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Etudes psychophysiques des mécanismes et processus de structuration dans la perception des formes
Abstract
Review article in french language
... Spatial facilitation with contour inducers has also been found to be influenced by the alignment of targets and inducers. While contrast thresholds for a circular target are facilitated by a collinear line inducer, it has been found that the facilitatory effect diappears when an orthogonal context element, destroying perceptual alignment, is presented between the target and the inducer (Dresp, 1991, 1993). ...
... Moreover, the dependency of contrast thresholds on alignment and spatial separation was found to be invariant across different global orientations, including curves (Polat and Sagi, 1994b). In configurations with edge-like spatial inducers, increasing the gap between two collinar inducers decreases facilitatory effects on the detection of a small light increment presented within that gap (Dresp and Bonnet, 1991, 1993). Detection facilitation disappears completely at a target-inducer separation of about 1.75 deg of visual angle with edge-like inducers of constant size. ...
Psychophysical thresholds for the detection of luminance targets improve significantly when the targets are presented in a specific context of spatially separated, collinear inducing stimuli defining visual contours. This phenomenon is generally referred to as a special case of detection facilitation called spatial facilitation. Spatial facilitation has been observed with luminance-defined. achromatic stimuli on achromatic backgrounds as well as with targets and inducers defined by colour contrast. This paper reviews psychophysical results from detection experiments with human observers showing the conditions under which spatially separated contour inducers facilitate the detection of simultaneously presented target stimuli. The findings point towards two types of spatial mechanisms: (i) Short-range mechanisms that are sensitive to narrowly spaced stimuli of small size and, at distinct target locations, selective to the contrast polarity of targets and inducers. (ii) Long-range mechanisms that are triggered by longer stimuli, generate facilitation across wider spatial gaps between targets and inducers, and are insensitive to their contrast polarity. Spatial facilitation with chromatic stimuli requires a longer inducer exposure than spatial facilitation with achromatic stimuli, which is already fully effective at inducer exposures of 30 ms. This difference in temporal dynamics indicates some functional segregation between mechanisms for colour and luminance contrast in spatial coding. In general, spatially induced detection facilitation can to a large extent be explained by mechanisms involving from-short-to-long-range interactions between cortical detectors.
Shepard's concept of internalization does not suggest mechanisms which help to understand how the brain adapts to changes, how representations of a steadily changing environment are updated or, in short, how brain learning continues throughout life. Neural mechanisms, as suggested by Barlow, may prove a more powerful alternative. Brain theories such as Adaptive Resonance Theory (ART) propose mechanisms to explain how representational activities may be linked in space and time. Some predictions of ART are confirmed by psychophysical and neurophysiological data. [Barlow; Shepard]
The thresholds of human observers detecting line targets improve significantly when the targets are presented in a spatial context of collinear inducing stimuli. This phenomenon is referred to as spatial facilitation, and may reflect the output of long-range interactions between cortical feature detectors. Spatial facilitation has thus far been observed with luminance-defined, achromatic stimuli on achromatic backgrounds. This study compares spatial facilitation with line targets and collinear, edge-like inducers defined by luminance contrast to spatial facilitation with targets and inducers defined by color contrast. The results of a first experiment show that achromatic inducers facilitate the detection of achromatic targets on gray and colored backgrounds, but tend to suppress the detection of chromatic targets. Chromatic inducers facilitate the detection of chromatic targets on gray and colored backgrounds, but tend to suppress the detection of achromatic targets. Chromatic spatial facilitation appears to be strongest when inducers and background are isoluminant. The results of a second experiment show that spatial facilitation with chromatic targets and inducers requires a longer exposure duration of the inducers than spatial facilitation with achromatic targets and inducers, which is already fully effective at an inducer exposure of 30 ms only. The findings point towards two separate mechanisms for spatial facilitation with collinear form stimuli: one that operates in the domain of luminance, and one that operates in the domain of color contrast. These results are consistent with neural models of boundary and surface formation which suggest that achromatic and chromatic visual cues are represented on different cortical surface representations that are capable of selectively attracting attention. Multiple copies of these achromatic and chromatic surface representations exist corresponding to different ranges of perceived depth from an observer, and each can attract attention to itself. Color and contrast differences between inducing and test stimuli, and transient responses to inducing stimuli, can cause attention to shift across these surface representations in ways that sometimes enhance and sometimes interfere with target detection.
