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Variations of the Craik-O'Brien, Muller-Lyer, reversed Muller-Lyer, orthogonal Muller-Lyer, and Ponzo illusions were created using 10 to 12 parallel lines. 20 participants judged which of two physically equal lines in the patterns appeared longer. For 4 of the 5 patterns, the parallel lines created illusions which corresponded to those produced by the template figures. The results suggest that length is encoded by frequency of neural response and that the parallel line illusions, as well as their classical counterparts, emerge from interactions which distort this frequency code. DOI 10.2466/PMS. 109.2.452-458

Previous research has shown that by placing nontargets around the endpoints of the shaft of the Mueller-Lyer illusion it is possible to markedly affect its normal strength. Using the same technique with the Ponzo illusion and the horizontal-vertical illusion, the current study demonstrated a similar ability to affect these other illusions of length. These findings provide varying amounts of support or challenges for different theories of illusion formation. In addition, the results highlight the importance of analyzing illusion data using comparative difference scores and examining separate overestimation and underestimation components in each illusion.

The authors describe a new visual illusion first discovered in a natural setting. A cyclist riding beside a pair of sagging chains that connect fence posts appears to move up and down with the chains. In this illusion, a static shape (the chains) affects the perception of a moving shape (the bicycle), and this influence involves assimilation (averaging) rather than opposition (differentiation). These features distinguish the illusion from illusions of motion capture and induced motion. The authors take this bicycle illusion into the laboratory and report 4 findings: Naive viewers experience the illusion when discriminating horizontal from sinusoidal motion of a disc in the context of stationary curved lines; the illusion shifts from motion assimilation to motion opposition as the visual size of the display is increased; the assimilation and opposition illusions are dissociated by variations in luminance contrast of the stationary lines and the moving disc; and the illusion does not occur when simply comparing two stationary objects at different locations along the curved lines. The bicycle illusion provides a unique opportunity for studying the interactions between shape and motion perception. Keywords: motion and shape perception, motion assimilation, visual illusion

This study frames anomalous functional brain organization in schizophrenia (SCZ) within an evolutionary model of brain development, the dual trends theory (DTT). The DTT argues that neural architecture develops along 2 separate pathways: the dorsal archicortical trend and the ventral paleocortical trend. The DTT dovetails with visual system organization, which is also composed of 2 independent pathways: a dorsal stream dedicated to visuomotor action and a ventral stream dedicated to perceptual processing. The present study examined the integrity of these pathways using a size-contrast visual illusion. Prior research has shown that, normally, perceptual estimations of object size are susceptible to visual illusions, whereas goal-directed actions are resistant. The authors hypothesized that, unlike control participants, SCZ patients' actions would be susceptible to the illusion, reflecting dorsal stream dysfunction. Here, 42 SCZ patients and 42 healthy controls grasped and estimated the size of target blocks in control and illusion conditions. During estimation, both groups were equally perturbed by the illusion; however, grasping movements of patients alone were influenced by the illusion. These results suggest disrupted dorsal brain circuitry in SCZ but relatively intact ventral circuitry. Keywords: schizophrenia, visual illusion, action and perception, dual cytoarchitectonic trends, evolution

We have implemented experimental code to compute a compensated phase-added stereogram (CPAS), which was proposed in a previous paper, on a graphic processing unit (GPU). In this paper, we show an acceleration method for CPAS computation by means of the GPU and compare the computation time between CPU-based and GPU-based calculations, which are programmed in our laboratories. In addition, we demonstrate their reconstructed images. As a result, we could achieve a performance gain of a factor of over 33 compared with a CPU-based computing environment and digital holograms can be displayed at 30 frames per second with 15,000 points. OCIS codes: 090.1760, 090.5694.

We propose an 'accurate' phase-added stereogram, which can be defined as an improved phase-added stereogram. Generally, the macroblock size transformed by the fast Fourier transform is the same as the segmentation size of the phase-added stereogram. However, the proposed method uses a lager macroblock size than does the conventional method to reduce quantization error in discrete spatial frequencies in the spatial frequency domain. Therefore, even when the fast Fourier transform is used for calculation, the quality of the reconstructed image can be improved to be as clear as the Fresnel hologram. OCIS codes: 090.1760, 090.2870.

