Question
How does ReSound implement research findings in cognition and hearing into its signal processing?
Answer
How the brain interprets the sounds delivered by the ears is a popular and exciting area of current research. The binaural processes that occur in the patient’s brain, along with individual cognitive factors, are becoming drivers of hearing instrument technology and the impetuses for ways to personalize the fitting for the patient.
ReSound has incorporated cognitive findings into its directionality, pinna restoration, compressor and frequency lowering options. Research about how the brain organizes sounds in the listening environment, to help individuals focus on speakers of interest while still monitoring other sounds around them, formed the basis for ReSound Binaural Directionality and Binaural Directionality II. What is known about localization abilities and the perception of spatial hearing drove the development of Spatial Sense and Directional Mix. Findings on individual cognitive factors promoted the introduction of adjustable compressor time constants. Research on frequency lowering techniques, as well as certain patient characteristics which may predict individual benefit or preclude the use of frequency lowering, influenced the implementation of Sound Shaper.
Cognition and ReSound Directionality
ReSound Binaural Directionality is designed to support auditory scene analysis (Bregman, 1990), which helps the brain create a virtual acoustic picture of the surrounding listening environment. From this mental construct, the brain is able to both detect current and new sounds as they arise, as well as maintain and redirect focus on signals that are the most interesting to the individual at a given point in time. The brain can best perform these tasks when the inputs from each ear provide information not only on the loudest speaker in the environment, but also on other environmental sounds.
ReSound Binaural Directionality and Binaural Directionality II provide an array of directional options within a single program to support the brain in these tasks. A bilateral omnidirectional response is provided in quiet or speech-only listening environments, as this response is strongly preferred by listeners (Walden, Surr, Cord, & Dyrlund, 1994; Walden et al., 2007). A bilateral directional response offers the best signal-to-noise ratio (SNR) benefit when the dominant speaker is in front of the listener and the noise is towards the back (Hornsby, 2008). An asymmetric directional response, with a directional response provided for one hearing instrument and an omnidirectional response provided for the other, offers directional benefit in a diffuse-noise environment (Cord, Walden, Surr, & Dittberner, 2007; Bentler, Egge, Tubbs, Dittberner, & Flamme, 2004; Kim & Bryan, 2011) or when the speaker is to one side (Hornsby & Ricketts, 2007; Coughlin, Hallenbeck, Whitmer, Dittberner, & Bondy, 2008; Cord, Surr, Walden, & Dittberner, 2011), while still offering environmental awareness around the listener. This asymmetric response has also been shown to improve ease of listening (Cord et al., 2007), as the person is not as acoustically “cut off” from hearing other sounds in the listening environment.
Cognition and ReSound Spatial Hearing Support
The ability to localize the origin of sounds, as well as to perceive sounds in their respective places and distances as related to the hearing instrument user, guided the implementation of ReSound Spatial Sense and Directional Mix. Spatial Sense incorporates pinna restoration and compressor synchronization to provide high-frequency monaural spectral cues and maintain interaural level difference cues. Directional Mix applies omnidirectional processing for low frequencies for every ReSound directional option, which supports horizontal localization abilities due to preservation of interaural time difference cues. Each of these cues is used by the brain to accurately place the individual in the sound scene, and to better identify the location of sound sources.
Cognition and ReSound Compressor Speed
The role of cognitive status and hearing instrument compressor speed has been studied quite extensively. Especially in difficult, variable listening environments, cognition plays a notable role in speech recognition in noise (Lunner & Sundewall-Thoren, 2007). Individuals with poor cognitive test scores, or poor working memory abilities, have been found to perform better in speech recognition tests with slow-acting compression, while those with better cognitive abilities generally perform better with fast-acting compression. Patients who prioritize speech intelligibility may perform better and prefer fast-acting compression, while those who prize listening comfort may benefit from slow-acting compression.
For better personalization to individual patient needs, ReSound offers 4 compression time constant options. From fastest to slowest, these time constant speeds are “Syllabic,” “Fast,” “Moderate” and “Slow.” The compression time constant can also differ among hearing instrument programs, which allows patients to experience different options in the real world and more accurately ascertain their personal preferences.
Cognition and ReSound Frequency Lowering
Hearing instrument frequency lowering techniques have one thing in common: they all introduce a trade-off between better audibility for high-frequency sounds and preservation of sound quality. Research has shown that some individuals may benefit from this processing, but group data has shown conflicting results. Further, no consensus exists for how to accurately identify individual patients who will benefit from frequency lowering.
