Ears are a peculiarly individual piece of anatomy. Those little fleshy seashells, whether they stick out or hang low, can be instantly recognizable in family portraits. And they aren’t just for show.
Researchers have discovered that filling in an external part of the ear with a small piece of silicone drastically changes people’s ability to tell whether a sound came from above or below. But given time, the scientists show in a paper published Monday in the Journal of Neuroscience, the brain adjusts to the new shape, regaining the ability to pinpoint sounds with almost the same accuracy as before.
Scientists already knew that our ability to tell where a sound is coming from arises in part from sound waves arriving at our ears at slightly different times. If a missing cellphone rings from the couch cushions to your right, the sound reaches your right ear first and your left ear slightly later. Then, your brain tells you where to look.
But working out whether a sound is emanating from high up on a bookshelf or under the coffee table is not dependent on when the sound reaches your ears. Instead, said Régis Trapeau, a neuroscientist at the University of Montreal and author of the new paper, the determination involves the way the sound waves bounce off outer parts of your ear.
Curious to see how the brain processed this information, the researchers set up a series of experiments using a dome of speakers, ear molds made of silicone and an fMRI machine to record brain activity.
Before being fitted with the pieces of silicone, volunteers heard a number of sounds played around them and indicated where they thought the noises were coming from. In the next session, the same participants listened to the same sounds with the ear molds in.
This time it was clear that something was very different.
“We would put a sound above the participant’s head, and he would say it’s below,” said Dr. Trapeau.
But when the volunteers returned for more testing, after a week wearing the little molds in their ears, most saw their scores go back up. We’re able to locate sound with our own ears because we know their shape, said Dr. Trapeau. When that shape changes, we need time and practice to adapt to it.
Putting the volunteers in the fMRI scanner at each point in the experiment and playing the sounds again gave the researchers a chance to look inside the participants’ brains to see how they were responding.
The researchers discovered that as sounds originate from higher locations, the neurons respond less and less. That means that the neurons are likely representing height by the magnitude of their response.
Additionally, when the volunteers first began wearing the ear molds and their test scores took a dive, their auditory neurons were firing in a much more disorganized manner.
The findings suggest that locating a sound in space requires your brain to incorporate not just the sound waves themselves but your own grasp, however unconscious, of how your ears modify that sound.
The researchers are interested in finding out more about how a sound’s height is perceived in the brain, especially sounds coming from behind. Such research could lead to better hearing devices, and also improve our understanding of how the ear, strange device that it is, contributes to our ability to understand the shape of the world around us. (https://www.nytimes.com/2018/03/06/science/ears-shape-hearing.html)