The latest paper from the departmental body representation reading group is here – and it’s free to read! The reading group focuses around the idea of how the body is represented by the mind. Key points include how the representation is constructed, how accurate it is, where it is maintained (if a stable representation is maintained at all), whether and how the representation is modified by tool use, and, relevant to this week’s paper, what information is used to construct it the representation.
This week’s paper examined whether the absence of sensory and motor feedback from the limbs as a result of spinal cord injury (SCI) affects the body scheme. As well as measuring disruption of the body scheme and a sense of body ownership using the rubber hand illusion (RHI), the paper also looked at whether SCI produces a sense of disembodiment and depersonalisation using the Cambridge Depersonalisation Scale (CDS), as the authors suggest there is increasing evidence that the foundations of the sense of self lie in the systems that represent the body. The authors proposed two hypotheses:
- Mismatch between a pre-existing body model and sensory input causes depersonalisation, thus patients with reduced sensorimotor input strength due to SCI would have higher depersonalisation scores.
- The rubber hand illusion is occurs because the visual perception of a rubber hand being stroked ‘captures’ the tactile perception of your hand being stroked, resulting in the perception that a rubber hand is, in fact, your own, and your arm is localised where the rubber arm is. Thus, patients with reduced somatosensory input will show a stronger effect, as they have to rely more on visual cues to localise affected body parts.
The study involved 16 healthy participants and 30 participants with SCI. SCI participants were grouped either by ability (paraplegic – impairment of lower limbs, or tetraplegic – impairment of all limbs). In further analysis, participants were grouped on the basis of either complete or reduced tactile sensation on the left hand, regardless of the presence or absence of a lesion.
In line with their prediction SCI patients had higher depersonalisation scores. However, there were only significant differences in scores on three out of 28 items on the scale. The items were: “parts of my body feel as if they don’t belong to me”, “I have to touch myself to make sure that I have a body or a real existence”, and “I seem to have lost some bodily sensations (e.g. of hunger and thirst) so that when I eat or drink, it feels like an automatic routine”. In my opinion, these questions do not seem to unambiguously indicate depersonalisation, especially question three: participants had literally lost some bodily sensations. In the absence of significant differences on the other items, I would be reluctant to conclude that SCI patients showed more depersonalisation
In contrast with their prediction, SCI patients were not more likely to experience the illusion did not show a greater effect of the RHI. There was some variation, but generally:
- Healthy people predominately experienced the complete illusion (qualitative perception that the rubber hand was their hand, and proprioceptive drift)
- Participants with paraplegia often experienced the rubber hand as their own, but did not show proprioceptive drift
- About half of participants with tetraplegia experienced the complete illusion, but about half experienced no aspect of the illusion.
When the amount of proprioceptive drift was examined statistically, healthy participants had significant effect of the illusion, showing more drift after synchronous stroking, participants with tetraplegia had only a non-significant trend in that direction, and patients with paraplegia showed no difference. However, despite the absence of an effect of the objective measure of the RHI, there was a significant effect on perceived body ownership in all groups.
Depersonalisation – a fair conclusion?
The authors suggested that a mismatch between online sensorimotor input and the cortical sensorimotor representation of the body in SCI results in depersonalisation. While broadly true, we might have expected a difference between participants with paraplegia and those with tetraplegia, which wasn’t the case. Although higher lesion results in higher scores in item 3, a correlation with one out of 28 items on a depersonalisation scale is scant evidence to conclude a link between lack of input and depersonalisation.
A mixed bag of rubber hands
The picture with the rubber hand illusion is a complex one. Although, almost half of the participants with tetraplegia did not experience the illusion, this was not borne out in the statistics, which suggested no difference between them and healthy participants. Further, even though participants with tetraplegia were less likely to report experiencing the illusion, they showed greater proprioceptive drift than participants with paraplegia. This is an interesting finding, as it suggests that while removing the sensorimotor input does affect formation of a proprioceptive representation, the accuracy or inaccuracy of this representation is independent of subjective feelings of body ownership. However, there is a more interesting finding: participants with paraplegia showed less drift than both participants with tetraplegia and healthy participants.
The authors suggest a possible explanation for the lack of proprioceptive drift in participants with paraplegia: cortical reorganisation. In the absence of lower limb sensorimotor input to the cortex, the representation of the hand expands to fill the space. A larger (and possibly as a result stronger) neural representation is more resistant to the effect of visual input, and reduces likelihood of drift, but has no effect on the illusion of ownership. This is a neat explanation, backed up by other studies that found less proprioceptive drift was associated with greater neural activity in the primary and somatosensory cortices.
The paper seems to lend weight to the idea that sensorimotor input contributes to the body scheme, but the way in which the input is used is complex, as disrupting that input does not necessarily disrupt the body scheme in predictable ways. Further, there is some suggestion that subjective feelings of ownership of a visible body-part and integration of that body-part into a body scheme are separate constructs.
Lenggenhager B, Pazzaglia M, Scivoletto G, Molinari M, & Aglioti SM (2012). The sense of the body in individuals with spinal cord injury. PloS one, 7 (11) PMID: 23209824 (free to access)