Synesthesia—from the Greek syn (union) and aisthaesis (sensation)—is a hereditary neurological condition in which ordinary activities trigger extraordinary experiences. A stimulus, such as a sound or a printed letter, produces a perception, such as color or taste. This condition has captured the popular imagination and has received considerable media attention, most recently following the pioneering identification of the first genetic regions linked to synesthesia reported earlier this year.1 In their new book, Wednesday Is Indigo Blue, neurologist Richard E. Cytowic—one of the founders of modern synesthesia research—and neuroscientist David M. Eagleman provide a fascinating introduction to synesthesia, synesthetes (people who have the condition) and research on this condition.
The book’s easy-to-digest overview of the growth of the field from relative obscurity to its current popularity enables people who are not familiar with synesthesia to dive in with relative ease. In addition to a thorough description and discussion of well-known forms of synesthesia, such as sound-color and grapheme-color (in which reading black text triggers the perception of color), the book contains the most cogent discussion to date of spatial sequence synesthesia (in which a person experiences sequences such as the days of the week as precisely ordered three-dimensional forms; see the illustration). This previously ignored variant is Eagleman’s specialty, and his enthusiasm is thoroughly engaging. His pioneering development of a virtual reality environment where synesthetes can place the forms they perceive in space around a computer-generated avatar has led to a dramatic increase in our understanding of this form of synesthesia, which experts now acknowledge to be among the most common.
Marti Pike, a synesthete, visualizes time as a series of overlapping, three-dimensional spirals. The spiral for hours varies in color along with the time; noon is bright white/yellow, while midnight is black. (Courtesy of The MIT Press. From Wednesday Is Indigo Blue by Richard E. Cytowic and David M. Eagleman, published by The MIT Press.)
The book’s discussion of taste synesthesia (encompassing taste both as a trigger and as a synesthetic response) is also particularly insightful, as we might expect given that taste-touch synesthesia was Cytowic’s entry into the field. After discussing that initial case study at considerable length, Cytowic deftly integrates his and others’ early explorations with more recent advances. Taste synesthesia is rare, but it has become an important research focus due to its implications for how synesthesia develops. For example, the tastes that synesthete James Wannerton experiences when he reads or hears words—the word maybe elicits the taste of baked beans, most tastes like toast and department tastes like jam tart—are based on Wannerton’s childhood diet. This implies that these synesthetic associations were cemented during that period of his life.
Because perception lies at the heart of synesthesia, the authors of any book on the subject must find a way to convey synesthetes’ perceptions to their primarily non-synesthetic audience. As neither author of Wednesday Is Indigo Blue is synesthetic, Cytowic and Eagleman wisely allow synesthetes to speak for themselves through quotations and anecdotes. While often less polished than the scientific portions of the text, these sections about real-life experiences offer a window into how synesthetes perceive the world. A sound-color synesthete describes her perception of her husband’s voice as “a wonderful golden brown, like crisp, buttery toast,” while a sound-taste synesthete describes the voices of people she knows as “barbecued pancakes,” “spaghetti with M&M’S” and “peanut butter.”
Most synesthetic perceptions are visual, and the frequent use of illustrations to show the reader what synesthetes “see” confirms that a picture really does speak a thousand words. This is especially the case when illustrating forms of synesthesia such as sequences in space, which are more difficult for readers to envision than are descriptions of synesthetes seeing a color when hearing a sound. The authors could have gone a step further, however; although the synesthetes’ hand-drawn sketches are charming, the use of computer graphics to render some of them would have enabled the authors to illustrate visually salient texture and three-dimensional structure as well as color.
Cytowic and Eagleman provide the most balanced discussion thus far of how synesthesia affects the lives of synesthetes. Most of us assume that the condition dominates synesthetes’ lives, but while their worlds may have a “different texture of reality,” according to the authors, most synesthetes do not orient their lives around their synesthesia any more than most people orient their lives purely around their ability to see. While the authors occasionally fall into the trap of describing synesthesia as an “astonishing gift,” they explicitly acknowledge that synesthesia is neither a gift nor a curse. This is particularly notable in light of the unfortunate tendency of some researchers and the popular media to overemphasize the condition’s positive aspects (such as grapheme-color synesthetes’ improved memory for names and numbers) while minimizing the very real disadvantages, such as sensory overload and cognitive difficulties. This popular bias distorts the public perception of synesthesia and does a tremendous disservice to synesthetes who find living with synesthesia challenging. It’s not hard to imagine that seeing colors when you hear speech would make it difficult to follow the content of a lecture, or that math could trouble young synesthetes who haven’t learned to separate synesthetic and symbolic meanings and thus find it perfectly natural to add a blue 3 and a yellow 2 to make a green 7. Eagleman and Cytowic also include anecdotes from several synesthetes who struggle with sensory overload. One woman describes her experience in London’s Piccadilly Circus thus: “Every one of my senses is being battered. . . . It’s like having nails at the back of my throat.”
