Proefschrift_vd_Beek

Theoretically, electrodes that produce more localized excitation would be more likely to provide better pitch acuity and less channel interaction. However, previous research showed no correlation between pitch ranking and eCAP SOE (Hughes & Abbas, 2006a). The attraction of eCAP measurements, in particular, is that these can usually be performed in young children, either intraor postoperatively, whereas psychophysical testing would often be impossible. Even in adults, psychophysical testing can be time-consuming and clinical eCAP procedures can be semi-automated and thereby made straightforward to perform clinically. Thus, it may be possible to identify the electrode contacts with narrower SOE, as part of the goal to identify an ideal subset of electrode contacts in an individual. eCAP recordings can be used in several ways to evaluate the SOE and are usually made using the back telemetry capabilities of the CI. A relatively basic method (termed “scanning” in this paper) is to stimulate one electrode contact and then measure the evoked response on all the other contacts along the array (Frijns et al, 2002; Cohen et al, 2004) (Figure 1, A). The amplitude profile of the responses thus obtained is not a direct measure of the SOE (as even a very narrow region of excitation can be detected some distance away) but wider regions of excitation would be expected to result in wider scanning profiles. For artefact reduction alternating polarity is incorporated in some clinical fitting software (used with Advanced Bionics cochlear implants), because it is a faster method than the subtraction method with masker and probe (Klop et al, 2004). A theoretically more precise method (termed “selectivity” in this paper) is analogous to the production of psychophysical forward-masking tuning curves, whereby a masker is applied to each electrode contact in turn, while a fixed “probe” contact is stimulated subsequently. The response amplitude indicates the amount of overlap between masker and probe (Figure 1, B). A limitation of this method is that the SOE of an electrode is derived from the neural excitation of two different electrodes (the masker and probe electrode). Residual charge from the masker pulse on the neural membranes of non-excited fibers could potentially change the region responding to the probe pulse, leading to a systematic deviation from the actual region of overlap. Additionally, the SOE of the masker contacts can vary along the array, which will affect the derived SOE of the electrode contact of interest. However, eCAP selectivity curves so produced have been shown to match behaviorally-obtained psychophysical tuning curves (Cohen et al, 2003), which theoretically suffer from the same limitation. Besides eCAP thresholds, eCAP-derived SOE measures are not a totally accurate reflection of the neural excitation. There are several characteristics of eCAP measures that might account for this. First, the position of the recording contact can affect certain characteristics of SOE measures. One limitation of intracochlear measurements is that the recording electrode needs to be located some distance away from the stimulating electrode contact. As the electrical pulse generated at the stimulating electrode is much larger than the neural response, the stimulus introduces a large artefact when the recording electrode is nearby. Another parameter, which may affect comparisons between different studies, is that not all researchers have used the same position of the recording contact. Indeed, there is even some discrepancy between publications

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