Mouse singers could offer hints about how the human brain handles the conversation

High in the cloud forests of Costa Rica, there is a kind of mouse that sings call / response duets, similar to high-speed round trips that men engage in a conversation. Scientists have now identified the precise brain circuit responsible for this behavior, which could give new insights into how humans converse, according to a new article in Science.

Co-author Michael Long, of the New York University School of Medicine, calls this conversational exchange "speech in turn," as if one were hitting a tennis ball over a net between two players. "If I had to summarize [the results] in one sentence, I would say this is the first demonstration of the neural mechanisms that lead to coordinated vocal speech in the mammalian brain, "he said. Our strong prediction of the study on the mouse is that a similar type of vocal coordination center may also exist in the human brain ".

Long's laboratory specializes in the study of voice communication, an area in which the human being stands out. We do not stop often to think of the complex neural processing that even a simple conversation requires. The pause time between when one speaker ends and another begins – called "floor transfer time" – is only 200 milliseconds. But one in ten people suffer from some form of communication disorder, whether it is due to a stroke or a developmental disorder such as autism.

It is difficult to study this with humans because of the enormous complexity of the human brain. However, some animal species also have eddies, including frogs, bottlenose dolphins, crickets and some primates. Alas, the typical species of the laboratory mouse (Mus musculus) does not exhibit this behavior. "They do not have significant vocal coordination," Long said.

Steve Phelps, an evolution biologist at the University of Texas at Austin, told him about the singing mice he had met during fieldwork in the cloud forest of Costa Rica. It is less singing and more of a call and answer behavior chirping, or "singing against". (It would be easy to confuse them with insects.) And it's noisy, which is unusual in mice. Typical lab mice sometimes emit vocalizations in the ultrasound field, but "it's a kind of random wheeze," Long said. "What is clear here is a sort of definite conversation exchange," probably as a means of marking a territory.

They are "kind of divas"

So they brought several mouse singers to New York. Long admits that mice require a little more luxurious accommodation: "They are singers, they are kind of divas." But give them bigger cages with better exercise equipment so they do not get fat, and a special diet of mealworms and insects, and they will thrive, even in captivity, while singing their little rodent heart.

Long and his team have carefully documented the singing behavior, noting in particular the elaborate duet. Not only do the mice take turns, but the singing of each mouse will change slightly in response to that of the other. Wanting to discover the brain mechanism behind this behavior, the team first had to determine what parts of the brain were involved. They used a technique called electromyography, stimulating different parts of the cortex and recording the activity of the vocal muscles at the same time.

It's like that et al. identified a "hot spot": a motor region at the front of the brain called orofacial motor cortex (OOM) that appears to be responsible for bending the vocal muscles. In addition to the region that requires the muscles to take notes, there are other distinct circuits in the motor cortex that make quick starts and stops possible, that is, the ability of the mice to interact in turn. role and have a conversation. . "

"The mouse actually divides this complex problem into two parts," said Long. "There is one part of the brain that creates the song itself and another, the MOC, which is important for voice coordination."

"Two individual mice can look like Simon and Garfunkel and create a duet with each other."

To test this hypothesis, Arkarup Banerjee, a postdoc and co-author of Long, measured the "hot spot" with muscimol, a psychoactive component found in certain types of fungi. This turned off the hot spot for several minutes. Once this happened, the mice could still sing but could no longer coordinate their songs in a duet in turn speaking with other mice.

They also used a technique called focal cooling. They used it to selectively cool a small part of a bird's brain to determine if this area could serve as a "timer" to birdsong. "We do not turn it off, we take what seems to be acting like a clock and slow it down," said Long. "Imagine running in molasses or something like that." And the birds actually began to sing slowly, more and more slowly each time Long and his cohorts lowered the temperature by a few degrees. Focal cooling has also been used on human epileptic subjects undergoing neurosurgery, when Long's laboratory partnered with neurosurgeon Jeremy Greenlee of the University of Iowa.

With the singing mice, they cooled only the region of the MOC, the part playing the role of coordinator of the song. "When we do that, the mice can still sing the same song, but now they come out a lot more slowly," Long said. This is an obvious proof that the song itself is actually stored in a different place, and that the OMC regulates it. "It's a way of controlling how the song works, so that two individual mice look like Simon and Garfunkel and create a duet with each other," he said. .

"From an evolutionary point of view, the study … supports the hypothesis that cortical control over vocal motor networks has been steadily increasing during the evolution of motor systems." voice in mammals, "wrote Steffan Hage of the University of Tubingen in Germany. the implications of the results of the study. "However, it seems to have started much earlier than expected." Hage thinks that the singer mouse "adds a new model system" to continue her research on voice communication in animals.

This does not mean that they have found the solution to complex communication disorders resulting from a stroke or autism. "Have we solved this kind of trouble? We do not have it. Do we have a new tool to fight? We are sure of it," said Long. "That's what fills me with hope."

So, what's next for Long and his team? "We've been studying the human brain for a few years now, and what the mouse study has taught us is that we really need to study the human brain in the context of interaction," said Long. His lab is currently working with human subjects, recording the activity of the brain as subjects talk to each other, rather than simply repeating a syllable or chain of words, or similar language tasks, commonly used in the classroom. study of language in the brain. "Just tell me what you ate for lunch today," Long said. "Let's talk back and see how the brain actually creates this type of conversational exchange."

It has long been regretted that over the past two decades, the diversity of laboratory animals studied by researchers has shrunk to focus primarily on the standard laboratory mouse, in part because many technologies have been specially designed to work with these creatures. If he had not turned to another rodent, this new knowledge may never have been exploited. "I think that over-an animal is dangerous," said Long. "It's clearly a case where the lab mouse does not have the behavior that we find absolutely fascinating.I hope that in the coming years, people will take risks and re-adopt the larger world of biology. "

DO I: Science, 2019. 10.1126 / science.aau9480 (About DOIs).