Abstract

The default mode network has been hypothesized based on the observation that specific regions of the brain are consistently activated during the resting state and deactivated during engagement with task. The primary nodes of this network, which typically include the precuneus/posterior cingulate, the medial frontal and lateral parietal cortices, are thought to be involved in introspective and social cognitive functions. Interestingly, this same network has been shown to be selectively impaired during epileptic seizures associated with loss of consciousness. Using a wide range of neuroimaging and electrophysiological modalities, decreased activity in the default mode network has been confirmed during complex partial, generalized tonic-clonic, and absence seizures. In this review we will discuss these three seizure types and will focus on possible mechanisms by which decreased default mode network activity occurs. Although the specific mechanisms of onset and propagation differ considerably across these seizure types, we propose that the resulting loss of consciousness in all three types of seizures is due to active inhibition of subcortical arousal systems that normally maintain default mode network activity in the awake state. Further, we suggest that these findings support a general “network inhibition hypothesis”, by which active inhibition of arousal systems by seizures in certain cortical regions leads to cortical deactivation in other cortical areas. This may represent a push-pull mechanism similar to that seen operating between cortical networks under normal conditions.

Highlights

  • Epileptic seizures can vary widely in terms of onset, propagation, and clinical manifestation, and often dramatically impair the patient’s state of consciousness

  • Elucidation of the default mode network originally arose from studies of healthy subjects; it seems clear that this network plays a central role in the clinical manifestations of seizure activity as well

  • The thalamus is strongly involved in all three seizure types, suggesting corticothalamic networks are at play

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Summary

Introduction

Epileptic seizures can vary widely in terms of onset, propagation, and clinical manifestation, and often dramatically impair the patient’s state of consciousness. In absence seizures, it has been proposed that the variable impairments observed are likely the result of focal disruptions of specific brain networks [12,38]. This view has been supported by mounting evidence that even so-called “generalized”. External awareness can be thought of as responses to environmental stimuli, whereas self-awareness is a contained process requiring no external input This distinction is important given the hypothesized function of the DMN in self-awareness [16,60]. We propose that loss of consciousness in seizures is caused by abnormal activity in subcortical and cortical regions including the default mode network

The default mode network
Default mode network involvement in complex partial seizures
Default mode network involvement in generalized tonic-clonic seizures
Default mode network involvement in absence seizures
Findings
Future directions and conclusions
Full Text
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