Research Roundup updates HR readers on some of the latest research and clinical findings related to hearing health care. Where appropriate, sources and original citations are provided, and readers are encouraged to refer to the primary literature for more detailed information. Additionally, related articles can be found and keywords can be searched in the HR Online Archives.

Simple Sounds Not Memorized Better Than Complex Sounds

The human brain is capable of detecting the slightest visual and auditory changes. Whether it is the flash of a student’s hand into the air or the faintest miscue of a flutist, the brain instantaneously and effortlessly perceives changes in our environment. Several studies have indicated, however, that even a small span of time between pre- and post-change images can disturb the brain’s ability to detect visual discrepancies.

“The pre-change scene must be memorized in some way,” explains psychologists Laurent Demany, Wiebke Trost, Maja Serman, and Catherine Semal from the University of Bordeaux and the French National Center for Scientific Research (CNRS). “In the visual domain, numerous experiments have shown that even a very short gap of less than 100 ms can dramatically disrupt our ability to detect a local change in complex images. Following such a gap, local changes can be detected only in very simple images. This phenomenon is known as ‘change blindness’.”

In a recent study, the aforementioned psychologists assessed the effect of time gaps on change detection in audition. Their goal was to determine if the brain uses similar mechanisms to perceive auditory changes as it does with vision. Participants had to detect a pitch change in one tone presented together with other tones. The complexity of the pre-change sound was varied, as well as the duration of the silent interval between the pre- and post-change sounds.

The experimenters reasoned that, if auditory change detection is similar to the visual process, a complex sound (including many tones) should be remembered less well than a simple sound (including few tones).

The psychologists discovered, however, that this was not the case. The participants were able to remember even the most complex sounds—reaching up to 12 tones—despite the time delays.

The results of the study, which appear in the January 2008 issue of Psychological Science, a journal of the Association for Psychological Science, indicate that the brain uses more efficient mechanisms in auditory memory than in visual memory. To that extent, the human brain appears to be a keener detective of auditory change than visual change. Source: Association for Psychological Science.

Original Article
Demany L, Trost W, Serman M, Semal C. Auditory change detection: simple sounds are not memorized better than complex sounds. Psychol Sci. 2008;19(1):85–91.

Your Ears, Your World

Scientists funded by the Biotechnology and Biological Sciences Research Council (BBSRC) in England are working out how the human ear and the brain come together to help us understand our acoustic environment. They have found that the auditory cortex is adapted in each individual and tuned to the world around us. We learn throughout our lives how to localize and identify different sounds—meaning that, if you could hear the world through someone else’s ears, it would sound very different to what you are used to.

The research, published in the current issue of BBSRC Business, could help to develop more sophisticated hearing aids and more effective speech recognition systems. The University of Oxford Auditory Neuroscience Group, led by Jan Schnupp, PhD, studied the auditory cortex of the brain and discovered that its responses are determined not merely by acoustical properties, like frequency and pitch, but by statistical properties of the soundscape. In the real world, loudness and pitch are constantly changing. The random shifts in sounds are underpinned with a statistical regularity. For example, subtle and gradual changes are statistically more regular than large and sudden changes. Schnupp’s team has found that our brains are adapted to the former; the neurons in the auditory cortex appear to anticipate and respond best to gradual changes in the soundscape. These are also the patterns most commonly found in both nature and musical compositions.

“Our research to model speech sounds in the lab has shown that auditory neurons in the brain are adaptable, and we learn how to locate and identify sounds, ” says Schnupp. “Each person’s auditory cortex in their brain is adapted to ways their ears deliver sound to them and their experience of the world. If you could borrow someone else’s ears, you would have real difficulty in locating the source of sounds, at least until your brain had relearned how to do it.”

Schnupp has also found that the auditory cortex does not have neurons sensitive to different aspects of sound. When the researchers look at how the auditory cortex responds to changes in pitch, timbre, and frequency, they saw that most neurons reacted to each change. “In the closely related visual cortex, there are different neurons for processing color, form, and motion,” explains Schnupp. “In the auditory cortex, the neurons seem to overwhelmingly react to several of the different properties of sound. We are now investigating how they distinguish between pitch, spatial location, and timbre.

“If we can understand how the auditory cortex has evolved to do this, we may be able to apply the knowledge to develop hearing aids that can blot out background noise and speech recognition systems that can handle different accents.”

The Oxford team’s current project is using BBSRC funding to fit trained ferrets with harmless auditory implants. The animals are trained to respond to different sounds, and the implants enable the team to observe the auditory neurons as the ferret responds to different sounds. Source: American Association for the Advancement of Science.

Original Article
Schnupp J. Modelling melodies: understanding the neurological basis of sound perception. BBCSRC Business. 2008; January. Available at: www.bbsrc.ac.uk/publications/…business.html. Accessed January 25, 2008.

