Lexical tone and word recognition in noise of Mandarin-speaking children who use cochlear implants and hearing aids in opposite ears

Abstract

The benefits of bimodal hearing (cochlear implant and hearing aid in opposite ears) in children are well documented in English-speaking populations (Ching et al., 2000; Holt et al., 2005) but not much evidence has been reported from populations using tonal languages. The lexical tones in tonal languages are heavily loaded with semantic and grammatical information, which are essentially represented by the fundamental frequency (F0) and low-order harmonics of the speech signal. This unique linguistic feature means that tonal language-speaking CI recipients may achieve more bimodal benefits than their non-tonal language peers may. Twenty Mandarin-speaking children using the Nucleus 24 cochlear implant system and a hearing aid on the non-implant ear were assigned to either one of the two groups to investigate the head-shadow and binaural redundancy effects. A computerized speech test - MAPPID-N (Yuen et al., 2007) was used to present Mandarin lexical tones in monosyllabic words, and disyllabic words with a four- and eight-alternative forced choice picture-identification task, respectively. Individualized signal-to-noise ratio was used to capture the speech scores in the 30%-70% range and was fixed throughout the CI alone and bimodal experimental conditions. Hearing aid fitting was optimized before the first test phase, which was followed by the second test phase after three months. Significant head shadow but not binaural redundancy benefits were observed, suggesting that subjects have not yet developed central binaural processing abilities to improve speech recognition when speech and noise are mixed, in the bimodal condition, in this group of Mandarin-speaking paediatric CI recipients. No subject experienced any degradation of performance in the bimodal versus the CI-only test condition. This may be the first study that demonstrated the bimodal benefits in CI paediatric recipients speaking tonal language, particularly in lexical tone perception. Hearing aid amplification for the non-implant ear should be a standard for the paediatric tonal-language CI population.