Success rates, near-response patterns, and learning trends with free-fusion stereograms

Free-fusion stereograms are routinely used for demonstrating various stereoscopic effects. Yet, untrained observers find it challenging to perform this task. This study showed that only less than 1/3rd of sixty-one pre-presbyopic adults with normal binocular vision could successfully free-fuse random-dot image pairs and identify the stereoscopic shapes embedded in these patterns. Another one-third of participants performed the task with poor success rates, while the remaining could not perform the task. There was a clear dissociation of vergence and accommodative responses in participants who were successful with free-fusion, as recorded using a dynamic infrared eye tracker and photorefractor. Those in the unsuccessful cluster either showed strong vergence and accommodation or weak vergence and strong accommodation during the task. These response patterns, however, were specific to the free-fusion task because all these participants generated good convergence/accommodation to real-world targets and to conflicting vergence and accommodative demands stimulated with prisms or lenses. Task performance of the unsuccessful cluster also improved significantly following pharmacological paralysis of accommodation and reached the performance levels of the successful cluster. A minority of participants also appeared to progressively learn to dissociate one of the two directions of their vergence and accommodation crosslinks with repeated free-fusion trials. These results suggest that successful free-fusion might depend upon how well participants generate a combination of volitional and reflex vergence responses to large differences in disparity with conflicting static accommodative demands. Such responses would require that only one direction of the vergence-accommodation crosslinks be active at any given time. The sequence of near-responses could also be learnt through repeated trials to optimize task performance.

Comparison of 3D Sensors for Automating Bolt-Tightening Operations in the Automotive Industry

Machine vision systems are widely used in assembly lines for providing sensing abilities to robots to allow them to handle dynamic environments. This paper presents a comparison of 3D sensors for evaluating which one is best suited for usage in a machine vision system for robotic fastening operations within an automotive assembly line. The perception system is necessary for taking into account the position uncertainty that arises from the vehicles being transported in an aerial conveyor. Three sensors with different working principles were compared, namely laser triangulation (SICK TriSpector1030), structured light with sequential stripe patterns (Photoneo PhoXi S) and structured light with infrared speckle pattern (Asus Xtion Pro Live). The accuracy of the sensors was measured by computing the root mean square error (RMSE) of the point cloud registrations between their scans and two types of reference point clouds, namely, CAD files and 3D sensor scans. Overall, the RMSE was lower when using sensor scans, with the SICK TriSpector1030 achieving the best results (0.25 mm ± 0.03 mm), the Photoneo PhoXi S having the intermediate performance (0.49 mm ± 0.14 mm) and the Asus Xtion Pro Live obtaining the higher RMSE (1.01 mm ± 0.11 mm). Considering the use case requirements, the final machine vision system relied on the SICK TriSpector1030 sensor and was integrated with a collaborative robot, which was successfully deployed in an vehicle assembly line, achieving 94% success in 53,400 screwing operations.

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.

From pictures to reality: modelling the phenomenology and psychophysics of 3D perception

The dominant inferential approach to human 3D perception assumes a model of spatial encoding based on a physical description of objects and space. Prevailing models based on this physicalist approach assume that the visual system infers an objective, unitary and mostly veridical representation of the external world. However, careful consideration of the phenomenology of 3D perception challenges these assumptions. I review important aspects of phenomenology, psychophysics and neurophysiology which suggest that human visual perception of 3D  objects and space is underwritten by distinct and dissociated spatial encodings that are optimized for specific regions of space. Specifically, I argue that 3D perception is underwritten by at least three distinct encodings for (1) egocentric distance perception at the ambulatory scale, (2) exocentric distance (scaled depth) perception optimized for near space, and (3) perception of object shape and layout (unscaled depth). This tripartite division can more satisfactorily account for the phenomenology, psychophysics and adaptive logic of human 3D perception. This article is part of a discussion meeting issue 'New approaches to 3D vision'.

Testing geometry and 3D perception in children following vision restoring cataract-removal surgery

As neuroscience and rehabilitative techniques advance, age-old questions concerning the visual experience of those who gain sight after blindness, once thought to be philosophical alone, take center stage and become the target for scientific inquiries. In this study, we employ a battery of visual perception tasks to study the unique experience of a small group of children who have undergone vision-restoring cataract removal surgery as part of the Himalayan Cataract Project. We tested their abilities to perceive in three dimensions (3D) using a binocular rivalry task and the Brock string task, perceive visual illusions, use cross-modal mappings between touch and vision, and spatially group based on geometric cues. Some of the children in this study gained a sense of sight for the first time in their lives, having been born with bilateral congenital cataracts, while others suffered late-onset blindness in one eye alone. This study simultaneously supports yet raises further questions concerning Hubel and Wiesel's critical periods theory and provides additional insight into Molyneux's problem, the ability to correlate vision with touch quickly. We suggest that our findings present a relatively unexplored intermediate stage of 3D vision development. Importantly, we spotlight some essential geometrical perception visual abilities that strengthen the idea that spontaneous geometry intuitions arise independently from visual experience (and education), thus replicating and extending previous studies. We incorporate a new model, not previously explored, of testing children with congenital cataract removal surgeries who perform the task via vision. In contrast, previous work has explored these abilities in the congenitally blind via touch. Taken together, our findings provide insight into the development of what is commonly known as the visual system in the visually deprived and highlight the need to further empirically explore an amodal, task-based interpretation of specializations in the development and structure of the brain. Moreover, we propose a novel objective method, based on a simple binocular rivalry task and the Brock string task, for determining congenital (early) vs. late blindness where medical history and records are partial or lacking (e.g., as is often the case in cataract removal cases).