This article introduces an experimental paradigm to selectively probe the multipie levels of visual processing that influence the formation of object contours, perceptual boundaries, and illusory contours. The experiments test the assumption that, to integrate contour information across space and contrast sign, a spatially short-range filtering process that is sensitive to contrast polarity inputs to a spatially long-range grouping process that pools signals from opposite contrast polarities. The stimuli consisted of thin subthreshold lines, flashed upon gaps between collinear inducers which potentially enable the formation of illusory contours. The subthreshold lines were composed of one or more segments with opposite contrast polarities. The polarity nearest to the inducers was varied to differentially excite the short-range filtering process. The experimental results are consistent with neurophysiological evidence for cortical mechanisms of contour processing.and with the Boundary Contour System model, which identifies the short-range filtering process with cortical simple cells, and the long-range grouping process with cortical bipole cells.
The idea that color is a powerful cue to figure-ground organization preceded the Gestalt movement and color psychophysics by almost half a century. In 1839, well before Katz published his observations on color experience (1911), and about 12 years before color stereopsis was discovered by Brewster (1851), Chevreul published his observations on the perceptual modifications produced by the mutual proximity of colors (De la loi du contraste simultané des couleurs et de l’assortiment des objets colorés, Chevreul, 1839). He defined what later has become known as simultaneous contrast effects. This book chapter reviews recent psychophysical research into the functional role of color, luminance contrast and form geometry in the genesis of figure-ground (pictorial depth) percepts. Theoretical implications of Grossberg's FACADE theory are brought forward.
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 Chevreul's 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.
Strategies for finding one's 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 naive 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.
The figure–ground context provided by a Kanizsa square did not facilitate the identification of the orientation of a line segment by 3 observers. This finding leads to the conclusion that figures with illusory contours do not seem to engender a configural superiority effect similar to the one reported by E. Wong and N. Weisstein (see record
1983-11835-001) for E. Rubin's (1921) reversible figure. (PsycINFO Database Record (c) 2012 APA, all rights reserved)
A simple working taxonomy with three classes of pictorial
completion is proposed as an alternative to Pessoa et al.'s
classification: area, surface, and contour completion. The
classification is based on psychophysical evidence, not on the
different phenomenal attributes of the stimuli, showing that
pictorial completion is likely to involve mechanistic interactions
in the visual system at different levels of processing. Whether
the concept of “filling-in” is an appropriate metaphor
for the visual mechanisms that may underlie perceptual completion
is questioned.
The heuristic value of Pylyshyn's cognitive
impenetrability theory is questioned in this commentary, mainly
because, as it stands, the key argument cannot be challenged
empirically. Pylyshyn requires unambiguous evidence for an effect of
cognitive states on early perceptual mechanisms, which is impossible
to provide because we can only infer what might happen at these
earlier levels of processing on the basis of evidence collected at the
post-perceptual stage. Furthermore, the theory that early visual
processes cannot be modified by cognitive states implies that it
is totally pointless to try to investigate interactions between
consciousness and neurosensory processes.
Shepard's concept of internalization does not suggest mechanisms which help to understand how the brain adapts to changes, how representations of a steadily changing environment are updated or, in short, how brain learning continues throughout life. Neural mechanisms, as suggested by Barlow, may prove a more powerful alternative. Brain theories such as Adaptive Resonance Theory (ART) propose mechanisms to explain how representational activities may be linked in space and time. Some predictions of ART are confirmed by psychophysical and neurophysiological data. [Barlow; Shepard]
Lehar's Gestalt Bubble model introduces a computational approach to holistic aspects of three-dimensional scene perception. The model as such has merit because it manages to translate certain Gestalt principles of perceptual organization into formal codes or algorithms. The mistake made in this target article is to present the model within the theoretical framework of the question of consciousness. As a scientific approach to the problem of consciousness, the Gestalt Bubble fails for several reasons. This article addresses three of these: (1) the terminology surrounding the concept of consciousness is not rigorously defined; (2) it is not made evident that three-dimensional scene perception requires consciousness at all; and (3) it is not clearly explained by which mechanism(s) the “picture-in-the-head,” supposedly represented in the brain, would be made available to different levels of awareness or consciousness.
Scientific studies have shown that non-conscious stimuli and representations influence information processing during conscious experience. In the light of such evidence, questions about potential functional links between non-conscious brain representations and conscious experience arise. This article discusses neural model capable of explaining how statistical learning mechanisms in dedicated resonant circuits could generate specific temporal activity traces of non-conscious representations in the brain. How reentrant signaling, top-down matching, and statistical coincidence of such activity traces may lead to the progressive consolidation of temporal patterns that constitute the neural signatures of conscious experience in networks extending across large distances beyond functionally specialized brain regions is then explained.
This paper describes a procedure for obtaining conditional accuracy functions(CAFs) from naive observers and a restricted
number of trials. The method permits the experimenter to counter the subjects’ tendency to favor accuracy in tasks in which
stimulus discrimination is easy. Each time a block of 12 trials contains less than three errors, observers are instructed,
by means of a speed-up signal, to respond faster. The subject is continuously informed about her/his effective reaction time.
