The sweet smell of purple
New Scientist, 13 August 1994
still remembers when her father painted the family house. The paint was
white. But it smelled unmistakably blue. Then there's Kristen, who
tastes words, complete with texture and temperature. Often the spelling
affects the flavour. 'Lori', for instance, tastes like a pencil eraser,
but 'Laurie' tastes lemony.
These two women have synaesthesia,
which means that their senses mingle. A stimulus to one sense gives
rise to entirely unexpected responses in others. A scent, in Muriel's
case, triggers not only smell but colour. Grass might smell purple - or
roses grey. The taste of chocolate might bring on a prickly feeling, of
pins jabbing into flesh. Sounds, too, evoke colours and shapes. One
synaesthetic can picture the green loops of a helicopter's drone;
another is assaulted with red daggers on hearing a bell.
people with the condition - there are an estimated one in 25 000 -
simply hear letters, numbers and words in colour. To them, it is a
natural, unobtrusive part of life, like seeing. Synaesthetics aren't
impaired, don't 'suffer' and for the most part enjoy their little gift.
Amusing and quaint though it is, synaesthesia, neurologists agree, is
rather unimportant - at least, it would be if it hadn't fired a debate
that goes to the core of how the brain works.
The problem is
that synaesthesia is not easily explained. How does this sensory mix-up
happen? And where in the brain does it take place? According to the
widely-accepted view, the various functions of the brain, senses
included, are handled in specialised neural 'modules' - theoretical
constructs which make it easier to understand how the brain works. And
each module consists of a number of areas of the brain wired together
by neural pathways. Information to do with hearing, for instance, is
processed in one module, while vision is processed in another.
neural pathways involved in this division of labour are believed to
centre on the cortex, the sheet of tissue that forms the outer shell of
the brain's two distinctive hemispheres. The unusually large size of
the cortex in humans compared to other animals suggests that it is the
seat of the rational thinking that sets humans apart from other
creatures; in other words, that it is the human brain's 'highest' and
most sophisticated structure.
But synaesthesia seems to defy
this segregation: at least two senses are interacting. When Muriel
'smells' the colour of paint, does the activity take place in the
brain's 'smell centre' or in the module for vision? Or somewhere else?
Is there something special about her brain or could any old brain do
this? Or could it be that ideas of modularity and cortical supremacy
are off the mark?
A team of psychologists and neurologists in
London, headed by Simon Baron-Cohen of the Institute of Psychiatry, has
been looking at the condition for six years. They have come to the
tentative conclusion that the brains of synaesthetics are biologically
distinct. 'In people with synaesthesia,' suggests John Harrison, a
neuropsychologist at Charing Cross Hospital and a leading member of the
team, 'perhaps unusual pathways link centres of the brain that process
auditory inputs to areas processing vision.' In other words, their
brains are cross-wired. 'It's a perfectly good explanation.'
not according to an American neurologist. Richard Cytowic of Capitol
Neurology, a private clinic in Washington DC, says that explaining
synaesthesia within the standard view of the brain is like explaining
the orbit of Mars by adding another 'sphere' to accommodate the theory
that the Universe is Earth-centred. Modules do not exist structurally.
So why, asks Cytowic, are researchers going out of their way to protect
them? Why invent 'special pathways' to explain synaesthesia? Why should
hypothetical modules drive experimentation? It is phenomenology, he
believes, which should drive theory. 'The fact is that the current
model can't explain it,' he insists. 'So that model has to be
The brains of synaesthetics, Cytowic proposes, may
not be fundamentally different at all. Synaesthesia may instead be
telling us something new about human brain physiology. Far from being a
hierarchy, dominated by a 'rational' cortex where functions such as
sensory perception are localised and separated, the human brain may be
a much more integrated and democratic unit.
In trying to explain
synaesthesia and sensory perception, says Cytowic, researchers in the
past have placed too little emphasis on the limbic, or 'emotional',
brain. This is a collection of neural structures that lies buried
beneath the cortex and has strong channels of communication with other
parts of the brain and with hormonal systems. Cytowic believes the
limbic system not only has a hand in coordinating communication in the
brain, but also determines the salience of the information being
processed. Far from being subordinate to the cortex, the limbic system
is central to human brain function. He calls this revised model,
largely based on the ideas of other neurologists, the 'multiplex'
brain. And the quest to explain synaes-thesia, he says, has helped to
Scarlet trumpetsSynaesthesia has been a
popular medical mystery for almost 300 years. John Locke, the
17th-century philosopher, knew a blind man who described scarlet, a
colour he'd never seen, as ' . . . the sound of a trumpet'. An
18th-century English ophthalmologist noted a blind patient who claimed
to 'hear' colour. The first systematic study was done by the geneticist
Francis Galton, of University College London, at the end of the last
century, when he compiled the reported experiences of a number of
synaesthetics. Then, for almost a hundred years, with scientists
lacking the tools to discover more, synaesthesia fell out of fashion.
