The motor cortex and auditory cortex work together to hear words or sounds in noisy environments. The Inserm team having recently proved this has shown that the former identifies the rhythm of the sound and helps the latter to hear it better.
The motor cortex helps the auditory system to hear better. This is what a team from Inserm observed when studying the impact of rhythms on auditory perception.
"When you're out driving and the traffic light changes to amber, you know you need to stop and you get ready for it. You anticipate. Our question was along similar lines: when it comes to our hearing, what is it that allows us to anticipate what we hear and thereby improve how we perceive it? ", explains Benjamin Morillon*, co-author of this paper with Sylvain Baillet from McGill University in Montreal. "We already know that a rhythmic sound is better integrated than disorganized noise. We also know that the motor cortex is involved in the perception of all the other senses: sight, touch, hearing and smell. It allows moving the sensory organs to improve perception, particularly the eyes, tongue, fingers and, in animals, the nose. It also makes it possible to capture temporality in that it's sensitive to a recurring or predictable signal and therefore makes it possible to anticipate an upcoming event. For example, we see an object better if we know when it will appear. So, what we wanted to know was whether the motor cortex could also play a role in the link between rhythmic sound and how well it is heard. Can it capture a certain temporality of sound to increase auditory perception? In other words, could sound rhythm or voice flow be captured by the motor cortex to improve hearing? ", explains the researcher.
Use of magnetoencephalography
To verify this, the researchers exposed volunteers to various sound frequencies emitted at regular intervals and recorded their brain activity using magnetoencephalography, a technique which measures the very weak magnetic field produced by neural activity. They observed that when sound is emitted rhythmically, the motor cortex is activated and neural oscillations are directed towards the auditory cortex, increasing the perception of this sound in relation to background noise: "Our results demonstrate that there exists a form of active perception when hearing, with a motor system forming an integral part of auditory processing, clarifies Benjamin Morillon. The communication between the auditory and motor cortexes is essential for high-quality hearing", he specifies. This means that when you focus on a voice, the motor cortex captures its flow and rhythm, which helps you understand what is being said. It captures a rhythmic signal and, in a way, prepares the auditory cortex to hear the next sound. In addition, this helping hand given by the motor cortex is observed more in a noisy environment. "When it is difficult to distinguish one sound from others, such as at a party, the activation of this region is important to help the auditory system to hear. And in a quiet environment in which sounds are clearly distinguished, there is little to no activation", explains the researcher.
This discovery could have clinical implications for the hard of hearing or people with dyslexia: "Providing people who are hard of hearing with the keys to better segment speech and integrate its rhythm, could help them hear better. In the case of people with dyslexia, their brain distinguishes sounds poorly. Having them concentrate on a rhythm associated with these sounds could help them to integrate them better. Our colleague, Daniele Schön, has also observed that the symptoms of children with dyslexia decrease when they make rhythmic music. Generally speaking, moving to the rhythm of the sound stimulus in question may be a key to hearing better", concludes Benjamin Morillon.
*Inserm unit 1106/Aix-Marseille University, Brain Dynamics Institute, Marseille
B. Morillon and S. Baillet, Motor origin of temporal predictions in auditory attention. PNAS USA, online edition of October 2, 2017