Proefschrift_vd_Beek

al., 2012]. In our clinic, T-levels are measured and established for each individual electrode contact, thus allowing us to analyze the course of the T-levels along the cochlea. When a positioner was used, the T-levels along the array exhibited a relatively flat profile. However, the patients without a positioner in our study population showed a patient-independent increase in the T-level profile towards the basal end of the cochlea [Chapter 3,5,6] . Indeed, with deeper insertions, these increases were no longer evident [Chapter 3,6] . When this basal increase in T-levels is not fitted, the basal dynamic range is reduced [Chapter 6] . To facilitate future fittings, a model was developed wherein the standard T- level profile was combined with a T-level measurement for an individual patient at just one electrode contact. Although the retrospective study by van der Beek et al. (2015) did not include an intervention and thus did not assess the effect on speech perception, Zhou et al. (2014) revealed that setting proper T-levels for electrode contacts with worse modulation detection thresholds could improve speech perception [Zhou and Pfingst, 2014]. Additionally, T-level increases did not exclusively occur in the most basal portion of the cochlea; ranges of up to 300 degrees insertion depth were associated with increases [Chapter 6] . Furthermore, this increase was not caused by a larger distance from the modiolus; the distance to the modiolus of the basalmost electrodes was smaller than the distance to the more apically located electrodes [Chapter 6] . Furthermore, the basal increase in T-levels was independent of the duration of deafness and was stable the first year of cochlear implant use [Chapter 6] , indicating that neural degeneration and increased scar tissue formation are less-likely underlying causes. Differences in the shape and position of the array along the cochlea are needed Differences in threshold levels [van der Beek F.B. et al., 2014;Vargas et al., 2012;van der Beek et al., 2015a] [Chapter 3,5,6] and SOE [van der Beek et al., 2012;Hughes and Stille, 2010] [Chapter 4] and clear differences in anatomy (Figure 1) indicate that different electrode designs along the array may be useful. However, with a few exceptions, electrodes are not designed to adapt to different anatomical situations along the array. Moreover, the effect of the anatomy of the cochlea on the electrode-neural interface has not been studied in detail. These effects are difficult to study because of a number of restraints (e.g., considerable variability in outcomes among patients and differences in human and animal anatomy). However, a large collection of outcome data can be acquired by studying patients implanted with different cochlear implant electrodes. Future research Future research should quantify the clinical effects of electrode design, especially differences along the array. Performance data from everyday clinical practice is available for all cochlear implant centers. The enormous number of patients with cochlear implants makes studies with large samples feasible. With clinical data from large study populations, even small effects of various parameters can be identified. The effects of those parameters can be further quantified in more fundamental, laboratory-based settings as a prelude to improved electrode design in the future.

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