Establishing the behavioural limits for countershaded camouflage

Countershading is a ubiquitous patterning of animals whereby the side that typically faces the highest illumination is darker. When tuned to specific lighting conditions and body orientation with respect to the light field, countershading minimizes the gradient of light the body reflects by counterbalancing shadowing due to illumination, and has therefore classically been thought of as an adaptation for visual camouflage. However, whether and how crypsis degrades when body orientation with respect to the light field is non-optimal has never been studied. We tested the behavioural limits on body orientation for countershading to deliver effective visual camouflage. We asked human participants to detect a countershaded target in a simulated three-dimensional environment. The target was optimally coloured for crypsis in a reference orientation and was displayed at different orientations. Search performance dramatically improved for deviations beyond 15 degrees. Detection time was significantly shorter and accuracy significantly higher than when the target orientation matched the countershading pattern. This work demonstrates the importance of maintaining body orientation appropriate for the displayed camouflage pattern, suggesting a possible selective pressure for animals to orient themselves appropriately to enhance crypsis

Is countershading camouflage robust to lighting change due to weather?

Countershading is a pattern of coloration thought to have evolved in order to implement camouflage. By adopting a pattern of coloration that makes the surface facing towards the sun darker and the surface facing away from the sun lighter, the overall amount of light reflected off an animal can be made more uniformly bright. Countershading could hence contribute to visual camouflage by increasing background matching or reducing cues to shape. However, the usefulness of countershading is constrained by a particular pattern delivering ‘optimal’ camouflage only for very specific lighting conditions. In this study, we test the robustness of countershading camouflage to lighting change due to weather, using human participants as a ‘generic’ predator. In a simulated three-dimensional environment, we constructed an array of simple leaf-shaped items and a single ellipsoidal target ‘prey’. We set these items in two light environments: strongly directional ‘sunny’ and more diffuse ‘cloudy’. The target object was given the optimal pattern of countershading for one of these two environment types or displayed a uniform pattern. By measuring detection time and accuracy, we explored whether and how target detection depended on the match between the pattern of coloration on the target object and scene lighting. Detection times were longest when the countershading was appropriate to the illumination; incorrectly camouflaged targets were detected with a similar pattern of speed and accuracy to uniformly coloured targets. We conclude that structural changes in light environment, such as caused by differences in weather, do change the effectiveness of countershading camouflage

Structures de Hodge mixtes et fibrés sur le plan projectif complexe

Le développement de la théorie de Hodge a permis une compréhension plus profonde de nombreux invariants topologiques dans le cadre de la géométrie algébrique complexe. Dans ce travail de thèse, nous nous proposons de géométriser la notion de structure de Hodge mixte (SHM). Dans ce but, nous généralisons la construction de Rees et son inverse qui permettent d'associer anneaux gradués et anneaux filtrés par une chaîne d'idéaux. Nous établissons des équivalences entre catégories d'espaces vectoriels filtrés munies de morphismes strictement compatibles et catégories de faisceaux cohérents équivariants pour l'action d'un tore. Le fait que des filtrations soient opposées se traduit géométriquement par une condition de semi-stabilité forte des fibrés associés. Cette correspondance est appliquée pour exhiber une équivalence entre la catégorie des SHMs et une catégorie de fibrés vectoriels semi-stables sur le plan projectif complexe. Nous vérifions que cette dernière catégorie est abélienne, ce qui nous donne donc une démonstration géométrique du fait que la catégorie des SHMs est abélienne, un des points de départ de la théorie de Hodge mixtes, démontré par P.Deligne. Un nouvel invariant des SHMs, le niveau de R-scindement, est alors défini et ses propriétés sont étudiées. Cet invariant est calculé pour des SHMs sur les premiers groupes de cohomologie de courbes de genre 0 et 1 possiblement singulières et non-complètes. Nous étudions aussi une version relative de la correspondance pour l'appliquer aux variations de SHMs. Cette correspondance ne fonctionne que modulo une stratification adéquate de la baseHodge theory has provided a deeper understanding of many topological invariants in complex algebraic geometry. The proposal of this thesis is to find a geometric equivalent of mixed Hodge structures (MHSs). Therefore, we generalize a construction by Rees that associates a graded ring to a ring filtered by a chain of ideals. This allows us to establish equivalences between categories of filtered vector spaces endowed with morphisms that are strictly compatible and categories of coherent sheaves that are equivariant for the action of a torus. The fact that filtrations are opposed translates into a strong semistability condition for the associated vector bundles. The equivalence is next applied to MHSs and yields an equivalence between the category of MHSs and a category of semistable vector bundles on the complex projective plane. This last category is shown to be Abelian, which provides a geometric proof that the category of MHSs is Abelian. We next define a new invariant of MHSs, the R-split level, and study its properties. We compute this invariant for singular and non-complete curves of genus 0 and 1. We study a relative version of the equivalence, which aims at describing variations of MHSs geometrically. This correspondence only works provided a good stratification of the base is chosen

