Perceiving pitch - a special human skill?

15 June 2019

MACAQUE-BABY

A baby macaque

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Human perception of pitch is unique and different to macaque monkeys, our closest evolutionary relative.

When it comes to understanding how the brain functions, scientists have done a great deal of work on studying macaque monkeys, with whom we share 93% of our DNA. Even for high level operations such as learning, memory and decision-making, our brains work in similar ways.

However, when it comes to sound, scientists have discovered that humans seem to have a unique edge in how we perceive, how low or high sounds are, something called pitch.

The brain of the macaque monkey is often treated as a small human brain, and a lot of biomedical research uses this as a model for how we expect human brains to work.

Neuroscientists like Bevil Conway, from the National Institute of Health, Bethesda, Maryland, USA, have done extensive work comparing the brains of monkeys and humans.

“In our work, we have shown that macaque monkeys and humans have very similar organisation in the visual cortex, so they have similar responses to colour and motion and faces,” says Conway.

But what about our response to sounds? To test this, the scientist played a series of harmonic sounds, with varying pitch, to both human volunteers and to macaque monkeys, while monitoring their brain activity.

For a comparison, they then played a series of pitchless sounds at the same frequencies. A pitchless sound is like a whisper; although it has a frequency, it has no pitch. For these sounds, the brain scans showed similar results.

However, when it came to the harmonic sounds, the human brain scans showed much more activity, indicating that we have a much higher sensitivity to pitch.

So what is pitch and how is it different to frequency? Frequency is a physical measurement of how “high” or “low” a sound is, it separates a high, lilting violin from a low, rumbling bass.

However, pitch is a psychological phenomenon which describes how “high” or “low” something sounds to us. It is why we can appreciate harmonies in music, and pick up on the “tone” of someone’s voice.

According to Conway, “When we hear a piece of music, we hear these rousing chords and sonorous melodies and so on. What the monkey hears is just the noise component.”

These results indicate that, although macaque monkeys can be very useful to learn about humans, in many ways we are still unique.

Pitch is a crucial part of how we communicate with each other, how we emotionally respond to music and our experience of the world around us.

Conway suggests that this work may “point to something very special about what pitch does for us and about the kinds of meanings we are looking for”. The next steps are to study marmoset monkeys as they have a much more complicated social structure, and see whether the perception of pitch plays a role in that.

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