The team, based at the National Institute of Neurological Disorders and Stroke in Maryland, USA, studied the differences in how humans and macaques hear the world. “We found that a certain region of our brains has a stronger preference for sounds with pitch than macaque monkey brains,” said Dr Bevil Conway.
The area of the brain that processes sounds is called the auditory cortex. By studying the activity of the auditory cortex in a non-invasive brain scan, the team could measure how strongly the brain responded to certain sounds.
The sounds were split into two types: noise without pitch, and pitch without noise. Specifically, ‘noise’ is a sound containing lots of different frequencies within a particular range, like the ‘fuzzy’ sound of an untuned radio. Humans can make sounds with noise but no pitch by whispering. A sound with pitch but no noise, however, has a single frequency, like a sung note. Regular human speech and macaque calls have both a pitch and a noise component.
To compare how humans and macaques responded to each type of sound, the team first played the participants a series of tones followed by a series of noises made up of similar frequencies. In humans, the auditory cortex lit up much more brightly when the tones were played, compared to the noises. However, macaques showed barely any difference in how they responded to these two sounds.
Then, the researchers separated the pitch and noise components of macaque calls, to test whether the monkeys were more responsive to sounds they would encounter naturally. However, even when tested with sounds based on their calls, the macaques showed no preference to pitched sounds – but humans did.
“These results suggest the macaque monkey may experience music and other sounds differently,” said Conway. “In contrast, the macaque’s experience of the visual world is probably very similar to our own. It makes one wonder what kind of sounds our evolutionary ancestors experienced.”
There are two important human behaviours which could explain the difference between the human and monkey responses: language and music. “The results raise the possibility that these sounds, which are embedded in speech and music, may have shaped the basic organization of the human brain,” explained Conway. Since we rely so heavily on speech to communicate, and music is culturally important, our brains evolved to accommodate these.
“It may also help explain why it has been so hard for scientists to train monkeys to perform auditory tasks that humans find relatively effortless,” he said.