Tech Topic | April 2016 Hearing Review

by Tammara Stender, AuD

A look at signal processing options for the contralateral ear in a bimodal fitting, and individual patient considerations that may indicate when a certain feature or signal processing choice is advantageous.

The benefits of binaural hearing have been established, and extend to bimodal fittings of a cochlear implant and a hearing instrument. Further, recent research reveals many patient benefits of simultaneous wireless audio streaming to both the cochlear implant and the hearing instrument. Yet precise protocols for programming the hearing instrument in the bimodal fitting are not currently determined. Aside from the need to balance loudness between the cochlear implant and the hearing instrument, little guidance exists for the contralateral ear’s signal processing choices.

This paper explores ReSound hearing instrument signal processing options for the contralateral ear in a bimodal fitting, and the individual patient considerations that may indicate when a certain feature or signal processing choice would be advantageous.

Binaural & Wireless Advantages

Binaural hearing provides benefits that exceed those of monaural hearing. These benefits include better localization abilities, or the ability to decipher the location of sound sources;1-5 improved speech intelligibility in noise,5-8 in part due to better spatial release from masking;9 prevention of auditory deprivation of the unaided ear, when that ear also has aidable hearing loss;13-17 possible relief from tinnitus;18 and improved sound quality with respect to music perception.19

In the same way, the benefits of bimodal fittings—in this paper, with a cochlear implant on one side and a hearing instrument fitted for the contralateral ear—exceed those of a unilateral cochlear implant or hearing instrument fitting. In addition to the general binaural benefits stated above, bimodal studies indicated that low-frequency information provided by the hearing instrument assisted with speech perception in noise, by helping the listener segregate speakers based on fundamental frequency cues.10-12 Further, the binaural benefit of improved sound quality and enhanced music perception was noted to be especially beneficial for bimodal users.19

Despite these binaural advantages in bimodal fittings, patients may experience the same difficulties encountered by bilateral users of hearing instruments. Discriminating speech in background noise, in reverberant environments, over a longer distance, or over the phone are common situations in which bilateral and bimodal users may not be completely satisfied with their ear-level devices.20 The directional processing capabilities of the ear-level hearing instrument or cochlear implant cannot extend more than several feet in front of the user, and cannot fully account for echoes in the environment as sound reverberates off of surfaces. The lack of visual cues on the phone or over longer distances rob the user of the speechreading advantages they would have in a typical one-to-one conversation.

For these difficult yet commonplace scenarios, wireless connectivity through dedicated accessories or direct-to-iPhone capabilities can make an enormous difference in the user’s ability to understand speech. The Smart Hearing Alliance between Cochlear and ReSound allows for simultaneous audio streaming of the same wireless accessories for the cochlear implant and the hearing instrument. By delivering the speaker’s voice directly to the user’s hearing instrument and cochlear implant, these accessories improve the signal-to-noise ratio (SNR) significantly, even in the most difficult situations—in background noise, in reverberant conditions, over distance, and over the phone.

This article will not focus on wireless connectivity advantages and benefits, which are becoming increasingly established through current research. Instead, it will take a step back and discuss some of the options for hearing instrument signal processing that are available in a bimodal fitting with a ReSound hearing instrument. Of the gain and feature options available in hearing instrument programming, the best researched area regards the hearing instrument’s gain settings.

Bimodal Considerations for Setting Hearing Instrument Gains

Research indicates that best outcomes are achieved when loudness is generally balanced between the cochlear implant and the hearing instrument in a bimodal fitting.21,22 The benefits of loudness balance between the cochlear implant and hearing instrument include better localization abilities23 and speech perception.24 When the hearing instrument gains are programmed to the same or slightly lower perceived loudness level as the cochlear implant, the highest levels of performance are expected.22

Typically, the bimodal loudness balance procedure entails programming the cochlear implant to optimal settings, and then fine-tuning the hearing instrument gains to achieve this loudness balance. For many bimodal patients, the goal is to maximize the low-frequency hearing instrument gains, as acoustic amplification of severe high-frequency thresholds has been noted to be detrimental for speech perception.25-27

However, the procedure and judgment of loudness balance between electric and acoustic stimulation may be difficult for some patients. For these patients, setting the hearing instrument gains to most comfortable levels (MCLs) may result in satisfactory performance levels.22

Figure 1

Figure 1. Difference in default gain settings between two programs: the All-around/Basic program vs the Restaurant program.

