Stewart Hulse never thought he would apply his interest in
music to experimental psychology.
But about 15 years ago it occurred to Hulse that music might be an ideal way to study how the brain arranges sights and sounds into an order, enabling people and animals to communicate and navigate their environments.
Hulse's idea of using music in his research evolved into an unlikely strategy: studying songbirds to learn whether animals, like people, might have the mysterious ability to pick out one specific sound amid a cacophony of many sounds, a phenomenon known as the "cocktail party effect."
It may seem intuitively simple, but scientists have been puzzled by how the human brain is able to focus selectively 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.
Hulse discovered that European starlings were able to accurately pick out specific bird songs when mixed with other songs and even when accompanied by the "dawn chorus," the combination of sounds heard in the forest during a spring morning, when all the birds are singing at once.
"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 Hulse, a professor in the Department of Psychology. "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."
The findings were detailed in a paper 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 and graduate students Scott MacDougall-Shackleton and Amy Wisniewski.
"I'm not an ornithologist, but I am slowly becoming one," Hulse said.
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.
"The birds were able to maintain this discrimination, between the song pairs, despite this really enormous amount of environmental noise," Hulse said.
Hulse began working with birds to study 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 communicate and find their way around.
"The idea is that we must all solve the question of remembering how things happened one after another," he said. "Language is based on certain sounds and a grammar, which describes the rules by which one thing comes after another. We all have a grammar if we are going to speak, and so that's an example of the problem of serial order."
The same principle applies to sights. For example, Hulse originally worked with rats, studying how they learned landmarks to navigate 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 identify immediately the pitch of an isolated tone.
About 3 percent of the human population has it. "And many of those are musicians, who find it a curse because if music is played off-pitch, it sounds wrong," Hulse noted. "Anyway, I thought that maybe this was relevant for bird song."
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--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 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.
"Much of the work we are doing is really borrowed from research that's been done with humans," Hulse said. "The question is whether non-human animals have the same capacity."
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