title: Biases and sensitivities in the Poggendorff effect when driven by subjective contours.
abstract: OBJECTIVE
  A consensus in the existing literature suggests that the Poggendorff effect (a perceptual misalignment of two collinear transversal segments when separated by a pair of parallel contours) persists when the parallels are defined by Kanizsa-like subjective contours. However, previous studies have often been complicated by a lack of quantitative measures of effect size, statistical tests of significance, appropriate measures of baseline and control biases, or stringent definition of subjective contours. The aim of this study was thus to determine whether subjective contours are capable of driving the Poggendorff effect once other factors are accounted for.
  
  
  METHODS
  Twenty participants were tested on a number of test and control figures incorporating first-order (luminance-defined) and subjective parallels using the method of adjustment. All figures were tested at two different orientations, and observer sensitivities and observer biases were assessed.
  
  
  RESULTS
  A systematic response bias (in the direction of the classical effect) was found for Poggendorff figures that incorporated subjective parallels. The effect was highly significant and greater than for control figures. There was no concomitant change in judgment sensitivity (positional certainty). Finally, there was a positive correlation between the effect size for figures incorporating first-order and subjective parallels.
  
  
  CONCLUSIONS
  The findings reported demonstrate conclusively that true Kanizsa-like subjective contours are capable of driving the Poggendorff effect. Further, the data are consistent with a growing body of evidence that suggests both first-order and subjective contours are processed at early loci in the visual pathways when position is encoded.
authors.affiliation: Department of Optometry and Visual Science, City University, Northampton Square, London, United Kingdom. mtibber@yahoo.com
authors.forename: Marc S
authors.initials: MS
authors.lastname: Tibber
authors.forename: Dean R
authors.initials: DR
authors.lastname: Melmoth
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Morgan
mesh_terms: D000328:Adult; D015350:Contrast Sensitivity / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D015588:Observer Variation; D009903:Optical Illusions / Q000502:physiology*; D010364:Pattern Recognition, Visual / Q000502:physiology*; D010469:Perceptual Distortion / Q000502:physiology*; D012684:Sensory Thresholds / Q000502:physiology; D013028:Space Perception; D014795:Visual Pathways / Q000502:physiology