Within the brain’s frontal lobe lies the primary motor cortex, a sliver of neurons that coordinates movement. Beginning in the 1930s, scientists developed a map of this brain region called a homunculus map, depicting how different sections of the primary motor cortex controlled specific parts of the body.
But now, in a paper published Wednesday in the journal Nature, researchers suggest the homunculus map is missing some key parts. Based on fMRI data, they argue the primary motor cortex also has distinct regions that play a role in whole-body action planning, rather than corresponding to a single body part.
“This study is very interesting and very important,” Michael Graziano, a neuroscientist at Princeton University who did not contribute to the research, tells Nature News’ Max Kozlov. The primary motor cortex appears not to be “just a simple roster of muscles down the brain that control the toes to the tongue.”
That description is essentially what the homunculus map looks like—it depicts a warped human body stretched over a slice of the brain, with each body part controlled by the neurons that lie below it. Researchers created the map by electrically stimulating the brains of surgical patients and noting where patients experienced corresponding feeling or movement. Body parts that humans have fine control over, such as our fingers and tongue, appear larger-than-life on the map, because they take up a disproportionate amount of the primary motor cortex. Meanwhile, areas we have less complex control over, such as ankles, appear smaller.
But for decades now, research has suggested that the primary motor cortex could also be responsible for more coordinated movements.
“There is a whole cohort of people who have known for 50 years that the homunculus isn’t quite right,” Evan Gordon, a co-author of the new study and a neuroscientist at Washington University, tells Science News’ Nora Bradford.
In the new study, the researchers scanned participants’ brains while they lay in an MRI machine. Seven people were measured while lying still, and two of those participants were also monitored while performing simple movements, such as winking or flexing a muscle.
The results suggest that the three general sections of the classic homunculus map—which control the face, arms and lower body, respectively—should be interspersed with three areas more related to whole-body control. These newly proposed sections may be connected to movements involving multiple body parts or activate during movements related to the midsection, writes New Scientist’s Jason Arunn Murugesu.
The whole-body control regions were connected to each other, as well as to parts of the brain in charge of action planning, pain and blood pressure regulation, per Nature News.
“We thought we knew everything about this region,” Angela Sirigu, a neuroscientist at the Institute of Cognitive Science Marc Jeannerod in France who wasn’t involved in the study, tells Nature News. “But its organization is much more complex than we have traditionally thought.”
To support their findings, the team also looked at previous data from three large fMRI studies. By examining these brain scans from about 50,000 people, they found more evidence for the existence of the new brain regions, per New Scientist.
Finally, the researchers looked at children’s brains to study how the primary motor cortex develops. A newborn baby did not have the three whole-body control regions, but an 11-month-old baby showed signs of them. Scans of a 9-year-old child looked similar to those of adults in the study, suggesting that these newly identified brain regions develop early in life.
A deeper understanding of the homunculus could help improve treatments for stroke- or injury-related damage to the primary motor cortex, Nature News reports. And the team wonders if the whole-body control areas could one day play a role in treating Parkinson’s disease, which has movement-related symptoms, per New Scientist.