Sensory cross-wiring causes people to 'hear' movement

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.