Ibn al-Haytham's ground theory of distance perception

The 11th-century Arab scholar, Ibn al-Haytham, in his Optics, offers a detailed, rigorous, empirically oriented explanation of distance perception that may be the first essentially modern, scientific theory of distance perception. Based on carefully described experiments, he argues that for distance to be perceived accurately: (1) the distance must lie along a continuous surface such as the ground; (2) the continuous surface must be visible; (3) the magnitudes of distances along the surface must be perceived and calibrated through bodily interaction (walking and reaching) with them; and finally (4) the distance must be moderate. Al-Haytham's work reached Europe early in the 13th century, and his was the dominant theory of distance perception there for about 400 years. It was superseded early in the 17th century by a theory, based on cues such as convergence and accommodation, of distance seen through empty, mathematized space. Around 1950, an explanation of distance perception strikingly like that of al-Haytham was independently developed by J. J. Gibson, who called his theory the “ground theory” of space perception.

More than a feeling: Tactile exploration informs relative size perception

Esref Armagan is a blind painter. Cognitive and neural scientists have described his use of line structure when drawing and his visual cortex activation during tactile exploration. But what remains unknown is how Esref, without ever having sight, may be able to understand the use of depth cues such as relative size in a picture. To examine whether tactile experience provides information about relative size, we blindfolded sighted individuals and asked them to haptically explore a set of objects in either near or far locations before drawing the objects on paper. Objects explored with an arms extended position were drawn significantly smaller than those explored just in front of the face. Our results provide the first evidence that haptic object interaction can provide information about relative size, akin to that garnered visually. It can be used to inform spatial layout or, in Esref's case, to artistically render objects in three dimensions.

An ecological approach to binocular vision

An ecological approach to binocular vision was already demonstrated in Wheatstone's initial stereograms and was explicitly called for by J. J. Gibson, but detailed analysis and experimentation supporting this approach has been more recent. This paper discusses several aspects of this more recent research on environmentally occurring spatial layouts that can influence binocular vision. These include gradients of depth and regions that can be seen by only one eye. The resolution of local stereoscopic ambiguity by more global factors is also discussed.

Depth from blur and grouping under inattention

Previous studies provided evidence in support of attention operating in three-dimensional space, and the iterative and multistage nature of organizational processes in relation to attention and depth. We investigated depth perception and attentional demands in grouping organizations that contain blur as a depth cue. Contrary to previous studies, in our displays, no depth from occlusion could be implied from a shared border between groups or surfaces. To evaluate depth perception, subjective reports were collected where participants indicated which elements, blurry or sharp, they perceived as closer. To examine whether depth perception from blur can alleviate attentional demands, we used an inattention paradigm. We presented displays of grouping organizations by collinearity or color similarity that were previously found to require attention and added blur to the figure or the background elements to generate depth perception. In addition, we presented similar displays containing grouping by blur similarity as a single cue. We hypothesized that adding blur would facilitate the segmentation of element groups due to their perceived depth, which might lead to a diminished demand for attention. Our results confirmed that blur led to depth perception, and that sharp elements were perceived as closer more frequently than blurry elements. Thus, these results provide novel evidence for depth from blur in grouping where no inference of occlusion can be derived from a border. However, although the results suggest that blur information was processed under inattention, little evidence was found for decreased attentional demands for grouping processes in the presence of blur.

The Role of Specular Reflections and Illumination in the Perception of Thickness in Solid Transparent Objects

Specular reflections and refractive distortions are complex image properties of solid transparent objects, but despite this complexity, we readily perceive the 3D shapes of these objects (e.g., glass and clear plastic). We have found in past work that relevant sources of scene complexity have differential effects on 3D shape perception, with specular reflections increasing perceived thickness, and refractive distortions decreasing perceived thickness. In an object with both elements, such as glass, the two optical properties may complement each other to support reliable perception of 3D shape. We investigated the relative dominance of specular reflection and refractive distortions in the perception of shape. Surprisingly, the ratio of specular reflection to refractive component was almost equal to that of ordinary glass and ice, which promote correct percepts of 3D shape. The results were also explained by the variance in local RMS contrast in stimulus images but may depend on overall luminance and contrast of the surrounding light field.

Form and Function in Information for Visual Perception

Visual perception involves spatially and temporally coordinated variations in diverse physical systems: environmental surfaces and symbols, optical images, electro-chemical activity in neural networks, muscles, and bodily movements—each with a distinctly different material structure and energy. The fundamental problem in the theory of perception is to characterize the information that enables both perceptual awareness and real-time dynamic coordination of these diverse physical systems. Gibson's psychophysical and ecological conception of this problem differed from that of mainstream science both then and now. The present article aims to incorporate Gibson's ideas within a general conception of information for visual perception. We emphasize the essential role of spatiotemporal form, in contrast with symbolic information. We consider contemporary understanding of surface structure, optical images, and optic flow. Finally, we consider recent evidence about capacity limitations on the rate of visual perception and implications for the ecology of vision.

Gibson and Pictures in Perspective: Reverse the Directions

In his extensive writing about pictures, James J. Gibson offered perspective formulae for square tiles projecting trapezoids onto a picture plane, foreshortening to zero height with distance. I reverse the claim: as distance decreases, the trapezoids increase to infinite height, in marginal distortion, or forelengthening. I also reverse the direction of projection. Usually considered to be incoming, from the distant tile to the picture plane, in reverse—outgoing—the tiles have directions from the center of projection, with implications for haptics and people who are blind. A drawing of a cube illustrates the argument. It is by an adult who is blind. It includes foreshortening, and shows directions of surfaces from a vantage point.

