In some of Messiaen's scores, he notated the colours in the music (notably in "Couleurs de la cité céleste" and "Des canyons aux étoiles...")—the purpose being to aid the conductor in interpretation rather than to specify which colours the listener should experience. The importance of colour is linked to Messiaen's synaesthesia, which he said caused him to experience colours when he heard or imagined music (he said that he did not perceive the colours visually). In his multi-volume music theory treatise "Traité de rythme, de couleur, et d'ornithologie" ("Treatise of Rhythm, Colour and Birdsong"), Messiaen wrote descriptions of the colours of certain chords. His descriptions range from the simple ("gold and brown") to the highly detailed ("blue-violet rocks, speckled with little grey cubes, cobalt blue, deep Prussian blue, highlighted by a bit of violet-purple, gold, red, ruby, and stars of mauve, black and white. Blue-violet is dominant").