Washington | Marmosets, ancient monkeys, appear to use auditory cues similar to humans to distinguish between low and high notes, say scientists who found pitch perception may have evolved over 40 million years ago to enable vocal communication and song-like vocalisations.
Pitch perception is essential to our ability to communicate and make music, but until now, we didn’t think any animal species, including monkeys, perceived it the way we do, said Xiaoqin Wang, a professor at the Johns Hopkins University School of Medicine in US. Now we know that marmosets, and likely other primate ancestors, do, said Wang.
The discovery infers that aspects of pitch perception may have evolved more than 40 million years ago to enable vocal communication and song-like vocalisations. Marmosets are small monkeys native to South America that are highly vocal and social. Wang has been studying their hearing and vocalisations for the past 20 years.
A decade ago, he said, researchers identified a region in the marmoset brain that appears to process pitch. Nerve cells in that region, on the edge of the primary auditory cortex, only ‘fired’ after marmosets were exposed to sounds with pitch, like the shifting in high and low notes associated with a melody, not those without, such as noise.
Human brains show similar activity in that region, as other researchers have reported, he said. Researchers spent years developing behavioural tests and electrophysiological devices designed to monitor subtle changes in the monkeys’ neural activity. Part of their work was to train a group of marmosets to lick a waterspout only after hearing a change in pitch.
Wang said that other animal species have been reported to show pitch perception, but none have shown the three specialised features of human pitch perception. First, people are better at distinguishing pitch differences at low frequencies than high.
Second, humans are able to pick up on subtle changes in the spread between pitches at low frequencies or hertz, so they notice if a series of tones is increasing by 100 hertz each time and then introduces a tone only 90 hertz higher.
Third, at high frequencies, peoples’ ability to perceive pitch differences among tones played simultaneously is related to how sensitive they are to the rhythm, or timed fluctuations, of sound waves.
Through a series of hearing tests, with waterspout licks as a readout, researchers led by graduate student Xindong Song, determined that marmosets share all three features with humans, suggesting that human components of pitch perception evolved much earlier than previously thought.
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