Proefschrift_vd_Beek

M-levels. This corresponds to a DR of 9–14 dB. This is ob-viously less than the 20 dB DR obtained when T-levels are set at 10% of the M-levels, which is the default set-ting in SoundWave, the manufacturer’s fitting software. The 9to 14-dB electrical DR is in line with the data reported by Wesarg et al. [2010] for Nucleus cochlear implant recipients. In previous studies, even lower DRs of around 8 dB were described [Pfingst and Xu, 2005]. Although research showed that speech perception is not negatively influenced by lower T-levels, at least not directly, some recipients may prefer strategies with higher T-levels [Spahr and Dorman, 2005]. Moreover, higher T-levels were shown to be beneficial for speech understanding at low sound levels and other challeng-ing listening situations [Holden et al., 2011]. However, even if programs with higher T-levels lead to better per-ception of soft speech, they carry an increased risk of inducing buzzing sounds in quiet, and a trade-off must be made. Normative data about levels in our study population can help in evaluating the overall level for an individual recipient. When a profile in a pediatric subject is fitted on the basis of an estimated level at a single electrode contact, the Tand M-levels can be held against the per-centiles of our adult cochlear implant users ( fig. 1 ). If the measurement for subjective levels performed in the sub-ject is clear, no adjustments have to be made. However, if the audiologist is in doubt about the subject’s reaction and stimulation is at a high level, it may be prudent to set the levels in a normal or average range. However, it is important to realize that the data presented here are for adults, and that a similar study with pediatric subjects still has to be done. Zwolan [2005] and Wesarg et al. [2010] showed that a lot of differences exist between dif-ferent implant centers, highlighting the large influence of the local audiologists’ practice. Further, Zwolan [2005] showed that children got used to higher M-levels easily, introducing the risk that M-levels are set higher and higher on consecutive fittings, thereby ultimately risking overstimulation. To deal with this risk, some groups pro- pose the use of eSRT measures [Allum et al., 2002; Gor-don et al., 2004; Caner et al., 2007]. However, in our cen-ter, the behaviorally determined M-levels of 43 children under 5 years of age were not significantly higher than the M-levels in the adults of the present study (mean 277 vs. 226 CU; p = 0.069). Additionally, figure 4 a shows that using the manufacturer’s default to set T-levels to 10% of M-levels will result in understimulation in the majority of cases. Therefore, it is worthwhile to measure actual T-levels, as this will most likely improve the perception of soft speech. Figure 2 c shows that the M-level is not a good predic-tor for the T-/M-level ratio, as these values are not cor-related. However, overall T-levels have been shown to hold at least some predictive value for overall M-levels ( fig. 2 b), which is in line with Wesarg et al. [2010]. This might be due to the fact that (even in a monopolar mode) T-levels give information about the neural excitability of the region around the electrode contact. If this region is easily excited, it is likely that the neighboring area will also be easily excited with an increasing current, resulting in a relatively low M-level. The M-level, however, gives infor-mation about a very wide region of excitation along the cochlea. It might well be that it does not reflect the neural status of the region nearby the electrode, which is directly influencing the T-level for that electrode. When comparing the relationships between Tand M-levels reported here with the findings in other publications, one should keep in mind that the results can be influenced by the way of setting M-levels.

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