Research on people born without a left or right hand proposes a new theory on the way the brain compensates: In the absence of a hand, the brain’s “hand area” will support the activity of diverse body parts that serve as a hand.
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The study, recently published in the journal Current Biology, was conducted by researchers from the Weizmann Institute of Science in Rehovot and University College London.
The study was led by Dr. Tamar Makin from UCL and Ph.D. candidate Avital Hahamy from the laboratory of Prof. Rafi Malach, who also took part in the study. They suggest that the brain is much more adaptive than previously thought. In the study, the researchers tried to determine what happens in brain regions associated with different organs when that organ is missing.
“The old and familiar argument that we use only a small fraction of our brain isn’t true,” Hahamy says.
Many studies that have focused on supposedly inactive regions of the brain such as visual centers in blind people or auditory centers in deaf people have shown that the brain doesn't waste resources.
“In a blind or deaf person, an entire system receives no information. What’s interesting in people without a hand is that it’s not the entire motor system that’s deprived. The system works normally except for the parts associated with that hand. Only a small defined brain area is inactive, not the entire system, and that makes this model particularly interesting,” Hahamy says.
“Thus, resources devoted to the hand region may be split up among other areas of the motor system. What are the guiding principles for dividing up these resources? We suggest that the principles are functional – body parts that compensate for the absence of a hand will enjoy resources that are normally devoted to brain regions controlling that hand.”
Based on studies of amputees, it has been argued for years that inactive brain regions are “invaded” by adjacent areas responsible for the operation of existing organs or limbs.
This theory was put to the test by Makin and Hahamy. They argued that the brain’s flexibility and ability to compensate is larger and more diverse than assumed. Their study included 17 subjects who were born with only one hand, as well as a control group of 24 people born with both hands.
First, subjects were assigned routine tasks such as folding laundry, opening water bottles, opening a wrapper from around a snack, wrapping a gift, taking money out of a wallet, leafing through a book and folding a letter and placing it in an envelope.
“Our hypothesis was that if your normal behavior requires two hands but you have only one, the brain finds ways to compensate, helping you complete the task, presumably using resources normally devoted to the missing hand,” Hahamy says.
The performance of these tasks was videotaped and shown to two independent observers who were not involved in the study. They were asked to note which limbs or organs the one-handed subjects used to complete their tasks. It turned out that the main limbs or organs used were the stump, the feet and the lips.
In the study’s second stage, researchers scanned the subjects’ brains using an MRI. They asked the subjects to then perform the tasks in the same way. The results were surprising. The researchers found that in one-handed subjects, all the body parts they used to compensate for the missing hand (the stump, the feet, the lips) also activated brain areas normally associated with the missing hand.
Based on these findings, the researchers propose that the brain area normally representing the hand in one-handed subjects doesn’t necessarily represent a hand, since these were subjects without a hand. Rather, it’s a region that represents the hand’s function. In the absence of a hand, this area will support the activity of diverse body parts that serve as a hand.
“It’s amazing to think that for almost a century we misinterpreted the way different parts of the brain are organized,” Makin says. “If our theory is correct, the brain doesn’t represent body parts but the functions of such parts. This is a significant change in our understanding of the way the brain works, and of the way it can reorganize when one body part is missing.”