Abstract
Epilepsy has been historically seen as a functional brain disorder associated with excessive synchronization of large neuronal populations leading to a hypersynchronous state. Recent evidence showed that epileptiform phenomena, particularly seizures, result from complex interactions between neuronal networks characterized by heterogeneity of neuronal firing and dynamical evolution of synchronization. Desynchronization is often observed preceding seizures or during their early stages; in contrast, high levels of synchronization observed towards the end of seizures may facilitate termination. In this review we discuss cellular and network mechanisms responsible for such complex changes in synchronization. Recent work has identified cell-type-specific inhibitory and excitatory interactions, the dichotomy between neuronal firing and the non-local measurement of local field potentials distant to that firing, and the reflection of the neuronal dark matter problem in non-firing neurons active in seizures. These recent advances have challenged long-established views and are leading to a more rigorous and realistic understanding of the pathophysiology of epilepsy.
Highlights
Coordinated neuronal activity and interactions between neurons and neuronal populations are the basic features of brain function
Spatial extension of penumbras that are large relative to the seizure focus can explain why many microelectrode studies in humans (Wyler et al 1982; Babb et al 1987; Truccolo et al 2011; Bower et al 2012) failed to record the expected neural signature of a seizure. These studies highlight that EEG and local field potential synchronization may be dissociated from spike synchronization. This context is important to keep in mind when investigating the spatial heterogeneity of seizure activity, and for interpreting observed synchronization of EEG signals between sites at differing spatial scales
This review demonstrates that synchronization in epilepsy is very complex, and may appear different depending on the spatial scale, the definition of synchrony and the signals being measured
Summary
Coordinated neuronal activity and interactions between neurons and neuronal populations are the basic features of brain function. Because of the rapid and extensive distribution of the powerful synaptic currents generated during a seizure, the size of the penumbra (region in which local field potentials and neural firing are dissociated) can be arbitrarily large and even multilobar (Schevon et al 2012). This context is important to keep in mind when investigating the spatial heterogeneity of seizure activity, and for interpreting observed synchronization of EEG signals between sites at differing spatial scales.
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