The Hearing Review Cross-Currents are staff-reviewed articles, features and news items that relate to hearing care issues from a variety of sources and disciplines. If you are interested in a particular item, we strongly encourage you to obtain a copy of the cited publication. —KES

Asymmetric Cochlear Processing Mimics Hemispheric Specialization

YS Sininger & B. Cone-Wesson
Science, September 10, 2004
Vol. 305, No. 5690, p 1581

Although HR reported briefly on this article in our December 2004 News, the information in this 1-page Science article warrants further attention. In the article, Sininger & Cone-Wesson explain that it’s fairly well accepted that the left hemisphere of the brain generally does the lion’s share of the processing of speech sounds and language functions, while the right hemisphere processes tonal stimuli and music. Speech signals that change rapidly are usually sorted out in the left hemisphere for enhanced temporal resolution; tonal signals are sorted out in the right hemisphere for enhanced spectral resolution. Due to the brain’s anatomy, the left ear has greater connections with the right hemisphere, and the right ear has greater connections with the left hemisphere. Hence, it has been found that reaction times and recognition are best when speech is presented on the right side.

A total of 1593 infants who had clear TEOAE and DPOAE responses (average SNR of 3 dB or more) were analyzed from an original pool of 3011 infants. Mean DPOAE amplitudes were found to be greater in the left ears of the infants (generated by the tones of DPOAEs), while the mean TEOAE scores were found to be greater in the right ears (generated by the clicks of the TEOAEs). The differences in the ear-by-OAE-type were found to be highly significant, and “indicate a tendency for the cochlea to provide greater amplification to stimuli that will also be preferentially processed in the auditory areas of the contralateral [brain] hemisphere” note the authors.

The message of the article is important to hearing care professionals on at least two levels. First, from the perspective of hearing aid fittings and fitting algorithms, it shows that we may need to consider the binaural nature of hearing and the specific weighting of tasks borne by each ear. The authors point out that “stimulus-guided asymmetry is present at the level of the cochlea before it is evident in the auditory cortex” [italics added]. Future hearing aids (and cochlear implants) and fitting methods may take into account these asymmetries and reflect the differences between a patient’s ears. Second, the article is one more bit of information in what is becoming a mountain of evidence for the value of binaural amplification. It demonstrates that humans definitely hear with two ears, and more to the point, each ear may have built-in specialized functions for processing sound—even before it reaches the brain for further temporal and spectral resolution processing.

How to Regrow Lost Hearing

New Scientist, September 18, 2004
Vol. 305, No. 2465, p 13

A technique for growing auditory nerve cell endings—which could greatly benefit cochlear implant wearers—is discussed in this brief news article. Graeme Clark of the Bionic Ear Institute in Melbourne, Australia, recently received the Prime Minister’s Prize for Science for his work on cochlear implants, and discussed his research team’s ideas about regrowing auditory nerve cell endings. Although cochlear implants bypass hair cells by directly stimulating the auditory nerve, the health of the auditory nerve depends on growth factors from hair cells. So, cochlear implant use can be limited by deterioration of these cells.

Using guinea pigs that had their hair cells destroyed, Clark and his team has infused growth factors called neuroptrophin 3 and BDNF into the cochlea. This resulted in not only the cessation of nerve cell death, but also the growth of branch-like extensions of nerve cells within a month.

Although the article points out several challenges before the technique can be used clinically, the finding presents hope for those people who wish to have a cochlear implant but may have damage to the auditory nerve.

Music and the Brain

Norman M. Weinberger
Scientific American, November 2004
Vol. 291, No. 5, pgs 89-95

Music is basic to all peoples and all cultures, and we know that humans have been playing musical instruments for at least 30,000 years. The pleasure centers of the brain light up in response to cherished music the same way as when “eating chocolate, having sex, or taking cocaine,” says the Norman Weinberger, PhD.

So how does the brain process music? Dr. Weinberger, a pioneering researcher of learning, memory, and the auditory system who conducts research at the University of California-Irvine, provides a fascinating discussion of what we know about music and the brain. Weinberger’s ultimate answer: it’s complicated.

He notes that research involving people with brain injuries, debilitating diseases (eg, Alzheimer's disease), and the imaging of healthy individuals suggest that there isn’t any single processing center for music: “music engages many areas distributed throughout the brain, including those that are normally involved in other kinds of cognition,” says Weinberger. Furthermore, it appears that training and experience can radically influence how the brain handles music.

There are several good examples of the ubiquitous processing of the brain for music. One case is that of the famous French composer Maurice Ravel, author of the popular Bolero. Ravel was afflicted by what is thought to have been focal cerebral degeneration in which certain areas of the brain atrophy. Although he could still hear and remember old compositions, he could not write music. In contrast, the Russian composer Vissarion Shebalin, could no longer talk or understand speech, yet he could write music for 10 years after being afflicted. This suggests that music and speech are processed independently.

However, imaging findings by Aniruddh D. Patel at the Neurosciences Institute in San Diego indicate that “a region of the frontal lobe enables proper construction of the syntax of both music and language, whereas other parts of the brain related aspects of language and music processing.”

Weinberger discusses the frequency-tuning aspects of the auditory cortex that most hearing care professionals are familiar with. However, the author also provides an interesting discussion of his own 1980s research on the topic of contour (ie, the rising and falling of pitch in a melody). With researcher Thomas M. McKenna, he found that cells in the auditory cortex discharge differently to various contours, and responses depend on where a particular note might fall within a melody. In other words, cells fire more vigorously if a specific tone is preceded by other specific tones. Additionally, the researchers found that the cells react differently to the same tone depending on if it is part of an ascending or descending contour. The conclusion: melody matters, and “processing in the auditory system is not like the simple replaying of sound in a telephone or stereo system.”

Other attributes of music like rhythm, harmony, and timbre may be more related to a particular brain center, says Weinberger. Rhythm is probably associated more with one hemisphere of the brain than another, although there is conflicting evidence to suggest which hemisphere is more involved. Imaging studies of the cerebral cortex show that the harmony is associated with the right temporal lobe. Likewise, people who have had their right temporal lobes removed (eg, to eliminate seizures) have difficulty resolving timbre.

Scientists have found that the brain is remarkably trainable when it comes to listening. The tuning of the auditory cortex is not fixed, but instead can be re-tuned to react to important acoustic cues. This retuning is durable and can grow stronger with time. It’s possible that this is why people with Alzheimer’s can recall music from the past even though they have little apparent working memory. Additionally, every person has the ability to “play back music” in their head by simply thinking about it. One study shows that, when brain scans of nonmusicians who were told to do this were examined, many of the same areas of the temporal lobe involved in listening to a particular musical piece were the same as when that musical piece is only imagined.

Weinberger points out that musicians are especially good at processing tonal information. One study shows that musicians listening to a piano exhibit about 25% more activity in their brain’s left hemisphere compared to nonmusicians, and that this activity is exclusively related to the musical tones (ie, they exhibited the same brain activity when presented with nonmusical tones). It also has been found that 4-5 year old children who have been exposed to a lot of music exhibit enhanced brain auditory activity, and the volume of the auditory cortex of trained musicians is larger (by 130% in one study) than that of nonmusicians.

The article also discusses emotional reactions to music, including specific tones and the underlying brain mechanisms thought to be associated with pleasure and music.