To her eyes, ears and tastebuds, for instance, a major sixth is low-fat cream and a minor sixth is full-fat cream, while C is red and F sharp is violet. Surprisingly, these sensory links only go one-way: the 27-year-old does not hear tone intervals or see colours when exposed to tastes. Unsurprisingly, she uses her uniquely linked sensations to identify tone intervals, doing so faster than five other musicians the team tested.

"We found that E.S.'s tone-interval identification was perfect," the researchers report today in the journal Nature.

For University of Melbourne cognitive neuroscientist Anina Rich, E.S.'s case is instructive. "It reminds us of how inherently subjective perception is," says Rich, who has compiled a database of 270 Australian synaesthetes.

Roughly one in 2000 people worldwide have some form of synaesthesia, from coloured hearing, letters and numbers to sounds with shape and weight. The condition is inherited and favours women three to one. Although Rich doesn't focus on musical synaesthesia as Jaencke does, she recently found that synaesthetes often end up in the arts.

"Musicians, visual artists, sculptors and so on - we found many more synaesthetes involved in artistic professions than you'd expect based on Australian census data," she claims.

Since synaesthesia was first identified by Charles Darwin's cousin Francis Galton in the late 19th century, numerous artistic luminaries have been thought to have experienced the phenomenon or at least been influenced by it.

That includes poets Charles Baudelaire and Arthur Rimbaud, artist Wassily Kandinsky and writers Vladimir Nabokov and Anton Chekhov. Composer Alexander Scriabin wasn't a true synaesthete but he used colour and sound synaesthete-style in orchestral works such as Prometheus. Nikolai Rimsky-Korsakov was supposedly the real thing.

Among contemporary synaesthetes are post-minimal composer Michael Torke, drummer Manu Katche, oboist Jennifer Paull and electronic musician Richard James.

Like all synaesthetes, their sensory blends are consistent. For example, the letter H may be green or the sound of high C pink. Still, individual associations vary between people ... with one wrinkle.

Rich and colleagues have just found that there's a "striking consistency" in the colours induced by certain letters and numbers among 150 synaesthetes: R often triggering red, Y yellow and D brown. That fits with E.S.'s reactions. She links dissonant intervals to unpleasant tastes and consonant ones to pleasant tastes.

The brain plasticity behind such mixing-and-matching is what drew Jaencke to study the intellectual, or cognitive, skills of musicians, especially those with synaesthesia.

"For instance, many [non-synaesthetic] musicians are mentioning that they use concurrent perceptions for memorising - though not as automatic and implicit as synaesthetes," he says.

"In some ways, synaesthesia can be used as an analogy for learning memory," he adds, noting that scientists have long-known that people remember things better if they're associated with other items. The classic case is the memory trick of linking a person's name with an object or action. The association is an artificial synaesthesia.

Jaencke also thinks synaesthesia may be useful in other cognitive activities. What are they? "This is the $1 million question," replies Jaencke, who's trying to discover the answer.

According to Jaencke, E.S.'s unique sensory blend was discovered by accident during a study of 12 synaesthetes, conducted by his doctoral student Gian Beeli.

Beeli measured brain waves and discovered that synaesthete brains work fast, connecting a stimulus (tone) with a perception (colour) virtually instantaneously, in just 100 milliseconds.

That's fast. It also poses the final question: Did Messiaen and co instantly know when they created a musical masterpiece?