Martha - Duncan Carmichael, a synaesthesia researcher at the University of Edinburgh is with us. Duncan, what's happening in James's brain to produce these strange sensory experiences?
Duncan - Hi, Martha. Well, we think synaesthesia is caused by a top of crosstalk between areas of the brain which wouldn't normally communicate with each other. So in most people, there's no real functional connection between areas of the brain that process different forms of sensory input, they don't activate each other. But in people with synaesthesia for example, there is clear evidence that these areas of the brain do communicate. So, if you take the example of James in the previous clip, when he reads or hears words, they activate not only the language part of the brain, but also the part of his brain which deals with taste. Whereas in people without synaesthesia, those parts of the brain are very separate and wouldn't activate each other at all, so it's this kind of cross activation between what a normally separate brain areas that seems to give rise to this synesthetic experience really.
Martha - And so, what's our current knowledge about the neural basis of synaesthesia?
Duncan - Well, evidence is starting to supports this idea of cross activation. There are some functional MRI studies that show this simultaneous activity does indeed occur actually. So, colour-letter synasthetes for example, when they read black and white letters, the colour regions of their brain become active. So in effect, they're experiencing colour without actually seeing it. And there is more recent work, which has looked at whether there are differences in the underlying neural structure of synesthetes brains and indeed, this appears to be the case. The areas which show this simultaneous functional connectivity, this cross activation. There's more white matter connectivity between those areas and synesthetes and so, we can almost say that synesthetes brains are wired up differently to non-synesthetes.
Martha - And is this a consistent phenomenon? Is it consistent within an individual and is there any consistency between individuals? For example, is a certain word associated with a certain taste or colour for example in different people or does each person have their own individual take on it?
Duncan - People often have their own individual takes. I mean, there is some kind of themes which do occur. I mean for example, 'A' often appears to be red. There is patterns which emerge, but often, people will have their own individual experiences. Often members of the same family who experience synaesthesia will disagree on the colours of letters they see. There is some kind of commonality but often, it's a very personal experience.
Martha - And is this something that varies with culture?
Duncan - I'm not so sure really. Someone did a study in Australia that took all the kind of books that had been published with different colours and looked to see whether there was themes that emerged, whether the books that people read at children kind of linked to their synesthetic experience, and I think that didn't turn out to be the case actually.
Martha - What can this tell us about the neuro-typical brain?
Duncan - Yeah, absolutely. I mean, even though synaesthesia is quite an unusual phenomenon, it can tell us quite a lot about how the normal brain processes information. I mean, the normal adult cortex is quite kind of specialised and quite modular, so the part of your brain that deals with vision for example only responds to input from the visual system or the part of your brain which deals with auditory input would only deal with input from the ears. But in synaesthesia, this doesn't appear to be the case. There's cross-talk between these different areas, and this may tell us quite a lot about how the brain actually develops. So, when you're born, these specialisations don't exist. So, what we think the study of synaesthesia can give us is a real insight into how this specialisation occurs because the synesthetic brain is obviously quite different and potentially develops along a different route. So, why do people with synaesthesia retain this capacity to have communication between different brain areas, which people without synaesthesia lose as they get older?
Martha - And so, what do we know about the inheritance and the genetics of this condition?
Duncan - People with synaesthesia typically, 40% of them have a first-degree relative with synaesthesia in their family. And there seems to be particular inheritance patterns. For example, mothers are quite likely to pass synaesthesia onto either sons or daughters, whereas fathers seem to pass it onto daughters only. One of the studies we're running in Edinburgh at the moment, really trying to identify genes that might be linked to the development of synaesthesia. Some work's been done in this area to date, but not very much. We've identified a few areas on chromosomes, so a few areas that contain hundreds and hundreds of genes that might be implicated in synaesthesia, but there's no specifically genes that have emerged that may be directly linked, and that's what we're trying to do with our current study, is recruit a lot of people with colour-grapheme synaesthesia, so people who see letters or numbers in colour and sample their DNA and see which genes might be specifically linked to the development of synaesthesia. So that's one thing we're trying to do at the moment. We suspect there's a collection of genes that are definitely involved in this process, but to date, nobody's had the ability to identify them. That's one thing we're trying to do with this particular study.
Martha - Duncan Carmichael at the University of Edinburgh.