Imagine that, reading this open question, you could not only see the text in black and white, but in different colors and eventually even hear a melody or smell flowers as you look at the shapes on the screen. Maybe you can. In that case, you (along with another 1-4% of the population) are likely to be diagnosed with “Synaesthesia”. Synaesthetes experience the world differently. For them, a stimulation of one sense triggers additional, anomalous perceptual experiences.
Strictly speaking, “synaesthesia” is an umbrella term representing a plethora of the most diverse diagnoses. About 150 varieties, combinations of sensory impulses (including the spatial perception of weekdays or months – February above the right shoulder – or the experience of being touched when watching other people being touched), are currently estimated. What makes this phenomenon even more complex is that not all synaesthetes indicated with one type of synaesthesia share the exact same experience: while one grapheme-color synaesthete might “only” experience a mental representation of the color red when seeing the letter A, others will actually see the letter in a crimson tinge. In addition, types of synesthesia often come in pairs. Research into which variants co-occur and why is just in the beginning.
Of course, although being far from fully understanding the causes, functions and logics of synaesthesia, science is not completely clueless about it either. After the topic had become d?class? for a while, in recent years, research on synesthesia has had a renaissance. New technologies like neuroimaging enabled scientists to see (feel? taste?) what happens in a synaesthetic brain as it experiences the (experimentally administered) world. Currently, the most common explanation holds hyper-linkages between sensory regions of the cerebral cortex responsible. Usually, those links would have been ‘pruned’ in the development of the brain. In adult synaesthetes, those links still work, and more actively so than in the lesser-gifted population.
Knowing that, the question remains: Why is there synaesthesia, what is its (evolutionary) function (if any)? Calling it a defect, error or anomaly would seem like the usual way to go. Genetic research, however, may hint towards an evolutionary benefit associated with synaesthesia. For one thing, it commonly runs in families, and it appears to be passed on genetically (the definitive genes or genome areas responsible have yet to be identified), in a dominant manner. For another thing, synaesthesia is assumed and reported to come along with cognitive advantages such as enhanced overall perception, better memory, possibly even heightened creativity. So why aren’t we all synaesthetes? In a way, we are actually: the model of linkages between brain areas sets out with the hypothesis of a synaesthetic state of the brain common in all neonates. They just grow out of it. But why? Further studying the special way synaesthetic brains work will improve our general understanding of how the brain works, and possibly also elucidate why and how sometimes linkages between perceptional regions lead to conditions like depression, autism or schizophrenia.
Cytowic, Richard E. Synesthesia: A Union of the Senses, Second Edition. MIT Press, 2002.
Novich, Scott, Sherry Cheng, and David M. Eagleman. “Is Synaesthesia One Condition or Many? A Large-scale Analysis Reveals Subgroups.” Journal of Neuropsychology 5.2 (2011): 353–371.