Since the 1930s, the so-called homunculus map has shown how different parts of the brain's motor cortex may control movement to different parts of the body. But it may be missing an important network  

Human brain with a highlighted precentral gyrus, the location of the motor cortex

Our movements may be controlled by two distinct networks in our brain, rather than just one.

For nearly a century, we have known that the motor cortex – a relatively thin strip of tissue in the centre of the brain that runs across both hemispheres – controls our body movements.

In the 1930s, neuroscientists Wilder Penfield and Edwin Boldrey electrically stimulated the brains of people undergoing brain surgery, showing that different parts of the primary motor cortex control different parts of the body. They also found that these control areas are arranged in the same order as the body parts they direct, with the toes at one end and the face at the other, as depicted by the so-called homunculus map.

Evan Gordon at Washington University School of Medicine in Missouri and his colleagues wanted to use modern technology to look into the Penfield-Boldrey idea in more detail. They therefore took high-resolution MRI scans from seven people for between 12 and 15 hours while they lay in a scanner looking at a cross symbol.

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Analysing the participants’ brains while they were largely stationary and not engaged in a task, such as reading, meant that their brain activity data was less complex, allowing the researchers to better observe which parts of the brain work together, says Gordon.

The Penfield-Boldrey research suggests that the primary motor cortex is just made up of the homunculus map. But Gordon and his team found that the cortex also has three regions interspersed within the map that appear to work together to coordinate movements to the centre of the body, such as the shoulders and abdomen.

Next, the researchers scanned the brains of two of the participants again as they carried out 25 movements, such as winking and swallowing, while lying down in an MRI scanner.

These scans generally support the information outlined in the homunculus map. In simple terms, they show that one region of the primary motor cortex is linked to movements in the lower body, one to hand-related movements, which includes shoulders, and another to facial movements. This corresponds with what Penfield and Boldrey described, and makes up one of the two distinct networks in our brain that control movement.

The homunculus map, depicting how different regions of the motor cortex control movement in different parts of the body

From these two participants’ scans, the researchers have shown the regions between those on the homunculus map in more detail, with these becoming particularly active when they moved their midsections.

The researchers have named these in-between regions the somato-cognitive action network (SCAN), which makes up our brain’s second distinct network.

They then looked for signs of the SCAN in brain activity data from scans taken from several major databases, such as the Human Connectome Project and the UK Biobank, finding that it appears to be present in all of the samples they analysed.

The SCAN network seems to work alongside the first network in the primary motor cortex, which controls movements related to the hands, feet and mouth, says Gordon. Movements to other parts of our body, such as our hips or back, may be controlled by other regions of the motor cortex, he says.

In another part of the experiment, the researchers scanned the brains of three children – a newborn, an 11-month-old and a 9-year-old – to see if they had the SCAN.

While they didn’t see the network in the newborn, it was in the 11-month-old and 9-year-old, suggesting that it develops as a baby grows. “An 11-month old can purposefully move his arms and legs around,” says Gordon. “Whereas a neonate has very little control of their body’s movements.”

The team wonders if SCAN may be involved in conditions such as Parkinson’s disease, which affects movement, leading to symptoms like tremors. While further research is required, SCAN could be a treatment target for Parkinson’s disease and other movement-related conditions, such as ataxia, says Gordon.

“This is a novel contribution to the long-standing neuroscientific question in neuroscience regarding the scheme that neurons in [the] motor cortex use to control movement,” says Patrick Haggard at University College London. To verify these results, it would be useful to record SCAN neurons in experiments where people or other animals are free to move, not confined to a scanner, he says.

Journal reference: NatureDOI: 10.1038/s41586-023-05964-2