Over the history of the study of visual perception there has been great success at discovering countless visual illusions. There has been less success in organizing the overwhelming variety of illusions into empirical generalizations (much less explaining them all via a unifying theory). Here, this article shows that it is possible to systematically organize more than 50 kinds of illusion into a 7 x 4 matrix of 28 classes. In particular, this article demonstrates that (1) smaller sizes, (2) slower speeds, (3) greater luminance contrast, (4) farther distance, (5) lower eccentricity, (6) greater proximity to the vanishing point, and (7) greater proximity to the focus of expansion all tend to have similar perceptual effects, namely, to (A) increase perceived size, (B) increase perceived speed, (C) decrease perceived luminance contrast, and (D) decrease perceived distance. The detection of these empirical regularities was motivated by a hypothesis, called 'perceiving the present,' that the visual system possesses mechanisms for compensating neural delay during forward motion. This article shows how this hypothesis predicts the empirical regularity. Keywords: Illusions; Systematization; Generalization; Extrapolation; Ecology; Unification; Evolution; Compensation; Neural delay; Flash-lag effect; Vision; Perceiving the present

The flash-lag effect is a visual illusion wherein intermittently flashed, stationary stimuli seem to trail after a moving visual stimulus despite being flashed synchronously. We tested hypotheses that the flash-lag effect is due to spatial extrapolation, shortened perceptual lags, or accelerated acquisition of moving stimuli, all of which call for an earlier awareness of moving visual stimuli over stationary ones. Participants judged synchrony of a click either to a stationary flash of light or to a series of adjacent flashes that seemingly bounced off or bumped into the edge of the visual display. To be judged synchronous with a stationary flash, audio clicks had to be presented earlier--not later--than clicks that went with events, like a simulated bounce (Experiment 1) or crash (Experiments 2-4), of a moving visual target. Click synchrony to the initial appearance of a moving stimulus was no different than to a flash, but clicks had to be delayed by 30-40 ms to seem synchronous with the final (crash) positions (Experiment 2). The temporal difference was constant over a wide range of motion velocity (Experiment 3). Interrupting the apparent motion by omitting two illumination positions before the last one did not alter subjective synchrony, nor did their occlusion, so the shift in subjective synchrony seems not to be due to brightness contrast (Experiment 4). Click synchrony to the offset of a long duration stationary illumination was also delayed relative to its onset (Experiment 5). Visual stimuli in motion enter awareness no sooner than do stationary flashes, so motion extrapolation, latency difference, and motion acceleration cannot explain the flash-lag effect. Resume L'effet << flash lag >> est une illusion visuelle selon laquelle des stimuli immobiles semblent trainer derriere un stimulus visuel en mouvement, tous intermittents, meme s'ils se produisent en synchronie. Nous avons mis a l'essai des hypotheses voulant que l'effet << flash lag >> soit cause par une extrapolation spatiale, un delai de perception moindre ou une acquisition acceleree des stimuli en mouvement, qui semblent tous indiquer une prise de conscience plus hative des stimuli en mouvement que des stimuli immobiles. Les participants devaient juger de la synchronie d'un clic et sor d'un clignotement lumineux immobile, soit d'une serie de clignotements adjacents qui semblaient rebondir ou entrer en collision contre le bord d'un ecran de visualisation. Pour etre juges synchrones a un clignotement immobile, les clics audio devaient etre produits avant--et non apres--les clics associes aux evenements, comme la simulation du rebond (experience 1) ou de la collision (experiences 2 a 4) d'une cible visuelle en mouvement. La synchronie d'un clic et de l'apparition initiale d'un stimulus en mouvement n'etait pas differente de celle d'un clic et de l'apparition d'un stimulus immobile. Par contre, les clics devaient etre retardes de 30 40 ms pour sembler synchrones aux positions finales (collision) des stimuli en mouvement (experience 2). L'ecart temporel etait constant pour une vaste gamme de vitesses de mouvement (experience 3). L'interruption du mouvement apparent par l'omission de deux positions de clignotement avant la position finale n'a pas altere la synchronie subjective, pas plus que leur occlusion. Le decalage de la synchronie subjective ne semble donc pas etre cause par le contraste de luminosite (experience 4). La synchronie d'un clic par rapport h l'ecart d'un long stimulus immobile etait egalement decalee par rapport a son debut (experience 5). Les stimuli visuels en mouvement ne font pas l'objet d'une prise de conscience plus hative que celle des stimuli immobiles, et, donc, l'extrapolation du mouvement, la difference de latence et l'acceleration du mouvement ne peuvent pas expliquer l'effet << flash lag >>.

