Size Illusion Research Articles

Viewpoint Invariance of Eye Size Illusion Caused by Eyeshadow.

Previous research found that application of eyeshadow on the upper eyelids induces overestimation of eye size. The present study examined whether or not this eyeshadow illusion is dependent on viewpoint. We created a three-dimensional model of a female face and manipulated the presence/absence of eyeshadow and face orientation around the axis of yaw (Experiment 1) or pitch (Experiment 2) rotation. Using the staircase method, we measured perceived eye size for each face stimulus. Results showed that the eyeshadow illusion occurred regardless of face orientation around axes of both yaw and pitch rotations. Crucially, the illusion’s magnitude did not vary across face orientations; lack of interaction between the illusion’s magnitude and face orientation was confirmed by small values of Bayes factors. These findings ruled out the hypothesis that eyeshadow serves as a depth cue and leads to overestimation of eye size due to size-distance scaling. Alternatively, the present findings suggest that the eyeshadow illusion can be well explained by the assimilation between the eyes and eyeshadow, which also facilitates assimilation between the eyes and eyebrows. Practical implications and the present findings’ generalizability are also discussed.

The moon size illusion does not improve perceptual judgments.

Recent studies suggest that the accuracy of perceptual judgments can be influenced by the perceived illusory size of a stimulus, with judgments being more accurate for increased illusory size. This phenomenon seems consistent with recent neuroscientific findings that representations in early visual areas reflect the perceived (illusory) size of stimuli rather than the physical size. We further explored this idea with the moon illusion, in which the moon appears larger when it is close to the horizon and smaller when it is higher in the sky. Participants (n=230) adjusted the orientation of an image of the moon on a smartphone to match the perceived orientation of the moon in the sky. Contrary to previous studies that investigated accuracy and size illusions, we found slightly lower perceptual judgment accuracy when the moon appeared large (close to the horizon) compared to when it appeared small (high in the sky).

Some Illusions Are More Inconsistent Than Others.

Illusions are characterized by inconsistencies. For instance, in the motion aftereffect, we see motion without an equivalent change in position. We used a simple pencil-and-paper experiment to determine whether illusions that influence an object’s apparent size give rise to equivalent changes in apparent positions along the object’s outline. We found different results for two equally strong size illusions. The Ebbinghaus illusion affected perceived positions in a way that was consistent with its influence on perceived size, but a modified diagonal illusion did not affect perceived positions. This difference between the illusions might explain why there are so many conflicting reports about the effects of size illusions on the maximum grip aperture during reach-to-grasp movements.

Asymmetry of pictorial space: A cultural phenomenon.

Art experts have argued that the mirror reversal of pictorial artworks produces an alteration of their spatial content. However, this putative asymmetry of the pictorial space remains to be empirically proved and causally explained. Here, we address these issues with the "corridor illusion," a size illusion triggered by the pictorial space of a receding corridor. We show that mirror-reversed corridors-receding respectively leftward and rightward-induce markedly different illusion strengths and thus convey distinct pictorial spaces. Remarkably, the illusion is stronger with the rightward corridor among native left-to-right readers (French participants, n = 40 males) but conversely stronger with the leftward corridor among native right-to-left readers (Syrian participants, n = 40 males). Together, these results demonstrate an asymmetry of the pictorial space and point to our reading/writing habits as a major cause of this phenomenon.

Guppies, Poecilia reticulata, perceive a reversed Delboeuf illusion.

