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.