A decade ago, S. Aglioti, J. F. X. DeSouza, and M. A. Goodale (1995) published an experiment that has had a big influence on the way that visual information is thought to control human behavior. Their findings have often been simplified as suggesting that action is immune to perceptual illusions. The current authors critically analyze the 4 steps involved in this simplification and argue that research during the last 10 years has shown that the validity of 3 of the 4 steps is doubtful. They conclude that this experiment cannot be regarded as firm support for the 2-visual-systems hypothesis (i.e., that the ventral stream is for perception and the dorsal stream is for visually guided actions). Keywords: visual illusion, selection, action, grasping, dorsal pathway

The magnitude of the Muller-Lyer illusion was investigated using the Brentano figure by varying the chromaticities of the shaft and the fins along the cone-excitation axes and by varying the occlusion status of the shaft by the fins. 72 college students participated using the staircase method. The stimuli whose shaft and the fin chromaticities were the same produced larger illusions than stimuli with different chromaticities regardless of the luminance contrast between the figure and the surround. When the shaft appeared in front of the fins, the illusion effect was stronger than when the shaft was occluded by the fins.

Extensive research has identified individual differences associated with sex in a range of visual task performances, including susceptibility to visual illusions. The aim of this study was to identify the locus of sex differences within the context of the Poggendorf illusion. 79 women and 79 men participated within a mixed factorial design. Analyses indicated that sex differences were only present in the stimulus context with the full inducing element present. This finding replicates recent research and provides qualifying evidence as to the locus of the effect. The findings are discussed within the functional framework of perceptual processes involved in extrapolating 3-dimensional characteristics from 2-dimensional visual stimuli

One of the most intriguing of the many discrepancies between perceived spatial relationships and the physical structure of visual stimuli is the Poggendorff illusion, when an obliquely oriented line that is interrupted no longer appears collinear. Although many different theories have been proposed to explain this effect, there has been no consensus about its cause. Here, we use a database of range images (i.e., images that include the distance from the image plane of every pixel in the scene) to show that the probability distribution of the possible locations of line segments across an interval in natural environments can fully account for all of the behavior of this otherwise puzzling phenomenon. geometrical illusions | natural scene statistics | range images | visual perception

Several brightness illusions indicate that borders can affect the perception of surfaces dramatically. In the Cornsweet illusion, two equiluminant surfaces appear to be different in brightness because of the contrast border between them. Here, we report the existence of cells in monkey visual cortex that respond to such an 'illusory' brightness. We find that luminance responsive cells are located in color-activated regions (cytochrome oxidase blobs and bridges) of primary visual cortex (V1), whereas Cornsweet responsive cells are found preferentially in the color-activated regions (thin stripes) of second visual area (V2). This colocalization of brightness and color processing within V1 and V2 suggests a segregation of contour and surface processing in early visual pathways and a hierarchy of brightness information processing from V1 to V2 in monkeys. Cornsweet | optical imaging | thin stripes

Antigravity hills, also known as spook hills or magnetic hills, are natural places where cars put into neutral are seen to move uphill on a slightly sloping road, apparently defying the law of gravity. We show that these effects, popularly attributed to gravitational anomalies, are in fact visual illusions. We re-created all the known types of antigravity spots in our laboratory using tabletop models; the number of visible stretches of road, their slant, and the height of the visible horizon were systematically varied in four experiments. We conclude that antigravity-hill effects follow from a misperception of the eye level relative to gravity, caused by the presence of either contextual inclines or a false horizon line.

When a spoked wheel is observed rolling behind a picket fence, a striking pattern (the Roget Illusion) is observed that arises as a result of the motion and the persistence of vision. The mathematical description of the illusion is developed and an apparatus briefly described that demonstrates it. The inverse process of de-convoluting a distorted object utilizing motion and the persistence of vision is employed in a device called the 'anorthoscope.' The analysis of this device is discussed and a simple instrument is shown. [DOI: 10.1119/1.1575766]

The Poggendorff illusion is one of the most prominent geometrical-optical illusions and has attracted enduring interest for more than a hundred years. Most modern theories explain the illusion by postulating various kinds of distortion of the 'test' component of the figure by the context or the inducing component. They make no reference to the importance of processes involved in three-dimensional scene perception for understanding the illusion. We measured the strength of the Poggendorff illusion in configurations containing solid inducing surfaces as opposed to the usual parallel lines. The surface, oblique-line, and background luminances were manipulated separately to create configurations consistent with modal completion of the obliques in front of the surface. The marked decrease in the size of the illusion in conditions favoring modal completion is consistent with claims that perceived spatial layout is a major determinant of the Poggendorff illusion.

