Independent effects of lighting, orientation, and stereopsis on the hollow-face illusion

The turn of the dice: Patrick Hughes' Hollow Dice and Reverspectives

Patrick Hughes' Reverspectives demonstrate the importance of perspective as a source of information about the structure and layout of the three-dimensional (3D) world. More recently, he has created a new work of art-Hollow Dice-in which the actual concave structure of the dice is seen as convex. In this article, we examine the similarities and differences between these two perceptual phenomena as well as attempting to explain how and why they arise. Popular interest in both effects is based on the fact that "what we perceive" does not correspond to "what the reality is." As a consequence, Reverspectives and Hollow Dice are often categorized and labeled as "illusions." However, if we consider the information that is available in patterns of light reaching our eyes-rather than the "actual" 3D structure of the Reverspectives and the Hollow Dice-we are in a better position to explain how the size, the viewing distance, the perspective features, the convexity bias, and observer movements determine what we see when viewing these novel and fascinating visual effects.

Minimal theory of 3D vision: new approach to visual scale and visual shape

Since Kepler and Descartes in the early-1600s, vision science has been committed to a triangulation model of stereo vision. But in the early-1800s, we realized that disparities are responsible for stereo vision. And we have spent the past 200 years trying to shoe-horn disparities back into the triangulation account. The first part of this article argues that this is a mistake, and that stereo vision is a solution to a different problem: the eradication of rivalry between the two retinal images, rather than the triangulation of objects in space. This leads to a 'minimal theory of 3D vision', where 3D vision is no longer tied to estimating the scale, shape, and direction of objects in the world. The second part of this article then asks whether the other aspects of 3D vision, which go beyond stereo vision, really operate at the same level of visual experience as stereo vision? I argue they do not. Whilst we want a theory of real-world 3D vision, the literature risks giving us a theory of picture perception instead. And I argue for a two-stage theory, where our purely internal 'minimal' 3D percept (from stereo vision) is linked to the world through cognition. This article is part of a discussion meeting issue 'New approaches to 3D vision'.

State anxiety influences P300 and P600 event-related potentials over parietal regions in the hollow-mask illusion experiment

The hollow-mask illusion is an optical illusion where a concave face is perceived as convex. It has been demonstrated that individuals with schizophrenia and anxiety are less susceptible to the illusion than controls. Previous research has shown that the P300 and P600 event-related potentials (ERPs) are affected in individuals with schizophrenia. Here, we examined whether individual differences in neuroticism and anxiety scores, traits that have been suggested to be risk factors for schizophrenia and anxiety disorders, affect ERPs of healthy participants while they view concave faces. Our results confirm that the participants were susceptible to the illusion, misperceiving concave faces as convex. We additionally demonstrate significant interactions of the concave condition with state anxiety in central and parietal electrodes for P300 and parietal areas for P600, but not with neuroticism and trait anxiety. The state anxiety interactions were driven by low-state anxiety participants showing lower amplitudes for concave faces compared to convex. The P300 and P600 amplitudes were smaller when a concave face activated a convex face memory representation, since the stimulus did not match the active representation. The opposite pattern was evident in high-state anxiety participants in regard to state anxiety interaction and the hollow-mask illusion, demonstrating larger P300 and P600 amplitudes to concave faces suggesting impaired late information processing in this group. This could be explained by impaired allocation of attentional resources in high-state anxiety leading to hyperarousal to concave faces that are unexpected mismatches to standard memory representations, as opposed to expected convex faces.

Correlations Between Hysteretic Categorical and Continuous Judgments of Perceptual Stimuli Supporting a Unified Dynamical Systems Approach to Perception

We report from two variants of a figure-ground experiment that is known in the literature to involve a bistable perceptual domain. The first variant was conducted as a two-alternative forced-choice experiment and in doing so tested participants on a categorical measurement scale. The second variant involved a Likert scale measure that was considered to represent a continuous measurement scale. The two variants were conducted as a single within-subjects experiment. Measures of bistability operationalized in terms of hysteresis size scores showed significant positive correlations across the two response conditions. The experimental findings are consistent with a dualistic interpretation of self-organizing perceptual systems when they are described on a macrolevel by means of so-called amplitude equations. This is explicitly demonstrated for a Lotka-Volterra-Haken amplitude equation model of task-related brain activity. As a by-product, the proposed dynamical systems perspective also sheds new light on the anchoring problem of producing numerical, continuous judgments.

No Evidence of Narrowly Defined Cognitive Penetrability in Unambiguous Vision

The classical notion of cognitive impenetrability suggests that perceptual processing is an automatic modular system and not under conscious control. Near consensus is now emerging that this classical notion is untenable. However, as recently pointed out by Firestone and Scholl, this consensus is built on quicksand. In most studies claiming perception is cognitively penetrable, it remains unclear which actual process has been affected (perception, memory, imagery, input selection or judgment). In fact, the only available "proofs" for cognitive penetrability are proxies for perception, such as behavioral responses and neural correlates. We suggest that one can interpret cognitive penetrability in two different ways, a broad sense and a narrow sense. In the broad sense, attention and memory are not considered as "just" pre- and post-perceptual systems but as part of the mechanisms by which top-down processes influence the actual percept. Although many studies have proven top-down influences in this broader sense, it is still debatable whether cognitive penetrability remains tenable in a narrow sense. The narrow sense states that cognitive penetrability only occurs when top-down factors are flexible and cause a clear illusion from a first person perspective. So far, there is no strong evidence from a first person perspective that visual illusions can indeed be driven by high-level flexible factors. One cannot be cognitively trained to see and unsee visual illusions. We argue that this lack of convincing proof for cognitive penetrability in the narrow sense can be explained by the fact that most research focuses on foveal vision only. This type of perception may be too unambiguous for transient high-level factors to control perception. Therefore, illusions in more ambiguous perception, such as peripheral vision, can offer a unique insight into the matter. They produce a clear subjective percept based on unclear, degraded visual input: the optimal basis to study narrowly defined cognitive penetrability.