The built environment and its patterns – a view from the vision sciences

Visual patterns are all around us. Despite overwhelming evidence from the visual sciences that some visual patterns, in particular highly geometric and repetitive patterns, can be aversive, patterns in our visual environment are rarely considered with regard to their impact on brain, behaviour, and well-being. Yet, attempts toward developing healthier, more inclusive cities recently attracted negative headlines, for example for their use of dazzling floor patterns in public spaces that lead to discomfort, avoidance behaviours and falls, particularly in older citizens. Recent developments in analysis now allow us to measure and predict adverse effects of patterns in the real world. Here, we show that aversive patterns are rare in natural scenes but prevalent in modern man-made settings. They occur at every spatial scale, partly because of modular construction, partly because of artistic expression. We review the evidence that visual discomfort and other adverse neurological and behavioural effects arise from aversive patterns, and hypothesise that this is because of the way our visual system has evolved to analyse scenes from nature. We finish our review with an outlook for future research and by proposing some simple ways of preventing adverse effects from visual environments, using urban design as example

Chromatic induction in migraine

Funding: This work is partially supported by the Spanish Ministerio de Economía, Industria y Competitividad, Gobierno de España through research project DPI2017-89867-C2-1-R, by the Agen- cia de Gestió d’Ajuts Universitaris i de Recerca (AGAUR) through 2017-SGR-649, and CERCA Programme/Generalitat de Catalunya.The human visual system is not a colorimeter. The perceived colour of a region does not only depend on its colour spectrum, but also on the colour spectra and geometric arrangement of neighbouring regions, a phenomenon called chromatic induction. Chromatic induction is thought to be driven by lateral interactions: the activity of a central neuron is modified by stimuli outside its classical receptive field through excitatory–inhibitory mechanisms. As there is growing evidence of an excitation/inhibition imbalance in migraine, we compared chromatic induction in migraine and control groups. As hypothesised, we found a difference in the strength of induction between the two groups, with stronger induction effects in migraine. On the other hand, given the increased prevalence of visual phenomena in migraine with aura, we also hypothesised that the difference between migraine and control would be more important in migraine with aura than in migraine without aura. Our experiments did not support this hypothesis. Taken together, our results suggest a link between excitation/inhibition imbalance and increased induction effects.Publisher PDFPeer reviewe

A mechanistic account of visual discomfort

Much of the neural machinery of the early visual cortex, from the extraction of local orientations to contextual modulations through lateral interactions, is thought to have developed to provide a sparse encoding of contour in natural scenes, allowing the brain to process efficiently most of the visual scenes we are exposed to. Certain visual stimuli, however, cause visual stress, a set of adverse effects ranging from simple discomfort to migraine attacks, and epileptic seizures in the extreme, all phenomena linked with an excessive metabolic demand. The theory of efficient coding suggests a link between excessive metabolic demand and images that deviate from natural statistics. Yet, the mechanisms linking energy demand and image spatial content in discomfort remain elusive. Here, we used theories of visual coding that link image spatial structure and brain activation to characterize the response to images observers reported as uncomfortable in a biologically based neurodynamic model of the early visual cortex that included excitatory and inhibitory layers to implement contextual influences. We found three clear markers of aversive images: a larger overall activation in the model, a less sparse response, and a more unbalanced distribution of activity across spatial orientations. When the ratio of excitation over inhibition was increased in the model, a phenomenon hypothesised to underlie interindividual differences in susceptibility to visual discomfort, the three markers of discomfort progressively shifted toward values typical of the response to uncomfortable stimuli. Overall, these findings propose a unifying mechanistic explanation for why there are differences between images and between observers, suggesting how visual input and idiosyncratic hyperexcitability give rise to abnormal brain responses that result in visual stress