The Poggendorff displacement effect with only three dots

Two experiments showed that apparent displacement from exact vertical bisection, a variant of the Poggendorff effect reported by Tolansky (1964), occurs not only with three vertical lines or three dots on oblique parallels but with three vertical lines or three dots alone. The size of the effect without parallels was no different from that with parallels. The second of the two experiments, with separate groups of subjects, was a control for the possible influence of one condition on another in the first. The occurrence of the bisection form of the Poggendorff displacement effect is interpreted in terms of a perceptual compromise between exact bisection relative to the vertical and that relative to the main direction of the figure. This direction can be delineated by either parallel lines or obliquely positioned elements.