Researchers have found in experiments that neurons within the cerebellum, which literally means “little brain” and is thought to do little more than helping us balance and breathe, respond to and learn to anticipate rewards.
Tucked in the back of the brain and believed mostly just keeping our muscles running smoothly, the cerebellum is believed to be helping control muscles mostly because of what happened when it got injured. While the neurons that make up most of the cerebellum, known as granule cells, account for 80 percent of the neurons in the brain, they only account for about 10 percent of the brain's volume; and with that density, they are difficult to study.
The density means that conventional techniques for recording cell activity don't work well.
In comparison, the cerebellum's larger neighbor, the cerebrum, gets all the attention as the most anterior part of the brain.
“If you have disruption of the cerebellum, the first thing you see is a motor coordination defect,” said Liqun Luo, an investigator at the Howard Hughes Medical Institute, a professor of biology at Stanford University and senior author of a paper published Monday in the journal Nature.
Mark Wagner, a postdoctoral fellow in Luo's lab, worked with colleagues initially to study how the cerebellum controls muscles in mice using a new technique that would allow him to record granule cells in real time. He had earned his PhD working with Mark Schnitzer, an investigator at the Howard Hughes Medical Institute and an associate professor of biology and of applied physics, who develops pioneering methods for imaging neuronal activity in fruit flies, mice and other living animals.
One method, known as two-photon calcium imaging, had the resolution Wagner needed to study mouse granule cells in action.
To get the mice to move and then to study their motor control, the team gave mice sugar water about a second after they pushed a little lever. While the mice pushed levers and received their rewards, Wagner recorded activity in each mouse's granule cells, expecting to find that activity in those cells would be related to planning and executing arm movements. The results: some granule cells did fire when the animals moved. But other granule cells fired when the mice were waiting for their sugary rewards. And when Wagner sneakily took away their rewards, still other granule cells fired.
“It was actually a side observation, that, wow, they actually respond to reward,” Luo was quoted as explaining in a news release from Stanford.
The discovery is something of a revelation, and is believed by the researchers to be a first step toward a much more exciting future for the cerebrum's largely overlooked little brother and one that could open up new avenues of research for neuroscientists interested in the roots of cognition.
“Given what a large fraction of neurons reside in the cerebellum, there's been relatively little progress made in integrating the cerebellum into the bigger picture of how the brain is solving tasks, and a large part of that disconnect has been this assumption that the cerebellum can only be involved in motor tasks,” Wagner said. “I hope that this allows us to unify it with studies of more popular brain regions like the cerebral cortex, and we can put them together.”