(© 2005 Peter U. Tse)
Attention-induced brightness changes occur over bistable transparent surfaces. Fixate any of the points above and shift your attention to one disk or other without moving your eyes. The attended disk appears to change brightness. We believe that this happens because attention biases figure formation such that filling-in happens differently within the attended region than in the unattended region. In particular, the features from the overlap region spread within the boundaries of the attended figure, and not within the boundaries of the unattended region. This happens only for bistable transparent surfaces because only then is it ambiguous over which surface or layer the visual system should carry out the filling-in operation.
Read more about the illusion and possible explanations
Voluntary attention modulates the brightness of overlapping transparent surfaces Peter U. Tse Vision Research. 2005. 45:1095-8