Functional connections in the human temporal lobe. II. Evidence for a loss of functional linkage between contralateral limbic structures

ER-detect: A pipeline for robust detection of early evoked responses in BIDS-iEEG electrical stimulation data

BACKGROUND

Human brain connectivity can be measured in different ways. Intracranial EEG (iEEG) measurements during single pulse electrical stimulation provide a unique way to assess the spread of electrical information with millisecond precision. However, the methods used for the detection of responses in cortico-cortical evoked potential (CCEP) data vary across studies, from visual inspection with manual annotation to a variety of automated methods.

NEW METHOD

To provide a robust workflow to process CCEP data and detect early evoked responses in a fully automated and reproducible fashion, we developed the Early Response (ER)-detect toolbox. ER-detect is an open-source Python package and Docker application to preprocess BIDS structured iEEG data and detect early evoked CCEP responses. ER-detect can use three early response detection methods, which were validated against 14 manually annotated CCEP datasets from two different clinical sites by four independent raters.

RESULTS AND COMPARISON WITH EXISTING METHODS

ER-detect's automated detection performed on par with the inter-rater reliability (Cohen's Kappa of ∼0.6). Moreover, ER-detect was optimized for processing large CCEP datasets, to be used in conjunction with other connectomic investigations.

CONCLUSION

ER-detect provides a highly efficient standardized workflow such that iEEG-BIDS data can be processed in a consistent manner and enhance the reproducibility of CCEP based connectivity results for both research and clinical purposes.

Mapping the Epileptogenic Brain Using Low-Frequency Stimulation: Two Decades of Advances and Uncertainties

Cortical stimulation is the process of delivering brief pulses of electrical current and visualizing the distributed pattern of evoked responses across the brain. Compared to high-frequency stimulation, which has long been used for seizure provocation and functional mapping, low-frequency stimulation (<1-2 Hz) is rarely incorporated into the epilepsy surgery evaluation. Increasingly, researchers have demonstrated that various cortico-cortical evoked potential (CCEP) features, including early and delayed responses, evoked high-frequency oscillations, and derived network metrics, may be useful biomarkers of tissue excitability and abnormal connectivity. Emerging evidence also highlights a potential role of CCEPs in guiding neuromodulatory therapies like responsive neurostimulation. In this review, we examine the past two decades of innovation in low-frequency stimulation as it pertains to pre-surgical evaluation. We begin with a basic overview of single-pulse electrical stimulation and CCEPs, including definitions, methodology, physiology, and traditional interpretation. We then explore the literature examining CCEPs as markers of cortical excitability, seizure onset, and network-level dysfunction. Finally, the relationship between stimulation-induced and spontaneous seizures is considered. By examining these questions, we identify both opportunities and pitfalls along the path towards integrating low-frequency stimulation into clinical practice.

ER-detect: a pipeline for robust detection of early evoked responses in BIDS-iEEG electrical stimulation data

Human brain connectivity can be measured in different ways. Intracranial EEG (iEEG) measurements during single pulse electrical stimulation provide a unique way to assess the spread of electrical information with millisecond precision. To provide a robust workflow to process these cortico-cortical evoked potential (CCEP) data and detect early evoked responses in a fully automated and reproducible fashion, we developed Early Response (ER)-detect. ER-detect is an open-source Python package and Docker application to preprocess BIDS structured iEEG data and detect early evoked CCEP responses. ER-detect can use three response detection methods, which were validated against 14-manually annotated CCEP datasets from two different sites by four independent raters. Results showed that ER-detect's automated detection performed on par with the inter-rater reliability (Cohen's Kappa of ~0.6). Moreover, ER-detect was optimized for processing large CCEP datasets, to be used in conjunction with other connectomic investigations. ER-detect provides a highly efficient standardized workflow such that iEEG-BIDS data can be processed in a consistent manner and enhance the reproducibility of CCEP based connectivity results.

Physiological and pathological neuronal connectivity in the living human brain based on intracranial EEG signals: the current state of research

Over the past decades, studies of human brain networks have received growing attention as the assessment and modelling of connectivity in the brain is a topic of high impact with potential application in the understanding of human brain organization under both physiological as well as various pathological conditions. Under specific diagnostic settings, human neuronal signal can be obtained from intracranial EEG (iEEG) recording in epilepsy patients that allows gaining insight into the functional organisation of living human brain. There are two approaches to assess brain connectivity in the iEEG-based signal: evaluation of spontaneous neuronal oscillations during ongoing physiological and pathological brain activity, and analysis of the electrophysiological cortico-cortical neuronal responses, evoked by single pulse electrical stimulation (SPES). Both methods have their own advantages and limitations. The paper outlines available methodological approaches and provides an overview of current findings in studies of physiological and pathological human brain networks, based on intracranial EEG recordings.

Multimodal anatomy of the human forniceal commissure

Ambiguity surrounds the existence and morphology of the human forniceal commissure. We combine advanced in-vivo tractography, multidirectional ex-vivo fiber dissection, and multiplanar histological analysis to characterize this structure’s anatomy. Across all 178 subjects, in-vivo fiber dissection based on the Human Connectome Project 7 T MRI data identifies no interhemispheric connections between the crura fornicis. Multidirectional ex-vivo fiber dissection under the operating microscope demonstrates the psalterium as a thin soft-tissue membrane spanning between the right and left crus fornicis, but exposes no commissural fibers. Multiplanar histological analysis with myelin and Bielchowsky silver staining, however, visualizes delicate cruciform fibers extending between the crura fornicis, enclosed by connective tissue, the psalterium. The human forniceal commissure is therefore much more delicate than previously described and presented in anatomical textbooks. This finding is consistent with the observed phylogenetic trend of a reduction of the forniceal commissure in non-human primates compared to non-primate eutherian mammals.

Anatomical dissection and tractography elucidate the delicate nature of the human forniceal commissure, an interhemispheric white matter circuit.

