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.     

Global factors in the Hermann grid illusion

Most explanations of the Hermann grid illusion are local in nature. For example, in Baumgartner's model the effect is generated by the response of cells having concentric on-off or off-on receptive fields. Such models predict that the magnitude of the illusion at a given intersection should be the same whether that intersection is viewed in isolation or in conjunction with other intersections in a grid. Two experiments are reported. The first demonstrates that illusion magnitude grows with the number of intersections. The second shows that this growth is seen when the intersections are arranged in an orderly grid but not when they are placed irregularly. These results suggest that a purely local model for the Hermann grid illusion is not a complete explanation. Global factors must be involved.     

Ponzo错觉解释的合理性：倾斜恒常性理论的局限

倾斜恒常性理论是一种新的解释Ponzo错觉的理论,但是似乎存在一些局限。本研究采用调节法通过两个实验考察了四种Ponzo错觉版本在各种条件(视角和水平线段间距)下的错觉量情况,以此来检验倾斜恒常性理论。实验一中,对称Ponzo错觉变异版本在50mm时的错觉量情况和Prinzmetal(2001)的结果相似,但是在85mm和120mm时得到了较多的错觉量,这和倾斜恒常性理论的理论预期不符。实验二对不对称Ponzo错觉的考察得到了和实验一类似的结果,只有50mm时的情形符合倾斜恒常性理论预期。通过以上实验得到水平线间距和视角因素都是产生Ponzo错觉的重要因素,而倾斜恒常性理论过分强调了背景线的倾斜诱导效应,忽略了其他结构因素对Ponzo错觉的影响,所以倾斜恒常性理论具有一定的局限性,不能有效地解释Ponzo错觉的产生机制。     

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.     

The effects of location,orientation, and cumulation of boxes in the Baldwin illusion

Three experiments were conducted on variants of the Baldwin illusion. Experiment 1 showed that placing a box on each side of the standard line produced a larger illusion than placing both boxes on the same side of the line. These results failed to support the assimilation theory proposed by Brigell, Uhlarik, and Goldhorn (1977). In Experiment 2, the side of a rectangular box was varied when that side was either parallel or perpendicular to the standard line. The parallel rectangle produced a function that was similar to the one found with the classical Baldwin figure, but the perpendicular boxes produced a monotonically decreasing function with a reversal of illusion evident at the large sizes. The latter function did not support any version of the assimilation theory. The findings of Experiment 3 replicated previous findings that showed that cumulating contours as box size increased had no effect on the illusion. These findings explain two longstanding puzzles about the Müller-Lyer illusion: why a multifinned form is not the sum of its single-finned parts and why the shrinkage form produces a smaller effect than does the expansion form.     

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.     

Assimilation and contrast illusions: Differences in plasticity

Three experiments show differences in the plasticity of the contrast and assimilation portions of the Delboeuf, Ebbinghaus, and Ponzo illusions. Contrast illusions show decrement in illusion magnitude with free inspection, whereas assimilation illusions do not. A model to explain both the original distortion and the differential susceptibility of the two classes of illusion to decrement is offered.     

The effect of line-figure information on the magnitude of the dot forms of the Poggendorff and Müller-Lyer illusions

The magnitudes of the dot and line forms of the Poggendorff illusion and the Brentano version of the Müller-Lyer illusion were assessed in two groups of subjects: the informed group was given information about the implied figure configuration in the dot pattern, the uninformed group was not. The informed group produced a significantly greater dot illusion than the uninformed group, and there was no difference between the two groups in the magnitudes of the line illusions. The experiments are discussed in the context of Coren and Porac's proposal that illusion-inducing mechanisms can be divided into structural and cognitive components. The results suggest that about 64% of the magnitude of the Poggendorff illusion and about 54% of the Müller-Lyer illusion can be attributed to the involvement of cognitive factors.     

