SYDNEY: Due to cross-wiring in their sensory parts of their brains, some people with synaesthesia perceive numbers or letters as having colours. Now researchers have discovered synaesthetes who perceive movements as sounds, such as tapping, beeping or whirring.

Psychologists already knew about visual, tactile, and taste synaesthesias – another common example is attributing personalities to days of the week – but this is the first ever example of an auditory synaesthesia.

"Enhanced soundtrack in life"

"These individuals have an enhanced soundtrack in life, rather than a dramatically different experience, compared to others," said Melissa Saenz who studies computation and neural systems at the California Institute of Technology in Pasadena, USA.

Saenz and co-author Christof Koch report the find – which they made quite by accident – this week in the journal Current Biology.

"While I was running an experiment at the Caltech Brain Imaging Centre, a group of students happened to pass by on a tour, and I volunteered to explain what I was doing," said Saenz.

"As part of the experiment, a moving display was running on my computer screen with dots rapidly expanding out, somewhat like the opening scene of Star Wars," she said (see link to video below). "Out of the blue, one of the students asked, 'does anyone else hear something when you look at that?'"

Experienced all his life

After talking to him further, Saenz realised that his experience had all the characteristics of a synaesthesia: an automatic sensory cross-activation that he had experienced all of his life.

A search of the synaesthesia literature revealed that auditory synaesthesia – of any kind – had never been reported. Intrigued, Saenz began to look for other individuals with the same ability, using the original movie seen by the student as a test.

After querying several hundred people and showing possible candidates the moving dots movie, Saenz discovered three more individuals

It would have been less successful had the participants just been asked "'Do you hear sounds when you see things move or flash?' because in the real environment, things that move often really do make a sound," said Saenz, for example, a buzzing bee.

This may be why auditory synaesthesia has never been detected by scientists before.

Only in their minds

However, when asked, all of the synaesthetes could name examples of daily visual events that caused sounds that they logically knew to be only in their minds, such as seeing a fluttering butterfly or watching television with the sound turned off.

Saenz and Koch now suspect that as much as one per cent of the population may experience auditory synaesthesia.

In fact, they believe that the brain may normally transfer visual sensory information over to the auditory cortex, to help predict the associated sound. And it could be this process that results in actual perception of sounds in synaesthetes, perhaps due to stronger than normal connections, says Saenz.

"We might find that motion processing centres of the visual cortex are more interconnected with auditory brain regions than previously thought, even in the 'normal' brain," she adds.

"At this point, very little is known about how the auditory and visual processing systems of the brain work together. Understanding this interaction is important because in normal experience, our senses work together all the time."