This is a post i stole from one of my favorite blogs mindhacks.com
Cognitive Daily coversa super-elegant study that helps us understand whethersynaesthesia is really just a case of 'crossed senses' or whether the perceptual blending effect requires the person to have processed some of the meaning of the triggering experience.
The traditional explanation of most types of synaesthesia is that the brain's sensory areas are overly connected, so activation of one sense triggers activity in another area which causes the experience. However, there has been some recent evidence that synaesthesia doesn't work purely at this basic sensory level.
For example, a recent study by neuropsychologist Jamie Ward and colleaguesreported that letter-colour synaesthetes needed to be concentrating on the letters to trigger colours - seeing them 'out of the corner of the eye' didn't work - suggesting there must be some involvement of focus and concentration and not just a reliance on incoming sensory information.
To test the idea further, it would be ideal to be able to separate out experiences where we process just sensory information and experiences where we also understand meaning but as we tend to deal with both at once, this is not easy to do.
But this new study managed to do exactly this in people with colour-speech synaesthesia, where affected people experience colours when they hear specific words, using a perceptual illusion called the McGurk effect.
It's an intriguing effect that you can see in action on the Cognitive Daily page, but essentially, it shows that seeing someone mouth a word affects what we hear, so if we are played the syllable 'gah', but see someone mouthing 'bah', the brain makes a compromise and we experience hearing the syllable 'dah'.
So the effect is a perfect tool to separate out sensory information and meaning, because the researchers can play exactly the same sound but change what word the participants hear simply by showing clips of people mouthing different words.
If colour-speech synaesthesia works only through crossed-senses then the McGurk effect should make no difference to the colours because the exact same sound is played each time, but if this form of synaesthesia is triggered by meaning, the colours should differ because the McGurk effect changes which words are perceived and understood, despite the identical sound.
This is exactly what the researchers found, providing additional evidence that synaesthesia is not just a sensory confusion, it is based in how the brain understands meaning.
It's an incredibly elegant study and the Cognitive Daily piece covers it equally as elegantly.
Link to CogDaily on 'Synesthesia and the McGurk effect'.
Link to PubMed entry for study.
pdf of study full text.