Face-induced gamma oscillations and event-related potentials in patients with epilepsy: an intracranial EEG study

Background

To examine the pathological effect of a mesial temporal seizure onset zone (SOZ) on local and inter-regional response to faces in the amygdala and other structures of the temporal lobe.

Methods

Intracranial EEG data was obtained from the amygdala, hippocampus, fusiform gyrus and parahippocampal gyrus of nine patients with drug-refractory epilepsy during visual stimulation with faces and mosaics. We analyzed event-related potentials (ERP), gamma frequency power, phase-amplitude coupling and phase-slope-index and compared the results between patients with versus without a mesial temporal SOZ.

Results

In the amygdala and fusiform gyrus, faces triggered higher ERP amplitudes compared to mosaics in both patient groups and higher gamma power in patients without a mesial temporal SOZ. In the hippocampus, famous faces triggered higher gamma power for both groups combined but did not affect ERPs in either group. The differentiated ERP response to famous faces in the parahippocampal gyrus was more pronounced in patients without a mesial temporal SOZ. Phase-amplitude coupling and phase-slope-index results yielded bidirectional modulation between amygdala and fusiform gyrus, and predominately unidirectional modulation between parahippocampal gyrus and hippocampus.

Conclusions

A mesial temporal SOZ was associated with an impaired response to faces in the amygdala, fusiform gyrus and parahippocampal gyrus in our patients. Compared to this, the response to faces in the hippocampus was impaired in patients with, as well as without, a mesial temporal SOZ. Our results support existing evidence for face processing deficits in patients with a mesial temporal SOZ and suggest the pathological effect of a mesial temporal SOZ on the amygdala to play a pivotal role in this matter in particular.

Differential Hemispheric Lateralization of Emotions and Related Display Behaviors: Emotion-Type Hypothesis

There are two well-known hypotheses regarding hemispheric lateralization of emotions. The Right Hemisphere Hypothesis (RHH) postulates that emotions and associated display behaviors are a dominant and lateralized function of the right hemisphere. The Valence Hypothesis (VH) posits that negative emotions and related display behaviors are modulated by the right hemisphere and positive emotions and related display behaviors are modulated by the left hemisphere. Although both the RHH and VH are supported by extensive research data, they are mutually exclusive, suggesting that there may be a missing factor in play that may provide a more accurate description of how emotions are lateralization in the brain. Evidence will be presented that provides a much broader perspective of emotions by embracing the concept that emotions can be classified into primary and social types and that hemispheric lateralization is better explained by the Emotion-type Hypothesis (ETH). The ETH posits that primary emotions and related display behaviors are modulated by the right hemisphere and social emotions and related display behaviors are modulated by the left hemisphere.

MRIES: A Matlab Toolbox for Mapping the Responses to Intracranial Electrical Stimulation

Propose

Directed cortical responses to intracranial electrical stimulation are a good standard for mapping inter-regional direct connectivity. Cortico-cortical evoked potential (CCEP), elicited by single pulse electrical stimulation (SPES), has been widely used to map the normal and abnormal brain effective network. However, automated processing of CCEP datasets and visualization of connectivity results remain challenging for researchers and clinicians. In this study, we develop a Matlab toolbox named MRIES (Mapping the Responses to Intracranial Electrical Stimulation) to automatically process CCEP data and visualize the connectivity results.

Method

The MRIES integrates the processing pipeline of the CCEP datasets and various methods for connectivity calculation based on low- and high-frequency signals with stimulation artifacts removed. The connectivity matrices are saved in different folders for visualization. Different visualization patterns (connectivity matrix, circle map, surface map, and volume map) are also integrated to the graphical user interface (GUI), which makes it easy to intuitively display and compare different connectivity measurements. Furthermore, one sample CCEP data set collected from eight epilepsy patients is used to validate the MRIES toolbox.

Result

We show the GUI and visualization functions of MRIES using one example CCEP data that has been described in a complete tutorial. We applied this toolbox to the sample CCEP data set to investigate the direct connectivity between the medial temporal lobe and the insular cortex. We find bidirectional connectivity between MTL and insular that are consistent with the findings of previous studies.

Conclusion

MRIES has a friendly GUI and integrates the full processing pipeline of CCEP data and various visualization methods. The MRIES toolbox, tutorial, and example data can be freely downloaded. As an open-source package, MRIES is expected to improve the reproducibility of CCEP findings and facilitate clinical translation.

Acute aerobic exercise enhances cortical connectivity between structures involved in shaping mood and improves self-reported mood: An EEG effective-connectivity study in young male adults

There seems to be a general consensus among researchers that acute aerobic exercise (exercise hereafter) improves mood, but the neural mechanisms which drive these effects are far from being clear. The current study investigated the cortical connectivity patterns that underlie changes in mood after exercise. Twenty male adults underwent three different experimental protocols that were carefully controlled in terms of underlying metabolism and were administered in a randomized order: moderate-intensity continuous exercise, high-intensity interval exercise, and seated rest condition. Before and after each experimental protocol, we collected data on the participants' mood using the UMACL questionnaire and recorded their resting-state EEG. We focused on the effective connectivity patterns exerted by the dorso-lateral prefrontal cortex (dlPFC) over the temporal region (TMP), as these are important cortical structures involved in shaping mood. The cortical connectivity patterns in the resting-state EEG were evaluated using the directed transfer function (DTF), which is an autoregressive effective connectivity method. The results showed that both moderate-intensity exercise and high-intensity interval exercise improved participants' self-reported mood. Crucially, this improvement was accompanied by stronger influences of dlPFC over TMP. The observed changes in the effective connectivity patterns between dlPFC and TMP might help to better understand the effects of exercise on mood.

Hippocampal vascularization: Proposal for a new classification

Background:

Anatomy of the hippocampal arterial supply is key to successful surgeries in this area. The goal of the current study is to present the results we obtained from our microsurgical dissections of the temporal lobe and to propose a new classification for the hippocampal arteries (HAs).

Methods:

Fifty-six brain hemispheres were analyzed. All dissections in this study were made using 3–40× at the surgical microscope.

