Pictorial Illusions Research Articles

Coming to grips with reality: Real grasps, but not pantomimed grasps, resist a simultaneous tilt illusion

Investigations of grasping real, 3D objects subjected to illusory effects from a pictorial background often choose in-flight grasp aperture as the primary variable to test the hypothesis that the visuomotor system resists the illusion. Here we test an equally important feature of grasps that has received less attention: in-flight grasp orientation. The current study tested a variant of the simultaneous tilt illusion using a mirror-apparatus to manipulate the availability of haptic feedback. Participants performed grasps with haptic feedback (real grasps) and without it (pantomime grasps), reaching for the reflection of a real, 3D bar atop a background grating that induced a 1.1° bias in the perceived orientation of the bar in a separate sample of participants. Analysis of the hand's in-flight grasp orientation at early, late, and end stages of the reach showed that at no point were the real grasps biased by the illusion. In contrast, pantomimed grasps were affected by the illusion at the late and end stages of the reach. At each stage, the effect on the real grasps was significantly weaker than the effect of the illusion as measured by the mean point of subjective equality (PSE) in a two-alternative forced-choice task. In contrast, the effect on the pantomime grasps was statistically indistinguishable from the mean PSE at all three stages of the reach. These findings reinforce the idea that in-flight grasp orientation, like grasp aperture to pictorial illusions of target size, is refractory to pictorial backgrounds that bias perceived orientation.

Sculpture and representation: apprehending marble portrait sculpture in the eighteenth century

Beginning with a question about how the spectator apprehends a figure or bust in marble as a representation, this article uses eighteenth-century portrait busts in marble to explore how competing theories about representation and the apprehension of art might be applied to a distinctive class of sculpture in a particular material from a specific period. It explores how the terms of a debate formulated by E. H. Gombrich and Richard Wollheim about pictorial illusion might be applied to our perception of sculpture, and goes on to examine the ways in which contemporary accounts of viewing eighteenth-century portrait sculpture might be understood within this context.

Changing the Real Viewing Distance Reveals the Temporal Evolution of Size Constancy in Visual Cortex

Our visual system provides a distance-invariant percept of object size by integrating retinal image size with viewing distance (size constancy). Single-unit studies with animals have shown that some distance cues, especially oculomotor cues such as vergence and accommodation, can modulate the signals in the thalamus or V1 at the initial processing stage [1-7]. Accordingly, one might predict that size constancy emerges much earlier in time [8-10], even as visual signals are being processed in the thalamus. So far, the studies that have looked directly at size coding have either used fMRI (poor temporal resolution [11-13]) or relied on inadequate stimuli (pictorial illusions presented on a monitor at a fixed distance [11, 12, 14, 15]). Here, we physically moved the monitor to different distances, a more ecologically valid paradigm that emulates what happens in everyday life and is an example of the increasing trend of "bringing the real world into the lab." Using this paradigm in combination with electroencephalography (EEG), we examined the computation of size constancy in real time with real-world viewing conditions. Our study provides strong evidence that, even though oculomotor distance cues have been shown to modulate the spiking rate of neurons in the thalamus and in V1, the integration of viewing distance cues and retinal image size takes at least 150ms to unfold, which suggests that the size-constancy-related activation patterns in V1 reported in previous fMRI studies (e.g., [12, 13]) reflect the later processing within V1 and/or top-down input from other high-level visual areas.

The Sander parallelogram illusion dissociates action and perception despite control for the litany of past confounds

The two visual systems hypothesis proposes that human vision is supported by an occipito-temporal network for the conscious visual perception of the world and a fronto-parietal network for visually-guided, object-directed actions. Two specific claims about the fronto-parietal network's role in sensorimotor control have generated much data and controversy: (1) the network relies primarily on the absolute metrics of target objects, which it rapidly transforms into effector-specific frames of reference to guide the fingers, hands, and limbs, and (2) the network is largely unaffected by scene-based information extracted by the occipito-temporal network for those same targets. These two claims lead to the counter-intuitive prediction that in-flight anticipatory configuration of the fingers during object-directed grasping will resist the influence of pictorial illusions. The research confirming this prediction has been criticized for confounding the difference between grasping and explicit estimates of object size with differences in attention, sensory feedback, obstacle avoidance, metric sensitivity, and priming. Here, we address and eliminate each of these confounds. We asked participants to reach out and pick up 3D target bars resting on a picture of the Sander Parallelogram illusion and to make explicit estimates of the length of those bars. Participants performed their grasps without visual feedback, and were permitted to grasp the targets after making their size-estimates to afford them an opportunity to reduce illusory error with haptic feedback. The results show unequivocally that the effect of the illusion is stronger on perceptual judgments than on grasping. Our findings from the normally-sighted population provide strong support for the proposal that human vision is comprised of functionally and anatomically dissociable systems.