first encountered synaesthesia in 1980 when he met Michael Watson, a
man who boasted of coloured orgasms and could taste the points of a
lemony flavour. Vaguely familiar with the condition - from long hours
spent in mouldy medical school libraries - Cytowic's curiosity was
piqued. The joint mission of doctor and subject to understand
synaesthesia forms the backbone of Cytowic's second book on the
subject, The Man Who Tasted Shapes. It also prompted him to turn his
back on orthodox brain physiology
Just a few years later, a
76-year-old painter from Wales, Elizabeth Stewart-Jones, presented
herself to the British Psychological Society. In an intriguing letter,
she described her lifelong ability to hear words and sounds in colour.
Each word in her vocabulary had its own distinct complexion. Simon
Baron-Cohen invited her in for some tests - and has been leading
mainstream British research on the condition ever since. He and his
team believe that synaesthesia can be explained without overturning the
'division of labour' model of how the brain works.
agree that the condition is genuine and diagnosable. When Cytowic
squirted flavoured liquids onto Michael Watson's tongue, for instance,
the res-ponses were both vivid and consistent. Quinine always felt
'like polished wood' and Angostura bitters was 'an organic sphere with
tendrils'. Similarly, one simple test with Stewart-Jones convinced
Baron-Cohen that synaesthesia was real. He read out 103 words and
letters and asked her to describe the colours they evoked. The word
'Moscow', she said, was darkish grey, with spinach-green and a pale
blue in places. 'Daniel' was deep purple, blue and red and shiny. By
contrast, the control subject gave simple and sometimes predictable
responses: 'fridge', unsurprisingly, was white. When retested two weeks
later, the control subject replied with 17 per cent accuracy.
Stewart-Jones, on the other hand, after 10 weeks and with no prior
warning, described every word exactly as she had the first time.
published his results in the journal Perception. When he reported the
findings in an interview on Radio 4, over 200 women (and two men) wrote
in, claiming to have syn-aesthesia - an astonishing response, given
that it was a science programme with at least equal numbers of male and
female listeners. Others responded to newspaper articles. Anecdotal
evidence from this databank of people who claim to have synaesthesia -
the London team has the names and details of nearly 600 to date -
sketches a portrait of the condition.
Objects of ridiculeThe
majority of synaesthetics are female. Most have experienced their
coloured words, sounds, smells, touches and what-not from as far back
as they can remember, and the associations have remained consistent
over time. Elizabeth Stewart-Jones's first name, for example, has
always been the 'horrid glistening yellow' that it is today. The
experiences are involuntary and cannot be suppressed. Synaesthetics
don't actively try to taste the colour of spearmint; it just happens.
Finally, synaesthetics tend to keep quiet about their sensory
spillover. At first believing that everyone perceived the world in this
multi-sensory way, most were shocked and embarrassed, some ridiculed,
when they learnt otherwise.
Cytowic agrees with the overall
sketch but says his sample group of about fifty have a number of other
shared traits. Females dominate, he agrees, but by far less - perhaps
three times - than the 10:1 prevalence Baron-Cohen and his colleagues
suggest. A larger than average proportion, at least 10 per cent and
perhaps more, are gay or lesbian, he finds. And there is a
preponderance of non-righthandedness. Synaesthetics also tend to have
extraordinary memories and they are inclined, he says, to 'unusual'
experiences - like deja vu, precognitive dreams and clairvoyance. But
they experience difficulties with maths and with finding their way
Both teams suspect that synaesthesia has a genetic
basis, though evidence is far from conclusive. Baron-Cohen reports that
a number of synaesthetics have relatives - every last one of them
female - who share the trait.
The particular constellation of
traits - difficulties with navigation and maths, the prevalence of
females - could prove significant. Some scientists believe that even
ordinary men and women show differences in their 'cerebral dominance' -
that the left and right hemispheres of their brains have strengths and
weaknesses that vary according to sex. Could synaesthesia reflect
differences between male and female minds? Toss in the possibility that
synaesthetics have a tendency to left-handedness and the puzzle becomes
even more intriguing. Most familial traits associated with
left-handedness - among them dyslexia and stuttering - are
predominantly male. A female trait associated with lefthandedness, says
Cytowic, 'is an unusual pattern'.
