Depth perception of interfering periodic patterns: a possible contribution to disorientation on escalators.

Incongruous and illusory depth cues, arising from 'interference patterns' produced by overlapping linear grids at the edges of escalator treads, may contribute to the disorientation experienced by some escalator users, which in turn may contribute to the causes of some of the many escalator accidents which occur. The apparent depth of the interference pattern from the viewer is analysed in terms of the cues deriving from size and viewer motion. Both of these cues support the depth of the target being infinite. Preliminary observations are reported which confirm this analysis. Remedies for the problem are suggested. The possible contribution of this illusion to disorientation on escalators, due to misjudgment of depth, is compared with another recently reported factor which is due to stereoscopic miscorrespondence of periodic targets.     

Three-dimensional Müller-Lyer illusion

Three-dimensional (3-D) variants of the Müller-Lyer pattern were created to address the question of where along the path of information flow in the visual system the illusion might occur. These variants, which yielded a robust illusion, included dihedral angles in place of the arrowheads of the classical pattern. The enormous difference in the shape of the resulting retinal image, compared with that of the classical pattern, makes it difficult to explain the present illusion by resorting to image-processing theories such as selective filtering (Ginsburg, 1984, 1986) or depth processing (Gregory, 1963, 1966, 1968). It was also shown that this 3-D illusion is homologous with the classical illusion, and that the two may thus share a common causal mechanism. A new type of 3-D figure, which yielded the same retinal image as did the classical pattern, was then employed. However, since the figure was 3-D, its shape in spatial coordinates was very different compared to that of the classical pattern. The magnitude of the illusion obtained with this figure was half that of the classical pattern. This finding suggests that the illusion might be caused by processes that occur after the computation of depth. All three experiments indicated that the illusion may be produced later in the processing stream than has previously been suggested.     

APPARENT PROPERTIES OF INSPECTION FIGURES AS DETERMINERS OF FIGURAL AFTER-EFFECTS

Figural after-effects (FAE) were studied using the Hering, the Ehrenstein, and the Orbisson illusion, patterns in two ways. In one condition, the total pattern was used as inspection figure (IF), and that part of the pattern in which the illusion normally appears, as test figure (TF). The resulting FAE was in a direction opposite to that of the illusion. In the second condition, the illusion producing part of the total pattern served as IF, and the illusion suffering part as TF. This produced an after-effect, the direction of which was the same as that of the illusion.     

The illusion of transparency and chromatic subjective contours

Subjective contours can be produced that include an illusion of edge and an extension of color throughout the area of the illusion. The phenomenological appearance is of a transparent colored shape in front of the background. Two explanations of this illusion are proposed. The first is that there is an assimilation of color analogous to brightness assimilation. The second is a variant of the stratification of depth theory of subjective contours. In it, the pattern elements lead to the illusion of a surface in front of the pattern elements. We thus predicted that an illusion of transparency would enhance the subjective contour, Metelli’s model of transparency was used to quantify our prediction, and it was found that the possibility of transparency was a powerful predictor of the chromatic subjective contour.     

Visual illusions affect planning but not control

Much debate has arisen over how to account for the pattern of effects of visual illusions on action - that is, the findings that illusions affect actions in some circumstances but not others. I propose that this pattern can best be explained by postulating that visual illusions affect the planning of actions but do not affect the on-line control of actions. Strong evidence for this viewpoint comes from recent studies that show 'dynamic illusion effects': a large illusion effect early in a movement, but a decreasing effect as the hand approaches the target. These findings pose difficulties for other models of illusion effects on action.     

The Poggendorff and its variants do arouse the same perceptual process

The first two of five experiments reported show that the complete Poggendorff and three of its variants all produce the same illusory effect and that, for all four patterns, that effect is increased as the angle of interception of the obliques decreases and as the separation of the parallels increases. The third experiment shows that the differences in the magnitude of illusion produced by the variants is most probably due to the components retained in a particular variant and not to the simple reduction in the total amount of illusion pattern retained, which also correlates with magnitude of illusion. The final two experiments show that equal or greater positive transfer to the complete pattern is obtained from the variants as from the complete pattern after equal amounts of practice. The conclusion is reached that the complete pattern and the variants arouse the same single perceptual process. The data do not permit identification of that process.     