One consideration when fitting the contralateral ear with more than one program is the possibility of gain offsets in multiple programs. Small gain deviations from the “All-Around” program may be employed as default settings in a “Restaurant,” “Party,” or other program. Certain ReSound programs may increase high-frequency gains with respect to the “All-Around” program in an effort to maximize speech intelligibility in noisy or difficult situations. Figure 1 shows default gain settings for the “All-Around” and “Restaurant” program, based on the same audiogram.

What does this mean for the bimodal user? If the loudness balance procedure is performed while the patient is in the first program, the loudness balance they perceive in the second program during daily use may be different than what was predicted in the clinic. To rectify this concern, the gains for the second or other programs may be increased or decreased to match the gains for the first program, where loudness balance was approximated.

For some bimodal patients, these small changes in default gains among programs may not make a significant impact on the perceived loudness balance in daily use. This may be especially true in the case of severe or profound high-frequency hearing loss, and whether these high-frequency gain targets are achievable or usable for the patient.

Aside from loudness balance, there is little guidance currently for setting any of the other hearing instrument signal processing or feature choices available in a bimodal fitting. Hearing instrument fitting software in a typical bimodal fitting may program the hearing instrument as a monaural device, with default signal processing and feature options that do not take the cochlear implant processing into account.

Research into optimal settings for the hearing aid signal processing and features are in progress; however, at the present time it is useful to explore the options available as bimodal fittings occur in today’s clinics. “Cookbook” approaches to setting up the hearing instrument signal processing may provide good starting points for fittings. Yet, as with any default setting, patient needs and characteristics will continue to help derive the best clinical solutions for the individual.

The hearing instrument signal processing options discussed in this article focus on ReSound technology. However, generalizations might be made in some cases to other commercially available hearing aid technology and features.

Bimodal Considerations for Directionality

Directionality in hearing instruments is widely regarded as a way to improve the SNR in noisy listening environments. There has been much discussion in recent years about how to support the brain’s natural processing abilities in these complex listening situations via bilateral directional, bilateral omnidirectional, or asymmetric directional responses. While this binaural directionality discussion is outside of this paper’s scope, it is still appropriate to consider how directionality is programmed for the cochlear implant side when selecting a directional or omnidirectional response for the hearing instrument side.

When fitting the contralateral ear for a bimodal fitting with ReSound hearing instrument technology, the options include SoftSwitching, Adaptive Directionality, Fixed Directionality, and Omnidirectionality. Some of these options may be limited by the program; for example, the All-Around program only offers SoftSwitching and Omnidirectionality, as the focus of that program is, by definition, to provide some auditory awareness to all directions in at least some listening situations.

SoftSwitching automatically applies an omnidirectional response or a directional response, depending on the location of speakers and the presence or absence of noise in the listening environment. For example, SoftSwitching will apply a directional response when there is a speaker in front of the listener in a noisy situation; it will apply an omnidirectional response in the case of a quiet environment or a diffuse noise environment where there is no predominant speaker in front of the listener. When a directional response is warranted by the environment, the directionality will be adaptive—meaning the null or nulls of the directional response will be directed to the strongest concentration of noise. However, the directional beam will always be front-facing, so that audibility for the look direction is not reduced.

SoftSwitching is the most automatic and adaptive directional option available for monaural hearing instrument programming sessions, which currently include bimodal fittings. As it provides SNR benefit in noise and auditory awareness in quiet, it is recommended for most listening situations and users. Its more comprehensive array of directional responses may make it a good choice for fittings that only include a single program, or for users who prefer not to change programs over the course of the day.

Adaptive Directionality is similar to SoftSwitching, except that the device is always in a directional response. There is no switching to an omnidirectional response for the Adaptive Directionality option. Adaptive Directionality is the default directional setting for the “Restaurant” program, since it may be most advantageous in noisy situations.

Figure 2

Figure 2. AutoScope applies a wider beamwidth for easier SNR environments, and a narrower beamwidth for more difficult SNR environments. This figure shows that in Adaptive Directionality, with less noise, the beamwidth is wider (black dotted lines); when three noise sources are introduced, the beamwidth narrows (red dotted lines).

Both Adaptive Directionality and SoftSwitching also have beamwidth options: AutoScope, Narrow, Medium or Wide. AutoScope allows for automatic beamwidth adjustment, based on the listening environment. As the SNR becomes more challenging, AutoScope automatically applies a narrower beamwidth in an effort to reduce more of the background noise (Figure 2). When a Narrow, Medium or Wide beamwidth is programmed, the beamwidth will remain fixed to that width regardless of the listening environment. The availability of different beamwidth programming options is dependent on the hearing instrument technology level.