The authors provide similar balance in their discussion of the debate about whether synesthetes are more creative than non-synesthetes. The authors rightly point out that although people often assume that synesthetes are more artistic, this is an artefact of sampling bias. Such claims stem from studies focusing on a few high-profile individuals such as composer Franz Liszt and artist David Hockney and ignoring the vast majority of non-artistic synesthetes. The confusion stems in large part from the use of the term synesthesia to describe an early 20th-century artistic movement whose followers attempted to meld visual art and music. There is some evidence that synesthetes are more creative in the broader sense of the word; for example, they score higher than non-synesthetes on measures of flexibility and originality. However, creative doesn’t necessarily equal artistic, and creative people can be found in any profession in which thinking outside the box is helpful.
Cytowic and Eagleman do a superb job of situating synesthesia and its research within the larger context of cognitive neuroscience. They note that although there is something very different about the brains of synesthetes, in many ways it is the similarities to non-synesthetes’ brains that are most fascinating. Substantial evidence indicates that all humans are born synesthetic.2, 3 While only some people retain explicit awareness of synesthetic perception, there are fundamental patterns (called form constants) underlying both synesthetic and non-synesthetic perception that will be readily apparent to any reader—for example, high musical tones are smaller and brighter while low tones are larger and darker. In addition to offering insight into the workings of the human brain, this discussion moves the book from being about “them” (synesthetes) to being about “us” (all humans).
While Wednesday Is Indigo Blue is an excellent introduction to the field, it is not without flaws. Discussion of the genetics of synesthesia is rather thin; it is limited to only a few pages focusing on a single paper about inheritance patterns. While this is an interesting introduction to heritability, the omission of the published molecular genetics work in this area is disappointing, particularly given the discussion of a putative “synesthesia gene”; the only mention of molecular genetic studies is a reference to preliminary results from Eagleman’s research. Timing no doubt made it difficult to include the results, published this year,1 of the first genetic study identifying candidate regions for synesthesia, but a discussion of the work done by other groups in this area and the existing molecular genetic studies on identical twins4, 5 (which have important implications for Cytowic and Eagleman’s discussion of inheritance patterns) would have added substance to the text.
Synesthesia research is a young and dynamic field with a number of active and interesting controversies. Unfortunately, the authors gloss over the central debate about the prevalence of synesthesia—which is, at its heart, a debate about the most fundamental question of all: What is synesthesia? Not all researchers agree about which phenomena should be considered forms of synesthesia, and prevalence figures vary widely depending on the definition used. The “broad tent” advocates argue for the inclusion of virtually any cross-modal phenomenon, whereas the “separate tent” advocates push for a more rigorous definition in the interest of clarity and preventing synesthesia from becoming a meaningless, catch-all term.
The authors’ main flaw lies in treating the work of Julia Simner and colleagues on the prevalence of synesthesia6 as definitive. Simner’s estimate includes phenomena such as associations between the “notion of a person” and color, but these associations are not universally accepted as forms of synesthesia. Ironically, Simner and her colleagues explicitly acknowledge the uncertainty surrounding the definition of synesthesia, and Cytowic and Eagleman themselves express doubt about some forms of “personification” synesthesia. For example, is cross-sensory perception (such as combining sound and color) really part of the same phenomenon as cross-conceptual connections (such as linking numerical sequences to three-dimensional space) or the personification of inanimate objects? Would it make more sense from cognitive and neurobiological perspectives to classify some of these conditions as “related cross-modal phenomena” rather than as types of synesthesia? This is one of the key debates in the field, and the authors’ failure to engage explicitly with it is disappointing.
Moreover, the authors’ focus on their personal journeys through the field and their (considerable) contributions to it sometimes results in a less than balanced historical perspective. In particular, the contributions of Simon Baron-Cohen of the University of Cambridge (the other founder of the field) are given short shrift; Baron-Cohen’s development of the test of “genuineness” to diagnose synesthesia7 forms the foundation of modern research in this field and deserves more than a vague one-line mention.
On a structural note, distracting references to chapter 9 (“as we will see in chapter 9” and “which we will return to in chapter 9,” for example) raises the question of whether the authors should have integrated that material into other parts of the book. It is probably the book’s densest chapter, focusing primarily on the science rather than the experience of synesthesia, which might explain why it was left until the end. However, it’s also one of the most interesting, with a discussion of synesthesia’s potential neurological underpinnings. Integrating this material into the earlier chapters would have strengthened them and the work as a whole.Despite its flaws, Wednesday Is Indigo Blue is an entertaining and informative introduction to synesthesia. It offers a window into a vastly different way of experiencing the world—and, more important, provides insight into what synesthetes’ experiences can tell us about the human brain. It also casts light on the subjectivity of what we consider reality and reminds us that everyone experiences the world in a slightly different way; synesthesia is merely an extreme on that spectrum. Reality, as the authors say, is not one size fits all—and that’s not a bad thing.