The cost-benefit analysis of providing frequency lowering for a given patient currently depends on many factors, including the patient’s hearing loss, tolerance for distortion and cognitive characteristics. Older listeners with hearing loss and poorer working memory abilities have been shown to be more prone to difficulties with distortion in hearing instruments (Arehart, Souza, Baca, & Kates, 2013). A feature such as frequency lowering, which distorts high frequency sounds by moving them to a lower frequency range, may not be recommended for individuals with these characteristics.
ReSound introduced Sound Shaper with current findings from the large body of frequency lowering research in mind. As there are no clear candidacy criteria for who may benefit from frequency lowering as compared to traditional amplification, Sound Shaper is set as a default to “Off” for every patient, in an effort to preserve sound quality. This allows the clinician to choose to enable Sound Shaper if it is deemed an appropriate option for the individual patient.
Sound Shaper is a proportional frequency compression method, which compresses frequencies above a threshold to promote better audibility. However, the relationships among the compressed frequencies are maintained in a proportional fashion, as opposed to the non-proportional approach used by nonlinear frequency compression. Speech intelligibility and other outcomes have been revealed to be comparable between proportional and nonlinear frequency compression methods (Kokx-Ryan et al., 2015). But the advantage of proportional frequency compression, in contrast to the nonlinear method, is that distortion for the compressed frequencies is reduced.
The growing body of research on cognition as it relates to hearing and hearing instrument fitting outcomes is shedding light on the need to promote optimal hearing instrument personalization, flexibility and signal processing choices. ReSound technology, designed to emulate natural hearing and support the brain in its unparalleled cognitive abilities, will continue to implement features that meet the needs of each unique patient.
For more information on this topic, please refer to the CEU webinar, Cognition and ReSound: Applying What We Know About the Brain to Signal Processing.
References
Bregman, A.S. (1990). Auditory scene analysis. Cambridge, MA: MIT Press.
Walden, B., Surr, R., Cord, M., & Dyrlund, O. (2004). Predicting hearing aid microphone preference in everyday listening. Journal of the American Academy of Audiology,15, 365-396.
Walden, B., Surr, R., Cord, M., Grant, K., Summers, V., & Dittberner, A. (2007). The robustness of hearing aid microphone preferences in everyday environments. Journal of the American Academy of Audiology, 18, 358-379.
Hornsby, B. (2008, June-July). Effects of noise configuration and noise type on binaural benefit with asymmetric directional fittings. Seminar presented at: 155th Meeting of the Acoustical Society of America, Paris, France.
Cord, M.T., Walden, B.E., Surr, R.K., & Dittberner, A.B. (2007). Field evaluation of an asymmetric directional microphone fitting. Journal of the American Academy of Audiology, 18, 245-256.
Bentler, R.A., Egge, J.L.M., Tubbs, J.L., Dittberner, A.B., & Flamme G.A. (2004). Quantification of directional benefit across different polar response patterns. Journal of the American Academy of Audiology, 15, 649-659.
Kim, J.S., & Bryan, M.F. (2011). The effects of asymmetric directional microphone fittings on acceptance of background noise. International Journal of Audiology, 50, 290-296.
Hornsby, B., & Ricketts, T. (2007). Effects of noise source configuration on directional benefit using symmetric and asymmetric directional hearing aid fittings. Ear & Hearing, 28, 177-86.
Coughlin, M., Hallenbeck, S., Whitmer, W., Dittberner, A., & Bondy, J. (2008, March). Directional benefit and signal-of-interest location. Seminar presented at American Academy of Audiology Convention, Charlotte, NC.
Cord, M.T., Surr, R.K., Walden, B.E., & Dittberner, A.B. (2011). Ear asymmetries and asymmetric directional microphone hearing aid fittings. American Journal of Audiology, 20, 111-122.
Lunner, T., & Sundewall-Thoren, E. (2007). Interactions between cognition, compression, and listening conditions: effects on speech-in-noise performance in a two-channel hearing aid. Journal of the American Academy of Audiology, 18(7), 604-17.
Arehart, K.H., Souza, P., Baca, R., & Kates, J.M. (2013). Working memory, age, and hearing loss: Susceptibility to hearing aid distortion. Ear & Hearing, 34(3), 251-60.
Kokx-Ryan, M., Cohen, J., Cord, M.T., Walden, T., Makashay, M.J., Sheffield, B.M., & Brungart, D.S. (2015). Benefits of nonlinear frequency compression in adult hearing aid users. Journal of the American Academy of Audiology, 26, 838-55.