A spatial illusion from motion rivalry

A new dynamic visual illusion is reported: contrast reversal of a horizontal and vertical plaid pattern (produced by adding two orthogonal sinusoidal gratings) causes the pattern to appear as an array of lustrous diamonds, cut by sharp lines into a diagonal lattice structure. On the basis of computer simulations it is suggested that the illusion results from rivalrous interaction of motion detectors tuned to opposing directions of motion.     

Integration biases in the Ouchi and other visual illusions

A texture pattern devised by the Japanese artist H Ouchi has attracted wide attention because of the striking appearance of relative motion it evokes. The illusion has been the subject of several recent empirical studies. A new account is presented, along with a simple experimental test, that attributes the illusion to a bias in the way that local motion signals generated at different locations on each element are combined to code element motion. The account is generalised to two spatial illusions, the Judd illusion and the Z?llner illusion (previously considered unrelated to the Ouchi illusion). The notion of integration bias is consistent with recent Bayesian approaches to visual coding, according to which the weight attached to each signal reflects its reliability and likelihood.     

Constancy scaling and conflict when the Zöllner illusion is seen in three dimensions

If a standard Z?llner illusion is seen as a staircase in depth, pairs of long lines flanking convex stair edges appear to diverge as usual, but divergence in pairs flanking concave edges can appear reduced. If the stair is reversed perceptually in the manner of the Schr?der staircase, convex and concave shapes exchange and the extent of apparent divergence in the long line pairs exchanges with them. The effect is enhanced if explicit stair edges are added, and reduced if the standard Z?llner pattern is replaced by one in which segments of the long lines are offset in the direction of the usual illusory effect. The observations suggest that the three-dimensional potential of the pattern cannot be excluded from explanations of the illusion, and are compatible with the view of Gregory and Harris that inappropriate constancy scaling is its primary cause, triggered 'bottom-up' by pattern properties or 'top-down' by cognitive inference. However, these two mechanisms would have to be acting in conflict to generate suppression of divergence in the concave steps. Pattern processing for properties, such as orientation, that are not associated with the potential of the Z?llner illusion as a three-dimensional configuration, but that have been suggested as sources of the illusion in recent studies, could also be acting in opposition to hypothesis scaling in the concave steps.     

Structural alterations of an ambiguous musical figure: the scale illusion revisited.

The scale illusion (Deutsch, 1975) shows the importance of frequency range in the perceptual organization of a sequence of notes. This paper includes three experiments on the scale illusion. Experiments 1 and 2 demonstrated that if the structure of the pattern of notes used in the original scale illusion study is altered slightly, by adding or subtracting a pair of notes from the ends of the sequence, there is a significant decrease in the rate of frequency-based responses, suggesting a weaker illusion. Experiment 3 investigated two features of the note patterns that may have led to this change. Specifically, it asked whether the decrease in the strength of the illusion is due to (1) the nature of the notes at the extremes of the frequency range and/or (2) the nature of the notes at the crossing point of the two scales. While both sources were found to affect the strength of the scale illusion, the former had a greater influence.     

Structural alterations of an ambiguous musical figure: The scale illusion revisited

The scale illusion (Deutsch, 1975) shows the importance of frequency range in the perceptual organization of a sequence of notes. This paper includes three experiments on the scale illusion. Experiments 1 and 2 demonstrated that if the structure of the pattern of notes used in the original scale illusion study is altered slightly, by adding or subtracting a pair of notes from the ends of the sequence, there is a significant decrease in the rate of frequency-based responses, suggesting a weaker illusion. Experiment 3 investigated two features of the note patterns that may have led to this change. Specifically, it asked whether the decrease in the strength of the illusion is due to (1) the nature of the notes at the extremes of the frequency range and/or (2) the nature of the notes at the crossing point of the two scales. While both sources were found to affect the strength of the scale illusion, the former had a greater influence.     

