There is still no consensus on the mechanisms behind the illusion of sloping mortar lines and wedged shaped tiles in the Café Wall Illusion. On one leading theory, McCourt’s (1983) Brightness Induction Model, the illusion is produced in several stages. In the first stage, lateral inhibition in retinal ganglion cells induces illusory brightness variations in the uniform mortar strips. Those areas of the mortar bordering dark tiles are perceived as lighter than those areas of the mortar bordering light tiles, similar to the way that the uniform gray bar looks darker on the right than on the left in the image below.
The induced brightness variations are then seen as connecting opposing corners of like-colored tiles creating illusory diagonal lines within the mortar.
Finally, when global processes (which are not yet well understood) use all of the localized representations to construct a simplest-explanation whole image, they treat the alternating diagonal lines as resulting from non-parallel lines. On this account, the apparently tilted mortar lines result from illusory “Twisted Cords” (below) induced in the mortar.
Because the effect is dependent on the luminance contrast between the various elements, it will change slightly over time as the cherry naturally darkens and the walnut lightens.
Gregory, R.L. and Heard, P. (1979) “Border Locking and the Café Wall Illusion.” Perception, 8, pp 365-80.
Kitaoka, A., Pinna, B., & Brelstaff, G. (2004) “Contrast Polarities Determine the Direction of Café Wall Tilts.” Perception, 33, pp 11-20.
McCourt, M. (1983) “Brightness Induction and the Café Wall Illusion.” Perception, 12, pp 131-42.