The study, titled Global remapping of the sensory homunculus emerges early in childhood development, was created in collaboration with several research teams in the field of neuroscience and developmental psychology from the University of Cambridge, Durham University and University College London. On behalf of the Czech Technical University, Assoc. Prof. Matěj Hoffmann and Dr. Zdeněk Straka from the Humanoid Robotics Group at the Department of Cybernetics participated in it.
Robotics from EPFL Lausanne also participated in the experiments, who developed a special pneumatic device that allows stimulating touch during brain scanning in an MRI scanner.
BRAIN BODY MAP The brain contains the so-called somatosensory body map, sometimes referred to as the sensory homunculus. It is an arrangement of areas of the cerebral cortex that process touch and other sensory information from different parts of the body.
This map was described in the mid-20th century by the Canadian neurosurgeon Wilder Penfield during operations on patients with epilepsy. Individual parts of the body are represented in the brain by areas of different sizes according to their sensitivity – for example, the hands or face take up significantly more space than the back. However, this map is much more dynamic than was long assumed.
HOW THE BRAIN ADAPTS
A MISSING LIMB Researchers used functional magnetic resonance imaging (fMRI) to monitor brain activity in children aged 5–7 and adults – both in people without one upper limb and in a control group. The results showed that in people born without a hand, there is a large reorganization of the brain map of the body. The area of the brain that would normally represent the hand does not remain inactive – instead, it begins to respond to signals from other parts of the body. The changes are not limited to the immediate surroundings of this area, but affect a wider part of the somatosensory cortex.
HOMEOSTATIC PLASTICITY: THE BRAIN MAINTAINS ITS BALANCE The key to understanding these changes is a mechanism called homeostatic plasticity. “Each neuron in the brain ‘guards’ itself to some extent to be active – to receive an adequate amount of input signals. When it loses some inputs, it starts to amplify others to maintain this level of activity,” explains doc. Matěj Hoffmann from CTU. So if a neuron in the area of the brain designated for the hand does not receive signals from the missing limb, it starts to amplify inputs from other parts of the body – for example, from the arm, wrist or sometimes even from the face.
“The brain is actually actively seeking balance in the entire network. It is not just a simple rule of ‘use it or lose it’, but a more complex regulatory mechanism,” adds dr. Zdeněk Straka.
COMPUTATIONAL MODEL FROM CTU It is this mechanism that helped to explain the computational model developed at CTU. The model created by Dr. Zdeněk Straka simulated the behavior of neurons in the somatosensory cortex. Each neuron in the model had a simple rule of homeostatic plasticity – it tried to maintain a stable level of activity.
When the researchers “removed” inputs from the hand to the model, i.e. simulated the situation of a congenital limb difference, the activity in the network began to rearrange in a way that corresponded very well to the results measured in brain scans.
“It turned out that even a relatively simple model can explain the main experimental results very well. The reviewers of the article evaluated this approach as one of its strengths,” says Dr. Straka.
CONNECTING ROBOTICS AND NEUROSCIENCE
The involvement of the team from CTU is related to their long-term research on the representation of the body and touch in the brain – and also in robotic systems.
For example, the researchers previously developed a model that allowed the humanoid robot iCub to create its own “body map” based on tactile stimuli from artificial skin.
“In robotics, we are solving a similar question: How does a system learn where on the body a touch has occurred and how does it create a map of its own body? These principles are surprisingly close to what happens in the human brain,” says Prof. Hoffmann.
ONE PIECE OF A BIGGER PUZZLE
For the humanoid robotics research group, this study is one step towards a deeper understanding of how the brain represents the body and how this representation develops.