A scaling analysis of the snake lightness illusion

Logvinenko and Maloney (2006) measured perceived dissimilarities between achromatic surfaces placed in two scenes illuminated by neutral lights that could differ in intensity. Using a novel scaling method, they found that dissimilarities between light surface pairs could be represented as a weighted linear combination of two dimensions, "surface lightness" (a perceptual correlate of the difference in the logarithm of surface albedo) and "surface brightness" (which corresponded to the differences of the logarithms of light intensity across the scenes). Here we attempt to measure the contributions of these dimensions to a compelling lightness illusion (the "snake illusion"). It is commonly assumed that this illusion is a result of erroneous segmentation of the snake pattern into regions of unequal illumination. We find that the illusory shift in the snake pattern occurs along the surface lightness dimension, with no contribution from surface brightness. Thus, even if an erroneous segmentation of the snake pattern into strips of unequal illumination does happen, it reveals itself, paradoxically, as illusory changes in surface lightness rather than as surface brightness. We conjecture that the illusion strength depends on the balance between two groups of illumination cues signaling the true (uniform) illumination and the pictorial (uneven) illumination.     

A new phenomenon of apparent movement and its after-effect

It is found that a stationary spiral pattern gives an appearance of movement in a flickering light, and, furthermore, that this apparent rotation gives rise to the same kind of after-effect as a spiral actually rotating. The illusion is obtainable over a wide range of conditions. Detailed results are given in the case of six subjects, but a large number of subjects experienced the illusion. The experimenter has as yet found no one who was not subject to the illusion itself. The after-effects are not universally experienced.     

A new variant of the Ouchi illusion reveals Fourier-component-based processing

We report that anomalous motion illusion in a new variant of the Ouchi figure is well predicted by the strength of its Fourier fundamentals and harmonics. The original Ouchi figure consists of a rectangular checkerboard pattern surrounded by an orthogonal rectangular checkerboard pattern, in which illusory relative motion between the two regions is perceived. Although this illusion has been explained in terms of biases in integrating one-dimensional motion signals to determine the two-dimensional motion direction, the physiological mechanism has not been clarified. With our new stimuli, which consisted of thin lines instead of rectangles, we found that the perceived illusion is drastically reduced when the position of each line element is randomly shifted. This is not predicted by simple models of local motion integration along the visible edges. We demonstrate that the relative amplitude of the relevant Fourier fundamentals and harmonics leads to a quantitative prediction. Our analysis was successfully applied to other variants of the Ouchi figure (Khang and Essock 1997 Perception 26 585-597), closely predicting the reported rating. The results indicate that the underlying physiological mechanism is sensitive to the Fourier components of the stimuli rather than the visible edges.     

Decrement of the Müller-Lyer and Poggendorff illusions: the effects of inspection and practice

Five experiments assessed the decline or decrement in illusion magnitude for the wings-out form and the combined or Brentano form of the Müller-Lyer illusion, and for the Poggendorff illusion. Judgments were obtained under conditions of either continuous or intermittent inspection of the illusion figure. In the continuous-inspection conditions observers scanned the illusion figure during the inter-trial intervals whereas in the intermittent-inspection conditions they did not. Substantial illusion decrement was found in all continuous-inspection conditions and in intermittent conditions with short inter-trial intervals (upto 20 s) but not with longer inter-trial intervals. However, intermittent-inspection with a long inter-trial interval (40 s) produced illusion decrement but only when observers were instructed during the decrement session to ignore the wings, and pay attention to the shaft, of the Müller-Lyer figure. Taken together, the pattern of results does not support the claim that illusion decrement is primarily a product of practice or repeated trials.     

