Types of Synesthesia Explained

Maybe you’ve heard of people having strange responses to certain senses — they’re seeing sounds, hearing smells, feeling sights. It sounds strange, but this uncommon phenomenon is called synesthesia. But what is synesthesia, exactly?

Synesthesia is a neurological phenomenon in which stimulation of one sense involuntarily activates another — like feeling a certain sensation when hearing a specific instrument.

Types of Synesthesia

Synesthesia is rare, and it’s hard to get a good estimate of how many people have it. People have different types of synesthesia based on which two senses are activated together. Often, people with synesthesia — they’re called synesthetes — have more than one type.

The different types of synesthesia are defined by the associated senses. The two most common types are:

  • Color-graphemic synesthesia, which occurs when certain letters or numbers are associated with a color or pattern
  • Color-auditory synesthesia, which occurs when sounds lead the person to see colors, textures or shapes

A Boston University researcher estimates that there could be more than 35 subtypes of synesthesia. Almost any pairing of senses — such as taste and hearing, or sound and touch — is possible.

What Causes Synesthesia?

“Synesthesia is rare, and some researchers are just beginning to understand this phenomenon,” said Jayashiri Srinivasan, MD, PhD, FRCP, a neurologist at Lahey Hospital & Medical Center in Burlington, Mass. She said some research suggests that the brains of people with synesthesia may have excessive neural connectivity and don’t go through the standard pruning process. Scientists theorize that we’re born with all of our senses connected, and as our brains naturally begin to prune neurons, these sensory connections are severed. It’s possible this doesn’t happen for synesthetes.

It’s not a medical problem requiring a doctor visit, so someone may have the condition and go decades without realizing his or her perception is different from others. Or, they may realize early on that something is different and try to hide it. Psychology Today estimates 3 to 5 percent of the population experiences some type of synesthesia.

Synesthesia seems to run in families, and it starts at birth. Women also tend to experience it more often.

What It’s Like Having Synesthesia

Perhaps the biggest question after “What is synesthesia?” is “What is it like to be a synesthete?” For synesthetes, the connection of senses isn’t something they can turn on or off. It’s just always there. For example, a person may always see a name or a letter in a particular color. This doesn’t change, even as the person ages.

There’s no standard test for diagnosing synesthesia. A diagnosis comes more from consistent responses and an individual’s reported experiences.

“Researchers have tried to learn more about synesthetes with functional MRI,” Dr. Srinivasan said. A functional MRI uses a dye that binds to glucose, which causes neurons to light up during an MRI. “A synesthete will have different parts of the brain light up in response to a stimulus than people without synesthesia.”

It might not be surprising, but many synesthetes choose artistic or creative fields. It’s thought that Vincent Van Gogh, Tori Amos, Vladimir Nabokov and Pharrell Williams are or were synesthetes.

“Synesthesia is an interesting phenomenon and illustrates how different people’s brains can be,” Dr. Srinivasan said.

*The content on this website is for informational purposes only and is not medical advice. Please consult a physician regarding your specific medical condition, diagnosis and/or treatment.

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