Vladimir Nabokov once called his famed fictional creation Lolita ‘a little ghost in natural colours’. The natural colours he used to paint his ‘little ghost’ were especially vivid in part because of a neurological quirk that generated internal flashes of colour whenever letters of the alphabet appeared within his mind. In his memoir Speak Memory (1951), he described a few of them: ‘b has the tone called burnt sienna by painters, m is a fold of pink flannel, and today I have at last perfectly matched v with “Rose Quartz” in Maerz and Paul’s Dictionary of Color’. The condition he had was synaesthesia, a neurological oddity that mixes up the senses, making those who possess it see as well as hear music, or taste the shapes they set their eyes upon.

Synaesthetes such as Nabokov see letters and numbers wreathed in fixed, seemingly idiosyncratic colours. Grapheme-colour synaesthesia, the term for this variety, is the most common sub-type of synaesthesia, occurring among four people in 100. It’s also the most widely studied. Other common varieties are chromaesthesia, in which tones or notes set off flashes of colour and a symphonic wall of sound can summon a three-dimensional landscape, and spatial-sequence synaesthesia, in which seconds, weekdays, months or years encircle those who experience it, like planetary rings. Some have lexical-gustatory synaesthesia, which lends every word or name a strong, specific taste, making some delicious, and others too bitter to utter. Still other synaesthetes report ordinal-linguistic personification, in which they ascribe distinct genders, colours or personality types to letters and numbers: ‘4’ might be an ill-tempered, ungenerous man, constantly heckling his wife, while ‘6’ turns out to be a dignified, genteel woman with exquisite manners.

Nothing could be more intensely subjective or taken-for-granted than the ineffable way that each of us perceives the world. This is why many synaesthetes go through a lifetime without realising that their everyday sense experience is exceptional or strange. Those who do, report a moment of startled self-awareness when friends respond with an uncomprehending: ‘What do you mean, my name tastes of split-pea soup?’ Such eureka moments have grown increasingly common since the 1980s, when cognitive tests were first developed to judge the authenticity of the reports through to the mid-1990s, when brain scans and brain-wave measurements began tracking the physiology of synaesthesia’s various forms. Writing in The Oxford Handbook of Synesthesia in 2013, Richard Cytowic, a neurologist and synaesthesia researcher at George Washington University, describes the ‘astonishment and enthusiasm’ reported by synaesthetes after tests validated that they weren’t ‘making it all up’.

As an increasing number of synaesthetes recognise for the first time that they are unusual, new forms of the phenomenon emerge. In 2008, two neuroscientists at the University of California, San Diego, V S Ramachandran and his then-student David Brang, stumbled on the first recorded ‘tactile-emotion’ synaesthete: a young woman who reacted viscerally to textures. Her eureka moment occurred when she told Brang how she’d cry as a child every time her parents dressed her in denim, which repulsed and depressed her. The feel of wax, on the other hand, deeply embarrassed her, while silk left her in a state of gurgling contentment.

Researchers have also realised that one individual can experience different forms of synaesthesia. The same person who feels revolted by a texture can see letters and numbers swathed in colours. It’s common for synaesthesia to recur within families. Nabokov’s father and mother saw letters and numbers tinted in hues, and his mother also saw clouds of colours accompany the music she heard. Nabokov’s wife Véra and their son Dmitri shared the same version of synaesthesia he and his parents had – grapheme‑colour synaesthesia. As Nabokov put it: ‘One letter which he [Dmitri] sees as purple, or perhaps mauve, is pink to me and blue to my wife. This is the letter M. So the combination of pink and blue makes lilac in his case. Which is as if genes were painting in aquarelle.’

But now it turns out that synaesthetes might not belong to a club as exclusive as once thought. Their rich palette and vivid sensations might be accessible to us all. Even though not kin to Nabokov, we too could be reading our books in aquarelle. The under-examined complexities of ordinary perception, some neuroscientists and developmental psychologists contend, suggest that, like the Nabokovs, we all inhabit the synaesthetic spectrum – we just need to look back in time, to when we were infants with developing brains.

The first glimmer of this idea appeared in a thought experiment in Emile (1762), a novel by the Enlightenment-era philosopher Jean-Jacques Rousseau. He hypothesised that a child born fully grown, the size of a man and with the faculties of a baby, would have the barest amount of self-awareness and mingled sense-impressions, amounting to a muddled proto-synaesthesia. Rousseau wrote: ‘His eye would not perceive colour, his ear sounds, his body would be unaware of contact with neighbouring bodies, he would not even know he had a body. All his sensations would be united in one place, they would exist only in the common “sensorium”.’

