The Relationship Between Visual Illusion and Aesthetic Preference – an Attempt to Unify Experimental Phenomenology and Empirical Aesthetics

Experimental phenomenology has demonstrated that perception is much richer than stimulus. As is seen in color perception, one and the same stimulus provides more than several modes of appearance or perceptual dimensions. Similarly, there are various perceptual dimensions in form perception. Even a simple geometrical figure inducing visual illusion gives not only perceptual impressions of size, shape, slant, depth, and orientation, but also affective or aesthetic impressions. The present study reviews our experimental phenomenological work on visual illusion and experimental aesthetics, and examines how aesthetic preference is influenced by stimulus factors determining visual illusions including anomalous surface and transparency as well as geometrical illusion. Along with line figures producing geometrical illusions, illusory surface figures inducing neon color spreading and transparency effects were used as test patterns. Participants made both of psychophysical judgments and of aesthetic judgments for the same test pattern. Both of geometrical illusions and aesthetic preferences were found to change similarly as a function of stimulus variables such as the number of filling lines and the size ratio of the inner and outer figural components. Also, following specific stimulus variables such as lightness contrast ratio and spatial interval between inducing figural elements (so called ``packmen''), strong effects of color spreading and transparency were accompanied with strong preferences. It seems that the paradigm to investigate aesthetic phenomena along with perceptual dimensions is useful to bridge the gap between experimental phenomenology and experimental aesthetics.     

Anomalous contours and illusion of angularity: phenomenal and theoretical comparisons.

Many experimental comparisons between real and anomalous contours have proven the functional equivalence of the two conditions; however, there are some contradictory findings. One of these is obtained by analyzing the anomalous contours in the light of a new illusion, called the 'illusion of angularity'. A circle becomes a polygon when it covers the centre of a radial arrangement of black stripes, and a polygon changes its perceptual shape depending on its orientation with respect to the same radial arrangement. Phenomenally, it appears like a very pointed polygon, in which every side is concave or, alternatively, a shape that looks like a circle with angles added in the spaces between the radial stripes, or a polygonal shape in which every side is convex. The reciprocal anomalous counterparts of these conditions, obtained by removing the geometrical/polygonal contours, reveal different results. In the first case, one sees a perfect circle; in the second case, a polygon with blunted vertices, or a circular shape with angular protrusions; in the third case, a deformed circle. These results are inconsistent with some theoretical models proposed to explain the emergence of anomalous contours, namely, all the top-down models expressed in terms of cognitive constructions and perceptual hypotheses, or in terms of global figural organizations. Rather, these comparisons suggest a different interpretation for the two phenomena (the illusion of angularity and anomalous contours). This interpretation is based on dynamic interactions or on network computations that synthesize both real and anomalous contours.     

Altering Mueller-Lyer illusion magnitude using figural additions at the wing-shaft intersections

The Mueller-Lyer (ML) illusion has been used to study the way in which perceived length is affected by processes of information extraction when a visual target of interest (the ML shaft) is surrounded by other nontarget figural elements (inward-or outward-turning wings). It is argued that the perception of length is computed in terms of the center of gravity or centroid of figural elements at the wing-shaft intersection. The outward-turning wings shift the computational centroid away from the shaft end, giving rise to an erroneous overestimation of shaft length, while the inward-turning wings have the opposite effect. In three experiments, we observed that figural changes, which theoretically shifted the center of gravity of figural elements at the wing-shaft intersection, also increased or decreased the magnitude of the ML illusion.     

Is amodal completion necessary for the formation of illusory figures?

Kanizsa's hypothesis suggests that the creation of an anomalous surface is due to the amodal completion of the inducers. In the present paper a new pattern that is able to disconfirm this explanation is presented. According to the Helmholtz-Ratoosh law amodal completion only occurs when the borders of two adjacent surfaces meet forming T-shaped junctions. When the borders of the two adjacent surfaces have Y-shaped junctions, amodal completion is absent. However, when a pattern inducing an anomalous figure has the latter figural characteristics, in spite of the absence of amodal completion, an illusory figure is still visible. In this paper a set of experimental results (carried out by means of a magnitude estimation procedure as well as the method of constant stimuli) supporting the aforementioned observations is presented.     

The phantom illumination illusion

A novel brightness illusion in planar patterns is reported. The illusion occurs, for example, when surfaces with a luminance ramp shaded from black to white are positioned on a black homogeneous background, so that each white end of the surfaces faces a single point of the plane of the pattern. The illusion consists of the enhancement of the brightness of the background in a relatively wide area around the white ends of the surfaces. A parametric study was conducted in which participants were asked to rate the difference in brightness between the parts of the background inside and outside a virtual circle formed by disks with different luminance ramps. The results show that mean ratings of brightness depended on the luminance of the background, the luminance range of ramps, and the kind of ramp. Discussion of these results with reference to other brightness illusions (assimilation, neon color spreading, anomalous surfaces, visual phantoms, grating induction, and the glare effect) shows that t hephantom illumination illusion derives from processes producing the perception of ambient illumination.     

