It has long been an accepted fact that a small test field presented against a large background may change its colour appearance because the test-field background contrast is attenuated by the receptor colour channels unequally (Willmer, 1944 Nature 153 774–775; Hartridge, 1947 Philosophical Transactions of the Royal Society of London, Series B 232 519–671). Such an effect is usually called small-field tritanopia. However, as shown in the present report, a similar colour illusion can be achieved with a large test field as well, provided its spatial-frequency content is high enough to reveal the differential drop of contrast sensitivity for the receptor colour channels (high-spatial-frequency tritanopia). A few demonstrations are presented which show that a traditional explanation of high-spatial-frequency tritanopia (including small-field tritanopia), based on the hypothetical process of filling-in, is not correct. An alternative account, based on spatial filtering within the receptor colour channels, is put forward.
