Retinal projection or size constancy as determinants of the horizontal-vertical illusion?

To compare the influence of the projected retinal size and of the figure size on the perception of the horizontal-vertical illusion, the target size, the viewing distance, and the slant of an illusion figure were varied. In the first experiment the illusion produced by two figures of the same object size but of different retinal size was compared with that of two figures projecting the same retinal size but differing in object size. The illusion diminished when the size of the retinal projection was increased, whereas a change in figure size did not change the illusion. In Exp. II the illusion figure was tilted backwards which reduced the retinal projection of the 'vertical' figure limb. The illusion decreased and became negative as a function of the retinal projection, but this decrease was relatively small compared with the reduction of the retinal image. The results are interpreted as supporting a retinal origin as an explanation of the illusion. Although there is strong evidence for size-constancy scaling in a tilted figure, constancy scaling is considered of minor importance as a determinant of the usual illusion.     

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.     

Computation of mean size is based on perceived size

The present study investigated whether computation of mean object size was based on perceived or physical size. The Ebbinghaus illusion was used to make the perceived size of a circle different from its physical size. Four Ebbinghaus configurations were presented either simultaneously (Experiment 1) or sequentially (Experiment 2) to each visual field, and participants were instructed to attend only to the central circles of each configuration. Participants’ judgments of mean central circle size were influenced by the Ebbinghaus illusion. In addition, the Ebbinghaus illusion influenced the coding of individual size rather than the averaging. These results suggest that perceived rather than physical size was used in computing the mean size.     

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 illusion of clarity: Image segmentation and edge attribution without filling-in

It comes as no surprise that viewing a high-resolution photograph through a screen reduces its clarity. Yet when a coarsely quantized (i.e., pixelated) version of the same photo is seen through a screen its clarity is increased. Six experiments investigated this illusion of clarity. First, the illusion was quantified by having participants rate the clarity of quantized images with and without a screen (Experiment 1). Interestingly, the illusion occurs both when the wires of the screen are aligned with the blocks of the quantized image and when they are shifted horizontally and vertically (Experiments 2 and 3), casting doubt on the hypothesis that a local filling-in process is involved. The finding that no illusion occurs when the photo is blurred rather than quantized (Experiment 4) and that the illusion is sharply reduced when visual attention is divided (Experiment 5) argue for an image segmentation process that falsely attributes the edges of the quantized blocks to the screen. Finally, the illusion is larger when participants adopt an active rather than a passive cognitive strategy (Experiment 6), pointing to the importance of cognitive control in the illusion.     

Motion versus position in the perception of head-centred movement

Abstract. Observers can recover motion with respect to the head during an eye movement by comparing signals encoding retinal motion and the velocity of pursuit. Evidently there is a mismatch between these signals because perceived head-centred motion is not always veridical. One example is the Filehne illusion, in which a stationary object appears to move in the opposite direction to pursuit. Like the motion aftereffect, the phenomenal experience of the Filehne illusion is one in which the stimulus moves but does not seem to go anywhere. This raises problems when measuring the illusion by motion nulling because the more traditional technique confounds perceived motion with changes in perceived position. We devised a new nulling technique using global-motion stimuli that degraded familiar position cues but preserved cues to motion. Stimuli consisted of random-dot patterns comprising signal and noise dots that moved at the same retinal 'base' speed. Noise moved in random directions. In an eye-stationary speed-matching experiment we found noise slowed perceived retinal speed as 'coherence strength' (ie percentage of signal) was reduced. The effect occurred over the two-octave range of base speeds studied and well above direction threshold. When the same stimuli were combined with pursuit, observers were able to null the Filehne illusion by adjusting coherence. A power law relating coherence to retinal base speed fit the data well with a negative exponent. Eye-movement recordings showed that pursuit was quite accurate. We then tested the hypothesis that the stimuli found at the null-points appeared to move at the same retinal speed. Two observers supported the hypothesis, a third partially, and a fourth showed a small linear trend. In addition, the retinal speed found by the traditional Filehne technique was similar to the matches obtained with the global-motion stimuli. The results provide support for the idea that speed is the critical cue in head-centred motion perception.     

