Can you learn to taste music?

Probably not. But you can learn to perceive black letters in colour.

Or so says an eyebrow-raising study published in Scientific Reports by a team from the University of Sussex. The team was studying a trippy condition called synesthesia, a condition trademarked by a crossing of senses. For synesthetes, a car honk may trigger them to "see" bright red, a whistle may make them "feel" a light touch, a given word – book, for example – may cause them to "taste" buttery waffles in their mouths. By far, however, the most common type of sensory mix & match is graphene-color synesthesia (GCS), in which a person consistently associates a letter or number with a certain colour*.

Synesthesia affects up to 1 in 23 people (!!!), and scientists currently believe that it's mostly congenital. Yet there are cases where only one individual in a pair of maternal twins is affected, suggesting that factors other than genetics are also at play.

Research with GCS synethestes supports this idea. For example, the specific number-colour associations that they have in adulthood can often be traced back to coloured number-shaped fridge magnets or toys that they played with as kids. The way such associations develop also points to learning as a crucial factor: GCS emerges during early school years when kids first encounter reading and numeracy in school. Their early associations are ever-changing and chaotic, and only gradually cement into consistent number-colour pairings with learning and time. An even more intriguing case study describes an adult woman who spent eight years of her life learning digit-colour associations for cross-stitching. When tested on a battery of synesthesia-related cognitive tests, she performed similarly to natural-born GCS individuals.

These data tantalizingly suggest that synesthesia-like traits can be learned. To test this idea, researchers recruited a small sample of 14 teenagers (2 male, 12 female), and enrolled them in an intensive 9-week training program. Rather than random pairings, the researchers decided to teach them 13 of the most common letter-colour associations found in born synesthetes and normal populations. To keep motivations up, the researchers frequently changed the learning tasks and also rewarded the participants small monetary prizes for scoring high on the tasks.

At the end of training, the participants were bombarded with a collection of tests designed to fish out genuine synesthesia traits. The Colour Naming Stroop, for example, presented letters from the training task in different colours – sometimes the colour would match the trained association ("congruent"), sometimes not ("incongruent"). The participants had to quickly name the actual colour of the letter, while ignoring the association. With training, response time for congruent trials dropped (blue bar in the graph above, read from left to right), suggesting that the letter-colour association had become highly automatic. An online standardized test also showed that the pairings were consistent (below).

Mean CIELUV distance is a mathematical way to measure consistency. The lower the number, the higher the consistency.

Perhaps more exciting is that most participants began automatically experiencing the trained letters differently in their daily lives. "When reading a sign on campus I saw all the letter E's coloured green on the sign," one participant said halfway through the training session. Others reported associating letters with personas: "x" was aggressive, while "u" induced feelings of pity. Although this phenomenon, called ordinal linguistic personification, is it's own type of synesthetic experience, it is often present in those with GCS. The authors stressed that these reflections were genuine, given that the participants were never told that they were in a study about synesthesia – although I'd argue the true nature of the study is not hard to guess.

This radical change in perception had an expiry date: three months after the last testing session, these synesthesia-type experiences had largely faded away in the participants. After all, the participants spent their lives learning that print letters don't usually have colour, the authors reasoned, and so when training stopped, the colour associations were overwritten.

Finally, as a side note, the researchers also included an IQ test to look at generalized cognitive function and unexpectedly found that training triggered a 12-point jump on average. Assuming that IQ scores are an adequate measure for general intelligence, that's really quite the leap, particularly for young, healthy individuals already in the higher IQ range.

In the end, can synesthesia be learned? The question's probably unanswerable until we have a better definition of what synesthesia actually is on the neurobiological level. But it's likely that even natural-born synesthestes "develop" their superpower with extensive environmental input. And with enough effort, cross-modality perception of the world may be in our reach – now isn't that a trippy thought?

*Random fun fact: several memory champions may have used this association to help them remember tens of thousands of digits of the number PI.

#For those that stayed with me – I once tasted music, hence the title.

Bor D, Rothen N, Schwartzman DJ, Clayton S, & Seth AK (2014). Adults can be trained to acquire synesthetic experiences. Scientific reports, 4 PMID: 25404369