Convexity Bias and Perspective Cues in the Reverse-Perspective Illusion

The present experiment was designed to examine the roles of painted linear perspective cues, and the convexity bias that are known to influence human observers’ perception of three-dimensional (3D) objects and scenes. Reverse-perspective stimuli were used to elicit a depth-inversion illusion, in which far points on the stimulus appear to be closer than near points and vice versa, with a 2 (Type of stimulus) × 2 (Fixation mark position) design. To study perspective, two types of stimuli were used: a version with painted linear perspective cues and a version with blank (unpainted) surfaces. To examine the role of convexity, two locations were used for the fixation mark: either in a locally convex or a locally concave part of each stimulus (painted and unpainted versions). Results indicated that the reverse-perspective illusion was stronger when the stimulus contained strong perspective cues and when observers fixated a locally concave region within the scene.

Experiments with a Hollow Mask and a Reverspective: Top-down Influences in the Inversion Effect for 3-D Stimuli

Earlier psychophysical and physiological studies, obtained mostly with two-dimensional (2-D) stimuli, provided evidence for the hypothesis that the processing of faces differs from that of scenes. We report on our experiments, employing realistic three-dimensional (3-D) stimuli of a hollow mask and a scene, that offer further evidence for this hypothesis. The stimuli used for both faces and scenes were bistable, namely they could elicit either the veridical or an illusory volumetric percept. Our results indicate that the illusion is weakened when the stimuli are inverted, suggesting the involvement of top-down processes. This inversion effect is statistically significant for the facial stimulus, but the trend did not reach statistical significance for the scene stimulus. These results support the hypothesis that configural processing is stronger for the 3-D perception of faces than it is for scenes, and extend the conclusions of earlier studies on 2-D stimuli.

The Hollow-Face Illusion: Object-Specific Knowledge, General Assumptions or Properties of the Stimulus?

The hollow-face illusion, in which a mask appears as a convex face, is a powerful example of binocular depth inversion occurring with a real object under a wide range of viewing conditions. Explanations of the illusion are reviewed and six experiments reported. In experiment 1 the detrimental effect of figural inversion, evidence for the importance of familiarity, was found for other oriented objects. The inversion effect held for masks lit from the side (experiment 2). The illusion was stronger for a mask rotated by 908 lit from its forehead than from its chin, suggesting that familiar patterns of shading enhance the illusion (experiment 2). There were no effects of light source visibility or any left/right asymmetry (experiment 3). In experiments 4 – 6 we used a ‘virtual’ hollow face, with illusion strength quantified by the proportion of noise texture needed to eliminate the illusion. Adding characteristic surface colour enhanced the illusion, consistent with the familiar face pigmentation outweighing additional bottom - up cues (experiment 4). There was no difference between perspective and orthographic projection. Photographic negation reduced, but did not eliminate, the illusion, suggesting shading is important but not essential (experiment 5). Absolute depth was not critical, although a shallower mask was given less extreme convexity ratings (experiment 6). We argue that the illusion arises owing to a convexity preference when the raw data have ambiguous interpretations. However, using a familiar object with typical orientation, shading, and pigmentation greatly enhances the effect.

Pinhole Viewing Strengthens the Hollow-Face Illusion

A hollow (concave) mask appears convex when viewed from beyond a certain distance even when viewed stereoscopically—this is the hollow-face illusion. At close viewing distances, the same mask is seen as hollow even when disparity information is eliminated by monocular viewing. A potential source of nonpictorial, monocular information that favors a veridical percept at close distances is accommodation in conjunction with focus blur. In this article, we used pinhole viewing to minimize this potential source of information and test whether it affects whether a hollow mask is seen as veridical (concave) or illusory (convex). Since monocular viewing also facilitates the illusory (convex) percept, it was included in the design both as a comparison and to test whether any effect of accommodation depends on vergence. Pinhole viewing was found favor the illusory percept, and its effect was at least as large as, and added to, that of monocular viewing. A control experiment using tinted glasses that attenuate illumination at least as much as the pinholes did not strengthen the illusion ruling out explanations in terms of reduced luminance. For pinhole viewing, there was no difference between monocular and binocular conditions. The results are interpreted as evidence that focus driven depth information affects perceived three-dimensional shape at close distances even when other sources of depth information are available. The lack of a difference between monocular and binocular pinhole viewing suggests that, by disrupting accommodation, pinholes may also interfere with linked vergence cues to depth.

Shading Beats Binocular Disparity in Depth from Luminance Gradients: Evidence against a Maximum Likelihood Principle for Cue Combination

Perceived depth is conveyed by multiple cues, including binocular disparity and luminance shading. Depth perception from luminance shading information depends on the perceptual assumption for the incident light, which has been shown to default to a diffuse illumination assumption. We focus on the case of sinusoidally corrugated surfaces to ask how shading and disparity cues combine defined by the joint luminance gradients and intrinsic disparity modulation that would occur in viewing the physical corrugation of a uniform surface under diffuse illumination. Such surfaces were simulated with a sinusoidal luminance modulation (0.26 or 1.8 cy/deg, contrast 20%-80%) modulated either in-phase or in opposite phase with a sinusoidal disparity of the same corrugation frequency, with disparity amplitudes ranging from 0'-20'. The observers' task was to adjust the binocular disparity of a comparison random-dot stereogram surface to match the perceived depth of the joint luminance/disparity-modulated corrugation target. Regardless of target spatial frequency, the perceived target depth increased with the luminance contrast and depended on luminance phase but was largely unaffected by the luminance disparity modulation. These results validate the idea that human observers can use the diffuse illumination assumption to perceive depth from luminance gradients alone without making an assumption of light direction. For depth judgments with combined cues, the observers gave much greater weighting to the luminance shading than to the disparity modulation of the targets. The results were not well-fit by a Bayesian cue-combination model weighted in proportion to the variance of the measurements for each cue in isolation. Instead, they suggest that the visual system uses disjunctive mechanisms to process these two types of information rather than combining them according to their likelihood ratios.

