The Optical-Geometrical Illusion “Corner Poggendorff” Re-Examined

In 1988 Greene noted, if a straight pair of obliques are drawn outside of two orthogonal lines, the segments appear to be angled slightly one relative to the other. This illusion, designated as “corner Poggendorff,” is different from the Poggendorff effect (the two obliques seem to lie on a parallel path). The results of the present experiments ( N = 76 students) suggest that the general conditions for the corner Poggendorff are similar to those for the Judd illusion.

Angular Induction as a Function of Contact and Target Orientation

The Poggendorff effect is seen as misalignment of two obliques, or misprojection of one, when the obliques are placed outside a set of parallel lines. To understand better the mechanisms behind this effect, the orientation of the lines which are normally parallel was systematically manipulated. The results indicate that projection bias is affected by the orientation of either line, is at a minimum where the line is orthogonal to the oblique, and is maximal at small angles. This is in line with classic theories which attribute the illusion to misperception of angular size. However, such explanations presuppose that in order to be effective the induction line must be proximal to the oblique so that an angle can be formed. Results are reported which show that the angle formed by the oblique and a line placed at a distance from the oblique, serving as the target of the projection, follows an angular rule of effectiveness similar to what is seen when the line is placed directly in contact with the oblique. The underlying process is described as ‘angular induction’.

The Corner Poggendorff

With the classic Poggendorff illusion a set of parallel ‘induction lines’ will cause a set of oblique line segments to look misaligned even though they are collinear. A different kind of misalignment can be produced by placing the induction lines so that they form a corner. Under these conditions the obliques will appear to be angled slightly, one relative to the other. The effects are small, but can be seen and reliably reported by a group of naive subjects. The influence of the induction lines drops sharply as their relative position is moved from parallel to orthogonal, but there is a small residual influence which may be called the corner Poggendorff effect.

The Relative Contribution of Contact and Target Lines in the Magnitude of the Poggendorff Effect

It is well known that a set of parallel lines can cause misperception of the projected path of an oblique. Most studies of this effect have emphasized either the proximal or the distal stimulus components—the line with which the oblique makes contact, or the line that serves as the target of the projection. An experiment is reported in which the relative contribution of the contact and target lines was examined. The results indicate that rotation of either line can determine the magnitude of the projection error.