The art of the float.

For more than 2000 years, artists have exploited cast shadows to influence how objects appear to be positioned in a scene. A contact cast shadow can anchor an object to the ground and a detached cast shadow can make an object appear to float. However, there is a period of approximately 1000 years when there were virtually no cast shadows in art. How were states of contact versus floating depicted by artists without cast shadows? Here, we survey various techniques used by artists to anchor relative position with and without cast shadows. We then conduct experimental tests of the hypothesized surface attraction principles that underlie these techniques. In the absence of cast shadows, an object (a wooden box) was often seen as resting on a surface as long as that surface offered information about ground orientation and support (a tiled floor). When the ground surface was ambiguous and cloud-like (1/f noise), the box was more likely to be seen to float. The presence of cast shadows made the box appear to contact the ground whether it was well-defined or ambiguous. Both shadows and surface support also increased the accuracy with which participants detected when the box was tilted up from the ground. These results indicate that artists long ago discovered the important power of support relationships to anchor objects to surfaces in the absence of shadows.

Lost in pandemic time: a phenomenological analysis of temporal disorientation during the Covid-19 crisis.

People have experienced many forms of temporal disorientation during the Covid-19 crisis. For this study, we collected a rich corpus of reports on the multifaceted experiences of disorientation during the pandemic. In this paper, we study the resulting corpus using a descriptive approach. We identify six emerging themes: temporal rift; temporal vertigo; impoverished time; tunnel vision; spatial and social scaffolding of time; suspended time. We offer a phenomenological analysis of each of the themes. Based on the phenomenological analysis, we draw a key distinction between episodic and existential forms of temporal disorientation, and we argue that the Covid-19 crisis is best conceptualised as a period of suspended time.

Scaling depth from shadow offset.

When an object casts a shadow on a background surface, both the offset of the shadow and the blur of its penumbra are potential cues to the distance between the object and the background. However, the shadow offset and blur are also affected by the direction and angular extent of the light source and these are often unknown. This means that the observer must make some assumptions about the illumination, the expected distribution of depth, or the relation between offset and depth in order to use shadows to make distance judgments. Here, we measure human judgments of perceived depth over a range of shadow offsets, blurs, and lighting directions to gain insight into this internal model. We find that distance judgments are relatively unaffected by blur or light direction, whereas the shadow offset has a strong and linear effect. The data are consistent with two models, a generic shadow-to-depth model and a Bayesian model.

A Slow Impossible Mirror Picture.

A new type of impossible picture is presented and described. The picture involves an object along with its reflection in a plane mirror, delivering two apparently irreconcilable views of the object itself when seen simultaneously in its flesh and in the mirror. Contrary to other, more familiar impossible pictures, its interpretation requires explicit reasoning about the represented reality. It is a slow impossible picture.

Social and temporal disorientation during the Covid-19 pandemic: An analysis of 3306 responses to a quantitative questionnaire.

The societal hallmark of the Covid-19 pandemic was a set of mitigation measures such as lockdowns and curfews. The cognitive impact on the public of the resulting spatial, social and temporal constraints is still being investigated. While pandemic time has been extensively studied and mostly described as slowed down and elongated, opposite experimental patterns across national and social contexts leave open an important explanatory gap in order to understand which factor has been causally fundamental in determining the phenomenology of the crisis. In this paper, we use a quantitative questionnaire developed for measuring temporal and social disorientation on a sample of 3306 respondents during an acute phase of restrictions in France. We show that social disorientation greatly contributed to the temporal disruptions experienced during the pandemic. This result reinforces the importance for public authorities to address the compounding effect of feeling isolated during crises.

Disorientation and time distortions during the metro commute: An analysis of 456 responses to a questionnaire distributed in real time on Twitter during traffic disruptions in the Paris area.

Public transport disruptions are conducive to disorientation narratives in which the temporal aspects of the experience are central, but it is difficult to collect psychometric data at the moment of disruption to quantify the occurring underlying feelings. We propose a new real-time survey distribution method based on travellers' interaction with disruption announcements on social media. We analyse 456 responses in the Paris area and find that travellers experience time slowing down and their destination feeling temporally farther away when undergoing traffic disruptions. Time dilation is more pronounced for people filling out the survey while still presently experiencing the disruption, suggesting that over time people remember a compressed version of their disorientation. Conflicted time feelings about the disruption, e.g., both faster and slower feelings of the passage of time, appear the longer the recollection delay. Travellers in a stopped train seem to change their itinerary not because the alternative journey feels shorter (it does not), but because it makes time pass faster. Time distortions are phenomenological hallmarks of public transport disruptions, but these distortions are poor predictors of confusion per se. Public transport operators can alleviate the time dilation experienced by their travellers by clearly stating whether they should reorient or wait for recovery when incidents occur. Our real-time survey distribution method can be used for the psychological study of crises, where a timely and targeted distribution is of paramount importance.

Experiencing a slow passage of time was an indicator of social and temporal disorientation during the Covid-19 pandemic.

Time dilation was experienced in most countries and across the several years of the Covid-19 crisis: the passage of time was deemed slower than before the pandemic, and the distance to the beginning of the pandemic seemed longer than it really was. An outstanding question is how these two aspects of time judgements relate to other temporal, social and affective disturbances. We developed and validated a 59-item questionnaire to explore these questions. 3306 participants completed the questionnaire in France in May and June 2021. Here, we analyse group differences and find that both slow passage of time and long distance judgements were associated with larger disturbances across all domains under study. These included temporal disruptions-the aptness to project oneself into the future, the sense of a rift between pre-pandemic and pandemic time, the ability to locate oneself in time, the capacity to recall the order of past events-, as well as an overall sense of social disorientation, and trauma-specific disturbances. In contrast, both fast passage of time and short distance judgements were associated with beneficial effects across all of the mentioned domains. Our results indicate that perceived passage of time and temporal distance judgements are key indicators of social and temporal disorientation.