Gravity and Known Size Calibrate Visual Information to Time Parabolic Trajectories

Catching a ball in a parabolic flight is a complex task in which the time and area of interception are strongly coupled, making interception possible for a short period. Although this makes the estimation of time-to-contact (TTC) from visual information in parabolic trajectories very useful, previous attempts to explain our precision in interceptive tasks circumvent the need to estimate TTC to guide our action. Obtaining TTC from optical variables alone in parabolic trajectories would imply very complex transformations from 2D retinal images to a 3D layout. We propose based on previous work and show by using simulations that exploiting prior distributions of gravity and known physical size makes these transformations much simpler, enabling predictive capacities from minimal early visual information. Optical information is inherently ambiguous, and therefore, it is necessary to explain how these prior distributions generate predictions. Here is where the role of prior information comes into play: it could help to interpret and calibrate visual information to yield meaningful predictions of the remaining TTC. The objective of this work is: (1) to describe the primary sources of information available to the observer in parabolic trajectories; (2) unveil how prior information can be used to disambiguate the sources of visual information within a Bayesian encoding-decoding framework; (3) show that such predictions might be robust against complex dynamic environments; and (4) indicate future lines of research to scrutinize the role of prior knowledge calibrating visual information and prediction for action control.

Bilateral Symmetry Has No Effect on Stereoscopic Shape Judgments

A single experiment is reported that measured the apparent stereoscopic shapes of symmetric and asymmetric objects at different viewing distances. The symmetric stimuli were specifically designed to satisfy the minimal conditions for computing veridical shape from symmetry. That is to say, they depicted complex, bilaterally symmetric, plane-faced polyhedra whose symmetry planes were oriented at an angle of 45° relative to the line of sight. The asymmetric stimuli were distorted versions of the symmetric ones in which the 3D position of each vertex was randomly displaced. Prior theoretical analyses have shown that it is mathematically possible to compute the 3D shapes of symmetric stimuli under these conditions, but those algorithms are useless for asymmetric objects. The results revealed that the apparent shapes of both types of objects were expanded or compressed in depth as a function of viewing distance, in exactly the same way as has been reported in many other studies, and that the presence or absence of symmetry had no detectable effect on performance.

A Ubiquitous Illusion of Volume: Are Impressions of 3D Volume Captured by an "Additive Heuristic"?

Several empirical approaches have attempted to explain perception of 2D and 3D size. While these approaches have documented interesting perceptual effects, they fail to offer a compelling, general explanation of everyday size perception. Here, we offer one. Building on prior work documenting an "Additive Area Heuristic" by which observers estimate perceived area by summing objects' dimensions, we show that this same principle-an "additive heuristic"-explains impressions of 3D volume. Observers consistently discriminate sets that vary in "additive volume," even when there is no true difference; they also fail to discriminate sets that truly differ (even by amounts as much as 30%) when they are equated in "additive volume." These results suggest a failure to properly integrate multiple spatial dimensions, and frequent reliance on a perceptual heuristic instead.

J. J. Gibson's "Ground Theory of Space Perception"

J. J. Gibson's ground theory of space perception is contrasted with Descartes' theory, which reduces all of space perception to the perception of distance and angular direction, relative to an abstract viewpoint. Instead, Gibson posits an embodied perceiver, grounded by gravity, in a stable layout of realistically textured, extended surfaces and more delimited objects supported by these surfaces. Gibson's concept of optical contact ties together this spatial layout, locating each surface relative to the others and specifying the position of each object by its location relative to its surface of support. His concept of surface texture-augmented by perspective structures such as the horizon-specifies the scale of objects and extents within this layout. And his concept of geographical slant provides surfaces with environment-centered orientations that remain stable as the perceiver moves around. Contact-specified locations on extended environmental surfaces may be the unattended primitives of the visual world, rather than egocentric or allocentric distances. The perception of such distances may best be understood using Gibson's concept of affordances. Distances may be perceived only as needed, bound through affordances to the particular actions that require them.

Theoretical Treatment of Limitations Inherent in Simple 3D Stimuli: Triangles and the P3P Problem

Understanding the visual stimulus in a psychophysical experiment, theoretically, is critical for controlling the experiment, for interpreting the empirical results of the experiment, and for discussing the mechanisms the visual system used to get these results. This fact encourages visual scientists to use “simple” visual stimuli in their experiments. A triangle is one of the simplest stimuli that has been used by psychophysicists to study 3D perception. It has also been used to compose the polygonal meshes that represent complex 3D surfaces in computer graphics. The relationship between the shape, orientation, and retinal image of a triangle has also been studied as the Perspective-3-Point problem (P3P). In this study, the statistical properties of this relationship between the 2D retinal image of a triangle and its recovered 3D orientation were tested in a simulation experiment whose results showed that a triangle is qualitatively different from more complex shapes that have been used to recover 3D information from their retinal images. This raises an important question, namely, how many, if any, inferences about our visual system can be generalized to our perceptions in everyday life when they are based on psychophysical experiments that used very simple visual stimuli such as triangles.

A Live Experience of Four-Dimensional Structure

Selective attention is well known for 2D patterns and perceptual alternations are well established for 3D structures projected into 2D, such as the Necker cube. Here, these concepts are extended to the spatial fourth dimension in the form of the mathematical structure of the 4D hypercube. In orthographic projection, its 2D outline figure has multiple and highly dynamic percepts of up to 28 different 3D interpretations, which correspond to local 3D views of the 4D hypercube. Thus, the spontaneous operations of perceptual processing can provide direct insight into conceptual structure in the fourth dimension.