The data show that the desired speed-accuracy tradeoff was obtained within each of the 7 observers. The mean percent error
was around 25%.
In two experiments, brightness enhancement of the illusory surface in the Kanizsa square was investigated by means of a brightness
matching procedure. The results show that specific properties of the inducing elements such as size, spacing, and luminance
have effects on the matching threshold that are similar to those previously obtained in experiments on simultaneous con trast.
The data from a third experiment demonstrate that increment thresholds measured within the Kanizsa square are elevated when
the target is flashed on a position close to the inducing elements. The thresholds decrease considerably in the center of
both test and control figures (representing or not representing an illusory square). These observations suggest that low-level
mechanisms are likely to explain local brightness differences within the configurations but not global figure brightness.
In other words, local contrast seems to generate brightness information that “sketches out” surfaces at their surrounds but
does not “fill” them “in.”
In this study a new type of state observer for dynamic systems containing non-linear polynomials is proposed. The stability of the structure and also the uniform convergence of the state estimates are analyzed. The simple and efficient algebraic criteria of Naslin normal damping polynomials permit synthesis of the parameters. A series of simulations illustrates the proposed developments based on a Van der Pol equation.
The visual perception of many birds extends into the near-ultraviolet (UV) spectrum and ultraviolet is used by some to communicate. The beak horn of the King Penguin (Aptenodytes patagonicus) intensely reflects in the ultraviolet and this appears to be implicated in partner choice. In a preliminary study, we recently demonstrated that this ultraviolet reflectance has a structural basis, resulting from crystal-like photonic structures, capable of reflecting in the near-UV. The present study attempted to define the origin of the photonic elements that produce the UV reflectance and to better understand how the UV signal is optimized by their fine structure. Using light and electron microscopic analysis combined with new spectrophotometric data, we describe here in detail the fine structure of the entire King Penguin beak horn in addition to that of its photonic crystals. The data obtained reveal a one-dimensional structural periodicity within this tissue and demonstrate a direct relationship between its fine structure and its function. In addition, they suggest how the photonic structures are produced and how they are stabilized. The measured lattice dimensions of the photonic crystals, together with morphological data on its composition, permit predictions of the wavelength of reflected light. These correlate well with experimentally observed values. The way the UV signal is optimized by the fine structure of the beak tissue is discussed with regard to its putative biological role.
We measured detection of a thin vertical line (target) in the presence of a slightly thicker collinear, adjacent line (inducer). Sign and strength of contrast of the inducer were varied. Test lines could be either bright or dark. Detection thresholds were obtained through a temporal two-alternative forced-choice (2AFC) procedure with the method of constant stimuli. When target and inducer had equal contrast polarity, low thresholds of target lines were observed for low inducer contrasts and increased with increasing inducer contrast. With opposite contrast polarity of target and inducer, thresholds were high for low inducer contrasts and decreased for increasing contrast thereof. Our results support the hypothesis that cortical mechanisms with different sensitivity to the sign and strength of contrast participate in the detection facilitation of line contours.
choose mates on the basis of these characters. Organism: King penguins, Aptenodytes patagonicus. Field site: Colony of approximately 16,000 breeding pairs on Possession Island in the Crozet Archipelago, southern Indian Ocean. Methods: We measured foot-web swelling in males resulting from experimental injection of a novel mitogen (phytohaemagglutinin, PHA) and compared that measure of immunocompetence with colours of the beak spot and plumage. Conclusions: Breast plumage colour is a reliable indicator of immunocompetence in king penguins. Breast plumage colour appears to rely on the presence of pterin pigments, and the fact that pterins are implicated in immune function may underlie the honesty of this signal. Descriptors: Article Subject Terms Body conditions | Breast | Colour | Evolution | Hemagglutinins | Immunocompetence | Islands | Marine birds | Oceans | Ornamentation | Pigments | Plumage | Reproductive behaviour | Sexual selection | Article Taxonomic Terms Aptenodytes patagonicus | Article Geographic Terms PSE, Indian Ocean, Crozet Is., Possession I.
We question the ecological plausibility as a general model of cognition of van der Velde's & de Kamps's combinatorial blackboard architecture, where knowledge-binding in space and time relies on the structural rules of language. Evidence against their view of the brain and an ecologically plausible, alternative model of cognition are brought forward
Evidence from neurophysiology and from behavioral studies on non-human vertebrates and invertebrates suggests that the early processing of illusory contour information relies on adaptive visual mechanisms. The significance of such mechanisms for the survival of various species is about to become an increasingly important issue in our understanding of newly discovered phenomena of visual adaptation and perceptual learning. Neurophysiological and computational approaches together with a consistent body of evidence from visual psychophysics have helped to shed light on the mechanisms underlying contour integration, contour completion, virtual borders, and the perceptual emergence of illusory contours. This article reviews some of the major empirical evidence suggesting a common functional significance of real and illusory contours and then sets out to demonstrate how these functional aspects can be studied by finely tuned psychophysical techniques.
