Threshold predictions of different pulse shapes using a human auditory nerve fibre model containing persistent sodium and slow potassium currents

The ability of a human auditory nerve fibre computational model to predict threshold differences for biphasic, pseudomonophasic and alternating monophasic waveforms was investigated. The effect of increasing the interphase gap, interpulse interval and pulse rate on thresholds was also simulated. Simulations were performed for both anodic-first and cathodic-first stimuli. Results indicated that the model correctly predicted threshold reductions for pseudomonophasic compared to biphasic waveforms, although reduction for alternating monophasic waveforms was underestimated. Threshold reductions were more pronounced for cathodic-first stimuli compared to anodic-first stimuli. Reversal of the phases in pseudomonophasic stimuli suggested a threshold reduction for anodic-first stimuli, but a threshold increase in cathodic-first stimuli. Inclusion of the persistent sodium and slow potassium currents in the model resulted in a reasonably accurate prediction of the non-monotonic threshold behaviour for pulse rates higher than 1000 pps. However, the model did not correctly predict the threshold changes observed for low pulse rate biphasic and alternating monophasic waveforms. It was suggested that these results could in part be explained by the difference in the refractory periods between real and simulated auditory nerve fibres, but also by the lack of representation of stochasticity observed in real auditory nerve fibres in our auditory nerve model.

Research Highlights
► Correct prediction trends of threshold reductions for pseudomonophasic waveforms.
► Persistent INa and slow IK currents can in part simulate non-monotonic behaviour.
► Inclusion of persistent INa current has the greatest influence in threshold changes