ABSTRACT Early studies on the modeling of electromagnetic (EM) field interactions with the human ... more ABSTRACT Early studies on the modeling of electromagnetic (EM) field interactions with the human head have shown that induced current densities in the brain depend on both tissue geometry and its conductive properties. However, no head model of sufficient complexity for studying the physics of induced brain activation has been developed which provides well-defined smooth boundaries between tissues of different conductivities and orientations. In our study, we generated a detailed finite element model of the head that includes structural details to the level of cerebral gyri and sulci as well as axonal fiber tracts by combining different imaging modalities, namely computed tomography, magnetic resonance and diffusion tensor imaging. The anisotropic properties of brain tissues accompanying these details have also been included.
Neuronal populations in the brain achieve levels of synchronous electrophysiological activity as ... more Neuronal populations in the brain achieve levels of synchronous electrophysiological activity as a consequence of both normal brain functions as well as during pathological states such as in epileptic seizures. Understanding the nature of this synchrony and the dynamics of neuronal oscillators in the brain is a critical component towards decoding such complex behaviors. We have sought to achieve a more in-depth understanding of the dynamics underlying the evolution of seizures in limbic epilepsy by analyzing recordings of local field potentials from three subcortical nuclei that are part of the circuit of Papez in a kainic acid rat model of temporal lobe epilepsy using the empirical mode decomposition technique. The empirical mode decomposition allows for an adaptive and nonlinear decomposition of the local field potentials into a series of finite oscillatory components. We calculated the frequencies, power, and measures of phase synchrony of these oscillatory components as seizures evolve in the brain and discovered patterns of phase synchrony that varies between the different stages of the seizures.
IEEE Transactions on Neural Systems and Rehabilitation Engineering, Feb 1, 2008
Experimental reports have appeared which challenge the dogma that epileptic seizures arise as a c... more Experimental reports have appeared which challenge the dogma that epileptic seizures arise as a consequence of neuronal hypersynchronization. We sought to explore what mechanisms that desynchronize neuronal firing could induce epileptic seizures. A computer model of connections in a mammalian hippocampal slice preparation was constructed including two recently-reported distinct inhibitory feedback circuits. When inhibition by interneurons that synapse on pyramidal dendrites was decreased, highly localized seizure-like bursting was observed in the CA3 region similar to that which occurs experimentally under GABAergic blockade. In contrast, when inhibition by interneurons that synapse in the axosomatic region was similarly decreased, no such bursting was observed. However, when this transient inhibition was increased, normal coordinated spread of excitation was interrupted by high-frequency localized seizure-like bursting. The increase of this inhibitory input resulted in decreased cell coupling of pyramidal neurons. A decrease in phase coherence was initially observed until seizure-like activity initiated causing a net increase in coherence as has been observed in epileptic patients. These results provide a possible pathway in which a decrease in synchronization could provide the trigger for inducing epileptiform activity.
ABSTRACT Early studies on the modeling of electromagnetic (EM) field interactions with the human ... more ABSTRACT Early studies on the modeling of electromagnetic (EM) field interactions with the human head have shown that induced current densities in the brain depend on both tissue geometry and its conductive properties. However, no head model of sufficient complexity for studying the physics of induced brain activation has been developed which provides well-defined smooth boundaries between tissues of different conductivities and orientations. In our study, we generated a detailed finite element model of the head that includes structural details to the level of cerebral gyri and sulci as well as axonal fiber tracts by combining different imaging modalities, namely computed tomography, magnetic resonance and diffusion tensor imaging. The anisotropic properties of brain tissues accompanying these details have also been included.
Neuronal populations in the brain achieve levels of synchronous electrophysiological activity as ... more Neuronal populations in the brain achieve levels of synchronous electrophysiological activity as a consequence of both normal brain functions as well as during pathological states such as in epileptic seizures. Understanding the nature of this synchrony and the dynamics of neuronal oscillators in the brain is a critical component towards decoding such complex behaviors. We have sought to achieve a more in-depth understanding of the dynamics underlying the evolution of seizures in limbic epilepsy by analyzing recordings of local field potentials from three subcortical nuclei that are part of the circuit of Papez in a kainic acid rat model of temporal lobe epilepsy using the empirical mode decomposition technique. The empirical mode decomposition allows for an adaptive and nonlinear decomposition of the local field potentials into a series of finite oscillatory components. We calculated the frequencies, power, and measures of phase synchrony of these oscillatory components as seizures evolve in the brain and discovered patterns of phase synchrony that varies between the different stages of the seizures.
IEEE Transactions on Neural Systems and Rehabilitation Engineering, Feb 1, 2008
Experimental reports have appeared which challenge the dogma that epileptic seizures arise as a c... more Experimental reports have appeared which challenge the dogma that epileptic seizures arise as a consequence of neuronal hypersynchronization. We sought to explore what mechanisms that desynchronize neuronal firing could induce epileptic seizures. A computer model of connections in a mammalian hippocampal slice preparation was constructed including two recently-reported distinct inhibitory feedback circuits. When inhibition by interneurons that synapse on pyramidal dendrites was decreased, highly localized seizure-like bursting was observed in the CA3 region similar to that which occurs experimentally under GABAergic blockade. In contrast, when inhibition by interneurons that synapse in the axosomatic region was similarly decreased, no such bursting was observed. However, when this transient inhibition was increased, normal coordinated spread of excitation was interrupted by high-frequency localized seizure-like bursting. The increase of this inhibitory input resulted in decreased cell coupling of pyramidal neurons. A decrease in phase coherence was initially observed until seizure-like activity initiated causing a net increase in coherence as has been observed in epileptic patients. These results provide a possible pathway in which a decrease in synchronization could provide the trigger for inducing epileptiform activity.
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