EVALUATING THE DECAY GRADIENT FOR COLLINEARITY BIAS WITH LATERAL DISPLACEMENT FROM THE AXIS OF INDUCTION

The misperception of alignment which is found in many geometric illusi ons can be quantified using relatively simple stimulus configurations. Perceived collinearity of one segment (designated as the test segment ) is biased by a second segment (designated as the induction segment), with the size of effect being a function of the relative angle betwee n the two segments. The process can be described as angular induction, The strength of bias is greatest when the Induction segment is center ed al the tip of the test segment. Tong and Weintraub have reported th at lateral displacement from the tip, i.e., at right angles to the axi s of the induction segment, produces a sharp drop in the strength of e ffect. This decline is described as a ''decay gradient'' for the angul ar induction. One experiment replicates and provides better quantifica tion of this ''decay gradient''. Two other experiments examine the dec ay gradient using a pair of induction segments, one on each side of th e tip of the test segment. Displacement of the segments (either in the same direction or in opposite directions) produces substantially the same gradient of effect. Therefore, previous evidence of ''tandem boos ting'' of effect for segment pairs does not depend on collinearity amo ng the stimulus components. Finally, a fourth experiment finds that an induction segment which is at a fixed position and orientation differ entially affects the influence of a variable induction segment. At som e angles the influence of the variable segment is augmented: and at ot hers it is suppressed. These findings are discussed in a neuroreductio nist context, and a simple model for angular induction is presented.