Effects of a moving distractor object on time-to-contact judgments

The effects of moving task-irrelevant objects on time-to-contact (TTC) judgments were examined in 5 experiments. Observers viewed a directly approaching target in the presence of a distractor object moving in parallel with the target. In Experiments 1 to 4, observers decided whether the target would have collided with them earlier or later than a standard (absolute identification task). A contrast effect was observed: If the distractor arrived later than the target, it caused a bias toward early responses, relative to the condition without a distractor. The early-arriving distractor had no significant effect. The pattern of results was unaltered when potentially confounding information from individual visual cues was removed. The availability of stereoscopic information reduced the effect. The contrast effect was also observed if target and distractor were abstract geometric objects rather than simulations of real-world vehicles, rendering less likely a simple safety strategy activated by a potentially threatening distractor. Experiment 5 showed that the effect of the late-arriving distractor generalized to a prediction-motion task. The results indicate that task-irrelevant information in the background has to be considered in revision of time-to-contact theory.     

The effect of body posture on long-range time-to-contact estimation

On Earth, gravity accelerates freely moving objects downward, whereas upward-moving objects are being decelerated. Do humans take internalised knowledge of gravity into account when estimating time-to-contact (TTC, the time remaining before the moving object reaches the observer)? To answer this question, we created a motion-prediction task in which participants saw the initial part of an object's trajectory moving on a collision course prior to an occlusion. Observers had to judge when the object would make contact with them. The visual scene was presented with a head-mounted display. Participants lay either supine (looking up) or prone (looking down), suggestive of the ball either rising up or falling down toward them. Results showed that body posture had a significant effect on time-to-contact estimation, but only when occlusion times were long (2.5 s). The effect was also rather small. This lack of immediacy in the posture effect suggests that TTC estimation is initially robust toward the effect of gravity, which comes to bear only as more time is allowed for post-processing of the visual information.     

The impact of spatiotemporal sampling on time-to-contact judgments

When motion in the frontoparallel plane is temporally sampled, it is often perceived to be slower than its continuous counterpart. This finding stands in contrast to humans' ability to extrapolate and anticipate constant-velocity motion. We investigated whether this sampling bias generalizes to motion in the sagittal plane (i.e., objects approaching the observer). We employed a paradigm in which observers judged the arrival time of an oncoming object. We found detrimental effects of time sampling on both perceived time to contact and time to passage. Observers systematically overestimated the time it would take a frontally approaching object to intersect their eye plane. To rule out artifacts inherent in computer simulation, we replicated the experiment, using real objects. The bias persisted and proved to be robust across a large range of temporal and spatial variations. Energy and pooling mechanisms are discussed in an attempt to understand the effect.     

A simple cell model with dominating opponent inhibition for robust contrast detection

Ein Modell kortikaler Einfachzellen mit dominanter opponenter Inhibition zur robusten Kontrastdetektion Zusammenfassung. Im primären visuellen Pfad wird Information in zwei getrennten, komplementären Domänen repräsentiert, den on- und off-Zellen. In dieser Arbeit untersuchen wir die Interaktion von on- und off-Zellen zur Generierung der Eingabe für eine kortikale Einfachzelle. Basierend auf physiologischen Studien schlagen wir einen Mechanismus vor, bei dem eine kortikale Einfachzelle aus beiden Domänen eine Eingabe erhält, wobei die Eingabe aus dem opponenten Pfad stärker gewichtet wird. Mit diesem Mechanismus der dominanten opponenten Inhibition können Antworten von kortikalen Einfachzellen auf Hell-Dunkel-Balken simuliert werden, die im primären visuellen Kortex der Katze gemessen wurden. Bei der Verarbeitung synthetischer und natürlicher Bilder können mit dem neuen Modell schärfere Antworten und bessere Rauschunterdrückung erreicht werden. Wir geben eine stochastische Analyse der Rauschunterdrückungscharakteristika des vorgeschlagenen Mechanismus und präsentieren detaillierte numerische Simulationen mit systematischen Parametervariationen. Die Resultate zeigen, dass das Modell kortikaler Einfachzellen mit dominanter opponenter Inhibition robuster gegenüber verrauschten Eingaben wird, weitgehend unabhängig von der Stärke des Rauschens. Diese Eigenschaft ist möglicherweise der Grund für die physiologisch gemessene dominante Inhibition und für die Repräsentation von Kontrastinformation in zwei komplementären Domänen. Basierend auf diesen Ergebnissen stellen wir die Hypothese auf, dass dominante opponente Inhibition im visuellen System verwendet wird, um in verrauschten Umgebungen Kontraste robust extrahieren zu können. Summary. In the primary visual pathway, information is represented in two distinct, complementary domains, namely "on" and "off" cells. In this work we examine how on and off cells may interact to form the input to simple cell subfields. On the basis of physiological evidence, we propose a mechanism of dominating opponent inhibition, where a simple cell subfield is driven by both on and off domains, receiving more heavily weighted input from the opponent pathway. We demonstrate that the model can account for physiological data on luminance gradient reversal recorded from simple cells in cat striate cortex. Next, we use the model for the processing of synthetic and natural images, showing that sharpness of response and robustness to noise can be increased by dominating opponent inhibition. Finally, we present a stochastic analysis of the noise-suppression characteristics of the proposed mechanism accompanied by detailed simulations with systematic parameter variations. Results show that dominating opponent inhibition makes the simple cell responses more robust to noise, largely independently of the amount of noise added. This property may give a rationale for the strong inhibition measured physiologically and for the representation of contrast information in two complementary domains. On the basis of these findings, we hypothesize that the visual system uses dominating opponent inhibition in order to robustly extract features in noisy environments.     

