When a young Mary Shelley sat down to describe the first conscious moments of Frankenstein’s monster, her imagination flew to synesthesia. “I saw, felt, heard and smelt, at the same time,” she wrote, “and it was, indeed, a long time before I learned to distinguish between the operation of my various senses.” Jolted awake for the first time, in Shelley’s vision, the mind emerges from a primordial unity in which sight and sound are one.

Artists have always been sensitive to hidden links between the senses. Nabokov, for example, claimed grapheme-color synesthesia, a condition he shared with Rimbaud, whose poem Voyelles paired the primary colors to the sound of vowels. Both thought that the letter A was black. To some degree, we all rely on cross-sensual metaphors to describe th...