In the boundary extension illusion, subjects recollect more of a photographed scene than was originally shown. In this study, first- and fifth-grade children, young adult college students, and older adults studied 4 one-object or 4 two-object picture stimuli for 15 s each. Immediately after each visual scene was shown, the subjects drew it from memory inside a rectangle that was the same size as the previous picture. This study demonstrated that all age groups, from young children to older adults, were susceptible to the boundary extension illusion. This finding is discussed in terms of Intraub&apos;s perceptual schema hypothesis and Johnson&apos;s source-monitoring hypothesis.

Numerous studies have reported that repetitive or extensive inspection leads to short-term decrement of the horizontal-vertical illusion. The present experiment explored whether this decrement reflects a gradual decline, as previously assumed, or is better described as a singular drop-off beyond the initial evaluation. 111 student participants adjusted vertical or horizontal lines such that they appeared equally long with a perpendicular standard. There were 8 successive adjustment trials for each subject. The results suggest that a substantial component of the illusion depends on the first impression assessment. The earlier anatomical and cognitive theories of the horizontal-vertical illusion cannot incorporate this datum. However, recent findings suggest that a motor theory of illusion might accommodate the observed one-step decrement in the illusion.

Byline: B. V. Ermolayev (1) Abstract: Interactions between visual and motor spaces were studied using tracking movements in a virtual action space paradigm. Disturbances of the regular structure of the visual space was done using computer modeling of the Hering visual illusion. The distribution of the cinematic characteristics of tracking movements in the coordinate system, associated with the target of the motor task, revealed the dominance of topological characteristics over metric characteristics during the formation of the motor field. Factors producing the appropriate topology were determined. It was found that motor space is more susceptible to these factors than visual field. Author Affiliation: (1) Moscow State Open University, ul. P. Korchagina 22, Moscow, 129805, Russia Article History: Registration Date: 12/10/2004

47 subjects adjusted the extended vertical lines of 20 inverted-T figures to make them appear equal to a horizontal line and rated the confidence in their accuracy after each trial. One group viewed figures of varying sizes, a second group viewed figures of standard size, and a third group viewed figures of standard size but received feedback on their accuracy immediately after completing Trial 5. Except for a significant increase in accuracy on Trial 6 for the Feedback Group, there were no differences in performance among groups. Subjects made the vertical line significantly shorter than the horizontal line on initial trials but their accuracy improved over trials. In contrast, there was no consistent increase in confidence, and several analyses indicated that confidence ratings were unrelated to accuracy. These results suggest that the subjects were unaware of the decrement in illusion that occurred over trials.

It is argued that an unwarranted dichotomization of geometrical illusions has produced theories whose explanatory scope is limited to either assimilation or contrast. Since recent attempts at integration lack precision, a 1977 unified model of the perception of extent by Brigner was revised to predict accurately both assimilative and contrastive phases of the parallel lines illusion. Application of the revised model to other geometrical illusions was discussed and a means of parsimoniously accounting for variations in assimilative illusions occurring with age and with lightness of the stimulus figure was suggested.

If Mach bands arise as an empirical consequence of real-world luminance profiles, several predictions follow. First, the appearance of Mach bands should accord with the appearance of naturally occurring highlights and lowlights. Second, altering the slope of an ambiguous luminance gradient so that it corresponds more closely to gradients that are typically adorned with luminance maxima and minima in the position of Mach bands should enhance the illusion. Third, altering a luminance gradient so that it corresponds more closely to gradients that normally lack luminance maxima and minima in the position of Mach bands should diminish the salience of the illusion. Fourth, the perception of Mach bands elicited by the same luminance gradient should be changed by contextual cues that indicate whether the gradient is more or less likely to signify a curved or a flat surface. Because each of these predictions is met, we conclude that Mach bands arise because the association elicited by the stimulus (the percept) incorporates these features as a result of past experience.

