"For Matisse in particular it often served to separate areas of contrasting colour, assisting in the vibrant activation of such juxtaposed blocks. While the Impressionist use of colour contrasts had concentrated mainly on the complementary yellow and violet-blue pair, because these most aptly imitated the effects of sunlight and shadow in nature, Matisse shifted to the red-green complementaries. This pair creates the greatest optical vibration when juxtaposed because the two colours are closest in tone of any on the colour circle. As the eye tires of reading, say, the red as dominant, the green at once appears to come forward and dominate. This vacillation of the eye between the two colours vying for dominance sets up an optical vibration, which enhances the colour properties of each simultaneously. By focusing upon the red-green pair—which Matisse often biased towards pink-turquoise—he avoided the emphasis on the naturalistic representation associated with the Impressionists' use of colour. It was also a pair which, again because of tonal equivalence, affirmed the flatness of the picture surface by negating the illusion of depth."