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Do you hear what I'm seeing?

By Brian Alexander, contributing health writer

Imagine you're watching some incomprehensible kid cartoon on TV with the sound off. But you realize that every time there's a flash or some character runs across the screen, you hear a loud pop or a whoosh coming from the set. You double-check. Yes, it's on mute.

You may have a brain condition called "auditory synesthesia," a condition described for the first time in the journal Current Biology in August. People who experience it actually hear movement.

Synesthesia is a kind of cross-wiring of the senses. For example, when some synesthetes hear music, they also perceive colors. The most well-known form of synesthesia is called "grapheme" where someone might see, say, the number 5 on a page, but will also the color red in connection to the numeral.

The discovery of auditory synesthesia occurred serendipitously during a visiting student group tour at the California Institute of Technology. The students stopped by the lab of neuroscientist Melissa Saenz while she was running an experiment involving moving images — white dots emanating from a central point — on a computer display. 

One of the students asked, "Does anybody else hear that?" Saenz was intrigued. The computer program was running silently.

After speaking further with the student, Saenz realized the young man had all the characteristics of synethesia. "It is something he experiences all the time, not something he turns on or off, and he has experienced it since childhood," she said.

Because the auditory condition had never been described in research, Saenz went about questioning a few hundred people. She found three, indicating that auditory synesthesia is not so rare. When the three looked at the white dots, they tended to hear whooshing, bubbling or scratching sounds.

Research indicates, Saenz said, that about 1 in 100 people may have some form of synesthesia, either seeing colors with certain numbers or letters of the alphabet or picking up a smell in response.

While it was once believed that many more women had it than men, new research suggests much of that discrepancy can be attributed to the fact that men tend to keep it to themselves. They would just rather not go around telling people that Hendrix's "Purple Haze" makes them actually see purple haze.

In fact, Saenz's own patient zero had long been aware that "he was probably hearing things that other people did not hear, that logically did not make sense," Saenz says. "But it was not something he had mentioned to many people."

Lucky for him, he ended up touring one of the leading experimental neuroscience labs.

Synesthetes are not crazy, or even damaged. It's usually a benign condition. You could actually argue synesthetes have an advantage over the rest of us. Saenz believes it's possible that synesthesia is just part of the range of normal human perception.

In fact, scientists use the competitive advantages of synesthesia to verify it exists. To confirm that her auditory synesthetes were not just imagining the sounds, Saenz developed a test designed to prove synethesia. The test is based on the idea that two senses combined ought to be better than one.

The three synesthetes and control subjects were exposed to moving visual patterns and auditory patterns that were either subtly different or exactly the same. Subjects were asked to tell whether each pair of visual or sound patterns were the same or different. On the sound pattern quiz, control subjects picked the right answer — same or different? — 85  percent of the time. But during the visual pattern test, they performed with only 55 percent accuracy, that is, no better than chance.

The synesthetes performed just as well on the sound pattern test. But on the visual test, they were able to distinguish visual patterns that were exactly the same or subtly different 75 percent of the time.

Why? Because they got an aid by also "hearing" the moving visual patterns.

Are these sounds real?

"It depends on how you define sound," Saenz answers. "If define it as a perceptual event that occurs in the mind, indeed the sounds are there. If you call it a physical transmission of sound waves through air, there was no sound."