Thresholds for the detection of a small light target (increment thresholds), measured at the ends of white lines and small luminance edges, are lower than when the target is presented on a plain field. This facilitation effect disappears when: (1) the line-end is 'stopped' by another line with perpendicular orientation; (2) the inducing line is black instead of white; and (3) when the inducer does not carry information about orientation (e.g., a small dot). These observations suggest that polarity specific and orientation selective neural activation, extending collinearly from the inducing lines and edges, produces a local increase in visual sensitivity. The possible role of such a mechanism in contour completion and the formation of illusory contours is discussed.
The objective of this multidisciplinary communication is to investigate the popularity of virtual globes while questioning their practice in the pre-condition of spatial experience for devices currently employing virtual space prior to physical experience. Data from a comparative experiments suggest that, on a complex path, subjects who had a pre-experience in virtual space are less efficient than subjects who saw a 2D map or subjects who were given a direct tour. Analysis of GPS tracks allows insight into the nature of decision errors (wrong turns) and the navigation times in the different conditions. Solutions for improving devices for a virtual pre-experience of space are suggested. This communication concludes on the democratization of augmented reality (AR) devices and forecasts our future relationship to space, where real space and cyberspace will be merged into a mobile terminal.
The impact of human activity on the biosphere has produced a global society context in which scarcity of natural resources and risks to ecological health such as air pollution and water contamination call for new solutions that help sustain the development of human society and all life on earth. This review article begins by recalling the historical and philosophical context from which contemporary environmental engineering has arisen as a science and domain of technological development. Examples that deal with some of the core issues and challenges currently faced by the field, such as problems of scale and complexity, are then discussed. It is emphasized that the sustainability of the built environment depends on innovative architecture and building designs for optimal use and recycling of resources. To evaluate problems related to global climate change, storms, floods, earthquakes, landslides and other environmental risks, the behaviour of the natural environment needs to be taken into account. Understanding the complex interactions between the built environment and the natural environment is essential in promoting the economic use of energy and waste reduction. Finally, the key role of environmental engineering within models of sustainable economic development is brought forward.
The human visual system is capable of integrating luminance contrasts defining object contours across relatively large spatial gaps. This capacity produces facilitating effects of a contour segment on the detection of a spatially separated co-linear target segment of either positive or negative contrast sign. Bayesian theories suggest that spatial integration in human vision is governed by probabilistic inference. For contour integration, a probabilistic mechanism implies that contour segments with the same contrast sign are more likely to belong to the contour of the same object than segments with opposite contrast signs. Thus, segments with a given contrast sign should produce stronger facilitating effects for detecting co-linear targets with the same contrast sign compared to targets with the opposite contrast sign. Using the method of constant stimuli, we measured contrast thresholds of ten human observers for the detection of a line target spatially separated from a clearly visible, co-linear contour line of constant luminance. The contrast signs of target and contour lines were varied to produce four conditions representing all possible combinations. In the control condition, thresholds for detecting the target without a co-linear contour line were measured. The results show that all target/contour line combinations produce a facilitating effect on the detection of the target. Target-contour line combinations with the same contrast sign produce facilitating effects twice as strong as those produced by combinations with opposite contrast signs. This result is consistent with a probabilistic mechanism of contour integration. We show that the findings are predicted by a model where the ratio between the luminance necessary to detect the target by itself and the luminance necessary to detect the target in the presence of a co-linear contour line is weighted by a conditional probability factor.
Conceptual design relies on internal representation for cognitive manipulation of the morphological properties of real or virtual objects. This study aims at investigating the nature of the perceptual information that could be retrieved in different representation modalities for learning a complex structure. An abstract and complex object was presented to two study populations, experts and non-experts, in three different representation modalities: 2D view; digital 3D model; real object. After viewing, observers had to draw some parts of the structure into a 2D reference frame. The results reveal a considerable performance advantage of digital 3D compared with real 3D in the expert population. The results are discussed in terms of the nature of the morphological cues made available in the different representation modalities.
