Thursday, 2 January 2014

PhD Thesis: Improving binaural hearing with cochlear implants and hearing aids (December 2013)


Worldwide over 200.000 profoundly deaf people have been implanted with a cochlear implant (CI). A CI directly simulates the auditory nerve fibers by means of electrical pulses, in order to partially restore the hearing.  Due to the success of CIs, implantation criteria have been relaxed. As a result, an increasing number of people receiving a CI still has residual hearing in the other ear.
Users of a CI with residual hearing in the non-implanted ear can benefit from using a hearing aid (HA) in this ear. The combined use of a CI and a HA in the contralateral ear is called bimodal stimulation. Although bimodal listeners receive binaural input of sound, they show poor speech understanding in noise and sound localization abilities. This is partly caused by the fact that current clinical CIs and HAs were not designed to work together. The independent fitting and processing of the devices as well as differences in the way the sound is transmitted in both ears, limit the ability of bimodal listeners in using binaural information. Binaural cues such as the interaural time difference (ITD) and the interaural level difference (ILD) are the basic cues for sound localization and contribute to speech understanding in noise. Previously, it has been shown that bimodal listeners can be sensitive to ITDs and ILDs in simple narrow band stimuli. In order to improve localization performance for bimodal listeners, there is a need for optimized signal processing to improve the perception of binaural cues through their clinical devices.

In the first part of this PhD thesis, multiple studies were performed with the aim to further characterize ITD perception with bimodal stimulation using more realistic stimuli. Using these results, in the second part of this thesis a sound processing strategy, the modulation enhancement strategy (MEnS), was developed to improve ITD perception with bimodal stimulation. MEnS enhances modulations in the electric signal, synchronously with modulations in the acoustic signal presented to the non-implanted ear, based on measurement of the peaks occurring at the rate of the fundamental frequency in voiced speech. The modulations are provided synchronous across electrode channels. JNDs in ITD were determined for five bimodal listeners using an artificial vowel processed by either the clinical standard speech processing strategy (ACE) or MEnS. All subjects were sensitive to ITD with MEnS. With MEnS, performance was significantly better for all subjects and within the range of physically available ITDs.

The results are encouraging and suggest that the concept of enhancing modulations at the CI side is promising for the improvement of ITD perception with bimodal stimulation. Improved perception of binaural cues could lead to better localization performance and speech understanding in noise.
Overall the studies described in this thesis provide valuable new information with regard to the characterization of ITD sensitivity with bimodal stimulation and provide an important step towards the development of a binaural strategy for bimodal stimulation.

For further information the phd thesis is available through the following link: thesis.pdf

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