Title: Computational Study of Synchrony in Fields and Microclusters of Ephaptically Coupled Neurons

1 Neuronal hyper-synchrony is implicated in epilepsy and other diseases. The low frequency, spa2 tially averaged electric fields from many thousands of neurons have been shown to promote 3 synchrony. It remains unclear whether highly transient, spatially localized electric fields from 4 single action potentials (ephaptic coupling) significantly affect spike timing of neighboring cells 5 and, in consequence, population synchrony. In this study, we simulated the extracellular poten6 tials and the resulting coupling between neurons in the NEURON environment and generalized 7 their connection rules to create an oscillator network model of a sheet of ephaptically coupled 8 neurons. Using both models, we explained several aspects of epileptiform behavior not previously 9 modeled by synaptically coupled networks. Importantly, reduction of neuron spacing induced 10 synchronization via single spike ephaptic coupling, agreeing with seizure suppression seen clin11 ically and in vitro via extracellular volume adjustment. Further reduction of neuron spacing 12 yielded locally synchronized clusters, providing a mechanism for recent in vitro observations of 13 localized neuronal synchrony in absence of synaptic and gap junction coupling. 14