Biases and Sensitivities in the Poggendorff Effect when Driven by Subjective Contours

Abstract

Purpose-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.The Poggendorff effect (1860) is a well-documented geometric illusion in which there is a perceptual misalignment of two collinear oblique lines when separated by a pair of parallel contours (henceforth referred to as the transversals and parallels respectively; see Fig. 1A). Several explanations have been put forward to account for the effect, including a misestimation of the orientation of the transversals through lateral inhibition, obtuse angle contraction, or angular induction,1-5 a bowing of the transversals at points of intersection with the parallels,6 a misestimation of the orientation of a projected line connecting the transversals as a result of neural blurring,7,8 a perceptual contraction or expansion of the space between the parallels,9-11 a misinterpretation of the transversals as receding in depth,

Geometrical features underlying the perception of collinearity

“…Such variants include amputations of lines, replacements of lines by dots, replacement of lines by subjective contours (Tibber, Melmoth, and Morgan, 2008), figures emphasizing perspective cues (Gillam, 1971) and horizontal rotation of the Poggendorff figure itself, which shows a smaller bias than the upright version (Hotopf and Hibberd, 1989). In this paper we renounce the term 'illusion' in favour of 'bias' and we refer to the 'P-bias' as any bias in the perception of collinearity in the same direction as that seen in the traditional, upright 4-line Poggendorff figure.…”

The Neural Correlates of Visuospatial Perceptual and Oculomotor Extrapolation

“…the LOC and FEFs) is consistent with overlapping neural mechanisms of extrapolation for perception and action. In support of this position we have previously shown that perceptual VSE judgements and eye movements share common attentional resources [54], and that pointing movements and perceptual extrapolation judgements are susceptible to a common geometric illusion [27], [75]. While the lack of interaction in our present study was unambiguous in the LOC, the pattern was not so clear cut in the FEFs where the main effect of target type only just reached statistical significance in the ROI analysis, and seemed largely to be driven by differences between mEXT and mEXP task conditions (see Figure 3 and Figure 5).…”

An investigation into perceived motion paths by means of the Slalom Illusion

“…However, despite the repeated replication of this phenomenon, it has also been found that the magnitude of the misalignment bias is smaller in subjective-contour Poggendorff displays, when compared to the bias that can be observed using a classical display with real contours. Tibber et al (2008) suggest that the attenuation in the Poggendorff effect when driven by subjective contours could be explained by the lower salience of the subjective contours. That is, that subjective contours are detected by a smaller subset of cortical neurons in the early visual areas than real contours and that, consequently, lateral inhibition between orientations columns in V1 is weaker than when the entire stimulus is defined by real contours.…”

“…In all of these illusions, static lines that should have been straight or parallel, appear as bent (Wundt, Hering) or at an angle with each other (Poggendorff, Zollner). The Poggendorff and Zollner illusion have both been shown to not only occur when the crossing lines causing the illusion are drawn as full lines, but also when they are induced as subjective contours (Tibber, Melmoth, & Morgan, 2008); for the Hering and Wundt illusion comparable data are not available. The slalom illusion can be compared to these four classic geometric illusions.…”

“…The Poggendorff illusion, in particular, has in a number of studies been adapted to a version where the real contours of the inducing rectangle are replaced with Kanizsa-like illusory ones (Figure 2.1). The misalignment effect typical of the illusion was robustly replicated in a number of studies (Gregory, 1972;Meyer & Garges, 1979;Westheimer & Wehrhahn, 1997;Tibber et al, 2008). One study (Day, Dickinson, & Jory, 1977) did not confirm the effectiveness of illusory contours as inducers to the Poggendorff illusion.…”