Abstract
The effect of the relative timing between pairs of same-polarity monophasic pulses has been studied extensively in single-neuron animal studies and has revealed fundamental properties of the neurons. For human cochlear implant listeners, the requirement to use charge-balanced stimulation and the typical use of symmetric, biphasic pulses limits such measures, because currents of opposite polarities interact at the level of the neural membrane. Here, we propose a paradigm to study same-polarity summation of currents while keeping the stimulation charge-balanced within a short time window. We used pairs of mirrored pseudo-monophasic pulses (a long-low phase followed by a short-high phase for the first pulse and a short-high phase followed by a long-low phase for the second pulse). We assumed that most of the excitation would stem from the two adjacent short-high phases, which had the same polarity. The inter-pulse interval between the short-high phases was varied from 0 to 345 μs. The inter-pulse interval had a significant effect on the perceived loudness, and this effect was consistent with both passive (membrane-related) and active (ion-channel-related) neuronal mechanisms contributing to facilitation. Furthermore, the effect of interval interacted with the polarity of the pulse pairs. At threshold, there was an effect of polarity, but, surprisingly, no effect of interval nor an interaction between the two factors. We discuss possible peripheral origins of these results.
Highlights
Cochlear implants (CIs) treat cases of severe-to-profound sensorineural hearing loss by electrically stimulating the spiral ganglion neurons (SGNs)
At Most Comfortable Levels (MCLs), the anodic stimuli required less current than cathodic stimuli to achieve the same loudness (+ 2.50 dB, paired t test, t(5) = 7.16, p G 0.001), leading to a positive polarity effect, defined as the cathodic MCL minus the anodic MCL
At the longest inter-pulse interval (IPI) tested here (172 μs), the MCLs of the paired pulses were smaller than that of the single pulses, and this difference was greater for cathodic than anodic stimulation (2.2 dB vs 0.9 dB, respectively)
Summary
Cochlear implants (CIs) treat cases of severe-to-profound sensorineural hearing loss by electrically stimulating the spiral ganglion neurons (SGNs). Amongst the first stages of the auditory pathway, both the myelination and the diameter of the spiral ganglion neurons (SGNs) can decrease following sensorineural hearing loss (Leake and Hradek 1988; Nadol 1997) These morphological changes affect how the SGNs integrate the electrical charge delivered by CIs (Bostock et al 1983; Colombo and Parkins 1987; Smit et al 2008; Resnick et al 2018). This is because the passive behaviour of the neuronal membrane is that of a leaky integrator (Lapicque 1907), and both the diameter and the amount of myelination can strongly affect its capacitive-resistive properties
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More From: Journal of the Association for Research in Otolaryngology
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