A systematic review of the impact of adjusting input dynamic range (IDR), electrical threshold (T) level and rate of stimulation on speech perception ability in cochlear implant users.

OBJECTIVE: To systematically review the evidence of how adjustments of the electrical threshold (T) level, input dynamic range (IDR) and electrical stimulation rate impact on speech perception for cochlear implant (CI) users. DESIGN: Systematic review. STUDY SAMPLE: A search of two electronic data sources yielded 32 studies, which met the inclusion criteria. A quality assessment and two evidence-based practice (EBP) review rating schemes were used to grade studies. RESULTS: Due to the heterogeneity of speech perception measures, CI device type and study design, comparisons were made by structured review. CONCLUSION: The quality of studies was found to be moderate to poor. Increasing T levels above behavioural threshold, or as a proportion of electrical dynamic range (EDR), has been demonstrated to improve perception of monosyllables in quiet and sentences in both quiet and in noise. Specific IIDR and IDR setting may improve perception of monosyllables in quiet and sentences in noise. However, no recommendation could be determined for setting rate of stimulation as speech perception varied significantly across rates examined. To optimise speech perception, a bespoke approach to parameter setting providing an individualised CI fitting is recommended; however, detail of how to optimise settings and the interactions between parameters is as yet unknown.

STARR: a speech test for evaluation of the effectiveness of auditory prostheses under realistic conditions.

OBJECTIVES: The objective was to develop and evaluate a new sentence test, the Sentence Test with Adaptive Randomized Roving levels, intended to emulate everyday listening experience, using both normal-hearing (NH) and cochlear implant (CI) groups, examining practicality, learning, test-retest variability, and interlist variability. DESIGN: In experiment 1, each of 25 NH adults was tested using five lists, each comprising 30 sentences. One male and one female speaker each spoke 15 sentences. Ten sentences were presented at each of three presentation levels: 50, 65, and 80 dB SPL. The relative level of a speech-shaped noise was varied adaptively to estimate the speech reception threshold (SRT). Counterbalance for list order was achieved by staggering the allocation of lists to participants. To allow assessment of learning effects, no practice was given. The variability of mean SRTs across lists was small, but correction factors were derived for each list so that, after correction, all lists gave the same mean SRT. Test-retest variability was estimated by examining the corrected SRTs for each subject's five lists. In experiment 2, 25 CI users each received one test list after a small amount of practice. Experiment 3 examined the effect of speech rate using time-compressed speech, for age-matched NH and CI users. RESULTS: The mean SRT for the NH participants was approximately -6 dB and was similar for the male and female speakers. There was a small but significant improvement in SRTs between the first and later lists administered, but no further improvement for subsequent lists. On the basis of the variability of the corrected SRTs within each participant, a 2.2 dB difference in SRT is meaningful for comparisons using one test list per condition, for a single participant. The percentage of key words correct varied with presentation level over a 13% range, being best at 65 dB SPL. Only 40% of the CI group achieved an SRT lower than 20 dB for both speakers. There was large individual variability in the SRTs, and SRTs were higher for the female than for the male speaker. For the CI participants, the percentage of key words correct varied markedly with level, from 19% at the lowest level to 57% at the medium level. Time compression had a small effect for NH participants but a very large effect for CI participants. CONCLUSIONS: The Sentence Test with Adaptive Randomized Roving levels seems practical to administer and is reasonably sensitive. For NH participants, a 2.2 dB difference in SRT is meaningful for a single list per condition and a single participant. Although learning effects were small for NH participants, it seems prudent to provide some practice sentences when testing hearing-impaired or CI participants. The very large effect of time compression for the CI group has implications for live voice testing of children, because speech rate is only poorly controlled in such testing.

The effect of Gaussian noise on the threshold, dynamic range, and loudness of analogue cochlear implant stimuli.

The deliberate addition of Gaussian noise to cochlear implant signals has previously been proposed to enhance the time coding of signals by the cochlear nerve. Potentially, the addition of an inaudible level of noise could also have secondary benefits: it could lower the threshold to the information-bearing signal, and by desynchronization of nerve discharges, it could increase the level at which the information-bearing signal becomes uncomfortable. Both these effects would lead to an increased dynamic range, which might be expected to enhance speech comprehension and make the choice of cochlear implant compression parameters less critical (as with a wider dynamic range, small changes in the parameters would have less effect on loudness). The hypothesized secondary effects were investigated with eight users of the Clarion cochlear implant; the stimulation was analogue and monopolar. For presentations in noise, noise at 95% of the threshold level was applied simultaneously and independently to all the electrodes. The noise was found in two-alternative forced-choice (2AFC) experiments to decrease the threshold to sinusoidal stimuli (100 Hz, 1 kHz, 5 kHz) by about 2.0 dB and increase the dynamic range by 0.7 dB. Furthermore, in 2AFC loudness balance experiments, noise was found to decrease the loudness of moderate to intense stimuli. This suggests that loudness is partially coded by the degree of phase-locking of cochlear nerve fibers. The overall gain in dynamic range was modest, and more complex noise strategies, for example, using inhibition between the noise sources, may be required to get a clinically useful benefit.