Proefschrift_vd_Beek

Most currently available cochlear implant (CI) systems use silicone electrode carriers that are positioned in the scala tympani of the cochlea and support typically between 12 and 22 electrodes spread over some 20–30 mm of the cochlea, i.e. about 60%–90% of its total length from the round window. The longitudinal arrangement of stimulating electrodes is designed to retain the tonotopic organization of the cochlea, whereby high frequency components of the incoming acoustic signal are delivered to electrodes near the base and lower frequency components towards the apex. Use of multichannel implants in this way therefore attempts to provide “place-pitch” spectral information, whereas characteristics of the stimulation patterns delivered to individual electrodes provide information on loudness changes over time, i.e. temporal information. The resolution of the spectral information provided by the CI depends on several factors. One factor is the number of electrodes used or, more specifically, the spacing between them. However, the assumption that the use of more closely spaced electrode contacts might provide greater frequency resolution has not been supported by experimental evidence (Friesen et al, 2001; Frijns et al, 2003). Better electrode discrimination has been shown to be correlated with better speech understanding (Zwolan et al, 1997), but it is clear from experimental studies that use of a larger number of electrodes does not always result in better speech understanding. Indeed, when other parameters are kept constant several studies have found no improvement in speech discrimination in quiet for electrode numbers greater than about seven (Friesen et al, 2001; Garnham et al, 2002; Frijns et al, 2003). One likely explanation for this is that every electrode does not necessarily produce a pitch percept that is distinct from the others. As shown by psychometric tuning curves, the regions of neuronal excitation produced by adjacent electrodes overlap significantly, particularly when monopolar stimulation (referenced to a remote electrode outside the cochlea) is used (Boex et al, 2003). Several studies have shown improvements in speech discrimination when such electrodes with considerable overlap are de-activated (Boex et al, 2003; Frijns et al, 2003; Arnoldner et al, 2007). Apart from this limited spatial selectivity, electrical channel interaction also has an influence. Therefore, many strategies, such as continuous interleaved sampling (CIS) attempt to minimize channel interaction by interleaving of the stimulus pulses between channels (Wilson et al, 1991). A better understanding of the spread of excitation (SOE) in the implanted cochlea would be beneficial in many ways, including the identification of which electrodes to de-activate and to improve electrode design in future cochlear implants. Measuring the SOE can be performed through psychophysical testing (Chatterjee & Shannon, 1998; Boex et al, 2003; Cohen et al, 2003; Dingemanse et al, 2006; Hughes & Stille, 2008; Nelson et al, 2008), from recordings of the electrically-evoked compound action potential (eCAP) of the auditory nerve (Cohen et al, 2003, 2004; Abbas et al, 2004; Hughes & Abbas, 2006a, 2006b; Klop et al, 2009; Hughes & Stille, 2010), or simulated using computer models (Frijns et al, 2001; Cohen, 2009).

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