Effects of orientation-selective adaptation on the Z?llner illusion

The model of inhibitory interaction between orientation detectors was examined by prolonged presentation of grating patterns (which was expected to induce orientation-selective adaptation) before measurement of the Z?llner illusion. Adaptation effects were measured under conditions which excluded intrusion by the tilt aftereffect. In experiment 1, illusion magnitude greatly decreased only when the orientation of the adapting grating was the same as that of the inducing lines, which confirmed the first prediction deduced from the model. There was no effect of adapting grating when it was oriented more than 20 degrees away from the inducing lines. In experiment 2, adaptation effects were selective not only to orientation but also to spatial frequency. In experiment 3 it was shown that illusion reduction was mediated neither by lowered apparent contrast of the inducing lines nor by retinal adaptation. The results are discussed with respect to the nature of adaptation and possible physiological correlates.     

Luminance gradient can break background-independent lightness constancy

A display with a luminance gradient was shown to induce a strong lightness illusion (Logvinenko, 1999 Perception 28 803-816). However, a 3-D cardboard model of this display was found to produce a much weaker illusion (less than half that in the pictorial version) despite the fact that its retinal image is practically the same. This is in line with the hypothesis that simultaneous lightness contrast is solely a phenomenon of pictorial perception (Logvinenko et al, 2002 Perception 31 73-82). The residual lightness illusion in the 3-D model can be accounted for by the fact that this model is a hybrid display. Specifically, while it is a real object, a pictorial representation (of the illumination gradient) is superimposed on it. Thus, lightness in the 3-D display is a compromise between two opposite tendencies: the background-independent lightness constancy and the lightness illusory shift induced by the luminance gradient.     

The influence of optical aberrations on the magnitude of the Poggendorff illusion

The amount of blurring of the retinal image can be reduced by proper selection of an artificial pupil and a chromatic filter. Reduction of the amount of blurring due to optical aberrations by viewing through a 1-mm artificial pupil and an interference filter in the eye, results in a concomitant reduction in the magnitude of the Poggendorff illusion. The magnitude of the reduction is smaller, however, than would be expected if the illusion was predominantly determined by blur due to optical aberration.     

An acceleration illusion caused by underestimation of stimulus velocity during pursuit eye movements: Aubert-Fleischl revisited

When the eyes pursue a fixation point that sweeps across a moving background pattern, and the fixation point is suddenly made to stop, the ongoing motion of the background pattern seems to accelerate to a higher velocity. Experiment I showed that this acceleration illusion is not caused by the sudden change in (i) the relative velocity between background and fixation point, (ii) the velocity of the retinal image of the background pattern, or (iii) the motion of the retinal image of the rims of the CRT screen on which the experiment was carried out. In experiment II the magnitude of the illusion was quantified. It is strongest when background and eyes move in the same direction. When they move in opposite directions it becomes less pronounced (and may disappear) with higher background velocities. The findings are explained in terms of a model proposed by the first author, in which the perception of object motion and velocity derives from the interaction between retinal slip velocity information and the brain's 'estimate' of eye velocity in space. They illustrate that the classic Aubert-Fleischl phenomenon (a stimulus seems to be moving slower when pursued with the eyes than when moving in front of stationary eyes) is a special case of a more general phenomenon: whenever we make a pursuit eye movement we underestimate the velocity of all stimuli in our visual field which happen to move in the same direction as our eyes, or which move slowly in the direction opposite to our eyes.     