The effects of luminance contrast and spatial frequency on the motion aftereffect were investigated. The point of subjective equality for velocity was measured as an index of the motion aftereffect. The largest effect was observed when a low contrast grating (5%) was presented as a test stimulus after adaptation to a high contrast grating (100%) in the low spatial frequency condition (0.8 cycle [deg.sup.-1]). On the whole, the effect increased with increasing adapting contrast and with decreasing test contrast or spatial frequency. Small effects were observed at high test contrasts. These results were inconsistent with those of Keck, Palella, and Pantle in 1976. Analysis showed that there was no saturation on velocity of the motion aftereffect above 5% of the contrast although Keck, et al. (1976) found that the incremental increases of the effect above 3% adapting contrast were small.

Research was conducted to examine the effects of perceived space on spatial attention using a cuing method aimed at analyzing space- versus object-based attention. The objective was to show that a perceptual illusion that changes the perceived length of two lines also influences spatial attention. Results indicate that spatial attention is distributed in space as it is perceived and transformed by perceptual organization.

Naive observers (N = 75) were asked to saccade in the dark across a point light source blinking on and off at 200 Hz and to describe the resultant phantom array (Hershberger, 1987). The vast majority represented this perisaccadic illusion of visual direction essentially as Hershberger described it. Replicable features of the phantom array imply that the perisaccadic shift of retinal local signs (i.e., spatiotopic coordinates) is discontinuous, comprising two separate parts, a discrete partial shift that occurs before the eyes start to move, and a continuous partial shift that is completed at about the same time as the eye movement. Although these implications are consistent with recent experimental findings, they are inconsistent with the received view that retinal local signs shift sluggishly. The phantom array implies that the faster, presaccadic, partial shift has a time constant of 5 ms or less., If one saccades in the dark across a point source of light blinking rapidly on and off, one will see a spatially extended series of lights called the 'phantom array' [...]

The present study examined the effect of a size-contrast illusion (Ebbinghaus or Titchener Circles Illusion) on visual perception and the visual control of grasping movements. Seventeen right-handed participants picked up and, on other trials, estimated the size of 'poker-chip' disks, which functioned as the target circles in a three-dimensional version of the illusion. In the estimation condition, subjects indicated how big they thought the target was by separating their thumb and forefinger to match the target's size. After initial viewing, no visual feedback from the hand or the target was available. Scaling of grip aperture was found to be strongly correlated with the physical size of the disks, while manual estimations of disk size were biased in the direction of the illusion. Evidently, grip aperture is calibrated to the true size of an object, even when perception of object size is distorted by a pictorial illusion, a result that is consistent with recent suggestions that visually guided prehension and visual perception are mediated by separate visual pathways., INTRODUCTION Vision provides us with a vast array of information about the world around us - information that can direct our thoughts and guide our actions. Yet our perception of [...]

This study investigated the effects of figure context on the apparent length of a line. In Exp. 1, ten participants were asked to adjust the length of a comparison line to match a standard line enclosed within a rectangle. The participants consistently overestimated the length of the standard line, demonstrating the stretching effect of figure context on the apparent length of a line. In Exp. 2 (12 participants), the size of the context figure was varied and it had no significant influence on the magnitude of the context effect. In Exp. 3 (nine participants), the context effect was shown not only for squares and rectangles but also for other shapes of figures such as circles and 5-pointed stars. We discuss the possible mechanism of the figure-context effect within Gregory's (1970, 1978) misapplied constancy theory of visual illusions.

When a brief lateral cue precedes an instantaneously presented horizontal line, observers report a sensation of motion in the line propagating from the cued end toward the uncued end. This illusion has been described as a measure of the facilitatory effects of a visual attention gradient (O. Hikosaka, S. Miyauchi, & S. Shimojo, 1993a). Evidence in the present study favors, instead, an account in which the illusion is the result of an impletion process that fills in interpolated events after the cue and the line are linked as successive states of a single object in apparent motion.

In this research, the McCollough effect was observed by using an ambiguous test pattern of coarse gratings made out of fine gratings. Coarse gratings meet at right angles to fine gratings. The global features (coarse gratings) in the test pattern became more salient if the test pattern was blurred, and the frequency of the McCollough effect, corresponding to the global orientation of the test pattern, increased with the increase of the extent of blur. The McCollough effects corresponding to the local orientation of the test pattern occurred frequently if the subjects adapted to fine gratings before viewing the test pattern, whereas the McCollough effects corresponding to the global orientation of the test pattern occurred frequently if the subjects adapted to coarse gratings. Our results indicate that the perceptual organization is an important determinant in the McCollough effect.