The role of linear perspective cues on the perceived depth magnitude of the surfaces they are painted on: proper-, reverse-, and flat-perspective paintings

Evidence from several studies suggests that perspective cues influence the perceived three-dimensional (3-D) layout of the surface they are painted on. The purpose of this study was to test how perspective cues influence the perceived depth magnitude. We rendered the same linear-perspective scene on three different surfaces: proper perspective, with a 3-D structure that was congruent with the painted scene; flat perspective, incongruent with the scene; reverse perspective, opposite to the scene, producing two competing stable percepts (veridical and illusory). We varied binocular disparity by using three different sizes for each type of stimulus. Observers assessed the magnitude of the perceived depth within each of these stimuli. Accuracy improved with increasing stimulus size that covaries with binocular-disparity magnitude. Generally, the magnitude of the perceived depth of stimuli painted with perspective cues was larger than the physical depth of the stimulus regardless of stimulus type (proper, reverse, flat). Further, depth magnitude tended to be larger when depth cues were congruent (proper) as compared with opposite (reverse) or incongruent (flat). There was no difference in perceived depth under the different percepts (veridical and illusory) for the reverse-perspective stimulus, suggesting that depth is assessed by the stimulus structure rather than by the percept obtained.

Instability of the perceived world while watching 3D stereoscopic imagery: A likely source of motion sickness symptoms

Watching 3D content using a stereoscopic display may cause various discomforting symptoms, including eye strain, blurred vision, double vision, and motion sickness. Numerous studies have reported motion-sickness-like symptoms during stereoscopic viewing, but no causal linkage between specific aspects of the presentation and the induced discomfort has been explicitly proposed. Here, we describe several causes, in which stereoscopic capture, display, and viewing differ from natural viewing resulting in static and, importantly, dynamic distortions that conflict with the expected stability and rigidity of the real world. This analysis provides a basis for suggested changes to display systems that may alleviate the symptoms, and suggestions for future studies to determine the relative contribution of the various effects to the unpleasant symptoms.

The hollow-face illusion monocularly observed in a box

Under monocular conditions, 40 students observed the reverse of polychrome and monochrome masks and judged them to be concave, convex or flat. The mask was presented upright and illuminated from above, below, right and left and in the upside down position illuminated from below. The magnitude of the perceived depth or relief was estimated using a retractable tape measure. Regardless of color, lighting and orientation, the majority of responses indicated that the hollow masks were perceived to be convex. No significant differences were observed between the depth or convexity of the metric magnitudes and scalar magnitudes of the concave masks in relation to variations in the light source direction, color, and position. The illusory depth, seeing the concave mask as convex, is a robust phenomenon that suggests the predominant role of higher-order processes over the low-order processes in visual face perception.

What's in a face? The role of depth undulations in three-dimensional depth-inversion illusions

Upright hollow human faces produce among the strongest depth-inversion illusions (DIIs), but why? We considered the role of depth undulations by comparing four types of hollow objects: an ellipsoid, a human mask, and two symmetric 'Martian'masks, which wavered in depth like the human mask but which lacked face-like features. Illusion strength was quantified either as the critical viewing distance at which the 3-D percept switched between convex and concave (experiment 1) or as the proportion of time ('predominance') that observers experienced DII from a fixed intermediate viewing distance (experiment 2). Critical distances were smallest--and hence the illusion was strongest--for the upright human mask; the remaining objects produced undifferentiated critical distance values. The predominance results were more fine-grained: illusions were experienced most often for the upright human mask, least often for the hollow ellipsoid, and to an intermediate extent for the Martian and upside-down human masks. These results suggest: (1) an upside-down human mask and a surface with nonface features undulating in depth are equivalent for the purposes of generating DIIs; (2) depth undulations contribute to DII; and (3) such undulations are most effective when structured into an upright human face.

Percepção monocular da profundidade ou relevo na ilusão da máscara côncava na esquizofrenia

Este trabalho foi desenvolvido com o propósito de investigar a percepção monocular da profundidade ou relevo da máscara côncava por 29 indivíduos saudáveis, sete indivíduos com esquizofrenia sob uso de antipsicótico por um período inferior ou igual a quatro semanas e 29 sob uso de antipsicótico por um período superior a quatro semanas. Os três grupos classificaram o reverso de uma máscara policromada em duas situações de iluminação, por cima e por baixo. Os resultados indicaram que a maioria dos indivíduos com esquizofrenia inverteu a profundidade da máscara côncava na condição de observação monocular e perceberam-na como convexa, sendo, portanto, suscetíveis à ilusão da máscara côncava. Os indivíduos com esquizofrenia sob uso de medicação antipsicótica pelo período superior a quatro semanas estimaram a convexidade da máscara côncava iluminada por cima em menor comprimento comparados aos indivíduos saudáveis.