In 1818, this idea inspired Mary Shelley’s description of the early sensory experience of another strange newborn. ‘A strange multiplicity of sensations seized me,’ says the monster in her novel Frankenstein, ‘and I saw, felt, heard, and smelt, at the same time; and it was, indeed, a long time before I learned to distinguish between the operations of my various senses.’

The psychologist William James conjured a similar picture of the baby’s sensory world in his Principles of Psychology (1890). He wrote (and the excited capitalisations are all his) that ‘any number of impressions, from any number of sensory sources, falling simultaneously on a mind WHICH HAS NOT YET EXPERIENCED THEM SEPARATELY, will fuse into a single undivided object for that mind’. As a consequence, ‘The baby, assailed by eyes, ears, nose, skin, and entrails at once, feels it all as one great blooming, buzzing confusion; and to the very end of life, our location of all things in one space is due to the fact that the original extents or bignesses of all the sensations which came to our notice at once, coalesced together into one and the same space.’ If the Jamesian perceptual model held true, if most of us separate out the senses as we mature, could synaesthetes simply be continuing the process of fusion the majority leave behind? In other words, are all babies synaesthetic?

This extraordinary concept was picked up again in 1988, in the book The World of the Newborn. Its authors, the developmental psychologist Daphne Maurer of McMaster University in Ontario and her writer husband Charles Maurer, conjured a sense-world of the newborn that recalled the proto-synaesthesia of Rousseau’s fully grown child:



His world smells to him much as our world smells to us, but he does not perceive odours as coming through his nose alone. He hears odours, and sees odours, and feels them too. His world is a melee of pungent aromas – and pungent sounds, and bitter-smelling sounds, and sweet-smelling sights, and sour-smelling pressures against the skin. If we could visit the newborn’s world, we would think ourselves inside a hallucinogenic perfumery.

The Maurers proposed that all infants were synaesthetic, with exuberant connections snaking between the parts of their brains that transformed various stimuli to perception. The pitched crosstalk between these various brain areas, they said, likely resulted in a synaesthetic infantile sensorium. The idea had first struck Charles Maurer while reading the Soviet psychologist Alexander Luria’s The Mind of a Mnemonist (1968), whose subject recalled the ‘vague synaesthetic sensations’ of his early childhood, where ‘a mass of fog, then of colours’ meant there was noise, possibly a conversation. Maurer asked his wife, who was then studying how sight developed among infants, if all newborns could be synaesthetic.

Over an interview on Skype, Daphne Maurer recalled her response: ‘I said, uh, I don’t know… Let me think about how that’d be manifest and go back and look at the data on infants.’ Her voice had the woozy, dream-like tones of someone spending a lifetime trying to get across to tiny babies. ‘And when I went back to look, I thought it makes sense.’

The evidence she found came from a body of studies of brain anatomy from the 1980s and ’90s, all charting how the number and extent of neural connections waned from infancy to adulthood. Spurred by anatomical tracing – a technology that helped mark groups of neurons and follow their developmental trajectory – the studies chronicled how networks of neurons in the brains of kittens, infant rhesus monkeys and humans thinned out as some skills were emphasised over others. More recent studies, which Maurer now cites as additional proof, show that in humans neural pruning appears to be especially pronounced between the ages of seven and nine.

The pruning faded the psychedelic phenomena – except for synaesthetes, whose thicket of brain connections were strengthened and reinforced

While sifting through behavioural experiments, Maurer found persuasive hints to support the hypothesis that synaesthesia declined as the infant brain matured and neural connections were pruned. An early example was a 1974 study by researchers at Harvard Medical School. ‘If you put electrodes on the newborn’s head and stimulate the wrist, you’ll see an increase in activity in the tactile cortex,’ she told me. ‘And if you turn on white noise at the same time, you’ll get larger changes in the tactile cortex.’ In other words, in the infant brain, touch and sound amplify each other. But the same part of an adult’s brain, which doesn’t process sound, doesn’t experience a corresponding increase in activity.

The study reinforced impressions made through many years spent in hospital wards studying sight development in newborns. Maurer recalled that babies in frenetic wards ‘started to cry and shut down’, while those who received a single, gentle sensation – a soft voice or a light down blanket smoothed atop them – broke into ‘some very gentle smiles. Babies seem to respond to the overall level of stimulation, regardless of where it was coming from. It’s as if the nervous system was just summing up sound and sight and touch.’