Results:

The hippocampal arterial vasculature can be divided into six groups, according to their frequencies: Group A: mixed arterial vasculature originating from the anterior choroidal artery anterior choroidal artery, posterior cerebral artery (PCA), anterior infratemporal artery (AIA), and splenic artery (SA). Group B: Main origin at the temporal branches – main inferotemporal trunk, middle inferotemporal artery, posterior inferotemporal artery, AIA, or main branch of PCA. Group C: AIA as the main branch of the hippocampus. Group D: HAs originating from the main branch of PCA. Group E: A single hippocampal artery with the origin at the main branch of PCA. This single artery covered all of the structure and is named Ushimura’s artery. Group F: The hippocampal vessels arose exclusively from the parieto-occipital artery, calcarine artery (CA), and the SA.

Conclusion:

This study proposes a new classification for the hippocampal vascularization, according to the origin of HAs. One of the groups has not yet been described in the literature – in which the HAs arise from the parieto-occipital artery, SA, and CA.

Transfer Function Models for the Localization of Seizure Onset Zone From Cortico-Cortical Evoked Potentials

Surgical resection of the seizure onset zone (SOZ) could potentially lead to seizure-freedom in medically refractory epilepsy patients. However, localizing the SOZ can be a time consuming and tedious process involving visual inspection of intracranial electroencephalographic (iEEG) recordings captured during passive patient monitoring. Cortical stimulation is currently performed on patients undergoing invasive EEG monitoring for the main purpose of mapping functional brain networks such as language and motor networks. We hypothesized that evoked responses from single pulse electrical stimulation (SPES) can also be used to localize the SOZ as they may express the natural frequencies and connectivity of the iEEG network. To test our hypothesis, we constructed patient specific transfer function models from the evoked responses recorded from 22 epilepsy patients that underwent SPES evaluation and iEEG monitoring. We then computed the frequency and connectivity dependent “peak gain” of the system as measured by the H∞  norm from systems theory. We found that in cases for which clinicians had high confidence in localizing the SOZ, the highest peak gain transfer functions with the smallest “floor gain” (gain at which the dipped H∞   3dB below DC gain) corresponded to when the clinically annotated SOZ and early spread regions were stimulated. In more complex cases, there was a large spread of the peak-to-floor (PF) ratios when the clinically annotated SOZ was stimulated. Interestingly for patients who had successful surgeries, our ratio of gains, agreed with clinical localization, no matter the complexity of the case. For patients with failed surgeries, the PF ratio did not match clinical annotations. Our findings suggest that transfer function gains and their corresponding frequency responses computed from SPES evoked responses may improve SOZ localization and thus surgical outcomes.

Four-dimensional tractography animates propagations of neural activation via distinct interhemispheric pathways

OBJECTIVE

To visualize and validate the dynamics of interhemispheric neural propagations induced by single-pulse electrical stimulation (SPES).

METHODS

This methodological study included three patients with drug-resistant focal epilepsy who underwent measurement of cortico-cortical spectral responses (CCSRs) during bilateral stereo-electroencephalography recording. We delivered SPES to 83 electrode pairs and analyzed CCSRs recorded at 268 nonepileptic electrode sites. Diffusion-weighted imaging (DWI) tractography localized the interhemispheric white matter pathways as streamlines directly connecting two electrode sites. We localized and visualized the putative SPES-related fiber activation, at each 1-ms time window, based on the propagation velocity defined as the DWI-based streamline length divided by the early CCSR peak latency.

RESULTS

The resulting movie, herein referred to as four-dimensional tractography, delineated the spatiotemporal dynamics of fiber activation via the corpus callosum and anterior commissure. Longer streamline length was associated with delayed peak latency and smaller amplitude of CCSRs. The cortical regions adjacent to each fiber activation site indeed exhibited CCSRs at the same time window.

CONCLUSIONS

Our four-dimensional tractography successfully animated neural propagations via distinct interhemispheric pathways.

SIGNIFICANCE

Our novel animation method has the potential to help investigators in addressing the mechanistic significance of the interhemispheric network dynamics supporting physiological function.

Laminar distribution of electrically evoked hippocampal short latency ripple activity highlights the importance of the subiculum in vivo in human epilepsy, an intraoperative study

OBJECTIVE

The goal of this study was to define the pathology and anesthesia dependency of single pulse electrical stimulation (SPES) dependent high-frequency oscillations (HFOs, ripples, fast ripples) in the hippocampal formation.

METHODS

Laminar profile of electrically evoked short latency (<100 ms) high-frequency oscillations (80-500 Hz) was examined in the hippocampus of therapy-resistant epileptic patients (6 female, 2 male) in vivo, under general anesthesia.

RESULTS

Parahippocampal SPES evoked HFOs in all recorded hippocampal subregions (Cornu Ammonis 2-3, dentate gyrus, and subiculum) were not uniform, rather the combination of ripples, fast ripples, sharp transients, and multiple unit activities. Mild and severe hippocampal sclerosis (HS) differed in the probability to evoke fast ripples: it decreased with the severity of sclerosis in CA2-3 but increased in the subiculum. Modulation in the ripple spectrum was observed only in the subiculum with increased fast HFO rate and frequency in severe HS. Inhalational anesthetics (isoflurane) suppressed the chance to evoke HFOs compared to propofol.

CONCLUSION

The presence of early HFOs in the dentate gyrus and early fast HFOs (>250 Hz) in the other subregions indicate the pathological nature of these evoked oscillations. Subiculum was found to be active producing HFOs in parallel with the cell loss in the hippocampus proper, which emphasize the role of this region in the generation of epileptic activity.

Corpus Callosum: Molecular Pathways in Mice and Human Dysgeneses

The corpus callosum is the largest of the 3 telencephalic commissures in eutherian (placental) mammals. Although the anterior commissure, and the hippocampal commissure before being pushed dorsally by the expanding frontal lobes, cross through the lamina reuniens (upper part of the lamina terminalis), the callosal fibers need a transient interhemispheric cellular bridge to cross. This review describes the molecular pathways that initiate the specification of the cells comprising this bridge, the specification of the callosal neurons, and the repulsive and attractive guidance molecules that convey the callosal axons toward, across, and away from the midline to connect with their targets.

