The objective determination of an optimal stimulation rate for CI users could save time and take the uncertainty out of choosing a rate based on patient preference. Electrically evoked compound action potential (ECAP) temporal response patterns vary across stimulation rates and cochlear regions, and could be useful in objectively predicting an optimal rate. Given that only one rate of stimulation can be used for current CI devices, we propose two potential ways to investigate whether a rate that produces stochastic ECAP responses (termed stochastic rate) can be used to predict an optimal stimulation rate. The first approach follows that of Hochmair et al. (2003), which compared performance across three cochlear regions using limited electrode sets. This approach, which has inherent limitations, may provide insight into the effects of region-specific stochastic rates on performance. The second, more direct, approach is to compare speech perception for full-array maps that each uses a stochastic rate from a different region of the cochlea. Using both of these methods in a set of two acute experiments, the goal of the present study was to assess the effects of stochastic rate on speech perception.
Speech-perception stimuli included the Hearing in Noise Test (HINT sentences), Consonant-Nucleus-Consonant (CNC) phonemes, and Iowa Medial Consonants. For Experiment 1, 22 ears in 20 CI recipients were tested in three map conditions (basal-only, middle-only, and apical-only electrode sets) using the subject’s daily-use stimulation rate to first explore the level of performance possible with region-specific maps. A one-way repeated-measures analysis of variance (RM ANOVA) was used to examine the effect of electrode region on performance. A subset of nine subjects was tested with three additional maps (basal-only, middle-only, and apical-only electrode sets) using the region-specific stochastic rate, as measured in a previous study. A two-way RM ANOVA was used to assess the effects of electrode region and per-channel stimulation rate on performance for this subgroup. For Experiment 2, the same subset of nine subjects was tested with four full-array maps that each used either the daily-use stimulation rate or one of the stochastic rates. A one-way RM ANOVA was used to examine the effect of stimulation rate on performance.
For Experiment 1, average performance with the daily-use rate and the stochastic rate was significantly better using the middle electrode set for HINT sentences and CNC phonemes. Perception of medial consonants was similar using the basal and middle electrode sets, and both of these were better than consonant perception with the apical region. For the subgroup of subjects tested with both the daily and stochastic rates, results revealed that stimulation rate did not have a significant effect on performance. For Experiment 2, results revealed no significant differences in performance using full-array maps with different stochastic rates or with the daily-use rate.
Speech-perception scores were higher using the middle electrode set than with the basal or apical sets; however, this may have resulted from less spectral compression for the middle-region map. The effect of using stochastic rate as an optimal stimulation rate requires further investigation. A longer acclimatization period may be more likely to show differences in performance using stochastic rates versus daily-use rates.