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Thinking merrily about what might be on the lunch menu, you walk towards the café, one step after another. Noticing footsteps behind you, you turn around to greet your friend.
In this seemingly mundane interaction, your brain performs a fascinating function: it suppresses your awareness of your own footsteps while heightening your sensitivity to other sounds, such as your friend’s footsteps. Similarly, you hear others’ voices over your own and, if you are a musician, hear other instruments over your own so that you can excel as an ensemble.
Schneider et al., a team of scientists at Duke University, sought to understand how the brain predicts and turns down sounds that arise during self-movement. To achieve this, they built a treadmill for mice that played tone pips of a certain pitch in sync with their running speed (Figure 1). Each mouse heard only one pitch during training, such that it learned to associate that pitch with its locomotion. Concurrently, the scientists recorded the mice’s brain activity from the auditory cortex, the brain region that processes sound information.
As one might anticipate based on our daily experiences, activity in the auditory cortex in response to tone pips was much reduced during run as compared to during rest. Interestingly, the brain only tuned out the tones at the pitch that the mice were accustomed to, and actually enhanced its response to tones of other pitches. This is reminiscent of our capacity to tune out our own footsteps and voices so that we can better hear those of others!
How does the brain suppress predictable sounds during movement while still responding to new sounds? This time, the scientists recorded the activity of a special type of cell in the auditory cortex: inhibitory neurons, whose job is to quiet the activity of other neurons. Recordings revealed that activity of some inhibitory neurons was heightened in sync with running speed. This suggested that running was somehow activating inhibitory neurons in the auditory cortex, and that these inhibitory neurons in turn were suppressing the activity of other auditory neurons. These inhibitory neurons could help turn down the auditory response to the familiar tone during running.
How, then, does movement turn on inhibitory neurons in the sound-processing region of the brain? The scientists hypothesized that motor cortex, the brain region that processes and controls movement, may be signaling movement information to inhibitory neurons in the auditory cortex. To test this possibility, they stimulated neurons in the motor cortex and observed the activity of inhibitory neurons in the auditory cortex. Indeed, motor cortex neurons triggered inhibitory neurons in the auditory cortex, confirming the scientists’ hypothesis that motor cortex neurons communicate with auditory cortex inhibitory neurons!
These neural recordings from motor and auditory cortices are great, but does motor cortex activity actually diminish the mice’s capacity to hear the tone paired to their running? To answer this question, the scientists trained the mice to report whenever they heard tone pips. While mice cannot verbally communicate with humans about the tone pips, they can stick out their tongues to report hearing the sound. The mice were trained to do exactly this: water-deprived mice were rewarded with water only when they correctly licked the water port in response to the tone pips. The scientists then artificially stimulated motor cortex neurons while the mice performed the tone detection task. As anticipated, the mice were less likely to report hearing the tone when their motor cortices were stimulated. Notably, this effect seemed to match that caused by running.
The scientists thus concluded that the auditory suppression that they observed during running originated from the motor cortex, which drove inhibitory neurons in the auditory cortex, which in turn suppressed the predictable pitch (schematized in Figure 2).
So next time you hear a friend approach you from behind, remember that the sounds of your own steps are being actively turned down in your auditory cortex by your motor cortex. Maybe you could even tell your friend about how this occurs on your walk to lunch (and of course, your motor and auditory cortices will be hard at work)!
Edited by Isabel Low
Schneider, David M., Janani Sundararajan, and Richard Mooney. "A cortical filter that learns to suppress the acoustic consequences of movement." Nature 561.7723 (2018): 391.