Fixed Directionality, as the name implies, applies a hypercardioid front-facing directional response at all times. As a simple directional solution, it provides an alternative directional option.

Omnidirectionality provides audibility to all directions, which reduces the chance of listeners “missing out” on sounds arriving from different locations around them. It is the default setting in the “Music” program, and research shows that it is strongly preferred by users in quiet environments.28,29

What Directional Setting Should You Choose for Bimodal Fittings?

As with any other clinical decision, the best hearing instrument feature settings for the contralateral ear in a bimodal fitting are dependent on the individual’s communication needs and listening environments. In many cases, the cochlear implant features may not perform identically to their correlates on the hearing instrument side. Future research will help determine how bimodal users benefit from signal processing choices on each side. Yet, for today, knowledge of both the individual’s characteristic needs combined with knowledge of how the cochlear implant and hearing instrument features perform in different environments will aid in selecting the optimal feature default settings.

For example, both the cochlear implant and the hearing instrument may have environmental classification that determines when a directional response will be provided versus an omnidirectional response. It is possible, however, that the criteria for this automatic directionality switching—the SNR of the environment, the location of dominant speakers, the overall level of the background noise—may be slightly different between the cochlear implant and the hearing instrument. This means that, in some environments, the cochlear implant may switch to directional processing while the hearing instrument is still in omnidirectional, or vice versa. Whether this is acceptable for the bimodal patient is difficult to determine at the first fitting session.

For some bimodal users, asymmetric settings may be advantageous; for others, a symmetric response on each side may be more beneficial. A more static response such as Adaptive Directionality, Fixed Directionality, or Omnidirectionality may prevent directional switching issues. However, static responses such as these will not provide the all-in-one program benefits in multiple environments that SoftSwitching provides.

A possible good solution at the present time might be to fit the first hearing instrument program, the ReSound “All-Around” program, with the default directional setting of SoftSwitching. A second program, already preselected in Aventa fitting software as the “Restaurant” program, applies an Adaptive Directional response. Fitting the bimodal patient with each setting over two programs may give patients time after the fitting to determine which program is preferred for their individual lifestyles.

Bimodal Considerations for Frequency Lowering and Restricted Bandwidth

Studies are emerging regarding performance benefits of wideband fittings, restricted high frequency fittings and frequency lowering. Davidson et al30 compared performance and patient preference for speech recognition and localization among these three options, using a nonlinear frequency compression strategy for the frequency lowering condition. The results indicated no significant differences in group data except for localization, where the wideband and frequency lowering conditions provided better performance. However, individual differences in performance and preference were noted. These results are similar to the findings of numerous studies comparing frequency lowering with other fitting options, in which group results may or may not predict individual performance and preference.

The appropriateness of frequency lowering or restricted bandwidth for the individual patient may be assessed through a fine tuning procedure, taking into account the user’s sound preferences.31 However, the patient’s satisfaction with one response as compared to another may not be easily gauged at the first fitting. In some cases, a patient’s history of satisfaction or dissatisfaction with frequency lowering or restricted bandwidth fitting can ease the decision-making process. If no other directive exists and the patient is not able to discern his or her preference at the first fitting, providing a few different options in separate hearing instrument programs may be helpful for the patient to evaluate preferences during daily use.

Figure 3

Figure 3. Spectrograms for a swept pure-tone, in the unprocessed condition (upper- and lower-left panels); nonlinear frequency compression (lower-right panel) and Sound Shaper (upper-right panel). Greater blurring of the response indicates more distortion.

If frequency lowering is deemed appropriate in a ReSound bimodal fitting, Sound Shaper is available. Sound Shaper is a proportional frequency compression technique. As with nonlinear frequency compression, frequencies above a frequency compression threshold are compressed. However, with Sound Shaper, the compressed frequencies maintain the same proportional relationship to one another, which results in lower distortion (Figure 3). It has been shown that outcomes with this technology are similar to those using a nonlinear method.32 Proportional frequency compression is a method which gains greater audibility for the patient in the high-frequency region, while also aiming to preserve the sound quality of the output. (For more information, see Stender and Groth.33)

Bandsplit Directionality and Bimodal Fittings

Many directional options available across most hearing instrument manufacturers include bandsplit directional processing. Bandsplit directionality applies an omnidirectional response below a crossover frequency, and some type of directional response above the crossover frequency. The reasons why bandsplit directionality is popular and useful are many. First, applying an omnidirectional response for the low frequencies precludes the need for artificial low-frequency gain equalization (“bass boost”) to overcome the directional gain reduction for low frequencies. Artificial gain equalization can lead to sound quality degradations, while providing an omnidirectional response for the low-frequencies maintains sound quality integrity. Second, bandsplit directionality provides some protection from wind noise, as wind is not over-amplified by the artificial bass boost used by full directionality. Finally, bandsplit directionality affords the listener with better horizontal localization, as interaural time differences of sounds arriving at the head are maintained via the low-frequency omnidirectional response.

