Siblings of those diagnosed with autism are more than 20 times as likely as members of the general population to also have autism. Some of these siblings also show evidence of autism-like but less marked cognitive and social communication problems. This suggests that autism has either an environmental cause typically found in all siblings during development or childhood or a strong heritable component, but there is not a known genetic link or a well established biological marker. A biological marker other than observed behavioral deficits would be a neurological phenotype such as might be seen in brain imaging.
Spencer et al, working from various units of Cambridge University seem to have identified such a marker.
… we show, for the first time, that the neural response to facial expression of emotion differs between unaffected siblings and healthy controls with no family history of autism. Strikingly, the functional magnetic resonance imaging (fMRI) response to happy versus neutral faces was significantly reduced in unaffected siblings compared with controls within a number of brain areas implicated in empathy and face processing. The response in unaffected siblings did not differ significantly from the response in autism. Furthermore, investigation of the response to faces versus fixation crosses suggested that, within the context of this study, an atypical response specifically to happy faces, rather than to faces in general, accounts for the observed sibling versus controls difference and is a clear biomarker of familial risk.
Autism is associated with limited reactions to emotional states of others. This study seems to identify the neurological component of that phenomenon, and sees it in individuals who are not otherwise diagnosed as autistic. This could suggest that the neural phenotype seen in this fMRI study is the root cause of the condition of autism, though of course that would have to be confirmed somehow with developmental studies.
The study included 40 adolescents between 12 and 18 years of age who were diagnosed with autism spectrum disorder, specifically with either autism or Asperger syndrome, as well as 40 unaffected siblings of those affected and 40 controls who did not seem to have autism of any kind. They were then given a task of observing faces displaying different emotional states while their brains were being scanned. The results are rather stunning. The control group showed differences in brain response when comparing happy vs. neutral faces, while both the sibling and autism groups simply did not. When comparing fearful and neutral faces, the control group and sibling group showed similar responses, but the autism group showed no change.
The lack of brain response to facial stimuli in autistic subjects was expected and has been demonstrated in previous studies. What was not expected, apparently, is the similarity between non-autism spectrum disorder siblings and the autism-diagnosed individuals. Various methods were used to ensure that the observed pattern had to do with emotional aspects of the faces being observed rather than the overall ability to parse what was being seen.
Press reports of this study emphasize that finding a biomarker in siblings proves that autism is genetic (and not, for instance, caused by vaccines). This is in fact what attracted me to the paper to begin with. However, this is simply not demonstrated by the paper. Merely having a set of observations inside the head, which seems somehow more deeply biological, does not make the observation more about genes than when the same phenomenon was only observed outside the head!
Putting it another way, The Brain Is Not Made of DNA! (That’s a post by Neuroskeptic on the same paper.)
It is important, however, that autism may include a feature that exists in non-diagnosed individuals and apparently not in the population at large, though this should be confirmed with a larger sample. If siblings of autistic individuals show this neurological “endophenotype” then there must be people out there who have the endophenotype but no close relatives with autism. If so, then the reason that some individuals express diagnosable autism could be investigated. This study thus both broadens the potential expression of an important phenotype and provides an avenue for research in figuring out what it is that causes it, and may encourage a closer look at sibs with and without diagnosed autism.
Spencer, M., Holt, R., Chura, L., Suckling, J., Calder, A., Bullmore, E., & Baron-Cohen, S. (2011). A novel functional brain imaging endophenotype of autism: the neural response to facial expression of emotion Translational Psychiatry, 1 (7) DOI: 10.1038/tp.2011.18