A quantitative method for evaluating cortical responses to electrical stimulation

BACKGROUND

Electrical stimulation of the cortex using subdurally implanted electrodes can causally reveal structural connectivity by eliciting cortico-cortical evoked potentials (CCEPs). While many studies have demonstrated the potential value of CCEPs, the methods to evaluate them were often relatively subjective, did not consider potential artifacts, and did not lend themselves to systematic scientific investigations.

NEW METHOD

We developed an automated and quantitative method called SIGNI (Stimulation-Induced Gamma-based Network Identification) to evaluate cortical population-level responses to electrical stimulation that minimizes the impact of electrical artifacts. We applied SIGNI to electrocorticographic (ECoG) data from eight human subjects who were implanted with a total of 978 subdural electrodes. Across the eight subjects, we delivered 92 trains of approximately 200 discrete electrical stimuli each (amplitude 4-15 mA) to a total of 64 electrode pairs.

RESULTS

We verified SIGNI's efficacy by demonstrating a relationship between the magnitude of evoked cortical activity and stimulation amplitude, as well as between the latency of evoked cortical activity and the distance from the stimulated locations.

CONCLUSIONS

SIGNI reveals the timing and amplitude of cortical responses to electrical stimulation as well as the structural connectivity supporting these responses. With these properties, it enables exploration of new and important questions about the neurophysiology of cortical communication and may also be useful for pre-surgical planning.

Dysfunctional Brain Networking among Autonomic Regulatory Structures in Temporal Lobe Epilepsy Patients at High Risk of Sudden Unexpected Death in Epilepsy

BACKGROUND

Sudden unexpected death in epilepsy (SUDEP) is common among young people with epilepsy. Individuals who are at high risk of SUDEP exhibit regional brain structural and functional connectivity (FC) alterations compared with low-risk patients. However, less is known about network-based FC differences among critical cortical and subcortical autonomic regulatory brain structures in temporal lobe epilepsy (TLE) patients at high risk of SUDEP.

METHODS

32 TLE patients were risk-stratified according to the following clinical criteria: age of epilepsy onset, duration of epilepsy, frequency of generalized tonic-clonic seizures, and presence of nocturnal seizures, resulting in 14 high-risk and 18 low-risk cases. Resting-state functional magnetic resonance imaging (rs-fMRI) signal time courses were extracted from 11 bilateral cortical and subcortical brain regions involved in autonomic and other regulatory processes. After computing all pairwise correlations, FC matrices were analyzed using the network-based statistic. FC strength among the 11 brain regions was compared between the high- and low-risk patients. Increases and decreases in FC were sought, using high-risk > low-risk and low-risk > high-risk contrasts (with covariates age, gender, lateralization of epilepsy, and presence of hippocampal sclerosis).

RESULTS

High-risk TLE patients showed a subnetwork with significantly reduced FC (t = 2.5, p = 0.029) involving the thalamus, brain stem, anterior cingulate, putamen and amygdala, and a second subnetwork with significantly elevated FC (t = 2.1, p = 0.031), which extended to medial/orbital frontal cortex, insula, hippocampus, amygdala, subcallosal cortex, brain stem, thalamus, caudate, and putamen.

CONCLUSION

TLE patients at high risk of SUDEP showed widespread FC differences between key autonomic regulatory brain regions compared to those at low risk. The altered FC revealed here may help to shed light on the functional correlates of autonomic disturbances in epilepsy and mechanisms involved in SUDEP. Furthermore, these findings represent possible objective biomarkers which could help to identify high-risk patients and enhance SUDEP risk stratification via the use of non-invasive neuroimaging, which would require validation in larger cohorts, with extension to patients with other epilepsies and subjects who succumb to SUDEP.

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

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

The Sensed Presence as Right Hemispheric Intrusions into the Left Hemispheric Awareness of Self: An Illustrative Case Study

The hypothesis of vectorial hemisphericity predicts that left hemispheric intrusions of the right hemispheric equivalent of the sense of self should be associated with the experience of a “presence” of someone else. The neurophenomenological profile of a woman whose medical history satisfied these theoretical criteria (verified electrical anomalies that could encourage phasic discharges within the right temporal lobe and atrophy within the left temporoparietal region) is presented. In addition to interactions between electrical seizures and thinking, she reported a long history of sensed presences, ego-alien intrusions, and “sudden knowing of the subsequent sequences of seizures” before they occurred clinically. The existence of these neurocognitive processes demands a reevaluation of the psychiatric default explanations of “hysteria” and questions the belief that “awareness during seizures” or “premonition of subsequent somatosensory experience” contraindicates an epileptic process.

Direct Visualization and Mapping of the Spatial Course of Fiber Tracts at Microscopic Resolution in the Human Hippocampus

While hippocampal connectivity is essential to normal memory function, our knowledge of human hippocampal circuitry is largely inferred from animal studies. Using polarized light microscopy at 1.3 µm resolution, we have directly visualized the 3D course of key medial temporal pathways in 3 ex vivo human hemispheres and 2 ex vivo vervet monkey hemispheres. The multiple components of the perforant path system were clearly identified: Superficial sheets of fibers emanating from the entorhinal cortex project to the presubiculum and parasubiculum, intermixed transverse and longitudinal angular bundle fibers perforate the subiculum and then project to the cornu ammonis (CA) fields and dentate molecular layer, and a significant alvear component runs from the angular bundle to the CA fields. From the hilus, mossy fibers localize to regions of high kainate receptor density, and the endfolial pathway, mostly investigated in humans, merges with the Schaffer collaterals. This work defines human hippocampal pathways underlying mnemonic function at an unprecedented resolution.

