ILLUSIONS are anomalies that can reveal clues about the mysterious workings of the brain to neuroscientists in much the same way as the fictional Sherlock Holmes can solve a crime puzzle by homing in on a single out-of-the-ordinary fact. Think of the phrase “the dog that did not bark” (in Sir Arthur Conan Doyle's short story “Silver Blaze”) or of the missing dumbbell (in Conan Doyle's Valley of Fear).