Motivation and Objectives

Improvements in technology of CI and speech recognition outcomes have led to some extension of the implantation criteria to include individuals with some residual hearing in one or both ears. Although recent studies have reported that CI have successfully restored hearing performance in quiet in severely hearing loss individuals. However, these individuals still remains several significant difficulties in understanding speech in noise or the presence of competing talkers [13].

First, for supra-segmental speech information perception, such as intonation, vocal emotion,

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voice gender and talker identity continues to be challenging for CI users, who have impoverishment spectral resolution [14-16]. The spectral resolution in CI is not high enough to resolve the fundamental frequency (F0) and its harmonics, due to the limited number of electrode number and neural survival. Although the CI encodes the temporal envelope of the acoustic signal and temporal correlates of voice pitch are preserved at least in the low frequency channels [17], the temporal envelope pitch is relatively weak and insufficient for challenging listening environment [18]. The dynamic changes in pitch across an utterance are the primary indicators of intonation and play a critical role in conveying talker‟s voice and emotional states [19] which relayed by the F0 of the target speech, is known to be impoverishment in CIs of today [15-20] but is available within the low frequency acoustic signal for those with residual acoustic hearing. One of the main deficiencies of CIs is that the spectral resolution of the signal is reduced and the lack of pitch perception and another possible is partly because they do not have access to binaural hearing when they only implanted the unilateral side, which these two factors was believed can enable listener to segregation the target speech from noise.

Second, a tonal language is a language that uses tonal cues, which is called lexical tones, to identify word. For example, the syllable /ba/ can have one of four different meanings depending on the F0 contour. Taiwanese Mandarinis a tonal language that variantedof Mandarin and derived from the official Standard Mandarin spoken in Taiwan. Consider into speech perception of Taiwanese Mandarin in CI users, it would be expected that poor frequency resolution in CI would led poor pitch perception and affect the ability of implant listeners to correctly identify this tonal language.

Third, the fluctuating and irregular nature of the background noise also allows for NH listeners to potentially listen for the target speech in the temporal and spectral „„dips” of the background, to perception the masking release, which also referred as “glimpsing.” While the poor frequency resolution in implanted listeners would reduce their ability to listen in spectral „„dips” as

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compared to NH [21]. In both listeners with HI [22-23] and CI [14, 20, 24-26] have been suggested that the lack of masking release in fluctuating maskers is partly due to an inability to code F0. In a fluctuating masker, F0 may help by providing stress cues and acoustic landmarks to give a framework that aid listener to fuse the glimpsed target information into a meaningful way. This inability likely arises from reduced temporal fine structure processing [26-32]. The inability of listeners with HI to glimpsing by makes use of temporal fine structure cues is currently a topic of considerable study.

For CI users with low frequency residual hearing may improve pitch perception ability by combined the electric and acoustic stimulation via using the HA with their CI in the same ear, which known as electric-acoustic stimulation (EAS) [33-34], or in opposite ears, which is known as Bimodal stimulation. This benefit is generally attributed to the addition of low frequency acoustic information that is provided by the hearing aid. Despite the low frequency sound is almost unintelligible when presented alone; it is largely unavailable in the electric stimulation with CI.

However, it remains a matter of debate which specific cues contained within the low frequency acoustic signal are responsible for the benefits observed. Much of the discussion regarding the Bimodal benefit to CI speech perception has focused on the role of F0.

In this thesis, the overall objective was to evaluate the benefits of speech recognition in Taiwanese Mandarin in noise background with simulation Bimodal stimulation, which combined unprocessed low frequency information and CI simulation in NH listeners. The first aim was to evaluate the hypothesis that residual low frequency acoustic hearing provides phonetic information which, when combined with the CI, produces an improvement in speech intelligibility beyond that of the CI alone. Furthermore, access to the binaural hearing effect in Bimodal stimulation with sound source in different spatial direction, to reveal the important of Bimodal stimulation in binaural hearing. The second aim was to evaluate the hypothesis that the F0 information which

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available within the low frequency acoustic information can provides the improvement in speech intelligibility with a CI, when comparing the results from low frequency acoustic information.

Further addressed a tone that follows the dynamic variation of F0 or amplitude envelope of the low pass speech, extracted from target speech to provide insight into the components of the low frequency region information that are important for achieving this benefit. The third aim was to evaluate the hypothesis that the Bimodal stimulation can benefit from masking release, by using the several acoustic cues within the low frequency acoustic information, to improve the ability of

“glimpsing” in CI users.