The links between contrast detection facilitation and so-called "pedestal effects" is discussed in the light of psychophysical experiments with square-shaped targets and inducers
Stimulus cues such as contrast, interposition, or partial occlusion critically influence the perception of apparent depth in pictorial (2-D) displays. We (Dresp, Durand, and Grossberg Spatial Vision in press) have shown recently that contrast cues may either cooperate or compete, depending on the spatial configuration, with other depth cues such as interposition or partial occlusion. The stimuli we used were briefly presented pairs of achromatic line drawings. Subjects had to decide which figure of a given pair seemed 'nearer' to them. Here, we present data from new experiments using the same experimental procedure. The stimuli were achromatic and coloured line drawings. The results show that, in the same way as achromatic contrast, colour either cooperates or competes, depending on the spatial configuration, with interposition cues and cues of partial occlusion. Colour, however, is found to be the stronger depth cue than an achromatic contrast cue of the same luminance. How the relative weight of a given depth cue may produce cooperation or competition with other cues is discussed in the light of cortical mechanisms of visual grouping.
The recent work of James Aronson and his colleagues (e.g. Aronson and Le Floc’h, 2000; Aronson et al., 2006; Clewell and Aronson, 2006) has contributed to a nascent scientific debate between ecologists and economists in the hope of promoting a more global awareness of the urgency to invest in the restoration of “natural capital” along with environmental protection, biological conservation, and ecological engineering. Such an expansion of awareness could, for example, have immediate and radical impacts on the pricing of goods and services
derived from “natural capital”, or on the cost to polluters and spoilers of the pollution and degradation for which they are responsible.
We investigated the categorical perception of photographic images of mutilated and intact human bodies in the specific context of competitive sports. Twenty photographs of mutilated and intact female and male bodies, all either actively or passively involved in competitive sports of various kinds such as swimming, marathon running, or handball, were presented in random order on a computer screen to four groups of twenty young students (ten females and ten males) each. In a two-alternative forced-choice task, observers of each group had to assign each photograph to one of two possible perceptual categories: 'beautiful' versus 'ugly', 'natural' versus 'artificial', 'familiar' versus 'strange', or 'dynamic' versus 'static'. The results show that, in general, categorical judgments of 'beautiful' are positively correlated with 'familiar', 'natural', and 'dynamic'. They also reveal that positively connoted perceptual judgments such as 'beautiful' do not depend on whether a body represented in a given image is visibly mutilated or intact, but on whether the activity represented in the image is likely to be perceived as 'natural' or as 'dynamic'. The findings suggest that the nature of subjectively connoted perceptual judgments can be predicted on the basis of specific ecological constraints, which is discussed.
To find our way through the real world, most of us rely on spatial skills developed on the basis of visual experience. Visual experience is, however, not the only knowledge source upon which spatial cognition is built, and neither the visually impaired, nor the blind necessarily have severe deficiencies in spatial ability. In an experimental field study, we examined the relative importance of tactile sensory compensation for navigating without sight through an unfamiliar urban environment in visually impaired, congenitally blind, late blind, and visually intact blindfolded individuals. The results show a statistically significant effect of the type of visual deficit on times taken to find the way from departure to destination. It is concluded that visual experience is neither a pre-requisite for spatial cognition nor for navigation success. Congenitally blind participants showed superior capability to compensate for the lack of visual input compared with sighted blindfolded or late blind individuals and are able to navigate successfully after a rapid tactile
analysis of geometric characteristics of a complex and unfamiliar environment. Different, to a large extent functionally equivalent, non-visual cognitive cues to physical space conveyed by the spoken word or touch enable the blind to navigate successfully in a complex real-world environment.
Poorly saturated colors are closer to a pure grey than strongly saturated hues and, therefore, appear less "colorful". Color saturation is effectively manipulated in the visual arts for balancing conflicting sensations and moods and for inducing the perception of relative distance in the pictorial plane. While perceptual science has proven quite clearly that the luminance contrast of any hue acts as a self-sufficient cue to relative depth in visual images, the role of color saturation in such figure-ground organization has remained unclear. We presented configurations of colored inducers on grey 'test' backgrounds to human observers. Luminance and saturation of the inducers was uniform on each trial, but varied across trials. We ran two separate experimental tasks. In a relative background perception task, the perceptual judgments indicated whether the apparent brightness of the grey test background contrasted with, assimilated to, or appeared equal (no effect) to that of a comparison background with the same luminance contrast. Contrast polarity and its interaction with color saturation affected response proportions for contrast, assimilation and no effect. In a figure-ground (relative depth) perception task, perceptual judgments indicated whether the inducers appeared to lie in front of, behind, or in the same depth with the background. Strongly saturated inducers produced larger proportions of foreground effects indicating that these inducers stand out as figure against the background. Weakly saturated inducers produced significantly larger proportions of background effects, indicating that these inducers are perceived as lying behind the backgrounds. Saturation and contrast polarity interacted on foreground and background response proportions. We infer that color saturation modulates figure-ground organization, both directly by determining relative inducer depth, and indirectly, and in interaction with contrast polarity, by affecting apparent background brightness. The results point towards a hitherto undocumented functional role of color saturation in the genesis of form, and in particular figure-ground percepts in the absence of chromatic stereopsis.
