Increasing numbers of cochlear implant (CI) patients are able to combine residual acoustic hearing (AH) with electric hearing (EH) in the same ear (electric-acoustic stimulation, or EAS), across ears (bimodal), or both (biEAS). The benefits of acoustic-electric hearing (AEH) are variable; some CI users even experience interference between acoustic and electric stimulation patterns. In the clinic, there is little effort to optimize CI and/or hearing aid signal processing with regard to combined acoustic and electric hearing, and it is unclear whether acoustic and electric patterns are combined differently within or across ears. EAS and biEAS patients may experience energetic interference between the current spread from EH and the spread of excitation (SOE) from AH. Bimodal, EAS, and biEAS patients may also experience informational interference because the same acoustic input may be delivered to different cochlear places (tonotopic mismatch) within and/or across ears. To mitigate these adverse effects, the optimal fitting for CI and/or hearing aid devices might be quite different for AEH than for EH and/or AH. The long-term goals of this proposal are to understand the mechanisms that underlie integration of acoustic and electric hearing, which can be used to improve AEH benefits for CI users. We hypothesize that acoustic and electric stimulation patterns are integrated differently within and across ears. Specifically, we hypothesize that integration for EAS listeners is limited by both energetic and informational interference; reducing the current spread/SOE between AH and EH and reducing tonotopic mismatch may improve integration. We also hypothesize that integration for bimodal listeners is limited by informational interference, due to tonotopic mismatch and sound quality differences across ears; improving the sound quality of EH (e.g., focused stimulation) while reducing the tonotopic mismatch between AH and EH may improve integration. Acoustic-electric integration will be evaluated in pairs of parallel experiments with normal-hearing (NH) subjects listening to simulations of AEH and in real EAS, bimodal, and biEAS CI listeners. Speech measures (vowel and sentence recognition in quiet and in noise, vocal emotion recognition) and frequency resolution will be collected with AH, EH, and AEH, and evaluated in terms of overall AEH performance, AEH benefit (performance difference between AEH and AH or EH), and integration efficiency (the ratio between observed and predicted AEH performance).
Aim 1 will primarily explore aspects of electric hearing that affect acoustic-electric integration while Aim 2 will focus on aspects of acoustic hearing that affect acoustic-electric integration. The proposed research is of great theoretical interest, as it will provide greater insight into acoustic-electric integration for AEH. The proposed research is also of great clinical value, as the results may provide better guidance for optimizing CI signal processing for AEH.
This project will provide important insights into integration of acoustic and electric hearing in EAS (electric- acoustic stimulation in the same ear), bimodal (electric and acoustic hearing in opposite ears), and biEAS (electric stimulation in one ear and acoustic stimulation in both ears) cochlear implant (CI) patients. The data from this research will shed light on the factors affecting integration efficiency of acoustic and electric hearing. Information from this research will provide useful guidance towards optimizing CI signal processing to maximize the benefits of combined acoustic and electric hearing.