The human auditory nerve can be electrically stimulated by cochlear implants (CIs) with pulse trains consisting of biphasic pulses with small interphase gaps (IPGs). In animal experiments, lower electrically evoked compound action potential (ECAP) thresholds in implanted animals were found for increasing IPGs (2.1, 10, 20, 30 μs). ECAP thresholds may correlate with loudness thresholds. Therefore, in this study, the IPG effect on loudness and dynamic range was investigated in nine CI subjects.
A loudness-matching procedure was designed with three different IPGs (2.1, 10, 30 μs) at three different pulse rates (200, 600, 1000 pps). An adaptive loudness-balancing test was performed at the 50% stimulus amplitude level of the dynamic range and most comfortable loudness level (MCL).
Increasing the IPG or increasing the pulse rate led to a significant decrease in stimulus amplitude for 50% level and MCL in the adaptive test. Because the stimulus amplitudes for 50% level and MCL decreased in a different manner, the calculated upper dynamic range between MCL and 50% level significantly decreased for increasing IPG between 0.24 and 0.38 dB. This decrease in the upper dynamic range was observed for all pulse rates.
It is possible to reduce the stimulus amplitude level for the same loudness impression using larger IPGs in CIs; however, larger IPGs decrease the dynamic range. These findings could help during the fitting process of CIs to find the balance between saving battery and a proper dynamic range.