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77.4 - Summer 2004

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By Lekshmi Santhosh

Jane Mackay painted “Tchaikovsky’s First Piano Concerto” to creat a synaesthetic composition that bridges the gap between sight and sound. (Credit: Jane Mackay)

Every morning is unique and different. Simply reading the newspaper elicits dazzling flashes of color at every word. Listening to a Beethoven concerto on the radio triggers a symphony of color. The ringing of the doorbell conjures up images of several triangles geometrically arranged in space, while the barking of the neighbor’s dog produces a pattern of red concentric circles. Meanwhile, the colorful rainbow elicits a cacophony of sounds. These are not hallucinations of a drug abuser but the day-to-day experiences of a synesthete, one who is afflicted with synesthesia.

The word synesthesia literally translates into “joint sensation,” a combination of the Greek words “syn” for together and “aesthesis” for perception. Synesthesia refers to the phenomenon in which the stimulation of one sense involuntarily induces perception of another sense. To put it simply, synesthetes can hear colors, feel shapes, taste voices, or experience other types of cross-modular sensory blending.

There are two types of synesthesia: acquired and idiopathic. Acquired synesthesia is usually associated with epilepsy, occurring in four percent of temporal lobe seizures. Idiopathic synesthesia is the rarer, more interesting form. Found in around one in every 25,000 individuals, it is highly variable because each synesthete has different fusions of the senses. In some cases, temporary synesthesia may also be induced by head trauma, hallucinogens, and lesions to the medial temporal lobe of the brain.

Although synesthetes’ experiences vary greatly, “there is a certain constellation of behaviors,” according to Lawrence Marks, Yale professor and director of the John B. Pierce Laboratory, an interdisciplinary research institute studying effects of the environment on health. He explained that synesthetes display similar behaviors both in their childhoods and their adulthoods. As children, many are “closet synesthetes” because of the looks of disbelief or even punishment they might have received for trying to explain their condition. However, as they grow into adulthood and discover that the disorder has a name, many cease to consider themselves abnormal.

All synesthetes report “generic” experiences: They do not taste cinnamon or see gorgeous, detailed scenery — they experience pleasant or bitter tastes, see colors and patterns, and feel rough or smooth surfaces. Also, synesthetes report consistency in their experiences; for example, the C note always means red.

Several factors are linked with synesthesia, even though there is no particular “type” of person who is predisposed to it. Females are much more likely than males to be afflicted, and non-right-handed people are more likely than right-handed. Synethetes seem to have superior memories and diminished mathematical and spatial abilities, indicating that synesthesia is predominantly a “left-brained” function. Furthermore, neuroimaging studies have shown that the limbic cortex and the hippocampus are often over-activated in synesthetes.

Each sense is controlled by a defined region in the brain. (Credit: Massachusetts General Hospital)

Thanks to neuroimaging and cognitive psychology studies, we now view synesthesia as a neurological phenomenon and have confirmed the experiences of synesthetes. This, however, was not always the case. For years, scientists, psychologists, and families alike dismissed synesthesia as a mental illness or as the product of a vivid and overactive imagination. Synesthesia was first reported in the nineteenth century, but interest waned with the rise of behaviorism and experimental psychology. Though some experiments were performed in the 1950s to see if synesthesia could be induced by classical conditioning, nothing proved definitive and interest died out. With increased technology and publicity, however, both popular and scientific interest has been rekindled. Now, the American Synesthesia Association educates people and investigates the science of synesthesia. Even Yale has hosted one of its meetings.

While studying the effects of synesthesia on mental processing, well-known psychological tests, such as the Stroop test, have silenced many skeptics. In the Stroop test, subjects are asked to name the color of the word they see. Individuals take longer to name words that describe a color different from the color of the text — this is known as Stroop interference. Lexical synesthetes, however, usually associate individual letters with specific colors; moreover, they have fixed ideas about which color represents which letter. Because the associations vary from one synesthete to another, there have been heated debates between synesthetes on such topics. For example, one synesthete may maintain that the letter B is white while the other associates it with blue. These synesthetes display an interesting form of Stroop interference: they will name colors more slowly for letters printed in colors that do not match their “synesthesia color” than for letters that do match.

In the heated debate on the etiology of synesthesia, some argue that it is acquired and caused by environmental factors while others argue that it is inborn through genetic causes. Marks points out, however, that since letters are a cultural convention, lexical synesthesia may not be inherited. This also raises the question as to what happens when one switches languages. Do the same sounds express the same colors, or is the entire color-alphabet switched? Synesthesia has been proven to run in families, but Marks cautions that “there have been behavioral links found but no true evidence of genetic transmission.” Some scientists speculate that since more women are affected, the gene may be sex-linked, while still others wonder if it is an autosomal dominant gene.

Most scientists agree that there is definitely a developmental component. Marks hypothesizes that “maybe different people have different developmental time courses.” Synesthesia has been linked to hypotheses on how infants view the world. The Neonatal Synesthesia Hypothesis, for example, claims that infants view all sensory inputs as synesthetes do. On the other hand, the Cross-Modal Transfer Hypothesis proposes that infant integration of the senses only occurs much later in development. Perhaps adult synesthesia is merely a problem in the differentiation of the perceptual system, as in infants. “We really still don’t have a lot of evidence — it is all suggestive but not definitive,” Marks said.

While researchers strive to uncover the clues to this mystery, many synesthetes regard their condition as a gift especially conducive to art. In fact, synesthesia has been an inspiration for many famous artists and poets affected, including the French poet Arthur Rimbaud, who wrote about how each letter represented a different color for him in his poem Voyelles (vowels). Rimbaud first learned that he had synesthesia by repeatedly looking at the letters in a children’s book and finding that the same colors appeared every time he looked at certain letters. Similarly, writer Vladimir Nabokov complained as a toddler to his mother that the colors of his toy blocks were all mixed up. Russian compuser Alexander Scriabin even built a “color-organ” to please his synesthetic eye and ear.

Despite advances in both neuroimaging and psychology, synesthesia is still very much an exotic condition – a terrain ripe with questions and unsolved mysteries. Thanks to research in the field by scientists like Marks, the experiences of synesthetes around the world are being legitimized. “There is a new position of synesthesia in the scientific community. It’s being taken seriously now and viewed as a real neurological condition. We hope that it can some day shed light on the nature of the nervous system and the link between perception and cognition.”

Science Links

Present your research to New Haven high school students (Synergy Outreach) Astronomy Picture of the Day (NASA) BBC Science & Nature (BBC) Learn how our lifestyles can change the way our genes work, examine a yet-to-be-broken code on a sculpture called Kryptos, see preserved dinosaur blood vessels (NOVA ScienceNow) Watch Health Videos: Weight-loss surgery to caffeine on the brain (CNN Health News) Science, Health, Environment & Technology (ScienceDaily)

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