The MIT “Checker Shadow Illusion” Brought to Life

The video you’re watch­ing is a real-life demon­stra­tion of an opti­cal illu­sion devel­oped in 1995 by Edward Adel­son, a pro­fes­sor in MIT’s Depart­ment of Brain and Cog­ni­tive Sci­ences. The Check­er Shad­ow Illu­sion, as Adel­son calls it, shows that our “visu­al sys­tem is not very good at being a phys­i­cal light meter.” But more impor­tant­ly, the opti­cal illu­sion offers impor­tant insight into how our visu­al sys­tem tries to break down “image infor­ma­tion into mean­ing­ful com­po­nents, and there­by per­ceive the nature of the objects in view.” Adel­son’s full expla­na­tion of the illu­sion and what it reveals appears below the jump (or here). H/T 3 Quarks Dai­ly

The visu­al sys­tem needs to deter­mine the col­or of objects in the world. In this case the prob­lem is to deter­mine the gray shade of the checks on the floor. Just mea­sur­ing the light com­ing from a sur­face (the lumi­nance) is not enough: a cast shad­ow will dim a sur­face, so that a white sur­face in shad­ow may be reflect­ing less light than a black sur­face in full light. The visu­al sys­tem uses sev­er­al tricks to deter­mine where the shad­ows are and how to com­pen­sate for them, in order to deter­mine the shade of gray “paint” that belongs to the sur­face.

The first trick is based on local con­trast. In shad­ow or not, a check that is lighter than its neigh­bor­ing checks is prob­a­bly lighter than aver­age, and vice ver­sa. In the fig­ure, the light check in shad­ow is sur­round­ed by dark­er checks. Thus, even though the check is phys­i­cal­ly dark, it is light when com­pared to its neigh­bors. The dark checks out­side the shad­ow, con­verse­ly, are sur­round­ed by lighter checks, so they look dark by com­par­i­son.

A sec­ond trick is based on the fact that shad­ows often have soft edges, while paint bound­aries (like the checks) often have sharp edges. The visu­al sys­tem tends to ignore grad­ual changes in light lev­el, so that it can deter­mine the col­or of the sur­faces with­out being mis­led by shad­ows. In this fig­ure, the shad­ow looks like a shad­ow, both because it is fuzzy and because the shad­ow cast­ing object is vis­i­ble.

The “paint­ness” of the checks is aid­ed by the form of the “X‑junctions” formed by 4 abut­ting checks. This type of junc­tion is usu­al­ly a sig­nal that all the edges should be inter­pret­ed as changes in sur­face col­or rather than in terms of shad­ows or light­ing.

As with many so-called illu­sions, this effect real­ly demon­strates the suc­cess rather than the fail­ure of the visu­al sys­tem. The visu­al sys­tem is not very good at being a phys­i­cal light meter, but that is not its pur­pose. The impor­tant task is to break the image infor­ma­tion down into mean­ing­ful com­po­nents, and there­by per­ceive the nature of the objects in view.

This video now appears in our col­lec­tion of 125 Great Sci­ence Videos.

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