Research PaperSimultaneous masking between electric and acoustic stimulation in cochlear implant users with residual low-frequency hearing
Introduction
For users of cochlear implants (CI), determining an appropriate signal representation at the auditory periphery is critical for improving the performance (e.g., Wilson, 2015). To optimize CI stimulation parameters, the research field has relied on auditory masking estimates (e.g., Wegel and Lane, 1924, Zwislocki, 1972) to decompose the acoustic input signal into electrical signals delivered by implantable electrodes along the basilar membrane.
For instance, previous CI masking studies have determined that the threshold of an electric probe stimulus is raised in the presence of an electric masker stimulus, and that the amount of across-channel interaction (Shannon, 1983) is dependent on the electrode positions (Lim et al., 1989), spatial spread profiles (Cohen et al., 2004), amplitude levels (Nelson et al., 2008), and modes of stimulation (McKay, 2012). These findings have fundamentally influenced the positioning of electrode arrays (Fraysse et al., 2006, James et al., 2005) and the assignment of speech processing parameters for modulation (Wilson et al., 1991), channel selection (Seligman and McDermott, 1995, Nogueira et al., 2005, Büchner et al., 2008), and current steering (Koch et al., 2007, Nogueira et al., 2009). With the advent of softer surgical techniques (von Ilberg et al., 1999, Lenarz et al., 2009) and more flexible arrays (Hochmair et al., 2015), it will be very important to extend the CI masking protocol to characterize the masking patterns for those with newly developed implants.
One area of CI research with increasing relevance due to relaxing implantation criteria is the combination of electric-acoustic stimulation (EAS; Kiefer et al., 2005, Skarzynski and Lorens, 2010, von Ilberg et al., 2011). Several subjective studies have investigated speech intelligibility with EAS (e.g., Büchner et al., 2009, Gifford et al., 2013). However, few studies have investigated masking between electric and acoustic stimulation.
Auditory masking refers to changes in hearing threshold levels of a sound due to the presence of another sound. Masking may be explained by peripheral or by central effects. Masking is not only a phenomenon of the normal hearing system, it also occurs with electric stimulation. It can therefore be assumed that masking exists in electric-acoustic stimulation, which is supported by the findings from Lin et al. (2011).
James et al. (2001) was the first to show electric-acoustic masking in CI users with residual hearing in the opposite ear. Lin et al. (2013) later showed that the presence of bilateral EAS masking is dependent upon the type of stimulation modality, i.e. electric or acoustic stimuli. To this day, ipsilateral EAS masking in the same ear has only been documented in a few participating subjects. Lin et al. (2011) showed electric masking on acoustic tones presented in the same ear in a single subject with a relatively long electrode array (24 mm). In all other subjects provided with a short electrode array (10 mm), electric masking was not observed. The authors hypothesized that for short electrode arrays the distance between the electric and acoustic stimulation sites in the cochlea may be too far apart to cause any interaction between electric and acoustic stimulation. The present study expands the previous masking research from Lin et al. (2011) by specifically investigating psychophysical ipsilateral EAS masking in relation to the distance between electric and acoustic stimulation sites. Moreover, the study presented here investigates masking between electric and acoustic stimulation in a larger population of subjects with residual hearing for frequencies up to around 1000 Hz and 16 mm electrodes that may maximize the interaction between both stimulation modalities.
For a better understanding of EAS interactions, it will be important to characterize not only psychophysical or perceptual EAS masking but also the physical interactions inside the cochlea. Recent advances in clinical imaging can be used to estimate the relative electrode position in the cochlea using cone beam computer Tomography (CBCT; Würfel et al., 2014). From the electrode position, it is possible to estimate the associated perceptual frequency using a place-to-frequency transformation based on the cochlear tonotopy (Greenwood, 1990, Stakhovskaya et al., 2007). Several studies have used place-to-frequency transformations to investigate pitch perception in single sided deafened CI users (Carlyon et al., 2010), the relationship between insertion angles and default clinical electrode frequency allocations (Landsberger et al., 2015) and to compare forward masking tuning curves between CI users and normal hearing listeners (Nelson et al., 2008). Moreover, thanks to clinical imaging it was possible to observe large differences between individual cochlea (Würfel et al., 2014) and it became also apparent that a large variability in insertion angles between and across electrode types do exist (Landsberger et al., 2015). These results indicate that the location of excitation along the cochlea between electric and acoustic stimulation may differ from subject to subject. In Lin et al., 2011, Lin et al., 2013 masking was characterized in terms of electrode numbers. Electrode numbers are well suited to determine intra-subject place-dependencies, but they do not allow inter-subject comparisons, because of large variability in electrode positions among subjects (Landsberger et al., 2015). Therefore, the present study proposes a novel analysis of electric-acoustic masking by defining the difference between the frequency delivered through electric stimulation and the frequency delivered through acoustic stimulation as the electric-acoustic frequency difference (EAFD). The EAFD in combination with perceptual masking experiments can be very useful to characterize EAS interaction across subjects and to determine the spread of masking in the frequency domain.