Though Baron-Cohen has not yet
assessed the mathematical and navigation skills of synaesthetics and
therefore cannot comment, he has inquired about lefthandedness. 'It
would have a direct bearing on neural development,' he says. 'But we
haven't found any evidence of it.'
clear that the brains of synaesthetics operate differently,' says
Cytowic. 'What we quibble over is the interpretation of this.' The
central dispute is over what is happening in the brain when a person
experiences synaesthesia. To help answer that question both Cytowic and
the London group have turned to imaging blood flow in the brains of
people with synaesthesia. Blood flow can be used as a measure of neural
activity. Both sides are cautious about the reliability of brain
imaging techniques - there is too much scope for error in measurement
and interpretation - and tend to consider behavioural in-formation more
compelling. Yet it has been brain imaging results that have polarised
In 1981, with the help of David Stump, an expert in
measuring brain metabolism, Cytowic induced a synaesthetic experience
in Watson's brain using a technique called xenon inhalation. Watson
inhaled radioactive xenon gas which quickly dissolved in his
bloodstream. The xenon-saturated blood could then be tracked by
detectors monitoring 16 different brain regions. The result was a
series of snapshots of his brain metabolism. The presumption is that
the greater the blood flow the greater the activity.
Cytowic and his colleagues established what Watson's brain looked like
normally; then they induced synaes-thesia with fragrances such as
spearmint and wintergreen; finally, the researchers augmented the
synaesthesia by giving Watson a dose of amyl nitrite, a drug that
enhances sensuality. What they discovered was astonishing: during
synaes-thesia, both with and without the amyl boost, blood flow in the
left hemisphere of Watson's brain was greatly reduced, falling as low
as three times below an average person's lower limit. There was a
slight but insignificant decrease in the right hemisphere. In parts of
his cortex, it appeared that the blood didn't circulate at all.
findings pushed Cytowic further towards the new model. Watson's sensory
fusion, he concluded, had not taken place in the cortex, as the
standard view might have predicted. Indeed, the left hemisphere of his
cortex seemed to shut down during the experience. But while average
cortical blood flow had dropped, the brain's overall blood flow had
increased. Cytowic drew a bold conclusion: syn-aesthesia took place in
the left side of the limbic system, a chunk of brain buried deep in the
temporal lobe, and the seat of emotions and memory. If senses merged in
the limbic brain, he reasoned, it no doubt played a far greater role in
perception than traditional brain theory had so far allowed.
just an idea,' he says, enjoying the ripple of controversy. But a bold
idea indeed, considering that the experiment was conducted on only one
individual - and an unusual one at that. The xenon inhalation process
revealed that Michael Watson's resting blood flow was inhomogenous to
begin with and that his hemispheric flow was not symmetrical, which it
should be. Moreover, given that it could only measure activity in the
cortex, observers would naturally assume that whatever was going on in
Watson's brain was taking place in the limbic system. 'We would be
cautious about generalising about that (the single case study),'
Cytowic is among a growing number of
neurologists who see the brain as a multiplex. Alongside the hard-wired
circuitry of classical neuroanatomy, say proponents, there are
additional channels of electrical and chemical communication. 'Volume
transmission', as the usage of these additional channels has become
known, is more lax, or 'leaky', than its more physically-contrained
counterpart, 'wiring transmission', the brain chatter conducted by
axons, synapses and dendrites.
In volume transmission, over
fifty messenger molecules, among them hormones and peptides, transmit
information through the extracellular fluid surrounding the nervous
system, in what is in effect an alternative, chemical network of
communication. This kind of transmission makes communication in the
brain much more versatile than classically imagined. The speed of
diffusion can be slow and the distances travelled long. Cytowic enjoys
likening it to travelling along a railway track (wiring transmission)
versus leaving the track and floating along unrestrained towards a
target. Neural impulses do not flow in the strictly linear,
hierarchical fashion traditionalists have long supposed. Rather,
information travels in all sorts of ways and in unexpected directions.
At the very heart of the process and regulating this cacophony of
systems is the limbic brain.
suggestion that the brain harbours some kind of coordination centre is
not a new one. Antonio Damasio of the University of Iowa College of
Medicine has suggested that recall is reconstructed from numerous
fragments of memories, all stored separately within the cortex, but
activated in an integrated way. This coordination site, perhaps located
in the hippocampus, may even be allowing the brain to 'read' the
sensory information that is packed away in the cortex.
Cytowic proposes, is an inside peep at this coordination process, which
he believes takes place in the limbic brain. He offers the analogy of a
television broadcast. Most people perceive only the final picture, but
it's as though people with synaesthesia are tapping in prematurely and
seeing the picture as it is being formed. The mixing of senses that
occurs in the limbic system, he believes, is a 'fundamental mammalian
'I believe that synaesthesia is actually a normal
brain function in every one of us, but that its working reaches
conscious awareness in only a handful,' he writes in his book. In most
people, sensory crossovers occur as a normal part of perception without
their knowledge. In people with synaesthesia, perception itself is
bared to consciousness.
Such are the vagaries of science that
about the time his book was being penned, the London team was
conducting its own experiments on cerebral blood flow in people with
synaesthesia. Its findings, using the more sophisticated positron
emission tomography (PET), are somewhat at odds with those of Cytowic.
