Ah, The Blue Smell Of It!

Synesthetes who mix up their senses and the scholars who study them cross paths to compare notes

When Sean Day hears the wail of a saxophone, he sees a writhing mass of neon-purple snakes hovering in the air. The hum of a harmonica, on the other hand, has a pleasantly greenish hue, while plunking pianos evoke a fine blue mist. Eating is colorful too. When Day takes a spoonful of mango sorbet, the wall before him turns lime green, rippled with cherry-red stripes.

Day is no human kaleidoscope. Nor is the Taiwan-based linguist afflicted with anything you would call a disorder. He's a synesthete--one of a small group of otherwise ordinary citizens who perceive the world in extraordinary...