Dealing effectively with risks in complex projects is difficult and requires management interventions that go beyond simple analytical approaches. This is one finding of a major field study into risk management practices and business processes of 35 major product developments in 17 high-technology companies. Almost one-half of the contingencies that occur are not being detected before they impact project performance. Yet, the risk-impact model presented in this article shows that risk does not affect all projects equally but depends on the effectiveness of collective managerial actions dealing with specific contingencies. The results of this study discuss why some organizations are more successful in detecting risks early in the project life cycle, and in decoupling risk factors from work processes before they impact project performance. The field data suggest that effective project risk management involves an intricately linked set of variables, related to work process, organizational environment, and people. Some of the best success scenarios point to the critical importance of recognizing and dealing with risks early in their development. This requires broad involvement and collaboration across all segments of the project team and its environment, and sophisticated methods for assessing feasibilities and usability early and frequently during the project life cycle. Specific managerial actions, organizational conditions, and work processes are suggested for fostering a project environment most conducive to effective cross-functional communication and collaboration among all stakeholders, a condition important to early risk detection and effective risk management in complex project situations.
Thresholds for line contrast detection(experiment2) were measured with a two-alternative
temporal forced-choice procedure as a function of the spatial position of a vertical target line with regard to two co-linear context lines. The different spatial positions of the target line corresponded to values near the position discrimination threshold (experiment 1) refl ecting the just detectable lateral offset, or non-co-linearity, between the context lines which were vertically separated by about 100 minutes of visual arc. Target and context lines were vertically separated by about 30 minutes of arc, had equal contrast polarity in one case, and opposite contrast polarity in the other. Strong line contrast detection facilitation is found at perceptually co-linear target locations. This facilitation decreases noticeably at a horizontal target offset that corresponds to the alignment threshold measured with the context lines. The effects are independent of the relative contrast polarity of target and context and, as
shown in a third experiment, also independent of both the relative length or number of lines, and the magnitude of their absolute co-axial separation. This independence seems to hold, provided individual line length and co-axial distance between lines are larger than what appears to be the lower limit of the long-range spatial domain for orientationor contour integration(i.e. 20 minutes of arc), as determined by previous studies. The ndings reported here suggest that alignment thresholds are likely to define a critical lateral boundary in long-range detection facilitation with co-linear lines. They support models of contour integration based on interactions between neural mechanisms that integrate local signals of contrast, orientation, and relative position or end-to-end alignment. Such mechanismsmay help to
explain the formation of representationsof virtual contours and object contours in human perception.
In the two experiments, the use of a psychophysical procedure of brightness/darkness cancellation shed light on interactions
between spatial arrangement and figure-ground contrast in the perceptual filling in of achromatic and colored surfaces. Achromatic
and chromatic Kanizsa squares with varying contrast, contrast polarity, and inducer spacing were used to test how these factors
interact in the perceptual filling in of surface brightness or darkness. The results suggest that the neuronal processing
of surfaces with apparent contrast, leading to figure-ground segregation (i.e., perceptual organization), is governed by mechanisms
that integrate both luminance contrast and spatial information carried by the inducing stimuli, while discarding information
on contrast polarity or color. The findings are discussed in relation to earlier observations on brightness assimilation and
contrast. They support theories of nonantagonistic neural mechanisms suppressing local contrast or color signs in brightness-based
figure-ground percepts. Such mechanisms might be necessary to cancel potentially conflicting polarities in geometrically complex
visual stimuli so that perceptual filling in resulting in the most plausible representation of figure and ground can be achieved.
The functional characteristics of the spatial regime of long-range interaction between collinear orientations are revealed at coaxial separations of 15 min of arc or more between orientations. At these separations, the contrast detection of oriented targets is facilitated by collinear orientations regardless of the contrast polarity or contrast intensity of the stimuli (Tzvetanov and Dresp, 2002 Vision Research 42 2493 - 2498). Whether background luminance affects these facilitating long-range interactions is not known. We measured contrast thresholds for the detection of line targets separated from collinear inducing lines by a gap of about 20 min of arc as a function of the contrast polarity of the stimuli and the luminance of the background on which they were presented. The effects of dark and bright inducing lines with identical Michelson contrast were tested. Interactions between the effect of contrast polarity and the effect of background luminance on long-range spatial interactions were found. When the target line and the inducing line have the same contrast polarity, facilitating effects independent of background luminance were observed. When the target and the inducing line have opposite contrast polarity, the effects of the inducing line on the contrast detection of the target line were found to vary with background luminance. The asymmetry between effects found on dark and light backgrounds with inducing lines of equivalent Michelson contrast suggests that interactions between ON and OFF pathways influence long-range spatial interactions between orientations.
