The number “6” is a bright shade of pink. Listening to a cello smells like chocolate. And eating a slice of pizza creates a tickling sensation on the back of your neck.
If you have experiences like this, you may be one of the special people with an unusual sensory condition called synesthesia (pronounced sin-uhs-THEE-zha).
People with synesthesia experience a “blending” of their senses when they see, smell, taste, touch or hear. Such people have specially wired brains, so that when something stimulates one of the five senses, another sense also responds. This blending can cause people to see sound, smell colors or taste shapes.
Dozens of different sensory combinations exist. In the most common form of synesthesia, numbers, letters or even days of the week appear in their own distinct color.
If you’ve encountered these types of events, you’re not alone. Scientists say as many as one in every 200 people may be a synesthetes, as people with this condition are called. The phenomenon is known to run in families, and may occur more often among women than men. Many famous people have had synesthesia, including Russian writer Vladimir Nabokov and physicist Richard Feynman.
One thing is certain; most synesthetes treasure their unusual ability to take in the world with an additional sense. After all, who wouldn’t want to experience the world in full, glorious color or sound?
“It’s absolutely a positive experience,” says Patricia Lynn Duffy, a synesthete who has talked to hundreds of others with the condition while writing a book on the subject. “If you proposed to take away someone’s synesthetic ability, I think they would say, ‘No, I like it this way.’’’
What Color is my “i”?
Most synesthetes learn about their amazing gift by accident. They are surprised to learn that everyone does not experience the world as they do.
Though it may sound strange to many people, Duffy says the experiences are not scary. The people who have synesthesia have always experienced life that way.
“For as long as I could remember, each letter of the alphabet had a different and distinct color. This is just part of the way alphabet letters look to me,” says Duffy. “Until I was 16, I took it for granted that everyone shared those perceptions with me.”
Synesthetes do not actively think about their perceptions — they just happen. Some synesthetes report that they see such colors internally, in “the mind’s eye.” Others, such as Duffy, see their visions projected in front of them, like watching an image on a movie screen.
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Scientists know that in synesthesia, those colors are real, not just figments of an active imagination. How? Studies show that the colors synesthetes see are highly specific and consistent over time. If the letter “b” is lime green, it will always be lime green.
Studies done in the mid-1990s showed that synesthesia also can be measured by brain-scanning techniques. For synesthetes who perceive colors when hearing words, a certain part of the brain involved with vision is active in response to sound. That type of activity didn’t occur in non-synesthetes.
So how can the sound of a musical instrument lead to color?
Scientists are still trying to discover exactly how information from the senses merge together in the brain. But this much is known:
Messages gathered from the eyes, ears, mouth, nose and nerves involved in the sense of touch travel to the brain for processing. Much of this sensory processing occurs in an area of the brain called the cortex, the outermost part of the brain that organizes and enables us to respond to the incoming messages.
Information from each of senses is first processed in its own special region. It’s then sent on to “higher” regions in the cortex for further processing. At certain points in the brain, these various senses converge.
One theory is that synesthesia may be caused by “cross-wiring” between areas of the brain that process different sensations, such as color, sound or taste. This theory draws on the fact that children are born with many nerve connections between nearby parts of the brain.
“During our first few years of life, our brain makes more connections than it needs, and then eventually prunes some of those away,” says Edward Hubbard, a post-doctoral researcher at the French National Institute for Health and Medical Research who studies what causes synesthesia.
One thing that may happen in synesthesia, Hubbard says, is that some of these connections don’t get pruned away. If so, then people may see specific colors with particular letters because they have extra connections between the brain areas involved in word and color perception.
Last summer, a group of scientists in the Netherlands found direct evidence of these types of extra connections.
The researchers used a method called DTI to scan the brains of 18 people with synesthesia. They also looked at the brains of 18 non-synesthetes.
DTI (which stands for diffusion tensor imaging) measures how water flows in the brain. Within certain brain tissues, or nerve fibers, water flows more freely in one direction than the other. This is especially true in a type of nerve fiber, or axon, that carries messages from brain cell to brain cell. Commonly called “white matter,” these axons connect different parts of the brain to each other.
By measuring the water flow through these tissues, the scientists could measure how many of these axons there were in each brain region. Brain regions that are highly connected will have more white-matter axons.
In synesthetes who saw colored letters, the scientists found higher levels of white matter in three different brain regions. One was in the letter and word region of the brain, known as V4. The other highly connected areas were found in brain regions involved in consciousness — the awareness that you’re thinking, feeling, seeing, hearing or any number of other things your brain enables you to do.
“We have lots of things impinging upon our senses, and some of them become conscious and some of them don’t,” says Hubbard. “Activity in this area might make a person more consciously aware of a synesthetic experience.”
These findings don’t rule out other possible causes of synesthesia, says Hubbard. Still, he is now working to see if this type of “cross-wiring” occurs in other forms of synesthesia. Other scientists are looking to see whether other parts of the brain are also involved in synesthesia.
Hubbard is also developing better ways to identify the various processing regions of the brain. “Everybody’s brain differs a little bit in its exact organization,” he says.
Duffy notes that these variations in nerve connections occur not only in synesthetes, but in all people.
“Everybody develops a neural pattern that’s kind of unique, just like a fingerprint,” she says. “That’s why no two people are seeing the world in exactly the same way.”Power words:
axon The long portion of a nerve cell that carries impulses away from the body of the cell.
cortex The outer layer of an organ or body part, such as the cerebrum or the adrenal glands.
nerves Any of the bundles of fibers made up of nerve cells that carry information in the form of electrical impulses throughout the body. Nerves send sensory information to the brain and spinal cord and carry impulses to the muscles, organs and glands.
brain The part of the nervous system in vertebrates that is enclosed within the skull, is connected with the spinal cord and is composed of gray matter and white matter. It receives and interprets impulses from sense organs, and it coordinates and controls body functions and activities, such as walking and talking. The brain is also the center of memory, thought and feeling.
stimulus Something that causes a response in a body part or organism. A stimulus may be internal or external. Sense organs, such as the ear, and sensory receptors, such as those in the skin, are sensitive to external stimuli such as sound and touch.