May floats over my left shoulder

Marie-Catherine Mousseau explains how the scientific study of the condition called synaesthesia opens a window on the mind and brain

Letter ‘Q’ is purple, number 7 is angry at number 8, my husband’s voice tastes like butter, what else…? Oh yes, May floats over my left shoulder. This is not nonsense talk from a deluded mind, but some of the 150 or more possible types of perception mingling experienced by synaesthetes.
Synaesthesia is a mixing of the senses experienced by some people, where for instance sounds can trigger colours or tastes. Curiously, in addition to basic sensory stimuli, it often involves cognitive concepts and learned categories, such as letters or words, numbers or time units, space location and forms. As these sensory perceptions and concepts are cross paired, part of the reality of the synaesthetes is distorted, appearing differently from the reality of other people.
Those affected rarely talk about their peculiar sensory experience, mainly because they think that everybody else senses the world exactly in the same way — or, on the contrary, that they are the only ones experiencing this. However, it is not as rare as previously thought.
It is now believed that up to one in 20 people possesses some form of synaesthesia.
Philosophy to the lab
Synaesthesia has only recently come back into the focus of science. For many years throughout the past century, the behaviourists dismissed it as a subjective phenomenon not worth investigating. But synaesthesia is now recognised as part of the human experience and, as such, is worth investigating. What is more, it provides an ideal model to understand how we construct reality. Unfolding the process underlying this cross-pairing of senses and concepts opens a window on the mind and brain — a window that might shed some light on the ancient philosophical quest for the nature of perception, how closely it mirrors reality and to what extent different people see the world in different ways.
“There’s an old question in philosophy about whether what your internal experience of the colour we call ‘blue’ is the same as my experience of blue. Maybe I see blue the way you see red. It’s an old question, but it turns out that the truth may run even deeper. With synaesthesia, we can study and quantify the way that someone’s reality can be different from someone else’s. It has moved from philosophy to the laboratory,” said Dr David Eagleman, Director, Center for Synesthesia Research, Baylor College of Medicine, USA.
According to Dr Kevin Mitchell, a researcher at TCD’s Developmental Neurogenetics, there might currently be between 10 an 12 groups doing work on synaesthesia worldwide.
Dr Mitchell is part of the Dublin group, but because it is such a small community he has regular contacts with the others, including Dr Eagleman, a US neuroscientist and author of a recent book on synaesthesia. “I know David well. His research is very overlapping with my own, although we disagree on the likely basis of the condition,” he said.
What could this basis be? The researchers combined several scientific approaches in an attempt to find out more about the underlying mechanism of synaesthesia.
Testing
First of all, they had to prove that synaesthesia was a real condition and not an invention of some creative minds – a task that is not so straightforward when you are dealing with a subjective experience.
To that end, the scientists used two interesting properties of synaesthesia: it is very stable – most synaesthetes report that it ‘has always been there’– and it is idiosyncratic, that is very specific to each individual (‘J’ can appear green to someone and red to someone else). As a result, a real synaesthete will always pinpoint the exact same green matching the letter ‘J’ at any time from among a wide range of different greens.
Dr Eagleman thus developed the Synesthesia Battery (www.synesthete.org), a free online test by which people can determine whether they are synaesthetic.
Brain scan
A second approach used brain scans to confirm that something different was indeed going on in the synaesthetic brain. For instance, when studying brain activation to linguistic-colour synaesthesia (i.e. when letters or words are seen or heard as colours), it was discovered that the visual area V4 responsible for colours was activated in response to a word as well as the adjacent area specific to words.
This finding gave some clues as to the underlying mechanisms. “The hypothesis has thus emerged that the condition results from cross-activation of one specialised cortical area by another, normally separate one,” explained Dr Mitchell. He went on to highlight the two major mechanistic explanations – that is, two possible ways by which this cross-wiring might appear: “The first is that there are extra axonal connections between the relevant cortical areas in synaesthetes; the second is that such connections exist in all people, but are disinhibited in synaesthetes.”
Supporting the disinhibition model is the fact that even ‘normal’ people (that is non-synaesthetes) have a tendency to match stimuli from one sense to another – for instance, matching higher-pitched sounds with brighter colours. Also, it is possible to induce a ‘pseudo-synaesthetic’ state in non synaesthetes with some hallucinogens such as LSD. This also supports the functional model, whereby cross-wired connections present in everybody are normally inhibited in non synaesthetes (and some drugs could suppress this inhibition).
But while Dr Eagleman is a proponent of the disinhibitory (functional) theory, Dr Mitchell is a proponent of the structural one, which hypothesises excess neuronal connections in synaesthetes.
“I think the disinhibition model lacks a mechanism for specificity. I would expect epilepsy, if anything, to result from widespread disinhibition.”