Neural substrates of successful working memory and long-term memory formation in a relational spatial memory task

Working memory (WM) tasks may involve brain activation actually implicated in long-term memory (LTM). In order to disentangle these two memory systems, we employed a combined WM/LTM task, using a spatial relational (object-location) memory paradigm and analyzed which brain areas were associated with successful performance for either task using fMRI. Critically, we corrected for the performance on the respective memory task when analyzing subsequent memory effects. The WM task consisted of a delayed-match-to-sample task assessed in an MRI scanner. Each trial consisted of an indoor or outdoor scene in which the exact configuration of four objects had to be remembered. After a short delay (7–13 s), the scene was presented from a different angle and spatial recognition for two objects was tested. After scanning, participants received an unexpected subsequent recognition memory (LTM) task, where the two previously unprobed objects were tested. Brain activity during encoding, delay phase and probe phase was analyzed based on WM and LTM performance. Results showed that successful WM performance, when corrected for LTM performance, was associated with greater activation in the inferior frontal gyrus and left fusiform gyrus during the early stage of the maintenance phase. A correct decision during the WM probe was accompanied by greater activation in a wide network, including bilateral hippocampus, right superior parietal gyrus and bilateral insula. No voxels exhibited supra-threshold activity during the encoding phase, and we did not find any differential activity for correct versus incorrect trials in the WM task when comparing LTM correct versus LTM incorrect trials.     

Security of multi-agent systems: A case study on comparison shopping

The multi-agent-systems paradigm is becoming more and more popular as a basis for realizing net-based solutions. This development is accompanied by an increasing relevance of security issues. For instance, the potential loss of privacy and other assets is a major concern for, both merchants and customers, in Internet-based commerce and, without being properly addressed, such very legitimate concerns hamper the growth of e-commerce.This article uses a comparison-shopping scenario to introduce a general methodology for formally verifying the security of multi-agent systems. Following the approach of possibilistic information flow security, the flow of information between and within agents is restricted in order to ensure that secrets will not be disclosed to unauthorized meddlers. The security requirements for the overall system are then decomposed into requirements for the individual agents that can be verified independently from each other. Exploiting the modular structure of a multi-agent system considerably reduces the complexity of the overall security analysis. The techniques for decomposing security requirements, for verifying individual agents, and for deriving global security guarantees for the entire system from locally verified properties are all generic in the sense that they apply also to many other systems and security requirements than the ones that appear in the example scenario.     

Judging body weight from faces: the height-weight illusion

Being able to exploit features of the human face to predict health and fitness can serve as an evolutionary advantage. Surface features such as facial symmetry, averageness, and skin colour are known to influence attractiveness. We sought to determine whether observers are able to extract more complex features, namely body weight. If possible, it could be used as a predictor for health and fitness. For instance, facial adiposity could be taken to indicate a cardiovascular challenge or proneness to infections. Observers seem to be able to glean body weight information from frontal views of a face. Is weight estimation robust across different viewing angles? We showed that participants strongly overestimated body weight for faces photographed from a lower vantage point while underestimating it for faces photographed from a higher vantage point. The perspective distortions of simple facial measures (e.g., width-to-height ratio) that accompany changes in vantage point do not suffice to predict body weight. Instead, more complex patterns must be involved in the height-weight illusion.     

Visual discrimination thresholds for time to arrival

In a seminal article, Todd (Journal of Experimental Psychology: Human Perception and Performance 7:795-810, 1981) reported a difference threshold of about 50 ms to discriminate the times of arrival of two differently sized objects that simultaneously approached head-on at constant but different velocities. Subsequent investigators, however, have often found much higher thresholds. We did one complete replication of Todd’s experiment, and then modified his stimuli and experimental regime, which we hypothesized may have been responsible for some of the discrepancies reported in the literature. Unlike Todd and most other researchers, we exclusively used untrained observers. Several of our participants performed almost as well as the trained observers used by Todd and others, but the performance of most of our participants fell short of this standard. Furthermore, thresholds were affected by the experimental regimes, with large differences between objects’ sizes and speeds compromising performance. Analyses of the response patterns revealed that the responses were driven mainly by the objects’ relative apparent sizes.     

Slope estimation and viewing distance of the observer

The overestimation of geographical slant is one of the most sizable visual illusions. However, in some cases estimates of close-by slopes within the range of the observer’s personal space have been found to be rather accurate. We propose that the seemingly diverse findings can be reconciled when taking the viewing distance of the observer into account. The latter involves the distance of the observer from the slope (personal space, action space, and vista space) and also the eye-point relative to the slope. We separated these factors and compared outdoor judgments to those collected with a three-dimensional (3D) model of natural terrain, which was within arm’s reach of the observer. Slope was overestimated in the outdoors at viewing distances between 2 m and 138 m. The 3D model reproduced the errors in monocular viewing; however, performance was accurate with stereoscopic viewing. We conclude that accurate slant perception breaks down as soon as the situation exits personal space, be it physically or be it by closing one eye.     

The future of workplace commitment: key questions and directions

This position paper presents the state-of-the art of the field of workplace commitment. Yet, for workplace commitment to stay relevant, it is necessary to look beyond current practice and to extrapolate trends to envision what will be needed in future research. Therefore, the aim of this paper is twofold, first, to consolidate our current understanding of workplace commitment in contemporary work settings and, second, to look into the future by identifying and discussing avenues for future research. Representative of the changing nature of work, we explicitly conceptualize workplace commitment in reference to (A) “Temporary work”, and (B) “Cross-boundary work”. Progressing from these two themes, conceptual, theoretical and methodological advances of the field are discussed. The result is the identification of 10 key paths of research to pursues, a shared agenda for the most promising and needed directions for future research and recommendations for how these will translate into practice.