In vivo visualization of connections among revised Papez circuit hubs using full q-space diffusion spectrum imaging tractography

Structural connections among the hubs of the revised Papez circuit remain to be elucidated in the human brain. As the original Papez circuit failed to explain functional imaging findings, a more detailed investigation is needed to delineate connections among the circuit's key hubs. Here we acquired diffusion spectrum imaging (DSI) from eight normal subjects and used data from the Human Connectome Project (HCP) to elucidate connections among hubs in the retrosplenial gyrus, hippocampus, mammillary bodies, and anterior thalamic nuclei. Our results show that the ventral hippocampal commissure (VHC) was visualized in all eight individual DSI datasets, as well as in the DSI and HCP group datasets, but a strictly defined VHC was only visualized in one individual dataset. Thalamic fibers were observed to connect with both the posterior cingulate cortex (PCC) and retrosplenial cortex (RSC). The RSC was mainly responsible for direct hippocampal connections, while the PCC was not. This indicates that the RSC and PCC represent separate functional hubs in humans, as also shown by previous primate axonal tracing studies and functional magnetic resonance imaging observations.

Connection between bilateral temporal regions: Tractography using human connectome data and diffusion spectrum imaging

Temporal lobe epilepsy often propagates inter-hemispherically. Although the pathway of the propagation was verified by electrophysiology, the trajectory remains poorly defined. DTI can depict fiber trajectory but it has limited angular resolution and cannot adequately assess cortical regions. We visualized potential pathways of bitemporal epilepsy propagation using diffusion spectrum imaging (DSI) with data consisting of 8 groups of 514 directions and diffusion templates of 842 subjects from the human connectome project (HCP). We verified the results with reference to the axonal-tracing literature. Both the large population overall and individual connection properties were investigated. In both the HCP 842 atlas and DSI individual data, the bilateral temporal pole was found to connect via the anterior commissure. The splenium of the corpus callosum was divided into 3 subregions (CS1, CS2, CS3) according to the form of connections. CS1 was predominately located at the rostral third and the dorsal part of middle third of the splenium; it communicated with the bilateral parietal lobe. SC2 was predominately located at the ventral middle third of the splenium. Fibers passed through the lateral wall of the lateral ventricle and connected to regions lateral of the occipitotemporal sulci. CS3 was located at the caudal third of the splenium. Together with the hippocampal commissure, its fibers constituted the medial wall of the lateral ventricle and distributed medially to the occipitotemporal sulci. The trajectory of bilateral temporal connections was visualized in this study; the results might help in the understanding and treatment of inter-hemispherical propagation of temporal-lobe epilepsy.

Depressive rumination and the emotional control circuit: An EEG localization and effective connectivity study

Ruminations are repetitive thoughts associated with symptoms, causes, and consequences of one's negative feelings. The objective of this study was to explore the neuronal basis of depressive rumination in a non-clinical population within the context of emotional control. Participants scoring high or low on the tendency to ruminate scale took part in the EEG experiment. Their EEG data were collected during a state of induced depressive ruminations and compared with positive and neutral conditions. We hypothesized that both groups would differ according to the level of activation and effective connectivity among the structures involved in the emotional control circuit. Clustering of independent components, together with effective connectivity (Directed Transfer Function), was performed using the EEG signal. The main findings involved decreased activation of the left dorsolateral prefrontal cortex (DLPFC) and increased activation of the left temporal lobe structures in the highly ruminating group. The latter result was most pronounced during the ruminative condition. Decreased information from the left DLPFC to the left temporal lobe structures was also found, leading to the conclusion that hypoactivation of the left DLPFC and its inability to modulate the activation of the left temporal lobe structures is crucial for the ruminative tendencies.

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

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

Increased BOLD activation in the left parahippocampal cortex after 1 year of medical school: an association with cumulative verbal memory learning

Although several studies have shown left-right hippocampus asymmetry during learning, it is unclear whether such asymmetry also exists for the parahippocampal cortex, a structure within the limbic system that is also involved in memory and learning. Using a common mental navigation task known to activate the bilateral parahippocampal cortex, this study aimed at determining how BOLD activation in these two areas changes after 1 year of medical school, a program characterized by intensive verbal learning. Fifteen first-year medical students participated in this study and underwent two sessions of functional MRI, at a 1-year interval. In the first session, we observed marginal differences between left and right parahippocampal cortex activity. However, 1 year later, left parahippocampal activation significantly increased (+4.7%), whereas the right remained stable. These results bring new information as to how intensive learning can modify regional metabolism in the human brain and how the left parahippocampal region is particularly important for cumulative verbal memory.

Incidence of functional bi-temporal connections in the human brain in vivo and their relevance to epilepsy surgery

The incidence of functional connections between human temporal lobes and their latencies were investigated using intracranial EEG responses to electrical stimulation with 1 msec single pulses in 91 patients assessed for surgery for treatment of epilepsy. The areas studied were amygdala, hippocampus, parahippocampal gyrus, fusiform gyrus, inferior and mid temporal gyrus. Furthermore, we assessed whether the presence of such connections are related to seizure onset extent and postsurgical seizure control. Responses were seen in any region of the contralateral temporal lobe when stimulating temporal regions in 30 patients out of the 91 (32.96%). Bi-hippocampal or bi-amygdalar projections were seen in only 5% of temporal lobes (N = 60) and between both fusiform gyri in 7.1% (N = 126). All other bilateral connections occurred in less than 5% of hemispheres. Depending on the structures, latencies ranged between 20 and 90 msec, with an average value of 60.2 msec. There were no statistical difference in the proportion of patients showing Engel Class I between patients with and without contralateral temporal connections. No difference was found in the proportion of patients showing bilateral or unilateral seizure onset among patients with and without contralateral temporal projections. The present findings corroborate that the functionality of bilateral temporal connections in humans is limited and does not affect the surgical outcome.

