The Effect of Pulse Shape and Interphase Gaps on Speech Perception and Perceived Sound Quality in Electrical Hearing

Stimulation with triphasic pulses has been shown to reduce the occurrence of unwanted facial nerve stimulation (FNS) with cochlear implants (CIs). However, there is little data available on how different pulse shapes affect the hearing outcome with electrical hearing in general. The aim of the study was to evaluate the effects of different stimulation pulse shapes on speech perception in noise, as well as loudness perception and subjective sound quality.

Twenty experienced cochlear-implant users not suffering from FNS participated in a prospective single-visit study. Based on the subjects' current clinical fitting, six fitting maps with different pulse shapes (biphasic and triphasic) and different interphase gap (IPG) durations (2.1 µs, 10 µs, and 20 µs) were created. First, the loudness was balanced for each configuration by adjusting the stimulation charge amount. Then, speech perception in noise was measured with a German matrix sentence test (Oldenburg Sentence test). The perception of particular sound attributes of speech and music, as well as overall preference, was evaluated with visual analog scales.

Similar levels of speech perception were obtained with triphasic stimulation ( P = 0.891) and longer IPGs ( P = 0.361) compared to the subjects' clinical map settings. The stimulation amplitudes for equal loudness were significantly higher with triphasic stimulation compared to biphasic stimulation when keeping the IPG constant. Increasing the IPG had a significantly larger effect on perceived loudness ( P < 0.0001) and charge reduction for equal loudness with triphasic pulses compared to biphasic pulses. Triphasic configuration showed lower overall subjective sound quality ratings than biphasic for speech intelligibility, clarity, naturalness, and overall preference, as well as for music naturalness, and overall preference in the acute setting without adaptation time. Post-hoc pairwise comparisons against the clinical map revealed significantly lower speech naturalness ratings for triphasic with 2.1 µs IPG and for triphasic with 20 µs IPG only.

Although some sound quality attributes were rated lower compared to the clinical map in the acute test setting, stimulation with triphasic pulses does not affect speech perception in noise and can be considered as a valuable option in CI fitting.