As words and sounds were read aloud to 12 blindfolded subjects, PET
scanners mapped the flow of radioactively-labelled blood. The six
control subjects exhibited nothing unusual. Blood flow in the six
synaesthetics, however, was anomalous: although sound had been the only
stimulus, blood seemed to be flowing to areas of the cortex at the back
of the head that specialise in vision - and in particular to regions
believed to process information about colour. A paper detailing the
experiment is currently being reviewed for publication.
results seem to reinforce the idea that synaesthetics' brains harbour
unusual neural pathways. Findings from an earlier behavioural test by
the same team do so as well. In a test similar to the one given to
Stewart-Jones, nine synaesthetics and nine controls were asked to
describe the colours of 130 carefully-selected words and sounds. As
predicted, controls scored low, around 37 per cent, after only one
week, while one year later, the nine synaesthetics were over 92 per
cent accurate. But they noted something interesting. Though there was
little agreement about the colour of individual letters, three vowels
were clearly exceptions: eight of the nine synaes-thetics saw 'i' as
white to pale grey, 'o' as white and 'u' as yellow to light brown.
consensus was striking. The point was further strengthened by the fact
that the white 'o' was consistent historically; in 1883 Galton noted it
in his group of subjects, as did Vladimir Nabokov of himself in 1966.
That there should be consistency not only among subjects but across
time suggests that the specialised neural pathways that become
inadvertently crossed in synaesthesia are very particular ones.
question now preoccupying the London team is whether this unusual
connectivity develops specially during early life or is simply the
relic of a system of pathways that would normally fade away. Research
on infants by Daphne Maurer of the University of Toronto indicates that
we may all be born with a sort of synaesthesia but grow out of it.
Flashing a light in a newborn's eyes does not elicit a clear response
from anything akin to a vision centre. Do the same to the average adult
and blood clearly flows to the visual cortex. In studies of kittens and
baby macaque, researchers have also found examples of neural
connections between sensory pathways that disappear as the animals
While other brains mature and segregate, Harrison
speculates, the brains of people with synaesthesia retain childhood
bonds. Perhaps from among many ways of processing data the
synaesthetic's brain chooses fusion over discretion.
from here? Both teams are interested in knowing how learning affects
synaesthesia, and vice versa. The London group is launching a study on
the pre-literate offspring of people with synaesthesia. They want to
know why synaesthesia is so often letter-based, something acquired with
literacy, as opposed to sound-based. 'It is counter-intuitive,' says
Harrison, because we are exposed to sounds from birth.
alters the brain,' says Cytowic. Along with culture and environment,
what we learn and how we learn, he believes, sets the context for
perception. 'We've got perception inside-out,' he says. 'We are not
passive receivers. The brain goes out and grabs information, and the
limbic system decides what.' Inquiries into how we learn may help
explain why only some people experience synaesthesia - or at least
experience it strongly. For Cytowic questions whether a distinction
should be made between the synaesthetics and non-synaesthetics.
'Perhaps there is a continuum,' he muses.
Living with coloured namesAll
your life you've enjoyed the scents of foods like roast turkey or
oranges or rum sauce. Imagine your surprise if you were to discover
that other people could merely taste them. You describe the distinct
and powerful 'citrusy' smell of a lemon, but others can't fathom it.
For them, lemons only have flavour. You can't believe it. Discovering
synaesthesia can be a bit like this.
I was in my late teens when
I found out - not so much that I had synaesthesia but that the others
did not. I was discussing a short story with my high school English
teacher, who wanted me to change the name of a main character. I
protested. 'She needs a strong red name,' I said, appealing to reason.
The issue of the name was suddenly dropped, and we turned instead to
examining the way for me all letters and numbers were endowed with
colour. I hadn't known it was unusual. Years went by before I learnt
that this habit of mine had a name and, even more exciting, that other
people experienced it too.
People ask questions like 'how many
times have you experienced synaesthesia?', as though it were a rare
psychic encounter, or 'how does it feel?', as though it caused pain. I
experience synaesthesia any time words and numbers are around - the way
most people detect a smell any time there is a scent in the air - and
it feels every bit as natural. A more important question sometimes
asked is 'how does it affect your daily life?'
In my case, with
the most common and basic form of synaesthesia
('chromatic-graph-emic'), the effect is small. Occasionally, I am
unfairly biased against people with 'weak coloured' names - Phil,
Lydia, Iona - and in favour of 'strong' ones - Mark, Richard, Sarah. it
also serves as an aide memoire: I can quickly commit telephone numbers
to memory, can remember important page numbers weeks later and have
always been a first-class speller. But for me it has unquestionably had
its greatest impact as an ice-breaker: talk of synaesthesia can liven
even the dullest of dinner parties.