The ability to tell where our bodies end and the rest of the world begins comes so naturally we tend not to give it much thought. In fact the brain mechanisms underlying bodily-identity are a vital part of basic social functioning. Given that social difficulties are a central part of autism, a team of US researchers led by Carissa Cascio wondered whether autism might be associated with differences in these basic mechanisms underlying body ownership.
To find out, they performed the first ever published test of how children on the autism spectrum experience the rubber hand illusion – a well-known procedure in psychology that exploits the mechanisms that give rise to feelings of body ownership.
Twenty-one children diagnosed with autism spectrum disorder (ASD) and 28 neurotypical controls (aged 8 to 17 years) undertook the illusion with an experimenter who was blind to the aims of the study. Tested one at a time, each participant sat opposite the experimenter and placed their left forearm and hand on the desk, out of sight, within a purpose-built container. To the right of their concealed left hand was visible a realistic rubber left hand.
The experimenter stroked with a cosmetic brush the participant’s hidden left hand between the second and third knuckles of their index finger and at the same time, in full view, stroked the rubber hand in the equivalent location. For two 3-minute phases the stroking was done on the real hand and rubber hand in synchrony – to the person being stroked this often gives rise to the illusory sensation that the rubber hand is their own. For another two 3-minute phases, the stroking was done out of synch, which usually spoils or reduces the experience of the illusion.
The key finding is that, unlike the controls, the children with ASD didn’t experience the illusion after the first 3-minute phase of synchronous stroking; they only experienced it after the second phase. This was tested objectively by having the children close their eyes and indicate with their right index finger where they thought their left index finger was located. Mislocating their finger towards the location of the rubber hand was taken as a sign that they’d experienced the illusion. Children with ASD may be less susceptible to the rubber hand illusion during synchronous stroking because they prioritise proprioceptive (tactile) information over visual information (the sight of the stroking).
The children were also asked to say whether they’d experienced certain sensations during each stroking phase, such as “It seemed as though the touch I felt was caused by the brush touching the rubber hand”. Here another difference emerged between the groups, with some ASD children agreeing more to this statement after the asynchronous stroking. This suggests some of them experienced the stroking as being synchronous when it wasn’t, perhaps because they have a less fine-tuned sense of whether information from different sensory modalities is being experienced in time.
The clinical relevance of the results is hinted at by the fact that ASD children with more impaired empathy scores tended to experience the rubber hand illusion even less strongly (based on their being less likely to mislocate their left index finger towards the rubber hand).
“Our results suggest that the malleability of the sense of body ownership is compromised in ASD, which may correspond to an altered cortical representation of the bodily self,” the researchers said. “This in turn may give rise to diminished capability for perspective-taking and empathy, as is seen in ASD.”
Cascio, C., Foss-Feig, J., Burnette, C., Heacock, J., and Cosby, A. (2012). The rubber hand illusion in children with autism spectrum disorders: delayed influence of combined tactile and visual input on proprioception. Autism DOI: 10.1177/1362361311430404