Six-Month-Old Infants' Perception of the Hollow Face Illusion: Evidence for a General Convexity Bias

Corrow, Granrud, Mathison, and Yonas (2011, Perception, 40, 1376–1383) found evidence that 6-month-old infants perceive the hollow face illusion. In the present study we asked whether 6-month-old infants perceive illusory depth reversal for a nonface object and whether infants' perception of the hollow face illusion is affected by mask orientation inversion. In experiment 1 infants viewed a concave bowl, and their reaches were recorded under monocular and binocular viewing conditions. Infants reached to the bowl as if it were convex significantly more often in the monocular than in the binocular viewing condition. These results suggest that infants perceive illusory depth reversal with a nonface stimulus and that the infant visual system has a bias to perceive objects as convex. Infants in experiment 2 viewed a concave face-like mask in upright and inverted orientations. Infants reached to the display as if it were convex more in the monocular than in the binocular condition; however, mask orientation had no effect on reaching. Previous findings that adults' perception of the hollow face illusion is affected by mask orientation inversion have been interpreted as evidence of stored-knowledge influences on perception. However, we found no evidence of such influences in infants, suggesting that their perception of this illusion may not be affected by stored knowledge, and that perceived depth reversal is not face-specific in infants.

Processing convexity and concavity along a 2-D contour: figure-ground, structural shape, and attention

Interest in convexity has a long history in vision science. For smooth contours in an image, it is possible to code regions of positive (convex) and negative (concave) curvature, and this provides useful information about solid shape. We review a large body of evidence on the role of this information in perception of shape and in attention. This includes evidence from behavioral, neurophysiological, imaging, and developmental studies. A review is necessary to analyze the evidence on how convexity affects (1) separation between figure and ground, (2) part structure, and (3) attention allocation. Despite some broad agreement on the importance of convexity in these areas, there is a lack of consensus on the interpretation of specific claims--for example, on the contribution of convexity to metric depth and on the automatic directing of attention to convexities or to concavities. The focus is on convexity and concavity along a 2-D contour, not convexity and concavity in 3-D, but the important link between the two is discussed. We conclude that there is good evidence for the role of convexity information in figure-ground organization and in parsing, but other, more specific claims are not (yet) well supported.

Virtual slant explains perceived slant, distortion, and motion in pictorial scenes

Virtual slants of obliquely viewed figures consisting of skewed grids, computed as a function of depicted slant and slant of the physical surface, were compared with perceived slants in both monocular and binocular viewing conditions. Computations were based on an assumption of parallelism. Perceived slant was well-correlated with virtual slant, even though slants were generally underestimated. The qualitative relationship between virtual slants and physical surface slants was variable, and liable to produce different degrees and directions of apparent rotation as well as of apparent compensation, depending on the depicted slant of the stimulus. Contributions of screen-related cues were small and mainly limited to small slants. Remarkably, binocular disparity did not have any effect on perceived slant. The results imply that many past findings of both transformation and (apparent) compensation in pictorial viewing may in principle be straightforwardly explained as a function of the virtual stimulus.

Recovering 3-D shape: roles of absolute and relative disparity, retinal size, and viewing distance as studied with reverse-perspective stimuli

When viewing reverspective stimuli, data-driven signals such as disparity, motion parallax, etc, help to recover veridical three-dimensional (3-D) shape. They compete against schema-driven influences such as experience with perspective, foreshortening, and other pictorial cues that favor the perception of an illusory depth inversion. We used three scaled-size versions of a reverspective to study the roles of retinal size, binocular disparity, and viewing distance--that influences both vergence and accommodation--in recovering the true 3-D shape. Experiment 1 used three conditions, in each of which a parameter was kept fixed across the three stimulus sizes: (a) fixed retinal size, (b) fixed viewing distance, (c) fixed disparity. The predominance of the veridical percept was recorded. Generally, the illusion strength was the same when the viewing distance was fixed, despite significantly different disparities and retinal sizes; conversely, illusion strength changed significantly in fixed-disparity and fixed-retinal-size conditions. Experiment 2 confirmed the results of experiment 1b (roughly equal performances for fixed viewing distance, independent of size) for two additional distances. Viewing distance and "scaled disparity" (disparity divided by retinal size) are good predictors of the data trends. We propose that disparity scaling is supported by both mathematical and 3-D shape considerations.

The role of convexity in perception of symmetry and in visual short-term memory

Visual perception of shape is affected by coding of local convexities and concavities. For instance, a recent study reported that deviations from symmetry carried by convexities were easier to detect than deviations carried by concavities. We removed some confounds and extended this work from a detection of reflection of a contour (i.e., bilateral symmetry), to a detection of repetition of a contour (i.e., translational symmetry). We tested whether any convexity advantage is specific to bilateral symmetry in a two-interval (Experiment 1) and a single-interval (Experiment 2) detection task. In both, we found a convexity advantage only for repetition. When we removed the need to choose which region of the contour to monitor (Experiment 3) the effect disappeared. In a second series of studies, we again used shapes with multiple convex or concave features. Participants performed a change detection task in which only one of the features could change. We did not find any evidence that convexities are special in visual short-term memory, when the to-be-remembered features only changed shape (Experiment 4), when they changed shape and changed from concave to convex and vice versa (Experiment 5), or when these conditions were mixed (Experiment 6). We did find a small advantage for coding convexity as well as concavity over an isolated (and thus ambiguous) contour. The latter is consistent with the known effect of closure on processing of shape. We conclude that convexity plays a role in many perceptual tasks but that it does not have a basic encoding advantage over concavity.

Specular reflections and the estimation of shape from binocular disparity

Binocular stereopsis is a powerful visual depth cue. To exploit it, the brain matches features from the two eyes' views and measures their interocular disparity. This works well for matte surfaces because disparities indicate true surface locations. However, specular (glossy) surfaces are problematic because highlights and reflections are displaced from the true surface in depth, leading to information that conflicts with other cues to 3D shape. Here, we address the question of how the visual system identifies the disparity information created by specular reflections. One possibility is that the brain uses monocular cues to identify that a surface is specular and modifies its interpretation of the disparities accordingly. However, by characterizing the behavior of specular disparities we show that the disparity signals themselves provide key information ("intrinsic markers") that enable potentially misleading disparities to be identified and rejected. We presented participants with binocular views of specular objects and asked them to report perceived depths by adjusting probe dots. For simple surfaces--which do not exhibit intrinsic indicators that the disparities are "wrong"--participants incorrectly treat disparities at face value, leading to erroneous judgments. When surfaces are more complex we find the visual system also errs where the signals are reliable, but rejects and interpolates across areas with large vertical disparities and horizontal disparity gradients. This suggests a general mechanism in which the visual system assesses the origin and utility of sensory signals based on intrinsic markers of their reliability.

