Proefschrift_vd_Beek

auditory brainstem responses (eABRs), electrically evoked compound action potentials (eCAPs), and stapedius reflex measurements [Cords et al., 2000;Eisen and Franck, 2004;Firszt et al., 2003;Mens et al., 2003;Pasanisi et al., 2002;Wackym et al., 2004]. Moreover, these perimodiolar electrodes are implanted in many patients worldwide. However, surprisingly few studies have described the clinical effects of perimodiolar positioning [Basta et al., 2010;Saunders et al., 2002;van der Beek et al., 2005b;Holden et al., 2013;Esquia Medina et al., 2013;Hughes and Abbas, 2006;Fitzgerald et al., 2007;Filipo et al., 2008] [Chapter 3] . Significant variations in speech perception scores among cochlear implant populations make comparative research difficult. Because of the considerable variation among patients, which is mainly caused by factors other than differences in electrode arrays [Blamey et al., 2013;Holden et al., 2013], studies must include large groups of patients if they wish to identify the significant effects of electrode design. Moreover, in those large patient populations, factors other than the one being investigated must remain fixed. However, given the considerable length of time required to include numerous patients from one center in a study, new factors - such as newly developed electrodes, new fitting strategies or even different indications for implantation - are likely to be developed before such studies can be conducted. Combining groups from different clinics to obtain larger study populations is also difficult given that fitting is performed differently at different centers [Zwolan, 2005;Wesarg et al., 2010]. In addition, many centers, especially European centers, use different speech reception tests. These differences among centers can be partially addressed by ranking the speech performances of groups [Holden et al., 2013;Blamey et al., 2013]. To continually improve electrode array designs, it is important to study the effects of the electrodes that are currently clinically available. The number of implanted patients makes it possible to conduct studies with large patient populations. However, studies that include more than 1000 cochlear implant patients are rare [Blamey et al., 2013], and even outcome studies that include more than 100 subjects are limited [van der Beek F.B. et al., 2014;Holden et al., 2013;Vargas et al., 2012] [Chapter 5] . Although numerous patients have been implanted with perimodiolar electrodes and the clinical effects of these electrodes could be studied in great detail, new designs are often introduced without a clear understanding of their clinical outcomes. In addition, their designs may be altered without regard for how the changes might impact their beneficial effects. Measuring spectral resolution It is important that adequate methods are used to analyze the potential improvement in the spectral resolution of the optimized location in the cochlea. The measurement of the electrically evoked action potentials (i.e., eCAPs) of the cochlear nerve fibers in patients with cochlear implants presents promising possibilities [Abbas et al., 1999;Frijns et al., 2002]. Various studies using spread of excitation (SOE) measures have demonstrated that the SOE was smaller in the basal portion of the cochlea [Eisen and Franck, 2005;van der Beek et al., 2012] [Chapter 4] . These findings indicate enhanced spectral resolution in the

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