Science Watch

When a toddler pounds his fists on piano keys, all but the most tolerant of parents will quickly end the cacophony. That's because clusters of notes with close-together pitches sound terrible, especially if we hear them for any extended period of time.

"There's a saying in German, 'It tears my socks off,'" says Tom Fritz, PhD, a psychologist at the Max Planck Institute, in Leipzig. "Permanently dissonant music is really hard to bear."

In fact, dissonance can be so hard to bear, it once sparked a riot. Audiences at the 1913 Paris premier of Igor Stravinsky's "Rite of Spring" started grumbling early on, unsettled by clashing melodies played by a bassoon and a flute. As the introduction gave way to a jagged, pulsing chord, the audience's unease turned violent and fistfights erupted in the aisles.

What is it about dissonant sounds that grate on our nerves? Psychologists have long argued that the physics of sound and our ears' limitations can explain much of our displeasure, but new research is complicating the picture--and finding that Western ears may be culturally trained to find dissonance particularly annoying.

Making sense of sound

Musicologists define a dissonant interval as two notes that sound "rough" when played together. Press two adjacent keys on a piano--an interval called a "second"--and it sounds as if the notes are fighting each other. Move your finger up just one note, so that there's one key between them, and you get the sonorous sound of a "third"--an interval a small child could bang for a while without bothering the neighbors.

So what's the big difference between a second and a third?

Classic research by psychologists in the 1960s suggested that seconds and other dissonant intervals sound rough because our ears can't easily separate the tones that make up the pitches. They are too close together, so they clash, like a bright red shirt paired with a skirt that's reddish-orange. A study by Danish researchers, published in the Journal of the Acoustical Society of America (Vol. 38, No. 1), found that this difficulty arises because the ear's basilar membrane--a sea of hair-like cells that respond to particular pitches--overlaps for tones that are less than one-and-a-half piano keys apart.

However, recent research by Harvard neurologist Mark Tramo, PhD, locates the source of dissonance in the firing of individual hair cells. He and his colleagues found that the ear's nerves pulse in time with peaks and valleys of sound waves, according to a study described in a chapter from the book "The Cognitive Neuroscience of Music" (Oxford University Press, 2003). Waves for consonant sounds tend to be smooth and regular, while dissonant sounds produce jagged waves. Tramo and his colleagues found that, similarly, auditory nerve fibers fire rhythmically in response to consonant intervals and erratically when exposed to dissonant ones.

Both the location of the cells that respond to a sound and their rate of responding may contribute to a sense that an interval is rough or smooth, says Laurel Trainor, PhD, a psychology professor at McMaster University in Canada.

"Probably both processes are going on, and together they give a sense of dissonance," she says.

This basis for telling dissonance from consonance is so hard-wired, a wide variety of animals can do it--and so can 4-month-old infants--according to a study by Trainor, published in Infant Behavior and Development (Vol. 21, No. 1).

The infants in Trainor's study sat between two toys. A flashing light drew the baby's attention toward one of the toys, and as long as the baby looked toward it, a nearby speaker played either consonant or dissonant music. Infants showed a clear preference for the consonant music, gazing in its direction for eleven seconds; they looked toward dissonant sounds only eight seconds, on average.

"I didn't find it surprising that the infants discriminated consonance from dissonance because there are physical differences and physiological differences between the processing of these two categories of sound very early on in the ear," says Trainor. "But I was surprised that they would have that preference for consonance."

Your lazy brain

There may be an evolutionary basis for finding dissonance alarming, says Stephan Koelsch, PhD, another psychologist at the Max Planck Institute in Leipzig. When babies cry--or adults yell--the vocal tract contracts, and the resulting sound is rough.

"If we are really annoyed by our babies crying, we'll be really motivated to do something about it," Koelsch says. (And it's probably no coincidence that our ears are most sensitive to sounds around 3,000 hertz, the pitch a baby wails.)

But it's also possible that we prefer consonant chords because our ears more easily make sense of them, Trainor notes.

"With a consonant interval we can clearly separate the two or more tones," she says. "When we hear dissonance, the components are quite close together and interfering with each other."

The challenging nature of dissonant chords was recently illustrated by psychologist Ehud Bodner, PhD, and his colleagues at Bar-Ilan University in Israel. In their study, published in Psychology of Music (Vol. 35, No. 2, pages 286-305), Bodner and his colleagues had students complete a battery of cognitive tasks. Half listened to a pleasant Finnish folk tune as they worked, while the other half listened to a dissonant version of the song--rewritten to sound as if someone had moved the pianist's right hand down one key.

The students who listened to the dissonant music fared better than the control group, the researchers found. That may be because the more unusual sounding background music sharpened their attention, Bodner says.

The ease with which the brain can process consonant music may even inspire us to sing along, according to a recent functional magnetic resonance imaging study by Koelsch and his colleagues, published in Human Brain Mapping (Vol. 27, No. 1). They found that when participants listened to pleasant-sounding classical music, the area of the brain that controls vocal chords lit up, even though the participants were not singing. Those same songs rendered dissonant didn't have the same effect.

"If you listen to music you find pleasant, then you automatically, covertly sing along with it," says Koelsch.

A Western obsession?

While basic definitions of dissonance and consonance may be rooted in physics and physiology, musical traditions differ vastly in their use of sound categories. In medieval times, Western composers treated the interval of a fourth as perfectly consonant. Today, fourths need quick resolution before they start to annoy audiences.

"As soon as you look at music historically and cross-culturally, you begin to run into problems," says Paul Berry, PhD, a music history professor at the University of North Texas. "What counts as consonance and what counts as dissonance can change a great deal over time."

Physically, our ears are no different from those of our 12th century ancestors, he says. But just by listening to the radio, we've picked up the "grammar rules" of our culture's music, and one of them is that fourths aren't meant to be lingered on.

To test how preferences for dissonance and consonance hold up across cultures, psychologist Tom Fritz, PhD, recently traveled to northern Cameroon, where he climbed up into the mountains and lived among the Mafa people for two months.

Mafa music is quite different from Western tunes, says Fritz. For one thing, the Mafa people have no word for music. All songs are associated with rituals, and they use dissonance liberally.

"They have a flute ritual that's extremely disturbing to the Western ear," says Fritz. "It sounds like concerts from car horns in the '50s."

Using a solar-powered computer, Fritz played dissonant and consonant versions of Western compositions, and he found that Mafa participants could easily distinguish between the two, and they preferred the consonant versions.

"That really points to the fact that something universal is going on," says Koelsch.

However, the Mafa participants were not nearly as bothered by the dissonant versions of the songs as Western listeners were, Fritz found.

"It wasn't necessarily unpleasant for them to listen to," says Fritz. "It was just less pleasant than the original."

Western listeners would grimace when they heard the dissonant songs, while the Mafa people said, "This is kind of nice, this is interesting," Fritz says.

Such research suggests that Western listeners' obsession with consonance and dissonance may be not quite universal. In fact, many music historians believe Western culture became hyper-attuned to managing harmonies due to the early development of musical notation, says Berry. Other traditions focus more on features of musical sound such as timbre--the "texture" of particular notes--something that isn't easily put down on paper. And cultures such as the Mafa have woven ritual and music together so tightly, it's almost impossible to consider one without the other.

"One contribution that ethnography makes to the broader discipline is putting into context exactly how provincial our concepts of consonance and dissonance are," Berry says.