Study Shows How Movement Affects Hearing

Thu, 08/28/2014 - 8:30am
Neurons in the mouse motor cortex (green) project to the auditory cortex. As the mouse moves, these neurons suppress activity in the auditory cortex. (Source: Duke University/Anders Nelson)When we want to listen carefully to someone, the first thing we do is stop talking. The second thing we do is stop moving altogether. This strategy helps us hear better by preventing unwanted sounds generated by our own movements.
This interplay between movement and hearing also has a counterpart deep in the brain. Indeed, indirect evidence has long suggested that the brain's motor cortex, which controls movement, somehow influences the auditory cortex, which gives rise to our conscious perception of sound.
The findings contribute to the basic knowledge of how communication between the brain's motor and auditory cortexes might affect hearing during speech or musical performance. Disruptions to the same circuitry may give rise to auditory hallucinations in people with schizophrenia.
In the latest study, the team recorded electrical activity of individual neurons in the brain's auditory cortex. Whenever the mice moved- walking, grooming, or making high-pitched squeaks- neurons in their auditory cortex were dampened in response to tones played to the animals, compared to when they were at rest.
The team knew from previous experiments 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- a result that 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.

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