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.

Varieties of Pictorial Illusion

This article focuses on a potentially perplexing aspect of our interactions with pictorial representations (including film, paintings, pictures, drawings, photographs, even video games): in some cases, it seems that visual representations can play tricks on our cognitive faculties. We may either come to believe that objects represented in pictures are real or perhaps perceive them as such. The possibility of widespread pictorial illusions has been oft discussed, and discarded, in the aesthetics literature. I support this stance. However, the nature of the illusion is more complicated than is usually considered. I argue that there are five different types of potential illusions and present reasons for rejecting each. I also explore in detail the most persistent illusion: the “object recognition perceptual illusion thesis,” which states that we undergo a perceptual illusion while viewing pictorial representations simply in virtue of seeing objects in the representation. I contend that a rejection of this thesis depends on the nature of perceptual content, an issue with far-reaching consequences in aesthetics.

Rapid decrement in the effects of the Ponzo display dissociates action and perception.

It has been demonstrated that pictorial illusions have a smaller influence on grasping than they do on perceptual judgments. Yet to date this work has not considered the reduced influence of an illusion as it is measured repeatedly. Here we studied this decrement in the context of a Ponzo illusion to further characterize the dissociation between vision for perception and for action. Participants first manually estimated the lengths of single targets in a Ponzo display with their thumb and index finger, then actually grasped these targets in another series of trials, and then manually estimated the target lengths again in a final set of trials. The results showed that although the perceptual estimates and grasp apertures were equally sensitive to real differences in target length on the initial trials, only the perceptual estimates remained biased by the illusion over repeated measurements. In contrast, the illusion's effect on the grasps decreased rapidly, vanishing entirely after only a few trials. Interestingly, a closer examination of the grasp data revealed that this initial effect was driven largely by undersizing the grip aperture for the display configuration in which the target was positioned between the diverging background lines (i.e., when the targets appeared to be shorter than they really were). This asymmetry between grasping apparently shorter and longer targets suggests that the sensorimotor system may initially treat the edges of the configuration as obstacles to be avoided. This finding highlights the sensorimotor system's ability to rapidly update motor programs through error feedback, manifesting as an immunity to the effects of illusion displays even after only a few trials.

How preparation to touch or grasp alters visual size perception.

Prior studies have suggested that visually guided actions are resistant to the effects of some pictorial size illusions, e.g., the maximum grip aperture component of a grasp for an element of the Ebbinghaus illusion display. We present evidence that when participants prepare to grasp, the reduction in illusion magnitude observed for action components is also present for conscious perceptual judgments. Our studies characterize how visual size perception changes when we choose to engage in different size-mediated behaviors. Even when the stimuli used were identical for two different tasks, we found that available information was processed differently. In the studies, participants always selected which of the two targets was larger. In some conditions, the context in which the targets were presented induced a visual illusion of size. We varied the sizes of target pairs to assess the magnitude of these visual illusions. In some tasks, participants indicated their size choice verbally. For other tasks, participants reached to grasp or touch the target that they perceived as larger. Illusion magnitudes were smaller when participants engaged in actions directed at a target or when participants imagined performing those actions. This shift in visual processing persisted for several minutes after participants switched back to a verbal, non-grasping, non-touch task. A motor interference task eliminated the reduction in illusion magnitude.

Citizen Spectator: Art, Illusion, and Visual Perception in Early National America

Citizen Spectator: Art, Illusion, and Visual Perception in Early National America

Saccades and the Müller-Lyer illusion: implications for the two-visual-systems hypothesis