The Moses, mega-Moses, and Armstrong illusions: integrating language comprehension and semantic memory

This study develops a new theory of the Moses illusion, observed in responses to general knowledge questions such as, "How many animals of each kind did Moses take on the Ark?" People often respond "two" rather than "zero" despite knowing that Noah, not Moses, launched the Ark. Our theory predicted two additional types of conceptual error demonstrated here: the Armstrong and mega-Moses illusions. The Armstrong illusion involved questions resembling, "What was the famous line uttered by Louis Armstrong when he first set foot on the moon?" People usually comprehend such questions as valid, despite knowing that Louis Armstrong was a jazz musician who never visited the moon. This Armstrong illusion was not due to misperceiving the critical words (Louis Armstrong), and occurred as frequently as the Moses illusion (with critical words embedded in identical sentential contexts), but less frequently than the mega-Moses illusion caused when Moses and Armstrong factors were combined.     

The horizontal-vertical illusion in photographs of concrete scenes with and without depth information

On the basis of the hypothesis of misapplied constancy scaling, the perception of an abstract horizontal-vertical illusion figure embedded in photographs of natural scenes with depth cues is investigated. The effect is compared with that of a figure on photographs containing no depth information and with a figure on a neutral surface. It is shown that the magnitude of the illusion in the perspective scenes is greater than in the other two conditions. The results are considered compatible with a constancy theory of the illusion. Finally, the evidence for misapplied constancy scaling in the horizontal-vertical illusion in relation to a retinal theory is discussed.     

The effect of retinal location on the magnitude of the Poggendorff illusion

The effect of retinal locus on the magnitude of the Poggendorff illusion was investigated. A significant illusion was found to occur in the fovea and in undiminished magnitude at the peripheral locations horizontally displaced from the fovea. No significant illusion was induced at the vertically displaced positions. It is suggested that the results obtained at the positions displaced from the fovea may be attributable to the refracting surfaces of the cornea, and that these findings lend support to an account of the Poggendorff illusion which emphasizes the significant involvement of peripheral mechanisms.     

The Hermann grid illusion revisited

The Hermann grid illusion consists of smudges perceived at the intersections of a white grid presented on a black background. In 1960 the effect was first explained by a theory advanced by Baumgartner suggesting the illusory effect is due to differences in the discharge characteristics of retinal ganglion cells when their receptive fields fall along the intersections versus when they fall along non-intersecting regions of the grid. Since then, others have claimed that this theory might not be adequate, suggesting that a model based on cortical mechanisms is necessary [Lingelbach et al, 1985 Perception 14(1) A7; Spillmann, 1994 Perception 23 691 708; Geier et al, 2004 Perception 33 Supplement, 53; Westheimer, 2004 Vision Research 44 2457 2465]. We present in this paper the following evidence to show that the retinal ganglion cell theory is untenable: (i) varying the makeup of the grid in a manner that does not materially affect the putative differential responses of the ganglion cells can reduce or eliminate the illusory effect; (ii) varying the grid such as to affect the putative differential responses of the ganglion cells does not eliminate the illusory effect; and (iii) the actual spatial layout of the retinal ganglion cell receptive fields is other than that assumed by the theory. To account for the Hermann grid illusion we propose an alternative theory according to which the illusory effect is brought about by the manner in which S1 type simple cells (as defined by Schiller et al, 1976 Journal of Neurophysiology 39 1320-1333) in primary visual cortex respond to the grid. This theory adequately handles many of the facts delineated in this paper.     

Gender Differences in Illusion Response: The Influence of Spatial Strategy and Sex Ratio

