What Is Synaesthesia?

Forms Of Synaesthesia

History Of Research

Synaesthesia And The Arts

Further Reading

“Hearing colours?”
“Feeling sounds?”
“Tasting sights?”
“Seeing smells?”

…

There are a great number of types of synaesthesia, and within each type, individuals can report differing triggers for their sensations, and differing intensities of experiences. This variety means, defining synaesthesia in an individual is difficult, and indeed, the majority of synaesthetes are not aware that their sensory experiences have a name. However, despite the differences between individuals, there are a few common elements which define a true synaesthetic experience, from any other.

Neurologist Richard Cytowic, identifies the following diagnostic criteria of synaesthesia:

1. Synaesthesia is involuntary and automatic.

2. Synaesthetic images are spatially extended, (they often have a definite location).

3. Synaesthetic perceptions are consistent and generic, (I.E. simple rather than imagistic).

4. Synaesthesia is highly memorable.

5. Synaesthesia is laden with affect.

Synaesthetes often claim, they were unaware their experiences were unusual, until they realized other people did not have them; whilst others report feeling as if they had been keeping an unexplainable secret their entire lives. The automatic and ineffable nature of a synaesthetic experience means, the pairing may not seem out of the ordinary. This involuntary and consistent nature helps define synaesthesia as a “real” experience, rather than just an illusion. Most synaesthetes describe their experiences as being pleasant, or simply there, although in rare cases, synaesthetes report that their experiences can lead to a degree of sensory overload.

Despite the commonalities which permit definition of the broad phenomenon of synaesthesia, it should also be noted, individual experiences vary greatly in numerous ways. This variability was first noticed during the early days of synaesthesia research, (Flournoy 1893), but has only recently come to be appreciated by modern researchers.

Synaesthesia can occur between nearly any two of the human senses, or perceptual modes. Given the large number of forms of synaesthesia, researchers have adopted a convention of indicating the type of synaesthesia, by using this notation: “X-Y” - where “X” is the "inducer", or trigger experience; and “Y” is the "concurrent”, or additional experience.

For example, perceiving letters and numbers, (X), as being coloured, would be indicated as “Grapheme Colour Synaesthesia”. Similarly, when synaesthetes see colours and movement as a result of hearing musical tones, (Y), it would be indicated as “Tone (Colour, Movement) Synaesthesia”. While nearly every possible combination of experiences is logically possible, several types are more common than others.

“Grapheme Colour Synaesthesia” - identifies how someone with synaesthesia might perceive certain letters and numbers. This is one of the most common forms of synaesthesia, where individual letters of the alphabet, and numbers, (collectively referred to as graphemes), are "shaded" or "tinged" with a colour. While no two synaesthetes will report the same colours for all letters and numbers, studies of large groups of synaesthetes find that there are some commonalities.

A grapheme colour synaesthete will often associate certain letters and numbers with a particular colour. When reading and writing, the words on the page will appear in different colours and shades. Many grapheme colour synaesthetes also claim, that their perception assists them with their spelling, (in some cases, their only way of spelling), due to the specific colour scheme associated with each word.

Some grapheme colour synaesthetes have attempted to explain, that the colours they experience seem to be "projected" outwards into the world, whereas, most others have said that the colours are experienced in their "mind's eye”. Additionally, some grapheme colour synaesthetes also report, they experience their colours strongly, and which show perceptual enhancement on the perceptual tasks; whilst others, and perhaps the majority, do not, possibly due to differences in the stage at which colours are evoked.

“Music Colour Synaesthesia” - identifies how someone with synaesthesia might perceive musical sound and tones. In music colour synaesthesia, individuals experience colours in response to tones or other aspects of musical stimuli, (e.g., timbre or key). Like Grapheme Colour Synaesthesia, there is rarely agreement amongst synaesthetes that a given tone will be a particular colour, but individuals are internally consistent - tested months later, synaesthetes will report the same experiences as they had done previously.

Colour changes in response to pitch, may involve more than just the hue of the colour. Brightness, (the amount of white in a colour - as brightness is removed from red, for example, it fades into a brown and finally to black); saturation, (the intensity of the colour - fire red and sky blue are highly saturated, whereas gray, white, and black are unsaturated); and hue, may all be affected to varying degrees. Additionally, music colour synaesthetes, unlike grapheme colour synaesthetes, often report that the colours move, or stream into and out of their field of view.

