Have you ever heard the color blue? Or tasted the musical note F-sharp? For most people, in both cases the answer is no. But for some people blue has a particular sound or shape, and F sharp has a slightly sour taste. This unusual blending of the senses is called synesthesia, a rare neurological phenomenon occurring in roughly 1 in 100,000 people.
People with this condition do not merely associate sound with color or taste with sound, or imagine hearing a sound when they see a certain color. Rather, when a person with synesthesia encounters a particular sensory stimulus, say seeing the color red, she will hear a sound even when there is no outside source producing the sound.
In a similar manner, a particular sound may cause someone with synesthesia to see certain shapes or certain colors, even when there is no object in that person's line of sight. The same goes for taste and smell.
Scientists know very little about what causes synesthesia. While it appears to be hereditary, the actual neurological processes that account for the phenomenon remain a mystery. Although most scientists agree that the hippocampus, an area of the brain responsible for memory, plays a role, exactly how the hippocampus might cause synesthesia is not clear.
Otherwise, scientific knowledge about synesthesia is limited to several interesting observations. For example, synesthetic perceptions remain consistent over time. In other words, if a person with synesthesia sees blue and green flashes when he hears a C-sharp, he will always see those same colors upon hearing that note.