WONOEP appraisal: Network concept from an imaging perspective

Neuroimaging techniques applied to a variety of organisms—from zebrafish, to rodents to humans—can offer valuable insights into neuronal network properties and their dysfunction in epilepsy. A wide range of imaging methods used to monitor neuronal circuits and networks during evoked seizures in animal models and advances in functional magnetic resonance imaging (fMRI) applied to patients with epilepsy were discussed during the XIV Workshop on Neurobiology of Epilepsy (XIV WONOEP) organized in 2017 by the Neurobiology Commission of the International League Against Epilepsy (ILAE). We review the growing number of technological approaches developed, as well as the current state of knowledge gained from studies applying these advanced imaging approaches to epilepsy research.     

Imaging brain activity in conscious animals using functional MRI

Functional magnetic resonance imaging (fMRI) in humans has helped improve our understanding of the neuroanatomical organization of behavior. Unfortunately, fMRI in animal studies has not kept pace with the human work. Experiments are limited because animals must be anesthetized to prevent motion artifacts, precluding most studies involving neuroimaging of brain activity during behavior. The present study tested a newly developed head and body holder for performing fMRI in fully conscious animals. Significant changes in signal intensities were observed in the somatosensory cortex of conscious rats in response to electrical shock of the hindpaw. These changes in evoked signal ranged between 4 and 19% and were accompanied by significant increases in local cerebral blood flow. The fMRI study was performed with a 2.0-Tesla spectrometer. Using this non-invasive method of imaging brain activity in conscious animals, it is now possible to perform developmental studies in animal models of neurological and psychiatric disorders.     

Coupling of Cortical and Thalamic Ictal Activity in Human Partial Epilepsy: Demonstration by Functional Magnetic Resonance Imaging

Summary: Purpose: To localize metabolic coupling between a cortical seizure focus and other brain regions by using functional magnetic resonance imaging (fMRI) data of ictal events obtained in a patient with frequent partial seizures involving his right face.
Methods: Cross-correlation analysis was used to examine time-dependent alterations in regional signal intensity that correlated with signal-intensity changes from a well-characterized cortical seizure focus in a patient with frequent partial seizures.
Results: Signal changes in the left ventrolateral thalamus showed a high degree of temporal correlation with signal changes in the left frontal cortical seizure focus, demonstrating close corticothalamic coupling of metabolism.
Conclusions: A significant role for thalamocortical interactions in the pathophysiology of epilepsy has been suggested by studies in animal models and human patients. This finding provides further support for the integral involvement of the thalamus in human focal epilepsy and underscores the potential for identifying neuronal networks by using cross-correlation analysis of fMRI data.     

Simultaneous EEG-fMRI in human epilepsy

Electroencephalography (EEG) has been used to study and characterize epilepsy for decades, but has a limited ability to localize epileptiform activity to a specific brain region. With recent technological advances, high-quality EEG can now be recorded during functional magnetic resonance imaging (fMRI), which characterizes brain activity through local changes in blood oxygenation. By combining these techniques, the specific timing of interictal events can be identified on the EEG at millisecond resolution and spatially localized with fMRI at millimeter resolution. As a result, simultaneous EEG-fMRI provides the opportunity to better investigate the spatiotemporal mechanisms of the generation of epileptiform activity in the brain. This article discusses the technical considerations and their solutions for recording simultaneous EEG-fMRI and the results of studies to date. It also addresses the application of EEG-fMRI to epilepsy in humans, including clinical applications and ongoing challenges.     

Validation of an automated punctate mechanical stimuli delivery system designed for fMRI studies in rodents

Functional magnetic resonance imaging (fMRI) is increasingly being used for animal studies studying the transmission of nociceptive information. Application of noxious mechanical stimuli is widely used for animal and human assessment of pain processing. Any accessory hardware used in animal imaging studies must, however, be sufficiently small to fit in the magnet bore diameter and be non-magnetic. We have developed a system that can apply mechanical stimuli simultaneously with fMRI. This system consists of a standardized instrument to deliver mechanical stimuli (VonFrey monofilament) and a gas-pressured mechanical transducer. These components were integrated with a computer console that controlled the period of stimuli to match acquisition scans. Preliminary experiments demonstrated that the force-stimulus transducer did not influence MRI signal to noise ratio. Mechanical stimulation of the hindpaw significantly increased blood oxygen level dependent (BOLD) signal intensity in several midbrain regions involved in the processing of nociceptive information in the rat (p<0.001, uncorrected for multiple comparisons). This system can be applied to both animal and human imaging studies and has a wide range of applications for studies of nociceptive processing.     

