It may seem intuitively simple, but scientists have been puzzled by how the human brain is able to selectively focus on one voice from among the tangle of other voices and sounds. New findings are providing evidence that humans are not alone in that ability.
Like humans, birds can easily pick out and concentrate on a specific sound amid a cacophony of many sounds, a "cocktail party effect" for bird song, Johns Hopkins University psychologists have found.
"The issue here is, how can the bird hear these significant events in the midst of other noises, which are other potentially significant signals?" said Stewart Hulse, an experimental psychologist at Johns Hopkins. "It's just something the brain does. It can disambiguate these sounds, and yet the acoustical signal is an incredible mixture of many different sounds."
Hulse discovered that European starlings were able to accurately pick out specific bird songs mixed with other songs. They were successful even during the "dawn chorus," the combination of sounds heard in the forest during a spring morning, when all the birds are singing at once.
The findings are detailed in a paper to be published in the March issue of the Journal of Comparative Psychology. They may help scientists analyze how animals and people accomplish the feat and draw further parallels between human and non-human perception. The paper was written by Hulse, who is a professor in the Department of Psychology, and graduate students Scott MacDougall-Shackleton and Amy Wisniewski.
The scientists combined the tape-recorded songs of starlings, brown thrashers, nightingales and mockingbirds. The starling and brown thrasher songs were paired together, as were the songs of nightingales and mockingbirds.
When the starlings listened to the paired bird song recordings, they were easily able to tell the difference between the two pairs. They could do this even when they heard song examples that contained just one of the two species' songs in the pair, a bird song sound they had never heard before. They accurately chose the correct species more than 80 percent of the time, well above the 50 percent that would be expected from pure chance decisions.
When paired songs were mixed with the dawn chorus, the starlings were still nearly as successful, identifying the correct bird songs with an accuracy ranging from just under 80 percent to about 85 percent.
Hulse began working with birds about 15 years ago, studying how the brain is able to separate a series of sights or sounds into distinct objects, using them as landmarks. By learning how to arrange those sights and sounds into an order, called a serial order, animals and people are able to navigate their environments. For example, Hulse originally worked with rats, studying how they learned landmarks to find their way through mazes.
But he switched to birds, in part because he was allergic to the rodents. "I decided that maybe there was a better way to do research on this question," Hulse said.
He had always been interested in music, having taken courses in harmony and music theory as an undergraduate, and he is an amateur pianist.
"It occurred to me that music was a perfect example of something that happened in serial order," he said. "Since I was working with animals, I started looking around for a likely animal to begin studying this issue. I thought about songbirds because they sing. Their song is not music, but bird song has a grammar and a structure to it. And I picked starlings because they can learn to mimic other sounds, even sounds like human speech that are not part of their natural world. That meant I could work with synthetic as well as natural sounds to study their hearing abilities."
To begin, the scientist trained birds to peck two different keys, one when they heard a rising sequence of tones, and the other when they heard a descending sequence. To his astonishment, the birds were not especially sensitive to whether the sequences went up or down in pitch. Instead, they possess a trait rarely found in people, called absolute pitch, which enables them to immediately identify the pitch of an isolated tone. The implication is that the birds may use this talent to communicate with each other, recognizing individual birds by the pitch of some parts or all of their song.
Research into absolute pitch in birds led to his current work on the cocktail party effect, designed to learn whether birds have the ability to pay selective attention to one sound that occurs simultaneously with one or more other sounds.
Other scientists, including Peter Jusczyk at Johns Hopkins, have studied the cocktail party effect in humans. Jusczyk, an experimental psychologist, has studied how infants are able to concentrate on one voice that is mixed with background sounds. The ability may help them learn language quickly; by tuning out the extraneous noise, babies are able to focus on the grammar and rhythm of language.