Over time, specificity takes control. Here Maurer cites the work of Helen Neville, a neuroscientist at the University of Oregon, whose 1995 study found that speech sparked brain waves across auditory and visual regions of the brain in six-month-olds. But the effect tapered off, with speech mainly provoking activity in auditory regions by the time the children turned three. These studies and others have led Maurer to conclude that the hyper-connected neural networks observed in babies tapered down over time, or got ‘pruned’ by their environment and their experience. She reckoned that the pruning had the effect of fading out the attendant psychedelic phenomena – except among synaesthetes, in whose brains the thicket of connections got strengthened and reinforced.

It’s impossible to truly replicate the phenomenological experience of an infant, but two psychologists at the University of California in San Diego have recently shown the phenomenon in play for the first time. Katie Wagner and Karen Dobkins presented two-month-olds, three-month-olds, eight-month-olds and adults with dark outlines of circles and triangles against two sets of background colours: either red and green, or blue and yellow. In one arm of the study, they hypothesised that synaesthetic infants who associated triangles with red would be inclined to glance more at triangles set against a green background, much like adult counterparts.

After analysing the results of about 100 rounds of these tests per infant, Wagner and Dobkins confirmed the effect for two- and three-month-olds, while eight-month-olds and adults no longer demonstrated any strong association or preference. The results of the study, they wrote in Psychological Science in 2011, ‘demonstrate [that] synaesthetic associations’ start ‘early in life and … decline with age’, providing clear support for the Maurers’ big ideas.

Other research shows infantile synaesthesia reconfigures in toddlerhood, as babies grow. That work comes from Julia Simner, a neuroscientist at the University of Edinburgh who has tried to capture the process in time-lapse view. In a 2009 study published in Brain, she asked 615 six-year-old school children to match up 13 colours with 26 letters of the English alphabet and the numerals 0 to 9. Ten seconds later, they were tested on their matches. The 47 best scorers were tested again a year later. In the intervening time, their associations strengthened, providing a glimpse into how synaesthesia progresses in real-time. When Simner checked back in with her would-be synaesthetes three years later, when they were 10 or 11, she found the constancy of their fixed associations had further strengthened. The trajectory was clear: 34 per cent fixed their letters and digits by age seven, 48 per cent by age eight, and 71 per cent by age 11.

Daphne Maurer also has an ongoing study along these lines. She’s been following three children of female grapheme-colour synaesthetes since age three or four. These children have been asked to choose colours from a selection of 96 crayons to go with letters of the alphabet, single digits, and four basic shapes. The task is staggered over several weeks and repeated several times. So far, she has found that non-synaesthetic children choose different crayons every time, while the offspring of synaesthetic mothers consistently pick the same shades to go with letters, numbers and shapes. Consistency increases the next year again, going from 40 per cent to 75 per cent. Maurer has been struck by how nuanced these children’s associations become, much like their synaesthetic parents. One child, for instance, complained that the green attached to a letter was not quite the right shade.

Lifelong time-lapse studies have proven costly and challenging, so synaesthesia researchers today increasingly focus on what’s termed cross-modal integration: the ways in which the brain combines different sensory inputs, such as smells and sounds. You might think our senses work independently, honing in on an individual sight or sound like a pinhole camera or laser beam. In actuality, all of our senses are constantly, seamlessly melding together to capture the world in a sharper, more vivid way. Just recall the cottony inedible mush you ate when you last had a cold, or how you know precisely which direction to flee when you hear a dog let out a low-pitched growl. Those are sights, sounds, smells and a host of other perceptions coming together, crisscrossing in the brain.

Such interactions between the senses produce ‘extra-perceptions’ – a kind of undertone. For example, you might have the sense that the source of a high-pitched yap is a tinier, less intimidating shape than that of a low-pitched growl. The earliest such cross-sensory correspondence was discovered in 1929, by Edward Sapir, a linguist at the University of Chicago, and by Wolfgang Köhler, a psychologist at the University of Berlin. Sapir told participants in his study to attribute two nonsense words, mil and mal, to two tables, one of which was smaller than the other. All but one participant identified mil as the tinier table. Meanwhile, Köhler’s participants were asked to match two made-up words, takete and maluma, to two shapes, one a lumpy amoeboid, the other, a jagged shard. Most of them were certain that takete was the spiky one, and maluma the blob. Subsequent studies have found that children and adults tend to associate brightness with loudness, and a small ball with a high-pitched sound.