Synthetic brain imaging: grasping, mirror neurons and imitation.

The article contributes to the quest to relate global data on brain and behavior (e.g. from PET, Positron Emission Tomography, and fMRI. functional Magnetic Resonance Imaging) to the underpinning neural networks. Models tied to human brain imaging data often focus on a few "boxes" based on brain regions associated with exceptionally high blood flow, rather than analyzing the cooperative computation of multiple brain regions. For analysis directly at the level of such data, a schema-based model may be most appropriate. To further address neurophysiological data, the Synthetic PET imaging method uses computational models of biological neural circuitry based on animal data to predict and analyze the results of human PET studies. This technique makes use of the hypothesis that rCBF (regional cerebral blood flow) is correlated with the integrated synaptic activity in a localized brain region. We also describe the possible extension of the Synthetic PET method to fMRI. The second half of the paper then exemplifies this general research program with two case studies, one on visuo-motor processing for control of grasping (Section 3 in which the focus is on Synthetic PET) and the imitation of motor skills (Sections 4 and 5, with a focus on Synthetic fMRI). Our discussion of imitation pays particular attention to data on the mirror system in monkey (neural circuitry which allows the brain to recognize actions as well as execute them). Finally, Section 6 outlines the immense challenges in integrating models of different portions of the nervous system which address detailed neurophysiological data from studies of primates and other species; summarizes key issues for developing the methodology of Synthetic Brain Imaging; and shows how comparative neuroscience and evolutionary arguments will allow us to extend Synthetic Brain Imaging even to language and other cognitive functions for which few or no animal data are available.     

Simultaneous EEG and fMRI recordings (EEG‐fMRI) in children with epilepsy

By combining electroencephalography (EEG) with functional magnetic resonance imaging (fMRI) it is possible to describe blood oxygenation level–dependent (BOLD) signal changes related to EEG patterns. This way, EEG‐pattern–associated networks of hemodynamic changes can be detected anywhere in the brain with good spatial resolution. This review summarizes EEG‐fMRI studies that have been performed in children with epilepsy. EEG‐fMRI studies in focal epilepsy (structural and nonlesional cases, benign epilepsy with centrotemporal spikes), generalized epilepsy (especially absence epilepsy), and epileptic encephalopathies (West syndrome, Lennox‐Gastaut syndrome, continuous spike and waves during slow sleep, and Dravet syndrome) are presented. Although EEG‐fMRI was applied mainly to localize the region presumably generating focal interictal discharges in focal epilepsies, EEG‐fMRI identified underlying networks in patients with generalized epilepsies and thereby contributed to a better understanding of these epilepsies. In epileptic encephalopathies a specific fingerprint of hemodynamic changes associated with the particular syndrome was detected. The value of the EEG‐fMRI technique for diagnosis and investigation of pathogenetic mechanisms of different forms of epilepsy is discussed.     

Imaging epileptic activity using functional MRI

This review concentrates on functional MRI (fMRI) methods to identify the epileptic focus, i.e. ictal and interictal fMRI. First, established clinical applications of fMRI in the field of epilepsy are briefly introduced: fMRI of sensorimotor, language and memory function can already be considered as clinically relevant tools to identify the eloquent cortex and to predict postoperative functional deficits in patients considered for epilepsy surgery. fMRI offers a valid alternative to invasive methods like the Wada test for establishing language dominance, and it is likely that it will also replace the Wada test for assessing presurgical memory function in the nearer future. Ictal fMRI (fMRI studies of epileptic seizures) will remain confined to exceptional cases due to practical limitations. The generators of interictal epileptiform discharges (IED) can be studied with EEG-correlated fMRI. Despite its technical challenges, it has proved useful to provide insights into the generation of IED in patients with focal and generalized epilepsy. In selected patients with focal IED, EEG-correlated fMRI has the potential to reproducibly identify cortical areas involved in generating IED, i.e. the irritative zone. In patients with generalizes IED, suspension of functional networks due to the IED can be demonstrated. The utility of EEG-correlated fMRI in clinical epileptology cannot be definitely determined yet.     

