Stop and Listen

Why is it necessary to stop talking and moving to listen to someone else talk? It’s not just being polite. Cutting edge research in electrophysiology, optogenetics and behavior analysis shows the brain connections that control how movement affects hearing.

DURHAM — If you walk down the sidewalk or sit in a coffee shop and look around, it would be easy to deduce that everyone has a cell phone. And your deduction would be pretty accurate. There are new numbers from the Pew Research Center that show just how much we all depend on cell phones and how much a part of daily life the tiny technological devices have become.

The Pew Center reports 90% of American adults have a cell phone, and 58% of those devises are smart phones.

But as comfortable as we have all apparently become with cell phones, do you ever wonder why the phone’s keypad is designed to make a tapping sound when typing out a message?

It’s because our brains are programmed to connect sound and movement.

“There must be an ability for the brain’s motor systems to tell auditory systems, “I’m going to move and my movement is going to make sounds,” explains Dr. Richard Mooney, Professor of Neurobiology at Duke University School of Medicine. “And not only does the brain’s motor system tell the auditory system there is going to be movement and it is going to make noise, it also tells the auditory system to take that into account when you start to process the sound stream that is going to happen in the next milliseconds.”

That’s right, the parts of the brain that control movement and hearing talk to each other. Here’s another example.

When you want to listen carefully to someone, the first thing you do is to stop talking. The second thing you do is stop moving altogether. The reason you unconsciously do that is to prevent unwanted sounds created by your own movements from disrupting the conversation.

Scientists have always assumed the brain’s motor cortex, which controls movement, and the auditory cortex, which allows us to perceive sound, work together to make sense of the sounds around us. It actually helps us hear better by preventing any unwanted sounds caused by our own movement.

Now, Duke University researchers have proven the connection.

“The connection between movement and sound has to be cast in a framework of survival,” explains Dr. Mooney. “But survival has many features to it.”

Dr. Mooney points out the concept of basic survival, which means not getting killed or eaten. Dr. Mooney cites the example of an early human being stalked by a saber-toothed cat. As you enter the woods, the motor cortex tells the auditory cortex that the sound of leaves crushing beneath your feet is movement and it can ignore that. But it also tells the auditory cortex that if the sound of a stick snapping behind you is heard, that’s something else.

But there is also social survival, which is much more complex. In modern society, human social behavior and communication is key. There is a need to be able to acquire behavior and generate behavior that is important for social function.

To test the theory, the Duke team devised an experiment to monitor the electrical activity in the auditory cortex of a mouse's brain, and the signals coming from the brain’s motor cortex.

Researchers placed mice on a treadmill. They then attached electrodes to the mice to monitor brain activity while the animals were running. The system allowed researchers to record the activity of individual brain cells in the auditory cortex and use a video camera to record the mouse’s activity.

“What we found is that when the animal is moving on the treadmill, the cells in the auditory cortex enter a very different state,” explains David Schneider, a post-doctoral student in neurobiology at the Duke University Medical Center. “The cells essentially stop firing and become much less responsive to sounds in a speaker. It’s almost as if they’ve been turned down.”

The experiment showed, for the first time, how the motor cortex learns to mute the response, or turn down the volume in the auditory cortex, to sounds that are created by one’s own movement. In other words, the sounds of the treadmill did not register in the mouse brain as something to pay attention to. At the same time, when other sounds were introduced, the cells in the auditory cortex started firing, so there was increased sensitivity to other, unexpected sounds.

That experiment explains why all of the city sounds that are heard while walking down the street seem to fade into the background. The motor cortex is essentially telling the auditory cortex, “I’m walking on a sidewalk, expect to hear random street noise.” However when there is the sound of a nearby car horn, or a siren, or a person yelling, the auditory cortex pays attention because that is an unexpected sound.

Anders Nelson, a doctoral student at Duke University, explains the flip side of the research into the connection between sound and movement.

“You can imagine what will happen if you are walking in a wooded environment and you can’t dampen the sounds of your movement and a tiger comes up behind you,” says Nelson. “You would lose the sensitivity to those movements and you would be in trouble. But another interesting aspect to this interaction is that when you hear a startling sound, one of the first things you do is freeze.”

Nelson says the reason your first reaction is to freeze is because you have increased the sensitivity of your auditory system to sounds in your environment and you want to focus on sounds beyond those you are making with your movements.

Besides adding to the overall knowledge of how the parts of the brain that control hearing and movement communicate with each other, there are some additional applications for this research. Disruptions in the brain's circuitry that control the connections between the motor and auditory cortex is believed to called tinnitus, which is also known as a ringing in the ears. This research could be applied to understanding and treating tinnitus. Also, it’s believed the research could add to a better knowledge of schizophrenia.

“The more we understand this sound/movement connection, it becomes clear this has evolved because what our brain does is adaptive,” says Dr. Mooney. “But there are so many examples that when it malfunctions, it creates problems in society, our ability to function, to move, and so on, so we need to continue to learn more about it.”

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