Nerves in Head and Neck Linked to Tinnitus

Nerves that sense touch in your face and neck may be behind the racket in your brain, University of Michigan researchers say. Touch-sensing nerve cells step up their activity in the brain after hearing cells are damaged, a study by U-M Kresge Hearing Research Institute scientists shows. Hyperactivity of these touch-sensing neurons likely plays an important role in tinnitus. The study appears in the European Journal of Neuroscience.

The research findings were made in animals, but they suggest that available treatments, such as acupuncture, if used to target nerves in the head and neck, may provide relief for some people plagued by tinnitus, says Susan E. Shore, PhD, lead author of the study and research professor in the Department of Otolaryngology and the Kresge Hearing Research Institute at the U-M Medical School.

“The study shows that, in deafened animals, the somatosensory response is much stronger than in animals with normal hearing,” says Shore, whose research team knew from earlier research that some neurons in the cochlear nucleus become hyperactive after hearing damage, and this hyperactivity has been linked to tinnitus in animals.

“This study [also] shows that it is only those neurons that receive somatosensory input that become hyperactive,” she says, which should make the search for treatments for tinnitus in some people more straightforward. For example, many people with temporomandibular joint syndrome (TMJ), a condition that causes frequent pain in the jaw, experience tinnitus. Shore’s research could lead to a better understanding of this link. In people with TMJ, the somatosensory system is disrupted and inflamed. Shore says that it’s possible that, in this situation as with hearing loss, somatosensory neurons stir excessive neuron activity in the cochlear nucleus. Source: Anne Rueter, UMHS Public Relations.

Original Article
Shore SE, Koehler S, Oldakowski M, Hughes LF, Syed S. Dorsal cochlear nucleus responses to somatosensory stimulation are enhanced after noise-induced hearing loss. Eur J Neurosci. 2008;27(1):155-168.

Amazing Neurons Exceed Abilities of Auditory Nerve

Reporting in the January 10 issue of the journal Nature, Dr Itzhak Fried and colleagues at the Hebrew University in Jerusalem and the Weizmann Institute of Science show that, in humans, a single auditory neuron in the brain exhibits an amazing selectivity to a very narrow sound frequency range—roughly down to a tenth of an octave. In fact, the ability of these neurons to detect the slightest differences in sound frequencies far exceeds that of the auditory nerve that carries information from the hair cells of the inner ear to the cortex, by as much as 30 times more sensitivity. Indeed, such frequency tuning in the human auditory cortex is substantially superior to that typically found in the auditory cortex of nonhuman mammals (except bats).

It is quite a paradox, the researchers note, in that even musically untrained people can detect very small sound frequency differences, much better than the resolution of the peripheral auditory nerves. This is very different from other peripheral nerves, such as those in the skin, where human ability to detect differences between two points (say from the prick of a needle) is limited by the receptors in the skin. Not so in hearing.

The researchers, including senior author Israel Nelken and first author Yael Bitterman from the Hebrew University, determined how neurons in the human auditory cortex responded to various sounds by taking recordings from four consenting clinical patients at the UCLA Medical Center. These patients had intractable epilepsy, and were being monitored with intracranial depth electrodes to identify the focal point of their seizures for potential surgical treatment. Using clinical criteria, electrodes were implanted bilaterally at various brain sites that were suspected to be involved in the seizures; these included the auditory cortex. The recording of brain activity was carried out while the patients listened to artificial random chords at different tones per octave, and to segments from the film The Good, the Bad and the Ugly. Thus, the sounds the patients heard were both artificial—the random chords—and more natural noises.

The results surprised the researchers. A single auditory neuron from humans showed an amazing sensitivity to distinguish between very subtle frequency differences, down to a tenth of an octave. This compared to a sensitivity of about one octave in the cat, about a third of an octave on average in rats, and one-half to one octave in the macaque.

“This is remarkable selectivity,” says Fried, who is also the co-director of UCLA’s Seizure Disorder Center. “It is indeed a mystery why such resolution in humans came to be. Why did we develop this? Such selectivity is not needed for speech comprehension, but it may have a role in musical skill. The 3% frequency differences that can be detected by single neurons may explain the fact that even musically untrained people can detect such frequency differences.

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“There is also evidence that frequency discrimination in humans correlates with various cognitive skills, including working memory and the capability to learn, but more research is needed to clarify this puzzle,” says Fried.

Previous studies from Fried’s lab have identified single cells in the human hippocampus specific to places during human navigation, and single cells that can translate varied visual images of the same item, such as the identity of an individual, into a single instantly and consistently recognizable concept. Source: American Association for the Advancement of Science.

Original Article
Bitterman Y, Mukamel R, Malach R, Fried I, Nelken I. Ultra-fine frequency tuning revealed in single neurons of human auditory cortex. Nature. 2008;451:197-201.