Animals are often required to estimate object sizes during several fitness-related activities, such as choosing mates, foraging, and competing for resources. Some species are susceptible to size illusions, i.e. the misperception of the size of an object based on the surrounding context, but other species are not. This interspecific variation might be adaptive, reflecting species-specific selective pressures; according to this hypothesis, it is important to test species in which size discrimination has a notable ecological relevance. We tested susceptibility to a size illusion in the guppy, Poecilia reticulata, a fish species required to accurately estimate sizes during mate choice, foraging, and antipredator behaviours. We focussed on the Delboeuf illusion, in which an object is typically perceived to be larger when surrounded by a smaller object. In experiment 1, we trained guppies to select the larger of two circles to obtain a food reward and then tested them using stimuli arranged in a Delboeuf-like pattern. In experiment 2, we tested guppies in a spontaneous food choice task to determine whether the subjective size perception of food items is affected by the surrounding context. Jointly, our experiments indicated that guppies perceived the Delboeuf illusion, but in a reverse direction relative to humans: guppies estimated as larger the stimulus that human perceived as smaller. Our results indicated susceptibility to size illusions also in a species required to perform accurate size discrimination and support previous evidence of variability in illusion susceptibility across vertebrates.

Inversion produces opposite size illusions for faces and bodies

Faces are complex, multidimensional, and meaningful visual stimuli. Recently, Araragi, Aotani, & Kitaoka (2012) demonstrated an intriguing face size illusion whereby an inverted face is perceived as larger than a physically identical upright face. Like the face, the human body is a highly familiar and important stimulus in our lives. Here, we investigated the specificity of the size underestimation of upright faces illusion, testing whether similar effects also hold for bodies, hands, and everyday objects. Experiments 1a and 1b replicated the face-size illusion. No size illusion was observed for hands or objects. Unexpectedly, a reverse size illusion was observed for bodies, so that upright bodies were perceived as larger than their inverted counterparts. Experiment 2 showed that the face illusion was maintained even when the photographic contrast polarity of the stimuli was reversed, indicating that the visual system driving the illusion relies on geometric featural information rather than image contrast. In Experiment 2, the reverse size illusion for bodies failed to reach significance. Our findings show that size illusions caused by inversion show a high level of category specificity, with opposite illusions for faces and bodies.

Effects of Color and Luminance Contrast on Size Perception-Evidence from a Horizontal Parallel Lines Illusion.

The present study investigated a size illusion composed of two horizontal lines that were vertically separated and parallel to each other. When the two lines were of equal length, the upper line was consistently perceived to be a little longer than the lower line, therefore it was termed as horizontal parallel lines (HPL) illusion. We investigated the effect of color and luminance contrast on the HPL illusion by manipulating the color and luminance of the two lines. Results indicated the following: (1) differences in color between the two lines reduced the illusion; (2) differences in luminance between the two lines reduced the illusion; (3) Effect 1 was greater than Effect 2; (4) the illusory effect could not be affected as long as both of the lines were of the same color or luminance. The results suggest that the color or luminance contrast may contribute to the overall decrease in the illusory effect for lines with different colors/luminances, but generally the illusion decreases as the two lines are less similar to each other. These findings indicate that the similarity or ‘sameness’ effect dominates the effect of color/luminance contrast on the size illusion over the effect resulted from contrast difference or depth perception.

Implicit and explicit changes in body satisfaction evoked by body size illusions: Implications for eating disorder vulnerability in women.

Dissatisfaction with one’s body is a widespread issue in modern society and has been linked to vulnerability for developing eating disorders. Recent studies have demonstrated a direct relationship between body perception and body satisfaction by manipulating perceived body size using multisensory body illusions. However, how these body size illusions influence implicit affective experience has not previously been examined. The current experiment used an established full-body ownership illusion paradigm to induce feelings of illusory obesity in male and female participants. The effects of illusory obesity on explicit and implicit body satisfaction were measured in naïve participants across two separate experiments. In terms of explicit measures, owning an obese body decreased body satisfaction, and owning a slimmer body increased body satisfaction in females but not in males. However, implicit feelings regarding the body were only influenced by the synchrony of the touch and not the size of the body in the illusion. These results suggest that implicit and explicit affective experiences of the body may be mediated by different factors. In addition, these findings may have clinical implications because both implicit and explicit changes in affective experience of the body were related to behaviours and thoughts associated with disordered eating in a non-clinical sample.

Developmental trends in susceptibility to perceptual illusions: Not all illusions are created equal.

