Mach bands

(Left) Photograph of a real-world cube manifesting a photometric highlight and lowlight (see luminance profile beneath the photo). (Right) A computer- generated image of a similar object, but lacking the highlight and lowlight. Despite the objective absence of these adornments, brightness maxima and minima (Mach bands) are apparent in the positions of their photometric counterparts in A. (Image from: Mach bands as empirically derived associations. Lotto RB, Williams SM, Purves D., Proc. Natl. Acad. Sci. 96: 5245-5250 [1999])