|Colour and sensation @ Wikipedia
Synaesthesia according to the UK Synaesthesia Association is best described as a "union of the senses" whereby "two or more of the five senses that are normally experienced separately are involuntarily and automatically joined together".
In other words, some people might experience certain words as tastes or see colours when they hear music. The BBC follow-up their coverage of the Baron-Cohen paper with a story about a man who can taste the London Underground map and his efforts to re-design the Tube map according to 'sausage and eggs' and 'putrid meat'.
In their paper, the authors tested the suggestion of overlap between autism - adults with "high-functioning autism or Asperger's syndrome" - and synaesthesia by asking participants with autism and asymptomatic controls to complete a questionnaire on any experiences of synaesthesia alongside something called the Test of Genuineness-Revised (ToG-R) "to validate any self-reported auditory-visual forms of grapheme-colour (GC) and sound-colour (SC) synaesthesia and was sent to all participants in order to detect true and false positives and negatives". That being said, response rates to the ToG-R were not brilliant; as per another quote: "Telephone follow-up to find out why ToG-Rs were not being completed revealed that participants with autism reported fatigue from the 241 possible choices".
Based on their data however, they report that "The rate of synaesthesia in autism (18.9%) was almost three times greater than in the typical sample (7.22%)". They also suggest that this figure might be an under-estimate given that some participants with autism "claimed they did not have synaesthesia, but were judged by the experimenters to have synaesthesia on the basis of their questionnaire responses".
I have to say that I'm very interested in these results. Sensory issues with regards to the autism spectrum are something of growing importance given the effect that they can have on both how a person perceives the world around them and also how such perceptions can impact on day-to-day functioning and quality of life. Even the latest version of DSM saw fit to include sensory issues in their latest redefinition of the autism spectrum (see here).
That being said I still want to see more on this topic done before I totally commit to the likelihood that synaesthesia is frequently prevalent in cases of autism. Questions about how representative these results are for example, to those who present with a greater severity of autistic symptoms or with comorbid learning disability remain as does the question of mechanism and whether there are genetic or biological commonalities to be seen.
I'd also hazard a guess that synaesthesia is just the tip of the iceberg when it comes to sensory and visual issues and autism; indeed whether as per the exclusion criteria for acquired synaesthesia in the Baron-Cohen study "the person had any medical conditions affecting vision". Well, if the Ikeda results are anything to go on, yes would probably be the answer for anything up to 40% of those with autism. How such vision issues would impact on the presentation of synaesthesia remains to be seen. And finally there is the question of whether any other -sia conditions also reported in cases of autism might also show some involvement, as per the chatter about prosopagnosia (face-blindness)?
* Baron-Cohen S. et al. Is synaesthesia more common in autism? Molecular Autism 2013, 4:40
Simon Baron-Cohen, Donielle Johnson, Julian Asher, Sally Wheelwright, Simon E Fisher, Peter K Gregersen, & Carrie Allison (2013). Is synaesthesia more common in autism? Molecular Autism, 4 DOI: 10.1186/2040-2392-4-40