The Components of the Poggendorff Illusion

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In two experiments, subjects adjusted various pairings of the top and bottom boundaries of two obliquely oriented outline bars (Experiment 1) and those of two similarly oriented complete and incomplete squares (Experiment 2) to apparent alignment. The data from the first experiment showed that the misalignment effects were determined jointly by the directional properties of the bar ends (vertical, oblique, and semicircular) and the pair of boundaries that were aligned (both top boundaries, top of upper bar with bottom oflower bar, bottom of upper bar with top oflower bar). The results from the second experiment showed that the misalignment effects were the same for the oblique boundaries of solid and outline squares and persisted when the squares were reduced to two parallel lines. The effect was undiminished when the ends of the parallels were aligned, but was markedly reduced when pairs of parallels themselves were aligned, The outcomes of the two experiments are explained in terms of the apparent positions of the oblique boundaries. It is proposed that these vary with the positions of the elements (bar or square) relative to the visual field, the position of the boundaries relative to the stimulus elements, and the positions of the boundaries relative to axes that are delineated by the parallel adjacent ends of bars and sides of squares. This relative-position basis for apparent misalignment is held to be the basis of misalignment effects in other figures.In a previous series of experiments (Day, Stecher, & Parker, 1993), it was shown that the collinear edges of oblique bars with rounded ends appear to be slightly but reliably misaligned. This effect was nearly doubled when one of the bars was moved vertically in the course of adjusting it to apparent alignment with the other. When (as in Figure 1A) the ends of the bars and movements of one bar during adjustment were both vertical, the misalignment effect was about six times greater. However, it was reduced when the ends of the bars remained vertical while adjustment movements were orthogonal to the axis of the oblique bars (as is shown in Figure 1B). In the last of the four experiments, it was found that although the orientation and form of the inner pair of bar ends had a marked effect on apparent misalignment, those of the outer pair had no effect at all. Finally, when the inner ends of the bars abutted on vertical parallel lines, as in the conventional Poggendorff figure, the misalignment effect was about the same as that with vertical bar ends, This suggested that vertical ends and vertical parallels

Section: Discussionsupporting

confidence: 72%

On apparent misalignment of collinear edges and boundaries

Day

Halford

1994

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“…These results seem consistent with the perceptual compromise hypothesis in the modified version proposed above. (3) Concerning the explanations of the standard Poggendorff illusion in terms of modifications of the space between the parallels as a consequence of an apparent vertical elongation or horizontal shortening of the space itself (Day & Dickinson, 1976), our results can be viewed as preliminary. A direct measurement of the modifications of the apparent size of the space between the parallels would be required, but has not yet been accomplished.…”

Section: Discussionmentioning

confidence: 94%

“…In any case, the alignment error still present in the control condition seems to point out the occurrence of a general alignment error concerning the oblique orientation, or of an apparent elongation of the vertical extent between the obliques, although it is less evident than in Experiment I (d. Appelle, 1972;Bouma & Andriessen, 1968;Day & Dickinson, 1976).…”

Section: Resultsmentioning

confidence: 99%

“…The data from the control condition in Experiment 2 (aligning two obliques with no texture interposed) show an alignment error statistically significant in the usual direction of the illusion; this error is still about 1 mm. This can be considered to be a general alignment error, as has already been mentioned in several contributions (e.g., Bouma & Andriessen, 1%8, in terms of orientation illusion; Day & Dickinson, 1976, in terms of deformation of the space between the obliques; Appelle, 1972, in terms of visual acuity). This value is not significantly different from Condition e (0°with no verticals), thus contradicting the possibility that subjective contours exist in the absence of verticals.…”

Section: Discussionmentioning

confidence: 99%

See 1 more Smart Citation

The orientation of a parallel-line texture between the verticals can modify the strength of the Poggendorff illusion