The Intersection of Visual Science and Art in Renaissance Italy

In the time of the Renaissance, a major aspect of vision science was understanding how spatial information projected to the viewpoint of the observer, that is, visual perspective, which is one of the primary cues to depth perception. Perspective representation was thus an early form of virtual reality. Although accurate perspective representation was developed earlier in the 15th century, the first analytic perspective scheme was developed by Piero della Francesca, whose chef d'oeuvre is in the Church of San Francesco, Arezzo, in which the present lecture took place. The focus of the lecture was to evaluate some of the contributions of Piero della Francesca and his 15th-century contemporaries to the visual science, art and symbolism of his era, and its significance for the perception of depth structure from two-dimensional images.

3D shape estimation in a constraint optimization neural network

One of the most important aspects of visual perception is the inference of 3D shape from a 2D retinal image of the outside world. The existence of several valid mapping functions from object to data makes this inverse problem ill-posed and therefore computationally difficult. In human vision, the retinal image is a 2D projection of the 3D world. The visual system imposes certain constraints on the family of solutions in order to uniquely and efficiently solve this inverse problem. This work specifically focused on the minimization of standard deviations of 3D angles (MSDA) for 3D perception. Our goal was to use a Deep Convolutional Neural Network based on biological principles derived from visual area V4 to achieve 3D reconstruction using constrained minimization of MSDA. We conducted an experiment using novel shapes with human subjects to collect data and test the model. The performance of the network largely agreed with how humans estimated novel 3D shapes. The results show that the constraint of MSDA in 3D shape can be implemented in a neural network and produce human-like results. Additional visual constraints can be added to the network in the future to fully test the theory of visual constraints as a basis of 3D shape perception.

The Effect of Material Properties on the Perceived Shape of Three-Dimensional Objects

Perceiving the shape of three-dimensional objects is essential for interacting with them in daily life. If objects are constructed from different materials, can the human visual system accurately estimate their three-dimensional shape? We varied the thickness, motion, opacity, and specularity of globally convex objects rendered in a photorealistic environment. These objects were presented under either dynamic or static viewing condition. Observers rated the overall convexity of these objects along the depth axis. Our results show that observers perceived solid transparent objects as flatter than the same objects rendered with opaque reflectance properties. Regional variation in local root-mean-square image contrast was shown to provide information that is predictive of perceived surface convexity.

3D shape-from-shading relies on a light source prior that does not change with age

The light-from-above prior enables observers to infer an object's three-dimensional shape-from-shading information. Young, Western adults implicitly assume the light source is placed not only above, but also to the left of, the observer. Previous evidence reached conflicting conclusions regarding the development of the assumed light source direction. In the present study, we measured the light source prior cross-sectionally in children aged 5-11 years, using an explicit shape judgement task. The light-from-above prior, and the left bias, were present as soon as children became sensitive to shading information, regardless of their age. Global processing preference was not related to the ability to perform the task. Similarly, scanning habits, as measured by reading proficiency and starting position in a cancellation task, were not related to the magnitude of the left bias. Children's ability to report shape-from-shading judgements increased with age, but age did not affect the direction of light priors. Thus, we concluded that the development of the light-from-above prior and leftward bias do not require an extended maturation period, but rather the direction of the light-source priors may be developmentally stable once measurable.

bmlTUX: Design and Control of Experiments in Virtual Reality and Beyond

Advances in virtual reality technology have made it a valuable new tool for vision and perception researchers. Coding virtual reality experiments from scratch can be difficult and time-consuming, so researchers rely on software such as Unity game engine to create and edit virtual scenes. However, Unity lacks built-in tools for controlling experiments. Existing third-party add-ins requires complicated scripts to define experiments. This can be difficult and requires advanced coding knowledge, especially for multifactorial experimental designs. In this article, we describe a new free and open-source tool called the BiomotionLab Toolkit for Unity Experiments (bmlTUX) that provides a simple interface for controlling experiments in Unity. In contrast to existing tools, bmlTUX provides a graphical interface to automatically handle combinatorics, counterbalancing, randomization, mixed designs, and blocking of trial order. The toolbox works out-of-the-box since simple experiments can be created with almost no coding. Furthermore, multiple design configurations can be swapped with a drag-and-drop interface allowing researchers to test new configurations iteratively while maintaining the ability to easily revert to previous configurations. Despite its simplicity, bmlTUX remains highly flexible and customizable, catering to coding novices and experts alike.

Abutting Objects Warp the Three-Dimensional Curvature of Modally Completing Surfaces

Binocular disparity can give rise to the perception of open surfaces or closed curved surfaces (volumes) that appear to vary smoothly across discrete depths. Here I build on my recent papers by providing examples where modally completing surfaces not only fill in from one depth layer's visible contours to another layer's visible contours within virtual contours in an analog manner, but where modally completing surface curvature is altered by the interpolation of an abutting object perceived to be connected to or embedded within that modally completing surface. Seemingly minor changes in such an abutting object can flip the interpretation of distal regions, for example, turning a distant edge (where a surface ends) into rim (where a surface bends to occlude itself) or turning an open surface into a closed one. In general, the interpolated modal surface appears to deform, warp, or bend in three-dimensions to accommodate the abutting object. These demonstrations cannot be easily explained by existing models of visual processing or modal completion and drive home the implausibility of localistic accounts of modal or amodal completion that are based, for example, solely on extending contours in space until they meet behind an occluder or in front of "pacmen." These demonstrations place new constraints on the holistic surface and volume generation processes that construct our experience of a three-dimensional world of surfaces and objects under normal viewing conditions.

