Cell-type specific and multiscale dynamics of human focal seizures in limbic structures

Abstract

The relationship between clinically accessible epileptic biomarkers and neuronal activity underlying the seizure transition is complex, potentially leading to imprecise delineation of epileptogenic brain areas. In particular, the pattern of interneuronal firing at seizure onset remains under debate, with some studies demonstrating increased firing while others suggest reductions. Previous study of neocortical sites suggests that seizure recruitment occurs upon failure of inhibition, with intact feedforward inhibition in non-recruited territories. We investigated whether the same principles applied also in limbic structures. We analyzed simultaneous ECoG and neuronal recordings during 34 seizures in a cohort of 19 patients (10 male, 9 female) undergoing surgical evaluation for pharmacoresistant focal epilepsy. A clustering approach with five quantitative metrics computed from ECoG and multiunit data was used to distinguish three types of site-specific activity patterns during seizures, at times co-existing within seizures. 156 single-units were isolated, subclassified by cell-type, and tracked through the seizure using our previously published methods to account for impacts of increased noise and single-unit waveshape changes caused by seizures. One cluster was closely associated with clinically defined seizure onset or spread. Entrainment of high-gamma activity to low-frequency ictal rhythms was the only metric that reliably identified this cluster at the level of individual seizures (p < 0.001). A second cluster demonstrated multi-unit characteristics resembling those in the first cluster, without concomitant high-gamma entrainment, suggesting feedforward effects from the seizure. The last cluster captured regions apparently unaffected by the ongoing seizure. Across all territories, the majority of both excitatory and inhibitory neurons reduced (69.2%) or ceased firing (21.8%). Transient increases in interneuronal firing rates were rare (13.5%) but showed evidence of intact feedforward inhibition with maximal firing rate increases and waveshape deformations in territories not fully recruited but showing feedforward activity from the seizure, and a shift to burst-firing in seizure-recruited territories (p = 0.014). This study provides evidence for entrained high gamma activity as an accurate biomarker of ictal recruitment in limbic structures. However, our results of reduced neuronal firing suggest preserved inhibition in mesial temporal structures despite simultaneous indicators of seizure recruitment, in contrast to the inhibitory collapse scenario documented in neocortex. Further study is needed to determine if this activity is ubiquitous to hippocampal seizures or if it indicates a "seizure-responsive" state in which the hippocampus is not the primary driver. If the latter, distinguishing such cases may help refine surgical treatment of mesial temporal lobe epilepsy.

Aberrant fast spiking interneuronal activity precedes seizure transitions in humans

“…The original single-units—prior to template matching for tracking through the seizure—were subclassified into putative cell-types based on mean extracellular waveforms and cell-intrinsic firing pattern autocorrelation as previously described 42 . Mean waveforms were calculated as the action potential-triggered average from the original, unfiltered signal and then z-scored.…”

“…Z-scored firing rate changes within neurons (Fig. 3b) were calculated via the standard deviation of a Poisson distribution with the observed firing rates 42,84 .…”

“…MEAs were arranged in a 10 x 10 grid over 4 x 4 mm with either 1-or 1.5-mm length electrodes, and implanted into the presumptive seizure onset zone in neocortical gyri. 4,222 single units were isolated across 33 seizures (mean ± SD per seizure: 127.9 ± 67.7), with 11.98% of units being probabilistically subclassified as inhibitory interneurons 42 (see Methods). The numbers of cells belonging to each classification was robust, with the putative inhibitory population dropping only by 3.84% when restricting to a high confidence threshold of 95% (Fig.…”

“…As a result, ictal patterns on the EEG are not sufficient to determine if a region has been recruited into seizure-driving territory (the spatiotemporally dynamic ictal "core") or if intact inhibition is successfully restraining the intense excitatory drive at that moment ("penumbra") 22,26,44 . We have previously shown that a wavefront of continuous ("tonic") firing occurs at the boundary when penumbral tissue becomes recruited 26,28,[45][46][47] and that individual neurons undergo action potential waveform alterations upon recruitment 28,42,44 . This activity is spatially restricted, with these firing patterns coexisting within seizures, marking the propagation of ictal activity (Fig.…”

Cellular resolution contributions to ictal population signals

“…Experimentally, ictal signal can also be captured with population‐level optical imaging of neuronal activity. Microelectrode array‐based electrophysiological methods have been used to record the activity of individual neurons with high temporal and moderate spatial resolution during physiological activity and, in some cases, even during seizure onset 4–6 . However, as neuronal spiking does not sum in a linear manner into a population response, these methods cannot directly show how single neuron activities contribute to the population ictal response.…”