Functional connectivity between right and left mesial temporal structures

The aim of this study is to investigate functional connectivity between right and left mesial temporal structures using cerebrocerebral evoked potentials. We studied seven patients with drug-resistant focal epilepsy who were explored with stereotactically implanted depth electrodes in bilateral hippocampi. In all patients cerebrocerebral evoked potentials evoked by stimulation of the fornix were evaluated as part of a research project assessing fornix stimulation for control of hippocampal seizures. Stimulation of the fornix elicited responses in the ipsilateral hippocampus in all patients with a mean latency of 4.6 ms (range 2-7 ms). Two patients (29 %) also had contralateral hippocampus responses with a mean latency of 7.5 ms (range 5-12 ms) and without involvement of the contralateral temporal neocortex or amygdala. This study confirms the existence of connections between bilateral mesial temporal structures in some patients and explains seizure discharge spreading between homotopic mesial temporal structures without neocortical involvement.

Posterior cingulate epilepsy: clinical and neurophysiological analysis

OBJECTIVE

Posterior cingulate epilepsy (PCE) is misleading because the seizure onset is located in an anatomically deep and semiologically silent area. This type of epilepsy is rare and has not been well described yet. Knowledge of the characteristics of PCE is important for the interpretation of presurgical evaluation and better surgical strategy. The purpose of this study was to better characterise the clinical and neurophysiological features of PCE.

METHODS

This retrospective analysis included seven intractable PCE patients. Six patients had postcingulate ictal onset identified by stereotactic EEG (SEEG) evaluations. One patient had a postcingulate tumour. We analysed clinical semiology, the scalp EEG/SEEG findings and cortico-cortical evoked potential (CCEP).

RESULTS

The classifications of scalp EEG were various, including non-localisible, lateralised to the seizure onset side, regional parieto-occipital, regional frontocentral and regional temporal. Three of seven patients showed motor manifestations, including bilateral asymmetric tonic seizures and hypermotor seizures. In these patients, ictal activities spread to frontal (lateral premotor area, orbitofrontal cortex, supplementary motor area, anteior cingulate gyrus) and parietal (precuneus, posterior cingulate gyrus, inferior parietal lobule (IPL), postcentral gyrus) areas. Four patients showed dialeptic seizures or automotor seizures, with seizure spread to medial temporal or IPL areas. CCEP was performed in four patients, suggesting electrophysiological connections from the posterior cingulate gyrus to parietal, temporal, mesial occipital and mesial frontal areas.

CONCLUSIONS

This study revealed that the network from the posterior cingulate gyrus and the semiology of PCE (motor manifestation vs dialeptic/automotor seizure) varies depending upon the seizure spread patterns.

A decade of experience with deep brain stimulation for patients with refractory medial temporal lobe epilepsy

In this study, we present long-term results from patients with medial temporal lobe (MTL) epilepsy treated with deep brain stimulation (DBS). Since 2001, 11 patients (8M) with refractory MTL epilepsy underwent MTL DBS. When unilateral DBS failed to decrease seizures by > 90%, a switch to bilateral MTL DBS was proposed. After a mean follow-up of 8.5 years (range: 67-120 months), 6/11 patients had a ≥ 90% seizure frequency reduction with 3/6 seizure-free for > 3 years; three patients had a 40%-70% reduction and two had a < 30% reduction. In 3/5 patients switching to bilateral DBS further improved outcome. Uni- or bilateral MTL DBS did not affect neuropsychological functioning. This open study with an extended long-term follow-up demonstrates maintained efficacy of DBS for MTL epilepsy. In more than half of the patients, a seizure frequency reduction of at least 90% was reached. Bilateral MTL DBS may herald superior efficacy in unilateral MTL epilepsy.

Fiber tract stimulation can reduce epileptiform activity in an in-vitro bilateral hippocampal slice preparation

Mesial temporal lobe epilepsy (MTLE) is a common medically refractory neurological disease that has been treated with electrical stimulation of gray matter with limited success. However, stimulation of a white matter tract connecting the hippocampi could maximize treatment efficacy and extent. We tested low-frequency stimulation (LFS) of a novel target that enables simultaneous targeting of bilateral hippocampi: the ventral hippocampal commissure (VHC) with a novel in-vitro slice preparation containing bilateral hippocampi connected by the VHC. The goal of this study is to understand the role of hippocampal interplay in seizure propagation and reduction by commissural fiber tract stimulation. LFS is applied to the VHC as extracellular and intracellular recording techniques are combined with signal processing to estimate several metrics of epilepsy including: (1) total time occupied by seizure activity (%); (2) seizure duration (s); (3) seizures per minute (#); and (4) power in the ictal (V(2)Hz(-1)); as well as (5) interictal spectra (V(2)Hz(-1)). Bilateral epileptiform activity in this preparation is highly correlated between hippocampi. Application of LFS to the VHC reduces all metrics of epilepsy during treatment in an amplitude and frequency dependent manner. This study lends several insights into the mechanisms of bilateral seizure reduction by LFS of the VHC, including that depolarization blocking, LTD/LTP and GABA(A) are not involved. Importantly, enhanced post-stimulation 1-Hz spiking correlates with long-lasting seizure reduction and both are heightened by targeting bilateral hippocampi via the VHC. Therefore, stimulating bilateral hippocampi via a single electrode in the VHC may provide an effective MTLE treatment.

A NOVEL ELECTRICAL STIMULATION PARADIGM FOR THE SUPPRESSION OF EPILEPTIFORM ACTIVITY IN ANIN VIVOMODEL OF MESIAL TEMPORAL LOBE STATUS EPILEPTICUS

In this study, we present a novel low-frequency electrical stimulation paradigm for mesial temporal lobe epilepsy (MTLE). The paradigm utilizes the hippocampal commissure as a unique stimulation target to simultaneously influence large portions of the bilateral hippocampal network. When applied to an acute rat model of MTLE, animals that received stimulation exhibited an 88% reduction in the signal power of the bilateral epileptiform activity relative to the control group. In addition, the stimulation entrained the hippocampal network's spontaneous epileptiform activity and disrupted its bilateral synchrony.

Requirement of longitudinal synchrony of epileptiform discharges in the hippocampus for seizure generation: a pilot study

OBJECT

The goal in this study was to assess the role of longitudinal hippocampal circuits in the generation of interictal and ictal activity in temporal lobe epilepsy (TLE) and to evaluate the effects of multiple hippocampal transections (MHT).

