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The next time you attend a cocktail party, notice how quickly everyone instinctively raises their voice as the room begins to fill with many speakers, each focused on different conversational partners. Thanks to the so-called "cocktail party effect," most of us are able to filter out the background chatter and pick out that particular voice of greatest interest.
This ability to selectively respond even as infants to auditory stimuli embedded in noise is not unique to humans. Many animals, such as penguins, fish and frogs, share with us this remarkable skill.
Honking Penguins
King penguin chicks are adept at hearing their parents' voices over the din of the colony, where the combined noise made by hundreds of penguins can reach 70 decibels (dB).1 The penguins do not build nests, so when parents return from foraging they must find their chicks among hundreds of others.
Researchers from the University of Paris in Orsay recorded king penguin calls at different volumes in a colony and found that a chick can recognize its parents' calls even when the combined sounds of other penguins honking is 6 dB louder.
Humming Fish
Cornell biologists studied midshipman fish seeking mates.2 Male fish hide in cavity-like nests excavated under rocks and produce a hum averaging 100 Hz. All the humming males together sound like a huge hives of bees or a squadron of motor boats, and a female midshipman fish has to choose that one particular nest in which to deposit her eggs.
he biologists set up underwater loudspeakers with two or more synthesized fish hums which played together produced rhythmic, acoustic beats and, sure enough, females were able to directly locate one of the humming speakers.
Croaking Frogs
A University of Minnesota behavioral biologist, Mark Bee, and his student assistants are conducting research for a project that may eventually help scientists develop a better hearing aid.3 Bee is studying the chirping, croaking, trilling sounds of Minnesota's "frog chorus," which swells to deafening proportions during mating season.
Over several decades, scientists have found that female frogs have an uncanny ability to identify the calls and exact locations of male frogs of their own species, even with frogs of several species all croaking together and mixed with human-produced highway traffic. Some frogs can even learn to recognize the calls of other male frogs who may infringe upon their territory.
Bee's work on "acoustic signal recognition" during frog courtship aims to fill in details sought by scientists working to improve human hearing aids and cochlear implant devices-"to learn the basic principles of how auditory and neural systems solve these problems," he said.
The Left Hemisphere Dominates
Recent research is continuing to unravel the neural interactions that underlie the cocktail party effect. In 2007, German scientists along with colleagues in Japan and Canada reported on the use of a neuroimaging technique to follow underlying neural mechanisms and hemispheric differences as volunteers listened to different combinations of test and background sounds.4
By monitoring the brain's response to these different sound combinations, the team observed that the left hemisphere was the site of most neural activity associated with processing sounds in a noisy environment.
Sparse Neural Representation of Sounds
In a 2008 study, scientists at Cold Spring Harbor Laboratory (CSHL) reported that a very small minority of available auditory neurons react strongly when exposed to any specific sound.5 "This finding challenges the standard model of sound representations in the auditory cortex, which predicts that neural representations of stimuli often engage a large fraction of neurons," said Anthony Zador, PhD.
Using a new technique called "in vivo cell-attached patch clamp recording" in rats, which measures the reaction of individuals neurons, the team found only 5 percent of neurons in the auditory cortex had a "high firing rate" when receiving a range of sounds of varying length, frequency, and volume. Such selective and sparse neural representation of sounds may help explain our ability to focus on one particular sound among many in noisy environments.
"Sparse representations may make sensory stimuli easier to recognize and remember," explains Dr. Zador. This study is the first evidence of a correlation between sparse representations of auditory stimuli and selective hearing.
Analysis of the Acoustic Scene
In another 2008 study, German scientists proposed a neural mechanism for the cocktail party effect based upon the ability of the human auditory system to segregate into discrete information streams the temporal fine structure of the acoustic scene, including voices.6 Thus, different voices have unique fine characteristics over time, each of which is represented in different areas of the auditory cortex.
This acoustic scene analysis can even be achieved monaurally, for example in telephone conversations, where no directional information is available.
By means of a so-called "winner-take-all" algorithm, one of these competitive vocal representations gains control while all other representations are inhibited. As a result, one voice is heard above all others. Such findings may ultimately help improve the auditory experience of hearing aid wearers in noisy listening situations, and of older adults whose auditory processing in noise has slowed due to the aging process.
References
1. Penguins make great party animals (l998, September 26). New Scientist.com. Accessed online at www.newscientist.com/article.ns?id=mg15921534.300&print=true
2. Cornell University (l998, July 3). The cocktail party effect: fish and human brain peform 'auditory scene analysis' when looking for love in all the loud places. ScienceDaily. Accessed online at www.sciencedaily.com/releases/l998/07/980703092505.htm
3. Tyler, K. (2006, May 11). Can frogs teach us how to build a better hearing aid? UMNnews: U. of M. Accessed online at www1.umn.edu/umnnews/Feature_Stories/Lessons_from_the_frog_Chorus.html
4. BioMed Central (2007, November 18). Left brain helps hear through the noise. ScienceDaily. Accessed online at www.sciencedaily.com/releases/2007/11/071115083707.htm
5. Cold Spring Harbor Laboratory (2008, January 30). How does the brain attend to one voice in a noisy room? New findings on selectively interpreting sounds. ScienceDaily. Accessed online at www.sciencedaily.com/releases/2008/01/080129140634.htm
6. Public Library of Science (2008, March 8). More than meets the ear in successful cocktail party conversations. ScienceDaily. Accessed online at www.sciencedaily.com/releases/2008/03/080304200855.htm
Jess Dancer is professor emeritus of audiology at the University of Arkansas at Little Rock. Contact him at jedancer@ualr.edu regarding your experiences with the cocktail party effect.
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