Synesthesia: Why do people hear colors and taste words?
In the 19th century, researchers noted that certain people who were otherwise normal “saw” every number or letter tinged with a particular color, even though it was written in black ink. For the past two decades researchers have been studying this phenomenon, which is called synesthesia.
In an article and accompanying podcast published November 22, 2011, in the online journal PLoS Biology, David Brang and V. S. Ramachandran try to understand the evolutionary basis of synesthesia. In other words, what value does synesthesia have from an evolutionary standpoint?
Ramachandran and colleagues have demonstrated that synesthesia is an authentic and repeatable phenomenon, and that it has a sensory basis rather than a high-level mental association. For grapheme-color synesthesia (where letters and numbers evoke perceptions of colors), they suggested that this occurs through cross-activation between sensory brain regions concerned with color and number. However, showing that the phenomenon is valid and caused by enhanced connectivity in the brains of synesthetes still left open the questions of how and why synesthesia evolved in the population.
As Dr. Ramachandran points out, one possible answer comes from the fact that synesthesia is purported to be seven times more common in artists, poets and novelists than in the rest of the population. Dr. Ramachandran suggests that:
… if the mutant gene was expressed diffusely throughout the brain (not just in color and number regions) and concepts and ideas are also represented in distinct brain regions, then a more “cross-wired” brain would have a greater propensity to link seemingly unrelated ideas.
This “hidden agenda” of the synesthesia gene (making some outliers in the population more creative) gives rise to one possibility of why it has survived.