 23) with intractable focal epilepsy and a depth electrode stereotactically placed in the cingulate cortex, as part of their pre-surgical epilepsy evaluation from 2015 to 2017, were included. Cortical stimulation was performed and examined for grasping actions. Post-implantation volumetric T1 MRIs were co-registered to determine the exact electrode position.

Results

Five patients (male: female 4:1; median age 31) exhibited contralateral grasping actions during electrical stimulation. All patients had electrodes implanted in the ventral bank of the right cingulate sulcus adjacent to the vertical anterior commissure (VAC) line. Stimulation of other electrodes in adjacent regions did not elicit grasping.

Conclusion

Grasping action elicited from a localized region in the mid-cingulate cortex (MCC) directly supports the concept of the cingulate cortex being crucially involved in the grasping network. This opens an opportunity to explore this region with deep brain stimulation as a motor neuromodulation target for treatment in specific movement disorders or neurorehabilitation.

Subcortical Stimulation in Brain Tumor Surgery: A closer look beneath the surface

INTRODUCTION

Maximizing a patient's onco-functional balance is the central tenet of brain tumor surgery. As a result, numerous surgical adjuncts have been developed to facilitate identification of the tumor-brain interface and preservation of functional anatomy. Among these, intraoperative neurophysiologic monitoring (IONM) with direct cortical and subcortical stimulation remains the gold standard for real time, functional mapping of motor and language activity. However, stimulation techniques are not standardized and vary significantly across institutions. This is particularly true with subcortical stimulation for mapping of motor function.

METHODS

We review the state of subcortical IONM and mapping techniques. Historical and predicate literature were reviewed as well as new and emerging techniques. We discuss their evolution, clinical utility, and limitations to direct future research and application.

RESULTS

We evaluate and discuss the background and current clinical use of direct cortical and subcortical stimulation techniques and protocols and identify current trends and limitations. We focus specifically on methods of subcortical stimulation given the heterogeneity in the published literature. We also suggest directions to optimize the clinical utility of these tools.

CONCLUSION

Despite significant heterogeneity in published techniques, trends support the use of the Taniguchi method for subcortical stimulation. Novel dynamic stimulation techniques may improve accuracy. Prospective studies to define standardized guidelines are needed.

New Era of Electroceuticals: Clinically Driven Smart Implantable Electronic Devices Moving towards Precision Therapy

In the name of electroceuticals, bioelectronic devices have transformed and become essential for dealing with all physiological responses. This significant advancement is attributable to its interdisciplinary nature from engineering and sciences and also the progress in micro and nanotechnologies. Undoubtedly, in the future, bioelectronics would lead in such a way that diagnosing and treating patients' diseases is more efficient. In this context, we have reviewed the current advancement of implantable medical electronics (electroceuticals) with their immense potential advantages. Specifically, the article discusses pacemakers, neural stimulation, artificial retinae, and vagus nerve stimulation, their micro/nanoscale features, and material aspects as value addition. Over the past years, most researchers have only focused on the electroceuticals metamorphically transforming from a concept to a device stage to positively impact the therapeutic outcomes. Herein, the article discusses the smart implants' development challenges and opportunities, electromagnetic field effects, and their potential consequences, which will be useful for developing a reliable and qualified smart electroceutical implant for targeted clinical use. Finally, this review article highlights the importance of wirelessly supplying the necessary power and wirelessly triggering functional electronic circuits with ultra-low power consumption and multi-functional advantages such as monitoring and treating the disease in real-time.

Human brain mapping with multithousand-channel PtNRGrids resolves spatiotemporal dynamics

Electrophysiological devices are critical for mapping eloquent and diseased brain regions and for therapeutic neuromodulation in clinical settings and are extensively used for research in brain-machine interfaces. However, the existing clinical and experimental devices are often limited in either spatial resolution or cortical coverage. Here, we developed scalable manufacturing processes with a dense electrical connection scheme to achieve reconfigurable thin-film, multithousand-channel neurophysiological recording grids using platinum nanorods (PtNRGrids). With PtNRGrids, we have achieved a multithousand-channel array of small (30 μm) contacts with low impedance, providing high spatial and temporal resolution over a large cortical area. We demonstrated that PtNRGrids can resolve submillimeter functional organization of the barrel cortex in anesthetized rats that captured the tissue structure. In the clinical setting, PtNRGrids resolved fine, complex temporal dynamics from the cortical surface in an awake human patient performing grasping tasks. In addition, the PtNRGrids identified the spatial spread and dynamics of epileptic discharges in a patient undergoing epilepsy surgery at 1-mm spatial resolution, including activity induced by direct electrical stimulation. Collectively, these findings demonstrated the power of the PtNRGrids to transform clinical mapping and research with brain-machine interfaces.

