Schematic of participant setup and configuration. Credit: Proceedings of the National Academy of Sciences (2025). DOI: 10.1073/pnas.2503188122

Conventional wisdom among neuroscientists suggests that the brain's motor functions are organized around the body, meaning certain brain areas control the hand; others the foot. An emerging alternative theory is that parts of the brain may be organized by the types of action, like reaching or using tools, no matter which body part is used to complete the task.

Researchers at Georgetown University recently set out to understand these theories, because knowing how the brain is organized around function versus body part has profound implications for rehabilitation and a person's return to function following a brain injury.

The findings are published in the Proceedings of the National Academy of Sciences. The work is titled "Action-type mapping principles extend beyond evolutionarily-conserved actions, even in people born without hands."

"If motor control is partly based on actions rather than body parts, it's possible the brain can use this flexibility to compensate for the loss of specific limbs, " said the study's senior author, Ella Striem-Amit, Ph.D., an assistant professor of neuroscience at Georgetown University School of Medicine.

To gain a deeper understanding of the emerging theory, Striem-Amit and a team of neuroscientists conducted a novel study with volunteers who were born without hands, and instead use their feet for everyday tasks with and without tools.

Using fMRI brain scans, the researchers showed that in these individuals, brain areas typically involved in hand tool use are still active—even though the individuals were using their feet, not their hands. This finding is consistent with the same action preference for control participants, who perform the action with either their hands or feet.

"We found that some regions in the brain care about the type of action a person is doing and not whether this action was performed with the hand or with the foot, " said Florencia Martinez Addiego, a graduate student in Striem-Amit's lab who led the study.

She said it appears this organization can arise without typical motor experience, providing evidence for action-type as a core driving factor in motor organization and development.

Interestingly, this was not true for all brain areas.

"The primary motor cortex, which is tightly mapped to the body, did not reorganize for foot-based tool use, even in people who have been using tools with their feet their whole lives, " said Dr. Yuqi Liu, a former postdoctoral fellow who also led the study. "This suggests that some brain areas demonstrate more plasticity than others."

Still, the study reveals a kind of brain organization that goes beyond the body—one that is abstract and action-centered, and that develops even without typical experience. They are also expanding on previous findings of brain organization and plasticity in blindness and deafness.

"Many brain regions may be more flexible than previously thought, especially early in development, " Striem-Amit said. "Just as some areas involved in perception respond to information from any sense, parts of the motor system may represent actions in a way that generalizes across body parts—regardless of which ones a person was born with."

Study participant Alvin Law, age 65, was born without arms, a birth defect caused by his mother's use of thalidomide, a drug commonly prescribed for morning sickness in the 1950s and 1960s.

He says that while prosthetics aren't a good fit for him because he learned to use his legs and feet for daily tasks very young, he recognizes that they are important for people who lose their limbs and that the knowledge learned from this study can improve them.

"I can't even imagine what it would be like losing a limb of any kind, to whatever cause, " Law said. He hopes his contributions can help people regain a good quality of life through improved technologies.

More information: Florencia Martinez-Addiego et al, Action-type mapping principles extend beyond evolutionarily conserved actions, even in people born without hands, Proceedings of the National Academy of Sciences (2025). DOI: 10.1073/pnas.2503188122  Journal information: Proceedings of the National Academy of Sciences