The effect of increased channel interaction on speech perception with cochlear implants

Abstract

Cochlear implants (CIs) are neuroprostheses that partially restore hearing for people with severe-to-profound hearing loss. While CIs can provide good speech perception in quiet listening situations for many, they fail to do so in environments with interfering sounds for most listeners. Previous research suggests that this is due to detrimental interaction effects between CI electrode channels, limiting their function to convey frequency-specific information, but evidence is still scarce. In this study, an experimental manipulation called spectral blurring was used to increase channel interaction in CI listeners using Advanced Bionics devices with HiFocus 1J and MS electrode arrays to directly investigate its causal effect on speech perception. Instead of using a single electrode per channel as in standard CI processing, spectral blurring used up to 6 electrodes per channel simultaneously to increase the overlap between adjacent frequency channels as would occur in cases with severe channel interaction. Results demonstrated that this manipulation significantly degraded CI speech perception in quiet by 15% and speech reception thresholds in babble noise by 5 dB when all channels were blurred by a factor of 6. Importantly, when channel interaction was increased just on a subset of electrodes, speech scores were mostly unaffected and were only significantly degraded when the 5 most apical channels were blurred. These apical channels convey information up to 1 kHz at the apical end of the electrode array and are typically located at angular insertion depths of about 250 up to 500°. These results confirm and extend earlier findings indicating that CI speech perception may not benefit from deactivating individual channels along the array and that efforts should instead be directed towards reducing channel interaction per se and in particular for the most-apical electrodes. Hereby, causal methods such as spectral blurring could be used in future research to control channel interaction effects within listeners for evaluating compensation strategies.

Figure 1

Speech perception results as mean scores for the group of 8 CI recipients when using spectral blurring for all 15 electrode channels (ALL, left) and for 5 out of 15 electrode channels (5-of-15, right). Top: speech perception in quiet (SIQ, in percentage correct), Bottom: speech reception threshold in multi-talker noise (SIN, in dB SNR). Note that for the ALL condition, the map number indicates the blurring factor and in the 5-of-15 condition, all maps used a blurring factor of 6. Error bars indicate standard errors and the dashed and dotted lines were added for visual comparison between the ALL and the 5-of-15 conditions (*: p < 0.05; **: p < 0.01; ***: p < 0.001).

Figure 2

Illustration of the electrode stimulation pattern used for spectral blurring. The top left part shows the blurring factors of 1 to 6 simultaneous electrodes used in the experimental maps (centred at electrode 5 in this example). The ALL plot shows a condition using 6 simultaneous stimulation electrodes for each of the 15 active frequency channels. The 5-of-15 conditions (Mspaced, Mapical, Mmiddle and Mbasal) use clusters of 5 adjacent or 5 evenly-spaced electrodes with a blurring factor of 6 (grey).