In fact, they said that auditory synesthesia is not unusual and may simply represent an enhanced form of how the brain normally processes visual information.
While visual, tactile, and taste synesthesias, have already been reported, it was Caltech lecturer in computation and neural systems Melissa Saenz, who discovered the auditory synesthesia quite by accident.
"While I was running an experiment at the Caltech Brain Imaging Center, a group of students happened to pass by on a tour, and I volunteered to explain what I was doing," explained 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. Out of the blue, one of the students asked, "Does anyone else hear something when you look at that?" After talking to him further, I realized that his experience had all the characteristics of a synesthesia: an automatic sensory cross-activation that he had experienced all of his life," said Saenz.
On further research she observed that auditory synesthetes happened to hear sounds when they saw objects moving on screen, even when there wasn't any sound. But, this according to them was quite natural, because in the real environment, things that move often really do make a sound, for example, a buzzing bee. And that's why auditory synesthesia hadn't been detected earlier by neurobiologists.
"People with auditory synesthesia may be even less likely than people with other synesthetic associations to fully realize that their experience is unusual. These individuals have an enhanced soundtrack in life, rather than a dramatically different experience, compared to others," said Saenz.
The researchers found that the four synesthetes outperformed a group of nonsynesthetes on a simple test involving rhythmic patterns of flashes similar to visual Morse code. Normally, such patterns are easier to identify with sound (beeps) than with vision (flashes), so the researchers predicted that synesthetes would have an advantage with visual patterns because they actually heard a sound every time they saw a flash.
Saenz said: "Synesthetes had an advantage because they not only saw but also heard the visual patterns."
Saenz and her team suspect that as much as 1 percent of the population may experience auditory synesthesia. In fact, they believe that the brain may normally transfer visual sensory information over to the auditory cortex, to create a prediction of the associated sound.
"This translation might result in actual sound perception in synesthetes, perhaps due to stronger than normal connections," said Saenz.
She has already begun brain imaging experiments to study this connectivity in synesthetes and nonsynesthetes.
"We might find that motion processing centers of the visual cortex are more interconnected with auditory brain regions than previously thought, even in the 'normal' brain," said Saenz.
The findings of the study are reported in the latest issue of the journal Current Biology.