The brains of the amputees upon adapting to regular usage of prosthetics recognize these artificial limbs as ordinary human limbs, finds a new study. The findings of this study are published in the Journal of Brain.
The human brain is known to take advantage of brain resources to assign various tasks to limbs automatically. Among people with only one hand, the brain area that enables us to recognize hands can also recognize a prosthetic hand, particularly among those who use a prosthesis regularly, according to the new Brain paper.
The study provides the first account of how artificial limbs are represented in the brains of amputees.
"If we can convince a person's brain that the artificial limb is the person's real limb, we could make prostheses more comfortable and easier to use."
The study included 32 people with one hand - half of whom were born with one hand, and half had lost a hand due to amputation - alongside 24 people with two hands, used as a control group, most of whom were family or friends of the people with one hand. The participants were shown images of prosthetic hands (including photos of their own prostheses) as well as real limbs. A functional magnetic resonance imaging (fMRI) scan was used to assess the participants' neural responses.
Within the visual cortex of the brain is an area that enables people to recognize hands. This area displayed a stronger response to images of prostheses among the one-handed participants, compared to the controls, particularly among those who used a prosthesis most frequently in their daily lives. This part of the brain also responded to images of prostheses that are functional but do not look like a hand, such as a hook prosthesis.
The researchers also investigated the connectivity between the visual hand-selective area and the area of the sensorimotor cortex which would be expected to control the missing hand.
They found there was better connectivity between these two brain areas in those people who used their prostheses regularly.
"Our findings suggest that the key determinant of whether the brain responds similarly to a prosthetic hand as it does to a real hand is prosthetic use. As many of our study participants lost their hand in adulthood, we find that our brains can adapt at any age, which goes against common theories that brain plasticity depends on development early in life," said the study's first author, Fiona van den Heiligenberg (UCL Institute of Cognitive Neuroscience).
The researchers say their findings offer hope, as they have not found clear neural barriers to representing a prosthesis as a body part.
"Logically I know my prosthesis is not my missing hand - it's a tool, it's a new sensation, and I accepted that. The more I use my prosthesis, the more I feel like it becomes a part of me," said Clare Norton, a study participant who has had one hand amputated.
"We think the ultimate barrier is simply how much you use the prosthesis," said Dr. Makin.
"To me this is natural, having one hand is how it's always been. The prosthesis is part of me, I don't regard it as an addition - I consider it a hand," said another study participant, John Miller, who was born with only one hand and regularly uses his prosthesis.
The researchers say their findings could provide new insights to guide rehabilitation strategies as well as prosthesis design, and potentially guide other types of augmentation technology as well.