Ever wondered why we stop talking and moving altogether when we want to listen carefully to someone? 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, researchers have found.
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.
A new study by Duke University combines cutting-edge methods in electrophysiology, optogenetics and behavioural analysis to reveal exactly how the motor cortex, seemingly in anticipation of movement, can tweak the volume control in the auditory cortex.
The new lab methods allowed the group 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 the study's senior author Richard Mooney, a professor of neurobiology at Duke University School of Medicine.
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.
"This is a major step forward in that we've now interrogated the system in an animal that's freely behaving," said David Schneider, a postdoctoral associate in Mooney's lab.
Mooney suspects that the motor cortex learns 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.
"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," Schneider said.
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 research appears in the journal Nature.