Proefschrift_vd_Beek

may produce a truly asymmetric excitation of nerve fibers. Such real asymmetric spreading of excitation is, however, enhanced by limitations of the recording methodology.

The use of tonotopy is one of the key contributing factors to the speech perception potential of multichannel cochlear implants. Theoretically, a well controlled, limited SOE would allow for more independent information channels, which could ultimately lead to better speech understanding. Unfortunately, like other studies (Hughes & Abbas, 2006a; Hughes & Stille, 2008) we were not able to demonstrate any significant correlation between SOE and speech perception. In this perspective it is worthwhile to reconcile the fact that the data in the present study were obtained at high current levels, and that we observed no inter-level differences in excitation width for normalized data. Nevertheless, the eCAP amplitudes obtained at these current levels varied by up to a factor of two between the low and the high current levels, suggesting that the response was not in saturation. However, in some individual cases with low noise levels and relatively large eCAP amplitudes, a clear decrease in SOE width could be seen with decreasing current levels. Surprisingly, these findings do not confirm previous research that was able to demonstrate level effects on SOE in patients, tested at the upper portion of the behavioral dynamic range (Abbas et al, 2004; Hughes & Stille, 2010). For the subjects of our study measurements at current levels used in daily use may show narrower SOE curves, but with present hardware limitations (especially system noise levels) it was not possible to test this hypothesis by measuring at low current levels. Additionally, the fact that intraoperatively derived eCAP SOE measures are compared with speech perception measured two years later could account for the lack of correlation. eCAP measures are known to change over time (Hughes et al, 2000, 2001; Gordon et al, 2004) and SOE measured at the same time as the speech perception test would have given additional information. Unfortunately, however, it turned out to be not feasible in our clinical setting to re-assess SOE in the same patient group two years postoperatively. For the selectivity curves this study showed no significant differences between a roving masker (with fixed probe) position and roving probe (with fixed masker). This is in agreement with theoretical expectations as both probe or masker fixed should measure the same overlap between the areas excited by masker and probe and should therefore give the same response. The advantage of the first condition is that the distance between recording electrode and probe is constant, stabilizing possible artefacts in the recorded response. On the other hand, with a roving probe the distance of the probe to the recording contact is larger at locations remote from the contact of interest (i.e. the masker contact), resulting in a reduced artefact interference. The conformity in response with probe or masker fixed indicates that SOE curves are relatively robust in relation to artefacts and other secondary effects. In contrast to selectivity, scanning allows both forward masking as well as alternating polarity as the artefact rejection method. The choice of method did not appear to affect the widths of the recorded scanning curves. The alternating polarity artefact rejection takes slightly less time and is used in clinical practice (with Advanced Bionics cochlear implants), but has the disadvantage of averaging different latencies produced

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