The Effect of Vertical Line Position on the Poggendorff Illusion

Abstract

This study had two main focuses, to see whether age and the use of parallel lines could affect traditional Poggendorff model. Two hundred and forty-five first year psychology students were tested on a number of conditions. Each condition had a different degree of difficulty which was made by the use of parallel lines. The results found that only one of the hypotheses held true, as there was a significant difference between the results and the enhancement of vertical parallel lines. The other hypothesis that age would play a significant role in the results was deemed to be untrue, as it was only evident in one condition. The only real limitations placed upon this study were classroom environmental factors.

The effect of Vertical Line Position on the Poggendorff Illusion

The Pogendorff illusion is an optical illusion that has been the focus of many visual-spatial research projects for over one hundred years (Declerck & De Brabander, 2002). The illusion occurs when a diagonal line is intersected by two parallel lines that obstruct its continuation (for visual representation see Figure 1). As such the parallel lines appear to distort the alignment of the diagonal line in such a way that the distance between the intersection appears to be greater than it should be for a continuous straight line (Declerck & De Brabander, 2002; Jones-Buxton & Wall, 2001). The extent of the illusion depends on many factors, for example the width and texture of the intersecting plane, the angle of the diagonal line, the distance form the observer to the perceived picture and previous experience with this illusion (Day, Dickinson, & Jory, 1977; Masini, Sciaky, & Pascarella, 1992; Wilson, 1983). Drawing on such variations, possible explanations for the Poggendorff illusion and potential effects of individual differences will be discussed.

While many potential explanations of the Poggendorff illusion have been put forward, no single one has succeeded in explaining all of its different components (Declerck & De Brabander, 2002). As such, the commonly utilised exlanation of constancy or depth processing theory will be overviewed. According to depth processing theory, the parallel lines are perceived as a plane of constant distance from the viewer while the diagonal lines represent a picture plane cue to depth. The distortion is a result of conflicting distance cues between the depth plane and the constancy plane. As such, while we perceive a two-dimensional figure, we process it as if it were a three-dimensional object (Gillam, 1971; Jones-Buxton & Wall, 2001). Although, the underlying mechanisms are unclear, the illusion itself depends on the spatial context within which it is viewed.

The spatial context of the illusion can be altered by varying the way the lines are viewed. For example,