HOW WE HEAR DISTANCE , ECHOES ESSENTIAL
Mammals are good at
figuring out which direction a sound is coming from, whether it's a rabbit with
a predator breathing down its neck or a baby crying for its mother. But how we
judge how far away that sound is was a mystery until now. Researchers from
UConn Health report in the 1 April issue of the Journal of Neuroscience that echoes and fluctuations in
volume (amplitude modulation) are the cues we use to figure the distance
between us and the source of a noise.
"This opens up a
new horizon," says Duck O. Kim, a neuroscientist at UConn Health.
Researchers have long understood how we can tell a sound's direction--whether
it's to our left or right, front or back, and above or below us. But how we
tell how far away it is had remained a mystery. "The third dimension of
sound location was pretty much unknown," says Kim.
All natural sounds,
including speech, have amplitude modulation. Kim and his colleague Shigeyuki
Kuwada suspected that amplitude modulation, and how echoes muddy it, were
together key to our ability to perceive a sound's distance from us. To explore
the idea, they used tiny microphones to record the sounds inside rabbits' ears
as they played sounds at different locations. They used these recordings to
simulate modulated or unmodulated noise coming from different distances from
the rabbit. Then Kim and Kuwada 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, certain type of IC neurons fired more when the sound was
closer and the depth of modulation was higher--that is, when there was a bigger
difference between the sound's maximum and minimum amplitude.
Reverberations, or
echoes, tend to degrade amplitude modulation, smoothing out the amplitude's
peaks and valleys. Almost any environment has echoes as sounds bounce off of
objects such as walls or trees, the ground, et cetera. The farther away the
source of a sound is from a listener, the more echoes there are, and the more degraded
the depth of amplitude modulation gets. As you would expect, the neurons fired
less and less when the sound moved further away and the depth of amplitude
modulation degraded more and more.
Pavel Zahorik, a
researcher at the University of Louisville School of Medicine, tested the same
amplitude modulated noise using human volunteers and got the same results:
people need both amplitude modulation and reverberation to figure out how far
away a sound is. Without amplitude modulation, a person can't tell how far away
that noise is. Neither can she do it in an anechoic (echo-free) room.
"Reverberation is
usually considered a bad thing," detrimental to hearing clearly, says
Kuwada. "But it is necessary and beneficial in order to recognize
distance."
Judging sound distance
is a crucial survival skill, whether you're a bunny or a human--is that monster
breathing down my neck, or huffing and puffing 20 yards behind me? Do I have
time to cross the street before that car I hear in the distance pulls around
the bend? Kim and Kuwada suggest that getting a better understanding of the
acoustics and neuroscience of distance perception could contribute to making
better hearing aids and prostheses, and perhaps reveal more subtle aspects of
our sound perception. The importance of amplitude modulation is still poorly
understood. Laurel Carney, a colleague at University of Rochester, modeled the
ear and IC neural circuitry and replicated the neural firing patterns recording
by Kim and Kuwada. The researchers hope that tweaking the model will give them
more insight into the neurons' responses.
Kim and Kuwada's next
step will be to do a two-eared study, and tie together the perception of
distance, horizontal and vertical directions of sound.
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