Effects of Threshold Adjustment on Speech Perception in Nucleus Cochlear Implant Recipients

Objective: The objective of this study was to investigate the impact of using smaller and larger electric dynamic ranges on speech perception, aided thresholds, and subjective preference in cochlear implant (CI) subjects with the Nucleus (R) device. Design: Data were collected from 19 adults using the Nucleus CI system. Current levels (CLs) used to set threshold stimulation levels (T-levels) were set above or below the measured hearing thresholds to create smaller or larger electric output dynamic ranges, respectively, whereas the upper stimulation level (C-level) was fixed. The base (unadjusted) condition was compared against two conditions with higher T-levels (compression), by 30% and 60% of the measured hearing dynamic range, and three conditions with lower T-levels (expansion), by 30%, 60%, and 90% of the measured hearing dynamic range. For each subject, the clinical CL units were adjusted on each electrode to achieve these conditions. The slow-acting dynamic acoustic gains of ADRO (R) and Autosensitivity (TM) were enabled. Consonant-nucleus-consonant (CNC) word scores were measured in quiet at 50 dB and 60 dB SPL presentation levels. The signal-to-noise ratios (SNRs) for 50% understanding of sentences in noise were measured for sentences presented at 55 dB and 65 dB SPL in 4-talker babble noise. Free-field aided thresholds were measured at octave frequencies using frequency-modulated (warble) tones. Thirteen of the 19 subjects had take-home experience with the base and experimental conditions and provided subjective feedback via a questionnaire. Results: There were no significant effects of 30% expansion and 30% compression of the electric dynamic range on scores for words in quiet and SNRs for sentences in noise, at the two presentation levels. There was a significant decrement in scores for words in quiet for 60% and 90% expansion compared with the base condition at the 50 dB and 60 dB SPL presentation levels. The score decrement was much less at 60 dB SPL. For the 50 dB SPL presentation level, the decrements in word scores at 60% and 90% expansion were linearly related to the reduction in CL units required to achieve these experimental conditions, with a greater decrement in scores for a larger CL change. There was a significant increase in SNR for sentences in noise for 60% compression compared with the base condition at the 55 dB and 65 dB SPL presentation levels. There was also a significant increase in SNR for sentences at the 55 dB SPL presentation level for 90% expansion. Aided thresholds were significantly elevated for the three expansion conditions compared with the base condition, although the mean elevation at 30% expansion was only 4 dB. The questionnaire results showed no clear preference for any condition; however, subjects reported a reduced preference for the extreme compression (60%) and expansion (90%) conditions. Conclusions: The results showed that CI subjects using the Nucleus sound processor had no significant change in performance or preference for adjustments in T-levels by +/-30% of the hearing dynamic range. In quiet, speech perception scores were reduced for the more marked expansion (60% and 90%) conditions, whereas in noise, performance was poorer for the highest compression (60%) condition. Across subjects, the decrement in scores for words at 50 dB SPL for the 60% and 90% expansion conditions was related to the changes in CL units required for these conditions, with greater decrements for larger changes in levels.