Perception of the Ponzo illusion by rhesus monkeys, chimpanzees, and humans: Similarity and difference in the three primate species

In Experiment 1, 3 rhesus monkeys and 1 chimpanzee were tested for their susceptibility to the Ponzo illusion. The subjects were first trained to report the length of the target bar presented at the center of the computer display by touching either of the two choice locations designated as “long” or “short.” When inverted-V context lines were superimposed on the target bar, the subjects tended to report “long” more often as the apex of these upward-converging lines approached the target bar. The perception of the Ponzo illusion was thus demonstrated. In Experiment 2, the same 3 rhesus monkeys and 2 new chimpanzees were tested using two types of context lines that provided different strengths of linear perspective. The subjects showed a bias similar to that found in Experiment 1, but there was no difference in the magnitude of the bias between the two types of context in either species. This failed to support the classic account for the Ponzo illusion, the perspective theory, raised by Gregory (1963). In Experiment 3, the magnitude of the illusion was compared between the inverted-V context and the context consisting of short vertical lines having the same gap as the former in the same 3 rhesus monkeys and 2 of the chimpanzees from the preceding experiments. While the chimpanzees showed the illusion for both types of stimuli, the monkeys showed no illusion for the latter. In Experiment 4, 6 humans were tested in a comparable procedure. As in the nonhuman primates, the illusion was unaffected by the strength of linear perspective. On the other hand, the humans showed considerably larger illusion for the context consisting of vertical lines than for contexts consisting of converging lines. Thus, there was a great species difference in the effect of the gap itself on the magnitude of the Ponzo illusion. Similarity found at first turned out to be no more than superficial. Possible sources of this species difference are discussed.     

An illusion of eccentricity

The illusion investigated here is that two concentric arcs, drawn in different (though possibly overlapping) circular sectors and having the same angular extent, appear to be eccentric. Three possible explanations of the illusion are tested. The first hypothesis is that concentricity judgments are made by elongating the arcs to see if they intersect, the illusion being due to the tendency, when elongating a curve, to follow the end-tangent. The second hypothesis is that concentricity judgments are based on a test of coincidence of centers, the illusion being due to the overestimation of the radius of short arcs. The third hypothesis is that both of these factors contribute in equal measure. These hypotheses make different predictions about the effect (on the magnitude of the illusion) of the following variables: (1) the angular distance between the arcs; (2) the radial distance between the arcs; (3) the degree of curvature of the arcs; and (4) the angular extent of the arcs. The observed values of the illusion angle (obtained by the method of limits) in relation to these variables did not uniformly support any of the hypotheses. A more complex model that is consistent with the observed results is therefore proposed.     

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.     

A variant of the anomalous motion illusion based upon contrast and visual latency

We examined a variant of the anomalous motion illusion. In a series of experiments, we ascertained luminance contrast to be the critical factor. Low-contrast random dots showed longer latency than high-contrast ones, irrespective of whether they were dark or light (experiments 1 -3). We conjecture that this illusion may share the same mechanism with the Hess effect, which is characterised by visual delay of a low-contrast, dark stimulus in a moving situation. Since the Hess effect is known as the monocular version of the Pulfrich effect, we examined whether illusory motion in depth could be observed if a high-contrast pattern was projected to one eye and the same pattern of low-contrast was presented to the other eye, and they were binocularly fused and swayed horizontally. Observers then reported illusory motion in depth when the low-contrast pattern was dark, but they did not when it was bright (experiment 4). Possible explanations of this inconsistency are discussed.     

The effect of insstructions on judgment of the Müller-Lyer illusion with normal and haptically mediated visual inspection

An experiment with the Müller-Lyer figure is reported in which variation in instructions resulted in a change in the magnitude of the visual illusion under normal viewing conditions. Variation in instructions did not, however, have a differential effect on the magnitude of the illusion when Ss inspected the figure by means of a point source of light attached to one fingertip. These results are equivalent to differences found between the effect of instructions on visual and haptic illusions using the same illusion figure and support the view that variation in inspection patterns rather than differences in higher processingof sensory input might account for differences between the two modalities.     

Framing effects and the reversed Müller-Lyer illusion.

The enclosure hypothesis of the reversed Müller-Lyer illusion was examined in three experiments. In Experiment 1, the ingoing- and outgoing-wings forms of the illusion were measured separately, as a function of the size of the gap between the ends of the shaft and the apices of the wings. In Experiments 2 and 3, the effects of a square frame and of complete and amputated versions of a rectangle on the perceived length of an enclosed horizontal line were examined. For all non-Müller-Lyer illusion figures, an inverted U-shaped function describes the relationship between illusion magnitude and the length of the test line. The peak overestimation of the test line's length was obtained when the ratio of total figure length to test line length was about 3:2. Taken together, the results of the three experiments suggest that the reversed Müller-Lyer illusion can be explained within current theoretical frameworks, such as assimilation theory, without recourse to a special factor of enclosure.     

