Evidence that a sense of our physical selves can develop even without the sense of touch has been uncovered in a new study by researchers in the UK and the United States.
The research shows that if someone loses their sense of touch and 'proprioception' - their sense of body position - as an adult, they may learn compensatory skills using visual cues and conscious thought, or reasoning, to move their bodies.
Someone who has never had a sense of touch or proprioception, however, can find faster, unconscious ways of processing visual cues to move and orient themselves.
A team at the University of Birmingham collaborated with researchers at Bournemouth University and the University of Chicago on the study, published in Experimental Brain Research.
The team worked with two individuals - called Ian and Kim - who have had unique sensory experiences: Ian developed a complete loss of tactile sense and proprioception (sense of body position), together called somatosensation, below his neck after an autoimmune response to an illness as a teenager. Kim was born without somatosensation, lacking the sensory nerve fibres needed to feel her body.
The researchers were interested in learning how the human brain adapts to a loss of sensory information and how it might compensate if this information is not present in the first place.
"There are a lot of questions about how we form a sense of the body and of the self," said Peggy Mason, Professor of Neurobiology at the University of Chicago. "Body and self are very integrated, and your sense of your physical self is present when you close your eyes -- but without a sense of touch or proprioception, it really wouldn't be.
"Kim has a unique condition, where she is operating on vision, hearing, and the vestibular system. She doesn't have touch or proprioception and never has. Ian is in a very different situation, because he had these senses and lost them. We were interested in whether or not a person can take visual information that's not involved in visual perception and feed it into some place in the brain that is responsible for generating a sense of your body. Essentially, can you use that to have a sense of the body when you're seeing it?"
For the study, Kim and Ian came into the lab at the University of Birmingham, along with age-matched control subjects, to participate in a number of experiments designed to assess both their mental image of their bodies as well as their unconscious sense of their bodies in space. These included reporting on the shape and size of their hands by moving a cursor on a screen to locate landmarks like fingertips and knuckles, and estimating their 'reach' distance (the length of their arm).
The study found a number of similarities and, intriguingly, differences in how Kim and Ian performed in the experiments. In the hand experiment, for example, Kim's estimation of her hand shape and size was close to the control group's, being wider and shorter than her actual hand, whereas Ian's was much more accurate.
Lead researcher, Chris Miall, Professor of Motor Neuroscience at the University of Birmingham, says: "We think the differences between Ian's and Kim's responses relate to the visual control that both of them use to navigate their environment. For Ian, this is a very conscious process and he has learned to use visual cues to continually evaluate and monitor that environment. For Kim the process is much more unconscious. She still uses the visual information, but in a more instinctive and intuitive way."
Co-author Jonathan Cole, Professor of Clinical Neurophysiology at Bournemouth University, adds: "You and I have habits and skills that are not conscious, but Ian has to think about movement the whole time."
These results indicate that if someone loses their sense of touch and proprioception as an adult, they may be able to learn compensatory skills, using visual input and conscious thought to move their bodies. However, a person who never experiences somatosensation may be able to develop mechanisms to bypass the lack of sensation and instead use unconsciously processed visual information to exert motor control.
"What we can learn from this is that you might not do it in the way that others do it, but you will find a way to make a body schema," said Mason. "You will find a way to make a sense of yourself. Kim has found a way. It's not the way that you or I do it, or the way that anyone else on earth might do it, but it's absolutely critical to have that sense of self. You have to be located somewhere. We're not brains in vats!"
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Notes to editor:
* The University of Birmingham is ranked amongst the world's top 100 institutions. Its work brings people from across the world to Birmingham, including researchers, teachers and more than 6,500 international students from over 150 countries.
* Miall et al (2021). 'Perception of body shape and size without touch or proprioception: evidence from individuals with congenital and acquired neuropathy'. Experimental Brain Research.
Journal
Experimental Brain Research