“Exorcist Illusion”: Twisting Necks in the Hollow-Face and Hollow-Torso Illusions

We combine a convex facial mask with a concave torso—or vice versa—thus creating a single rigid object with a transition area at the neck, where convexity changes to concavity. This combination creates stunning illusions when the rigid object is set to motion. The two simplest effects are (1) when the object is rotated about its axis, the head appears to twist with respect to the torso, as in “The Exorcist” film; (2) when it is rotated around an axis parallel to the shoulders, the head appears to hinge around the torso. More complex illusory effects result from more complex motions. The involvement of higher-level perceptual processes may account for the illusory effects.

Impaired perceptual processing and conceptual cognition in patients with anxiety disorders: a pilot study with the binocular depth inversion paradigm

The binocular depth inversion test (BDIT) measures a common illusion of visual perception whereby implausible objects are seen as normal, e.g., a hollow face is perceived as a normal, convex face. Such inversion is frequent, especially for objects with a high degree of familiarity. Under normal conditions, cognitive factors apparently override the binocular disparity cues of stereopsis. This internal mechanism of "censorship" of perception, which balances "top-down" and "bottom-up" processes of perception to come to a cognitive coherence, which is congruent to previous experience and concepts, appears to be disturbed in (pro-)psychotic states. The BDIT has been shown to be a sensitive measure of impaired higher visual processing and conceptual cognition common to conditions including schizophrenia, cannabinoid-intoxication, and sleep deprivation but not depression. In this pilot study, we tested the performance of patients with anxiety disorders (ICD-10 F40 and F41) compared to matched controls using the BDIT paradigm. Anxiety patients scored significantly higher on the BDIT than controls, in a range comparable to propsychotic conditions. The findings suggest that anxiety patients could have abnormalities in central perceptual processing, top-down processing (conceptual cognition), and reality testing similar to (pro-)psychotic conditions. Implications of these findings are discussed in relation to therapeutic interventions with anxiety disorders.

Where do the eyes really go in the hollow-face illusion?

The hollow-face illusion refers to the finding that people typically perceive a concave (hollow) mask as being convex, despite the presence of binocular disparity cues that indicate the contrary. Unlike other illusions of depth, recent research has suggested that the eyes tend to converge at perceived, rather than actual, depths. However, technical and methodological limitations prevented one from knowing whether disparity cues may still have influenced vergence. In the current study, we presented participants with virtual normal or hollow masks and asked them to fixate the tip of the face's nose until they had indicated whether they perceived it as pointing towards or away from them. The results showed that the direction of vergence was indeed determined by perceived depth, although vergence responses were both somewhat delayed and of smaller amplitude (by a factor of about 0.5) for concave than convex masks. These findings demonstrate how perceived depth can override disparity cues when it comes to vergence, albeit not entirely.

Refractive Error and Monocular Viewing Strengthen the Hollow-Face Illusion

We measured the strength of the hollow-face illusion—the ‘flipping distance’ at which perception changes between convex and concave—as a function of a lens-induced 3 dioptre refractive error and monocular/binocular viewing. Refractive error and closing one eye both strengthened the illusion to approximately the same extent. The illusion was weakest viewed binocularly without refractive error and strongest viewed monocularly with it. This suggests binocular cues disambiguate the illusion at greater distances than monocular cues, but that both are disrupted by refractive error. We argue that refractive error leaves the ambiguous low-spatial-frequency shading information critical to the illusion largely unaffected while disrupting other, potentially disambiguating, depth/distance cues.

The effects of vertical gradient of disparity and combination mode of features on the occurrence of double fusion in Panum's limiting case

Panum's limiting case generally refers to the phenomenon that two features presented to one eye and a single feature presented to the other are combined and then perceived as two features at different depths. It is still not clear why experimental results derived from the Panum-type configuration (all lines parallel) support a double fusion viewpoint, but they do not for the Wheatstone-type configuration (one line not parallel to the others). Some experimental results support the double fusion theory, while others do not, even under a small disparity. Here we report that, under a small disparity, when the vertical gradients of the horizontal disparity ofdichoptic feature pairs in previous Wheatstone-type configurations were increased or decreased, the evidence which was considered to be very convincing in previous studies, either supporting or against the double fusion viewpoint, was challenged, and even turned to support the opposite view. Moreover, it was discovered that changes in the way features were arranged altered the results. Together, these results indicate that double fusion is the common basis for all kinds of Panum-type configurations. But for the Wheatstone-type configurations double fusion is also constrained by the vertical gradient of disparity of the configurations in addition to disparity and influenced by the degree of similarity/conflict between binocular cues and monocular cues resulting from different arrangements of features.

Reduced P300 and P600 amplitude in the hollow-mask illusion in patients with schizophrenia

Illusions provide a useful tool to study the mechanisms by which top-down and bottom-up processes interact in perception. Patients suffering from schizophrenia are not as subject to the hollow-mask illusion as healthy controls, since studies have shown that controls perceive a hollow mask as a normal face, while patients with schizophrenia do not. This insusceptibility to the illusion is indicating a weakened top-down processing in schizophrenia and little is understood about the neurobiology of this phenomenon. We used event-related potentials to investigate the hollow-mask illusion in patients with schizophrenia and healthy controls. We hypothesized that there would be a visible reduction of top-down processing in the patients' group and that this reduction would occur in the late stages of processing. We found significantly decreased amplitudes in the P300 and P600 components in the patients' group, indicating that visual information does not benefit from frontal, parietal or temporal activity for perceiving incoming stimuli. We propose that a deficit in functional connectivity may be responsible for impaired top-down visual processing in schizophrenia. These data further the understanding of the time course of top-down processing in patients with schizophrenia.

