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Our brain can be equipped with a noise reduction function: one that helps us to ignore the sound of our own steps or tightening our bites.
In a new mouse study, the mouse brain canceled the sound of its own footsteps. This ability has helped mice to better hear other sounds in their environment, researchers (Nature) reported today in their diary (12 September).
For a mouse walking in a field, "it's better to hear a cat than its own tracks," said lead author Richard Mooney, a professor of neurobiology at Duke University. [3D Images: Exploring the Human Brain]
Mooney and his team used mice to study their "acoustic virtual reality system". They implanted tiny electrodes in their auditory cortex – the region of the brain that processes the sound – and the mice ran on a treadmill under a microscope so they could also take live images of the brain.
To see how the brain processed the sounds associated with the movement of an animal, the researchers created artificial pitch sounds – sounds that mice would not encounter in nature. At each stage, the researchers played a quick note or a "tone". Imagine that mice run on a small piano, Mooney told Live Science. But "each key plays exactly the same note".
Mooney and his team found that after several thousand steps on two to three days, activity in the auditory cortex decreased.
But when the researchers changed the sound of the pip, the auditory cortex became much more active. It could also explain why you can hear your footsteps if, for example, you wear strong boots one day, and you usually do not, said Mooney.
"The experience can shape the way the brain suppresses the predictable sensations resulting from movement," he said.
Their imagery and measurements showed a strong coupling between the motor cortex – an area of the brain involved in the movement – and the auditory cortex. During training, the motor cortex begins to form synapses or connections to the auditory cortex. These connections eventually serve as an anti-noise filter.
So-called inhibitory neurons, or brain cells, in the motor cortex began to send signals to cancel the triggering of neurons in the auditory cortex that sensitized us to sound. This process is so fast that it is "predictive," said Mooney, which means that the cancellation signal occurs at the same time that the brain is controlling movement.
The researchers also found that mice that had been trained to ignore the sound of their own footsteps were better able to detect abnormal or new sounds when running, compared to those who had not followed the training.
Mooney thinks that the results could be very clearly translated for humans. Although the cortex is much more advanced in humans, "the basic architecture of the brain between the motor cortex and the auditory cortex is present in all the mammals studied," he said.
"The mice do not play the piano, at least no one I know," Mooney said. For them, the ability to suppress movement-related sounds is more of a survival advantage, for example to better detect potential predators.
This may also be true for humans, but this auditory adaptation may also allow humans to participate in complex tasks such as learning to speak, play an instrument or sing, Mooney said.
This type of system can cause your brain to wait for the notes you play or sing. "Once you have a very good prediction of what should happen … you are also very sensitive to the situation if it turns out to be different."
(In the human brain, similar systems exist with movement: take, for example, figure skaters, their brains learn what movements to expect and begin to cancel the reflexes that would prevent their whirlwind from turning their head, but if the figure skater does a wrong landing, the brain considers that something unexpected and does not trigger its inhibitory neurons – and fall reflexes are appearing.)
According to Mooney, understanding this system may be beneficial for studies on psychosis. A common symptom of schizophrenia, for example, is a hallucination similar to a voice that is thought to be caused by a "broken" prediction circuit in the brain, he said. In other words, the auditory brain cells are not removed as much and shoot too much, even if there are no external sounds to trigger them.
Originally published on Science live.
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