We hear sounds around us every day, and usually we are able to find the direction from which the sounds come. When hiking, a few notes of a bird’s song can lead us to locate the bird. When walking down the street, someone calling our name causes us to turn in the direction of the caller. How do we locate the source of a sound?
Let’s do a little experiment… First, find a friend to help you. As you sit in a chair with your eyes closed, have your friend shake a set of keys one of three places – above your head, at eye level, or near the ground. After each shake, guess the location of the keys. I bet you guessed correctly. Now, push on the back of your ears, just above the lobes. Push hard enough that you distort the shape of the ear canal but don’t completely block sound from reaching the ear. Have your friend shake the keys in various locations again. Did you guess correctly? If you didn’t fare as well on the second part of the experiment, there is a good reason.
How do we hear and interpret the sounds coming into the ears? It’s a question that Donata Oertel, professor and interim director of the Department of Physiology at the University of Wisconsin-Madison, has spent her career studying. I sat down with Oertel in an attempt to understand this sense that many of us take for granted every day.
Oertel explains that when we hear sounds, we receive two important pieces of information – where the source of the sound is and what that sound means. But the ear itself does not carry out these functions.
“From the signals that come from the ear, the brain has to extract the information it needs,” explains Oertel, “It turns out these are quite complicated computations.”
Our ears are far apart from each other on our heads for good reason. When sounds come from the side, they will reach the near ear a little bit earlier than they reach the far ear. If the sound is at a high frequency, it will also be louder in the near ear.
“These time and intensity differences sensed by the ear give our brains the cues to compute the location of the source of a sound in the horizontal plane,” says Oertel. “But that process doesn’t help us determine where the sound is coming from in the vertical plane.”
Oertel explains that to locate a sound in the vertical plane, the asymmetry of our ears is important. Our ears are asymmetrical, both top to bottom and front to back. As sounds coming from various locations impinge on the ear at different angles, they interact with the ear differently and give the brain spectral cues. The cues change as the elevation of the sound source changes, and we can then locate the source in the vertical plane.
Being able to hear a sound and locate the source is a great skill, but once a sound is heard, the brain must work to interpret it. The question of how this happens was a new concept when Oertel began working in the auditory system, a field dominated by engineers in the late 1970s.
“Engineers described, where as I wanted to ask. I wanted to ask why it was that these brain circuits were doing what they were doing,” says Oertel.
To find answers to the “why” questions, Oertel and her lab study the brains of mice. Electrophysiological recordings can be taken from the part of the brain where the auditory nerves terminate, giving Oertel and her colleagues information about the neurons and how they are processing auditory messages.
Oertel’s hard work and knowledge of the brain have led to a number of interesting findings, including a circuit in the auditory pathway of the brain which Oertel believes functions to suppress echoes. Sounds come directly from a source to our ears, but they also bounce off walls around us. To accurately hear the sound and locate its source, we must suppress the echoes.
“I think one part of that ability comes from a brain circuit which receives a first excitation and then inhibits further signals,” explains Oertel. “In this way, we can use the first signal but ignore the subsequent signals to accurately process the sound.”
So the next time you are walking through a crowd of people or trying to spot a bird you hear, stop and appreciate your ability to determine the direction from which sounds are coming. And when a friend calls your name from through the crowd, be grateful you can pinpoint that person without calling for a search party.