Otolaryngology - Theses
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ItemBilateral advantage and spatial unmasking for children with normal hearing, bimodal devices and bilateral implants( 2012)This thesis describes bilateral advantage and spatial unmasking for children using bimodal devices and for children using bilateral implants. It also describes the development of spatial unmasking with age in children with normal hearing. Spatial unmasking was measured with a speech detection in noise task, and bilateral advantage was measured with both speech detection and speech recognition tasks in background noise. The relationships between aided thresholds and bilateral advantage for children with bimodal devices were examined. The frequencies of the phonemes that contributed to bilateral advantage in children with bimodal devices and children with bilateral implants were investigated. Comparisons were made between children with bimodal devices and children with bilateral implants on bilateral advantage, head-shadow effect, and types of phonemes contributing to bilateral advantage. Results showed that spatial unmasking was demonstrated in children with normal hearing across three different age groups: infants, preschoolers, and school children. Spatial unmasking was also demonstrated in children with a hearing loss, both in children using bimodal devices and in children using bilateral implants. For children with normal hearing, spatial unmasking changed with age, with infants and preschoolers showing a greater amount of spatial unmasking than school children and/or adults. This change in spatial unmasking with age could imply that the development of the binaural processing of non-spatial cues is slower than that of spatial cues. Differences in attention and motivation between the age groups may also have contributed to the results. These results suggest that sources of noise and target speech should be spatially separated wherever possible for children with bimodal devices and bilateral implants in educational and home settings. For children with bimodal devices or bilateral implants, both speech detection and speech recognition in noise were better when listening with two hearing devices rather than one. This reinforces the importance of fitting a contralateral hearing aid or a second implant to children who use a single implant. Results showed that aided thresholds affect bilateral advantage in children with bimodal devices. Children with better aided thresholds at 250 and 500 Hz, and/or poorer aided thresholds at 4 kHz demonstrated greater bilateral advantage. This finding is possibly due to the fact that a hearing aid provides finer low-frequency spectral information than a cochlear implant, so the low-frequency information from a hearing aid could be complementary to the signal received from an implant in the opposite ear. There may also be a mismatch of high-frequency information provided by the hearing aid and the cochlear implant, resulting in the negative impact of aided thresholds at 4 kHz on bilateral advantage. Results of information transmission analyses for children with bimodal devices and bilateral implants showed that bilateral advantage in speech recognition was spread over a wide range of frequencies of phonemes for both groups. This research identified two main differences between children with bimodal devices and children with bilateral implants that were consistent across the speech detection and speech recognition experiments. Firstly, children with bilateral implants demonstrated a greater head-shadow effect than children with bimodal devices. Secondly, children with bimodal devices demonstrated a greater bilateral advantage than children with bilateral implants when signal and noise are both presented from the front. These results imply that bimodal devices and bilateral implants may each have their own advantages and disadvantages. Careful pre-operative counseling is therefore important for children with bimodal devices who are considering a second implant. The speech detection method for measuring spatial unmasking and bilateral advantage described in this thesis can be used for evaluating the outcome of bimodal devices and bilateral implants for young children with minimal language ability for speech recognition tasks.