Proefschrift_vd_Beek

from the same group: Cohen et al (2004) use varying the position of the recording contact as the main principle of what here is called the scanning method to measure SOE, while Cohen et al (2003) show negligible effect of the position of the recording contact. Hughes and Stille (2010) showed generally higher eCAP amplitudes with recording electrodes towards the apical end of the array, in agreement with the scanning data presented by Frijns et al (2002). However, this effect of recording position showed only a limited shift in selectivity measures (Hughes & Stille, 2010). eCAP-based SOE profiles become wider with increasing current levels. Most SOE measures, however, have been obtained using small numbers of awake CI users (Cohen et al, 2003, 2004; Abbas et al, 2004; Eisen & Franck, 2005; Hughes & Abbas, 2006a, 2006b; Hughes & Stille, 2008). Performing measurements in awake subjects limits the current range that can be used, due to loudness tolerance issues. In line with Abbas et al (2004), the recent study by Hughes and Stille (2010) has shown a significant influence of current levels on SOE, but this was only seen in one third of the measurements. Furthermore, asymmetry in SOE along the array has been reported, but not quantified in detail (Cohen et al, 2003; Hughes & Abbas, 2006b; Cohen, 2009; Hughes & Stille, 2010). Apart from effects of level, location along the array and recording electrode, comprehensively analysed in a recent study by Hughes & Stille (2010), there are other likely factors potentially influencing the SOE. Some of these factors have to our knowledge not been considered by previous studies, and will additionally be investigated in this study. For selectivity measures the position of the probe is usually fixed and that of the masker varied. It is not known whether the same SOE is obtained if the masker is fixed and the probe varied. Theoretically, one would expect that it would be, since the measurement in both cases is the neural response from the region of overlap between the regions of excitation produced by the masker and the probe. Another parameter which could influence data obtained with scanning measurements is the artefact rejection method used (alternating polarity, or the forward masking method). Two different artefact rejection methods were therefore compared in this study. To date, no clear correlation between SOE measures and speech understanding has been reported (Hughes & Stille, 2008), but this may have been due to inappropriate statistical methodology. Hughes and Stille (2008) averaged the normalized amplitudes across SOE functions and subsequently performed linear regression to look for correlation between SOE width and individual speech perception scores. However, several parameters, such as the use of different electrode contacts along the array, could have affected the analysed outcome. In order to investigate parameters separately and to quantify the effects of the individual parameters, linear mixed models were used in our study (Fitzmaurice et al, 2004). The principal aim of the present study was to compare the two eCAP-based methods (scanning and selectivity) to measure SOE using the “ neural response imaging (NRI) ” system of the Advanced Bionics HiRes 90K implant in a relatively large patient group. More specifi cally, we aimed to identify and quantify parameters that limit the ability of these methods to determine the true excitation area within the

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