Volume 94, No.3, May-June 2008

Duke Magazine-In Search of Music's Biological Roots by Ker Than

Seeking to understand the universal appeal of music, neuroscientist Dale Purves has discovered surprising similarities between the twelve-note chromatic scale and the universal tones found in speech.

Complex and sublime: Amateur guitarist Purves contemplates the complicated role music plays in our lives.
Complex and sublime: Amateur guitarist Purves contemplates the complicated role music plays in our lives.
Michael Zirkle

Dale Purves is not musical by nature. He's been trying to play the guitar for forty years-with limited success. He has no formal training and, if presented with a sheet of music, can't tell an F-sharp from a B-flat.

But even though he is not musical, Purves is deeply curious about music. Why, he wonders, do humans appear to be hard-wired to appreciate it despite its lack of a clear survival benefit? Why do we find some combinations of musical notes pleasing but can't stand others? And, perhaps most enticing, why do we think of some types of music as happy and bright, but others as dark and sad-in other words, how did music come to pack such an emotional wallop?

Purves (pronounced purr-VEHZ), a physician and neurobiologist who heads Duke's Center for Cognitive Neuroscience, has made a name for himself studying human vision. More recently, he has also turned his attention to hearing. "We started looking at audition to compare it to the theories we developed in vision," he says.

Over the last five years, he and his fellow researchers have provided compelling evidence that our species' fondness for music is linked to another human universal: language. The findings suggest that humans like music because, in subtle and unconscious ways, it reminds us of speech, arguably the most important social cue in our environment and a critical factor in our species' survival and success.

"The only vocalizations that count for us are the vocalizations we make for each other," Purves says. "Those are the tonal sounds in nature that we've always been exposed to." His studies of music are part of a broader theory he is developing about how our perceptions are shaped not only by our individual experiences, amassed over decades, but also by the collective experiences of our species, gathered over millions of years of evolutionary time.

With the possible exception of love, nothing in the human experience is as difficult to define-or has attracted so many attempts at definition-as music. Music has been described as mysterious, sublime, even divine. The French novelist Victor Hugo said music "expresses that which cannot be put into words and that which cannot remain silent." Music has been called an "echo of the invisible," the "speech of angels," a "shorthand of emotion," and "unconscious arithmetic." The American poet and musician Sidney Lanier considered the two mysteries equal, and called music "love in search of a word."

What everyone does agree upon is music's universal ability to transcend time, geography, and culture. Throughout history, people living in every corner of the globe have made and listened to music. And whether they coax music by plucking, striking, or blowing instruments crafted from wood, metal, or bone, people in just about every culture make music in the same general way, using subgroups of the same twelve notes.

These notes are known as the chromatic scale and can be heard on a piano by starting with any key and then playing the next twelve black and white keys in succession. On the thirteenth note, the scale begins again, but at a higher frequency. The interval between one piano key and a key of the same name either above or below it is called an octave.

No culture uses all twelve notes of the chromatic scale in its music, but nearly all musical traditions make music based on some combination of notes within it. Traditional Chinese music and much of American folk music, for example, are made using what's called the pentatonic scale, which uses five of the notes within the octave (F and B are not used). The five notes of the pentatonic scale are a subset of the seven-note diatonic scale used in Classical Western music. The latter includes the familiar "Do-Re-Mi-Fa-So-La-Ti-Do" taught in schools.

The widespread use of the chromatic scale is puzzling if you consider that the human auditory system is capable of distinguishing a very large number of notes, also called pitches, over the range of sound frequencies that humans can hear (about 20 to 20,000 "hertz" or cycles per second).

"Why is it, despite the fact that we can hear many, many different pitch relationships, we use just these twelve relationships in music pretty much universally?" Purves asks. "There are embellishments on this basic fact-Arabian and Indian music and American blues use some well-defined variations-but, basically, we humans all build music using the same bricks."

Not only do different cultures compose music using the same notes, they also agree on which note combinations sound pleasing and which rankle the ears-a phenomenon music theorists and auditory scientists call relative consonance. For example, given a choice, nearly everyone in the world will agree that C together with F, which is the musical interval called a fourth, is a more pleasing note combination than C together with F sharp, which is called a tritone.

Philosophers and scientists have struggled for centuries to explain why we find certain combinations so appealing. One of the earliest attempts looked at music's mathematical properties. Some 2,500 years ago, the Greek philosopher Pythagoras, who was obsessed with numbers and their significance, demonstrated a direct relationship between how pleasing or harmonious some tone combinations sounded and the physical dimensions of the object that produced them. For example, a plucked string will always sound a fourth lower in pitch than an identical string three-quarters its length, a fifth lower than a string two-thirds its length, and a full octave lower than a string half its length.

Pythagoras believed that the intervals of the fourth, the fifth, and the octave sounded beautiful because the ratio of the frequency of the two notes making up the sounds were small-number fractions such as 4/3, 3/2, or 2/1. "It's basically a mystical explanation," says David Schwartz, a Duke neuroscientist who has worked with Purves on his studies of music. "He thought that the gods in some sense preferred simple small numbers, and that the pleasure we take in the sounds of these intervals is a perceptual manifestation of the intrinsic beauty of small-number ratios."

"Pythagoras and many others had mystified music by making it seem that it had to do with celestial motions," Purves says. "That's just hocus-pocus.

"Others, up to the nineteenth century and beyond, have argued that it's all about physics, that you can explain consonance in terms of physical relationships having to do with these harmonic ratios."

The belief that math and music are closely interrelated is still widespread today. Indeed, an entire industry has been built around the so-called Mozart effect, the controversial claim that listening to Mozart or other complex music provides temporary boosts in mathematical abilities because the brain regions involved in processing music are also involved in other mental tasks, including math.

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