If you’ve ever heard anyone describe the sound of a voice as “crumbly and yellow” or the name Paul as “ugly and grey” or perhaps music as shapes and colours, then you could well have been in the company of a synesthete. Although considered a rare neurological trait, statistics show that 1 in 23 people have some form of synesthesia. In his book Synesthesia, American neurologist Richard E. Cytowic shares his findings that have been compiled from years of study involving hundreds of people with this trait, and tells us why continued research in this area is so important for all of us.

Synesthesia is clearly explained by Cytowic as “joined or coupled sensations”, and individuals who possess this neurological trait perceive the world through “cross-sensory experiences.” Some people might be able to feel, taste or see a sound as well as hear it, and flavours might have colour, for example. Elaborating on the intricacies of synesthesia and its various manifestations, the author gives us an insight into the 200-year recorded history of synesthesia, while expertly combining his detailed scientific analysis with the often-humorous personal accounts of synesthetes who experience life through this most vivid of multi-sensory lenses.

Cytowic’s first book, Synesthesia — A Union of the Senses was first published in 1989 and awakened new interest in the subject when his research showed that synesthesia was “brain-based and perceptual rather than being mind-based as is the case with memory.” In 2003, he wrote The Man who Tasted Shapes, which was inspired by a friend who at a dinner party once claimed that he could feel intense flavours.

“When I taste something with an intense flavour, I also feel it on my face and in my hands. A sensation sweeps down my arm, and I feel weight, shape, texture and temperature as if I’m actually grasping something.”

In 2009, Cytowic co-wrote Wednesday Is Indigo Blue: Discovering the Brain of Synesthesia with neuroscientist David M. Eagleman. Together they brought neuroscientific consideration to synesthesia. Oliver Sacks, with reference to Wednesday Is Indigo Blue, observed that “Twenty years ago, synesthesia — the automatic conjoining of two or more senses — was regarded by scientists (if at all) as a rare curiosity. We now know that perhaps one person in twenty is synesthetic, and so we must regard it as an essential, and fascinating, part of the human experience. Indeed, it may well be the basis and inspiration for much of human imagination and metaphor.” (p. xv)

Along with all the scientific data, tables, diagrams, figures and both coloured and black & white plates, this book is peppered with anecdotes from everyday people through to famous musicians and artists who have synesthesia. A seven-year-old girl tells her friend that “the letter A was the most beautiful pink she had ever seen” and composer, Franz Liszt, who, as the newly appointed Kapellmeister at Weimer, surprised his orchestra when he said, “Oh please, gentlemen, a little bluer, if you please! This tone type requires it!”

Other famous musicians and artists who have coloured hearing include Rimsky-Korsakov, Jean Sibelius, Olivier Messiaen, Stevie Wonder, and Lady Gaga, to name a few. In Chapter 7, titled ‘See with Your Ears’ (p. 129) Gyorgy Ligeti, whose music was featured on the soundtrack for 2001: A Space Odyssey, says he associates sounds with colours and shapes

“…when I say that C minor has a rust-brown colour and D minor is brown this does not come from the pitch but from the letters C and D.” (p. 142).

David Hockney and Wassily Kandinsky are both synesthetes. For Kandinsky, each colour has “an intrinsic sound” and after spending time with Hockney in Los Angeles to gain some insight into his particular form of synesthesia, Cytowic found that it was sound, and more directly, melody that triggered the seeing of colours and shapes for Hockney, who then injected this into his art. For composer Michael Torke, the key of D major has been the colour blue since he was five years of age, which is not surprising for Cytowic as his research tells us that synesthesia appears at an early age and is genetically determined.

Synesthesia is a comprehensive and succinct examination of this fascinating, and often scientifically ridiculed, phenomenon. As a new addition to The MIT Press Essential Knowledge series, Cytowic’s latest contribution is in keeping with their ethos of providing “specialized subject matter for non-specialists” by experts in their chosen field and is in the form of a tastefully produced pocket-size book. Useful as a primer for those who want to undertake further study into the realm of synesthesia, this book is also informative for synesthetes who may or may not know they have this trait, and for anyone who has ever wondered about it.

Cytowic believes that the study of synesthesia shows that “not everyone sees the world as you do.” More importantly, he believes the examination and understanding of synesthesia provides a “peephole onto this big expanse of how the mind works and how the brain is organized.” And who knows where that could take us. Perhaps on a journey into the green, velvety unknown.