The research in our lab focuses on improving patient outcomes with hearing devices, whether they are cochlear implants and/or hearing aids. We take a unique view of the brain as an active rather than passive participant in the brain-device interface. Our research focuses on how brain plasticity accommodates the limitations of hearing devices, and how experience with certain hearing device settings may lead to abnormal and thus impaired central auditory processing. We also study ways to improve hearing device programming and surgical outcomes to improve performance.
Pitch plasticity with combined cochlear implant (CI) and hearing aid (HA) experience
We have measured pitch perception and how it changes over time in cochlear implant (CI) listeners who also wear a hearing aid (HA). We found that some CI listeners adapt their pitch perception with the CI so that it matches what they hear in the other ear with the HA. Others show no adaptation. Paradoxically, a few have pitch perception that drops in pitch for all electrodes and worsens the pitch mismatch with the other ear. This dropping-pitch pattern may explain why some individuals do not receive optimum benefit from the CI.
Binaural spectral integration
Normal-hearing listeners can tell very easily if two sounds differ much in pitch between ears, for very small pitch differences. In contrast, many deaf individuals who wear CIs and/or HAs fuse sounds that differ by as much as 3-4 octaves in pitch between ears. They are unable to tell that these sounds differ in pitch when presented simultaneously, even though they can discriminate them easily when presented sequentially. For example, a single electrode with a pitch of 240 Hz in the CI ear can be perceptually fused with acoustic tones ranging from 125 to 2000 Hz played to the other ear (about 1 octave below to 3 octaves above).
This fusion also leads to an effect we call pitch averaging, in which two sounds of different pitch that are fused across ears lead to a new binaural pitch that is an average of the two original pitches, much like the way red and blue are averaged between the two eyes in the 3-D glasses you use at the movie theater. This may reflect the personal experience of many CI users that voice pitch and musical pitch seems to change depending on whether they are wearing the CI alone or the CI together with a HA (or a second CI for bilateral CI users).
New studies are underway to model how this process, which we call binaural spectral integration, affects speech perception when two hearing devices are combined across ears. These questions are being studied in bilateral HA users, bimodal CI+HA users, and bilateral CI users. This effect may explain why some individuals experience interference with two hearing devices compared to one worn alone (such as a CI with a HA, or two CIs compared to one CI worn alone). Abnormal binaural spectral integration could also limit binaural benefit from the second device, because interference can cancel any binaural benefits.
Better understanding of the factors leading to abnormal binaural spectral integration will help in eventually designing new device processing strategies or training programs that redirect the brain toward more “normal” integration. Studying these questions in children will also indicate which device combinations or programming strategies promote normal development of binaural spectral integration. The ultimate goal of this research will be to reduce interference and increase binaural benefit, especially for speech perception in background noise.
Speech perception with a cochlear implant and hearing aid: How can we optimize the benefits?
Due to changes in cochlear implant (CI) candidacy, people with more residual hearing are receiving cochlear implants. Therefore it is likely that people with a CI on one side may wear a HA on the other side. While some report that the sound quality improves in the bimodal condition (CI on one side and a HA on the other), others report decreased performance when both devices are worn together. Benefits of bimodal devices include increased performance in noise, improved satisfaction when listening to music, and improved localization ability. Additionally, many report a more natural sound quality when the CI and HA are worn together, due to the low frequency acoustic hearing that the HA provides.
Currently, bimodal CI patients are given the same one-size-fits-all program for their speech processor as the early patients who did not use HAs. However, each patient has a differing amount of residual hearing in the HA ear, and the HA models and programming settings also vary greatly across patients.
In this lab, we investigate how different CI and HA programming parameters and strategies affect speech perception benefit, with the ultimate goal of developing new guidelines that optimize benefit with bimodal devices.
Improving hearing preservation with Hybrid Electro-Acoustic Stimulation (EAS) Cochlear Implants
Our current research goals are to understand the mechanisms of hearing loss with “hybrid” or “electro-acoustic stimulation” cochlear implants (CIs), a new type of CI designed to preserve low-frequency hearing and allow combined acoustic-electric stimulation in the same ear. Hybrid CI users perform significantly better than standard CI users on musical melody recognition, voice recognition, and speech recognition in the presence of background talkers. However, approximately 10% of Hybrid CI patients lose all residual hearing, and another 20% lose 20-30 dB after implantation, which means that those with severe hearing loss or worse will not benefit from a Hybrid CI.
Potential mechanisms currently under study include surgical damage to the delicate blood supply in the cochlea and from electrical stimulation. The findings will guide the development of strategies to prevent hearing loss in Hybrid CI patients, and allow extension of the Hybrid concept to all cochlear implant recipients with usable residual hearing.