Aug. 20, 2008 -- Scientists at the California Institute of Technology have discovered a new form of synesthesia, a rare but benign condition in which some people see the world quite differently than the rest of us. To them, letters or numbers have specific colors and tastes, may have specific shapes, because one sensory perception mysteriously leads to an automatic experience in a second sensory pathway.

Now, it turns out, some synesthetes hear sounds when they see movement or flashes of light.

The discovery took neurologist Melissa Saenz by surprise. She was running a program on her computer in the Caltech Brain Imaging Center when a group of students passed by. The screen on her computer showed bright lights moving rapidly back and forth, and a student asked a question that has precipitated a whole new field of inquiry.

"Out of the blue, one of the students asked, 'Does anyone else hear something when you look at that?'" Saenz said in a telephone interview. None of the others heard anything, because there was no sound associated with the lights.

Saenz interviewed the student at length, and determined that the student did, indeed, have synesthesia. Along with Christof Koch, professor of cognitive and behavioral biology at Caltech, Saenz searched the scientific literature for similar cases, but "no one had published a paper on auditory synesthesia, so it hadn't been reported before," she said.

It didn't take long for Saenz to locate three other people affiliated with Caltech, either students or members of the community, who also have auditory synesthesia. A series of experiments have demonstrated that the condition is quite real, and even somewhat beneficial to the synesthetes.

"They have a slightly enhanced soundtrack in life," she said, and she suspects they are more numerous than had been thought, especially since a new form of the condition has been discovered.

Saenz believes auditory synesthetes have not been known about before because objects that move, and bright flashes of light, often occur with sounds, so it's a little harder to detect. In some cases, she said, synesthetes may realize they are different when they hear sounds where there clearly should be none, like sounds coming from a television set when the audio is turned off.

In fact, that's similar to what happened to the student who raised the question in her lab. He was at an art show, and "there was a very bright flashing light that made such a clear sound for him that he couldn't ignore it," Saenz said. "He asked his mom, 'Do you hear that, too?' Then he realized it was only him."

The study of synesthesia is so new that there are still many questions unanswered. Some studies have indicated that the experience is different for each synesthete, and this research reinforces that.

All four participants in her study reported hearing sounds from flashing lights that were similar, in that they were basic, uncomplicated sounds, yet different in that the sounds varied from beeps to scratching to tapping.

One experiment showed that the synesthetes benefited from what some call the "cross activation of brain regions," or simply cross fertilization of sensory pathways.

Several participants, including the synesthetes and others without synesthesia, observed patterns of rhythmic flashes similar to visual Morse code, and had to guess whether subsequent patterns were the same or different. When the flashes were accompanied by sound, like beeps, Saenz and Koch found no difference in the performance of those with synesthesia and those without.

But when the test was repeated without the audio track, the synesthetes outperformed the nonsynesthetes dramatically because they still "heard" the sounds. Two sensory pathways are better than one.

Although people with synesthesia are not handicapped, and do not suffer from their condition, they are fascinating to neuroscientists because of what their condition might tell about how the human brain functions.

"Very little is known about how the auditory and visual systems work together in the brain and communicate with each other," Saenz said. "We know that when we see and hear something at the same time, what we hear can influence what we see, and vice versa. But we don't really know why."

Lee Dye is a former science writer for the Los Angeles Times. He now lives in Juneau, Alaska.