From a clinical perspective, a proper analysis of EAS masking will be critical since EAS systems are fitted by setting the crossover frequency between electric and acoustic stimulation, applying gains to the acoustic component (hearing aid) as well as determining the comfortable and threshold levels of electric stimulation. Setting the crossover frequency determines the allocation of frequencies to single electrodes. However, there is no standardized consensus on a best fitting approach for EAS systems (Incerti et al., 2013). While in some studies (Helbig et al., 2011, Nopp and Polak, 2010) the crossover frequency is set to the frequency at which the hearing threshold reaches 65 dB HL, other studies used a crossover frequency at 80 dB HL (Fraysse et al., 2006, Gstoettner et al., 2008, James et al., 2005, Lenarz et al., 2009) or 85 dB HL (Gantz et al., 2009, Simpson et al., 2009). Since the frequencies associated to the electrode position can lead to deviations with respect to normal hearing listeners, the distortion from unintended effect of EAS masking may lead to a negative impact on speech intelligibility. For this reason, significant improvements for combined electric-acoustic stimulation might be obtained if masking were considered in signal processing strategies and fitting concepts. For instance, computational systems have shown that the EAS feature input has a significant effect on the automatic speech recognition performance (Rader et al., 2015). However, these interactions have not yet been investigated thoroughly enough to be clinically applicable.
In case of EAS, it can be distinguished between electric and acoustic masking. While electric masking refers to changes in detecting an acoustic probe stimulus in the presence of an electric masker stimulus, acoustic masking refers to changes in detecting an electric probe stimulus in the presence of an acoustic masker stimulus. Consequently, two experiments were carried out to characterize psychophysiological EAS masking. In the first experiment, we have investigated masking produced by unmodulated electric pulse trains on the detection of acoustic probe tones, while within the second experiment masking caused by acoustic pure tones on the detection of electric pulse trains were examined. The population of EAS users has typically some residual hearing in the apical part of the cochlea corresponding with low frequencies. Furthermore, if masking occurs because of peripheral interaction between electric and acoustic stimulation, increased masking is anticipated when decreasing the difference between masker and probe stimulation sites. Therefore, one can expect that masking is more likely to occur at the apex of the cochlea. Under this assumption, a subset of apical electrodes and a selection of audiometric frequencies, based on the subject's individual residual hearing, were used to present electric and acoustic stimuli. The results from these experiments were combined with the estimated difference between stimulation sites from CBCT imaging data to characterize electric-acoustic masking.
Section snippets
Subjects
Five CI users with ipsilateral residual acoustic hearing at low frequencies participated in this study. Fig. 1 presents their post-operative clinical audiograms. The audiograms represent the subject's unaided air conduction pure tone thresholds measured via headphones in a clinical environment at their last clinical appointment. All participants had at least one year experience of EAS with the Nucleus CI24RE implant in combination with a Hybrid-L electrode array. The array shows a total length
Electric masking
Figure 6 shows the threshold elevations of an acoustic probe tone in presence of an electric masker stimulus for all five subjects. Fig. 6a) – e) show electric masking on acoustic probe tones as a function of electrode number in dB. Fig. 6f) – j) shows the threshold elevation caused by electric masking in %DR.
For intra-subject comparisons the probe frequencies and masker electrodes were converted into EAFDs. Fig. 7a) and c) shows the acoustic threshold elevation in the presence of an electric
Discussion
This work investigated the interaction between electric and acoustic stimulation using psychophysical simultaneous masking experiments. Threshold elevations of acoustic probe tones in the presence of electric maskers were observed for CI users with significant residual low-frequency hearing. The largest threshold elevations were observed for probe tones having frequencies close to the characteristic frequencies of the cochlear places at the electrode positions. In contrast, for an acoustic
Conclusions
The findings of two psychophysical masking experiments involving the detection of electric probes under acoustic maskers and acoustic probes under electric maskers support the conclusion that ipsilateral electric and acoustic stimulation can mask each other. Furthermore, this study shows an asymmetry between these two stimulation modalities. A method termed EAFD has been developed to characterize the difference between electric and acoustic stimulation sites in CI users with residual
Acknowledgements
The authors are grateful for the support by the DFG Cluster of Excellence EXC 1077/1 “Hearing4all”’. The responsibility for the content of this paper is with the authors. Without the subjects who have participated in the experiments, this work would have not been possible, therefore the authors would like to thank the subjects for their invaluable contribution.
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