Shape to speed

Music is a complex form of art, so it is not surprising that visual images aroused by music often show a complexity that can itself be related to many features of the music. So we find that when synesthetes listen to music, their visual responses–color, form, movement–may shift, surge, and ebb according to musical key, to musical pattern, to musical progression. But here too results are divergent, with little specific agreement on the images reported by different individuals. There are, however, a few notable exceptions. Karwoski and Odbert (1938) discovered a systematic relation between the shapes of synesthetic visual forms and the tempos of the music. The faster the music, the sharper and more angular the visual image.

Lawrence E. Marks, The Unity of the Senses, 1978, 93.

Justin Hurwitz, Accident