Electroencephalography, electrocorticography, and cortical stimulation techniques

Electroencephalography (EEG) and electrocorticography (ECoG) are two important neurophysiologic techniques used in the operating room for monitoring and mapping electrical brain activity. In this chapter, we detail their principle, recording methodology, and address specifics of their interpretation in the intraoperative setting (e.g., effect of anesthetics), as well as their clinical applications in epilepsy and non-epilepsy surgeries. In addition, we address differences between scalp, surface, and deep cortical recordings that will help towards a more reliable interpretation of the significance of electrophysiologic parameters such as amplitude and morphology as well as in differentiation between abnormal and normal patterns of electrical brain activity. Electrical stimulation is used for intraoperative mapping of different cortical functions such as language, parietal, and motor. Stimulation paradigms used in clinical practice vary with regard to stimulation frequencies and probes being used. Parameters, such as the number of phases per pulse, pulse/phase duration, pulse frequency, organization, and polarity, define their characteristics, including their safety, propensity to trigger seizures, efficiency and reliability of stimulation, and the mapping thresholds. Specifically, in this chapter, we will address differences between monopolar and bipolar stimulation; anodal and cathodal polarity; monophasic and biphasic pulses; constant voltage, and constant current paradigms.

Intra-operative mapping and language protection in glioma

ABSTRACT

The demand for acquiring different languages has increased with increasing globalization. However, knowledge of the modification of the new language in the neural language network remains insufficient. Although many details of language function have been detected based on the awake intra-operative mapping results, the language neural network of the bilingual or multilingual remains unclear, which raises difficulties in clinical practice to preserve patients' full language ability in neurosurgery. In this review, we present a summary of the current findings regarding the structure of the language network and its evolution as the number of acquired languages increased in glioma patients. We then discuss a new insight into the awake intra-operative mapping protocol to reduce surgical risks during the preservation of language function in multilingual patients with glioma.

Reducing the Cognitive Footprint of Brain Tumor Surgery

The surgical management of brain tumors is based on the principle that the extent of resection improves patient outcomes. Traditionally, neurosurgeons have considered that lesions in “non-eloquent” cerebrum can be more aggressively surgically managed compared to lesions in “eloquent” regions with more known functional relevance. Furthermore, advancements in multimodal imaging technologies have improved our ability to extend the rate of resection while minimizing the risk of inducing new neurologic deficits, together referred to as the “onco-functional balance.” However, despite the common utilization of invasive techniques such as cortical mapping to identify eloquent tissue responsible for language and motor functions, glioma patients continue to present post-operatively with poor cognitive morbidity in higher-order functions. Such observations are likely related to the difficulty in interpreting the highly-dimensional information these technologies present to us regarding cognition in addition to our classically poor understanding of the functional and structural neuroanatomy underlying complex higher-order cognitive functions. Furthermore, reduction of the brain into isolated cortical regions without consideration of the complex, interacting brain networks which these regions function within to subserve higher-order cognition inherently prevents our successful navigation of true eloquent and non-eloquent cerebrum. Fortunately, recent large-scale movements in the neuroscience community, such as the Human Connectome Project (HCP), have provided updated neural data detailing the many intricate macroscopic connections between cortical regions which integrate and process the information underlying complex human behavior within a brain “connectome.” Connectomic data can provide us better maps on how to understand convoluted cortical and subcortical relationships between tumor and human cerebrum such that neurosurgeons can begin to make more informed decisions during surgery to maximize the onco-functional balance. However, connectome-based neurosurgery and related applications for neurorehabilitation are relatively nascent and require further work moving forward to optimize our ability to add highly valuable connectomic data to our surgical armamentarium. In this manuscript, we review four concepts with detailed examples which will help us better understand post-operative cognitive outcomes and provide a guide for how to utilize connectomics to reduce cognitive morbidity following cerebral surgery.

