What color is Wednesday? If the answer is obvious to you, you might have a form of synesthesia in which sequences such as numbers, days of the week, and months of the year are perceived as having colors (or else you might be from Thailand).

Initially, one may well be skeptical on being told that someone sees “Wednesday” as “apple green”, but the study of synesthesia has come a long way from the early days when researchers relied purely on subjective reports. Synesthesia has now been well established through more objective tests, and by verifying the reliability of e.g., color-letter or color-day of the week associations over time, which tend to be much more stable in true synesthetes.

With the reality of synesthesia established, researchers have turned to questions about its mechanisms. One immediate question concerns the origin of the reported associations. For people who experience letters or words as having color, what determines which color a letter or word is? One possibility is that there is something intrinsic about the shape or sound of a letter that might give rise to the link. Another possibility is that the associations are learned through experience associating letters and words with specific colors.

In one recent study, Witthoft, Winawer, and Eagleman examined associations between letters and colors in 6588 color-grapheme synesthetes (i.e., people who reported seeing letters as having different colors) who participated in the Synesthesia Battery. Was there any consistency with which colors went with which letters? The answer was a resounding ‘yes’. Some combinations were much more common than others. For example, the modal colors for ‘Y’, ‘B’, ‘G’, and ‘V’  were, respectively, yellow, blue, and green, and violet: a link apparently stemming from the first letter of the English color names. Other common associations were yellow for ‘C’, orange for ‘H’ and blue for ‘W’. Where did these associations come from? As is turns out, in the 1980s Fisher Price sold sets of magnetic letters in which the letters had just these colors. As many as 16% of the participants in the study by Witthoft and colleagues who grew up around the time that these sets were most popular had color-letter combinations well-predicted by the color pattern used by the Fisher Price. Interestingly, the Fisher Price colors appeared to compete with the colors suggested by the associations with color names. The “Fisher-Price kids” might see “B” as orange (the color in the set) rather than “blue”. (It is worth pointing out that the Fisher Price colors are simply the conventional rainbow-colors acronym ROYGBIV beginning with ‘A’ and repeating).

Ok, so experience matters. But of the millions of kids who grew up with Fisher-Price Alphabets, only a very small percentage ever developed color-grapheme synesthesia. Might some people be more sensitive to learning such associations than others?

A recent paper by Bankieris and Aslin, published in Psychonomic Bulletin & Review, addresses this question by comparing how word-to-color synesthetes and control participants learn probabilistic associations between shapes and colors. The authors recruited eight synesthetes who experienced colors for letters, numbers, days of the week, and/or months of the year and presented them with a visual search task in which they had to search for a specific “snowflake” shape and indicate whether it was on the left or right side of the screen (see below).

Some snowflakes were equally likely to occur in any one of six colors. Other snowflakes were biased, occurring in a particular color 5/6 of the time and in one of the other five colors 1/6 of the time.

This design allowed the authors to ask several questions. First, did synesthetes learn the color-shape associations faster than non-synesthetes? If they did, they should show greater facilitation for the congruent trials as the experiment went on. Second, did the synesthetes show greater interference when biased targets were presented in the ‘wrong’ color? If they did, they should show poorer performance on these trials as the experiment went on.

The results are shown in the “Visit 1” panel below. Not surprisingly, all participants improved with practice, but surprisingly, the non-synesthetes showed an earlier benefit from the congruent colors (the difference between the green and gray lines) than the synesthetes. On the other hand, the synesthetes showed greater interference when presented with color-biased trials that were in the non-frequent color (the difference between the gray and red trials).

Following Visit 1, participants were brought back two weeks later to do the same task, but, the color-biased snowflakes were now associated with a different color. For example, if in Visit 1 a snowflake was shown in purple 5/6 of the time, it might now be shown in a color that was previously neutral, e.g., yellow.

The results (Visit 2 panel above) revealed that the synesthetes showed a growing interference for the biased trials, suggesting, in the authors’ words, “that synesthetes’ heightened long-term memory for shape–color pairs leads to increased interference when learning new colors for those same shapes.”

Participants also completed an explicit color memory test in which they were asked to match the color-biased snowflakes with the colors they were presented with the most. The synesthetes outperformed the controls after the first session and after the second session, providing corroborating evidence that compared to the controls, the synesthetes learned the color associations more robustly. Taken together, the results show that synesthetes learn the implicitly presented color pairings a bit slower, but the knowledge persisted over a timespan of two weeks in a way it did not in the control participants. It is unclear whether these differences in learning shape-color associations are the result of being a synesthete or, at least partly, a cause of synesthesia.

One fascinating aspect of synesthesia, rarely discussed in the scientific literature, is that it is possible—indeed, probable—for people to go their entire lives not realizing that not everyone sees Wednesday as having a color or hears the sound of a blinking cursor. The internet is replete with discussions of synesthetes discovering—to their amazement—that not everyone has these experiences.

Such masked differences in subjective experience can also be seen in other domains. Some people have trouble seeing anything in their “mind’s eye”, others lack the capacity for mental time travel, while others have extraordinary abilities to recognize faces. That it is possible for these differences to go unnoticed suggests that we (researchers included) have a strong assumption that everyone’s mental lives are much the same as our own, when in fact there may be striking differences. Future investigations of individual differences in perception and cognition may hold many surprises.

Article focused on in this post:

Bankieris, K. R., & Aslin, R. N. (2016). Implicit associative learning in synesthetes and nonsynesthetes. Psychonomic Bulletin & Review. DOI: 10.3758/s13423-016-1162-y.