Effects of contextual and local factors on Ponzo illusion magnitude

Ponzo illusion has been explained by considering either just the inducing elements present in a restricted area of the visual field, the same area in which the test elements are located, or the stimulus configuration as a whole in which even the most distal figural elements – i.e., the external converging lines, here called “Ponzo wedge”– play a crucial role. The two studies reported here aimed at showing that both global configurational characteristics and inducing elements locally interacting with the test stimuli can independently affect the illusory effect. This hypothesis was tested using stimuli in which graphic-inducing elements giving rise to a herringbone pattern (Coren & Girgus, 1978) were drawn in the same area of the test segments. Results of Exp. 1 confirmed the effect of the two factors. In particular, both factors proved to determine the illusion, since they induced illusory effects either in isolation or in the same/opposite direction. In Exp. 2 the relative weight of these two factors was evaluated in relation to the width of the angle of the inducing elements and to the distance of the test segments from the vertex. Results showed no linear relationships between the distance of the test segments from the external inducing elements and the weight of the Ponzo wedge factor. Received: 25 June 1996 / Accepted: 1 October 1997     

Induction-, test-, and comparison-figure interactions under illusion and figural aftereffect conditions

Ganz (1966a, b) has argued that an induction figure will displace a test figure placed near it under both illusion and figural aftereffect conditions. The data from Experiments I and II show that most of the illusion produced by the figures studied by Ganz results from an interaction between the comparison and induction figures. The data from Experiment III suggest that both the test and comparison figures interact with the induction figure under figural aftereffect conditions. Although the induction-test figure interactions do not contradict Ganz’s model, the induction-comparison figure interactions cannot be explained by it. The data also suggest that researchers should be extremely cautious in drawing conclusions about the processes underlying illusions and figural aftereffects unless they are confident that there is no interaction between the induction and comparison figures.     

Two-dimensional filtering,oriented line detectors,and figural aspects as determinants of visual illusions

Quantitative data of Müller-Lyer illusions from the literature were analyzed according to three different models. All three models predict the illusion effect, although with different magnitude and different parameter dependency. First, a filter model describing a certain amount of blurring of the retinal picture seems partly responsible for the observed illusion. With reasonable estimation of the filter constants, however, a sufficient magnitude of illusion cannot be obtained. A second model of oriented line or bar receptors is even less effective in explaining the observed length illusions. A third model, consisting of a size-constancy operator triggered by depth cues, may predict effects larger than actually observed. It is concluded that figural aspects such as depth-inducing cues are mainly responsible for the illusion effects observed in Müller-Lyer figures.     

Pigeons perceive the Ebbinghaus-Titchener circles as an assimilation illusion

A target circle surrounded by larger "inducer" circles looks smaller, and one surrounded by smaller circles looks larger than they really are. This is the Ebbinghaus-Titchener illusion, which remains one of the strongest and most robust of contrast illusions. Although there have been many studies on this illusion in humans, virtually none have addressed how nonhuman animals perceive the same figures. Here the authors show that the Ebbinghaus-Titchener figures also induce a strong illusion in pigeons but, surprisingly, in the other direction; that is, all five successfully trained pigeons judged the target circle surrounded by larger circles to be larger than it really is and vice versa. Further analyses proved that neither the gaps between target and inducer circles nor the cumulative weighted surface of these figural elements could account for the birds' responses. Pigeons are known to show similarities to humans on various cognitive and perceptual tasks including concept formation, short-term memory, and some visual illusions. Our results, taken together with pigeons' previously demonstrated failure at visual completion, provide strong evidence that pigeons may actually experience a visual world too different for us to imagine.     

Figural after-effects, retinal size, and apparent size

The results of earlier experiments on the question of whether figural after-effects are affected by apparent as opposed to retinal size are shown to be inconclusive. A new hypothesis is proposed namely that both factors may be responsible for producing figural aftereffects, and four experiments have been made to test it. Situations were used in which the apparent sizes of the figures were determined by the size-constancy effect. It was found that where retinal sizes of test and inspection figures are the same and apparent sizes are different, figural after-effects in the direction which would be predicted on the basis of apparent sizes are obtained. It was further shown that where retinal and apparent sizes are in conflict, whether a figural after-effect is seen or not, and the direction of the figural after-effect, depends upon the balance between these two factors.     