The Efficacy of Intraoperative Passive Language Mapping for Glioma Surgery: A Case Report

Background: Recently, electrocorticographic (ECoG) studies have emphasized the importance of gamma band-based functional mapping in the presurgical localization of the eloquent cortex. Passive functional mapping using ECoG signals provides a reliable method for identifying receptive language areas without many of the risks and limitations associated with electrical cortical stimulation. We report a surgical case of left temporal malignant glioma with intraoperative passive language mapping. Case Description: A 78-year-old woman was diagnosed with left temporal glioma with inspection of her language difficulty. MRI showed a left temporal tumor measuring 74.6 × 50.0 × 51.5 mm in size. Real-time CortiQ-based mapping using high-gamma activity by word-listening and story-listening tasks was performed. Significant listening task-evoked high gamma activities were detected in 5 channels in the superior temporal gyrus and one channel in the middle temporal gyrus. The tumor was grossly removed except for the region corresponding to listening task-evoked high gamma activities. Postoperatively, the patient's symptoms of language comprehension difficulty improved, and no new neurological deficits were observed. Conclusion: Intraoperative passive language mapping was successfully performed, and the patient's language function was well-preserved. Intraoperative passive language mapping, which is applicable in a short time and under general anesthesia, can be an important tool for detecting language areas.

Evoking highly focal percepts in the fingertips through targeted stimulation of sulcal regions of the brain for sensory restoration

Background:

Paralysis and neuropathy, affecting millions of people worldwide, can be accompanied by significant loss of somatosensation. With tactile sensation being central to achieving dexterous movement, brain-computer interface (BCI) researchers have used intracortical and cortical surface electrical stimulation to restore somatotopically-relevant sensation to the hand. However, these approaches are restricted to stimulating the gyral areas of the brain. Since representation of distal regions of the hand extends into the sulcal regions of human primary somatosensory cortex (S1), it has been challenging to evoke sensory percepts localized to the fingertips.

Objective/hypothesis:

Targeted stimulation of sulcal regions of S1, using stereoelectroencephalography (SEEG) depth electrodes, can evoke focal sensory percepts in the fingertips.

Methods:

Two participants with intractable epilepsy received cortical stimulation both at the gyri via high-density electrocorticography (HD-ECoG) grids and in the sulci via SEEG depth electrode leads. We characterized the evoked sensory percepts localized to the hand.

Results:

We show that highly focal percepts can be evoked in the fingertips of the hand through sulcal stimulation. fMRI, myelin content, and cortical thickness maps from the Human Connectome Project elucidated specific cortical areas and sub-regions within S1 that evoked these focal percepts. Within-participant comparisons showed that percepts evoked by sulcal stimulation via SEEG electrodes were significantly more focal (80% less area; p = 0.02) and localized to the fingertips more often, than by gyral stimulation via HD-ECoG electrodes. Finally, sulcal locations with consistent modulation of high-frequency neural activity during mechanical tactile stimulation of the fingertips showed the same somatotopic correspondence as cortical stimulation.

Conclusions:

Our findings indicate minimally invasive sulcal stimulation via SEEG electrodes could be a clinically viable approach to restoring sensation.

Epilepsy Surgery: Monitoring and Novel Surgical Techniques

Drug-resistant epilepsy warrants referral to an epilepsy surgery center for consideration of alternative treatments including epilepsy surgery. Advances in technology now allow for minimally invasive neurophysiologic monitoring and surgical interventions, approaches that are attractive to families because large craniotomies and associated morbidity are avoided. This work reviews the presurgical evaluation process and discusses the use of invasive stereo-electroencephalography monitoring to localize seizure onset zones. Minimally invasive surgical techniques are described for the treatment of focal and generalized epilepsies. These approaches have expanded our capacity to palliate and cure epilepsy in the pediatric population.