The Poggendorff illusion with obtuse and acute angles

The Poggendorff misalignment illusion with the conventional figure and its obtuse- and acute-angle variants was investigated in five experiments. The method of adjustment was used in Experiments I–IV, and in Experiment V compared with a forced-choice procedure. The first experiment showed that the 45-deg acute-angle illusion is positive but smaller than those from the other two figures, the second that it is the same size as that with two 45-deg oblique lines without parallels, and the third that the 30-deg acute-angle illusion is also positive but smaller than that for the other two patterns. In the fourth experiment, the 30-deg acute-angle illusion was insignificant under the same condition in which it was positive in Experiment III, but significantly positive for a reversed contrast (white on black) figure. The results from the last experiment were paradoxical; the 45-deg acute-angle figure again gave a significantly positive illusion with the method of adjustment, but mainly reports of a negative illusion with a forced-choice technique. A possible basis for this difference is discussed in terms of psychophysical procedures and special features of the acute-angle pattern. The weakness of the 30-deg acute-angle illusion is also considered along with other issues.     

Mechanisms in the haptic horizontal-vertical illusion: Evidence from sighted and blind subjects

The haptic horizontal-vertical illusion was studied in two experiments. In Experiment 1, the illusion was relatively weak in sighted subjects and depended on stimulus size and the nature of the figure, that is, whether the pattern was an inverted-T or L shape. Experiment 2 compared early blind and late blind subjects. The illusion was present for an inverted-T figure but absent for an L figure in late blind subjects. However, the early blind subjects treated both the L and T figures as similar and showed the illusion to both. These results support the idea that visual experience may alter haptic judgments in sighted and late blind subjects.     

Moon illusion in pictures: a multimechanism approach

The existence of the moon illusion in pictorial representations was demonstrated in 6 experiments. Ss either judged the size of the moon in pictures, depicted as on the horizon or high in the sky, or drew horizon and elevated moons. The horizon moon was consistently judged to be larger than the elevated moon, independent of the angle at which the pictures are viewed. The distance paradox usually observed with the moon illusion (horizon moon apparently closer than the elevated moon) also exists in pictures. The magnitude of both size and distance effects depends on the salience of depicted depth cues. The pattern of results suggests that the moon illusion is caused by several interacting mechanisms and that use of pictorial stimuli may allow the separation of various cognitive from physiological contributions to the illusion.     

The cognitive basis of the Uznadze illusion

The present study examines the role of Luria's dimensions of simultaneous and successive synthesis in fixation and extinction of the Uznadze haptic illusion. The subjects were 53 grade 10 high school students. Extinction of the illusion correlated significantly with three measures which, according to Luria's criteria, are indicative of simultaneous synthesis. Correlations between extinction and measures of successive synthesis failed to reach significance. Fixation of the illusion was unrelated to measures of either simultaneous or successive synthesis. An exploratory factor analysis of the data suggested the hypothesis that extinction of the illusion is greatly facilitated when the successive presentations of the spheres are integrated into a simultaneous or contemporaneous pattern.     

Two-dimensional tilt illusions induced by orthogonal plaid patterns: effects of plaid motion, orientation, spatial separation, and spatial frequency

Tilt illusions occur when a drifting vertical test grating is surrounded by a drifting plaid pattern composed of orthogonal moving gratings. The angular function of this illusion was measured as the plaid orientation (and therefore its drift direction) varied over a 180 degrees range. This was done when the test and inducing stimuli abutted and had the same spatial frequency, and when the test and inducing stimuli either differed in frequency by an octave, or were spatially separated by a 2 deg blank annulus, or both differed in frequency and were also separated by the annulus (experiments 1-4). The obtained angular function was virtually identical to that obtained previously with the rod and frame effect and other cases involving orthogonal inducing components, with evidence for illusions induced both by real-line components and by virtual axes of symmetry. Although the magnitude of the illusion was very similar in all four experiments, there was evidence to suggest that largest real-line effects occurred in the abutting same-frequency condition, with a pattern of results similar to that obtained previously with the simple one-dimensional tilt illusion. On the other hand, virtual-axis effects were more prominent with gaps between test and inducing stimuli. A fifth, repeated-measures, experiment confirmed this pattern of results. It is suggested that this pattern-induced tilt effect reflects both striate and extrastriate mechanisms and that the apparent influence of spatially distal virtual axes of symmetry upon perceived orientation implies the existence of AND-gate mechanisms, or conjunction detectors, in the orientation domain.