A reversed Ebbinghaus–Titchener illusion in bantams (Gallus gallus domesticus)

A disk surrounded by smaller disks looks larger, and one surrounded by larger disks looks smaller than reality. This visual illusion, called the Ebbinghaus–Titchener illusion, remains one of the strongest and most robust illusions induced by contrast with the surrounding stimuli in humans. In the present study, we asked whether bantams would perceive this illusion. We trained three bantams to classify six diameters of target disks surrounded by inducer disks of a constant diameter into “small” or “large”. In the test that followed, the diameters of the inducer disks were systematically changed. The results showed that the Ebbinghaus–Titchener figures also induce a strong illusion in bantams, but in the other direction, that is, bantams perceive a target disk surrounded by smaller disks to be smaller than it really is and vice versa. Possible confounding factors, such as the gap between target disk and inducer disks and the weighted sum of surface of these figural elements, could not account for the subjects’ biased responses. Taken together with the pigeon study by Nakamura et al. (J Exp Psychol Anim Behav Process 34:375–387 2008), these results show that bantams as well as pigeons perceive an illusion induced by assimilation effects, not by contrast ones, for the Ebbinghaus–Titchener types of illusory figures. Perhaps perceptual processes underlying such illusory perception (i.e., lack of contrast effects) shown in bantams and pigeons may be partly shared among other avian species.     

Variables for designing cube folding tasks influencing sixth-graders' performance

This study investigated whether the three variables of task form, squares carried, and figural complexity, for designing cube folding tasks, affect sixth graders' cube folding performance. Two task forms were used to develop two versions of “cube folding test.” Each version was designed based on two levels of squares carried and three levels of figural complexity, and contained 18 multiple-choice items. The sample included 107 boys and 92 girls with an average age of 12.04 years who were randomly assigned to complete one of the two versions. A linear mixed model was carried out with three factors: task forms, squares carried, and figural complexity, in which squares carried and figural complexity were “within-subjects” factors, and task form was the “between-subjects” factor. The results showed the interaction effect of squares carried and task form, and the main effect of figural complexity, which differentiated the difficulty in cube folding tasks.     

Large errors in the perception of vertically are generated by luminance borders (integrated across space) not by subjective borders

The rod-and-frame illusion shows large errors in the judgment of visual vertical in the dark if the frame is large and there are no other visible cues (Witkin and Asch, 1948 Journal of Experimental Psychology 38 762-782). Three experiments were performed to investigate other characteristics of the frame critical for generating these large errors. In the first experiment, the illusion produced by an 11 degrees tilted frame made by luminance borders (standard condition) was considerably larger than that produced by a subjective-contour frame. In the second experiment, with a 33 degrees frame tilt, the illusion was in the direction of frame tilt with a luminance-border frame but in the opposite direction in the subjective-contour condition. In the third experiment, to contrast the role of local and global orientation, the sides of the frame were made of short separate luminous segments. The segments could be oriented in the same direction as the frame sides, in the opposite direction, or could be vertical. The orientation of the global frame dominated the illusion while local orientation produced much smaller effects. Overall, to generate a large rod-and-frame illusion in the dark, the tilted frame must have luminance, not subjective, contours. Luminance borders do not need to be continuous: a frame made of sparse segments is also effective. The mechanism responsible for the large orientation illusion is driven by integrators of orientation across large areas, not by figural operators extracting shape orientation in the absence of oriented contours.     

The hollow-face illusion: object-specific knowledge, general assumptions or properties of the stimulus?

The hollow-face illusion, in which a mask appears as a convex face, is a powerful example of binocular depth inversion occurring with a real object under a wide range of viewing conditions. Explanations of the illusion are reviewed and six experiments reported. In experiment 1 the detrimental effect of figural inversion, evidence for the importance of familiarity, was found for other oriented objects. The inversion effect held for masks lit from the side (experiment 2). The illusion was stronger for a mask rotated by 90 degrees lit from its forehead than from its chin, suggesting that familiar patterns of shading enhance the illusion (experiment 2). There were no effects of light source visibility or any left/right asymmetry (experiment 3). In experiments 4-6 we used a 'virtual' hollow face, with illusion strength quantified by the proportion of noise texture needed to eliminate the illusion. Adding characteristic surface colour enhanced the illusion, consistent with the familiar face pigmentation outweighing additional bottom-up cues (experiment 4). There was no difference between perspective and orthographic projection. Photographic negation reduced, but did not eliminate, the illusion, suggesting shading is important but not essential (experiment 5). Absolute depth was not critical, although a shallower mask was given less extreme convexity ratings (experiment 6). We argue that the illusion arises owing to a convexity preference when the raw data have ambiguous interpretations. However, using a familiar object with typical orientation, shading, and pigmentation greatly enhances the effect.     