MRI-Compatible and Conformal Electrocorticography Grids for Translational Research

Intraoperative electrocorticography (ECoG) captures neural information from the surface of the cerebral cortex during surgeries such as resections for intractable epilepsy and tumors. Current clinical ECoG grids come in evenly spaced, millimeter-sized electrodes embedded in silicone rubber. Their mechanical rigidity and fixed electrode spatial resolution are common shortcomings reported by the surgical teams. Here, advances in soft neurotechnology are leveraged to manufacture conformable subdural, thin-film ECoG grids, and evaluate their suitability for translational research. Soft grids with 0.2 to 10 mm electrode pitch and diameter are embedded in 150 µm silicone membranes. The soft grids are compatible with surgical handling and can be folded to safely interface hidden cerebral surface such as the Sylvian fold in human cadaveric models. It is found that the thin-film conductor grids do not generate diagnostic-impeding imaging artefacts (<1 mm) nor adverse local heating within a standard 3T clinical magnetic resonance imaging scanner. Next, the ability of the soft grids to record subdural neural activity in minipigs acutely and two weeks postimplantation is validated. Taken together, these results suggest a promising future alternative to current stiff electrodes and may enable the future adoption of soft ECoG grids in translational research and ultimately in clinical settings.

Convergence of heteromodal lexical retrieval in the lateral prefrontal cortex

Lexical retrieval requires selecting and retrieving the most appropriate word from the lexicon to express a desired concept. Few studies have probed lexical retrieval with tasks other than picture naming, and when non-picture naming lexical retrieval tasks have been applied, both convergent and divergent results emerged. The presence of a single construct for auditory and visual processes of lexical retrieval would influence cognitive rehabilitation strategies for patients with aphasia. In this study, we perform support vector regression lesion-symptom mapping using a brain tumor model to test the hypothesis that brain regions specifically involved in lexical retrieval from visual and auditory stimuli represent overlapping neural systems. We find that principal components analysis of language tasks revealed multicollinearity between picture naming, auditory naming, and a validated measure of word finding, implying the existence of redundant cognitive constructs. Nonparametric, multivariate lesion-symptom mapping across participants was used to model accuracies on each of the four language tasks. Lesions within overlapping clusters of 8,333 voxels and 21,512 voxels in the left lateral prefrontal cortex (PFC) were predictive of impaired picture naming and auditory naming, respectively. These data indicate a convergence of heteromodal lexical retrieval within the PFC.

Improvements on spatial coverage and focality of deep brain stimulation in pre-surgical epilepsy mapping

Objective.Electrical stimulation mapping (ESM) of the brain using stereo-electroencephalography (SEEG) intracranial electrodes, also known as depth-ESM (DESM), is being used as part of the pre-surgical planning for brain surgery in drug-resistant epilepsy patients. Typically, DESM consists in applying the electrical stimulation using adjacent contacts of the SEEG electrodes and in recording the EEG responses to those stimuli, giving valuable information of critical brain regions to better delimit the region to resect. However, the spatial extension or coverage of the stimulated area is not well defined even though the precise electrode locations can be determined from computed tomography images.Approach.We first conduct electrical simulations of DESM for different shapes of commercial SEEG electrodes showing the stimulation extensions for different intensities of injected current. We then evaluate the performance of DESM in terms of spatial coverage and focality on two realistic head models of real patients undergoing pre-surgical evaluation. We propose a novel strategy for DESM that consist in applying the current using contacts of different SEEG electrodes (x-DESM), increasing the versatility of DESM without implanting more electrodes. We also present a clinical case where x-DESM replicated the full semiology of an epilepsy seizure using a very low-intensity current injection, when typical adjacent DESM only reproduced partial symptoms with much larger intensities. Finally, we show one example of DESM optimal stimulation to achieve maximum intensity, maximum focality or intermediate solution at a pre-defined target, and one example of temporal interference in DESM capable of increasing focality in brain regions not immediately touching the electrode contacts.Main results.It is possible to define novel current injection patterns using contacts of different electrodes (x-DESM) that might improve coverage and/or focality, depending on the characteristics of the candidate brain. If individual simulations are not possible, we provide the estimated radius of stimulation as a function of the injected current and SEEG electrode brand as a reference for the community.Significance.Our results show that subject-specific electrical stimulations are a valuable tool to use in the pre-surgical planning to visualize the extension of the stimulated regions. The methods we present here are also applicable to pre-surgical planning of tumor resections and deep brain stimulation treatments.