Whether visual repetition priming by shape or colour occurs automatically or requires top - down processing is a debated issue. We investigated effects of shape and colour of primes on the identification of targets as a function of the instructions given. Four independent groups of ten observers were tested. Group 1 had to identify the shape of a target following a prime and was explicitly told that the prime preceding the target could be relevant for the task. Group 2 had to identify the colour of a target following a prime and was explicitly told that the prime preceding the target could be relevant for the task. Group 3 was given no further instructions than to identify the colour of the second of two successive stimuli. Group 4 was given no further instructions than to identify the shape of the second of two successive stimuli. Primes and targets could be congruent in regard to shape and colour, shape only, colour only, or completely non-congruent. In neutral trials, no prime preceded the target. Different conditions with a constant number of trials were presented in random order. Shape-congruent primes and targets produced significantly shorter response times (RTs) compared with neutral trials in the explicitly instructed group identifying target shape. Colour-congruent primes and targets produced significantly shorter RTs compared with neutral trials in both explicitly and implicitly instructed groups identifying target colour. Shape priming thus requires explicit instructions. Repetition priming by colour does not, occurs automatically, and is explained by spatiotemporal probability summation. Shape priming implies top - down matching (Grossberg, 1999 Consciousness and Cognition 8 1 - 44) of prime - target relationships where top - down expectation signals activated by the prime are matched to bottom - up signals activated by the target.
Spatial interactions between collinear orientation stimuli such as lines or Gabor patches may facilitate the contrast detection of visual targets. Such interactions are assumed to reveal mechanisms that integrate visual signals across space, and may be essential to the generation of representations of visual contour over smaller (short-range) and larger (long-range) gaps in the physical stimulus. We present a quantitative model for both short-range and long-range line contrast detection facilitation with collinear target and context lines. The spatial boundaries of short-range and long-range domains are defined on the basis of empirical studies on spatially induced contrast detection facilitation, also called spatial facilitation. Psychophysical data measuring contrast detection thresholds with a temporal 2AFC procedure as a function of the coaxial distance separating collinear target and context lines were recorded. A sigmoid function was found to provide the best fit describing transitions between short-range and long-range spatial regimes in line contrast detection performance of trained human observers. The function predicts facilitating effects for spatial separations up to 2.5 deg between collinear targets and inducing lines, which is consistent with several earlier findings on this kind of spatial facilitation. The possible interpretation of our findings in terms of interactions between orientation-selective neural mechanisms is discussed.
We present a biologically plausible neural network model for long-range contour integration based on current knowledge about neural mechanisms with orientation selectivity in the primate visual cortex. The network simulates diffusive cooperation between cortical neurons in area V1. Recent neurophysiological evidence suggests that the main functional role of visual cortical neurons, which is the processing of orientation and contour in images and scenes, seems to be fulfilled by long-range interactions between orientation selective neurons [5]. These long-range interactions would explain how the visual system is able to link spatially separated contour segments, and to build up a coherent representation of contour across spatial separations via cooperation between neurons selective to the same orientation across collinear space. The network simulates long-range interactions between orientation selective cortical neurons via 9 partially connected layers: one input layer, four layers selecting image input in the orientation domain by simulating orientation selectivity in primate visual cortex V1 for horizontal, vertical, and oblique orientations, and four connected layers generating diffusion-cooperation between like-oriented outputs from layers 2, 3, 4, and 5. The learning algorithm uses standard backpropagation, all processing stages after learning are strictly feed-forward. The network parameters provide an excellent fit for psychophysical data collected from human observers demonstrating effects of long-range facilitation for the detection of a target orientation when the target is collinear with another orientation. Long-range detection facilitation is predicted by the network's diffusive behavior for spatial separations up to 2.5 degrees of visual angle between collinear orientations.
The spatial facilitation effects of collinear stimuli with luminance contrast and isoluminant color contrast are compared. We find that isoluminant color displays do not facilitate the contrast detection of achromatic, colllinear lines. It is concluded that the color pathways in the human visual system do not contribute to spatial facilitation effects, which are known to reflect the very early genesis of form perception in the brain.
