Syenthesia

Synethesia (literally, a union of the senses) is the rare and very unusual condition of having one of the senses automatically triggering another sense.

An example of this could be looking at a grapheme (a letter or a number) and simultaneously experiencing a sensation of a color or sound coming from inside the brain. How and why these strange associations come into existence is not well understood but it is hypothesized that it develops during childhood when children first become intensively engaged with abstract concepts.