Language Mapping Using Stereo Electroencephalography: A Review and Expert Opinion

Stereo-electroencephalography (sEEG) is a method that uses stereotactically implanted depth electrodes for extra-operative mapping of epileptogenic and functional networks. sEEG derived functional mapping is achieved using electrical cortical stimulations (ECS) that are currently the gold standard for delineating eloquent cortex. As this stands true especially for primary cortices (e.g., visual, sensitive, motor, etc.), ECS applied to higher order brain areas determine more subtle behavioral responses. While anterior and posterior language areas in the dorsal language stream seem to share characteristics with primary cortices, basal temporal language area (BTLA) in the ventral temporal cortex (VTC) behaves as a highly associative cortex. After a short introduction and considerations about methodological aspects of ECS using sEEG, we review the sEEG language mapping literature in this perspective. We first establish the validity of this technique to map indispensable language cortices in the dorsal language stream. Second, we highlight the contrast between the growing empirical ECS experience and the lack of understanding regarding the fundamental mechanisms underlying ECS behavioral effects, especially concerning the dispensable language cortex in the VTC. Evidences for considering network architecture as determinant for ECS behavioral response complexities are discussed. Further, we address the importance of designing new research in network organization of language as this could enhance ECS ability to map interindividual variability, pathology driven reorganization, and ultimately identify network resilience markers in order to better predict post-operative language deficit. Finally, based on a whole body of available studies, we believe there is strong evidence to consider sEEG as a valid, safe and reliable method for defining eloquent language cortices although there have been no proper comparisons between surgical resections with or without extra-operative or intra-operative language mapping.

Neurobiological After-Effects of Low Intensity Transcranial Electric Stimulation of the Human Nervous System: From Basic Mechanisms to Metaplasticity

Non-invasive low-intensity transcranial electrical stimulation (tES) of the brain is an evolving field that has brought remarkable attention in the past few decades for its ability to directly modulate specific brain functions. Neurobiological after-effects of tES seems to be related to changes in neuronal and synaptic excitability and plasticity, however mechanisms are still far from being elucidated. We aim to review recent results from in vitro and in vivo studies that highlight molecular and cellular mechanisms of transcranial direct (tDCS) and alternating (tACS) current stimulation. Changes in membrane potential and neural synchronization explain the ongoing and short-lasting effects of tES, while changes induced in existing proteins and new protein synthesis is required for long-lasting plastic changes (LTP/LTD). Glial cells, for decades supporting elements, are now considered constitutive part of the synapse and might contribute to the mechanisms of synaptic plasticity. This review brings into focus the neurobiological mechanisms and after-effects of tDCS and tACS from in vitro and in vivo studies, in both animals and humans, highlighting possible pathways for the development of targeted therapeutic applications.

Crying with depressed affect induced by electrical stimulation of the anterior insula: A stereo EEG case study

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Anterior insular stimulation produces reproducible episodes of emotional crying.

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This is due to activation of complex neural network with its connectivity to the anterior cingulate cortex.

•

This study increases our understanding of the complex functionality of the insula.

Stereo-EEG (sEEG) is an invasive recording technique used to localize the seizure-onset zone for epilepsy surgery in people with drug-resistant focal seizures. Pathological crying reflects disordered emotional expression and the anterior insula is known to play a role in empathy and socio-emotional processing. We describe a patient where electrical stimulation mapping (ESM) of the anterior insula during sEEG generated pathological crying and profound sadness that was time-locked to the electrical stimulus.

We evaluated a 35-year-old left-handed female for repeat epilepsy surgery. The patient had drug resistant focal impaired awareness seizures despite a previous left temporal neocortical resection informed by an invasive study using subdural grid and strip electrodes seven years earlier. She was studied invasively with 10 sEEG electrodes sampling temporal, occipital, and insular targets. In the process of functional mapping, stimulation of the anterior insular cortex provoked tearful crying with sad affect, reproducible upon repeat stimulation.

Our case is unique in demonstrating transitory pathological crying with profound sadness provoked by ESM of the left anterior insula. Furthermore we demonstrate repeated time-synched crying from electrical stimulation, which supports the hypothesis that the anterior insula in the brain plays an important role in the biology of emotion, as implicated by previous studies.

