Sound localization and interaural time sensitivity with bilateral cochlear implants

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2006.Includes bibliographical references.Bilateral cochlear implantation is becoming more common as clinicians attempt to provide better sound-source localization and speech reception in noise for cochlear implant (CI) users. While some improvement over the abilities of monolateral implantees has been documented, bilateral performance for CI users is far from that achieved with normal hearing. Identifying factors that limit bilateral performance has been difficult because little is understood about CI listeners' localization abilities, their sensitivity to interaural cues, and the relationships between them. To better understand bilateral electric hearing, five bilateral CI users' abilities to locate sound sources and their sensitivities to interaural time difference (ITD) were studied in this thesis. Unlike past studies, monolateral and bilateral performance was recorded before and after exposure to daily, bilateral-CI listening using constant- and roving-level stimuli. For constant-level stimuli, increasing bilateral-listening experience improved all subjects' bilateral performance but degraded two subjects' monolateral performance. Using roving-level stimuli, increasing bilateral-listening experience also improved bilateral performance but did not alter monolateral performance.(cont.) Our results show that depending on the method of evaluation, the benefit of bilateral CIs over monolateral CI could be overstated for some subjects. A simple decision model was used to predict subjects' localization performance based on their sensitivity to interaural time and level differences (TD and ILD) measured through their sound processors. The predicted performance indicated that the measured performance could be accounted for by subjects' ILD sensitivity but not by their ITD sensitivity alone. Poor ITD sensitivity may be one reason that bilateral CI users' localization performance is poor compared to that of normal-hearing (NH) listeners. To improve ITD sensitivity, a first step is to characterize ITD sensitivity on single, interaural electrode pairs because data in the literature is incomplete. In particular, the dependence of ITD sensitivity on the repetition rate and the number of pulses in the unmodulated pulse trains was studied. Just noticeable difference (JND) of ITD was measured with four subjects on their most ITD-sensitive, interaural electrode pair. At low rate (50 pps), ITD JND improved with increasing number of pulses, indicating integration of ongoing ITD cues. The best ITD JNDs were 85 - 354 pts. Using 800-pps trains, two subjects' ITD JND degraded with increasing number of pulses.(cont.) Two subjects were insensitive to ITD up to 2 ms for 800-pps trains. To begin studying the impact of CI processing on ITD sensitivity, ITD JND was also measured using low-rate (50 pps) pulse trains delivered to the external input of the subjects' sound processors. ITD JND improved with increasing number of pulses. While subjects were insensitive to ongoing ITD in unmodulated, high-rate pulse trains delivered to single, interaural electrode pairs, they were sensitive to ongoing ITDs in the low-frequency modulator of high-rate pulse trains in the through-processor case. A next step toward greater understanding of bilateral electric hearing is to fully investigate the degree to which subjects are sensitive to ITD using modulated pulse trains. The results of this thesis show that there is significant localization benefit with bilateral CIs even though performance is not at the level of NL listeners. Further studies to improve ITD sensitivity may improve localization ability, which will further justify the risks and cost associated with bilateral implantation.by Becky Bikkei Poon.Ph.D