I have severe-profound bilateral hearing loss and have worn powerful hearing aids ever since I was diagnosed in early childhood. While intensive speech and listening therapy gave me the ability to communicate orally in quiet environments, I have always struggled with understanding speech in noisy places like busy restaurants.
As many people who wear hearing aids will tell you, the hearing aids help in quiet, but don’t help and often make things even worse in noisy environments. Until recently, it has not been clear why hearing-impaired people have so much difficulty with speech in noise.
I did not start out trying to address this problem directly, but some very interesting findings from my lab may finally explain a big part of the problem. We were looking at a different problem in bilateral cochlear implant users and the problem of pitch mismatches between the two ears due to asymmetries in surgical insertion of the implants. We found that many cochlear implant users had a big mismatch in pitch between ears, but did not perceive this in everyday listening.
For example, one bilateral implant user told me that the same voice would sound like a low pitch when one implant was on by itself, and like a high pitch in the other implant. Yet, when both cochlear implants were on at the same time, she did not perceive that mismatch and the voices sounded “just right”.
This got me thinking that there might be an adaptation in the brain that increases tolerance of mismatches between the ears and allows the cochlear implant users to be comfortable with their device.
It is well known that normal-hearing listeners have “sharp fusion” and will only tolerate small mismatches in pitch, of less than a tenth of an octave between the ears. Sounds of similar pitch are fused together into a single perception.
May is Better Speech & Hearing Month.
The Oregon Hearing Research Center is one of the top centers for auditory research, with expertise spanning the inner ear to the cortex. Learn more
In follow-up experiments, we found that both hearing aid users and cochlear implant users differ from normal-hearing listeners in how they fuse sounds. They can have abnormally “broad fusion”, with large tolerances for a mismatch in pitch between the ears, and fuse sounds differing by up to 3-4 octaves in pitch.
Further, the tolerance range predicts the ability to separate different voices in a noisy multi-talker environment, with broader fusion predicting poorer speech perception in noise ability.
This means that for someone like myself with broad fusion, different voices in a restaurant are fused and blended together and it is not possible to separate the voices out. This differs remarkably from what is experienced by normal-hearing listeners, who can still separate the different voices but may experience confusion about which words were spoken by which talkers.
It is not clear yet how the abnormal fusion arises, but preliminary studies suggest that early childhood onset or long duration of hearing loss may be risk factors.
In conducting these studies, I had a unique advantage over normal-hearing scientists in the field in being able to conduct pilot studies using myself as a study subject, and being able to directly experience the perceptions that hearing-impaired people experience. This is a direct example of the value of having people with the hearing loss in the field of hearing research. The Oregon Hearing Research Center at OHSU is unique in that is has the highest number of scientists (Fred Nuttall, John Brigande, Peter Steyger, and Lina Reiss) with hearing loss studying hearing.