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.     

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.     

Dependence of illusory motion on directional consistency in oblique components

Pinna and Brelstaff (2000 Vision Research 40 2091-2096) reported a motion illusion on viewing two concentric circles consisting of quadrangular components with black and white sides on a grey background. Our results suggest that the illusion is based on the integration of motion signals derived from oblique components, and on the consistency in the direction among those components. Furthermore, arrays of these oblique components can elicit the perception of motion not only for the oblique components themselves, but also for other objects in the picture. We propose that the motion illusion depends not only upon detection of the illusory motion signal at each local oblique component, but also upon the accumulation of the signal all over the stimulus configuration.     

Halt and recovery of illusory motion perception from peripherally viewed static images

We quantitatively investigated the halt and recovery of illusory motion perception in static images. With steady fixation, participants viewed images causing four different motion illusions. The results showed that the time courses of the Fraser-Wilcox illusion and the modified Fraser-Wilcox illusion (i.e., "Rotating Snakes") were very similar, while the Ouchi and Enigma illusions showed quite a different trend. When participants viewed images causing the Fraser-Wilcox illusion and the modified Fraser-Wilcox illusion, they typically experienced disappearance of the illusory motion within several seconds. After a variable interstimulus interval (ISI), the images were presented again in the same retinal position. The magnitude of the illusory motion from the second image presentation increased as the ISI became longer. This suggests that the same adaptation process either directly causes or attenuates both the Fraser-Wilcox illusion and the modified Fraser-Wilcox illusion.     

The directional tuning of the barber-pole illusion.

In order to study the integration of local motion signals in the human visual system, we measured directional tuning curves for the barber-pole illusion by varying two crucial aspects of the stimulus layout independently across a wide a range in the same experiment. These were the orientation of the grating presented behind the rectangular aperture and the aspect ratio of the aperture, which in combination determine the relative contributions of local motion signals perpendicular to the gratings and parallel to the aperture borders, respectively. The strength of the illusion, ie the tendency to perceive motion along the major axis of the aperture, obviously depends on the spatial layout of the aperture, but also on grating orientation. Subjects were asked which direction they perceived and how compelling their motion percept was, revealing different strategies of the visual system to deal with the barber-pole stimulus. Some individuals respond strongly to the unambiguous motion information at the boundaries, leading to multistable percepts and multimodal distributions of responses. Others tend to report intermediate directions, apparently being less influenced by the actual boundaries. The general pattern of deviations from the motion direction perpendicular to grating orientation--a decrease with aspect ratio approaching unity (ie square-shaped apertures) and with gratings approaching parallel orientation to the shorter aperture boundary--is discussed in the context of simple phenomenological models of motion integration. The best fit between model predictions and experimental data is found for an interaction between two stimulus parameters: (i) cycle ratio, which is the sine-wave gratings equivalent of the terminator ratio for line gratings, describing the effects from the aperture boundaries, and (ii) the grating orientation, responsible for perpendicular motion components, which describes the influence of motion signals from inside the aperture. This suggests that the most simple cycle (terminator) ratio explanation cannot fully account for the quantitative properties of the barber-pole illusion.     

Perception of the Ebbinghaus illusion in 5- to 8-month-old infants

The Ebbinghaus illusion is a geometric illusion based on a size-contrast between a central circle and surrounding circles. A central circle surrounded by small inducing circles is perceived as being larger than a central circle surrounded by large inducing circles. In the present study we investigated 5- to 8-month-old infants' perception of the Ebbinghaus illusion using a preferential-looking paradigm. We measured the preference between a central circle surrounded by small inducing circles (overestimated figure) and a central circle surrounded by large inducing circles (underestimated figure). Infants showed a significant preference for the overestimated figure when the central circle was flashing, but not when it was static. Furthermore, there was no preference between the two figures when the central circles were removed. These results suggest that infants' preference reflects their perception of the size illusion of the central circle. There is a possibility that 5- to 8-month-old infants perceive the Ebbinghaus illusion.     

Visual context integration is not fully developed in 4-year-old children

Long-range horizontal interactions supporting contour integration were found to be weaker in children than in adults (Kovács et al, 1999 Proceedings of the National Academy of Sciences of the USA 96 12204-12209). In the present study, integration on a larger scale, between a target and its context was investigated. Contextual modulation of the percept of a local target can be directly measured in the case of geometric illusions. We compared the magnitude of a size contrast illusion (Ebbinghaus illusion or Titchener circles) in children and adults. 4-year-old children and adults performed 2AFC size comparisons between two target disks in the classical Ebbinghaus illusion display and in two other modified versions. We found that the magnitude of the illusion effect was significantly smaller in children than in adults. Our interpretation is that context integration is not fully developed in 4-year-old children. Closer-to-veridical-size estimations by children demonstrate that the perception of the local target is less affected by stimulus context in their case. We suggest that immature cortical connectivity is behind the reduced contextual sensitivity in children.     

