For most people, hearing is just hearing and seeing is just seeing, but for people who have synesthesia, these senses are anything but.
Synesthesia is a condition where the stimulation of one sensory pathway causes the automatic stimulation of a second cognitive pathway. For example, when someone hears a bell, they might also see the color yellow.
Synesthesia as a condition encompasses the totality of these connections in senses, which means that someone who is a synesthete doesn't necessarily experience the same thing as another person with the same condition.
What are Synesthetes
Synesthetes, as you've likely already picked up on, are people that have involuntary connections between senses. They can see sounds, taste words, or feel smells. They also might be able to see abstract concepts like happiness projected on the world around them.
The intertwining of these senses can cause some interesting lifestyle changes for the people with the condition, but rarely in a manner that is negative to quality of life.
What's it like?
Neuroscientists often use photos of famous people to illustrate what being synesthetic is like. When you look at a photo of someone famous, your brain automatically populates your conscious with the name and other characteristics about that person that aren't related to their physical appearance. It's in this same way that a person who sees the number four might also suddenly smell roses.
The interesting thing about synesthesia though that you might not realize is that the senses aren't a hallucination. For example, synesthetes that see orange when they smell something are still able to see normally, the orange doesn't interfere with their visual processing as a hallucination would.
Synesthetic people can also have interesting reactions to silent videos, some being able to hear the sounds in the video when there isn't anything actually being played.
This is called a visually evoked auditory response or VEAR. From a technical definition this sense is not a form of synesthesia, but it seems to be very close from a neuroscientific standpoint.
If you're reading this and thinking that you must have VEAR because you hear sounds in silent situations too, you might be right. It's estimated that 20-30% of all people have this faint association of sounds and imagery.
What's the Cause?
40% of all synesthetes have a first-degree relative with the condition. This has lead scientists to believe that there is a strong genetic tie to it.
Notably, scientists studied the genetic conditions of people with synesthesia and found a commonality. Families with synesthesia did have different DNA variations, but they all had enhanced genes involved in cell migration and axonogenesis. Axonogenesis is a process that lets brain cells wire to their correct counterparts.
The conclusion here demonstrated that genetic variation can actually alter and modify our sensory experiences. Synesthesia then isn't as much a disorder as it is an example of neurodiversity.
How is it tested?
Because synesthesia isn't a condition associated with memory, doctors and scientists can "test" patients to see if they actually have the condition. For example, if a letter has a specific shade of blue related to it for you, then you'll be able to identify that shade over and over again to the tester. If you were just making the connection up, or trying to insert the connection, then capturing that color again wouldn't be consistent.
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We're also not just talking about short term consistency here, a synesthete would be able to identify the same blue for the same letter throughout the course of their lives.
Aside from practical sensory testing, researchers are also able to scan synesthete's brains using tomography and magnetic resonance imaging. People with the condition have been found to have increased activation in the areas of the brain associated with the senses they commonly experience synesthesia.
You have it, Now What?
The experience a person has with synesthesia varies greatly depending upon the patient. Some people report that it is quite bothersome and affects their quality of life, other's see it as an advantage.
For example, the people who associate letters with colors can often have problems with reading as sentences can appear to be rainbows. People who associate words with taste can also have issues getting bad tastes during conversation or study.
Most of the negative side effects of the condition are those that result in sensory overload.
However, the people that experience the condition in this light are generally in the minority.
There can be benefits to being synesthetic, such as the ability to quickly memorize strings of numbers and letters thanks to the associated senses. Or the ability to memorize information for tests or work quickly due to the increased senses associated with it.
In fact, Daniel Tammet set the European record for pi memorization in 2005 in large part due to his ability to see numbers with color, texture, and sound.
Notable historical synesthetes are Billy Joel, Van Gogh, Marilyn Monroe, and Pharrell Williams.