Background and aim: The cortical visual system is divided anatomically into the dorsal and ventral stream. Goodale and Milner in 1992 proposed that this division was functional: the dorsal stream processes ‘vision-for-action’ whereas the ventral stream processes ‘vision-for-perception’. This model of vision is known as the ‘two-visual-systems hypothesis’ (TVSH). Pictorial illusions, such as the Mu¨ ller-Lyer (ML) illusion, have been used by advocates of the TVSH to demonstrate dissociation between visionfor-action and vision-for-perception in healthy subjects. However, this literature is controversial, and not least in experiments investigating the magnitude of the illusion effect on saccades. This review aims to synthesise and critically evaluate evidence from saccade experiments on the effect of the ML illusion, in order to determine whether it provides support for the TVSH. Methods: Literature searches were performed using the Web of Science, Google Scholar and PubMed. Text books were referred to for additional information. Data were extracted from relevant papers and collated using Microsoft Excel. Statistical analysis was performed using SPSS. Results: Analysis revealed that saccades are affected by the ML illusion. The magnitude of the illusion effect on saccades is comparable to its effect on perception. Reflexive saccades are affected by the illusion, yet they receive little input from the ventral stream. This is inconsistent with the TVSH as it demonstrates that a dorsally controlled action is affected by a perceptual illusion. Conclusion: Evidence from experiments on the effect of the ML illusion on saccades provides no general support for the TVSH.

Visual Illusions and the Control of Ball Placement in Goal-Directed Hitting

When hitting, kicking, or throwing balls at targets, online control in the target area is impossible. We assumed this lack of late corrections in the target area would induce an effect of a single-winged Müller-Lyer illusion on ball placement. After extensive practice in hitting balls to different landing locations, participants (N = 9) had to hit a ball to a distant target specified by the vertex of a single-winged Müller-Lyer configuration. Impact velocity was not significantly “tricked™ by the pictorial illusion, suggesting that, even when late corrections in the target area are absent, some motor behaviors are not susceptible to the influence of the visual environment surrounding the target.

Visual Illusions and the Control of Ball Placement in Goal-Directed Hitting

When hitting, kicking, or throwing balls at targets, online control in the target area is impossible. We assumed this lack of late corrections in the target area would induce an effect of a single-winged Müller-Lyer illusion on ball placement. After extensive practice in hitting balls to different landing locations, participants (N = 9) had to hit a ball to a distant target specified by the vertex of a single-winged Müller-Lyer configuration. Impact velocity was not significantly “tricked™ by the pictorial illusion, suggesting that, even when late corrections in the target area are absent, some motor behaviors are not susceptible to the influence of the visual environment surrounding the target.

Abrupt stimulus motion eliminates task-specific immunity to pictorial illusions

Abrupt stimulus motion eliminates task-specific immunity to pictorial illusions

To Kirsti: ‘from Art History with Love…’ Perspective: Then and Now, and Artistic Practice

Perspective, (a central projection) is a method to convey space in the pictorial arts as well as in many other areas, where one wants to use images to convey information. It was invented/developed in the 15th century in an artistic ambience in Firenze, and it still plays an important role in art in the 21st century. Therefore it is of interest for art historians to follow both how perspective developed over the centuries in mathematics and to analyze how artists interpreted the basic mathematical construction in order to obtain pictorial illusion. I focus on two examples: One from around 1600 (Caravaggio), and another from around 1900 (Duchamp) in order to show how these artists adapted the basic mathematical construction to the motif of an image.

Division of labour within the visual system: fact or fiction? Which kind of evidence is appropriate to clarify this debate?

The perception versus action hypothesis of Goodale and Milner (Trends Neurosci 15:20-25, 1992) and Milner and Goodale (The visual brain in action. Oxford University Press, Oxford, 1995) postulated two different pathways within the visual system--one for action and one for perception. With the help of pictorial illusions, evidence for this dissociation was found in various studies. There is an ongoing debate, however, as to whether or not this evidence is biased by methodological issues. Indeed, relevant and decisive data can come only from those studies that (1) match conditions appropriately with respect to task demands, (2) use illusions that do not provide any potential obstacles for the hand, (3) do not risk that grasping is either memory driven (when the target is not visible) or online corrected (due to a direct comparison of the grip aperture with the size of the target object), (4) do not confound differences between perception and action conditions with differences in visual feedback, and (5) correct for differences in response functions between grasping and perception. In following all these points outlined above we found support for the perception versus action hypothesis: grip aperture follows actual size independent of illusory effects, while perceived length as indicated by finger-thumb span clearly was subject to the illusion.

Is simple reaction time affected by visual illusions?

A number of studies have shown that while perceptual judgment is deceived by pictorial illusions, grasping and other kinds of motor behaviour are not. This is in keeping with the existence of two different cortical systems: a ventral stream subserving vision-for-perception and a dorsal stream subserving vision-for-action. The former is sensitive to illusions, the latter is not. Given this dissociation of functions, one wonders whether simple visuomotor reaction time (RT) follows the ventral or the dorsal rule in perceiving illusory figures. Answering this question might contribute to a better understanding of the different functions of the two systems. We carried out two experiments, one with the Ponzo and the other with the Ebbinghaus-Titchener illusion and found that RT is sensitive to both illusions with faster responses to stimuli appearing illusorily bigger than the others. These results show that motor action is subserved by the ventral system when that action directly reports the presence or onset of a target rather than when that action requires a spatial adjustment that reflects the physical features of the target.

