Your brain is wired to quiet voices in your head
Nerve circuits let your brain turn down sounds generated from your own movements, and turn up other sounds you need to pay attention to, according to a new study published in The Journal of Neuroscience on September 4.
For example, when you are having a normal conversation with other people, your brain is constantly adjusting the volume to soften the sound of your own voice while boosting the voices of others in the room.
This ability to distinguish between sounds coming from your own actions and those coming from the outside world is important, not only for catching up on water cooler gossip, but also for learning how to speak or play a musical instrument.
As a result of this finding, researchers have now developed the first ever diagram of the brain circuitry that allows this complex interplay between the motor system and the auditory system to occur.
The research could lend insight into schizophrenia and other mood disorders that develop when this circuitry goes awry in individuals, causing them to hear voices that other people do not hear.
"Our finding is important because it provides the blueprint for understanding how the brain communicates with itself, and how that communication can break down to cause disease," said Richard Mooney, Ph.D., senior author of the study and professor of neurobiology at Duke University School of Medicine.
"Normally, motor regions would warn auditory regions that they are making a command to speak, so be prepared for a sound,” Mooney added. “But in psychosis, you can no longer distinguish between the activity in your motor system and somebody else's, and you think the sounds coming from within your own brain are external."
Researchers have long concluded that the neuronal circuitry conveying movement – to voice an opinion or hit a piano key – also feeds into the wiring that senses sound. However, the nature of the nerve cells providing that input – and how they interact to help the brain anticipate the impending sound – have remained largely unknown.
Accordingly, Mooney and his research team conducted a study, using a technology created by Fan Wang, Ph.D., associate professor of cell biology at Duke, to trace all of the inputs into the auditory cortex (the sound-interpreting region of the brain).
Although the researchers discovered that several different areas of the brain fed into the auditory cortex, they were most interested in a particular region, called the secondary motor cortex (or M2), because it is the region that is responsible for sending motor signals directly into the brain stem and the spinal cord.
"That suggests these neurons are providing a copy of the motor command directly to the auditory system," said David M. Schneider, Ph.D., co-lead author of the study and a postdoctoral fellow in Mooney's lab. "In other words, they send a signal that says 'move,' but they also send a signal to the auditory system saying 'I am going to move.'"
After discovering this connection, the research team then explored what kind of influence this interaction was having on auditory processing (or hearing). To do this, they took slices of brain tissue from mice and specifically manipulated the neurons leading from the M2 region to the auditory cortex.
As a result, the research team found that stimulating those neurons actually dampened the activity of the auditory cortex.
"It jibed nicely with our expectations," said Anders Nelson, co-lead author of the study and a graduate student in Mooney's lab. "It is the brain's way of muting or suppressing the sounds that come from our own actions."
Finally, the researchers tested this circuitry in live animals, artificially turning on the motor neurons in anesthetized mice and then looking to see how the auditory cortex responded. Mice usually sing to each other through a form of song known as ultrasonic vocalizations, which are too high-pitched for humans to hear. Therefore, the researchers played back these ultrasonic vocalizations to the mice after activating the motor cortex, and what they discovered was that the neurons became much less responsive to the sounds.
"It appears that the functional role that these neurons play on hearing is they make sounds we generate seem quieter," said Mooney. "The question we now want to know is if this is the mechanism that is being used when an animal is actually moving. That is the missing link, and the subject of our ongoing experiments."
Once the researchers find the missing link and have pinned down the basics of the circuitry, they could begin to investigate whether altering this circuitry could induce auditory hallucinations or perhaps even take them away in models of schizophrenia.
SOURCE: The Journal of Neuroscience, "A Circuit for Motor Cortical Modulation of Auditory Cortical Activity," Anders Nelson, David M. Schneider, Jun Takatoh, Katsuyasu Sakurai, Fan Wang, Richard Mooney (Sept. 4, 2013). DOI: 10.1523/JNEUROSCI.2275-13.2013