Lois F. A. Martin

Adaptive dynamic range optimization for cochlear implants: a preliminary study.

Objective The present study investigated the acceptability and the effect of Adaptive dynamic range optimization (ADRO) on speech perception for cochlear implant subjects. ADRO is a preprocessing scheme that continuously adjusts the gain in each frequency band to optimize the signal in the output dynamic range. Design Speech processor programs were created with and without ADRO processing. Nine subjects were tested in the laboratory and encouraged to use both programs in everyday listening situations. Take-home experience was assessed with preference questionnaires. Speech perception performance was compared for the standard and ADRO programs using City University of New York (CUNY) sentences, consonant-nucleus-consonant (CNC) words, and closed set spondees presented in quiet. A range of presentation levels were used; from 70 to 40 dB sound pressure level (unweighted RMS). CUNY sentences were also presented in multi-talker babble with +15 dB and +10 dB signal to noise ratios. Results There was a significant improvement in speech perception scores with the ADRO programs compared with the standard. At 50 dB, the mean open set sentence scores in quiet improved by 16% (p < 0.001). There was an improvement in mean word score for CNC words presented at 60 dB of 9.5% (p < 0.001) and a 20% improvement in mean score for spondees presented at 40 dB. There was no significant difference in sentence scores between the ADRO and the standard program for sentences presented in either noise condition. ADRO was the preferred program in 59% of listening situations, with five out of nine subjects indicating a strong overall preference and three subjects indicating a slight preference for ADRO. Conclusions Continual adjustment of channel gains using ADRO provided improved sound quality and improved speech perception performance. Therefore, ADRO is a viable alternative to fixed channel gain and offers a means for cochlear implantees to gain more benefit from their devices.

Tone discrimination in Cantonese-speaking children using a cochlear implant

Most tone perception tests for Cantonese-speaking cochlear implant users have been based on tone identification tasks which require significant cognitive development to be successfully completed. Results from such tests suggest that cochlear implant child users are performing at about chance level and may not be receiving much information about pitch using the implant. This paper reports on the ability of cochlear implant child users to discriminate pitch variations in Cantonese by using an experimental procedure based on play audiometry. As part of the study, the usefulness of higher rates of electrode stimulation for aiding tone discrimination is also examined. Cochlear implant users are shown to derive sufficient information about pitch to discriminate most tone contrasts relatively successfully, with performance being most variable for contrasts involving tones clustered in the lower register of the speakers fundamental frequency range. Contrary to hypothesis, higher electrode stimulation rates are not found to offer significant benefits for aiding pitch discrimination.

An investigation of input level range for the nucleus 24 cochlear implant system: speech perception performance, program preference, and loudness comfort ratings.

Objective Cochlear implant recipients often have limited access to lower level speech sounds. In this study we evaluated the effects of varying the input range characteristics of the Nucleus 24 cochlear implant system on recognition of vowels, consonants, and sentences in noise and on listening in everyday life. Design Twelve subjects participated in the study that was divided into two parts. In Part 1 subjects used speech processor (Nucleus 24 SPrint™) programs adjusted for three input sensitivity settings: a standard or default microphone sensitivity setting (MS 8), a setting that increased the input sensitivity by 10.5 dB (MS 15), and the same setting that increased input sensitivity but also incorporated the automatic sensitivity control (ASC; i.e., MS 15A) that is designed to reduce the loudness of noise. The default instantaneous input dynamic range (IIDR) of 30 dB was used in these programs (i.e., base level of 4; BL 4). Subjects were tested using each sensitivity program with vowels and consonants presented at very low to casual conversational levels of 40 dB SPL and 55 dB SPL, respectively. They were also tested with sentences presented at a raised level of 65 dB SPL in multi-talker babble at individually determined signal to noise ratios. In addition, subjects were given experience outside of the laboratory for several weeks. They were asked to complete a questionnaire where they compared the programs in different listening situations as well as the loudness of environmental sounds, and state the setting they preferred overall. In Part 2 of the study, subjects used two programs. The first program was their preferred sensitivity program from Part 1 that had an IIDR of 30 dB (BL 4). Seven subjects used MS 8 and four used MS 15, and one used the noise reduction program MS 15A. The second program used the same microphone sensitivity but had the IIDR extended by an additional 8 to 10 dB (BL 1/0). These two programs were evaluated similarly in the speech laboratory and with take-home experience as in Part 1. Results Part 1 Increasing the microphone input sensitivity by 10.5 dB (from MS 8 to MS 15) significantly improved the perception of vowels and consonants at 40 and 55 dB SPL. The group mean improvement in vowel scores was 25 percentage points at 40 dB SPL and 4 percentage points at 55 dB SPL. The group mean improvement for consonants was 23 percentage points at 40 dB SPL and 11 percentage points at 55 dB SPL. Increased input sensitivity did not significantly reduce the perception of sentences presented at 65 dB SPL in babble despite the fact that speech peaks were then within the compressed range above the SPrint processors automatic gain control (AGC) knee-point. Although there was a demonstrable advantage for perception of low-level speech with the higher input sensitivity (MS 15 and 15A), seven of the 12 subjects preferred MS 8, four preferred MS 15 or 15A, and one had no preference overall. Approximately half the subjects preferred MS 8 across the 18 listening situations, whereas an average of two subjects preferred MS 15 or 15A. The increased microphone sensitivity of MS 15 substantially increased the loudness of environmental sounds. However, use of the ASC noise reduction setting with MS 15 reduced the loudness of environmental sounds to equal or below that for MS 8. Results Part 2 The increased instantaneous input range gave some improvement (8 to 9 percentage points for the 40 dB SPL presentation level) in the perception of consonants. There was no statistically significant increase in vowel scores. Mean scores for sentences presented at 65 dB SPL in babble were significantly lower (5 percentage points) for the increased IIDR setting. Subjects had no preference for the increased IIDR over the default. The IIDR setting had no effect on the loudness of environmental sounds. Conclusions Given the fact that individuals differ in threshold (T) and comfort (C) levels for electrical stimulation, and preferred microphone sensitivity, volume control, and noise-reduction settings, it is essential for the clinician and recipient to determine what combination is best for the individual over several sessions. The results of this study clearly show the advantage of using higher microphone sensitivity settings than the default MS 8 to provide better speech recognition for low-level stimuli. However, it was also necessary to adjust other parameters such as map C levels, automatic sensitivity control and base level, to optimize loudness comfort in the diversity of listening situations an individual encounters in everyday life.

A multidimensional scaling analysis of tone discrimination ability in Cantonese-speaking children using a cochlear implant

Tone discrimination testing with two groups of profoundly hearing-impaired children using a cochlear implant and one group of normally hearing children suggests that pitch level is perceptually more salient than pitch contour. In this paper, the discrimination results from these children are submitted to a multidimensional scaling analysis to determine what differences if any exist between the children in the information they use to discriminate between tone types. Through the multidimensional scaling analysis it is shown that, average pitch height and pitch direction are the most salient acoustic features in tone with pitch height being perceptually more important than pitch level. Furthermore, these acoustic features are similarly salient for all three groups of children despite differences in the source of auditory information and linguistic experience. Finally, the two groups of cochlear implant users who employ different speech processing strategies with their implant are shown to be distinguishable by differences in the relative salience of pitch direction.