Synesthesia is a condition in which stimulation of one sense triggers another. For example, hearing something may cause you to experience specific visual images or tastes, or seeing something may cause you to taste something or physically feel something. This may seem like a common thing that all of us experience from time to time, but it's estimated that no more than 1 in 2,500 people actually experience synesthesia, as it is a neurological condition.
This video briefly describes the condition:
There are several different types of synesthesia. The most common type of synesthesia is called Grapheme - Color synesthesia. This is where a person would see individual letters or numbers as a specific shade or color.
The type of synesthesia I'm most interested is Color - Sound Synesthesia, where noises or music trigger colors or firework-like sensations. I've always found the idea of this very interesting because it is a condition that my boyfriend has. He describes it to me as, "Seeing different colors or shades dependent on music." His synesthesia determines which kinds of music he listens to. Music that causes him to visualize gray or rusty colors is extremely unappealing to him, where music that causes pleasant colors is very appealing to him. He told me that for most of his life he thought it was something that everyone experienced, until he tried talking about it with a friend. He then realized that it is something that not many people experience. I must admit that I've always been somewhat jealous of his condition, because I can't imagine what it must be like to have music cause me to see colors, and so the thought is somewhat appealing.
No one is really sure where synesthesia comes from, but some studies propose that it's possible that it may be genetic. Though this article talks about the possibility of a genetic factor, there hasn't been any proof as to where exactly the condition comes from.