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.