Synesthesia is a condition where one stimulus (like a letter) automatically is experienced with another attribute (like a colour) that is not actually present. About 4% of people have some form of this sensory mixing. It has been generally assumed that synesthesia is inherited because it runs in families. But it has been clear that some learning is involved in triggering and shaping synesthesia. “Simner and colleagues tested grapheme-color consistency in synesthetic children between 6 and 7 years of age, and again in the same children a year later. This interim year appeared critical in transforming chaotic pairings into consistent fixed associations. The same cohort were retested 3 years later, and found to have even more consistent pairings. Therefore, GCS (grapheme-color synesthesia) appears to emerge in early school years, where first major pressures to use graphemes are encountered, and then becomes cemented in later years. In fact, for certain abstract inducers, such as graphemes, it is implausible that humans are born with synesthetic associations to these stimuli. Hence, learning must be involved in the development of at least some forms of synesthesia.” There have been attempts to train people to have synesthetic experiences but these have not shown the conscious experience of genuine synesthesia.
In the paper cited below Bor and others managed to produce these genuine experiences in people showing no previous signs of synesthesia or a family history of it. They feel their success is due to more intensive training. “Here, we implemented a synesthetic training regime considerably closer to putative real-life synesthesia development than has previously been used. We significantly extended training time compared to all previous studies, employed a range of measures to optimize motivation, such as making tasks adaptive, and we selected our letter-color associations from the most common associations found in synesthetic and normal populations. Participants were tested on a range of cognitive and perceptual tasks before, during, and after training. We predicted that this extensive training regime would cause our participants to simulate synesthesia far more closely than previous synesthesia training studies have achieved. ”
The phenomenology in these subjects was mild and not permanent, but definitely real synesthesia. The work has shown that although there is a genetic tendency, in typical synesthetics the condition is learned, probably during intensive, motivated, developmental training. It also seems that the condition is simply an associative memory one and not ‘extra wiring’.
Here is the abstract:
“Synesthesia is a condition where presentation of one perceptual class consistently evokes additional experiences in different perceptual categories. Synesthesia is widely considered a congenital condition, although an alternative view is that it is underpinned by repeated exposure to combined perceptual features at key developmental stages. Here we explore the potential for repeated associative learning to shape and engender synesthetic experiences. Non-synesthetic adult participants engaged in an extensive training regime that involved adaptive memory and reading tasks, designed to reinforce 13 specific letter-color associations. Following training, subjects exhibited a range of standard behavioral and physiological markers for grapheme-color synesthesia; crucially, most also described perceiving color experiences for achromatic letters, inside and outside the lab, where such experiences are usually considered the hallmark of genuine synesthetes. Collectively our results are consistent with developmental accounts of synesthesia and illuminate a previously unsuspected potential for new learning to shape perceptual experience, even in adulthood.”
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