This study examined the development of the utilization of contextual information in visuospatial integration during childhood. We examined four contextual size illusions in children and adults asking whether young children's sensitivity to context is reduced or varies with the perceptual mechanisms or the levels of integration involved. We tested susceptibility to contextual illusions in four-year-olds, seven-year-olds, and adults, employing two psychophysical paradigms, perceptual estimation and a 2AFC discrimination task. We tested susceptibility to Ebbinghaus and Ponzo illusions to estimate the effect of the interaction of object size with its contextual background on the rescaling of its perceived size; we also tested susceptibility to the rectangle and 3D-cube illusions to estimate the effect of the interaction of two dimensions of the target object on the rescaling of its perceived size. While four-year-olds were affected by the Ebbinghaus and Ponzo illusions, they showed no susceptibility to the rectangle or 3D-cube illusion. The results show that, overall, sensitivity to context is not reduced in early childhood; rather, it varies with the perceptual mechanisms or the levels of integration involved. In particular, development is protracted for size illusions in which contextual effects entail the extraction of the relations of dimensions within an object.

Tool use produces a size illusion revealing action-specific perceptual mechanisms

In four experiments, participants estimated the sizes of target objects that were either out of reach, or that could be reached by a tool (a stylus or laser pointer). Objects reachable with the aid of a tool were perceived to be smaller than identical objects without a tool. Participants' responses to questioning rule out demand characteristics as an explanation. This new size illusion may reflect a direct impact of tool use on perceived size, or it may stem from the effects of tool use on perceived distance. Both possibilities support action specific accounts of perception.

Perception of contextual size illusions by honeybees in restricted and unrestricted viewing conditions.

How different visual systems process images and make perceptual errors can inform us about cognitive and visual processes. One of the strongest geometric errors in perception is a misperception of size depending on the size of surrounding objects, known as the Ebbinghaus or Titchener illusion. The ability to perceive the Ebbinghaus illusion appears to vary dramatically among vertebrate species, and even populations, but this may depend on whether the viewing distance is restricted. We tested whether honeybees perceive contextual size illusions, and whether errors in perception of size differed under restricted and unrestricted viewing conditions. When the viewing distance was unrestricted, there was an effect of context on size perception and thus, similar to humans, honeybees perceived contrast size illusions. However, when the viewing distance was restricted, bees were able to judge absolute size accurately and did not succumb to visual illusions, despite differing contextual information. Our results show that accurate size perception depends on viewing conditions, and thus may explain the wide variation in previously reported findings across species. These results provide insight into the evolution of visual mechanisms across vertebrate and invertebrate taxa, and suggest convergent evolution of a visual processing solution.

Human Occipital and Parietal GABA Selectively Influence Visual Perception of Orientation and Size.

GABA is the primary inhibitory neurotransmitter in human brain. The level of GABA varies substantially across individuals, and this variability is associated with interindividual differences in visual perception. However, it remains unclear whether the association between GABA level and visual perception reflects a general influence of visual inhibition or whether the GABA levels of different cortical regions selectively influence perception of different visual features. To address this, we studied how the GABA levels of parietal and occipital cortices related to interindividual differences in size, orientation, and brightness perception. We used visual contextual illusion as a perceptual assay since the illusion dissociates perceptual content from stimulus content and the magnitude of the illusion reflects the effect of visual inhibition. Across individuals, we observed selective correlations between the level of GABA and the magnitude of contextual illusion. Specifically, parietal GABA level correlated with size illusion magnitude but not with orientation or brightness illusion magnitude; in contrast, occipital GABA level correlated with orientation illusion magnitude but not with size or brightness illusion magnitude. Our findings reveal a region- and feature-dependent influence of GABA level on human visual perception. Parietal and occipital cortices contain, respectively, topographic maps of size and orientation preference in which neural responses to stimulus sizes and stimulus orientations are modulated by intraregional lateral connections. We propose that these lateral connections may underlie the selective influence of GABA on visual perception.SIGNIFICANCE STATEMENT GABA, the primary inhibitory neurotransmitter in human visual system, varies substantially across individuals. This interindividual variability in GABA level is linked to interindividual differences in many aspects of visual perception. However, the widespread influence of GABA raises the question of whether interindividual variability in GABA reflects an overall variability in visual inhibition and has a general influence on visual perception or whether the GABA levels of different cortical regions have selective influence on perception of different visual features. Here we report a region- and feature-dependent influence of GABA level on human visual perception. Our findings suggest that GABA level of a cortical region selectively influences perception of visual features that are topographically mapped in this region through intraregional lateral connections.