Seeing big things: overestimation of heights is greater for real objects than for objects in pictures

In six experiments we demonstrate that the vertical-horizontal illusion that is evoked when viewing photographs and line drawings is relatively small, whereas the magnitude of this illusion when large objects are viewed is at least twice as great. Furthermore, we show that the illusion is due more to vertical overestimation than horizontal underestimation. The lack of a difference in vertical overestimation between pictures and line drawings suggests that vertical overestimation in pictures depends solely on the perceived physical size of the projection on the picture surface, rather than on what is apparent about an object's represented size. The vertical-horizontal illusion is influenced by perceived physical size. It is greater when viewing large objects than small pictures of these same objects, even when visual angles are equated.     

The oblique line illusion: The poggendorff effect without parallels

The apparent misalignment of two oblique collinear lines was investigated in two experiments. In the first the effect with the lines at 45° to the median plane was compared with that for the same two lines separated by the conventional parallels of the Poggendorff figure. The illusion with the two lines was consistent and significant but about one-third the magnitude of that with the parallels. The two illusions were significantly correlated. In the second experiment the angle of the two oblique, collinear lines was varied in 15° steps. The misalignment illusion was maximal at 45° and smaller but significant at 60 and 75°. There was no significant effect at 15 and 30°.     

Similarity of the perimeters in the Ebbinghaus illusion

Coren and Miller (1974) and Coren and Enns (1993) argued that the magnitude of the Ebbinghaus illusion is a function of the rated or conceptual similarity of the inducing objects to the test object. In three experiments, we examined the convergence between conceptual similarity and illusion magnitude. The first failed to find support for this parallel. Two further experiments yielded support for an alternative hypothesis that the magnitude of the Ebbinghaus illusion is a function of the similarity of the perimeters of the inducing object to the test object. The similarity of the centers had no effect. These results suggest that the information used to estimate size is computed earlier in the visual system than suggested by Coren and colleagues and apparently does not involve the use of conceptual information.     

Visual field differences in the magnitude of the Oppel-Kundt illusion vary with processing time

Three experiments were performed to investigate hemispheric differences in susceptibility to the Oppel-Kundt illusion presented tachistoscopically to the two visual hemifields. Experiment 1 used a successive comparison mode, in which 16 undergraduate students indicated whether the second of two successive extents looked shorter or longer than the first. Experiment 2 (20 under-graduates) and Experiment 3 (1 commissurotomy patient) required judgments of two extents presented simultaneously. The first experiment found no significant visual field differences, although females were more susceptible than males. In the second experiment, the illusion magnitude was greater in the left visual field, and in the third experiment, it was greater in the right visual field. Post hoc analyses resolve the conflicting results and show that in all three experiments, the susceptibility of the right hemisphere declined more than that of the left hemisphere during illusion processing. An interpretation is offered in terms of two parallel processes, one that is fast, uses feature extraction, and is performed more effectively in the left hemisphere, and one that is slow, uses visuospatial analysis to compute distances between parts of the illusion figure, and is performed more effectively in the right hemisphere.     

Framing effects and the reversed Müller-Lyer illusion

The enclosure hypothesis of the reversed Müller-Lyer illusion was examined in three experiments. In Experiment 1, the ingoing- and outgoing-wings forms of the illusion were measured separately, as a function of the size of the gap between the ends of the shaft and the apices of the wings. In Experiments 2 and 3, the effects of a square frame and of complete and amputated versions of a rectangle on the perceived length of an enclosed horizontal line were examined, For all non-Müller-Lyer illusion figures, an inverted U-shaped function describes the relationship between illusion magnitude and the length of the test line. The peak overestimation of the test line’s length was obtained when the ratio of total figure length to test line length was about 3:2. Taken together, the results of the three experiments suggest that the reversed Müller-Lyer illusion can be explained within current theoretical frameworks, such as assimilation theory, without recourse to a special factor of enclosure.