What color is a piano note? How loud is purple? What’s the brightness of a melody? If these questions are leaving you a little confused, that just means you don’t have synesthesia, a rare sensory condition that’s estimated to affect just one out of twenty people. If you do have it, then your answer to those questions might be very different from anyone else’s, since it affects each person in a unique way. Synesthesia means “joined senses” in Greek, and that’s exactly what it is: a mixture of different senses within the brain so that people can taste the words they read on a page, feel numbers as if they were building blocks, or literally see time flowing around them.The most famous kind of synesthesia might be chromesthesia, a neurological link between sound and color that allows people with this condition to see each note as a separate color. The result can be both beautiful and a little dizzying to watch, and it reveals a deeper link between the senses, and between sound and light themselves, than most of us realize.You can find a link on my blog to a short video from Great Big Story and chromesthetic artist Melissa McCracken as she shows us how she turns the songs she hears into paintings:https://www.youtube.com/watch?v=zbh7tAnwLCYWhile synesthesia is a neurological condition that seems to have a genetic component, it isn't technically a disorder or a disability, and it definitely isn't a disease. If anything, it seems to help some of the people who have it. There are reports of "synesthetes"  having better memory and creativity than average, and for people with chromesthesia, the ability to see sound can make them more successful as musicians or artists. Leonard Bernstein famously wrote about his ability to see timbre as color and would often notice and correct changes in timbre without anyone else being able to tell the difference. Some historians suspect Isaac Newton may have also had chromesthesia, given that he devoted much of his life to exploring the relationship between musical notes and color.The cause of synesthesia is still a mystery, but it runs in families and seems to develop in early childhood while the brain's learning how to process language and symbols. One theory is that extra neurological connections are formed between different senses and the parts of the brain that process them, which might explain why having it seems to have other benefits like boosting memory and problem-solving skills. Another is that the connections are the same, but the boundaries between those different parts of the brain are weaker. That could explain why some brain injuries, and even certain hallucinogens, can also cause it. But no one's sure if that's really the same kind of synesthesia, or if all the different kinds out there are related to each other. It's still a very mysterious topic.Want to see a classical violin piece through the eyes of someone with chromesthesia? Check out my blog for a link to a YouTube 360° video of Kaitlyn Hova, a neuroscientist and musician with chromesthesia, who’s created a real-time VR simulation of how she sees music:https://www.youtube.com/watch?v=obrBAysVef0Most of us can't see music for ourselves, but that doesn't mean sound and color don't have a lot in common. Each one is a broad spectrum of wavelengths much larger than the part we can perceive, and both light and sound have certain wavelengths that are more comfortable than others. For light, we're most sensitive to a wavelength of 555 nanometers, which is bright green. The color our eyes are least sensitive to is blue, which is one reason why LED displays strain our eyes. On the other hand, blue light also helps us relax, so it's becoming a popular choice for street...