Illusion decrement for the Muller-Lyer and Horizontal-Vertical illusions was examined. The experiment consisted of an initial adjustment of an illusion followed by 20 test trials, each with an intervening 60-sec. intertrial interval during which a comparator line and a standard line set to equality were visually inspected for 0, 20, 40, or 60 sec. After each intertrial interval the length of the comparator line was reset by the experimenter to either 0 or 90 cm, and subjects then adjusted its length to perceived equality with the standard line (42 cm). Illusion decrement was inversely related to the duration of inspection for each illusion, with significant reductions in magnitude of illusion observed for all groups. These results support prior demonstrations that perceptual learning mechanisms are operative during brief periods of visual inspection, especially when these periods are followed by the opportunity to make repeated adjustments.

We report a new visual illusion of a perceptual boundary visible between two contiguous regions of equal luminance when the intensity is modulated with a temporal frequency that is higher than the critical fusion rate. Measurements of the luminance threshold of the perceptual border with various slopes of the luminance gradient yielded a function suggestive of the range of ocular instability. These findings raise the possibility that this new border illusion may be influenced by involuntary ocular motion during fixation.

The magnitude of the Ebbinghaus illusion has been reported to be greater when test element and context elements are figurally similar as opposed to figurally dissimilar. In the current investigation with 16 observers, illusion magnitude was greater for a figurally similar configuration even though the context elements of the figurally similar configuration were perceived as smaller than the context elements of a figurally dissimilar configuration. Hence, figural similarity appears to have a pre-potent effect in the Ebbinghaus illusion.

There is evidence that complex objects are decomposed by the visual system into features, such as shape and color. Consistent with this theory is the phenomenon of illusory conjunctions, which occur when features are incorrectly combined to form an illusory object. We analyzed the perceived location of illusory conjunctions to study the roles of color and shape in the location of visual objects. In Experiments 1 and 2, participants located illusory conjunctions about halfway between the veridical locations of the component features. Experiment 3 showed that the distribution of perceived locations was not the mixture of two distributions centered at the 2 feature locations. Experiment 4 replicated these results with an identification task rather than a detection task. We concluded that the locations of illusory conjunctions were not arbitrary but were determined by both constituent shape and color.

The inside of a picture of a uniformly gray tube drawn with black circles appears lighter than the outside. Coren and Komoda, who first described this illusion, argued that observers take illumination into account to infer that the inside is lighter. That is, the inside of the tube should receive less illumination than the outside but reflects the same amount of light into the eyes. Observers, therefore, infer that it must be lighter. The inside of a gray tube drawn with white circles should appear lighter as well according to this account, but the experiments reported here show that the outside appears lighter in such a tube. We believe that depth perception is involved in this illusion but that lightness constancy is not.

The jump effect occurs when an object, for example, a small square, moves on a homogeneous background at a constant velocity along its path except for a short section wherein the velocity suddenly changes to a higher value. In correspondence with this increase, observers may report that the object appears to jump. The experiments reported here explored the velocity conditions which determine the occurrence of this perceptual jump. A difference of at least 15 cm/sec. between the two velocities of the object was necessary for the effect to occur with probability 0.5. With slightly lower frequency, the effect also occurred when the object increased its velocity to a higher constant value in a short part of its path and then stopped. For the phenomenal jump to occur in all the different conditions used, there had to be an optimal length of the path in which the object increased its velocity. Finally, both velocities, before and after this increase, influenced the jump, although each with different weight.

Weak central coherence at low levels helps children with autism in making accurate judgements of illusory figures and prevents them from succumbing to visual illusions. Autistic children fail to combine the induced lines and the inducing context involved in the task of illusory figure perception. They succumb more to three-dimensional disembedded illusions rather than to two-dimensional conditions.

Wheels turning in the movies or in other forms of stroboscopic presentation often appear to be rotating backward. Remarkably, a similar illusion is also seen in continuous light. The occurrence of this perception in the absence of intermittent illumination suggests that we normally see motion, as in movies, by processing a series of visual episodes.

