Temporal interactions in electric hearing captured by reaction times

Although cochlear implants provide hearing restoration for the deaf, there is considerable variability in speech outcomes among recipients. The interface from electrode to auditory nerve is considered a major contributor to this variability with temporal and spatial interactions between electric pulses hindering channel independence. In this study, we investigated reaction times as a potential objective measure of temporal pulse interactions. Fourteen Nucleus cochlear implant users pressed a button as fast as possible to a train of eighteen pairs of closely timed pulses presented for one second to a medial electrode. Each pulse was biphasic with an anodic (A) and a cathodic (C) polarity phase. We quantified the effect of changes in pulse-pair parameters on the reaction times: the pulse-pair interval ranging between 59 and 350 µs, pulse amplitudes from threshold to supra-threshold levels, and the order of pulse polarity, alternating between CA-AC and AC-CA combinations. Our results show that reaction times decrease with increasing pulse-pair interval, that CA-AC pulse pairs lead to faster responses than AC-CA pulse pairs, and that reaction times decrease monotonically with increasing pulse amplitude. Reaction times thus reflect temporal interactions, polarity sensitivity and amplitude changes, in line with previous findings using psychophysical threshold measurements. At audible stimulation levels, these effects predominantly operate independently, except for an interaction between inter-pulse interval and polarity. We conclude that reaction times provide a valid assessment of electrical hearing.