THE HALF-ZOLLNER ILLUSION

The Zollner figure contains stacks of short parallel segments oriented obliquely to the direction of the stack. Adjacent parallel stacks of opposite polarity seem to diverge where their top segments form an arr owhead. To probe whether or not the opposite polarities are necessary to the illusion, three 'half-Zollner' configurations were designed, co ntaining stacks of a single polarity. The 'orientation profile' of the se configurations was studied, that is, the way the strength of the pe rceived illusion varies with the orientation of the stacks. The subjec ts had to align two stacks or align stacks with target segments situat ed at a slight distance from them. All three half-Zollner configuratio ns produced errors that could be assimilated to global-orientation mis judgments. These errors were of opposite sign for the two types of sta cks and varied with the orientation of the stacks as in the standard Z ollner illusion. A further study was conducted in which the effect of several configurational parameters was explored for a single observer. The standard Zollner illusion increases with the separation of the st acks. The illusion is also increased when the orientations of the segm ents in different stacks are orthogonal, independently of the particul ar longitudinal orientations of the stacks. When the ends of the short segments are curved so that at their endpoints they become precisely perpendicular to the axis of the stacks, the standard and half-Zollner illusions are reduced, but not abolished. Therefore, they cannot be e ntirely accounted for by a mechanism of alignment of illusory contours generated at these endpoints. The results are consistent with the exi stence of a single common mechanism at work in both the standard and t he half-Zollner illusion. It is suggested that the illusion itself is not a rotation of the stacks but either a shear deformation in which t he segments of a stack slide with respect to one another, or an expans ion of the stacks orthogonally to the segments.