The feasibility and value of extraoperative and adjuvant intraoperative stereoelectroencephalography in rolandic and perirolandic epilepsies

OBJECTIVE

The objective of this study was to illustrate the feasibility and value of extra- and intraoperative stereoelectroencephalography (SEEG) in patients who underwent resection in rolandic and perirolandic regions.

METHODS

The authors retrospectively reviewed all consecutive patients with at least 1 year of postoperative follow-up who underwent extra- and intraoperative SEEG monitoring between January 2015 and January 2017.

RESULTS

Four patients with pharmacoresistant rolandic and perirolandic focal epilepsy were identified, who underwent conventional extraoperative invasive SEEG evaluations followed by adjuvant intraoperative SEEG recordings. Conventional extraoperative SEEG evaluations demonstrated ictal and interictal epileptiform activities involving eloquent rolandic and perirolandic cortical areas in all patients. Following extraoperative monitoring, patients underwent preplanned staged resections guided by simultaneous and continuous adjuvant intraoperative SEEG monitoring. Resections, guided by electrode contacts of interest in 3D boundaries, were performed while continuous real-time electrographic data from SEEG recordings were obtained. Staged approaches of resections were performed until there was intraoperative resolution of synchronous rolandic/perirolandic cortex epileptic activities. All patients in the cohort achieved complete seizure freedom (Engel class IA) during the follow-up period ranging from 18 to 50 months. Resection resulted in minimal neurological deficit; 3 patients experienced transient, distal plantar flexion weakness (mild foot drop).

CONCLUSIONS

The seizure and functional outcome results of this highly preselected group of patients testifies to the feasibility and demonstrates the value of the combined benefits of both intra- and extraoperative SEEG recordings when resecting the rolandic and perirolandic areas. The novel hybrid method allows a more refined and precise identification of the epileptogenic zone. Consequently, tailored resections can be performed to minimize morbidity as well as to achieve adequate seizure control.

A data resource from concurrent intracranial stimulation and functional MRI of the human brain

Mapping the causal effects of one brain region on another is a challenging problem in neuroscience that we approached through invasive direct manipulation of brain function together with concurrent whole-brain measurement of the effects produced. Here we establish a unique resource and present data from 26 human patients who underwent electrical stimulation during functional magnetic resonance imaging (es-fMRI). The patients had medically refractory epilepsy requiring surgically implanted intracranial electrodes in cortical and subcortical locations. One or multiple contacts on these electrodes were stimulated while simultaneously recording BOLD-fMRI activity in a block design. Multiple runs exist for patients with different stimulation sites. We describe the resource, data collection process, preprocessing using the fMRIPrep analysis pipeline and management of artifacts, and provide end-user analyses to visualize distal brain activation produced by site-specific electrical stimulation. The data are organized according to the brain imaging data structure (BIDS) specification, and are available for analysis or future dataset contributions on openneuro.org including both raw and preprocessed data.

Awake glioma surgery: technical evolution and nuances

INTRODUCTION

Multiple studies have demonstrated that improved extent of resection is associated with longer overall survival for patients with both high and low grade glioma. Awake craniotomy was developed as a technique for maximizing resection whilst preserving neurological function.

METHODS

We performed a comprehensive review of the literature describing the history, indications, techniques and outcomes of awake craniotomy for patients with glioma.

RESULTS

The technique of awake craniotomy evolved to become an essential tool for resection of glioma. Many perceived contraindications can now be managed. We describe in detail our preferred technique, the testing paradigms utilized, and critically review the literature regarding functional and oncological outcome.

CONCLUSIONS

Awake craniotomy with mapping has become the gold standard for safely maximizing extent of resection for tumor in or near eloquent brain. Cortical and subcortical mapping methods have been refined and the technique is associated with an extremely low rate of complications.

Novel Use of Stimulating Fence-Post Technique for Functional Mapping of Subcortical White Matter During Tumor Resection: A Technical Case Series

Background

Maximal safe resection remains a key principle in infiltrating glioma management. Stimulation mapping is a key adjunct for minimizing functional morbidity while “fence-post” procedures use catheters or dye to mark the tumor border at the start of the procedure prior to brain shift.

Objective

To report a novel technique using stereotactically placed electrodes to guide tumor resection near critical descending subcortical fibers.

