Neon colour spreading in three-dimensional illusory objects in humans

We studied whether neon spreading can be induced within three-dimensional illusory triangles. Kanizsa triangles were induced by black pacman disks consisting of red sectors with curved sides. Viewing our stimuli monocularly produced two-dimensional illusory contours and surfaces as well as neon spreading in each figure. Triangles appeared concave or convex under stereoscopical viewing. Neon colour spreading was induced within illusory figures bending in three-dimensional space, suggesting that neural contour completion and surface filling-in interact across depth. Surprisingly, neon spreading was induced above the intervening surface even when the inducers were below the surface. Neon colour and illusory configuration were preserved behind the intervening surface only when it appeared transparent.