Evidence for a significant contribution of interactions between oriented line segments in the Tolansky version of the Poggendorff illusion

Interactions between line segments, including angle distortion, commonly are proposed as partial determinants of the Poggendorff illusion. Day and Kasperczyk (1985) found that replacing all transverse line segments with dots had no effect on alignment errors, thus rendering untenable explanations couched solely or largely in terms of line interactions. The illusions they obtained, using Tolansky variants of the basic illusion, were attributed to judgments that compromised between vertical and oblique bisection. In the present experiment, subjects were instructed to avoid such a compromise: Preliminary experiments had suggested that compromise judgments were more likely to affect displays with dot pointers than those with line pointers. It was found that the addition of parallel inducing lines significantly increased illusions, but only when transverse pointers were line segments rather than dots. This was consistent with previous research, which has shown that line interactions are a significant component of Poggendorff effects.