Understanding how the human brain works is one of the most important goals of science. And one of the first steps to uncovering its secrets lies in working out how the brain is actually organised. Our current view goes back to pioneering work by the neurosurgeon Wilder Penfield in the 1940s, which established the link between brain organisation and our ability to operate our body.
Penfield electrically stimulated certain brain areas of his patients and noticed that they involuntarily moved, or reported sensations from specific parts of their body in response. This was the foundation for the idea that the brain contains a map of the body, in which different regions control different body parts. But now, nearly 80 years later, a new study suggests an alternative idea.
There are many ways to study the link between brain organisation and its function. One is by observing the brain’s response to a profound physical change, such as losing a body part or being born without one. For example, what happens to the brain region controlling the hand once the hand no longer exists?
When scientists first started to explore this question, they observed that the brain area formerly representing the hand gets “taken over” by the neighbouring regions in the brain. In this way, the brain would reorganise through a battle for brain territory, with the winning neighbour region claiming the area for its own functional purpose.
Similarly, the unoccupied cortex in people born without a hand where sensory input was not lost, but never actually existed in the first place, gets recruited to represent the one-hander’s residual arm. These extra brain resources have been suggested to allow one-handers to utilise their residual arm in order to substitute for the missing hand function.
However, the new study by my colleagues, published in Current Biology, suggests one handers use a range of creative strategies to complete complex everyday tasks – involving body parts that are controlled by brain areas which are nowhere near the missing hand territory (refer to image). This made them question whether brain reorganisation is really determined by neighbourhood relationships in the brain.
The scientists studied 17 people born with just one hand and 24 people with two hands. Participants were asked to carry out tasks in the lab, from wrapping a present to opening a bottle or folding laundry. These seemingly mundane duties actually require immense coordination between our two hands, and therefore pose continuous challenges to people who only have one hand. One-handers successfully completed the tasks by using a diverse repertoire of compensatory strategies, ranging from using their legs, lips, prosthesis or residual arm to perform the tasks that normally involve two hands.
Since these acts included body parts that are not all controlled by areas close to that which controls the missing hand, the scientists wanted to know what is happening in the brains of these remarkable people. They asked the participants to carry out simple movements with these various body parts and tracked their brain activity.
Functional MRI scans revealed that the representations of different body parts used to replace hand function mapped right onto the cortical space that would have represented the hand. In other words, the brain area that would have represented the hand got used instead to support the lips, feet or arms.
Although this study is not the first to demonstrate a body part invading the region of the brain cortex of a different body part, it is the first to show multiple body parts benefiting from the same brain area – irrespective of whether these regions are close to each other in the brain.
So it seems there is much more flexibility in the brain than we previously thought, given that the available cortex can be allocated to body parts that share the missing hand’s functionality.
Reorganisation or reorientation?
One way to put these results into context is to ask, what if the hand area does not represent the hand area per se, but just the part of the brain in charge of the function normally carried by that hand?
If this interpretation is correct, then we have been misunderstanding brain organisation as being based on body parts rather than on body function. While speculative at this point, it is intriguing to think we could have been wrong about this for so long.
As exciting as these ideas are, we must exercise caution and remember that this is still a working theory at the moment. However, tying together these strings of discovery has important implications. Not only will it help us rewrite the theory of brain organisation, it will inform us on how we might harness brain reorganisation to make better interfaces between the brain and assistive and even augmentative technology.
Indeed, if the brain can take advantage of the missing hand territory to represent a multitude of other body parts, can it also be used to represent and control artificial body parts such as a prosthetic arm?