Proefschrift_vd_Beek

Djourno and Eyries, who began their experimental work in the 1950s, are considered the pioneers in the field of cochlear implants given their direct electrical stimulation of cranial nerve VIII [Eisen, 2003;Djourno and Eyries, 1957]. Based on their ideas, William House developed the first single channel cochlear implant [House, 1976]. This device merely functioned as a lip-reading aid. In the 1970s, multichannel implants, including devices designed by Ingeborg and Erwin Hochmair [Hochmair et al., 1979] and the first commercialized multielectrode device, developed by Graeme Clark [Clark, 1978;Mudry and Mills, 2013], were implanted for the first time. These multichannel implants also provided basic speech perception. In 1984, the FDA approved cochlear implants for adults, and approval for children followed in 1990. A next step in improving speech understanding with cochlear implants involved improving signal processing. A major step in that process was the development of continuous interleaved sampling (CIS), which yielded significant improvements in speech reception performance by preventing electrical interactions in the cochlea [Wilson et al., 1991]. Increasing numbers of both deaf adults and children have received implants since then. Optimization Although cochlear implantation can restore speech perception for many and numerous patients have been implanted, its results vary considerably among patients [Holden et al., 2013;Blamey et al., 2013]. Some patients merely experience closed-set speech recognition, and even well-performing patients experience hearing difficulties in real-life settings. Background noise remains a problem for cochlear implant patients [Spahr and Dorman, 2005;Fetterman and Domico, 2002]. Furthermore, tone recognition is only moderate in speakers of tonal languages, such as Chinese [Wei et al., 2004], and music appreciation remains poor for most cochlear implant users [McDermott, 2004]. Therefore, the optimization of cochlear implants is an ongoing process. The microphone is the first part of the cochlear implant that influences the quality of the captured sound. Directional microphones attenuate noise and increase the signal-to-noise ratio. Because hearing in noisy situations remains a problem for most cochlear implant patients, directional microphones are used to improve speech perception in noisy conditions [van der Beek et al., 2007;Wolfe et al., 2012]. In recent years, directional microphones have become routinely integrated into the external parts of cochlear implants. Further improvements have been obtained for speech processing. The greatest improvement in speech processing occurred with the introduction of CIS [Wilson et al., 1991], which decreases current interactions and thus increases channel independence. Further improvements have been attempted with the development of strategies that use higher stimulation rates to improve temporal resolution (HiRes, Advanced Bionics Corp., Sylmar, CA, USA; Fine Hearing, MedEl Corp., Innsbruck, Austria; MP3000, Cochlear Corp., Lane Cove, Australia) [Filipo et al., 2008a;Buechner et al., 2011] and virtual channels to improve spectral resolution (HiRes120, Advanced Bionics Corp., Sylmar, CA, USA). Additionally, the use of hearing aid technology to preprocess the speech signal in cochlear implants can facilitate hearing in specific circumstances.

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