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Lars Bramsløw

Research Engineer, PhD, Project Leader

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mail@eriksholm.com

This PhD project is concerned with bimodal hearing, i.e. users that have a cochlear implant in one ear and a normal hearing aid (HA) in the other ear (or even normal hearing in the other ear). Users of such systems should have the benefit of being able to combine the sounds from both ears but face difficulties.

For a quick overview of the project, please click on the video below:

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phd-bimodal-hearing

The CI's electrically stimulated signal is perceived quite different from the acoustic sound in the other ear and further the timing between the signals in both ears is different, sometimes perceived as an echo. There can also be large differences in the pitch, timbre (color) and/or loudness on the two sides. Thus, bringing together this information, fusing it, can be a challenge for some patients.

The aim of this PhD project is to understand the mechanisms that make it difficult to fuse the information so as to guide development of optimized methods for the fitting of both devices together - a bimodal fitting. With such a bimodal fitting, devices should be able to deliver binaural cues, that make the localization of sounds and the segregation of sound sources much easier. This should help patients to better understand speech in difficult situations (Most, Harel, Shpak, & Luntz, 2011; Sheffield & Zeng, 2012), make listening to music more enjoyable (El Fata, James, Laborde, & Fraysse, 2009; Looi, McDermott, McKay, & Hickson, 2008; McDermott, 2004) and identification of sounds occurring in everyday life easier (Sucher & McDermott, 2009).

Today cochlear implants often use fixed-rate pulse trains whose amplitude is modulated by the smoothed envelope of signals derived from a number of partially overlapping frequency bands in their processing strategies. This, together with a broader stimulation over the electrodes in the cochlear liquid, leads to pitch and temporal fine structure of sounds being represented coarser. Yet, other temporal cues such as attack, decay, sustain, release times and rhythm are transmitted more accurately.

On the other side, hearing aids are better at transmitting these temporal fine-structure and spectral cues reliably, while other sound aspects may be distorted due to sound processing such as time-varying gain functions, noise reduction, feedback cancellation and frequency lowering. Thus patients could profit from the synergy of both devices.

With a bimodal fitting, it could be easier for patients to exploit the benefits of both devices and of hearing with both hears. Until now audiology clinics are often specialized in only one hearing device. Hence bimodal patients have to visit two different places with two different audiologists in order to get their devices fitted – often with no consideration for the other device. Further, cochlear implants and hearing aids were traditionally manufactured by different companies. This changed only very recently, when two large hearing aid companies each bought a cochlear implant company (Phonak and Advanced Bionics, Oticon and Neurelec). With the combined knowledge and capabilities, we now have the chance to make true bimodal fitting a reality, based on the findings on how to create binaural cues for sound localization and segregation from this PhD project.

Without such, differences between auditory sources can only partly be perceived with one ear, making stream segregation and recognition of musical information more difficult (Marozeau, Innes-Brown, & Blamey, 2013; Marozeau, Vannson, Peretz, & Innes-Brown, 2013). Also here localization cues derived from binaural listening play a role but additionally the effect of binaural masking release or simply being able to pay more attention to the ear closer to the source make a difference.

Directly after activation of the cochlear implant, patients are reported to perceive a strong difference in perception between their ears, resulting from the different ways of stimulation. They have to get used to the way the implant sounds. This difference supposedly makes it harder to fuse the percepts from both ears. Given time, the patients can learn how to do this. However, with a better understanding of the underlying mechanisms for fusion, one could eventually match the percepts between devices better from the beginning on. This should decrease the patients' effort needed for learning how to fuse the percepts and eventually result in a more natural perception.

This Ph.D. will be supervised by A/Prof Jeremy Marozeau and Prof Torsten Dau in collaboration of Oticon Medical. Jeremy has a strong expertise in the field of cochlear implants and Torsten is world expert in the field of auditory scene analysis and hearing aid processing. On the side of Oticon, the project is supervised by Dr. Søren Riis and Dr. Lars Bramsløw.


Further reading

Camilleri, M., Marozeau, J., Innes-brown, H., & Blamey, P. (2010). Effect of sound localisation on melody segregation. In Australasian International Conference on Speech Science and Technology (pp. 197–200).

El Fata, F., James, C. J., Laborde, M. L., & Fraysse, B. (2009). How much residual hearing is “useful” for music perception with cochlear implants? Audiol Neurootol, 14 Suppl 1, 14–21. doi:000206491 [pii] 10.1159/000206491

Francart, T., & McDermott, H. J. (2012). Development of a loudness normalisation strategy for combined cochlear implant and acoustic stimulation. Hearing Research, 294(1-2), 114–24. doi:10.1016/j.heares.2012.09.002

Lazard, D. S., Marozeau, J., & McDermott, H. J. (2012). The sound sensation of apical electric stimulation in cochlear implant recipients with contralateral residual hearing. PLoS ONE, 7(6), e38687. doi:10.1371/journal.pone.0038687

Looi, V., McDermott, H., McKay, C., & Hickson, L. (2008). Music perception of cochlear implant users compared with that of hearing aid users. Ear and Hearing, 29(3), 421–34. doi:10.1097/AUD.0b013e31816a0d0b

Marozeau, J., Florentine, M., & Campbell, M. (2008). Binaural Loudness Summation in an Experienced Bimodally Aided Listener: a Case Study. In ’Medical Bionics - a new paradigm for human health. Lorne, VIC, Australia.

Marozeau, J., Innes-Brown, H., & Blamey, P. (2013). The acoustic and perceptual cues in melody segregation for listeners with a cochlear implant. Frontiers in Psychology, 4(790).

Marozeau, J., Vannson, N., Peretz, I., & Innes-Brown, H. (2013). Bimodal and bilateral cochlear implant users prefer fast tempos: aesthetic responses to dichotic, binaural and monaural melodies. In First conference on Music, Mind and Health. Melbourne, Vic, Australia.

McDermott, H. J. (2004). Music perception with cochlear implants: a review. Trends in Amplification, 8(2), 49–82.

Most, T., Harel, T., Shpak, T., & Luntz, M. (2011). Perception of suprasegmental speech features via bimodal stimulation: cochlear implant on one ear and hearing aid on the other. Journal of Speech, Language, and Hearing Research : JSLHR, 54(2), 668–78. doi:10.1044/1092-4388(2010/10-0071)

Sheffield, B. M., & Zeng, F.-G. (2012). The relative phonetic contributions of a cochlear implant and residual acoustic hearing to bimodal speech perception. The Journal of the Acoustical Society of America, 131(1), 518–30. doi:10.1121/1.3662074

Sucher, C. M., & McDermott, H. J. (2009). Bimodal stimulation: benefits for music perception and sound quality. Cochlear Implants International, 10 Suppl 1, 96–99. doi:10.1002/cii.398