Counterphase lightness induction.

When half-cycle wide mid-grey test elements are superimposed on a black and white square wave induction grating, their apparent lightness is a linear function of their phase relationship with the induction grating; being judged lightest when in phase with the black bars of the grating, and darkest when in phase with the white bars. This finding justifies the use of the term "counterphase induction" to describe the effect, and emphasizes the similarity between this effect and McCourt's (1982) [Vision Res. 22, 119-134] grating induction effect. The fact that the same linear function holds whether the test elements are square or rectangular favours a global over a local explanation.