METHODS

In 6 patients with TLE, the authors evaluated the synchrony of hippocampal interictal and ictal epileptiform discharges by using a cross-correlation analysis, and the effect of MHT on hippocampal interictal spikes was studied. Five of the 6 patients were studied with depth electrodes, and epilepsy surgery was performed in 4 patients (anterior temporal lobectomy in 1 and MHT in 3).

RESULTS

Four hundred eighty-two (95.1%) of 507 hippocampal spikes showed an anterior-to-posterior propagation within the hippocampus, with a fixed peak-to-peak interval. During seizures, a significant increase of synchronization between different hippocampal regions and between the hippocampus and the ipsilateral anterior parahippocampal gyrus was observed in all seizures. An ictal increase in synchronization between the hippocampus and ipsilateral amygdala was seen in only 24.1% of the seizures. No changes in synchronization were noticed during seizures between the hippocampi and the amygdala on either side. The structure leading the epileptic seizures varied over time during a given seizure and also from one seizure to another. Spike analysis during MHT demonstrated that there were two spike populations that reacted differently to this procedure--namely, 1) spikes that showed maximum amplitude at the head of the hippocampus (type H); and 2) spikes that showed the highest amplitude at the hippocampal body (type B). A striking decrease in amplitude and frequency of type B spikes was noticed in all 3 patients after transections at the head or anterior portion of the hippocampal body. Type H spikes were seen in 2 cases and did not change in amplitude and frequency throughout MHT. Type B spikes showed constantly high cross-correlation values in different derivations and a relatively fixed peak-to-peak interval before MHT. This fixed interpeak delay disappeared after the first transection, although high cross-correlation values persisted unchanged. All patients who underwent MHT remained seizure free for more than 2 years.

CONCLUSIONS

These data suggest that synchronized discharges involving the complete anterior-posterior axis of the hippocampal/parahippocampal (H/P) formation underlie the spread of epileptiform discharges outside the H/P structures and, therefore, for the generation of epileptic seizures originating in the H/P structures. This conclusion is supported by the following observations. 1) Hippocampal spikes are consistently synchronized in the whole hippocampal structures, with a fixed delay between the different hippocampal areas. 2) One or two transections between the head and body of the hippocampal formation are sufficient to abolish hippocampal spikes that are synchronized along the anterior-posterior axis of the hippocampus. 3) Treatment with MHT leads to seizure freedom in patients with H/P epilepsy.

How the serotonin transporter 5-HTTLPR polymorphism influences amygdala function: the roles of in vivo serotonin transporter expression and amygdala structure

The serotonin transporter-linked promoter region (5-HTTLPR) polymorphism of the serotonin transporter gene is associated with amygdala response during negative emotion. The aim of this study was to investigate whether this genotype effect on amygdala function is mediated by current serotonin transporter (5-HTT) levels or rather by genetically induced influences during neurodevelopment, shaping brain structure. A total of 54 healthy subjects underwent functional and structural magnetic resonance imaging, [(11)C]DASB positron emission tomography and 5-HTTLPR genotyping to analyze the interrelationships between amygdala activation during processing of unpleasant stimuli, 5-HTTLPR genotype, amygdala volumes and 5-HTT levels in the midbrain and in other brain regions. In line with previous research, carriers of the short allele (S) showed increased amygdala activation. Path analysis demonstrated that this genotype effect was not procured by current 5-HTT availability but by amygdala structure, with smaller amygdala volumes in the S than in the LL genotype, as well as smaller volumes being associated with increased amygdala activation. Our findings stress the role of genetic effects during neurodevelopment.

Uneven interhemispheric connections between left and right primary sensori-motor areas

To clarify the characteristics of interhemispheric connections, we investigated cortico-cortical evoked potentials (CCEP) in human. Fourteen patients with temporal lobe epilepsy who underwent invasive EEG monitoring with bilaterally implanted subdural electrodes were studied. Electric pulse stimuli were given in a bipolar fashion at two adjacent electrodes on and around the motor area (MA) or sensory area (SA), and CCEP responses were recorded by averaging electrocorticograms from the contralateral hemisphere. Seventy-two pairs of electrodes were stimulated, and 468 recordings were analyzed. Fifty-one of 468 recordings demonstrated CCEP responses. Of 51 responses, 16 consisted of an initial positive triphasic wave (Type 1), 27 had an initial negative biphasic wave (Type 2), and 8 showed an initial positive biphasic wave (type 3). The mean latencies of the earliest peaks were 13.1, 28.9, and 29.4 ms in Types 1, 2, and 3 responses, respectively. The responses were more frequently evoked by stimulating facial MA (f-MA) and nonfacial MA (nf-MA) than by stimulating SA or noneloquent area. In both f-MA and nf-MA stimulation, the responses were more frequently recorded at the contralateral f-MA than at the contralateral nf-MA or other areas. SA stimulation never evoked CCEP responses at the contralateral MA or SA. The amplitudes were maximal when f-MA was stimulated and responses recorded at the contralateral f-MA. These findings suggest that the interhemispheric connections are uneven. Both f-MA and nf-MA send dense interhemispheric connections to the contralateral f-MA. SA may have no or only rare direct connection with the contralateral MA or SA.

From intracerebral EEG signals to brain connectivity: identification of epileptogenic networks in partial epilepsy

Epilepsy is a complex neurological disorder characterized by recurring seizures. In 30% of patients, seizures are insufficiently reduced by anti-epileptic drugs. In the case where seizures originate from a relatively circumscribed region of the brain, epilepsy is said to be partial and surgery can be indicated. The success of epilepsy surgery depends on the accurate localization and delineation of the epileptogenic zone (which often involves several structures), responsible for seizures. It requires a comprehensive pre-surgical evaluation of patients that includes not only imaging data but also long-term monitoring of electrophysiological signals (scalp and intracerebral EEG). During the past decades, considerable effort has been devoted to the development of signal analysis techniques aimed at characterizing the functional connectivity among spatially distributed regions over interictal (outside seizures) or ictal (during seizures) periods from EEG data. Most of these methods rely on the measurement of statistical couplings among signals recorded from distinct brain sites. However, methods differ with respect to underlying theoretical principles (mostly coming from the field of statistics or the field of non-linear physics). The objectives of this paper are: (i) to provide an brief overview of methods aimed at characterizing functional brain connectivity from electrophysiological data, (ii) to provide concrete application examples in the context of drug-refractory partial epilepsies, and iii) to highlight some key points emerging from results obtained both on real intracerebral EEG signals and on signals simulated from physiologically plausible models in which the underlying connectivity patterns are known a priori (ground truth).

