Loudness growth in cochlear implants: effect of stimulation rate and electrode configuration

In cochlear implant speech processor design, acoustic amplitudes are mapped to electric currents with the intention of preserving loudness relationships across electrodes. Many parameters may affect the growth of loudness with electrical stimulation. The present study measured the effects of stimulation rate and electrode configuration on loudness growth in six Nucleus-22 cochlear implant users. Loudness balance functions were measured for stimuli that differed in terms of stimulation rate, electrode configuration and electrode location; a 2-alternative, forced-choice adaptive procedure (double-staircase) was used. First, subjects adaptively adjusted the amplitude of a 100-pulse-per-second (pps) pulse train to match the loudness of a 1000-pps standard pulse train. For a range of reference stimulation levels, the loudness of the 100-pps stimulus was matched to that of the 1000-pps standard stimulus; loudness balancing was performed for three electrode pairs [(20,22), (1,3), (1,22)]. The results showed that the loudness balance functions between the 100- and 1000-pps stimulation rates were highly subject-dependent. Some subjects' loudness balance functions were logarithmic, while others' were nearly linear. Loudness balance functions were also measured across electrode locations [(20,22) vs. (1,3)] for two stimulation rates (100, 1000 pps). Results showed that the loudness balance functions between the apical and basal electrode pairs highly depended on the stimulation rate. For all subjects, at the 1000-pps rate, the loudness balance functions between the two electrode locations were nearly linear; however, at the 100-pps rate, the loudness balance function was highly nonlinear in two out of six subjects. These results suggest that, for some cochlear implant patients, low-frequency stimulation may be processed differently at different electrode locations; for these patients, acoustic-to-electric amplitude mapping may need to be sensitive to this place-dependent processing when relatively low stimulation rates are used.