Illusory bands in orientation and spatial frequency: a cortical analog to Mach bands

Illusory bands at a luminance transition in space (ie an edge) are well known. Here we demonstrate illusory bands of enhanced orientations or spatial frequencies at transitions between higher-contrast and lower-contrast image content along the orientation and spatial-frequency dimensions--the dimensions of cortical spatial coding. We conclude that this illusion is a consequence of cortical-level suppression of units of similar orientations and spatial frequencies and serves to aid texture segmentation while providing efficient neural coding.     

Illusory motion induced by blurred red-blue edges

Visual patterns consisting of a red-and-blue region with a blurry edge yield illusory motion. Eye movements over a static pattern induced illusory motion of the edge in the direction opposite to the eye movement. The illusion also takes place for patterns in motion without eye movement. The illusion suggests the effect of colour combination on the spatial perception of a blurry edge.     

Lightness from contrast: a selective integration model

As has been observed by Wallach (1948), perceived lightness is proportional to the ratio between the luminances of adjacent regions in simple disk-annulus or bipartite scenes. This psychophysical finding resonates with neurophysiological evidence that retinal mechanisms of receptor adaptation and lateral inhibition transform the incoming illuminance array into local measures of luminance contrast. In many scenic configurations, however, the perceived lightness of a region is not proportional to its ratio with immediately adjacent regions. In a particularly striking example of this phenomenon, called White's illusion, the relationship between the perceived lightnesses of two gray regions is the opposite of what is predicted by local edge ratios or contrasts. This paper offers a new treatment of how local measures of luminance contrast can be selectively integrated to simulate lightness percepts in a wide range of image configurations. Our approach builds on a tradition of edge integration models (Horn, 1974; Land & McCann, 1971) and contrast/filling-in models (Cohen & Grossberg, 1984; Gerrits & Vendrik 1970; Grossberg & Mingolla, 1985a, 1985b). Our selective integration model (SIM) extends the explanatory power of previous models, allowing simulation of a number of phenomena, including White's effect, the Benary Cross, and shading and transparency effects reported by Adelson (1993), as well as aspects of motion, depth, haploscopic, and Gelb induced contrast effects. We also include an independently derived variant of a recent depthful version of White's illusion, showing that our model can inspire new stimuli.     

Boundary completion, contrast polarity, and the perception of illusory tilt

What we perceive as a unitary object can be the result of integrative processes that generate a whole from parts. Although this issue of visual perception has been widely explored, recent experimental findings demonstrate that our knowledge is still incomplete. In particular, the question whether contour binding is affected by the sign of contrast (contrast polarity) across edges requires more in-depth examination. Here we show the effects of edge bindings that originate from the merging of laterally displaced edges with the same contrast polarity. We have studied a particular context in which such effects may emerge: a checkerboard with a series of alternated dark and light shapes superimposed on the corners of the squares. The phenomenal observations and experimental findings support the theories according to which boundary completions are originated by phenomena of edge propagation within a 'field of completion' (eg Shipley and Kellman, 2003 Perception 32 985-999) adjacent to an edge ending. Our findings conform to the Shipley and Kellman theory that boundary completion results from the interaction of edges as well as from edges and shapes lacking in oriented contours, the latter serving as 'receiving units', anchoring the paths of activations generated by oriented edges. We propose to integrate this theory with the hypothesis that interactions sensitive to the contrast sign generate conjunction paths of edges that alter their perceived orientation. Based on this perspective we propose an alternative account for the Café Wall illusion that can be extended to other phenomena of orientation misperception and to a Café Wall inversion effect that has not been observed previously.     

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.     

Perception of the Ebbinghaus illusion in four-day-old domestic chicks (Gallus gallus)

In the Ebbinghaus size illusion, a central circle surrounded by small circles (inducers) appears bigger than an identical one surrounded by large inducers. Previous studies have failed to demonstrate sensitivity to this illusion in pigeons and baboons, leading to the conclusion that avian species (possibly also nonhuman primates) might lack the neural substrate necessary to perceive the Ebbinghaus illusion in a human-like fashion. Such a substrate may have been only recently evolved in the primate lineage. Here, we show that this illusion is perceived by 4-day-old domestic chicks. During rearing, chicks learnt, according to an observational-learning paradigm, to find food in proximity either of a big or of a small circle. Subjects were then tested with Ebbinghaus stimuli: two identical circles, one surrounded by larger and the other by smaller inducers. The percentage of approaches to the perceptually bigger target in animals reinforced on the bigger circle (and vice versa for the other group) was computed. Over four experiments, we demonstrated that chicks are reliably affected by the illusory display. Subjects reinforced on the small target choose the configuration with big inducers, in which the central target appears perceptually smaller; the opposite is true for subjects reinforced on the big target. This result has important implications for the evolutionary history of the neural substrate involved in the perception of the Ebbinghaus illusion.     

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.     