Towards understanding language organisation in the brain using fMRI

Functional magnetic resonance imaging (fMRI), which allows non-invasive mapping of human cognitive functions, has become an important tool for understanding language function. An understanding of component processes and sources of noise in the images is contributing to increased confidence in the reproductability of studies. This allows clinical applications, e.g., for pre-surgical lateralisation of language functions in patients with temporal lobe epilepsy. fMRI is a sensitive method for mapping regions involved in language functions. We recently have applied it to study the effect of word surface form on reading with a comparison of responses to Chinese characters or alphabetical Pinyin. Interpretation of fMRI activations must be made with caution; fMRI suggests task-associated activation, but does not independently confirm that such activity is necessary. However, complementary studies can be performed using transcranial magnetic stimulation (TMS), which can be used to interfere with brain activity in a specific region transiently for characterisation of the behavioural effects. We describe how TMS combined with fMRI has confirmed a role for the left inferior frontal cortex in semantic processing.     

Brain complex network analysis by means of resting state fMRI and graph analysis: Will it be helpful in clinical epilepsy?

Functional magnetic resonance imaging (fMRI) has just completed 20 years of existence. It currently serves as a research tool in a broad range of human brain studies in normal and pathological conditions, as is the case of epilepsy. To date, most fMRI studies aimed at characterizing brain activity in response to various active paradigms. More recently, a number of strategies have been used to characterize the low-frequency oscillations of the ongoing fMRI signals when individuals are at rest. These datasets have been largely analyzed in the context of functional connectivity, which inspects the covariance of fMRI signals from different areas of the brain. In addition, resting state fMRI is progressively being used to evaluate complex network features of the brain. These strategies have been applied to a number of different problems in neuroscience, which include diseases such as Alzheimer's, schizophrenia, and epilepsy. Hence, we herein aimed at introducing the subject of complex network and how to use it for the analysis of fMRI data. This appears to be a promising strategy to be used in clinical epilepsy. Therefore, we also review the recent literature that has applied these ideas to the analysis of fMRI data in patients with epilepsy.This article is part of a Special Issue entitled “NEWroscience 2013”.     

Disorders of reproduction in epilepsy--what can we learn from animal studies?

Several animal studies have shown that both the epilepsy itself and many antiepileptic drugs (AEDs) affect reproductive endocrine function in both males and females. Epileptic activity may lead to arrested ovarian cyclicity, anovulatory cycles, polycystic ovaries, and endocrine changes in female animals. In males, seizures disturb normal reproductive physiology by inducing endocrine changes, alterations in gonadal size, and hyposexuality. Several AEDs also affect endocrine function, fertility, and gonadal morphology in both sexes. This paper reviews the literature regarding animal studies related to reproductive disorders in epilepsy. Although care should always be taken when applying data from animal experiments to the human situation, animal models provide a unique possibility for investigating the independent effects of the epilepsy itself and the effects of AEDs in isolation, without confounding factors. By constantly comparing results from clinical and animal studies, and by developing appropriate animal models, several mechanistic questions regarding the complex interplay between epilepsy, hormones, and AEDs can be explored. Animal experiments should play an integral part in the study of reproductive endocrine disorders in epilepsy.     

The utility of functional magnetic resonance imaging in epilepsy and language

Functional magnetic resonance imaging (fMRI) is a viable presurgical tool for use with the pediatric epilepsy population as replacement for the intra-carotid sodium amobarbital test (IAT) used to identify hemispheric language dominance. This paper reviews the current imaging research on the identification of language cortex in pediatric epilepsy patients and in normal children. A review of the literature comparing fMRI to the IAT and electrocortical stimulation suggests that fMRI reliably identifies the dominant hemisphere, with pediatric and adult studies producing comparable results. Within-hemisphere localization of eloquent cortex with fMRI is more problematic. Paradigm selection, data analysis techniques, and considerations specific to imaging children are discussed. Utility of fMRI for studying neural plasticity as a result of brain insult (eg, epilepsy) is also considered.     

Workshop on Neurobiology of Epilepsy appraisal: New systemic imaging technologies to study the brain in experimental models of epilepsy

