Birgitta Dresp-Langley

Simultaneous Brightness and Apparent Depth from True Colors on Grey: Chevreul Revisited

We show that true colors as defined by Chevreul (1839) produce unsuspected simultaneous brightness induction effects on their immediate grey backgrounds when these are placed on a darker (black) general background surrounding two spatially separated configurations. Assimilation and apparent contrast may occur in one and the same stimulus display. We examined the possible link between these effects and the perceived depth of the color patterns which induce them as a function of their luminance contrast. Patterns of square-shaped inducers of a single color (red, green, blue, yellow, or grey) were placed on background fields of a lighter and a darker grey, presented on a darker screen. Inducers were always darker on one side of the display and brighter on the other in a given trial. The intensity of the grey backgrounds varied between trials only. This permitted generating four inducer luminance contrasts, presented in random order, for each color. Background fields were either spatially separated or consisted of a single grey field on the black screen. Experiments were run under three environmental conditions: dark-adaptation, daylight, and rod-saturation after exposure to bright light. In a first task, we measured probabilities of contrast, assimilation, and no effect in a three-alternative forced-choice procedure (background appears brighter on the left, on the right or the same). Visual adaptation and inducer contrast had no significant influence on the induction effects produced by colored inducers. Achromatic inducers produced significantly stronger contrast effects after dark-adaptation, and significantly stronger assimilation in daylight conditions. Grouping two backgrounds into a single one was found to significantly decrease probabilities of apparent contrast. Under the same conditions, we measured probabilities of the inducers to be perceived as nearer to the observer (inducers appear nearer on left, on right or the same). These, as predicted by Chevreuls law of contrast, were determined by the luminance contrast of the inducers only, with significantly higher probabilities of brighter inducers to be seen as nearer, and a marked asymmetry between effects produced by inducers of opposite sign. Implications of these findings for theories which attempt to link simultaneous induction effects to the relative depth of object surfaces in the visual field are discussed.

Principles of perceptual grouping: implications for image-guided surgery

The laws and principles which predict how perceptual qualities can be extracted from the most elementary visual signals were discovered by the Gestalt psychologists (e.g., Wertheimer, 1923; Metzger, 1930, translated and re-edited by Spillmann in 2009 and 2012, respectively). Their seminal work has inspired visual science ever since, and has led to exciting discoveries which have confirmed the Gestalt idea that the human brain would have an astonishing capacity for selecting and combining critical visual signals to generate output representations for decision making and action. This capacity of selection and integration enables the perception of form and space, and the correct estimation of relative positions, trajectories, and distances of objects represented in planar images. The Gestalt laws and principles were initially aimed at answering a single all-encompassing question: Why does the world look the way it does. They have subsequently been made operational in experimental studies (for an illustration of ongoing research see the international METHUSALEM project, coordinated by Johan Wagemans, at www.gestaltrevision.be) aimed at deepening our insights into the ways in which specific characteristics and qualities of visual configurations may determine perceptual organization and behavior at various levels of processing. Perceptual organization directly determines the ability of human observers to assess (1) which parts of an image belong together to form a unified visual object or shape, and (2) which parts should be nearer and which further away from the observer if the represented objects were seen in the real world. This opinion paper argues that the Gestalt principle of Pragnanz and the Gestalt law of good continuation address specific problems of perceptual organization with critical implications for visual interface design, and the design of image-guided surgery platforms in particular. The principle of Pragnanz relates to the general Gestalt postulate that objects in the visual field will produce the simplest and most complete perceptual solution possible under the conditions given. The Gestalt laws of perceptual organization, of which the law of good continuation is a particular example, describe the conditions under which specific perceptual solutions (groupings) are likely to occur. The question of how planar image structures are grouped into perceptual representations of figure and ground is one of the study grounds the Gestalt laws have been designed for. Figure-ground representation is a perceptual solution that enables the observer to assess which objects in the image would be likely to be nearer and which objects would be likely to be further away in a real world configuration. It is mediated by specific image cues to shape and to relative distance, involving local signals of contrast and orientation to fill in specific regions of an image and thereby enabling the perception of surfaces. The associated perceptual sensations of local contrast enhancement make visual objects in the image appear to stand in front of other objects represented in the same plane.

Neural Computation of Surface Border Ownership and Relative Surface Depth from Ambiguous Contrast Inputs

The segregation of image parts into foreground and background is an important aspect of the neural computation of 3D scene perception. To achieve such segregation, the brain needs information about border ownership; that is, the belongingness of a contour to a specific surface represented in the image. This article presents psychophysical data derived from 3D percepts of figure and ground that were generated by presenting 2D images composed of spatially disjoint shapes that pointed inward or outward relative to the continuous boundaries that they induced along their collinear edges. The shapes in some images had the same contrast (black or white) with respect to the background gray. Other images included opposite contrasts along each induced continuous boundary. Psychophysical results demonstrate conditions under which figure-ground judgment probabilities in response to these ambiguous displays are determined by the orientation of contrasts only, not by their relative contrasts, despite the fact that many border ownership cells in cortical area V2 respond to a preferred relative contrast. Studies are also reviewed in which both polarity-specific and polarity-invariant properties obtain. The FACADE and 3D LAMINART models are used to explain these data.

Wayfinding through an Unfamiliar Environment

Strategies for finding ones way through an unfamiliar environment may be helped by 2D maps, 3D virtual environments, or other navigation aids. The relative effectiveness of aids was investigated. Experiments were conducted in a large, park-like environment. 24 participants (12 men, 12 women; age range = 22–50 years; M = 32, SD = 7.4) were divided into three groups of four individuals, who explored a 2D map of a given route prior to navigation, received a silent guided tour by means of an interactive virtual representation, or acquired direct experience of the real route through a silent guided tour. Participants then had to find the same route again on their own. 12 observers were given a “simple” route with only one critical turn, and the other 12 a “complex” route with six critical turns. Compared to three people familiar with the routes, among the naïve participants, those who had a direct experience prior to navigation all found their way again on the simple and complex routes. Those who had explored the interactive virtual environment were unable to find their way on the complex route. The relative scale representation in the virtual environment may have given incorrect impressions of relative distances between objects along the itinerary, rendering important landmark information useless.

