Its name means “little brain” in Latin, but the cerebellum is anything but. The fist-sized orb at the back of the brain has an outsized role in social interactions, a study in mice suggests.
Once thought to be a relatively simple brain structure that had only one job, coordinating movement, the cerebellum is gaining recognition for being an important mover and shaker in the brain.
Early clinical observations of people with movement disorders pigeonholed the cerebellum, says neuroscientist Kamran Khodakhah of Albert Einstein College of Medicine in New York City. But the “cerebellum has more than half of the neurons in your entire brain,” he says. “It never made sense that the only thing it confines itself to do is motor coordination.”
Khodakhah’s new results on social behavior, described in the Jan. 18 Science, expand that view, and add to other work on the cerebellum’s role in memory, language and emotions. The results also offer clues to disorders such as autism and schizophrenia, both of which have been linked to an abnormal cerebellum.
By finding a connection between the cerebellum and a part of the brain involved in social behavior, Khodakhah and his colleagues “solve an important gap in our understanding of the circuitry underlying disorders such as autism and schizophrenia,” says pediatric neurologist and developmental biologist Mustafa Sahin of Boston Children’s Hospital. “We’ve known for a while that the cerebellum is involved in these disorders, but we really haven’t been able to connect it to other regions directly.”
Khodakhah and his colleagues went looking for connections to one such region — an area in the middle of the brain called the ventral tegmental area, or VTA, that’s heavily involved in feeling the thrill of reward. Using molecular tools that light up certain cells with fluorescent proteins, the researchers saw that nerve cells, or neurons, from the cerebellum sent message-sending axons directly to cells in the VTA in mice.
Not only were those connections there, they were important. Khodakhah and his colleagues used a method called optogenetics to control the activity of cerebellar nerve cells that sent messages to the VTA. Activating these cells made mice feel good, the researchers found. When the mice learned that the cells were turned on only when the animals were in a certain spot, the mice spent more time there.
These cells seem to be sending a particular sort of feel-good signal, one that comes from social interactions. The team found that the cells were active when mice were in contact with a companion. When the researchers artificially turned these cells off using lasers, mice no longer preferred to hang out with a fellow mouse over an empty room, instead spending equal amounts of time in the two areas. That social deficit suggests that this particular neural highway is involved in social behavior, Khodakhah says.
Linking the cerebellum to social behavior might also help explain some connections with autism. Damage to the cerebellum is known to increase the risk of autism, a disorder that comes with social deficits. Sahin has found some deficits in the cerebellar cells of patients with a certain form of autism. The newly described pathway, from cerebellum to the VTA, “adds to our understanding of the circuitry of social behavior and reward behavior in a very important way,” Sahin says.
The cerebellum has other jobs, too, says neurologist Jeremy Schmahmann of Massachusetts General Hospital in Boston, who has studied the brain structure for decades. Along with movement problems, people with damage to the cerebellum can have trouble with memory, planning, multitasking, creativity and language, he says. That constellation of symptoms, called cerebellar cognitive affective syndrome, shows that the cerebellum has wide-ranging jobs in the brain, Schmahmann says.
An example comes from experiments published in 2018 in NeuroImage, which show that the cerebellum is important for recognizing emotions. When researchers interfered with the structure using strong magnets, people grew worse at seeing emotions on other people’s faces.
Those results add to the growing realization that the cerebellum might have its hands in many aspects of the brain. Based on the growth of the field, these expanded roles for the cerebellum are “not unexpected, but almost required,” Schmahmann says.