Single‐unit activities during the transition to seizures in deep mesial structures

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Chronically implanted microelectrodes have offered a unique opportunity to study basic neural processes underlying human cognition1 and pathophysiological mechanisms in brain diseases.2 In the field of epilepsy, major advances have been carried out over the last decade to piece together multiple scales of epileptic processes.3 When recorded at the spatial scale of intracranial electroencephalogram (EEG; ∼10mm2), the characteristics of ictal discharges are often highly reproducible from seizure to seizure.5 Nevertheless, unlike these stereotypical macroscopic patterns, the temporal dynamics of microelectrode recordings are reported to exhibit more complex dynamics in spatially restricted microdomains (<1mm3).6 Using penetrating microelectrode arrays, previous analyses of single‐unit activity revealed heterogeneous neuronal firing patterns at seizure onset in the epileptic neocortex.10
However, whether similar heterogeneity in seizure‐onset zone (SOZ) occurs in mesial structures as it is in neocortex, given the phylogenetic and structural differences between the two regions, remains unclear.4 One main difficulty is attributed to the limited nature of the available data: It remains a technical challenge to follow with high fidelity single‐neuronal‐unit activity in these depth structures over the time span of days required to capture spontaneous seizures.12
In the present work, using depth electrodes, we present rare data of human single‐neuron activity from the SOZ during the transition from interictal to ictal states in deep mesial seizures, and we propose a description of relationships between intracranial EEG and single‐unit activities from these depth and mesial structures of temporal or frontal lobes. We report new insights into the microscale of the neurophysiology of human ictogenesis.
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