Researchers are interested in finding out if these interactions exist along a continuum – mild among non-synaesthetes and strong among synaesthetes, indicating that we all have some ability to access the sort of enriched perceptions that come with synesthesia. Daphne Maurer, who regards these correspondences as connections between brain areas that were spared during the pruning process, thinks this might be the case.

So does the neuroscientist Edward Hubbard of the University of Wisconsin-Madison. ‘Your actual experience of the world is highly integrated, and constantly combines all sorts of information from different sensory modalities,’ he told me. In synaesthesia, ‘we’re seeing a heightened version’ of it.

A famous instance of this is the McGurk effect, a trippy perceptual illusion discovered by the British psychologist Harry McGurk in 1976, in which an audio recording of a person repeatedly saying ‘ba’ is played along with video recording of a person repeatedly saying ‘ga’. The sound is ultimately reconciled in the brain as something in-between: ‘da’.

the universality of this cross-sensory quality might make us all ‘closet synaesthetes’

‘This shows that speech perception is actually this multisensory phenomenon, where we bring together inputs from various senses,’ Hubbard said.

Back in 2001, a set of experiments by Hubbard and his then-advisor Ramachandran revealed the likely cerebral crossroads through which such connections streamed back and forth: the angular gyrus, a tiny area sidling up to the major brain regions processing touch, hearing and vision. Hubbard and Ramachandran showed three patients with damaged angular gyri abstract shapes that they called bouba and kiki – their version of the maluma-takete task. ‘The patients were far less likely than healthy college undergraduates to readily recognise that the blob was bouba and the shard-like shape kiki. Hubbard said.

Hubbard and Ramachandran then asked the patients to explain metaphors or figures of speech such as ‘he stepped down as director’. Not surprisingly, the patients were generally impaired at a range of linguistic tasks. The damage to their angular gyri had wiped out their ability to sense the extra-sensoriness of language – it had damaged their ability to recognise that a jagged thing could be something you see with your eyes like a jagged shape or something you hear, like a jagged guitar chord, or the word kiki. In other words, they lacked the extrasensoriness most of us take for granted. Indeed, the researchers concluded that the universality of this cross-sensory quality, and the fact that it might be lodged in our anatomy, might make us all ‘closet synaesthetes’.

It could be that synaesthesia is the true engine of metaphor and art. Just ask Megan Hart. ‘The word love has always tasted like the scent of fresh ink and soft paper to me. Like a newly written poem,’ she wrote in her novel Tear You Apart (2013). The poem, like the novel, is synaesthetic at heart, and the synaesthetic self is the secret source.

That we are all on the synaesthetic spectrum hit home last November at the Society for Neuroscience meeting in Washington DC, where researchers said this talent just might explain why those with sensory deficits still function so well. Jenessa Seymour, a doctoral candidate at the University of Illinois at Urbana-Champaign, described an experiment proving that individuals born deaf have a highly sharpened sense of peripheral vision especially useful in low-light conditions. The additional brainpower for this supervision, she said, came from a brain region that would otherwise be used to combine sight and sound – the posterior superior temporal gyrus.

Ryan Stevenson, a neuroscientist at the University of Toronto, has honed in on the corner of this brain region: the superior temporal sulcus, which is involved in speech perception, facial perception, interpreting emotions, and understanding the intentions of others. All these functions, he noted, were impaired in the autistic. In an experiment, he tested children with and without autism on their ability to combine auditory and visual information. First, he measured how clearly the children were able to perceive simple non-speech sounds such as flash-beeps and whistles. The children were then asked to combine sound and vision in a test of the McGurk effect. They were shown a video of a person mouthing ‘ga’ alongside audio of someone saying ‘ba’. The researchers found that, while both groups were equally good at perceiving flash-beeps and whistles, children with autism were far less likely to report hearing the ‘combined’ sound ‘da’. Many of them reported just what they heard, a lone ‘ba’.

Stevenson concluded that children with autism were slower to put together what they saw with what they heard, particularly when it came to speech. ‘It’s kind of like how you can hear better in a noisy room if you see someone’s mouth,’ Stevenson explained. ‘But kids with autism have a harder time differentially putting together what they heard and saw.’

Taken together, the findings show that we are all closet synaesthetes to a greater or lesser degree. The further along the autistic spectrum, the less synaesthethetic we might be. Synaesthesia promotes connection within one’s own mind – and between minds as well. To lack the synaesthete’s skill is to stand apart. A circus of the senses forges our early development and drives our humanity. By thinking back to our infant mind, when our synaesthesia was at its height, we just might amplify the sensations flooding in from the world.