Characteristics of the indirect McCollough effect.

Induction of contingent color aftereffects with a single chromatic grid sometimes results in an illusory color on a grid different from the one presented during induction. Such illusory color, contingently elicited by a noninduced grid, has been termed the indirect McCollough effect (indirect ME). We show that the indirect ME occurs only when the color complementary to the grid color is present during induction (either physically present or as a color afterimage), and that the indirect ME is seen only on gratings that are orthogonal to the induction orientation. These findings are in accord with the account of the indirect ME proposed by Humphrey, Dodwell, and Emerson (1989). We also show that characteristics of the indirect ME (seen following one-grid induction), both on induced and orthogonal orientations, are similar to those observed with the direct ME (seen following the usual two-grid induction procedure). Both procedures result in contingent aftereffects that display substantial retention and that do not display interocular transfer.