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Bass notes lay down beats in music worldwide, and new research may reveal why that is — the ear responds better to rhythms set by deeper sounds, scientists say.
Although melodies in the foreground of music are often dominated by higher tones, rhythms in the background of music are often made up of by lower tones, such as the bass lines of jazz and blues, the left-hand, low-pitched rhythms of ragtime piano, and the pulses of bass drums in electronic, pop and dance music.
"Music in diverse cultures is composed this way, from classical East Indian music to Gamelan music of Java and Bali, suggesting an innate origin,"said study co-author Laurel Trainor, director of the McMaster Institute for Music and the Mind and a neuroscientist at McMaster University in Hamilton, Canada.
To find out why lower tones might dominate rhythms in music, Trainor and her colleagues had 35 volunteers listen to two simultaneous streams of computer-synthesized piano tones, each of a different pitch. A tenth of the time, the lower tone occurred 50 milliseconds too early; another tenth of a time, the higher tone occurred 50 milliseconds too early.
In one set of experiments, the researchers scanned the brain activity of the volunteers as they listened to these streams of tones, revealing the brains of the participants responded more strongly when the lower tones were off beat. In another set of experiments, the investigators had the volunteers tap their fingers in time with the streams of tones, and they discovered the participants altered their finger tapping to follow changes in timing of the lower tones more often than for the higher tones. [Incredible Technology: How to See Inside the Mind]
These findings suggest people are better at detecting mistakes in a rhythm's timing when they occur with lower tones, which could explain why lower tones dominate musical rhythms. The fact these findings were seen even with the eight volunteers in the experiments who did not have any musical training suggests this discovery may reflect innate aspects of human biology.
"There is a physiological basis for why we create music the way we do," Trainor told Live Science. "Virtually all people will respond more to the beat when it is carried by lower-pitched instruments."
The researchers suggest this effect might originate in the cochlea, the part of the inner ear that sends electrical signals to the brain in response to sound vibrations. Their computer models of the inner ear suggest the cochlea is more sensitive to changes in rhythms that are made up of lower tones.
Previous work by Trainor and her colleagues found that people are better at perceiving the higher-pitched of two simultaneous sounds. This could explain why melodies are commonly conveyed by higher tones, Trainor said.
The findings also suggest that "for some people with poor rhythm, the problems might arise actually in cochlea of the ear," Trainor said. "But at the same time, timing and rhythm are subsequently processed in many different cortical and sub-cortical areas [of the brain], so their problems could be in any of these regions as well."
The researchers do note that higher-pitched sounds can also contribute to rhythms. "Indeed, high-pitched instruments can carry important rhythmic aspects — for example, in jazz, higher-pitched instruments often add rhythmic interest by playing off the beat, so the rhythm is an interaction between different instruments," Trainor said. "What we are saying is that most typically the bass-range instruments lay down the basic beat, and that we are best at perceiving that beat when it is in the lower-pitched instruments."
In the future, the researchers would like to study how early this effect might appear in children. This work suggests this effect originates in the inner ear, meaning it should arise early in development, but there may be contributions from the brain as well "that are more dependent on learning and experience," Trainor said.
Trainor and her colleaguesMichael Hove, Celine Marie and Ian Bruce detailed their findings online today (June 30) in the journal Proceedings of the National Academy of Sciences.