Action without perception in human vision

In 1992, David Milner and I (Goodale & Milner, 1992) proposed a division of labour in the visual pathways of the primate cerebral cortex between a dorsal stream specialized for the visual control of action and a ventral stream dedicated to constructing our percepts of the visual world. Support for the perception–action distinction has come from neuroimaging experiments, human neuropsychology, and monkey neurophysiology. Differences in the timing and spatial metrics of vision-for-perception and vision-for-action have been studied in human psychophysical experiments, particularly in those that have looked at the way in which each system deals with pictorial illusions. Although the literature is not free from controversy, a large number of studies have found that actions such as grasping and reaching are often unaffected by high-level pictorial illusions, which by definition affect perception. Recent experiments have shown that for actions to escape the effects of such illusions, however, they must be highly practised actions, preferably with the right hand, and must be directed in real time at visible targets. But even though the behavioural evidence suggests that the dorsal and ventral streams make use of different timing, different metrics, and different frames of reference in carrying out their computations, there is a seamless interaction between the two streams in the production of adaptive behaviour. A full understanding of the integrated nature of visually guided behaviour will require that we specify the nature of the interactions and information exchange that occur between these two streams of visual processing.

Online corrections can produce illusory bias during closed-loop pointing

This experiment examined whether the impact of pictorial illusions during the execution of goal-directed reaching movements is attributable to ocular motor signaling. We analyzed eye and hand movements directed toward both the vertex of the Müller-Lyer (ML) figure in a closed-loop procedure. Participants pointed to the right vertex of a visual stimulus in two conditions: a control condition wherein the figure (in-ML, neutral, out-ML) presented at response planning remained unchanged throughout the movement, and an experimental condition wherein a neutral figure presented at response planning was perturbed to an illusory figure (in-ML, out-ML) at movement onset. Consistent with previous work from our group (Heath et al. in Exp Brain Res 158:378-384, 2004; Heath et al. in J Mot Behav 37:179-185, 2005b), action-bias present in both conditions; thus illusory bias was introduced into during online control. Although primary saccades were influenced by illusory configurations (control conditions; see Binsted and Elliott in Hum Mov Sci 18:103-117, 1999a), illusory bias developed within the secondary "corrective" saccades during experimental trials (i.e., following a veridical primary saccade). These results support the position that a unitary spatial representation underlies both action and perception and this representation is common to both the manual and oculomotor systems.

A Double Dissociation Between Action and Perception in the Context of Visual Illusions

The idea that there are two distinct cortical visual pathways, a dorsal action stream and a ventral perception stream, is supported by neuroimaging and neuropsychological evidence. Yet there is an ongoing debate as to whether or not the action system is resistant to pictorial illusions in healthy participants. In the present study, we disentangled the effects of real and illusory object size on action and perception by pitting real size against illusory size. In our task, two objects that differed slightly in length were placed within a version of the Ponzo illusion. Even though participants erroneously perceived the physically longer object as the shorter one (or vice versa), their grasping was remarkably tuned to the real size difference between the objects. These results provide the first demonstration of a double dissociation between action and perception in the context of visual illusions and together with previous findings converge on the idea that visually guided action and visual perception make use of different metrics and frames of reference.

Action Rules: Why the Visual Control of Reaching and Grasping is Not Always Influenced by Perceptual Illusions

It is generally accepted that vision first evolved for the distal control of movement and that perception or 'representational' vision emerged much later. Vision-for-action operates in real time and uses egocentric frames of reference and the real metrics of the world. Vision-for-perception can operate over longer time scales and is much more scene-based in its computations. These differences in the timing and metrics of the two systems have been examined in experiments that have looked at the way in which each system deals with visual illusions. Although controversial, the consensus is that actions such as grasping and reaching are often unaffected by high-level pictorial illusions, which by definition affect percetion. However, recent experiments have shown that, for actions to escape the effects of such illusions, they must be highly practiced actions, preferably with the right hand, and must be directed in real time at visible targets. This latter finding suggests that some of the critical components of the encapsulated (bottom-up) systems that mediate the visual control of skilled reaching and grasping movements are lateralised to the left hemisphere.