When Mario listens to Mozart, a symphony of colors is displayed in his head. Alicia associates the smell of fresh air with a rectangle, while the aroma of coffee is cloud shaped. Meanwhile, Alvaro says that he “sees” his name in the colour red because it begins with the letter A. These are not the results of an overactive imagination, but are examples of synesthesia, a neurological configuration that causes information from different senses to be combined. Medicine described this peculiar way of feeling in the nineteenth century, though interest in the phenomenon waned by the mid-twentieth century. More recently, modern neuroscience has delved into its mechanisms, demonstrating that all human beings are a little synesthetic to some extent, although we may not be aware of it.
Beyond being a condition that affects one in twenty-five people on the planet —that is to say, more common than autism, despite not being much spoken of— synesthesia has much to teach us about how the human brain works. For example, researchers at the University of Sussex (UK) recently analyzed how various synesthetes reacted when they were shown compound words such as scarecrow, basketball and rainbow. Specifically, they worked with some twenty subjects who were grapheme-colour synesthetes, which means that numbers and letters are identified with specific colours. In the experiment they had to identify which words were associated with one colour and which were linked with two. The results revealed that almost all words (75%) were assigned with two colours, and only those compound terms frequently used in daily speech were associated with a single colour. From this it follows that the human brain, both synesthetic and non-synesthetic, processes compound words as two units if they are rare and as a unit when they are common. Related to this is another study from the University of Waterloo (Canada) suggesting that, for synesthetes, those letters and numbers that they use daily shine more brightly.
“What color is Tuesday? Exploring synesthesia” Credit: Richard E. Cytowic / TED-Ed
While exploring the synesthetic mind, neuroscientists have also learned that their brains do not function incorrectly, but rather disproportionately. In other words, synesthetes have a hyper-excitable brain. Specifically, in the case of grapheme-colour synesthesia, in order to stimulate the neurons of the primary visual cortex —in charge of processing what we see— far fewer stimuli are needed than are required to activate those same nerve cells in a normal brain. It is even possible to eliminate or increase synesthetic experiences by lowering or raising the excitation threshold of neurons. “With a lower threshold, it is easier for the neurons to light up, and this gives access to a conscious experience of colour when we see letters or numbers,” says Cohen Kadish, co-author of the study.
In his laboratory at the Monell Chemical Senses Center in Philadelphia (USA), Johan Lundstrom has reached a similar conclusion. When electrically stimulating the visual cortex of “normal” subjects, he found something unexpected: an improvement in the capacity of their sense of smell. His experiment showed that, at a basic level, the relevant brain structures responsible for processing sight and smell information are connected in all of us. “The crossover between senses exists in all human beings, so we can be considered universally synesthetic to some degree,” argues Lundstrom.
This leads to a theory that synesthetes do not experience extraordinary associations, but simply become aware of them, while the rest of the population ignores them. In other words, any brain is able to connect the stimuli captured by different senses, but in 24 out of 25 individuals this happens at an unconscious level. The study of synesthetes thus sheds light on the brain mechanisms that underlie the conscious experience of all human beings.
It could also provide tips for improving memory. More than a decade ago, a team of Canadian researchers encountered a 21-year-old student with synesthesia, who was able to remember strings of numbers better than anyone else. Random chance? It seems not, according to data collected by British researcher Nicolas Rothen, who says it makes sense when you consider that “extra-perceptive” experiences increase the coding information and offer more opportunities to remember stimuli, especially as it affects the visual memory. Meanwhile, Clare Jonas at the University of East London, who is training non-synesthetes to make the same associations between letters as synesthetes do, has shown that their ability to remember series of letters improves. Jonas believes that “trained synesthesia” could even help patients recover from brain damage, and perhaps slow the cognitive decline found in Alzheimer patients.