THE ROLE OF EDGES AND LINE-ENDS IN ILLUSORY CONTOUR FORMATION

Illusory contours can be induced along directions approximately collin ear to edges or approximately perpendicular to the ends of tines. Usin g a rating scale procedure we explored the relation between the two ty pes of inducers by systematically varying the thickness of inducing el ements to result in varying amounts of ''edge-like'' or ''line-like'' induction. Inducers for our illusory figures consisted of concentric r ings with arcs missing. Observers judged the clarity and brightness of illusory figures as the number of arcs, their thicknesses, and spacin gs were parametrically varied. Degree of clarity and amount of induced brightness were both found to be inverted-U functions of the number o f arcs. These results mandate that any valid model of illusory contour formation must account for interference effects between parallel line s or between those neural units responsible for completion of boundary signals in directions perpendicular to the ends of thin lines. Line w idth was found to have an effect on both clarity and brightness, a fin ding inconsistent with those models which employ only completion perpe ndicular to inducer orientation.