When two neon-coloured illusory bands overlap each other, the filling-in at the region of overlap is perceived to be different from those of the overlapping bands, resulting in an additional illusory shape at the region of overlap.
In Figure 1 and 2 two coloured examples of displays with overlapping neon bands are shown. Each display consists of four overlapping neon-coloured bands. The perceived lightness at the regions of overlap appears to be different from the lightness of the overlapping bands. The overlapping yellow neon bands (1) result in a darker overlapping region whereas overlapping blue neon bands (2) result in a lighter overlapping region.
At the Perception
website these and various other colour examples can be seen.
See also a presentation of Daniel Dennet talking about this illusion here at about 16:15 minutes (TED talk), or here at about 36 minutes.