Studying network mechanisms using intracranial stimulation in epileptic patients

Patients suffering from focal drug-resistant epilepsy who are explored using intracranial electrodes allow to obtain data of exceptional value for studying brain dynamics in correlation with pathophysiological and cognitive processes. Direct electrical stimulation (DES) of cortical regions and axonal tracts in those patients elicits a number of very specific perceptual or behavioral responses, but also abnormal responses due to specific configurations of epileptic networks. Here, we review how anatomo-functional brain connectivity and epilepsy network mechanisms can be assessed from DES responses measured in patients. After a brief summary of mechanisms of action of brain electrical stimulation, we recall the conceptual framework for interpreting DES results in the context of brain connectivity and review how DES can be used for the characterization of functional networks, the identification of the seizure onset zone, the study of brain plasticity mechanisms, and the anticipation of epileptic seizures. This pool of exceptional data may be underexploited by fundamental research on brain connectivity and leaves much to be learned.

The corpus callosum, the other great forebrain commissures, and the septum pellucidum: anatomy, development, and malformation

There are three telencephalic commissures which are paleocortical (the anterior commissure), archicortical (the hippocampal commissure), and neocortical. In non-placental mammals, the neocortical commissural fibers cross the midline together with the anterior and possibly the hippocampal commissure, across the lamina reuniens (joining plate) in the upper part of the lamina terminalis. In placental mammals, a phylogenetically new feature emerged, which is the corpus callosum: it results from an interhemispheric fusion line with specialized groups of mildline glial cells channeling the commissural axons through the interhemispheric meninges toward the contralateral hemispheres. This concerns the frontal lobe mainly however: commissural fibers from the temporo-occipital neocortex still use the anterior commissure to cross, and the posterior occipito-parietal fibers use the hippocampal commissure, forming the splenium in the process. The anterior callosum and the splenium fuse secondarily to form the complete commissural plate. Given the complexity of the processes involved, commissural ageneses are many and usually associated with other diverse defects. They may be due to a failure of the white matter to develop or to the commissural neurons to form or to migrate, to a global failure of the midline crossing processes or to a selective failure of commissuration affecting specific commissural sites (anterior or hippocampal commissures, anterior callosum), or specific sets of commissural axons (paleocortical, hippocampal, neocortical commissural axons). Severe hemispheric dysplasia may prevent the axons from reaching the midline on one or both sides. Besides the intrinsically neural defects, midline meningeal factors may prevent the commissuration as well (interhemispheric cysts or lipoma). As a consequence, commissural agenesis is a malformative feature, not a malformation by itself. Good knowledge of the modern embryological data may allow for a good understanding of a specific pattern in a given individual patient, paving the way for better clinical correlation and genetic counseling.

Fables of the prefrontal cortex

On the basis of neuroiinaging studies, Posner & Raichle summarily report that the prefrontal cortex is involved in executive functioning and attention. In contrast to that superficial view, we briefly describe a testable model of the kinds of representations that are stored in prefrontal cortex, which, when activated, are expressed via plans, actions, thematic knowledge, and schemas.

Bright red spots or – the meaning of the meaning

There are methodological problems with the new techniques reviewed by Posner & Raichle. Some brain mechanisms are not detected by the temporal and spatial resolution. Questions are also raised by the stimulation paradigms.

Broca's area: Motor encoding in somatic space

Encoding articulate speech is widely accepted as the principal (or sole) role of the frontal operculum. Clinical observations of speech apraxia have been confirmed by brain-imaging studies of speech production. We present evidence that the frontal operculum also programs limb movements. We argue that this area is a ventral counterpart of the dorsal premotor area. The two are functionally distinguished by specialization for somatic and visual space, respectively.

Images in search of a theory

Images of mind is an exciting book, well-written and wellorganized, but many of the connections the authors draw between PET scan results and more general psychological issues are somewhat strained.

Mind mappers and cognitive modelers: Toward cross-fertilization

It is argued that current neuroimaging studies can provide useful constraints for the construction of models of cognition, and that these studies should be guided by cognitive models. A numberof challenges for a successful cross-fertilization between “mind mappers” and cognitive modelers are discussed in the light of current research on word recognition.

Brain imaging, ethology, and the nonhuman mind

Posner & Raichle's (1994) exciting, wonderfully illustrated book describes the past successes and future potential of the relatively noninvasive imaging of the nervous systems of living people. The focus has been on cognitive processes but there is no reason why emotional and motivational systems cannot also be tapped. Although the authors do not formally address such contentious issues as consciousness and the private experience of other species, imaging methods may hold promise for helping us to understand these phenomena, as well as to integrate psychological processes into ethological and phylogenetic research in general.

If a picture is worth a thousand words, how many pictures is a word worth?

Pictures of normal brain activity during human thought can be worth a great deal. Electrophysiology and functional neuroimaging together allow both temporal and spatial dimensions of neurocognitive functions to be explored. Although these techniqueshave their limitations, the Cognitive Neuroscience approach is well-suited to pursuing questions about how words are perceived, understood, and remembered.

Multiple scales of brain-mind interactions

Posner & Raichle's Images of mind is an excellent educational book and very well written. Some flaws as a scientific publication are: (a) the accuracy of the linear subtraction method used in PET is subject to scrutiny by further research at finer spatial-temporal resolutions; (b) lack of accuracy of the experimental paradigm used for EEG complementary studies.