Factors Influencing the Detection of Spatially-Varying Surface Gloss

In this study, we investigate the ability of human observers to detect spatial inhomogeneities in the glossiness of a surface and how the performance in this task depends on several context factors. We used computer-generated stimuli showing a single object in three-dimensional space whose surface was split into two spatial areas with different microscale smoothness. The context factors were the kind of illumination, the object's shape, the availability of motion information, the degree of edge blurring, the spatial proportions between the two areas of different smoothness, and the general smoothness level. Detection thresholds were determined using a two-alternative forced choice (2AFC) task implemented in a double random staircase procedure, where the subjects had to indicate for each stimulus whether or not the surface appears to have a spatially uniform material. We found evidence that two different cues are used for this task: luminance differences and differences in highlight properties between areas of different microscale smoothness. While the visual system seems to be highly sensitive in detecting gloss differences based on luminance contrast information, detection thresholds were considerably higher when the judgment was mainly based on differences in highlight features, such as their size, intensity, and sharpness.

Identifying Cortical Substrates Underlying the Phenomenology of Stereopsis and Realness: A Pilot fMRI Study

Viewing a real scene or a stereoscopic image (e.g., 3D movies) with both eyes yields a vivid subjective impression of object solidity, tangibility, immersive negative space and sense of realness; something that is not experienced when viewing single pictures of 3D scenes normally with both eyes. This phenomenology, sometimes referred to as stereopsis, is conventionally ascribed to the derivation of depth from the differences in the two eye's images (binocular disparity). Here we report on a pilot study designed to explore if dissociable neural activity associated with the phenomenology of realness can be localized in the cortex. In order to dissociate subjective impression from disparity processing, we capitalized on the finding that the impression of realness associated with stereoscopic viewing can also be generated when viewing a single picture of a 3D scene with one eye through an aperture. Under a blocked fMRI design, subjects viewed intact and scrambled images of natural 3-D objects, and scenes under three viewing conditions: (1) single pictures viewed normally with both eyes (binocular); (2) single pictures viewed with one eye through an aperture (monocular-aperture); and (3) stereoscopic anaglyph images of the same scenes viewed with both eyes (binocular stereopsis). Fixed-effects GLM contrasts aimed at isolating the phenomenology of stereopsis demonstrated a selective recruitment of similar posterior parietal regions for both monocular and binocular stereopsis conditions. Our findings provide preliminary evidence that the cortical processing underlying the subjective impression of realness may be dissociable and distinct from the derivation of depth from disparity.

Perceived speed of motion in depth modulates misjudgements of approaching trajectories consistently with a slow prior

Previous studies have shown that the angle of approach is consistently overestimated for approaching (but passing-by) objects. An explanation based on a slow-motion prior has been proposed in the past to account for this bias. The mechanism relies on the (less reliable) in-depth component of the motion being more attracted towards the slow motion prior than the (more reliable) lateral component. This hypothesis predicts that faster speeds in depth will translate into a greater bias if the perception of velocity in depth follows Weber's law. Our approach is different than the one used in previous studies where perceived speed and direction were measured in different experiments. To test our hypothesis, we conducted an experiment in which participants estimated approaching angles via a pointing device, while at the same time comparing the speed of the approaching object with a lateral velocity reference. This way, we couple perceived speed with perceived trajectory for each approaching angle in the same trial. Our results show that the directional bias is larger for faster objects, which is consistent with motion in depth following Weber's law. The differential biases can be accounted for by a Bayesian model that includes a slow motion prior.

Enhancing Motion-In-Depth Perception of Random-Dot Stereograms

Random-dot stereograms have been widely used to explore the neural mechanisms underlying binocular vision. Although they are a powerful tool to stimulate motion-in-depth (MID) perception, published results report some difficulties in the capacity to perceive MID generated by random-dot stereograms. The purpose of this study was to investigate whether the performance of MID perception could be improved using an appropriate stimulus design. Sixteen inexperienced observers participated in the experiment. A training session was carried out to improve the accuracy of MID detection before the experiment. Four aspects of stimulus design were investigated: presence of a static reference, background texture, relative disparity, and stimulus contrast. Participants' performance in MID direction discrimination was recorded and compared to evaluate whether varying these factors helped MID perception. Results showed that only the presence of background texture had a significant effect on MID direction perception. This study provides suggestions for the design of 3D stimuli in order to facilitate MID perception.

Searching for invariance: Geographical and optical slant

When we move through rigid environments, surface orientations of static objects do not appear to change. Most studies have investigated the perception of optical slant which is dependent on the perspective of the observer. We investigated the perception of geographical slant, which is invariant across different viewing perspectives, and compared it to optical slant. In Experiment 1, participants viewed a 3D triangular target surface with triangular phosphorescent texture elements presented at eye level at one of 5 slants from 0° to 90°, at 0° or 40° tilt. Participants turned around to adjust a 2D line or a 3D surface to match the slant of the target surface. In Experiment 2, the difference between optical and geographical slant was increased by changing the height of the surface to be judged. In Experiment 3, target surfaces were rotated by 50° (±25°) and viewed in both a dark and lighted room. In Experiment 1, the overall pattern of judgments exhibited only slight differences between response measures. In Experiment 2, slant judgments were slightly overestimated when the surface was at a low height and at 0° tilt. We compared optical slants of the surfaces to geographical slants. While sometimes inaccurate, participants' slant judgments remained invariant across changes in viewing perspective. In Experiment 3, judgments were the same in the dark and lighted conditions. There was no effect of target motion on judgments, although variability decreased. We conclude that participants' judgments were predicted by geographical slant, not optical slant.

