PARALLELS AND TRANSVERSAL SUBJECTIVE CONTOURS IN THE POGGENDORFF ILLUSION

  • Andrea Spoto
  • Alessia Bastianelli
  • Roberto Burro
  • Giulio Vidotto

Abstract

When the continuity of an obliquely oriented line is broken by a vertically oriented pair ofparallels, the position of the line segment on the other side of the interruption does not seemto be collinear, but vertically shifted (i.e. the Poggendorff illusion). Evidences from literatureproved that the Poggendorff illusion is still present when there are no parallels but Kanizsalikesubjective contours. The present study attempts to verify whether the Poggendorff illusionpersists when both the transversal segment and the parallels consist of Kanizsa-like subjectivecontour. Eight participants were tested using the method of constant stimuli a) on a number ofhorizontal subjective parallels and transversal line segments, b) on both horizontal parallelsand transversal segment Kanizsa-like subjective contours. The response bias in the directionof the classical effect and the threshold values for the two patterns are discussed.
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