After this training regime, people saw letters of the alphabet as being alive with colour

A training regime at the University of Sussex has successfully conditioned fourteen people with no prior experience of synesthesia – crossing of the senses – to experience coloured phenomena when seeing letters.

The regime took place over nine weeks, a half hour session every workday together with extra homework. Again and again, the trainees were encouraged to treat the letter “r” as red, or “e” as green, with a similar process repeated on 13 letters in all. This was tested every session using tasks such as viewing a sequence of letters and selecting all the associated colours, or completing a timed reading task where letters were omitted and replaced with squares of the relevant colours, for example: “Mann Mssn t Mas.”

Tasks became progressively harder and the group were financially incentivised to outperform their previous scores. No previous intervention has been as extensive as this one, as Daniel Bor and colleagues were seeking to go beyond learned colour-letter associations to try and produce a genuine subjective experience of synesthesia.

After the training, the group became better at those “Stroop” test trials where the entrained colour of a presented letter matched the ink colour it was written in, and the task was to name the ink colour as fast as possible. This suggests that the training had gone deep enough to help them make rapid, non-reflective decisions.

The majority of participants also reported gaining a subjective experience of synesthesia. By their own accounts, nine definitely experienced a coloured effect when seeing trained letters, which was mostly characterised as seeing the colour “in front of my mind’s eye” (only two participants definitely didn’t have this experience). Naturally occurring synesthetic effects can be stronger than this, with colours seen floating on the surface of the letter or number, but the reported experiences are nonetheless impressive.

In addition, participants got smarter, scoring an equivalent of 12 IQ points higher on a standard intelligence test administered pre- and post-training. We should make no firm conclusions from this, as the causal mechanism may be other aspects of the training process not directly related to synesthesia, such as the heavy load on working memory. Even so, achieving a 12-point increase in a normal- to high-functioning group is not something routinely delivered by psychology interventions.

Three months later, did the synesthesia stick? Not so much. The effect on the Stroop task was maintained, suggesting learned associations were going strong, but participants reported a weakening or total dissipation of the coloured experience itself, which is the defining feature of synesthesia proper.

Nevertheless, this work questions whether synesthesia is limited to a rare and genetically distinct group, and shows how learning and experience likely plays an important part too. We already know that young synesthetes experience a strengthening of their colour linkings during early school years. Perhaps early pairings – seen on coloured alphabet jigsaws or fridge magnets – provide the initial associations that some people develop into an ever-present feature of their world.


Bor, D., Rothen, N., Schwartzman, D., Clayton, S., & Seth, A. (2014). Adults Can Be Trained to Acquire Synesthetic Experiences Scientific Reports, 4 DOI: 10.1038/srep07089

Post written by Alex Fradera (@alexfradera) for the BPS Research Digest.