New motion illusion caused by pictorial motion lines

Motion lines (MLs) are a pictorial technique used to represent object movement in a still picture. This study explored how MLs contribute to motion perception. In Experiment 1, we reported the creation of a motion illusion caused by MLs: random displacements of objects with MLs on each frame were perceived as unidirectional global motion along the pictorial motion direction implied by MLs. In Experiment 2, we showed that the illusory global motion in the peripheral visual field captured the perceived motion direction of random displacement of objects without MLs in the central visual field, and confirmed that the results in Experiment 1 did not stem simply from response bias, but resulted from perceptual processing. In Experiment 3, we showed that the spatial arrangement of orientation information rather than ML length is important for the illusory global motion. Our results indicate that the ML effect is based on perceptual processing rather than response bias, and that comparison of neighboring orientation components may underlie the determination of pictorial motion direction with MLs.     

The Leaning Tower of Pisa effect: an illusion mediated by colour, brightness, and motion

We report a novel, easily observed, and extraordinarily striking optical illusion mediated by interactions of colour, brightness, form, and motion perception--the Leaning Tower of Pisa (LTOP) illusion. Under some circumstances, the perception of orientation of coloured forms is radically altered by rotary movement. We demonstrate that this kinetic effect--easily reproduced with a common record turntable--is optimised by particular colour and brightness differences between foreground and background with an illusory tilt of 8 degrees and more. The described illusions can be easily studied at home by downloading the colour figures from www.perceptionweb.com/perc1000/ditzinger, printing them on a common colour printer and placing them on a rotating record turntable.     

Objects, raised lines, and the haptic horizontal-vertical illusion

Experiments were conducted to determine whether the haptic horizontal-vertical illusion occurs with solid, three-dimensional objects as well as with tangible lines. The objects consisted of round or square bases, with dowel rods projecting above them at heights equal to the widths of the horizontal bases. A negative illusion, with overestimation of horizontals, was found with free haptic exploration, but not with tracing with the fingertip. The negative illusion occurred when subjects felt wooden Ls and inverted Ts with a grasping, pincers motion of the index finger and thumb. The presence or absence of illusory misperception was dependent upon exploration strategy, since the negative illusion vanished with finger tracing. A negative illusion was also found when subjects adjusted a vertical dowel so that it was judged to be equal in extent to a round or square base. A general overestimation of judged size derived from the pincers response measure, but was not found with the use of a tangible ruler. Comparable illusory results are most likely when drawings and objects promote similar haptic scanning methods. The results were consistent with the idea that the orientation of an edge or line is more important than whether one explores a tangible line or a three-dimensional object.     

More accurate size contrast judgments in the Ebbinghaus Illusion by a remote culture

The Ebbinghaus (Titchener) illusion was examined in a remote culture (Himba) with no words for geometric shapes. The illusion was experienced less strongly by Himba compared with English participants, leading to more accurate size contrast judgments in the Himba. The study included two conditions of inducing stimuli. The illusion was weaker when the inducing stimuli were dissimilar (diamonds) to the target (circle) compared with when they were similar (circles). However, the illusion was weakened to the same extent in both cultures. It is argued that the more accurate size judgments of the Himba derive from their tendency to prioritize the analysis of local details in visual processing of multiple objects, and not from their impoverished naming.     

Pattern orientation, not motion, determines the two-dimensional tilt illusion

In the usual tilt illusion (TI) configuration, an inducing stimulus which has a single orientation is used psychophysically to explore orientation analysis in the human visual system. Recently, this approach has been extended to the use of inducing stimuli which have two orientations. Such a two-dimensional (2-D) stimulus permits investigation of the low-level analysis of visual patterns. Prior experimentation has left it unclear whether it is the spatial or the motion properties of a moving crossed-grating plaid which determine two-dimensional tilt illusions (2-D TIs) because these two parameters previously were perfectly correlated. In the present experiments pattern orientation and motion were decoupled. It is shown that 2-D TIs are determined by the spatial properties of an inducing annulus and not by its motion properties. The results also support the existence of a mechanism which extracts axes of symmetry, and which is difficult to account for in terms of local cross-orientation domain inhibition.     

The line-motion illusion can be reversed by motion signals after the line disappears

In the line-motion illusion, a briefly flashed line appears to propagate from the locus of attention, despite being physically presented on the screen all at once. It has been proposed that the illusion reflects low-level visual information processing that occurs faster at the locus of attention (Hikosaka et al 1993 Vision Research 33 1219-1240; Perception 22 517-526). Such an explanation implicitly embeds the assumption that speeding or slowing of neural signals will map directly onto perceptual timing. This 'online' hypothesis presupposes that signals which arrive first are perceived first. However, other evidence suggests that events in a window of time after the disappearance of a visual stimulus can influence the brain's interpretation of that stimulus (Eagleman and Sejnowski 2000 Science 287 2036-2038; 289 1107a; 290 1051a; 2002 Trends in Neuroscience 25 293). If the online hypothesis were true, we should expect that events occurring after the flashing of the line would not change the illusion. Consistent with our hypothesis that awareness is an a posteriori reconstruction, we demonstrate that the perceived direction of illusory line-motion can be reversed by manipulating stimuli after the line has disappeared.