Ask any first-grader to describe the five primary senses and you're likely to get a succinct account of how the human body manages perception. We depend on our sensory organs—eyes, ears, nose, skin and tongue—to organize and navigate the world around us.
The nervous system typically handles each sense independently and discretely. But for as many as 1 in every 100 people, scientists now estimate, there is cross-talk between senses.
The condition, known as synesthesia, is understood to be harmless and can result in some fascinating perception-mixing. Synesthetes may see colors when hearing sounds or strongly associate flavors with shapes and language. A low note on the piano may look dark purple. The word "houseboat" tastes like a glazed doughnut. (Mmm, houseboat.)
Not until this summer, though, had there been any reported cases of synesthetic auditory perception. That is, no one had documented the particular experience of hearing sounds when prompted with other sensory stimuli.
Melissa Saenz, Ph.D., a neurobiologist at California Institute of Technology, had been running an unrelated experiment at the school's Brain Imaging Center. When she showed a handful of passers-by the silent video loop on her computer screen, which looked something like a bunch of dots converging and expanding, one student asked, "Does anyone else hear something when you look at that?"
"After talking to him further," said Saenz, "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."
The student has what Saenz describes as an enhanced soundtrack to his life. He has an auditory experience when he sees ants on the ground or sunlight dappled on water. Seeing birds hop on a lawn, he reports a soft, scooping sound. He hears motion.
Saenz hopes to learn more about synesthesia and about normal sensory experiences with the brain-imaging experiments she now has underway at Caltech. Her work depends on activating the parts of the brain's cortex (the "gray matter") where vision and sound are each processed. One hypothesis she's working on is that our brains routinely transfer visual information over to the auditory section to "predict" the associated sound. That is, our eyes provide a preview of what a motion will sound like.
A normal brain requires the actual sound to complete the perception of hearing. But a synesthetic brain may be able to "hear" the visual cue.
Learning that the visual and auditory areas of the cortex have such a strong connection would be big news in brain world. It's long been assumed those two functions work independently, like a strip of film with images on one track and sound on another. Currently, each sense is understood to operate within its own closed system until it reaches a multi-sensory region where perceptions are integrated.
See what I'm saying?
Saenz's work may also challenge existing theories about synesthesia. One holds that areas of the cortex usually differentiate early in life, but in the synesthetes' brain, they never do. As Daniel Levitin, professor of psychology and neuroscience at McGill University, describes it, "The neurological basis [of synesthesia] is that the sensory regions in the brain fail to differentiate during early neonatal development."
Another idea is that in a synesthetic brain, those multi-sensory regions where sensory experiences merge are not the one-way roads they usually are. Instead, some signals run back to the single-sensory regions, effectively producing hallucinations.
The acid test
This all sounds pretty trippy. In fact, it's no coincidence that synesthesia shares some hallmarks of an acid high, like being enveloped in swirling colors as music plays. LSD-induced synesthesia was never well understood, but the drug may have been similarly affecting areas of the cortex or the brain's limbic system.
While we wait to learn more, those with synesthesia can enjoy their enhanced soundtrack without pharmaceutical assistance. Turn on, tune in … taste an adjective.
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