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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).