Development and validation of a quantitative instrument for measuring temporal and social disorientation in the Covid-19 crisis.

We developed a quantitative Instrument for measuring Temporal and Social Disorientation (ITSD), aimed at major crises such as the Covid-19 pandemic. Disorientation has been identified as one of the central elements of the psychological impact of the Covid-19 era on the general public, but so far, the question has only been approached qualitatively. This paper offers an empirical, quantitative approach to the multi-faceted disorientation of the Covid-19 pandemic by operationalising the issue with the help of the ITSD. The ITSD was developed through multiple stages involving a preliminary open-ended questionnaire followed by a coder-based thematic analysis. This paper establishes the reliability and validity of the resulting ITSD using a 3-step validation process on a sample size of 3306.

Does pre-testing promote better retention than post-testing?

Compared with other learning strategies, retrieval practice seems to promote superior long-term retention. This has been found mostly in conditions where learners take tests after being exposed to learning content. However, a pre-testing effect has also been demonstrated, with promising results. This raises the question, for a given amount of time dedicated to retrieval practice, whether learners should be tested before or after an initial exposure to learning content. Our experiment directly compares the benefits of post-testing and pre-testing relative to an extended reading condition, on a retention test 7 days later. We replicated both post-testing (d = 0.74) and pre-testing effects (d = 0.35), with significantly better retention in the former condition. Post-testing also promoted knowledge transfer to previously untested questions, whereas pre-testing did not. Our results thus suggest that it may be more fruitful to test students after than before exposure to learning content.

Shadow-related concavity-convexity inversions reveal a very basic tolerance for impossible shadows.

The stimuli traditionally used for demonstrating shadow-related concavity-convexity inversions reveal a very basic tolerance for impossible shadows-namely, self shadows do not induce a visual request for geometrically mandatory cast shadows.

Some varieties of shadow illusions: split shadows, occluded shadows, stolen shadows, and shadows of shadows.

Shadows are visual objects and as such are subject to preference rules for segmenting the visual scene (such as Gestalt laws). These rules govern shadows along two dimensions: their general unity and individuation (eg they describe a certain area as a unitary shadow as opposed to a set of two distinct shadows), and their being the shadow of a given object, as opposed to being the shadow of any other object in the scene (the shadow correspondence problem Mamassian, 2004 Perception 33 1279-1290). I describe a few phenomena that indicate the interplay of preferences in determining the final visual aspect of scenes in which shadows are present.

Nonphysical visual objects generated by colour spreading are expected to cast shadows.

Purely visual objects created by colour spreading are expected to behave like any other physical object. In particular, they are expected to block light and to cast shadows.

The copycat solution to the shadow correspondence problem.

Cast shadows (henceforth simply 'shadows') depicted in artworks can fulfil some of their perceptual roles, such as helping the retrieval of 3-D structure and of relative positions of objects, without having to be geometrically or physically accurate. The visual system displays a wide tolerance for shadows that are mostly inaccurate, at times making use of the scarce accurate but relevant information that can still be retrieved from them. However, the extent of the tolerable inaccuracy is at present still unexplored. I propose that inaccuracy can be not only totally acceptable but in some cases is even likely to be preferable to accuracy if the main perceptual role of shadows is to help locate in a scene the objects that cast them. I examined a small but effective historical corpus of pictorial endeavours, from which it appears that in some cases painters have used a copycat strategy for drawing the terminator of a shadow, ie they have produced a replica of the visible profile of the caster, which in the norm yields an impossible shadow. The copycat strategy is perceptually effective for solving the correspondence problem of associating a shadow with its caster; copycatting can be more effective than other solutions, including the depiction of the geometrically correct shadow, and is complementary to coarser solutions to the correspondence problem. These phenomena provide insight into the computations effectively used by the brain to deal with space perception. In particular, the brain is not relying on some sort of simplified physical model of the world, for the shadows produced by the copycat effect would correspond to a more complicated physical situation than the one encountered in real life.

How I managed to hide my shadow.

A special type of visual stimulus is described which gives the perceiver the impression that a shadow has been hidden behind a surface in the image, contrary to expectations based on ecological situations in which covering a shadow only passes it on to the cover.

The shadow knows: a primer on the informational structure of cast shadows.

I list relevant properties of cast shadows that the visual system could exploit (although it may not exploit). The study concerns the various types of information that can be extracted from shadows by systems that are significantly like ours (including embodied artificial systems) in an environment that is significantly like ours.

On-road measurement of particle emission in the exhaust plume of a diesel passenger car.

Particle size distributions were measured under real world dilution conditions in the exhaust plume of a diesel passenger car closely followed by a mobile laboratory on a high speed test track. Under carefully controlled conditions the exhaust plume was continuously sampled and analyzed inside the mobile laboratory. Exhaust particle size distribution data were recorded together with exhaust gas concentrations, i.e., CO, CO2, and NO(x), and compared to data obtained from the same vehicle tested on a chassis dynamometer. Good agreement was found for the soot mode particles which occurred at a geometric mean diameter of approximately 50 nm and a total particle emission rate of 10(14) particles km(-1). Using 350 ppm high sulfur fuel and the standard oxidation catalyst a bimodal size distribution with a nucleation mode at 10 nm was observed at car velocities of 100 km h(-1) and 120 km h(-1), respectively. Nucleation mode particles were only present if high sulfur fuel was used with the oxidation catalyst installed. This is in agreement with prior work that these particles are of semivolatile nature and originate from the nucleation of sulfates formed inside the catalyst. Temporal effects of the occurrence of nucleation mode particles during steady-state cruising and the dynamical behavior during acceleration and deceleration were investigated.