The distance gradient in the after-effect of kinaesthetic width judgement

The combined effects of inspection-time and interfigural distance on a kinaesthetic figural after-effect were determined. The figural after-effect was defined as the degree to which a 2-in. width appeared to shrink following prolonged inspection of larger widths. The inspection-widths were 2.0, 2.5, 3.0, 3.5, 4.0, and 4.5 in. and the inspection-times were 10 and 50 sec. A control group which did not inspect any widths was also employed. 156 subjects were tested. The results indicated that the classical non-monotonic relationship between interfigural distance and figural after-effect was not present. Instead, the figural after-effect increased as a negatively accelerated function of interfigural distance. Increasing inspection-time increased the asymptotic level of the figural after-effect. These results were interpreted in terms of the effects of anchors on judgements of magnitude.     

Variation of the magnitude of the horizontal-vertical illusion with retinal eccentricity

The horizontal-vertical illusion was studied as a function of retinal eccentricity. It was found that the relation of illusion magnitude to vertical eccentricity is described by a U-shaped function with large amounts of reversed illusion for the more eccentric positions. Substantial effects due to horizontal eccentricity were also obtained, but these were not consistent across subjects. It is suggested that the flattening of the peripheral zones of the refracting surfaces of the eye may be involved in the variation of the illusion with retinal position, and that the astigmatic properties of the central portions of these surfaces may be a prime factor in the usual horizontal-vertical illusion.     

The tilted Hermann grid illusion: 'illusory spots' versus 'phantom bands'

The Hermann grid illusion became a cause célèbre, when it was reported that small figural changes from straight to curved bars abolish the dark illusory spots. We demonstrate that this is not an all-or-none effect; rather, the visual system tolerates some tilt/curviness. We transformed straight and curved Hermann grids to rhombic Motokawa grids by gradually tilting the horizontal bars. Initially, we observed only dark illusory spots, then dark spots combined with phantom bands traversing the rhomb along the minor axis, and finally dark phantom bands only. This shows that two kinds of illusions can coexist in the same grid pattern.     

Familiar shapes attract attention in figure-ground displays

We report five experiments that explore the effect of figure-ground factors on attention. We hypothesized that figural cues, such as familiar shape, would draw attention to the figural side in an attentional cuing task using bipartite figure-ground displays. The first two experiments used faces in profile as the familiar shape and found a perceptual advantage for targets presented on the meaningful side of the central contour in detection speed (Experiment 1) and discrimination accuracy (Experiment 2). The third experiment demonstrated the figural advantage in response time (RT) with nine other familiar shapes (including a sea horse, a guitar, a fir tree, etc.), but only when targets appeared in close proximity to the contour. A fourth experiment obtained a figural advantage in a discrimination task with the larger set of familiar shapes. The final experiment ruled out eye movements as a possible confounding factor by replicating the RT advantage for targets on the figural side of face displays when all trials containing eye movements were eliminated. The results are discussed in terms of ecological influences on attention, and are cast within the framework of Yantis and Jonides's hypothesis that attention is exogenously drawn to the onset of new perceptual objects. We argue that the figural side constitutes an "object" whereas the ground side does not, and that figural cues such as shape familiarity are effective in determining which areas represent objects.     

Selective attention and asymmetry in the Müller-Lyer illusion

Two experiments reexamined the effect of selective spatial attention on the magnitudes of the wings- in and wings-out forms of the Müller-Lyer (M-L) illusion and a version of the illusion in which the two forms are superimposed to produce a figure (XX) flanked at both ends by an X. For the XX figure, ignoring the outer wings produced significant underestimation of shaft length, whereas ignoring the inner wings had no significant effect. For the M-L figures, ignoring the wings was more effective in attenuating the magnitude of the wings-out than of the wings-in illusion. The results are discussed with reference to space-based approaches to visual attention and to claims that attentional modulation of illusion magnitudes implicates high-level or cognitive factors in the formation of the M-L illusion.     

The role of figure orientation and apparent depth in the perception of the horizontal-vertical illusion.

An experiment was performed which examined the role of figural orientation directly, and the role of an inappropriately invoked size-constancy mechanism indirectly, in the actuation and magnitude of the horizontal-vertical illusion. When the vertical line of the stimulus figure was aligned above the horizontal line, the illusory effect was significant and positive; in contrast, when the vertical line was located below the horizontal line, the illusion was negative. Under the assumption that a vertical line can appear as a foreshortened line in depth, these findings support an explanation based on the operation of a misapplied size-constancy mechanism.