During a conversation, the brain constantly makes adjustments in order to efficiently process the speech volume of others while recognizing self-made speech for what it is. According to a new report published this week in The Journal of Neuroscience, Duke researchers have mapped the complex brain interactions responsible for the ability to hold a conversation or play a musical instrument.
What’s more, the team believes that their findings could also provide insight into schizophrenia and other mental disorders that are marked by a confusion between external and internal voices.
“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 study author Richard Mooney, professor of neurobiology at the 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 said. “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.”
To understand the integration between the movements associated with speech and auditory processing, the Duke researchers traced neural inputs to the sound-processing region of the brain using a newly developed technique.
The team said they were particularly interested in an area called the secondary motor cortex, or M2. This brain region is responsible for transmitting 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,” explained co-author David M. Schneider, 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 establishing the nature of the connection, the neuroscientists took brain tissue from laboratory mice and manipulated tissue neurons that connected the M2 region to the auditory cortex. They discovered that stimulating these neurons actually reduced the activity of the auditory cortex.
“It jibed nicely with our expectations,” said co-author Anders Nelson, 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.”
In one final test, the research team switched on the motor neurons in anesthetized mice and watched for a response from the auditory cortex. After playing ultrasonic mouse vocalizations to their test mice with activated motor regions, the team found that the signaling neurons become much less responsive to the tiny voices.
“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.”
After some additional research, the team said they could begin to investigate whether faulty wiring in this circuitry is perhaps responsible for auditory hallucinations that mark conditions like schizophrenia.
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