What are body representations?
Our ability to know where our limbs are in space is important for making accurate movements.
Traditionally, this field, known as proprioception, has focused on its components that have well-characterised sensory receptors. For instance, we have receptors on the retina for vision, muscle receptors signalling joint angles and muscle force, and the vestibular system in the ear contributing to balance.
However, without a map to compare these signals, we are not able to interact with the world. These maps or ‘body representations’ exist in our heads and are mostly generated from experience. The most studied of these brain maps is in the primary somatosensory cortex (S1).
Evidence for body representations
There are a number of key pieces of evidence for the existence of body representations. You may have experienced yourself the feeling of enlarged lips from anaesthesia after a visit to the dentist. Some evidence suggests that the anaesthetic weakens the response of inhibitory neurons in the brain region that correspond to the lips, blurring the border between the lips and the surrounding skin. Similar mechanisms were proposed to explain the experience in amputees of feeling a limb that does not physically exist. This ‘phantom limb’ can persist for years, causing pain and discomfort. Additionally, touch strokes applied to the face in upper limb amputees are often felt on the phantom limb.
In the ’90s, scientists mapped S1 using magnetoencephalography (MEG) in upper-limb amputees. The MEG suggested that in upper-limb amputees, the facial regions invaded the arm regions. This was promising as, at the time, it confirmed observations in monkey studies of mass cortical reorganisation. However, a study conducted last year accounted for some potential methodological flaws in this string of experiments.
Researchers measured S1 activity using functional MRI to measure activity in the brain during a task where participants moved the lips or the hand. They found that while the lip representation moved towards the hand, it did not encroach upon it. The incongruence between these results prompts further study on the mechanisms of reorganisation in S1 and of higher-order body representations.
Sense of self and control of body actions
Another important aspect in proprioception is the sense that self is located in the body. A part of the driving force for research on this was a new illusion and measure of embodiment. Known as the rubber hand illusion, the participant’s arm is placed out of view while it receives touch in synchrony with a rubber hand that is in the view. Participants made ratings for statements about their limb on a seven-point scale from ‘disagree strongly’ to ‘agree strongly’. Ratings for ‘I felt as if the rubber hand were my hand’ fell between ‘agree’ and ‘agree strongly’. The coincidence in timing of the touch and visual input is integral to the illusion, as it degrades if the stimulation is asynchronous. It is known that connections between neurons (synapses) are strengthened when they receive sensory input at around the same time. Research is uncovering that the changes at the synaptic level drive the cortical reorganisation previously observed.
Additionally, we have a sense of agency or control over our limbs. It wasn’t until the turn of the millennium that the sense of self and agency were distinguished. There is more literature on this topic in the last five years than there was in the preceding 50 years. Since this time, scientists have used brain-machine interfaces to control robots and measured the contribution of agency to embodiment. The implications of further research in this field for prosthetics and human-robot interactions will be far-reaching.
Treating disorders of body representations
There are over 40 known medical conditions in which body representations are impaired, such as anorexia, schizophrenia and stroke. Little is known about the potential of experimentally induced illusions to treat these conditions. For instance, a recent meta-analysis conducted by scientists at the University of South Australia found that a number of illusions, including the rubber hand illusion, were ineffective at treating phantom limb pain. This highlights the need for better understanding of the neural mechanisms underlying phantom limb pain and other embodiment disorders. There is some good news, however, such as a stroke patient who completely regained the ability to correctly perceive touch location five years after stroke. Recent studies have been able to change perceived location using different stimulation patterns on the skin, but whether these recently discovered illusions speed recovery has yet to be tested.
Learning from mice
A team in Japan published the first study of its kind on body ownership in mice. Mice were stroked simultaneously on their tail and a rubber tail that was in their view, as in the rubber hand illusion. When the rubber tail was grasped, mice were more likely to respond by moving their head toward the rubber tail. This response was greatly reduced when the conditioning was asynchronous. These same scientists report in a separate study that c-Fos expression is increased in mice following temporally congruent stimulation. It is interesting to consider that in humans the parietal cortex is important for embodiment, but this region is substantially smaller in mice. Studies using this method will open the doors to understanding neural activity underlying embodiment and treating disorders of embodiment, as until this point it has been difficult to understand in humans.
This rapidly growing field requires further well-designed studies that document the relationship between brain maps and perception of our bodies. Once this is better understood, we will be able to tailor and track the effect of interventions on disordered body representations. Animal studies may lead the way in this regard, initiating collaborations between biochemists and sensory psychologists. To be of use, this research will have to account for anatomical and functional differences between humans and animals. Additionally, future studies on agency and body ownership will prove insightful for human-robot interactions.
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