Phenomenal coherence of moving visual patterns

When a moving grating is viewed through an aperture, only motion orthogonal to its bars is visible, as motion parallel to the bars causes no change in the stimulus. Because there is a family of physical motions of various directions and speeds that appear identical, the motion of the grating is ambiguous. In contrast, when two crossed moving gratings are superimposed, the resulting plaid pattern usually moves unambiguously and predictably. In certain cases, however, two gratings do not combine into a single coherent percept, but appear to slide across one another. We have studied the conditions under which coherence does and does not occur, and we report here that it depends on the relative contrasts, spatial frequencies and directions of motion of the gratings. These effects may reveal the previously unstudied properties of a higher order stage of motion analysis.