Neuronal Firing and Waveform Alterations through Ictal Recruitment in Humans

Abstract

Analyzing neuronal activity during human seizures is pivotal to understanding mechanisms of seizure onset and propagation. These analyses, however, invariably using extracellular recordings, are greatly hindered by various phenomena that are well established in animal studies: changes in local ionic concentration, changes in ionic conductance, and intense, hypersynchronous firing. The first two alter the action potential waveform, whereas the third increases the "noise"; all three factors confound attempts to detect and classify single neurons. To address these analytical difficulties, we developed a novel template-matching-based spike sorting method, which enabled identification of 1239 single neurons in 27 patients (13 female) with intractable focal epilepsy, that were tracked throughout multiple seizures. These new analyses showed continued neuronal firing with widespread intense activation and stereotyped action potential alterations in tissue that was invaded by the seizure: neurons displayed increased waveform duration (p , 0.001) and reduced amplitude (p , 0.001), consistent with prior animal studies. By contrast, neurons in "penumbral" regions (those receiving intense local synaptic drive from the seizure but without neuronal evidence of local seizure invasion) showed stable waveforms. All neurons returned to their preictal waveforms after seizure termination. We conclude that the distinction between "core" territories invaded by the seizure versus "penumbral" territories is evident at the level of single neurons. Furthermore, the increased waveform duration and decreased waveform amplitude are neuron-intrinsic hallmarks of seizure invasion that impede traditional spike sorting and could be used as defining characteristics of local recruitment.

Human interictal epileptiform discharges are bidirectional traveling waves echoing ictal discharges

“…In order to study the propagation patterns of IEDs relative to seizures, it was necessary to quantify spatial features of SDs and seizure expansion for each recorded seizure. These measures have also been described in previous publications (14,15,17,30). The ictal wavefront is the slowly expanding edge of the seizure representing the spatial signature of failure of feedforward inhibition, and therefore defines recruitment of the tissue surrounding an electrode into the ictal core (24).…”

“…The ictal wavefront is the slowly expanding edge of the seizure representing the spatial signature of failure of feedforward inhibition, and therefore defines recruitment of the tissue surrounding an electrode into the ictal core (Schevon et al, 2012b). This biomarker of seizure recruitment and expansion has thus far only been detected with recordings of multiunit firing rates (Smith et al, 2016), and can also be detected on single microelectrodes by observing widening of action potential waveforms (Merricks et al, 2020(Merricks et al, , 2015. We followed methods from our previous manuscripts to generate multiunit firing rates, i.e.…”

“…Microelectrode array recordings in epilepsy patients, which sample the activity of both individual neurons and field potentials, have revealed the spatiotemporal features of ictal selforganization (13)(14)(15)(16)(17). These studies reported two classes of recordings, those in which neuronal firing is recruited into the ongoing seizure, and those in which neuronal firing is relatively unaffected, despite seizure-like field potentials appearing on the same microelectrodes.…”

Distinct signatures of loss of consciousness during Focal Impaired Awareness versus Focal to Bilateral Tonic Clonic seizures

“…52 Waveforms from the peri-ictal period were sorted visually after k-means clustering using the UltraMegaSort2000 MATLAB toolbox, 53 and noise artifacts were removed. Ictal unit firing was then analyzed using methods of Merricks et al 54 . Spike waveforms from the ictal period were template-matched to sorted peri-ictal putative units if the principal component vector of the ictal spike fell within the convex hull of the peri-ictal unit's spikes in principal component space.…”

Single Unit Analysis and Wide-Field Imaging Reveal Alterations in Excitatory and Inhibitory Neurons in Glioma

“…A comprehensive understanding of how excitatory and inhibitory neuronal populations contribute to ictal events observed in tumoral epilepsy requires simultaneous observation and analysis of each population across an extended spatial region. This can be achieved with single unit analysis 18–20 and cell type-specific genetically encoded calcium indicators 21 . Implementing this methodology in a model of TAS also allows for a greater understanding of how targeted drug therapy could affect these populations.…”