Seeing virtual while acting real: Visual display and strategy effects on the time and precision of eye-hand coordination

Effects of different visual displays on the time and precision of bare-handed or tool-mediated eye-hand coordination were investigated in a pick-and-place-task with complete novices. All of them scored well above average in spatial perspective taking ability and performed the task with their dominant hand. Two groups of novices, four men and four women in each group, had to place a small object in a precise order on the centre of five targets on a Real-world Action Field (RAF), as swiftly as possible and as precisely as possible, using a tool or not (control). Each individual session consisted of four visual display conditions. The order of conditions was counterbalanced between individuals and sessions. Subjects looked at what their hands were doing 1) directly in front of them (“natural” top-down view) 2) in top-down 2D fisheye view 3) in top-down undistorted 2D view or 4) in 3D stereoscopic top-down view (head-mounted OCULUS DK 2). It was made sure that object movements in all image conditions matched the real-world movements in time and space. One group was looking at the 2D images with the monitor positioned sideways (sub-optimal); the other group was looking at the monitor placed straight ahead of them (near-optimal). All image viewing conditions had significantly detrimental effects on time (seconds) and precision (pixels) of task execution when compared with “natural” direct viewing. More importantly, we find significant trade-offs between time and precision between and within groups, and significant interactions between viewing conditions and manipulation conditions. The results shed new light on controversial findings relative to visual display effects on eye-hand coordination, and lead to conclude that differences in camera systems and adaptive strategies of novices are likely to explain these.

Colour for Behavioural Success

Colour information not only helps sustain the survival of animal species by guiding sexual selection and foraging behaviour but also is an important factor in the cultural and technological development of our own species. This is illustrated by examples from the visual arts and from state-of-the-art imaging technology, where the strategic use of colour has become a powerful tool for guiding the planning and execution of interventional procedures. The functional role of colour information in terms of its potential benefits to behavioural success across the species is addressed in the introduction here to clarify why colour perception may have evolved to generate behavioural success. It is argued that evolutionary and environmental pressures influence not only colour trait production in the different species but also their ability to process and exploit colour information for goal-specific purposes. We then leap straight to the human primate with insight from current research on the facilitating role of colour cues on performance training with precision technology for image-guided surgical planning and intervention. It is shown that local colour cues in two-dimensional images generated by a surgical fisheye camera help individuals become more precise rapidly across a limited number of trial sets in simulator training for specific manual gestures with a tool. This facilitating effect of a local colour cue on performance evolution in a video-controlled simulator (pick-and-place) task can be explained in terms of colour-based figure-ground segregation facilitating attention to local image parts when more than two layers of subjective surface depth are present, as in all natural and surgical images.

Perceptual categories derived from Reid's Common Sense Philosophy

The 18th-century Scottish ‘common sense’ philosopher Thomas Reid argued that perception can be distinguished on several dimensions from other categories of experience, such as sensation, illusion, hallucination, mental images, and what he called ‘fancy.’ We extend his approach to eleven mental categories, and discuss how these distinctions, often ignored in the empirical literature, bear on current research. We also score each category on five properties (ones abstracted from Reid) to form a 5 × 11 matrix, and thus can generate statistical measures of their mutual dependencies, a procedure that may have general interest as illustrating what we can call ‘computational philosophy.’

Editorial: Perceptual Grouping—The State of The Art

Editorial on the Research Topic Perceptual Grouping—The State of The Art Perceptual neuroscience has identified mechanisms of perceptual grouping which account for the ways in which visual sensitivity to ordered structure and regularities expresses itself, in behavior and in the brain. The need to actively construct order, notably representations of objects in depth, is mandated as soon as visual signals reach the retina, given the occlusion of retinal signals by retinal veins and other retinal elements or blur. Multiple stages of neural processing transform fragmented signals into visual key representations of 3D scenes that can be used to control effective behaviors. Since our survival depends on our ability to pick up order in the physical world, and since we conceive the physical world as an ordered one, our perception must somehow be sensitive to order. The articles from this collection illustrate how texture dissimilarity, boundary completion, surface filling-in, and figure-ground segregation generate perceptual order. Structural regularities are required for a visual texture to be discriminated from random noise by human observers, as shown in Katkov et al., who compare the performance of human observers to those of an Ideal Observer and an Order Observer model. The Order Observer model, based on the capture of statistical regularities in textures, is shown to closely reflect the sensitivity of the human visual system. Spatiotemporal boundary formation is critical to the perception of shape boundaries and global motion, as shown here by Erlikhman and Kellman, who demonstrate boundary integration from spatially and temporally sparse transformations of texture. The visual system uses positions and times of element transformations to extract local structural regularities such as edge fragments, which then connect by interpolation processes to produce larger shapes with a spatiotemporal integration constraint of ∼80 ms between elements. Structural ambiguity and uncertainty pose a specific problem here. Perceptual grouping not only accounts for the ways in which physically defined order is processed by the visual system, but also how such order may be inferred in cases of uncertainty, and how this may give rise to the perception of illusory or phantom shapes. Roncato et al. discuss a new phenomenon in which illusory streaks irradiate from the vertexes of angular contours, connecting pairs of figures, and forming phantom net shapes that are only clearly visible in a specific figural context, as discussed here in the light of several theories of object contour detection. …