Modern functional neuroimaging provides opportunities to visualize activity of the entire brain, making it an indispensable diagnostic tool for epilepsy. Various forms of noninvasive functional neuroimaging are now also being performed as research tools in animal models of epilepsy and provide opportunities for parallel animal/human investigations into fundamental mechanisms of epilepsy and identification of epilepsy biomarkers. Recent animal studies of epilepsy using positron emission tomography, tractography, and functional magnetic resonance imaging were reviewed. Epilepsy is an abnormal emergent property of disturbances in neuronal networks which, even for epilepsies characterized by focal seizures, involve widely distributed systems, often in both hemispheres. Functional neuroimaging in animal models now provides opportunities to examine neuronal disturbances in the whole brain that underlie generalized and focal seizure generation as well as various types of epileptogenesis. Tremendous advances in understanding the contribution of specific properties of widely distributed neuronal networks to both normal and abnormal human behavior have been provided by current functional neuroimaging methodologies. Successful application of functional neuroimaging of the whole brain in the animal laboratory now permits investigations during epileptogenesis and correlation with deep brain electroencephalography (EEG) activity. With the continuing development of these techniques and analytical methods, the potential for future translational research on epilepsy is enormous. A PowerPoint slide summarizing this article is available for download in the Supporting Information section here     

Where fMRI and electrophysiology agree to disagree: corticothalamic and striatal activity patterns in the WAG/Rij rat

The relationship between neuronal activity and hemodynamic changes plays a central role in functional neuroimaging. Under normal conditions and in neurological disorders such as epilepsy, it is commonly assumed that increased functional magnetic resonance imaging (fMRI) signals reflect increased neuronal activity and that fMRI decreases represent neuronal activity decreases. Recent work suggests that these assumptions usually hold true in the cerebral cortex. However, less is known about the basis of fMRI signals from subcortical structures such as the thalamus and basal ganglia. We used WAG/Rij rats (Wistar albino Glaxo rats of Rijswijk), an established animal model of human absence epilepsy, to perform fMRI studies with blood oxygen level-dependent and cerebral blood volume (CBV) contrasts at 9.4 tesla, as well as laser Doppler cerebral blood flow (CBF), local field potential (LFP), and multiunit activity (MUA) recordings. We found that, during spike-wave discharges, the somatosensory cortex and thalamus showed increased fMRI, CBV, CBF, LFP, and MUA signals. However, the caudate-putamen showed fMRI, CBV, and CBF decreases despite increases in LFP and MUA signals. Similarly, during normal whisker stimulation, the cortex and thalamus showed increases in CBF and MUA, whereas the caudate-putamen showed decreased CBF with increased MUA. These findings suggest that neuroimaging-related signals and electrophysiology tend to agree in the cortex and thalamus but disagree in the caudate-putamen. These opposite changes in vascular and electrical activity indicate that caution should be applied when interpreting fMRI signals in both health and disease from the caudate-putamen, as well as possibly from other subcortical structures.     

Physiological monitoring of small animals during magnetic resonance imaging

Maintaining a stable physiologic state is essential when studying animal models of epilepsy with simultaneous electroencephalograph (EEG) and functional magnetic resonance imaging (fMRI) or EEG and magnetic resonance spectroscopy (MRS). To achieve and maintain such stability in rats in the MRI environment, a minimally invasive but comprehensive system was developed to monitor body temperature, heart rate, blood pressure, blood oxygen saturation and end-tidal CO2 (ETCO2) of expired gas. All physiologic parameters were successfully monitored in Sprague-Dawley rats without interfering with EEG recordings during simultaneous fMRI and MRS studies. Body temperature, heart rate, blood pressure, blood oxygen saturation, and ETCO2, were maintained between 36.5 and 37.5 degrees C, 250-450 beats/min, 136+/-17 mmHg, >90%, and 20-35 mmHg, respectively for 6-8 h under inhalational anesthesia. This set-up could be extended to study in vivo applications in other laboratory animals with only minor modifications.     

Application of Coregistration for Imaging of Animal Models of Epilepsy

Summary:  The past decade has seen a surge in the utilization of small animal imaging for epilepsy research. In vivo imaging studies have the potential to provide important insights into the structural and functional correlates of the development and progression of epilepsy in these models. However, the small size of the rodent brain means that anatomic resolution is often relatively poor for many imaging modalities, particularly those providing functional information such as positron emission tomography. Coregistration of these images with those of higher structural resolution, such as MRI, provides an attractive approach to this problem, and also allows correlations between structural and functional imaging data. Image coregistration is commonly utilized in clinical research and practice. However, its application for small animal images has been, to date, relatively under utilized and largely unvalidated. The current review aims to provide an overview of image coregistration methods, particularly for MRI and PET, and their application to imaging of small animal models of epilepsy. Methodological advantages and potential traps are highlighted.     

