Virtual channel discrimination is improved by current focusing in cochlear implant recipients

Abstract

Cochlear implant users' spectral resolution is limited by both the number of implanted electrodes and channel interactions between electrodes. Current steering (virtual channels) between two adjacent monopolar electrodes has been used to increase the number of spectral channels across the electrode array. However, monopolar stimulation is associated with large current spread and increased channel interaction. Current focusing across three adjacent electrodes (tripolar stimulation) has been used to reduce electrode current spread and improve channel selectivity. In the present study, current steering and current focusing were combined within a four-electrode stimulation pattern (quadrupolar virtual channels), thereby addressing the need for both increased channels and reduced current spread. Virtual channel discrimination was measured in 7 users of the Advanced Bionics Clarion II or HiRes 90K implants; virtual channel discrimination was compared between monopolar and quadrupolar virtual channels at three stimulation sites. The results showed that quadrupolar virtual channels provided better spectral resolution than monopolar virtual channels. The results suggested that quadrupolar virtual channels might provide the "best of both worlds" improving the number of spectral channels while reducing channel interactions.

Figures

Figure 1

Illustration of different stimulation modes. Note that the amplitudes only represent the first phase of a biphasic pulse. The x-axis describes the electrode position (EC = extra-cochlear electrode).

Figure 2

Mean current amplitude needed to obtain different loudness levels for MP (filled circles) and QP stimulation modes (open circles). The different panels show three stimulation sites in the cochlea. The error bars show one standard deviation. The four different loudness levels are “Barely Audible” (BA), “Soft”, “Most Comfortable Level” (MCL), and “Maximal Acceptable Loudness” (MAL).

Figure 3

Adjacent electrode discrimination with MP or QP stimulation, for individual CI subjects. The left axis shows d' and the right axis shows percent correct. The different panels show different stimulation sites in the cochlea.

Figure 4

Cumulative d' for MPVC (filled circles) and QPVC discrimination (open circles) for individual subjects, as a function of current steering α values.

Figure 5

QPVC discrimination as a function of MPVC discrimination, in terms of cumulative d'.

Figure 6

Left panel: MPVC discrimination as a function of MP adjacent electrode discrimination. The different symbols represent different stimulation sites. The diagonal shows the unity (MPVC discrimination = MP electrode discrimination). Right panel: QPVC discrimination as a function of QP adjacent electrode discrimination.