The Delboeuf illusion consists in a change in the perceived (judged) size of one circle in the presence of another concentric circle. This illusion was presented and analyzed for the first time in 1865 and not, as stated by numerous investigators, in 1892 or 1893. This misconception reflects profound misreadings of Delboeuf&apos;s works. The present study examines the three Delboeuf articles on visual illusions (1865a, 1865b, 1892) and analyzes the author&apos;s data in the light of results obtained to date on concentric circle illusions., Joseph Delboeuf is primarily known in psychology for the optico-geometric illusion that today bears his name (Delboeuf concentric circles). However, perusal of specialized works (e.g., Robinson, 1972; Vurpillot, 1963) as [...]

The generation of illusory contours is closely related to distinct parts of a pattern such as dots, line ends, and corner points. On the other hand, the remarkable property is that gaze at one point of the contours diminishes the illusion and a return of gaze to the whole pattern restores it. Therefore, illusory contours depend on local parts and the whole pattern formed by the parts, and fitting data on the two aspects is necessary to clarify underlying mechanisms. We have obtained such data from the experiments performed to elucidate other visual phenomena. On the basis of the data, the concepts of strength of pattern, strength of its spread effect, ridgelines of the spread effect, and a hollow of the spread effect are introduced and then various phenomena on illusory contours, including the Kanizsa triangle, are explained in terms of these concepts.

When a wheel rolls along a flat surface, a point on its perimeter traces a cycloid trajectory, forming a sequence of adjacent semicircle-like scallops. However, when mentally visualizing this point's trajectory, participants erroneously describe the point's path as looping back on itself between each scallop or phase of the cycloid, a phenomenon called the curtate cycloid illusion. The studies supported the hypothesis that the curtate cycloid illusion occurs because the cognitive system sometimes does not have sufficient resources for simultaneously processing 2 components of the motion: its translation and its rotation about its current instant center. Four experiments using computer-animated rolling wheels found that participants who were high in spatial ability were less susceptible to the curtate cycloid illusion than were low-spatial participants, that high-spatial participants were not susceptible to the illusion if they could control the animated wheel display, and that the illusion was substantially decreased if the opportunity to compute instant centers was reduced.

Following earlier informal observations, the first of two experiments confirmed that, whereas the vertical and horizontal extents of a printed passage are more or less veridically perceived when the passage is surrounded by margins as is usual on a page, they appear reduced when the margins are removed by cropping them close to the edges of the passage. In Exp. 2 with a horizontal-line grating a similar reduction occurred vertically but not horizontally, suggesting that the effect occurs for patterned regions but less or not at all for unpatterned, i.e., uniform, regions. The possible relationship between this effect and the Delboeuf illusion of size is discussed.

The KoWa model described here gives a parametric account of the geometric conditions for 6 optical illusions from which prediction of illusory qualities of the figures in perceptual judgements may be accomplished using selected stimulus dimensions. The model is designed to allow integration of visual information.

Decrement, a time-related decrease in the magnitude of the Mueller-Lyer illusion, was measured separately for the wings-out and the wings-in variants of the Mueller-Lyer figure. There were significant reductions of wings-out illusion magnitude during the decrement period. Observers viewing the wings-in segment showed a nonsignificant decrement pattern. Analyses of individual decrement patterns showed that illusion magnitude did not decrease for a number of observers even when there were significant time-related trends at the group level. Data for 80 observers imply that the mechanisms of perceptual learning proposed by previous models of Mueller-Lyer illusion decrement are not sufficient explanations of the decrement process.

The misalignment which is seen in the Poggendorff illusion can be studied with better control by using a configuration which has only two line segments. Two experiments were conducted in which subjects judged collinearity of a test segment, this judgment being subjected to a biasing influence from a second (induction) segment. Exp. 1 held the test segment at one of three orientations relative to the observer (30 degrees, 45 degrees, and 60 degrees) and systematically varied the orientation of the induction segment in 15 degrees increments through the range of possible positions. The orientation of the page relative to the observer was varied as well. Exp. 2 varied the test segment through a greater range of angles and sampled more levels of induction segment orientation. Analysis indicated that projection errors follow orderly rules similar in kind to but different in magnitude from those observed for the Tilt Illusion, most notably, (a) misprojection is greatest when the orientation of the interfering line is similar to that of the line segment being projected and (b) the strength of this influence decreases as the relative angle becomes orthogonal. Also, the orientation of the segment being projected relative to the observer serves to modulate the strength of the basic induction effect. These perceptual interactions are discussed in relation to neural models for orientation selectivity.