Researchers at Duke University have drawn the first wiring diagram of the brain system which enables the intricate interaction between the motor system and the auditory system to happen.
In normal conversations, the brain is continuously adjusting the volume to soften the sound of your own voice and increase the voices of others in the room. That ability to distinguish between the sounds made by you and those coming from the outside world is vital not only for catching up on the latest gossip, but more importantly is needed for learning how to speak or play a musical instrument.
“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 senior author Richard Mooney, “Normally, motor regions would warn auditory regions that they are making a command to speak, so be prepared for a sound. 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."
Interactive Nerve Cells
Researchers have long theorised that the neural circuitry transmitting movement, for example, to object to an opinion or pluck a guitar string, also feeds into the wiring that senses sound. But the character of the nerve cells that gave that input, and how functionally they interacted to help the brain foresee the impending sound, was not known.
Mooney used tools created by Duke associate professor of cell biology Fan Wang, Ph.D., to trace all of the inputs into the auditory cortex, which is the sound-interpreting region of the brain. Although researchers found that a few different regions of the brain led into the auditory cortex, they were paying the most attention to one area called the secondary motor cortex, or M2, 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 co-lead author David M. Schneider, Ph.D. “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.’"
Nature’s Mute Button
After finding this connection, the researchers examined what influence that interaction had on hearing. They took slices of brain tissue from mice and expressly manipulated neurons leading from the M2 region to the auditory cortex. The researchers 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. “It is the brain’s way of muting or suppressing the sounds that come from our own actions."
“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 team have a good grip on the basics of the circuitry, they could start investigating whether altered circuitry could induce auditory hallucinations or perhaps even take them away in models of schizophrenia.
“A Circuit for Motor Cortical Modulation of Auditory Cortical Activity,” Anders Nelson, David M. Schneider, Jun Takatoh, Katsuyasu Sakurai, Fan Wang, Richard Mooney. The Journal of Neuroscience, Sept. 4, 2013. DOI: 10.1523/JNEUROSCI.2275-13.2013