Various psychophysical studies have shown that the contrast detection of line targets is facilitated by spatially separated, collinear, suprathreshold inducing lines or edges. This particular case of detection facilitation is now referred to as 'spatial facilitation' (Yu and Levi, 1997 Vision Research 37 3117 - 3127; Wehrhahn and Dresp, 1998 Vision Research 38 423 - 428). It is still unclear whether the visual mechanisms underlying spatial facilitation with collinear stimuli are connected to those underlying the visual coding of the relative spatial position of the target line with regard to the inducing line. We approached this question in psychophysical experiments with human observers measuring (i) the effect of the lateral separation of a line target and two collinear inducing lines on spatial facilitation, and (ii) the positional threshold at which a lateral displacement (to the left or to the right) of one of two inducing lines is detected. The length of the line stimuli, their spatial separation, and the exposure duration (30 ms) were identical in the two tasks. The results show that spatial facilitation disappears at a lateral separation between target and inducers that corresponds exactly to the spatial position where a lateral displacement between the two inducing lines is detected. The finding suggests that spatially induced detection facilitation and positional acuity have common underlying mechanisms. The extent to which these mechanisms may involve long-range interactions between cortical detectors is discussed.
It has been suggested (Livingstone and Hubel, 1988 Science240 740 – 749) that the ‘colour-blind’ magnocellular pathways generate the neurophysiological basis of surfaces with illusory contours since the latter do not seem to be perceived in inducing configurations of a given colour which is isoluminant with regard to the colour of the background (equiluminant colour contrast). However, psychophysical data allowing us to assess the relative visibility of illusory surfaces in coloured stimuli with luminance contrast compared to configurations with equiluminant colour contrast are not yet available.
We designed a colour-matching experiment where ten naive observers had to adjust the intensity of a red illusory surface so that it appeared to match the intensity of the red background. The configurations used were Kanizsa squares with green inducing elements, isoluminant or not with regard to the background. Isoluminance was assessed individually for each observer by means of a classical flicker test. A brightness-matching procedure was applied to configurations of achromatic inducers on a grey background. In this case, the inducers had either all the same contrast polarity (light), or both polarities (light and dark) within a given configuration. Luminance contrast in the achromatic configuration with only one polarity was the same as in the non-isoluminant colour condition. Luminance contrasts of light and dark inducers in the mixed-polarity condition were physically balanced.
The results show that the mean point of subjective equality (PSE) of the test surface corresponds to the physical intensity of the background with equiluminant colour contrast only, indicating the absence of an apparent surface in this condition. This result supports the idea that magnocellular pathways in the human visual system mediate the neurophysiological genesis of illusory surfaces. In all the other stimulus conditions, the PSE does not correspond to the physical intensity of the background. Matching ‘errors’ are significantly stronger in the achromatic conditions, but, paradoxically, strongest in the condition with balanced contrasts of opposite polarity. This finding suggests that luminance contrast is not the only determinant of the perceived strength of illusory surfaces.
Psychophysical detection thresholds were measured for small light targets superimposed on an illusory contour. Detection was considerably facilitated by the spatial context compared with conditions where the target was presented out of context. We concluded that this 'detection in context' paradigm probes early mechanisms of contour integration in the visual system.
We present results from forced-choice experiments which show that the perceived strength of illusory contours is conditionally dependent on luminance contrast and contrast polarity, suggesting at least two distinct mechanisms.
We discuss a computational model for illusory contour integration by visual cortex based on the functional properties of bi-pole detectors and cooperative neural mechanisms.
The functional implications of psychophysical observations of facilitated detection of light targets presented within contextual configurations are discussed. A possible explanation in terms of adaptive resonance mechanisms driving visual perceptual learning in the brain is given.
Psychophysical observations of facilitated contrast detection of small target lines flashed on illusory contours are discussed. An interpretation of the effects in terms of "subthreshold summation" of visual information generated by the line targets and the illusory contours in the brain is proposed.
The processes in the brain which enable humans to navigate independently through complex environments have been related to functionally identified neural structures involved in long-term memory representation. This study investigates the role of short-term memory representation, activated through brief exposure to the spatial layout of an unfamiliar complex environment. The effects of short-term memory processes on navigation performances of men and women were investigated. After two minutes of either 2D map or direct viewing (learning phase), individuals navigated from memory in Google Street View (test phase). Subjective ratings of psychological stress were recorded after testing. Satisfactory, and significantly better navigation performances compared with 2D map exposure, were found for both men and women after direct viewing. A significant effect of the sex factor was observed after 2D map exposure, where women produced the weakest performances, and the highest psychological stress ratings.We conclude that short-term memory representations of acomplex and novel spatial layout enable successful navigation, depending on the type of visual information made available through prior exposure. Direct visual experience is encoded more effectively by the brain than abstract map experience, especially by women, under the conditions investigated here.
We present a computational model for the hierarchical processing of collinear contour line segments. Visual processing of local feature detectors and grouping across collinear space is accounted for by computations which are consistent with neurophysiological data on orientation selective mechanisms in visual cortex V1.