In ReSound technology, all directional options (not including omnidirectional) include built-in bandsplit directionality, termed Directional Mix. The crossover frequency between omnidirectional and directional processing is prescribed based on the user’s hearing loss and the hearing instrument model selected. The Directional Mix setting is inversely proportional to the crossover frequency; for example, a “high” Directional Mix setting will have a lower crossover frequency, as more of the signal will be processed as directional.

An important implication for bandsplit directionality with respect to bimodal fittings is whether high-frequency gains can be adequately amplified for the user. An example would be the case of a bimodal user who has profound hearing loss above 1,500 Hz. If audibility cannot be achieved due to receiver limitations or cochlear “dead regions,” directionality may be limited to a smaller aidable high-frequency range above the bandsplit crossover frequency, and the patient may receive less SNR benefit.

It is for cases such as these that ReSound prescribes a Directional Mix setting based partially on the user’s hearing loss. This Directional Mix setting can be changed in the fitting software, but the prescribed level is recommended for most users as it generally balances directional benefit with sound quality integrity. As with any prescribed hearing instrument setting, audibility should still be verified; however, the prescribed Directional Mix setting helps prevent a situation where the crossover frequency is higher than the frequency where audibility is no longer achievable. If the bandsplit crossover frequency is too high (ie, the Directional Mix setting is inappropriately low), the user may not receive any directional processing in their aidable frequency range.

Bimodal Considerations for Automatic Environmental Gain and Noise Reduction Adjustments

ReSound Environmental Optimizer II is available in monaural and bimodal fittings, just as it is in binaural hearing instrument fittings, with one simple exception: there is no synchronization between devices in a monaural or bimodal fitting. Environmental Optimizer II applies automatic gain and/or noise reduction adjustments per the listening environment. For example, slightly more gain and less noise reduction may be beneficial in quiet environments, while less gain and more noise reduction may be advantageous for patient comfort in noisy environments. The benefit of automatic adjustments per environment is that patients may not need to make volume or program changes as they encounter different listening situations throughout the day.

The same convenience and reduction of manual adjustments provided by Environmental Optimizer II also applies to bimodal fittings. Yet since the hearing instruments are set to an equivalent or slightly softer level than the cochlear implant in the loudness balance procedure, these gain and noise reduction offsets per listening environment may result in different loudness balance results outside the clinic. A patient who is fitted with the default gain adjustments per environment may have less balanced loudness perception in varying listening situations (Figure 4).

To preserve the loudness balance procedure’s recommendations for hearing instrument gain in the bimodal fitting, the Environmental Optimizer II gain sliders may be set to zero in the Aventa fitting software. For the most accurate results, the gain sliders should be set to zero before the loudness balance procedure is conducted. This will ensure that the clinical results are more representative of what the patient will experience with daily usage.

The same deactivation of environmentally adjusted noise reduction (NoiseTracker II) settings may also be done for the fitting. However, the effect on loudness balance may not be as large as the gain adjustments per environment, especially if the cochlear implant is programmed with noise reduction activated. The NoiseTracker II benefits of improved listening comfort specifically in noisy environments may still be enjoyed by bimodal patients. With this in mind, the choice to deactivate or preserve per-environment noise reduction adjustments may depend largely on patient preference.

Figure 4

Figure 4. Environmental Optimizer II gain adjustment defaults in Aventa fitting software. The “Reset To” box in the upper right corner allows for deactivation of this feature, if desired, by enacting 0 dB gain adjustments per environment.

Summary

While the benefits of bimodal fittings and loudness balance between the cochlear implant and the hearing instrument are more established, recommended signal processing choices for the contralateral ear have not been researched to date. To fit today’s bimodal patients, it is beneficial to understand the signal processing options available for both the cochlear implant and the hearing instrument, in an effort to meet the individual’s amplification needs.

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Tammara Stender

Tammara Stender, AuD

Tammara Stender, AuD, is manager of Global Audiology-Chicago at GN ReSound in Glenview, Ill.

Correspondence can be addressed to HR or Dr Stender at: [email protected]

Original citation for this article: Stender T. What About the Contralateral Ear? Bimodal Programming Considerations. Hearing Review. 2016;23(4):32.?