Have you ever noticed that the first thing you do when you want to listen carefully to someone, is stop talking? The second thing you do is stop moving altogether.
These tactics helps us hear better by preventing unwanted sounds generated by our own movements. But how does our brain do this almost without our knowing?
This interchange between movement and hearing has an equivalent deep in the brain. In fact, oblique evidence has long implied that the brain’s motor cortex, which controls movement, somehow influences the auditory cortex, which gives rise to our conscious perception of sound.
Now, a new study from neurobiologists at Duke University joins cutting-edge electrophysiology methods to optogenetics and behavioral analysis to uncover just how the motor cortex, seemingly in anticipation of movement, can adjust the volume control in the auditory cortex.
New Causality-driven Brain Model
The group of researchers used the new lab methods to “get beyond a century’s worth of very powerful but largely correlative observations, and develop a new, and really a harder, causality-driven view of how the brain works,” said senior author Richard Mooney Ph.D., of Duke.
The findings add to our basic knowledge of how communication between the brain’s motor and auditory cortexes may affect hearing during speech or musical performance. In people with schizophrenia, disturbances to the same circuitry may give rise to auditory hallucinations.
Researchers led by Mooney, first characterized In 2013 connections between motor and auditory areas in mouse brain slices as well as in anesthetized mice. The new study answers the important question of how those connections operate in an awake, moving mouse.
Dr. Mooney believes that the motor cortex may learn how to mute responses in the auditory cortex to sounds that are expected to arise from one’s own movements while heightening sensitivity to other, unexpected sounds. The group is now testing this idea.
“Our first step will be to start making more realistic situations where the animal needs to ignore the sounds that its movements are making in order to detect things that are happening in the world,” said researcher David Schneider.
Dampened Auditory Cortex
During the study, the team monitored the electrical activity of individual neurons in the brain’s auditory cortex. Whenever the mice moved, whether grooming, walking, or making high-pitched squeaks, certain neurons in their auditory cortex were dampened in response to tones played to the animals, compared to when they were at rest.
In order to discover whether the actual movement was directly influencing the auditory cortex, researchers did experiments in awake animals using optogenetics.
Optogenetics is a powerful technique which uses light to control the activity of select populations of neurons that have been genetically sensitized to light. Like the game of telephone, sounds that enter the ear pass through six or more relays in the brain before reaching the auditory cortex.
The Penultimate Node
“Optogenetics can be used to activate a specific relay in the network, in this case the penultimate node that relays signals to the auditory cortex,” Mooney said.
Roughly half of the suppression during movement was found to originate within the auditory cortex itself.
“That says a lot of modulation is going on in the auditory cortex, and not just at earlier relays in the auditory system” Mooney said.
In fact, the team found that movement stimulates inhibitory neurons. These, in turn, suppress the response of the auditory cortex to tones.
That brought up the question of what turns on the inhibitory neurons. There were many theories.
“The auditory cortex is like this giant switching station where all these different inputs come through and say, ‘Okay, I want to have access to these interneurons,’ ” Mooney said. “The question we wanted to answer is who gets access to them during movement?”
From previous experiments, it was known that neuronal projections from the secondary motor cortex (M2) modulate the auditory cortex.
But to isolate M2’s relative contribution, something not possible with traditional electrophysiology, the researchers again used optogenetics, this time to switch on and off the M2’s inputs to the inhibitory neurons.
Turning on M2 inputs reproduced a sense of movement in the auditory cortex, even in mice that were resting, the group found.
“We were sending a ‘Hey I’m moving’ signal to the auditory cortex,” Schneider said.
Then the effect of playing a tone on the auditory cortex was much the same as if the animal had actually been moving. This result confirmed the importance of M2 in modulating the auditory cortex.
On the other hand, turning off M2 simulated rest in the auditory cortex, even when the animals were still moving.