Some people hear colors, see flavors and are generally prone to a mixing-and-matching of typically disparate perceptual domains.
To Ramachandran, the latter answer gets at the truth — but he stressed that what appears as metaphor is a literal sensory experience for synesthetes. That may explain, he said, why synesthesia is eight times more common among poets, artists and novelists than the general population.
Where does synesthesia come from? Maybe synesthetes are just lying. Perhaps they’re under the influence of hallucinogenic drugs — many research subjects are college kids, after all — or happened as children to play with colored alphabet blocks. Or maybe they’re simply good with metaphors.