Synaesthesia is the condition when the stimulation of one sensory modality automatically evokes a perception in another unstimulated modality, for example written and/or auditory stimuli triggering colours.This occurs in 4% of population. Developmental synaesthesia shows familial clustering and some studies suggest excessive neural connections between certain regions of the brain.What is this condition to do with autism? Is it more common in autism?
Simon Baron-Cohen and team at Cambridge carried out the first prevalence study of synaesthesia in autism to see whether these conditions are independent.
172 adults with autism and 123 typical adults participated in this study.They completed the Synaesthesia Questionnaire, Autism Spectrum Quotient and the test of Genuineness-Revised .
Results: 19% of those with autism met criteria for Synaesthesia as compared to 7% in controls. Grapheme -colour and sound-colour synaesthesia were the common forms.
One assumption is that autism is associated with a reduction in long-range neural connections, alongside an increase in local, short-range connectivity .This increase in short ranged connectivity might explain the increased prevalence of synaesthesia in autism.It has also been suggested that savants ( autistic individuals with exceptional skills) might have both autism and synaesthesia and that this could explain the enhanced/unusual skills.Linkage studies have indicated chromosome 2 involvement in both conditions.
Is this increased prevalence due to higher sensory sensitivity ( and hence note reporting) seen among adults with autism? Not all autistic individuals with synesthesia ( as judged by the questionnaire) claimed to have such experiences indicate this might not be the case.
Limitations– These findings are based on self report. Study included high-functioning adults only. Consistency tests ( to see that the unusual experiences are consistently reported in the tests) were completed by a small minority of the sample only.It is not known whether the link is specific to autism.
Conclusion: Synaesthesia is significantly more common in adults with autism .It is likely that these conditions share some underlying biological causal factor.(e.g.: local neural hyper-connectivity,faulty axonal pruning, differences in axon guidance etc).
Summary of the article:
Is synaesthesia more common in autism? Baron-Cohen S, Johnson D, Asher J, Wheelwright S, Fisher SE, Gregersen PK, Allison C. Mol Autism. 2013 Nov 20;4(1):40