"Synaesthesia is a re-mapping of sensory data. Signals coming
though one sense organ get re-routed in the brain and are received
as if coming through a different sense organ. Inside the brain all
sensory data are essentially the same—neural, electrical impulses.
The materials of the sense data are the same, but depending on the
sense organ that receives the original signal, the data are processed
differently."

"Sometimes these signals get crossed. Sound, for example might
enter the ear, but once in the brain the sensory data travels down
the visual pathways and color is seen. [Livingstone 2002] With
other routings one might taste a whisper or smell a tickle. If neural
data takes a detour then experience, while synchronized with the
external source event, is mapped and perceived atypically (but not
necessarily uninterestingly). Some people are born with these detours
built into their neural roadways. Most of us do not experience
this at all. Some of us try to simulate it. We make art."