Scientists have long believed that the brain contains a “map” of the body, with different parts of the brain responsible for the actions of specific body parts. Many refer to this ordered cortical representation of the body as the sensorimotor homunculus, a term proposed by American-Canadian neurosurgeon Wilder Penfield in the 1940s, which has since become one of the most famous conceptual maps in modern neuroscience.
However, a new study could shake up scientists’ fundamental understanding of how the brain is organized.
Tamar Makin, an associate professor at University College London, and Avital Hahamy, a Ph.D. student at Israel’s Weizmann Institute of Science, studied the brains of 17 individuals born with one hand and compared them to the brains of 24 two-handed individuals.
The participants were asked to complete five basic tasks: wrap a present, remove money from a wallet and place it inside a diary, write and post a letter, fold laundry, and handle cafeteria food. Participants were told to complete the tasks as they do in their daily lives. Two independent observers rated the one-handed individuals’ reliance on different body parts.
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MRI scans were then used to track brain activation evoked by movements of different body parts, while participants were asked to move their hand, arms, mouths, and feet. The level of activity in the missing hand area was measured during each action. Also, participants were asked to rest in the scanner while the level of coupling between the activity of the missing hand area and the areas of the lips and feet were examined. Finally, the level of GABA, which is the prime inhibitory neurotransmitter in the brain, was measured in the hand areas using MR spectroscopy.
The experiment revealed that the brains of one-handed individuals undergo extensive remapping. When another parts of the body pitched in for the missing hand’s function — for instance, when participants relied on, say, their lips or feet to carry out a task that normally requires bimanual coordination — the traditional hand area of the brain, which is quite large, was used up by a multitude of body parts.
As a result, these new findings, published in Current Biology, suggest that the sensorimotor homunculus might not represent body parts, but rather, the function that each body part fulfills. “If several body parts act as a hand, they will all be represented in the area defined by this function — the hand function area,” Hahamy explained.
This brings forth the revelation that the sensorimotor homunculus may not be “as strict as previously believed,” Hahamy said. It also suggests that far away — and not only neighboring — body-part representations can be expressed in the missing-hand area.
“The fact that we see such a striking different representation in that area in congenital one-handers might suggest that this is not actually the hand area,” Makin said. “It’s kind of mind-blowing for me to think we've been getting this wrong for so long.”
“The implications, if this interpretation is correct,” she added, “are massive. But this is just a theory.”
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The study also shows that one-handed individuals do not use their intact hand more than two-handed individuals use their dominant hand. While Makin said that more research needs to be done to understand why this happens, it’s an area that she would like to study further.
Malkin speculated that if the brain can take advantage of the missing hand area to represent a multitude of body parts, then perhaps it can also be used to represent and control artificial body parts, such as a prosthetic arm.
“This is cool because it will allow us to better interface assistive and even augmentative technology,” she said.
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