Behavioral and cortical responses to electric and acoustic stimulation in partial hearing cochlear implant model

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy by published work to the School of Engineering and Mathematical Sciences, Faculty of Science, Technology and Engineering, La Trobe University, Bundoora.

Animal behavioral studies make a significant contribution to hearing research and provide vital information that is not available from human subjects. Animal psychoacoustics is usually time consuming and labor intensive; in addition, animals may become stressed, especially if restraints or negative reinforcers such as electric shocks are used. To address these issues, a novel psychoacoustic experiment was designed and vigorously tested. Measured frequency discrimination thresholds were stable, repeatable, and comparable with previously published results. The same psychoacoustic experimental setup was used to study chronic performance of partially deafened cats using a cochlear implant. Four cats were systematically tested on different reference frequencies with intracochlear electrical stimulation turned on and off. We were able to show, for the first time, that cats can utilize information provided by a cochlear implant in performing a behavioral frequency discrimination task (3-way ANOVA, p is less than 0.001). Behavioral performance recorded during the chronic experiment was also compared to neural activation patterns in primary auditory cortices of the same animals. A new data analysis technique allowing direct comparison of these psychoacoustical and electrophysiological recordings was developed using signal detection theory’s d-prime (d’). Consistent with our previous report of improved behavioral discrimination with combined electric and acoustic stimulation (EAS) compared to acoustic alone, the cortical d’ was significantly higher for EAS compared to acoustic stimulation alone (3-way ANOVA, pair x on-off condition, p = 0.03). However, there was no correlation between behavioral performance and cortical d’ (Pearson: ρ = 0.3, P is greater than 0.3). The thesis describes a technique to perform behavioral and neurophysiological comparison of electric and acoustic stimulation of the cochlea in partially hearing subjects. This is of particular importance as more people with significant amount of residual hearing receive a combination of acoustic and electric auditory stimulation via hybrid cochlear implants.

Thesis with publications.