Psychophysics, Fitting, and Signal Processing for Combined Hearing Aid and Cochlear Implant Stimulation

In Brief The addition of acoustic stimulation to electric stimulation via a cochlear implant has been shown to be advantageous for speech perception in noise, sound quality, music perception, and sound source localization. However, the signal processing and fitting procedures of current cochlear implants and hearing aids were developed independently, precluding several potential advantages of bimodal stimulation, such as improved sound source localization and binaural unmasking of speech in noise. While there is a large and increasing population of implantees who use a hearing aid, there are currently no generally accepted fitting methods for this configuration. It is not practical to fit current commercial devices to achieve optimal binaural loudness balance or optimal binaural cue transmission for arbitrary signals and levels. There are several promising experimental signal processing systems specifically designed for bimodal stimulation. In this article, basic psychophysical studies with electric acoustic stimulation are reviewed, along with the current state of the art in fitting, and experimental signal processing techniques for electric acoustic stimulation. Current commercial hearing aids and cochlear implants were not designed to work together, leading to temporal synchronization problems and mismatch in place of stimulation between the two cochleas, which can lead to poorer localization ability and worse speech perception in noise than seems possible in principle. There are no generally accepted fitting methods available for bimodal or hybrid stimulation, and on theoretical grounds it does not seem practical to attain loudness balance with existing commercial devices for arbitrary signals and levels without supplementary signal processing. There are a few promising new speech processing strategies specifically designed for electric acoustic stimulation.