Functional rather than anatomic connectivity predicts seizure propagation in a multi-node model of focal neocortical epilepsy

Abstract

Seizures propagate through the brain either locally or via widespread networks of anatomically and functionally connected nodes. These sites can be manipulated in the surgical treatment of human patients through ablation or stimulation. However, we still lack a full understanding of how seizures, ablation, and stimulation recruit or alter recruitment of these distant sites. Here, we apply widefield calcium imaging in a non-anesthetized rodent multi-nodal bilateral neocortical network model of focal epilepsy to examine excitatory and inhibitory cell recruitment. When we initiate seizures in somatosensory cortex (S1), they preferentially spread to an ipsilateral node in frontal cortex (M2) rather than across the corpus callosum to contralateral mirror somatosensory cortex. On the other hand, seizures rapidly spread across the corpus callosum in regions that connect M2 with its mirror M2 focus, indicating that this frontal region acts an amplifier for secondary generalization. Accordingly, ablation of M2 radically altered seizure propagation. Electrical stimulation of S1 revealed that S1 preferentially recruits excitatory cells in ipsilateral M2 but inhibitory cells in contralateral S1, which may explain the preferred propagation pathway. We also observed that the stimulation frequency can differentially determine the response of excitatory versus inhibitory neurons. Altogether, our findings suggest that seizures do not propagate homogeneously through anatomically connected nodes but are preferentially "pulled" to specific locations by excitatory/inhibitory balance. Thus, functional connectivity rather than anatomic connectivity will be more predictive of ictal spread, and more informative for ablative and stimulation-based therapeutics. SignificanceUnderstanding how seizures spread outward from one brain region to another is critical to informing new therapies for epilepsy patients. We used widefield calcium imaging and electrical stimulation to examine seizure propagation patterns in a bilateral brain network in non-anesthetized animals. We found that seizures unexpectedly preferred certain pathways over others, despite strong anatomical connections, and that these preferences were dictated by the type of cell (excitatory or inhibitory) recruited in distant nodes. We were able to reproduce these electrical imbalances with focal stimulation at varying frequencies. Our findings highlight that anatomy by itself is insufficient to fully identify seizure networks and suggests that specific neurostimulation parameters will confer different effects on distant brain regions. Key PointsO_LIFunctional connectivity rather than anatomic connectivity determines ictal propagation C_LIO_LIContralateral seizure propagation from S1 favors cross callosal spread in via motor, rather than somatosensory pathways. C_LIO_LIHigh frequency electrical stimulation differentially affects excitatory and inhibitory recruitment across a neocortical network C_LIO_LILow frequency electrical stimulation emulates seizure-like activity propagation across a network C_LIO_LIConnectome data may be insufficient to identify critical nodes and pathways of a seizure network C_LI