Abstract
In the White effect (White, 1979) gray test patches of identical luminance placed on the black and white bars of a square-wave grating appear different in brightness. The gray patch on the black bar appears brighter than the gray patch on the white bar. The theoretical importance of the effect is that the direction of the brightness change is independent of the aspect ratio of the test patch. This means that, unlike contrast and assimilation effects, the White effect does not correlate with the amount of black or white border in contact with the test patch or in its general vicinity. White (1979) concluded that explanations must, therefore, depend on the directional (orientation) properties of the inducing grating. The effect of spatial frequency on the White effect, the shifted-White effect (White. 1981) and the checkerboard illusion (DeValois & DeValois, 1988) was measured for four subjects using a brightness matching paradigm. Brightness matching data indicate that the direction of the White effect is similar at all spatial frequencies and confirm (White, 1979) that the magnitude of the effect increases at high spatial frequencies. For both the shifted-White and the checkerboard stimuli, the direction of the brightness effect is the same as the White effect at high spatial frequencies but reverses direction at low spatial frequencies. Significantly, these findings represent a difficult challenge for explanations of the White effect and checkerboard illusion based on mid-level and high-level grouping characteristics since these explanations should not be sensitive to simple changes in spatial scale. Interestingly, however, preliminary modeling indicates that these brightness effects are predicted by an oriented-difference-of-Gaussians (ODOG) model (Blakeslee & McCourt, 1999; 2001) and may therefore not require an explanation beyond multiscale spatial filtering.
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