Front-Presented Looming Sound Selectively Alters the Perceived Size of a Visual Looming Object

In spite of accumulating evidence for the spatial rule governing cross-modal interaction according to the spatial consistency of stimuli, it is still unclear whether 3D spatial consistency (i.e., front/rear of the body) of stimuli also regulates audiovisual interaction. We investigated how sounds with increasing/decreasing intensity (looming/receding sound) presented from the front and rear space of the body impact the size perception of a dynamic visual object. Participants performed a size-matching task (Experiments 1 and 2) and a size adjustment task (Experiment 3) of visual stimuli with increasing/decreasing diameter, while being exposed to a front- or rear-presented sound with increasing/decreasing intensity. Throughout these experiments, we demonstrated that only the front-presented looming sound caused overestimation of the spatially consistent looming visual stimulus in size, but not of the spatially inconsistent and the receding visual stimulus. The receding sound had no significant effect on vision. Our results revealed that looming sound alters dynamic visual size perception depending on the consistency in the approaching quality and the front-rear spatial location of audiovisual stimuli, suggesting that the human brain differently processes audiovisual inputs based on their 3D spatial consistency. This selective interaction between looming signals should contribute to faster detection of approaching threats. Our findings extend the spatial rule governing audiovisual interaction into 3D space.

Three-Dimensional Representations of Objects in Dorsal Cortex are Dissociable from Those in Ventral Cortex

An established conceptualization of visual cortical function is one in which ventral regions mediate object perception while dorsal regions support spatial information processing and visually guided action. This division has been contested by evidence showing that dorsal regions are also engaged in the representation of object shape, even when actions are not required. The critical question is whether these dorsal, object-based representations are dissociable from ventral representations, and whether they play a functional role in object recognition. We examined the neural and behavioral profile of patients with impairments in object recognition following ventral cortex damage. In a functional magnetic resonanace imaging experiment, the blood oxygen level-dependent response in the ventral, but not dorsal, cortex of the patients evinced less sensitivity to object 3D structure compared with that of healthy controls. Consistently, in psychophysics experiments, the patients exhibited significant impairments in object perception, but still revealed residual sensitivity to object-based structural information. Together, these findings suggest that, although in the intact system there is considerable crosstalk between dorsal and ventral cortices, object representations in dorsal cortex can be computed independently from those in ventral cortex. While dorsal representations alone are unable to support normal object perception, they can, nevertheless, support a coarse description of object structural information.

Volume Completion Between Contour Fragments at Discrete Depths

Building on the modal and amodal completion work of Kanizsa, Carman and Welch showed that binocular stereo viewing of two disparate images can give rise to a percept of 3D curved, nonclosed illusory contours and surfaces. Here, it is shown that binocular presentation can also give rise to the percept of closed curved surfaces or volumes that appear to vary smoothly across discrete depths in binocularly fused images, although in fact only two binocular disparities are discretely defined between corresponding contour elements of the inducing elements. Surfaces are filled in from one depth layer's visible contours to another layer's visible contours within virtual contours that are interpolated on the basis of good contour continuation between the visible portions of contour. These single depth contour segments are taken not to arise from surface edges, as in Kanizsa's or Carman and Welch's examples, but from segments of "rim" where the line of sight just grazes a surface that continues behind and beyond the rim smoothly. When there are two or more surface-propagating contour segments, the propagated surfaces can continue away from the inferred rim, merge, and then close behind the self-occluding visible surface into an everywhere differentiable closed surface or volume. Illusory surfaces can possess a depth and perceived surface curvature that is consistent with all visible contour segments, despite the absence of local disparity cues at interpolated 3D surface locations far from any visible contour. These demonstrations cannot be easily explained by existing models of visual processing. They place constraints on the surface and volume generation processes that construct our 3D world under normal viewing conditions.

Dynamic Volume Completion and Deformation

A new class of dynamic volume completion is introduced, where image elements (e.g., occluding semi-ellipses placed at the edge of an object) can link across a gap between two or more objects, leading to the perception of illusory volumes that deform as those image elements are set into relative motion. These new demonstrations provide further evidence that volume completion is not dictated solely by contour relatability constraints, but is instead a dynamic process of 3D shape construction that also takes into account dynamic cues to object shape, even in the absence of any contour relatability whatsoever.

Depth Without a Surface: Observations From a "Finger Spinner" Depth Illusion

A trending novelty toy that is spun between the fingers induces a striking depth illusion from specular reflections. Further examination of the phenomenon suggests that when surface features are obscured by spinning, depth from disparity of reflections is enhanced.

Stereoscopic Slant Contrast and the Perception of Inducer Slant at Brief Stimulus Presentations

Slant contrast refers to a stereoscopic phenomenon in which the perceived slant of a test object is affected by the disparity of a surrounding inducer object. Slant contrast has been proposed to involve cue conflict, but it is unclear whether this idea is useful in explaining slant contrast at short stimulus presentations (<1 s). We measured both slant contrast and perceived inducer slant while varying the presentation duration (100-800 ms) of stereograms with several spatial configurations. In three psychophysical experiments, we found that (a) both slant contrast and perceived inducer slant increased as a function of stimulus duration, and (b) slant contrast was relatively stable across different test and inducer shapes at each short stimulus duration, whereas perceived inducer slant increased when cue conflict was reduced. These results suggest that at brief, not long stimulus presentations, the cue conflict between disparity and perspective plays a smaller role in slant contrast than other depth cues.

Correction to Figures: A Reply to Hwang and Peli (2014)

In Hwang and Peli (2014), few errors occurred in computing the angular disparities. The direction of peripheral depth distortion (the angular disparity differences between what it is in real-world 3D viewing and S3D viewing) is reversed when the computational errors were corrected, making the perception of the peripheral depth to be expanded, not compressed. This reply points to the error and provides the corrected figures. Correcting these errors does not affect the general conclusion that S3D viewed on single screen display induces peripheral depth distortion which may be a cause of visually induced motion sickness.