Methods

Navigated electrodes were placed prior to tumor resection along the deep margin bordering presumed eloquent tracts. Stimulation was administered through these depth electrodes for subcortical motor and language mapping.

Results

Twelve patients were included in this preliminary technical report. Seven patients (7/12, 58%) were in asleep cases, while the other 5 cases (5/12, 42%) were performed awake. Mapping of motor fibers was performed in 8 cases, and language mapping was done in 1 case. In 3 cases, both motor and language mapping were performed using the same depth electrode spanning corticospinal tract and the arcuate fasciculus.

Conclusion

Stereotactic depth electrode placement coupled with stimulation mapping of white matter tracts can be used concomitantly to demarcate the border between deep tumor margins and eloquent brain, thus helping to maximize extent of resection while minimizing functional morbidity.

Acute Effects and the Dreamy State Evoked by Deep Brain Electrical Stimulation of the Amygdala: Associations of the Amygdala in Human Dreaming, Consciousness, Emotions, and Creativity

Accurate localization of complex human experiences such as emotions, dreaming, creativity, and consciousness to specific cerebral structures or neural networks has remained elusive despite technological advances. We report the use of acute deep brain stimulation (DBS) to evoke behavioral and emotional effects by applying electrical stimulation (ES) at various voltage strengths to the basolateral and central subnuclei of the amygdala in addition to the head of hippocampus (HC) for two subjects with medically refractory post-traumatic stress disorder (PTSD). Our results suggest that the amygdala could be a node in a neural network responsible for the generation of complex vivid mental imagery and integrated sensory experiences similar to John Hughlings Jackson's "dreamy state" and "double consciousness," which have been classically associated with temporal lobe epilepsy during uncinate seizures. That we were able to elicit similar vivid, dynamic, complex, bizarre, and original mental imagery with ES in non-epileptic subjects suggests that Jackson's seizure related "dreamy state" and "double consciousness" may arise from heightened innate brain mechanisms with the amygdala acting as a node in the neural network responsible for physiologic dreaming and creative functions. Furthermore, our subjects experienced different emotions with different stimulation strengths at various electrode contacts. Our results suggest that higher voltage stimulation of the amygdala and HC at 4-5 V leads to predominantly negative responses and 2-4 V stimulation showed inversely coupled positive and negative responses of the amygdala in either hemisphere which may imply hemispheric dominance of emotional valences without relation to handedness. Due to the unique and complex responses dependent on location and strength of stimulation, we advise that all patients receiving DBS of the amygdala undergo acute stimulation mapping in a monitored setting before selecting therapeutic parameters for chronic stimulation.

Detection of after-discharges during intraoperative functional brain mapping in awake brain tumor surgery using a novel high-density circular grid

OBJECTIVE

To evaluate intraoperative use of a novel high-density circular grid in detecting after-discharges (AD) on electrocorticography (ECoG) during functional brain mapping (FBM).

METHODS

FBM during glioma surgery (10/2016 to 5/2019) recorded ADs using a 22-channel circular grid compared to conventional strip electrodes. ADs were analyzed for detection, duration, amplitude, morphology, histology, direction, and clinical signs.

RESULTS

Thirty-two patients (mean age 54.2 years; r = 30-75) with glioma (WHO grade II-IV; 20 grade IV) had surgery. ADs during FBM were more likely in patients with wild-type as opposed to IDH-1 mutants (p < 0.0001) using more contacts compared with linear strip electrodes (p = 0.0001). More sensors tended to be involved in ADs detected by the circular grid vs strips (6.61 vs 3.43; p = 0.16) at lower stimulus intensity (3.14 mA vs 4.13 mA; p = 0.09). No difference in the number of cortical stimulations before resection was present (38.9 mA vs 47.9 mA; p = 0.26). ADs longer than 10 seconds were 32.5 seconds (circular grid) vs 58.4 (strips) (p = 0.12).

CONCLUSIONS

High-density circular grids detect ADs in 360 degrees during FBM for glioma resection. Provocation of ADs was more likely in patients with wild-type than IDH-1 mutation.

SIGNIFICANCE

Circular grids offer high-resolution ECoG during intraoperative FBM for detection of ADs.