Beneath the Tip of the Iceberg in Auditory Nerve Fibers: Subthreshold Dynamics for Cochlear Implants

Refractoriness is a common neural phenomenon that has been shown to characterize some as-pects of the auditory nerve fiber (ANF) spike-dependent response to cochlear implant (CI) stim-ulation. However, for high-rate pulse trains, a greater drop-off in spike rate over time is often observed than can be explained by refractoriness alone. This is typically assumed to be caused by ongoing spike-dependent neural adaptation, but mounting evidence suggests that subthreshold stimulus-response behaviors may also play a crucial role in ANF stimulus-response electrophysi-ology. In this study, we explore two such mechanisms: facilitation in which a subthreshold stimulus increases the subsequent excitability, and accommodation in which the excitability is decreased. Progress has been made in the area of developing phenomenological models to predict the ef-fects of several of the aforementioned behaviors. However, up until now, no model has combined all four of these stimulus-response behaviors: refractoriness, spike-rate adaptation, facilitation and accommodation. In this study, we present a stochastic integrate-and-fire model that simultaneously considers all four phenomena using parameters from fits to data from paired-pulse experiments to model facilitation, accommodation (Dynes 1996, PhD Thesis) and refractoriness (Miller et al 2001