Many people with binaural hearing loss will benefit from wearing hearing devices, such as hearing aids or cochlear implants, in both ears. However, not everyone benefits from aiding both ears; some will experience binaural interference, in which sounds and speech are less clear when using a hearing device in both ears than in one ear only.
First documented in 1961, binaural interference can be measured as poorer performance on tests of speech recognition abilities, such as word and sentence recognition tests, using both ears compared with using either ear (J Speech Hear Res. 1961;4:137 http://bit.ly/2ie2hJ2). Estimates of the prevalence of binaural interference range from 20 percent to as high as 82 percent among hearing aid users (J Am Acad Audiol. 2012;23:171 http://bit.ly/2hRjlBF;J Am Acad Audiol. 2005;16:574 http://bit.ly/2hRmoKb). A survey of hearing aid users showed that nearly half preferred to use one hearing aid instead of two (Ear Hear. 2011;32:181 http://bit.ly/2hRob1D).
More recently, binaural interference was also reported in bimodal cochlear implant users, who wear a cochlear implant in one ear and a hearing aid in the other; 51 percent preferred to use the cochlear implant alone, and 21 percent reported binaural interference (Trends Amplif. 2010;14:199 http://bit.ly/2hRpRsa).
BINAURAL INTERFERENCE STUDIES
For more than a half-century, hearing scientists and audiologists have been puzzled as to why binaural interference occurs. Studies of how people fuse sounds between the two ears have been instrumental in increasing our understanding of binaural interference (J Assoc Res Otolaryngol. 2014;15:235 http://bit.ly/2inKDjb;J Assoc Res Otolaryngol. 2016;17:341 http://bit.ly/2hRcZlE;J Acoust Soc Am. in revision). In one study, we measured how precisely people fuse sounds between the ears and compared typical-hearing listeners’ ability to fuse sounds with hearing-impaired listeners’ ability to fuse sounds (J Acoust Soc Am. in revision). We found that listeners with normal hearing are more precise in how they fuse sounds, and only fuse sounds that are very similar and close in pitch (Fig. 1A). On the other hand, hearing-impaired listeners are less precise; many of them fuse sounds that are very different in pitch between the ears (Fig. 1B), such as a low-frequency sound in one ear (e.g., a man's voice or a low tone on the piano) with a high-frequency sound in the other ear (e.g., a woman's voice or a high tone on the piano). Some hearing-impaired listeners had an “abnormally broad” fusion and would fuse sounds that were as far apart in pitch as four octaves–over half of a piano's frequency range. These listeners differed comparatively from some normal-hearing listeners with “sharp” fusion, who only fused sounds that differed by less than 0.3 octaves. In another study, we showed that many cochlear implant users who wear a hearing aid in the other ear also experience abnormally broad fusion (J Assoc Res Otolaryngol. 2014;15:235 http://bit.ly/2inKDjb).
In yet another study, we found that this fusion leads to averaging and distortion of the different pitches between the ears, similar to the averaging of color between the eyes, and results in speech perception interference (J Assoc Res Otolaryngol. 2016 http://bit.ly/2hRcZlE). We tested the ability of hearing-impaired listeners to understand vowel sounds, such as “a” or “e,” using two ears compared with using either ear. Normal-hearing listeners experienced no interference and did just as well when listening with two ears compared with one (Fig. 2A). Hearing-impaired listeners with normal-like fusion also did not experience interference and were usually able to ignore a bad ear (Fig. 2B).
However, hearing-impaired listeners with abnormally broad fusion experienced binaural interference. In some cases, when there was one bad ear, the bad ear would “pull” the performance of the good ear down (Fig. 2C). In other cases, listeners had more difficulty with discriminating vowels when listening with both ears compared with listening with either ear (Fig. 2D). For some listeners, abnormally broad fusion only distorted a few sounds. For others, fusion distorted the majority of sounds, making speech perception so difficult with two ears that they chose not to wear a second hearing device.
The study also suggests a possible explanation for why hearing-impaired listeners have difficulty understanding speech in the presence of background noise, such as a room full of many people talking at once. If many voices of different pitches are fused together, it will be difficult to separate the voices and attend to a particular voice.
FURTHER RESEARCH NEEDED
This is the first study to investigate a potential underlying cause of binaural interference in hearing-impaired listeners. Further research is needed to fully understand what causes abnormally broad fusion. Additional investigation is also warranted to extend the findings on single vowels to consonant sounds, such as “s” or “p,” as well as to understanding speech in background noise. Once the causes of interference are fully understood, brain training tools, cochlear implants, and hearing aids can be designed in new ways to reduce binaural interference.