Masini

Sciaky

Pascarella

1992

In the present experiments, we attempted to evaluate the modification of the strength of the Poggendorff illusion as a function of the different orientation of a parallel-line texture filling the space between the vertical lines. In Experiment 1, the standard version of the PoggendorfI configuration was tested against four different parallel-line textures oriented at 0 0 , 4 5 0 , 90 0 , and 135 0 with respect to the obliques. The results showed that the illusory effect was a linear function of the progressive discrepancy between the angle of the lines of the texture and that of the obliques. In Experiment 2, we tested the same textures used in Experiment 1 after the elimination of the two vertical lines. The data obtained approximated a linear function, as in the previous experiment, but the alignment errors were consistently lower. The statistical analysis performed on the data of all eight experimental conditions shows that both factors-texture and presence/absence of verticals-were significant, but most of the effect was due to the texture factor. The results may be interpreted through the "perceptual compromise hypothesis," originally proposed for the bisection forms of the PoggendorfI illusion, but with important modifications. The data are also discussed in terms of their implications for other theories proposed for the PoggendorfI illusion.The Poggendorff illusion is shown in Figure I; in the original version of this optical-geometrical illusion, two collinear obliques separated by a pair of parallels appear displaced from their physical alignment. In this case, the upper right oblique appears too high to be aligned with the prolongation of the lower left oblique.There are several accepted fmdings on this illusion: The apparent displacement of the transversals is increased when the angles are more acute; increasing the separation between the parallels increases the effect; a horizontal or vertical orientation of the transversal reduces the illusion; and the distortion does not depend on the straightness of the separating elements (Coren & Girgus, 1978).Despite general agreement on these empirical findings, the illusory effect has been explained within different theoretical frameworks. One type of explanation involves "structural factors" like optical blur and lateral inhibition that could induce distortions in the region of the intersections between lines, for example, expansion of acute

“…The classical form of the Poggendorff illusion refers to the apparent misalignment of two objectively collinear oblique lines separated by a pair of vertical lines, as in Figure lAo Although it is frequently classified as an illusion of direction, several investigators (Day & Dickinson, 1976;Greist-Bousquet & Schiffman, 1981bPressey, 1971;Quina-Holland, 1977;Zanuttini, 1976) have suggested that the Poggendorff misalignment may be due, at least in part, to the apparent contraction of the distance between the parallels. Recently, Schiffman (1981b, 1985), following the early work of Judd (1899), proposed that the underestimation of interparallel extent is caused by the relative influence of the amputated forms of the wings-out and wings-in Miiller-Lyer figure contained within the Poggendorff figure, as shown in Figures IB and lC with the transversals and in Figures ID and IE without the transversals, respectively.…”

mentioning

confidence: 99%

Illusion decrement and transfer of illusion decrement in obtuse- and acute-angle variants of the Poggendorff illusion

Predebon

1990

Illusion-decrement and transfer-of-illusion-decrement procedures were used to examine the contribution of the obtuse-and acute-angle components of the Poggendorff pattern to the standard Poggendorff illusion. In the first four experiments, subjects were required to scan between the oblique lines of the Poggendorffpattern during the inspection phase of the decrement procedure. However, because of a possible confound associated with this procedure, a different decrement technique was used in Experiment 5. The results of Experiment 5 confirm and extend MacKay and Newbigging's (1977) finding that similar amounts of transfer to the standard pattern are obtained from the obtuse-and acute-angle patterns as from the standard pattern itself: In showing that the acute-and obtuse-angle components both contribute to the illusion, these findings question the plausibility of those theories of the Poggendorff illusion which do not assign any significant role to the acute-angle component. Furthermore, the potential confound associated with the decrement procedure of Experiments 1-4 suggests that the results of other studies obtained with similar procedures need to be reevaluated.The classical form of the Poggendorff illusion refers to the apparent misalignment of two objectively collinear oblique lines separated by a pair of vertical lines, as in Figure lAo Although it is frequently classified as an illusion of direction, several investigators (Day