Speed-size illusion correlates with retinal-level motion statistics.

It is a common perceptual experience that smaller objects appear to move faster than larger ones when their physical speeds are the same in either the laboratory or daily life. In this study, we show that the speed-size illusion is correlated with retinal image speed distribution bias. The illusion was quantified with a two-alternative, forced choice speed comparison paradigm, and retinal image speed distributions for different image sizes were obtained by simulation. Simulation results show that smaller retinal images tend to have slower projected speed, and the retinal image speed distribution bias correlates with the strength of the speed-size illusion. Furthermore, exposure to a training movie containing unnatural motion statistics tended to modulate the illusion in a way that was consistent with the speed distribution bias. We discuss how the data could be explained by empirical ranking theory, Bayesian theory, and motion adaptation.

Ebbinghaus figures that deceive the eye do not necessarily deceive the hand

In support of the visual stream dissociation hypothesis, which states that distinct visual streams serve vision-for-perception and vision-for-action, visual size illusions were reported over 20 years ago to ‘deceive the eye but not the hand’. Ever since, inconclusive results and contradictory interpretations have accumulated. Therefore, we investigated the effects of the Ebbinghaus figure on repetitive aiming movements with distinct dynamics. Participants performed a Fitts’ task in which Ebbinghaus figures served as targets. We systematically varied the three parameters which have been shown to influence the perceived size of the Ebbinghaus figure’s target circle, namely the size of the target, its distance to the context circles and the size of the context circles. This paper shows that movement is significantly affected by the context size, but, in contrast to perception, not by the other two parameters. This is especially prominent in the approach phase of the movement towards the target, regardless of the dynamics. To reconcile the findings, we argue that different informational variables are used for size perception and the visual control of movements irrespective of whether certain variables induce (perceptual) illusions.

Adaptation effects in grasping the Müller-Lyer illusion

Recent results have shown that effects of pictorial illusions in grasping may decrease over the course of an experiment. This can be explained as an effect of sensorimotor learning if we consider a pictorial size illusion as simply a perturbation of visually perceived size. However, some studies have reported very constant illusion effects over trials. In the present paper, we apply an error-correction model of adaptation to experimental data of N=40 participants grasping the Müller-Lyer illusion. Specifically, participants grasped targets embedded in incremental and decremental Müller-Lyer illusion displays in (1) the same block in pseudo-randomised order, and (2) separate blocks of only one type of illusion each. Consistent with predictions of our model, we found an effect of interference between the two types when they were presented intermixed, explaining why adaptation rates may vary depending on the experimental design. We also systematically varied the number of object sizes per block, which turned out to have no effect on the rate of adaptation. This was also in accordance with our model. We discuss implications for the illusion literature, and lay out how error-correction models can explain perception-action dissociations in some, but not all grasping-of-illusion paradigms in a parsimonious and plausible way, without assuming different illusion effects.

The Silhouette Zoetrope: A New Blend of Motion, Mirroring, Depth, and Size Illusions.

Here, we report a novel combination of visual illusions in one stimulus device, a contemporary innovation of the traditional zoetrope, called Silhouette Zoetrope. In this new device, an animation of moving silhouettes is created by sequential cutouts placed outside a rotating empty cylinder, with slits illuminating the cutouts successively from the back. This “inside-out” zoetrope incurs the following visual effects: the resulting animated figures are perceived (a) horizontally flipped, (b) inside the cylinder, and (c) appear to be of different size than the actual cutout object. Here, we explore the unique combination of illusions in this new device. We demonstrate how the geometry of the device leads to a retinal image consistent with a mirrored and distorted image and binocular disparities consistent with the perception of an object inside the cylinder.