Two experiments explored factors related to gender differences in Ponzo illusion susceptibility. In Experiment 1, 54 male and 54 female (predominantly white, middle class) undergraduates were administered Witkin's Embedded Figures Test (EFT) and, on 2 separate occasions, a form of the Ponzo illusion. Results showed the Ponzo to be quite reliable over several days. Females were significantly more field dependent (as shown by slower responses to the EFT), and significantly more susceptible to the Ponzo illusion, than males. Furthermore, EFT performance correlated significantly with Ponzo susceptibility for females, but not for males, suggesting that the difference between males and females in Ponzo response may be due not to differences in field independence per se, but rather to differences in the strategies used to solve the illusion task. In Experiment 2, 111 male and 148 female (predominantly white, middle class) undergraduates were administered the Ponzo illusion twice, the 2 administrations separated by about 90 min. Again, the illusion task showed good reliability, and females were significantly more susceptible to the illusion. Furthermore, the magnitude of the difference between males and females was systematically related to the sex ratio (the ratio of the number of males to the number of females) of the particular session in which each subject happened to be participating. It is suggested that social factors such as sex ratio might affect the strategies participants use when doing illusion tasks, and perhaps other spatial skills tasks as well.     

内源性空间注意对声音诱发闪光错觉的影响

采用经典的声音诱发闪光错觉范式,通过操纵集中和分散的空间注意的方式,考察内源性空间注意和刺激出现视野位置的交互作用对多感觉整合中听觉主导效应的影响。结果发现：(1)当空间注意处于分散状态时,下视野的裂变错觉量显著大于上视野,而集中条件下则没有差异。(2)闪光出现的位置是否随机不会影响裂变错觉。研究说明了声音诱发闪光错觉中的裂变错觉只会受到内源性空间注意和视野位置交互作用的影响。     

Ontogeny of the Ponzo illusion: Effects of age,schooling, and environment(1)

Subjects were 384 Moroccan males (age range 6–22 yrs.), divided into 16 equal groups, according to the factorial design: age (4) × schooling (2) × environment (2). Subjects were tested on four Ponzo configurations (differing in contextual information) from Leibowitz et al. (1969), the Ponzo perspective stimulus from Segall et al. (1966), the CEFT from Witkin et al. (1971), and a measure of pictorial depth perception. Individual measures of contact with mass-media and urban life were collected on each subject. Analyses indicated that all main factors of age, schooling, and environment played important, and differing, roles in inducing illusion susceptibility. Piaget's (1969) theory of primary and secondary illusions was found useful in understanding the results of the Ponzo configurations used in the study. Primary illusion configurations were found to be relatively insensitive to experiential variables, and illusion susceptibility decreased with chronological age. In contrast, secondary illusion configurations were affected by many experiential factors, and illusion susceptibility was mediated through perceptual development and pictorial depth perception rather than chronological age. It was concluded that single-factor theories of ontogenetic change in illusion susceptibility were inadequate to explain the complex interactions found in this study.     

Motion induction from biological motion

A new type of motion illusion is described in which ambiguous motion becomes unidirectional on superimposition of a human figure walking on a treadmill. A point-light walker in profile was superimposed on a vertical counterphase grating backdrop. Eleven na?ve observers judged the apparent direction of motion against the grating as left or right in a two-alternative forced-choice task and found that the grating appeared to drift in a direction opposite to the walking. The illusion disappeared when the point lights moved in scrambled configurations. This indicates that the illusion is caused by biological motion that provides recognition of gaits. A human figure walking backwards produced no illusion because of the difficulty in identifying the gait. This indicates that the illusion is determined by translational motion rather than form represented from biological motion.     

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.     

Wagon-wheel illusion under steady illumination: real or illusory?

Wheels turning in the movies sometimes appear to rotate backwards. This is called the wagon-wheel illusion (WWI). The mechanism of this illusion is based on the intermittent nature of light in films and other stroboscopic presentations, which renders them as a series of snapshots rather than a continuous visual data stream. However, there have been claims that this illusion is seen even in continuous light, which would suggest that the visual system itself may sample a continuous visual data stream. We examined the rate of this putative sampling and its variations across individuals while in different psychological states. We obtained two results: (i) WWI occurred in stroboscopic lights as expected, (ii) WWI was never reported by our subjects under continuous lights, such as sunlight and lamps with DC power source. Thus, WWI cannot be taken as evidence for discreteness of conscious visual perception.