COLLINEARITY JUDGMENT AS A FUNCTION OF INDUCTION ANGLE

The misalignment which is seen in the Poggendorff illusion can be stud ied with better control by using a configuration which has only two li ne segments. Two experiments were conducted in which subjects judged c ollinearity of a test segment, this judgment being subjected to a bias ing influence from a second (induction) segment. Exp. 1 held the test segment at one of three orientations relative to the observer (30-degr ees, 45-degrees, and 60-degrees) and systematically varied the orienta tion of the induction segment in 15-degrees increments through the ran ge of possible positions. The orientation of the page relative to the observer was varied as well. Exp. 2 varied the test segment through a greater range of angles and sampled more levels of induction segment o rientation. Analysis indicated that projection errors follow orderly r ules similar in kind to but different in magnitude from those observed for the Tilt Illusion, most notably, (a) misprojection is greatest wh en the orientation of the interfering line is similar to that of the l ine segment being projected and (b) the strength of this influence dec reases as the relative angle becomes orthogonal. Also, the orientation of the segment being projected relative to the observer serves to mod ulate the strength of the basic induction effect. These perceptual int eractions are discussed in relation to neural models for orientation s electivity.