The Effects of Spectral Smearing and Elevated Thresholds on Speech in Noise Recognition in Simulated Electric-Acoustic Hearing
Degree GrantorUniversity of Canterbury
Degree NameMaster of Audiology
Combined Electric-acoustic stimulation (EAS) is becoming an increasingly viable treatment option for individuals with sloping severe to profound high frequency hearing loss and residual low frequency hearing. Sound stimulation via EAS is delivered to the high frequency region electrically using cochlear implantation, and to the low frequency region acoustically with or without amplification from hearing aids. This combined mode of stimulation often results in improved speech recognition in background noise compared to either mode of stimulation in isolation. It is important to note that many EAS listeners have some degree of hearing loss in the low frequency region, and may experience associated effects such as reduced frequency selectivity and elevated audiometric thresholds. This study simulated EAS listening in 20 normal hearing listeners by combining vocoded high frequency sound with low frequency sound. Low frequency sound was further manipulated by applying varying degrees of spectral smearing and attenuation to the low frequency region in an EAS simulation, to simulate changes in frequency selectivity and sensitivity that usually accompany sensorineural hearing loss. The aim of this study was to investigate the effects of spectral smearing and attenuation of low frequency information on the identification of vocoded speech in noise. Participants were required to complete a sentence recognition task in the presence of competing talkers for six simulated listening conditions with varying degrees of processing in the low frequency region. Results indicated that the advantage for speech in noise of simulated combined EAS over simulated electric stimulation alone was 3.9 dB when low frequency sound was unprocessed, 2.9 dB when low frequency sound had spectral smearing of x3 applied, and 2.4 dB when low frequency sound had spectral smearing of x6 applied. When 30 dB attenuation was applied as well as x3 spectral smearing, no significant benefit was observed. When 60 dB attenuation was applied as well as x3 spectral smearing, a significant negative relationship was found, with a 3 dB disadvantage found for simulated EAS compared to simulated electric stimulation alone. Overall, the results of this study indicate that there is indeed a significant improvement in speech recognition in a background of competing speakers with simulated EAS compared to simulated electric stimulation only. However, when reduced hearing thresholds were simulated for the residual low frequency hearing, we found that this benefit was either absent or reversed. These results therefore support the use of amplification for individuals with reduced hearing thresholds in the low frequencies in order to utilize the benefit they are able to achieve with combined EAS.