Parkinson's patients can rely on perspective cues to perceive 3D space

3D perception, which is necessary for an optimal navigation in our environment, relies on 2 complementary kinds of cues; binocular cues allowing precise depth localization near the point of visual interest and monocular ones that are necessary for correct global perception of visual space. Recent studies described deficient binocular 3D vision in PD patients; here we tested 3D vision in PD patients when based on monocular cues (m3D). Sixteen PD patients and 16 controls had to categorize visual stimuli as perceived in 2D (flat) or 3D (with depth). Both performance and response times were measured. EEGs were recorded to extract Visual Evoked Potentials. Effects of PD were tested by comparing psychometric and electrophysiological data obtained in controls and PD patients evaluated without dopaminergic treatment. Effects of Levodopa were tested by comparing data in PD patients with and without dopaminergic treatment. We didn't find statistical differences between PD patients and controls' performance. Severity of PD (UPDRS III) in OFF condition is positively correlated with P1 amplitudes and latencies for both 2D and m3D stimuli. Levodopa administration didn't modify either PD patients' performances although it increases principal visual components latencies for both 2D and m3D stimuli. Unlike binocular 3D vision, monocular 3D vision does not seem to get affected by PD. However given the correlation between severity of PD and VEPs' modifications, alteration of visual cortical processing might have nonetheless begun. PD patients reporting trouble in perceiving space must rely more on m3D cues present in the environment.

Turbine Blade Illusion

In January 2017, a large wind turbine blade was installed temporarily in a city square as a public artwork. At first sight, media photographs of the installation appeared to be fakes – the blade looks like it could not really be part of the scene. Close inspection of the object shows that its paradoxical visual appearance can be attributed to unconscious assumptions about object shape and light source direction.

Aging and Haptic-Visual Solid Shape Matching

A total of 36 younger (mean age = 21.3 years) and older adults (mean age = 73.8 years) haptically explored plastic copies of naturally shaped objects (bell peppers, Capsicum annuum) one at a time for 7 s each. The participants' task was to then choose which of 12 concurrently visible objects had the same solid shape as the one they felt. The younger and older participants explored the object shapes using either one, three, or five fingers (there were six participants for each combination of number of fingers and age group). The outcome was different from that of previous research conducted with manmade objects. Unlike Jansson and Monaci (2006) , we found that for most objects, our participants' performance was unaffected by variations in the number of fingers used for haptic exploration. While there was no significant overall effect of the number of fingers, there was a significant main effect of age. The younger adults' shape matching performance was 48.6% higher than that of the older adults. When perceiving naturally shaped objects such as bell peppers, it appears that the usage of a single finger can be as effective as haptic exploration with a whole complement of five fingers.

Objects Versus Shadows as Influences on Perceived Object Motion

The motion trajectory of an object’s cast shadow has been shown to alter the perceived trajectory of a casting object, an effect that holds even if the cast shadow appears unrealistic. This raises the question of whether a cast shadow per se is necessary for this influence, a question that has been studied only with stationary targets. We examined the relative influence of a shadow and a spherical object on the perceived motion trajectory of an identical spherical object, using a paradigm similar to Kersten, Mamassian, and Knill's ball-in-box animation. We recorded both depth and height estimates of the perceived end-point of the target trajectory as a function of various target and context trajectories. Both shadows and objects significantly influenced the perceived trajectory of the target, though the influence of the shadow was overall stronger. We conjecture that the influence of the object reveals the assumption that similar objects moving at the same speed and in similar directions are perceived to move within the same plane, a plane subject to a fronto-parallel bias.

Stereoscopic Depth Contrast in a 3D Müller-Lyer Configuration: Evidence for Local Normalization

Depth contrast is a stereoscopic visual phenomenon in which the slant of an element is affected by that of adjacent elements. Normalization has been proposed to be a possible cause of depth contrast, but it is still unclear how depth contrast involves normalization. To address this issue, we devised stereograms consisting of a vertical test line accompanied by several inducer lines, like a three-dimensional variation of the well-known Müller-Lyer configuration. The inducer lines had horizontal binocular disparities that defined a stereoscopic slant about a horizontal axis with respect to the endpoints of the test line. The observer's task was to adjust the slant of the test line about a horizontal axis until it appeared subjectively vertical. The results of two psychophysical experiments found that slant settings were affected by the slant of local inducers, but not by the overall slant of the whole stimulus. These results suggest that, at least for line patterns, the stereo system normalizes depth locally.

The Star Wars Scroll Illusion

The Star Wars Scroll Illusion is a dynamic version of the Leaning Tower Illusion. When two copies of a Star-Wars-like scrolling text are placed side by side (with separate vanishing points), the two scrolls appear to head in different directions even though they are physically parallel in the picture plane. Variations of the illusion are shown with one vanishing point, as well as from an inverted perspective where the scrolls appear to originate in the distance. The demos highlight the conflict between the physical lines in the picture plane and perspective interpretation: With two perspective points, the scrolling texts are parallel to each other in the picture plane but not in perspective interpretation; with one perspective point, the texts are not parallel to each other in the picture plane but are parallel to each other in perspective interpretation. The size of the effect is linearly related to the angle of rotation of the scrolls into the third dimension; the Scroll Illusion is stronger than the Leaning Tower Illusion for rotation angles between 35° and 90°. There is no effect of motion per se on the strength of the illusion.

Do Individual Differences and Aging Effects in the Estimation of Geographical Slant Reflect Cognitive or Perceptual Effects?