The hollow-face illusion in infancy: do infants see a screen based rotating hollow mask as hollow?

We investigated whether infants experience the hollow-face illusion using a screen-based presentation of a rotating hollow mask. In experiment 1 we examined preferential looking between rotating convex and concave faces. Adults looked more at the concave—illusory convex—face which appears to counter rotate. Infants of 7- to 8-month-old infants preferred the convex face, and 5- to 6-month-olds showed no preference. While older infants discriminate, their preference differed from that of adults possibly because they don't experience the illusion or counter rotation. In experiment 2 we tested preference in 7- to 8-month-olds for angled convex and concave static faces both before and after habituation to the stimuli shown in experiment 1. The infants showed a novelty preference for the static shape opposite to the habituation stimulus, together with a general preference for the static convex face. This shows that they discriminate between convex and concave faces and that habituation to either transfers across a change in view. Seven- to eight-month-olds have been shown to discriminate direction of rigid rotation on the basis of perspective changes. Our results suggest that this, perhaps together with a weaker bias to perceive faces as convex, allows these infants to see the screen-based hollow face as hollow even though adults perceive it as convex.

Six-month-old infants perceive the hollow-face illusion

In this study we investigated infants' perception of the hollow-face illusion. 6-month-old infants were shown a concave mask under monocular and binocular viewing conditions and the direction of their reaches toward the mask was recorded. Adults typically perceive a concave mask as convex under monocular conditions but as concave under binocular conditions, depending on viewing distance. Based on previous findings that infants reach preferentially toward the parts of a display that are closest to them, we expected that, if infants perceive the hollow-face illusion as adults do, they would reach to the center of the mask when viewing it monocularly and to the edges when viewing it binocularly. The results were consistent with these predictions. Our findings indicated that the infants perceived the mask as convex when viewing it with one eye and concave when viewing it with two eyes. The results show that 6-month-old infants respond to the hollow-face illusion. Our finding suggests that, early in life, the visual system uses the constraint, or assumption, that faces are convex when interpreting visual input.

Binocular Disparity Magnitude Affects Perceived Depth Magnitude despite Inversion of Depth Order

The hollow-face illusion involves a misperception of depth order: our perception follows our top–down knowledge that faces are convex, even though bottom–up depth information reflects the actual concave surface structure. While pictorial cues can be ambiguous, stereopsis should unambiguously indicate the actual depth order. We used computer-generated stereo images to investigate how, if at all, the sign and magnitude of binocular disparities affect the perceived depth of the illusory convex face. In experiment 1 participants adjusted the disparity of a convex comparison face until it matched a reference face. The reference face was either convex or hollow and had binocular disparities consistent with an average face or had disparities exaggerated, consistent with a face stretched in depth. We observed that apparent depth increased with disparity magnitude, even when the hollow faces were seen as convex (ie when perceived depth order was inconsistent with disparity sign). As expected, concave faces appeared flatter than convex faces, suggesting that disparity sign also affects perceived depth. In experiment 2, participants were presented with pairs of real and illusory convex faces. In each case, their task was to judge which of the two stimuli appeared to have the greater depth. Hollow faces with exaggerated disparities were again perceived as deeper.

Estimating changes in lighting direction in binocularly viewed three-dimensional scenes

We examine human ability to detect changes in scene lighting. Thirteen observers viewed three-dimensional rendered scenes stereoscopically. Each scene consisted of a randomly generated three-dimensional "Gaussian bump" surface rendered under a combination of collimated and diffuse light sources. During each trial, the collimated source underwent a small, quick change of position in one of four directions. The observer's task was to classify the direction of the lighting change. All observers were above chance in performing the task. We developed a model that combined two sources of information, a shape map and a shading map, to predict lighting change direction. We used this model to predict patterns of errors both across observers and across scenes differing in shape. We found that errors in estimating lighting direction were primarily the result of errors in representing surface shape. We characterized the surface features that affected performance in the classification task.

Perceiving parts and shapes from concave surfaces

"A hole is nothing at all, but it can break your neck." In a similar fashion to the danger illustrated by this folk paradox, concave regions pose difficulties to theories of visual shape perception. We can readily identify their shapes, but according to principles of how observers determine part boundaries, concavities in a planar surface should have very different figural shapes from the ones that we perceive. In three experiments, we tested the hypothesis that observers perceive local image features differently in simulated 3-D concave and convex regions but use them to arrive at similar shape percepts. Stimuli were shape-from-shading images containing regions that appeared either concave or convex in depth, depending on their orientation in the picture plane. The results show that concavities did not benefit from the same global object-based attention or holistic shape encoding as convexities and that the participants relied on separable spatial dimensions to judge figural shape in concavities. Concavities may exploit a secondary process for shape perception that allows regions composed of perceptually independent features to ultimately be perceived as gestalts.

Binocular Disparities, Motion Parallax, and Geometric Perspective in Patrick Hughes's ‘Reverspectives’: Theoretical Analysis and Empirical Findings

Patrick Hughes's ‘reverspective’ artworks provide a novel way of investigating the effectiveness of different sources of 3-D information for the human visual system. Our empirical findings show that the converging lines of simple linear perspective can be as effective as the rich array of 3-D cues present in natural scenes in determining what we see, even when these cues are in conflict with binocular disparities. Theoretical considerations reveal that, once the information provided by motion parallax transformations is correctly understood, there is no need to invoke higher-level processes or an interpretation based on familiarity or past experience in order to explain either the ‘reversed’ depth or the apparent, concomitant rotation of a reverspective artwork as the observer moves from side to side. What we see in reverspectives is the most likely real-world scenario (distal stimulus) that could have created the perspective and parallax transformations (proximal stimulus) that stimulate our visual systems.

