The influence of response factors on the magnitude of angular induction

The angular induction which is manifested in geometric illusions such as the Poggendorff can be quantified by having the subject select a point which appears to be collinear to an oblique line segment. There may be some concern, however, that this operant procedure does not control the distance which is selected by the subject and may introduce nonperceptual sources of response bias. Four experiments, involving 14 subjects, provide evidence that the error, expressed as angular departure from true collinearity, is a constant irrespective of the location of the page which is chosen. Further, this operant procedure provides a valid measure of perceptual bias.