He continued: “I like the structural model because I know of many mutations in mice that do result in excess connections between areas devoted to different sensory modalities — from either excess production or misrouting of axons or from failure to prune them — either mechanism is perfectly plausible. The nice thing about those is that the cross-connections are very specific. In fact, we’ve a bet of a hundred dollars on which theory is correct!”
But there are no hard feelings. “We’re very friendly about it and [are] both willing to be proven wrong. We are both very open to the idea that either, or something else entirely, could be true.”
He also admitted that at the moment, leaning one way or the other was based more on hunches than any real data. “I think once we find the gene(s) involved, we will have a much better idea of what’s going on.” he noted.
Down to the genes
Indeed, the third approach, the genetic one, has added a critical thread to the unfolding of the mechanism underlying synaesthesia: the condition has been shown to often run in families.
Dr Mitchell and his team investigated inheritance patterns in synaesthesia. They found that 42 per cent of synaesthetes have a family member with it. What is more, they discovered that very different types of synaesthesia, such as linguistic-colour and taste-shape synaesthesia, can co-occur within the same family.
These are major findings. “They strongly suggest that the underlying predisposition to synaesthesia (regardless of type) is genetic,” the authors commented. “The fact that many variants of synaesthesia exist in the same family suggests that all forms of synaesthesia fit within a spectrum and share a single underlying genetic mechanism,” they concluded. This would be a mechanism whereby mutation of some gene(s) results in altered organisation or function of circuits mediating perception.
Other recent findings — this time from Dr David Eagleman’s lab — have modulated this result, indicating that the types of synaesthesia tend to cluster in individuals and well as families. “If you have coloured letters, you’re likely to have coloured months, weekdays or numbers – but you’re no more likely than anyone else in the population to have, say, hearing-to-taste synaesthesia,” Dr Eagleman explained.
“We hypothesise that there may be several different conditions — and perhaps several different genetic bases — underlying the different clusters of synaesthesia,” he concluded.
Dr Mitchell commented on his colleague’s result: “I think David’s findings on familial clustering seem to contradict ours somewhat, but actually both may be true — there may be some families with only one type of synaesthesia (indeed, most of ours are like this) and others with multiple types.”
In any case, this does not change his main conclusion: “The fact that any families exist with multiple types reinforces the idea that they are all related at a mechanistic level.”
Nature versus nurture
Then the question remains, what accounts for the differences between individuals even within the same family? Why is it that while a mutation in your gene makes you see, say, the letter ‘J’ as green, the same mutation in your mother results in her seeing the same letter as red, and in your brother perceiving coloured weekdays and months?
Dr Mitchell suggested that synaesthetes might inherit a general tendency to either cross-activate or disinhibit normally functionally separate cortical areas; but then developmental variation or early experience would play a major role in determining the final characteristics that emerge.
For instance, the heterogeneity of synaesthesia might be the result of ‘random cerebral variations’ between people, causing slightly different arrangement and connections between the cortical areas involved.
Environmental factors may also play a role. Although this role may not be that strong, Dr Mitchell thinks that cultural factors and experience can modify the outcome, and especially can bias some of the cross-associations that arise (like the letter ‘Y’ being more commonly yellow, in English speakers — still only 50 per cent, however).
So synaesthesia gives us some clue about the important interaction between nature and nurture. These are likely to be crucial in many other psychological or psychiatric conditions such as autism and dyslexia, where, as Dr Mitchell puts it, ‘cascading effects of a primary mutation over cognitive development determine the eventual phenotype’.
The binding problem
Thus, synaesthesia, though seemingly odd, has a lot to teach us. Not only has it provided a model accounting for the differences in the way people see the world — which, amazingly, could be down to a few genes — but on a more general note, it also serves as a powerful inroad into how the normal (non-synaesthetic) brain works.
In particular, it might help us to understand how the brain binds all the different senses together to come up with a unified reality… a mystery known as the binding problem, which, so far, has remained unsolved by scientists.
Multisensory learning aid
Complete separate research conducted in a CNRS laboratory in Grenoble, France, has highlighted the importance of multisensory stimulations in learning. For instance, we are learning much more efficiently the association of one letter (grapheme) to its corresponding sound (phoneme) if we can explore the letter with touch as well.
Touch would play the role of cement between vision and audition. Would synaesthesia be an extension of such multisensory integration? The question remains open.
l More on synaesthesia:
Kevin Mitchell’s website:
www.gen.tcd.ie/mitchell
David Eagleman’s website: www.eagleman.com
New Irish Synaesthesia Network: www.facebook.com/group.php?gid=119308514768834
Blog for discussion of synaesthesia and related topics in brain wiring: http://wiring
thebrain.blogspot.com/
l The views expressed above are those solely of the author and in no way may be deemed to reflect the views or policy of either MSD Science Centre or MSD.