Cor, iluminação e orientação do reverso de uma máscara facial não afetam a ilusão da máscara côncava

Em condição monocular, 40 observadores julgaram o reverso côncavo de uma máscara facial monocromada cinza e de outra policromada (iluminados por cima, por baixo, pela direita e pela esquerda na posição vertical e na posição vertical invertida, iluminado por baixo) como côncavos, planos ou convexos. Além disso, as magnitudes dos seus relevos percebidos foram reproduzidas ao se esticar uma trena retrátil. Independente da cor, iluminação e orientação das máscaras, a maioria das respostas indicou que os reversos das máscaras foram percebidos como convexos. E mesmo nas poucas respostas em que as máscaras foram classificadas como planas, houve atribuição de relevo mensurável. Estes resultados confirmam que a ilusória profundidade da máscara facial côncava como convexa é robusta sob influência de variáveis diversas, o que sugere atuação predominante de processos de alta ordem sobre os processos de baixa ordem na percepção visual de faces.

Understanding why patients with schizophrenia do not perceive the hollow-mask illusion using dynamic causal modelling

Patients suffering from schizophrenia are less susceptible to various visual illusions. For example, healthy participants perceive a hollow mask as a normal face, presumably due to the strength of constraining top-down influences, while patients with schizophrenia do not (Schneider, U., Leweke, F.M., Sternemann, U., Weber, M.M., Emrich, H.M., 1996. Visual 3D illusion: a systems-theoretical approach to psychosis. Eur. Arch. Psychiatry Clin. Neurosci. 246, 256-260; Scheider, U., Borsutzky, M., Seifert, J., Leweke, F.M., Huber, T.J., Rollnik J.D., Emrich, H.M., 2002. Reduced binocular depth inversion in schizophrenic patients. Schizophrenia Research 53, 101-108.; Emrich, H.M., Leweke, F.M., Schneider, U., 1997. Towards a cannabinoid hypothesis of schizophrenia: cognitive impairments due to a dysregulation of the endogenous cannabinoid system. Pharmacol. Biochem. Behav. 56, 803-807). However the neural mechanisms underpinning this effect remain poorly understood. We used functional magnetic resonance imaging to investigate the hollow-mask illusion in schizophrenic patients and healthy controls. The primary aim of this study was to use measures of effective connectivity arising from dynamic causal modelling (DCM) to explain differences in both the perception of the hollow-mask illusion and associated differences in neural responses between patients with schizophrenia and controls, which we hypothesised would be associated with difference in the influences of top-down and bottom-up processes between the groups. Consistent with this explanation, we identified differences between the two groups in effective connectivity. In particular, there was a strengthening of bottom-up processes, and weakening of top-down ones, during the presentation of 'hollow' faces for the patients. In contrast, the controls exhibited a strengthening of top-down processes when perceiving the same stimuli. These findings suggest that schizophrenic patients rely on stimulus-driven processing and are less able to employ conceptually-driven top-down strategies during perception, where incoming sensory data are constrained with reference to a generative model that entails stored information from past experience.

Eye vergence is susceptible to the hollow-face illusion

When an observer looks at a hollow mask of a face, a normal convex face is often perceived [the hollow-face illusion--Gregory 1973, in Illusion in Nature and Art (London: Duckworth) pp 49-96]. We show that in exploring an illusory face, the eyes converge at the illusory and not at the real distances of fixated targets like the tip of the nose. The 'vergence error' appears even though the resulting disparities of the two retinal images of the target provide feedback that would allow an immediate correction. It is presumably the success of recognising a familiar object (a face) which overrides the correction of convergence. This suggests that the brain strives for a congruency of eye vergence and distance perception.

Effect of Spatial Position on Visual Search for 3-D Objects

We examined whether the position of objects in external space affects the visual-search task associated with the tilt of 3-D objects. An array of cube-like objects was stereoscopically displayed at a distance of 4.5 m on a large screen 1.5 m above or below eye height. Subjects were required to detect a downward-tilted target among upward-tilted distractors or an upward-tilted target among downward-tilted distractors. When the stimuli consisted of shaded parallelepipeds whose upper/bottom faces were lighter than their side faces, the upward-tilted target was detected faster. This result was in accordance with the ‘top-view assumption’ reported in previous research. Displaying stimuli in the upper position degraded overall performance. However, when the shaded objects whose upper/bottom faces were darker than their side faces were displayed, the detection of a downward-tilted target became as efficient as that of an upward-tilted target only at the upper position. These results indicate that it is possible for the spatial position of the stimulus to promote the detection of a downward-tilted target when shading and perspective information are consistent with the viewing direction.

Dissociation of perception and action unmasked by the hollow-face illusion

It has been suggested that there are two separate visual streams in the human cerebral cortex: a ventral pathway that provides perceptual representations of the world and serves as a platform for cognitive operations, and a dorsal pathway that transforms visual information for the control of motor acts. Evidence for this distinction comes from neuropsychology, neuroimaging, and neurophysiology. There is also evidence from experimental psychology, with normal observers experiencing an illusion-where perception and action can be dissociated, although much of this evidence is controversial. Here, we report an experiment aimed at demonstrating a large dissociation between perception and fast action using the hollow-face illusion, in which a hollow mask looks like a normal convex face. Participants estimated the positions of small targets placed on the actually hollow but apparently normal face and used their fingers to 'flick' the targets off. Despite the presence of a compelling illusion of a normal face, the flicking movements were directed at the real, not the illusory locations of the targets. These results show that the same visual stimulus can have completely opposite effects on conscious perception and visual control of fast action.

