Scientists Discover How We Hear Distance

Hearing sounds from a distance is important part of everyday life for mammals. Whether it's identifying how far away prey may be or how close a predator is, animals need to be able to where a noise is coming from in order to react. Now, scientists have discovered exactly how we judge how far away a sound happens to be.

All natural sounds, including speech, have amplitude modulation. This is essentially echoes and fluctuations in volume. The researchers suspected that amplitude modulation, and how echoes muddy it, are key to perceive a sound's distance. That's why they explored the idea by using tiny microphones to record the sounds inside rabbits' ears as the researchers played sounds at different locations.

The researchers used the recordings to simulate modulated or unmodulated noise coming from different distances from the rabbit. Then, they played the simulated sounds back to the rabbit and measured the responses of neurons in the rabbit's inferior colliculus (IC), a region of the midbrain known to be important for sound perception.

When the rabbit heard the simulated sounds, a certain type of IC neurons fired more when the sound was closer and the depth of modulation was higher, which is when there was a bigger difference between the sound's maximum and minimum amplitude.

Reverberations, or echoes, tend to degrade amplitude modulation. They smooth out the amplitude's peaks and valleys. This allows animals to judge how far away something is; neurons fire less when the sounds moves further away.

"Reverberation is usually considered a bad thing," said Shigeyuki Kuwada. "But it is necessary and beneficial in order to recognize distance."

The findings reveal a bit more about how animals hear and judge distance. The researchers are currently planning to do a two-eared study as a next step, tying together the perception of distance, horizontal and vertical directions of sound.

The findings are published in the Journal of Neuroscience.

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