Several individual differences including age have been suggested to affect the perception of slant. A cross-sectional study of outdoor hill estimation (N = 106) was analyzed using individual difference measures of age, experiential knowledge, fitness, personality traits, and sex. Of particular note, it was found that for participants who reported any experiential knowledge about slant, estimates decreased (i.e., became more accurate) as conscientiousness increased, suggesting that more conscientious individuals were more deliberate about taking their experiential knowledge (rather than perception) into account. Effects of fitness were limited to those without experiential knowledge, suggesting that they, too, may be cognitive rather than perceptual. The observed effects of age, which tended to produce lower, more accurate estimates of hill slant, provide more evidence that older adults do not see hills as steeper. The main effect of age was to lower slant estimates; such effects may be due to implicit experiential knowledge acquired over a lifetime. The results indicate the impact of cognitive, rather than perceptual factors on individual differences in slant estimation.

Revisiting Motion Parallax as a Source of 3-D Information

There are many similarities between binocular disparity and motion parallax as sources of information about the structure and layout of 3-D objects and surfaces. The former can be thought of as a transformation that maps one eye's image onto the other while the latter is a transformation that maps the changes in one eye's image over time. There are many empirical similarities in the ways we use the two sources of information but there are also significant differences. A consideration of those differences leads to the conclusion that, rather than seeing motion parallax as a close analogue of binocular stereopsis, motion parallax is better thought of as a special case of the kinetic depth effect in which the depth order of the depicted 3-D object or surface can be disambiguated by vertical perspective information.

Design and Analysis of a Single-Camera Omnistereo Sensor for Quadrotor Micro Aerial Vehicles (MAVs)

We describe the design and 3D sensing performance of an omnidirectional stereo (omnistereo) vision system applied to Micro Aerial Vehicles (MAVs). The proposed omnistereo sensor employs a monocular camera that is co-axially aligned with a pair of hyperboloidal mirrors (a vertically-folded catadioptric configuration). We show that this arrangement provides a compact solution for omnidirectional 3D perception while mounted on top of propeller-based MAVs (not capable of large payloads). The theoretical single viewpoint (SVP) constraint helps us derive analytical solutions for the sensor’s projective geometry and generate SVP-compliant panoramic images to compute 3D information from stereo correspondences (in a truly synchronous fashion). We perform an extensive analysis on various system characteristics such as its size, catadioptric spatial resolution, field-of-view. In addition, we pose a probabilistic model for the uncertainty estimation of 3D information from triangulation of back-projected rays. We validate the projection error of the design using both synthetic and real-life images against ground-truth data. Qualitatively, we show 3D point clouds (dense and sparse) resulting out of a single image captured from a real-life experiment. We expect the reproducibility of our sensor as its model parameters can be optimized to satisfy other catadioptric-based omnistereo vision under different circumstances.

Distance perception in an open water environment: Analysis of individual differences

We investigated whether distance estimation accuracy over open water is influenced by the viewing direction of the observer. Twenty-two healthy students (9 male, 13 female) made 10 distance estimates ranging between 50 and 950 m actual distance in 2 viewing conditions: (1) from shore to boat and (2) from boat to shore. There were no consistent differences in estimation accuracy between viewing directions. The group data revealed a general tendency to underestimate actual distances (74%), but there was considerable interindividual variance (mean error of 74% ± 27%, range = 31% to 145%). A multilevel regression model of estimate accuracy suggests there were three subgroups of participants. One subgroup (N = 4, 18%) were consistent underestimaters, regardless of distance, whereas another subgroup (N = 5, 23%) consistently overestimated. However, the majority (N = 13, 59%) tended to underestimate at shorter distances (less than 400 m) and then overestimate at longer distances. These findings have important implications in survival situations in open water where an individual may need to judge an estimated distance against their perceived swimming capacity in order to self-rescue.

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.

Inferred motion perception of light sources in 3D scenes is color-blind

In everyday scenes, the illuminant can vary spatially in chromaticity and luminance, and change over time (e.g. sunset). Such variation generates dramatic image effects too complex for any contemporary machine vision system to overcome, yet human observers are remarkably successful at inferring object properties separately from lighting, an ability linked with estimation and tracking of light field parameters. Which information does the visual system use to infer light field dynamics? Here, we specifically ask whether color contributes to inferred light source motion. Observers viewed 3D surfaces illuminated by an out-of-view moving collimated source (sun) and a diffuse source (sky). In half of the trials, the two sources differed in chromaticity, thereby providing more information about motion direction. Observers discriminated light motion direction above chance, and only the least sensitive observer benefited slightly from the added color information, suggesting that color plays only a very minor role for inferring light field dynamics.

Detection of 3D curved trajectories: the role of binocular disparity

We examined the ability of observers to detect the 3D curvature of motion paths when binocular disparity and motion information were present. On each trial, two displays were observed through shutter-glasses. In one display, a sphere moved along a linear path in the horizontal and depth dimensions. In the other display, the sphere moved from the same starting position to the same ending position as in the linear path, but moved along an arc in depth. Observers were asked to indicate whether the first or second display simulated a curved trajectory. Adaptive staircases were used to derive the observers' thresholds of curvature detection. In the first experiment, two independent variables were manipulated: viewing condition (binocular vs. monocular) and type of curvature (concave vs. convex). In the second experiment, three independent variables were manipulated: viewing condition, type of curvature, and whether the motion direction was approaching or receding. In both experiments, detection thresholds were lower for binocular viewing conditions as compared to monocular viewing conditions. In addition, concave trajectories were easier to detect than convex trajectories. In the second experiment, the direction of motion did not significantly affect curvature detection. These results indicate the detection of curved motion paths from monocular information was improved when binocular information was present. The results also indicate the importance of the type of curvature, suggesting that the rate of change of disparity may be important in detecting curved trajectories.