Examination of factors affecting the intraoral perception of object size: a preliminary study.

Oral sensory function is essential for the successful performance of a range of ingestion. Although the perception of object size is important in determining the degree of manipulation, evidence suggests that people does not always perceive the size of the object in oral cavity accurately. The purpose of this study was to investigate the factors affecting the intraoral perception of object size. Twenty-three healthy young volunteers detected the size of cylinders inserted into oral cavity blindly and identified it from the reference set. The diameter of cross section varied from 10 to 14 mm in a gap of 1 mm and had three different temperatures (5, 36, 50 °C) in each. The perceived size was recorded, and the difference with the actual size was assessed. The required time for identification was also measured and compared between gender and between temperatures. The results demonstrated that the direction of size illusion was significantly affected by the required time for identification. Long manipulation led to overestimation, and short manipulation led to underestimation of object size irrespective of temperature and size. Gender was the other factor affecting intraoral size perception. The rate of overestimation was low in female participants comparing with male participants in this experiment, although the number of participants was limited. We therefore concluded that in order to detect the other factors affecting intraoral size perception, regulating oral manipulation time is indispensable.

Robust size illusion produced by expanding and contracting flow fields

A new illusion is described. Randomly positioned dots moved radially within an imaginary annular window. The dots’ motion periodically changed the direction, leading to an alternating percept of expanding and contracting motion. Strikingly, the apparent size of the enclosed circular region shrank during the dots’ expanding phases and dilated during the contracting phases. We quantitatively measured the illusion, and found that the presence of energy at the local kinetic edge could not account for the illusion. Besides, we reproduced the illusion on a natural scene background seen from a first-person point of view that moved forward and backward periodically. Blurring the boundaries of motion areas could not reverse the illusion in all subjects. Taken together, our observed illusion is likely induced by optic flow processing with some components of motion contrast. Expanding or contracting dots may induce the self-motion perception of either approaching or leaving way from the circle. These will make the circle appear smaller or larger since its retinal size remains constant.

Biological motion distorts size perception

Visual illusions explore the limits of sensory processing and provide an ideal testbed to study perception. Size illusions – stimuli whose size is consistently misperceived – do not only result from sensory cues, but can also be induced by cognitive factors, such as social status. Here we investigate, whether the ecological relevance of biological motion can also distort perceived size. We asked observers to judge the size of point-light walkers (PLWs), configurations of dots whose movements induce the perception of human movement, and visually matched control stimuli (inverted PLWs). We find that upright PLWs are consistently judged as larger than inverted PLWs, while static point-light figures do not elicit the same effect. We also show the phenomenon using an indirect paradigm: observers judged the relative size of a disc that followed an inverted PLW larger than a disc following an upright PLW. We interpret this as a contrast effect: The upright PLW is perceived larger and thus the subsequent disc is judged smaller. Together, these results demonstrate that ecologically relevant biological-motion stimuli are perceived larger than visually matched control stimuli. Our findings present a novel case of illusory size perception, where ecological importance leads to a distorted perception of size.

Eggs illusion: Local shape deformation generated by a grid pattern.

In this study, we report a new visual shape illusion, the eggs illusion, in which circular disks located at the midpoints between adjacent grid intersections are perceived as being deformed to ellipses. In Experiment 1, we examined the eggs illusion by using a matching method and found that grid luminance and patch size play a critical role in producing the illusory deformation. In Experiment 2, we employed several types of elliptic or circular patches to examine the conditions in which the illusory deformation was cancelled or weakened. We observed that the illusory deformation was dependent on local grid orientation. Based on these results, we found several common features between the eggs illusion and the scintillating grid illusion. This resemblance suggests a possibility that similar mechanisms underlie the two phenomena. In addition to the scintillating grid illusion, we also considered several known perceptual phenomena that might be related to the eggs illusion, i.e., the apparent size illusion, the shape-contrast effect, and the Orbison illusion. Finally, we discuss the role of orientation processing in generating the eggs illusion.