Temporal interaction in electrical hearing elucidates auditory nerve dynamics in humans

In cochlear implants, severe limitations arise from electrical crosstalk between channels. Therefore, the current trend in cochlear implants is to increase stimulation rates to encode signals with higher temporal precision. However, the fundamental question: "What is the limit of temporal precision due to inherent neuronal dynamics of the stimulated neurons?" has not yet been resolved. In this study we have developed a double-pulse method and, for the first time, reversed stimulus polarity systematically between consecutive pulses to elucidate subthreshold-induced temporal interaction effects. This method allowed us to determine the time-course of subthreshold temporal interaction in human subjects which identifies the limits of encoded temporal precision. Our results show significant temporal interaction up to 600 μs inter-pulse interval. In all the cases tested we saw a facilitation effect on threshold. Interaction effects at a 20% below threshold pre-conditioning stimulation showed up to 38% ± 6% threshold reduction. These results imply that there is significant temporal interaction between two subsequent pulses. This interaction diminishes the precision of amplitude coding. We predict interaction effects on temporal precision and channel interaction. For (interleaved) stimulation with short inter-pulse intervals it is interesting to consider our interaction results; and it may become important to consider them for future coding strategies where high temporal precision is required. In an increasing group of binaural implanted patients this will be the case when interaural time differences are encoded with μs precision.