Again, as with grapheme colour synaesthetes, no two music colour synaesthetes will report the same colour for a particular tone. However, generally speaking, many music colour synaesthetes will agree on higher tones appearing paler or brighter, and lower tones appearing duller or darker.

“Number-Form Synaesthesia” - is a mental map of numbers, which involuntarily, and automatically, appears whenever someone who experiences number-forms thinks of numbers. Number-forms were first documented and named by Francis Galton, in The Visions of Sane Persons, (Galton 1881).

Later research has identified number-forms as a type of synaesthesia, (Seron, Pesenti & Noël 1992). In particular, it has been suggested that number-forms are a result of "cross-activation" between regions of the parietal lobe, that are involved in numerical cognition and spatial cognition.

In addition to its interest as a form of synaesthesia, researchers in numerical cognition have begun to explore this form of synaesthesia, for the insights that it may provide into the neural mechanisms of numerical-spatial associations, present unconsciously in everyone.

“Ordinal-Linguistic Personification”, (Personification, or OLP) - is another form of synaesthesia, in which ordered sequences, such as ordinal numbers, letters, days, and months, are associated with personalities. Although this form of synaesthesia was documented as early as the 1890s, (Flournoy 1893 and Calkins 1893), modern research has, until recently, paid little attention to this form.

For some people, in addition to numbers and other ordinal sequences, objects can also be imbued with a sense of personality, sometimes referred to as a type of animism. This form of synaesthesia is harder to distinguish from non-synaesthetic associations. However, recent research has begun to show, that this form of synaesthesia co-varies with other forms, and is consistent and automatic as required, to be counted as a form of synaesthesia.

“Lexical Gustatory Synaesthesia” - is a rare form of synaesthesia, where individual words and phonemes of spoken language, evoke the sensations of taste in the mouth. University College London cognitive neuroscientist, Jamie Ward, along with Julia Simner, have extensively studied this form of synaesthesia, and have found that the synaesthetic associations are constrained by early food experiences, (refer to Ward & Simner 2003, and Ward, Simner & Auyeung 2005).

For example, Jamie has no synaesthetic experiences of coffee or curry, even though he consumes them regularly as an adult. Conversely, he tastes certain breakfast cereals and sweets that are no longer readily available.

In addition, these early food experiences are often paired with tastes based on the phonemes in the name of the word, (e.g., /I/, /N/, and /S/, may trigger the taste of mince); whereas others have less obvious roots, (e.g., /l/ may trigger the taste of strawberries). To show that phonemes, rather than graphemes, are the critical triggers of tastes, Ward and Simner showed that, for Jamie, the taste of egg is associated to the phoneme /K/, whether spelled with a C, (e.g., accept), K, (e.g., York), CK, (e.g., lucky) or X, (e.g., fax). Another source of tastes comes from semantic influences, so that food names tend to taste of the food they match, and the word blue, may taste "inky”.

“Mirror-Touch Synaesthesia” - is a more recently identified form of synaesthesia, (2007), where synaesthetes possess a heightened sensitivity to others experiencing a physical contact. A brain anomaly can make the saying "I know how you feel" literally true in hyper-empathetic people, who actually sense that they are being touched themselves, when they witness others being touched.

This condition is related to the activity of mirror neurons, cells recently discovered to fire, not only when some animals perform some behavior, such as climbing a tree, but also when they watch another animal perform the same behavior. For synaesthetes, it's as if their mirror neurons are on overdrive.

University College London cognitive neuroscientist, Jamie Ward, said: "We often flinch when we see someone knock their arm, and this may be a weaker version of what these synaesthetes experience”. Scientists find these synaesthetes possess an unusually strong ability to empathize with others. A non-mirror-touch synaesthete can experience what it may be like to have the condition, by, for example, watching themselves tap their hand in a mirror. Further research into this condition might shed light on the roots of empathy, which could help better understand autism, schizophrenia, psychopathy, and other disorders linked with empathy.