The importance of spatial scale in determining illusions of orientation.

The twisted-cord illusion is a powerful demonstration of interaction between 1st-order (luminance-defined) and 2nd-order (contrast-defined) orientation processing. The perceived orientation of contrast-defined objects is pulled towards their 1st-order orientation content when the difference in orientation is small (Fraser effect), yet is pushed away from the 1st-order content at large orientation differences (Zöllner effect). Here we show that the relative spatial scale of carrier and envelope represents a decisive factor in determining the magnitude and direction of such interactions. We conclude that the perceived 2nd-order structure of a stimulus is biased by the properties of the 1st-order structure in a manner that depends on relative, rather than absolute spatial scale.