The role of oriented filters and T-junctins in White's effect

Abstract

In White's Effect the gray target surrounded more by black than white appears darker than the target of the same physical luminance surrounded more by white than black. Blakeslee and McCourt (1999) have proposed a multiscale spatial filtering (ODOG) model of White's Effect while, according to Anderson (1997), the effect is the consequence of a perceptual scission determined by the contrast polarity of aligned contours at T-junctions. As consequence of its response normalization stage, the ODOG model predicts that White's Effect should be abolished if the ratio of horizontal to vertical structure in the image is equalized. To test this prediction we constructed a variant of White's stimulus in which the background consists of zig-zag rather than straight stripes. This new stimulus is also designed to eliminate any effects of T-junctions on the difference in perceived lightness between the gray patches lying on the black and white bars. All the target patches have T-junctions at each corner, two of which are black along the top and gray/white across the stem and two of which are white across the top and gray/black across the stem. The magnitude and direction of the lightness illusion in the standard and novel White's stimuli were measured by adjusting the luminance of comparison patches on a variegated background to match the perceived lightness of the gray target patches. We found a highly significant illusion in the novel White's configuration in the same direction as the standard White's Effect. Simulations show that the ODOG filtering model predicts instead a small effect in the opposite direction. This result is also hard to reconcile with accounts based on the structure and distribution of junctions within the image.