Research with marmosets has led to the conclusion that they perceive pitch much as we do, and that what scientists previously believed to be a uniquely human ability is really not.
The findings suggest that a skill we use to make music and communicate actually evolved in non-humans more than 40 million years ago.
“Pitch perception is essential to our ability to communicate and make music,” says Xiaoqin Wang, professor of biomedical engineering at the Johns Hopkins School of Medicine, “but until now, we didn’t think any animal species, including monkeys, perceived it the way we do. Now we know that marmosets, and likely other primate ancestors, do.”
Marmosets are small monkeys, native to South America; they are highly vocal and social. Wang, an auditory neuroscientist and biomedical engineer, has been studying their hearing and vocalizations for 20 years. Results of the research appear online in the Proceedings of the National Academy of Sciences.
Listen: Marmoset “phee” calls (above) and “trill-twitter” calls (below):
A decade ago, he says, his team identified a region in marmoset brains that appeared to process pitch. Nerve cells on the edge of the primary auditory cortex only “fired” after marmosets were exposed to sounds with pitch, like shifts between high and low notes associated with a melody. They did not fire when the monkeys were exposed to sounds without pitch, such as noise. Human brains show similar neuron activity in that brain region, he notes.
What was missing until now was behavioral evidence that the marmosets perceive and respond to pitch differences as humans do. Wang’s lab spent years developing behavioral tests and electrophysiological devices to monitor subtle changes in monkey neural activity. Part of the work was to train marmosets to lick a waterspout only after hearing a change in pitch.
Other species have been reported to have some pitch perception, Wang says, but none have shown three specialized features of human pitch perception. First, people are better at distinguishing pitch differences at low frequencies than high. For example, people can hear four separate sounds when tones of 100, 200, 300, and 400 hertz are played simultaneously. They hear only one sound, however, when tones of 1,100, 1,200, 1,300, and 1,400 hertz are played together, although the frequency intervals are the same in both cases.
Second, humans are able to pick up on subtle changes in the spread between pitches at low frequencies. If a series of tones increases by 100 hertz each time, but then a tone only 90 hertz higher, humans will notice.
Third, at high frequencies, people’s ability to perceive pitch differences among tones played simultaneously is related to how sensitive they are to the rhythm of sound waves.
Through a series of hearing tests, with waterspout licks as a readout, Wang’s team, led by graduate student Xindong Song, determined that marmosets share all three features with humans.
Those shared characteristics suggest that human pitch perception evolved much earlier than previously thought—in fact, that it evolved before there were humans.
The Americas, with marmosets in place, broke away from the African landmass approximately 40 million years ago, before humans appeared in Africa. It is possible that humanlike pitch perception had evolved before that break—so that the marmosets had it—and was maintained throughout primate evolution in Africa until it was inherited by modern humans.
Another possibility is that only certain aspects of pitch perception were in place before the split, with the rest of the mechanisms evolving in parallel in Old and New World monkeys. More work is needed to determine whether existing Old World monkeys perceive pitch like humans do, Wang says.
“In addition to the evolutionary implications of this discovery, I’m looking forward to what we will be able to learn about human pitch perception now that we have a primate relative we can study behaviorally and physiologically,” Wang says. “Now we can explore questions about what goes wrong in people who are tone deaf and whether perfect pitch is an inherited or learned trait.”
The National Institute on Deafness and Other Communication Disorders supported the work.
Source: Johns Hopkins University