Electroencephalography/functional MRI in human epilepsy: what it currently can and cannot do

PURPOSE OF REVIEW: Simultaneous recording of electroencephalogram and functional MRI is being increasingly applied to the investigation of normal cerebral processes and disorders, particularly epilepsies. We will summarize recent epilepsy-related studies and appraise the clinical and scientific value of EEG/fMRI. RECENT FINDINGS: Interictal and ictal EEG/fMRI can provide helpful information in the presurgical evaluation of epilepsy. At present, EEG/fMRI cannot supercede any of the current methods because validation studies are lacking, informative results are only obtained in some patients, and haemodynamic activation and deactivation patterns are not always of localizing value. EEG/fMRI data often identify distributed brain areas and can help to generate concepts of epileptogenic networks both in individual patients and groups with particular epilepsy syndromes. SUMMARY: Clinically, EEG/fMRI studies may influence further investigations such as more detailed structural imaging or the planning of intracranial electrophysiological studies by generating hypotheses about the location of epileptic foci. Validation studies are underway to determine whether such clinical applications are appropriate. EEG/fMRI can also assess epileptogenic networks and changes in brain state, leading to a new dimension of understanding of dynamic cerebral processes in health and disease.     

Functional magnetic resonance imaging in the treatment of epilepsy

Although structural magnetic resonance imaging (MRI) is now in routine use in the evaluation and management of epilepsy, functional MRI (fMRI) has recently begun to provide a noninvasive and widely available modality for assessing regional brain function. fMRI studies of language and memory are able to show discrete areas of activation in cerebral cortex, are useful in lateralizing language and memory during presurgical evaluation, and are providing further insight into the processes underlying cerebral plasticity in the brains of epilepsy patients. The use of fMRI for localization of ictal phenomena may also contribute to the localization of seizure foci and to a better understanding of the pathophysiology of electrographic spikes. The combination of fMRI with electroencephalogram and other advanced structural imaging techniques may not only improve seizure localization, but may also contribute valuable information towards a better understanding of the pathophysiology of epilepsy and its consequences on brain development.     

Pr?klinische Positronen-Emissions-Tomographie-Studien in Epilepsiemodellen

Mechanisms leading to the development of epilepsy as well as pharmacoresistance are still insufficiently understood and animal models remain essential to investigate the underlying pathologies. Positron emission tomography (PET) is a nuclear medical method allowing the visualization of neurobiological processes and hereby represents a valuable tool in clinical and preclinical epilepsy research. Meanwhile, the specific development of small animal PET scanners enables comprehensive longitudinal studies in rat and mouse epilepsy models for the whole process of epileptogenesis. Although possibilities for molecule labeling are theoretically unlimited, 2-[¹⁸F]-fluoro-2-deoxy-D-glucose ([¹⁸F]FDG), which serves as a marker of cellular glucose consumption, is the PET tracer predominantly used in epilepsy research to date. Development and preclinical evaluation of new PET tracers, which might serve as biomarkers for epileptogenesis or seizure-associated processes will be in focus of forthcoming research efforts. Moreover, such biomarkers can be suitable means for monitoring the success of therapeutic strategies in animal models and human patients.     

Critical evaluation of animal models for localization-related epilepsies

There are many forms of human partial seizures and many human localization-related epilepsies. Idiopathic epilepsies undoubtedly have pathophysiologic substrates different from those of symptomatic epilepsies, and there is evidence that some forms of limbic epilepsy involve different epileptogenic mechanisms than neocortical epilepsies. Although these mechanisms are best studied and understood by direct investigations of patients, this is often impractical and experimental animal models are also necessary. The use of experimental animals requires that the relevance of each model to a human condition be determined. Human epilepsies are comprised of multiple component parts which can be modeled independently. For instance, acute animal models provide opportunities to study epileptic seizures, but chronic models are necessary for investigation of processes relevant to epileptic conditions, such as epileptogenesis, transition from interictal to ictal state, and long-term consequences of epilepsy. Interactions between localized epileptic activity and cerebral maturation can also be studied in the animal laboratory. Experimental animal models of human partial seizures and localization-related epilepsies can be used to further investigations on basic mechanisms that cannot be pursued in patients, and to develop hypotheses concerning the fundamental neuronal processes underlying epilepsy and epilepsy-related phenomena that subsequently can be validated in patients. In addition, it would be of great clinical utility to develop animal models of partial seizures or localization-related epilepsy that could be used cost-effectively to screen potential antiepileptic drugs.Original research reported by the author was supported in part by Grants NS-02808, NS-15654 and GM-24839, from the National Institutes of Health, and Contract DE-AC03-76-SF00012 from the Department of Energy.