A inversão de profundidade visual em faces côncavas

Apresenta-se uma revisão de artigos sobre inversão de profundidade visual, principalmente relacionada à ilusão da face côncava (hollow face illusion). Enfatizam-se a supremacia dos processos de alta ordem (top-down) sobre os processos de baixa ordem (bottom-up) na percepção de profundidade invertida da face côncava, fatores e condições que a modulam, e plausíveis aplicações do fenômeno para se investigar condições comportamentais, psicopatológicas e psicofarmacológicas.

The Medawar Lecture 2001 knowledge for vision: vision for knowledge

An evolutionary development of perception is suggested-from passive reception to active perception to explicit conception-earlier stages being largely retained and incorporated in later species. A key is innate and then individually learned knowledge, giving meaning to sensory signals. Inappropriate or misapplied knowledge produces rich cognitive phenomena of illusions, revealing normally hidden processes of vision, tentatively classified here in a 'periodic table'. Phenomena of physiology are distinguished from phenomena of general rules and specific object knowledge. It is concluded that vision uses implicit knowledge, and provides knowledge for intelligent behaviour and for explicit conceptual understanding including science.

A new look at binocular stereopsis

We report a new phenomenon, which illustrates that the role of binocular disparity in 3D shape perception critically depends on whether the parts are interpreted as belonging to a single object. The nature of this phenomenon was studied in four experiments. In the first two experiments the subjects were shown a sequence of stereoscopic images of a cube, in which binocular disparity indicated that the individual parts move towards or away from one eye. However, when the parts of the cube were perceived as elements of a single object, they appeared to move in a rigid fashion and the direction of motion was orthogonal to that predicted by the binocular disparities. The third experiment generalized these results to more complex polyhedra. The last experiment showed that constraints related to motion, such as rigidity, are important, but not critical for this phenomenon to occur. All these results imply that the interpretation as to what corresponds to a single object affects the importance (weight) of binocular disparity and may even eliminate its contribution altogether; the percept of a 3D shape is dominated by a priori constraints, and depth cues play a secondary role.

Perceptual biases in the interpretation of 3D shape from shading

Two experiments are reported in which observers judged the sign and magnitude of surface curvature from shaded images of an indoor scene. The depicted surfaces were illuminated by an area light in the ceiling or floor, and the illumination was attenuated with distance in a physically correct manner. The displays were presented both with and without cast shadows, specular highlights and surface inter-reflections in all possible combinations. The overall pattern of results revealed a strong perceptual bias to interpret the images as convex rather than concave, and a weaker bias to prefer illumination from above rather than from below, though there were large individual differences in the relative strengths of these biases. For displays that did not contain cast shadows or specular highlights, the accuracy of observers' judgments about the sign of surface curvature was no greater than chance, but performance was significantly improved when those aspects of normal shading were included in the rendering model. An analysis of the apparent depth magnitudes revealed that convex surfaces produce much greater perceived depth than concave surfaces with comparable relief.

Object perception as Bayesian inference

We perceive the shapes and material properties of objects quickly and reliably despite the complexity and objective ambiguities of natural images. Typical images are highly complex because they consist of many objects embedded in background clutter. Moreover, the image features of an object are extremely variable and ambiguous owing to the effects of projection, occlusion, background clutter, and illumination. The very success of everyday vision implies neural mechanisms, yet to be understood, that discount irrelevant information and organize ambiguous or noisy local image features into objects and surfaces. Recent work in Bayesian theories of visual perception has shown how complexity may be managed and ambiguity resolved through the task-dependent, probabilistic integration of prior object knowledge with image features.

Bayesian modeling of cue interaction: bistability in stereoscopic slant perception

Our two eyes receive different views of a visual scene, and the resulting binocular disparities enable us to reconstruct its three-dimensional layout. However, the visual environment is also rich in monocular depth cues. We examined the resulting percept when observers view a scene in which there are large conflicts between the surface slant signaled by binocular disparities and the slant signaled by monocular perspective. For a range of disparity-perspective cue conflicts, many observers experience bistability: They are able to perceive two distinct slants and to flip between the two percepts in a controlled way. We present a Bayesian model that describes the quantitative aspects of perceived slant on the basis of the likelihoods of both perspective and disparity slant information combined with prior assumptions about the shape and orientation of objects in the scene. Our Bayesian approach can be regarded as an overarching framework that allows researchers to study all cue integration aspects-including perceptual decisions--in a unified manner.

Effects of different subanaesthetic doses of (S)-ketamine on psychopathology and binocular depth inversion in man

The role of the N-methyl-D-aspartate (NMDA) neurotransmitter system in relation to psychoses is not completely understood, but represent a challenge in neurobiological research. The psychotic states induced by NMDA antagonists such as phencyclidine and ketamine have been described as being most similar to schizophrenia and the NMDA system has been implicated in the pathogenesis of schizophrenia. Binocular depth inversion, an illusion of visual perception, has been shown to be impaired in psychotic and psychotomimetic states in healthy and schizophrenic subjects. In this study, pictures of natural and artificial objects were presented stereoscopically to 12 healthy male volunteers and depth perception assessed using an operationalized method. The effects of the psychotomimetic S-enantiomer of the anaesthetic ketamine in two different subanaesthetic doses were compared with those of a placebo. In spite of dose dependence and grave subjective and significant objective psychopathology, no significant impairment of binocular depth perception was found with (S)-ketamine. Implications related to memory function, perceptogenesis and 'bottom-up' processing in ketamine model psychosis and schizophrenia are discussed.