Synesthesia.
Main article: Number form

A number form from one of Francis Galton's subjects (1881).[10] Note how the first 12 digits correspond to a clock face.
A number form is a mental map of numbers that automatically and involuntarily appears whenever someone who experiences number-forms synesthesia thinks of numbers. Number forms were first documented and named in 1881 by Francis Galton in "The Visions of Sane Persons"
