Quite independently of what they represent, some images provoke discomfort, and even headaches and seizures in susceptible individuals. The visual system has adapted to efficiently process the images it typically experiences, and in nature these images are usually scale--invariant. In this work, we sought to characterize the images responsible for discomfort in terms of their adherence to low--level statistical properties typically seen in natural scenes. It has been conventional to measure scale invariance in terms of the one--dimensional Fourier amplitude spectrum, by averaging amplitude over orientations in the Fourier domain. However, this loses information on the evenness with which information at various orientations is represented. We therefore fitted a two--dimensional surface (regular circular cone 1/f in logarithmic coordinates) to the two--dimensional amplitude spectrum.The extent to which the cone fitted the spectrum explained an average of 18% of the variance in judgments of discomfort from images including rural and urban scenes, works of non--representational art, images of buildings and animals, and images generated from randomly disposed discs of varying contrast and size. Weighting the spectrum prior to fitting the surface to allow for the spatial frequency tuning of contrast sensitivity explained an average of 27% of the variance. Adjusting the shape of the cone to take account of the generally greater energy in horizontal and vertical orientations improved the fit, but only slightly. Taken together, our findings show that a simple measure based on first principles of efficient coding and human visual sensitivity explained more variance than previously published algorithms. The algorithm has a low computational cost and we show that it can identify the images involved in cases that have reached the media because of complaints.We offer the algorithm as a tool for designers rather than as a simulation of the biological processes involved.

Scenes from nature share in common certain statistical properties. Images with these properties can be processed efficiently by the human brain. Patterns with unnatural statistical properties are uncomfortable to look at, and are processed inefficiently, according to computational models of the visual cortex. Consistent with such putative computational inefficiency, uncomfortable images have been demonstrated to elicit a large haemodynamic response in the visual cortex, particularly so in individuals who are predisposed to discomfort. In a succession of five small-scale studies, we show that these considerations may be important in the design of the modern urban environment. In two studies we show that images from the urban environment are uncomfortable to the extent that their statistical properties depart from those of scenes from nature. In a third study we measure the haemodynamic response to images of buildings computed as having unnatural or natural statistical properties, and show that in posterior brain regions the images with unnatural statistical properties (often judged uncomfortable) elicit a haemodynamic response that is larger than for images with more natural properties. In two further studies we show that judgments of discomfort from real scenes (both shrubbery and buildings) are similar to those from images of the scenes. We conclude that the unnatural scenes in the modern urban environment are sometimes uncomfortable and place excessive demands on the neural computation involved in vision, with consequences for brain metabolism, and possibly also for health. *Blinded Manuscript with No Author Identifiers Click here to view linked References

Summary Orientation with respect to the sun has been observed in a wide range of species and has generally been interpreted in terms of thermoregulation and/or ultraviolet (UV) protection. For countershaded animals, orientation with respect to the sun may also result from the pressure to exploit the gradient of coloration optimally to enhance crypsis.Here, we use computational modelling to predict the optimal countershading pattern for an oriented body. We assess how camouflage performance declines as orientation varies using a computational model that incorporates realistic lighting environments.Once an optimal countershading pattern for crypsis has been chosen, we determine separately how UV protection/irradiation and solar thermal inflow fluctuate with orientation.We show that body orientations that could optimally use countershading to enhance crypsis are very similar to those that allow optimal solar heat inflow and UV protection.Our findings suggest that crypsis has been overlooked as a selective pressure on orientation and that new experiments should be designed to tease apart the respective roles of these different selective pressures. We propose potential experiments that could achieve this.

Countershading, the widespread tendency of animals to be darker on the side that receives strongest illumination, has classically been explained as an adaptation for camouflage: obliterating cues to 3D shape and enhancing background matching. However, there have only been two quantitative tests of whether the patterns observed in different species match the optimal shading to obliterate 3D cues, and no tests of whether optimal countershading actually improves concealment or survival. We use a mathematical model of the light field to predict the optimal countershading for concealment that is specific to the light environment and then test this prediction with correspondingly patterned model "caterpillars" exposed to avian predation in the field. We show that the optimal countershading is strongly illumination-dependent. A relatively sharp transition in surface patterning from dark to light is only optimal under direct solar illumination; if there is diffuse illumination from cloudy skies or shade, the pattern provides no advantage over homogeneous background-matching coloration. Conversely, a smoother gradation between dark and light is optimal under cloudy skies or shade. The demonstration of these illumination-dependent effects of different countershading patterns on predation risk strongly supports the comparative evidence showing that the type of countershading varies with light environment.camouflage | defensive coloration | animal coloration | shape-from-shading | shape perception M any animals, across diverse taxa and habitats, are darker on their dorsal than ventral side (1-8). One of the oldest theories of animal camouflage (9-13) suggests that this "countershading" has evolved to cancel the dorsoventral gradient of illumination across the body, thus obliterating cues to 3D form and enhancing background matching. Indeed, so common are dorsoventral gradients of pigmentation that Abbott Thayer branded his explanation as "The law which underlies protective coloration" (12). Countershading also became one of the most popular early tactics in military camouflage (14, 15). However, somewhat ironically, given that the theory was inspired by observations of nature, a role in biological camouflage remains equivocal. The present paper uses predation rate to test directly whether countershading affects detectability and the degree to which the pattern has to be tightly matched to the illumination conditions to be effective.Assessments of coat pattern in relation to positional behavior and body size in primates are consistent with the pattern functioning as camouflage (5, 16). Primate species that spend more time oriented vertically, and thus do not experience strong differential illumination between belly and back, are less intensely countershaded. The most powerful quantitative test to date used empirically derived predictions from the pattern of illumination on a model deer under different illumination conditions (1). This study found a broad correspondence between correlates of illumination and the observed coun...

Many animals have a gradation of body color, termed "countershading," where the areas that are typically exposed to more light are darker. One hypothesis is that this patterning enhances visual camouflage by making the retinal image of the animal match that of the background, a fundamentally two-dimensional theory. More controversially, countershading may also obliterate cues to three-dimensional (3D) shape delivered by shading. Despite relying on distinct cognitive mechanisms, these two potential functions hitherto have been amalgamated in the literature. It has previously not been possible to validate either hypothesis empirically, because there has been no general theory of optimal countershading that allows quantitative predictions to be made about the many environmental parameters involved. Here we unpack the logical distinction between using countershading for background matching and using it to obliterate 3D shape. We use computational modeling to determine the optimal coloration for the camouflage of 3D shape. Our model of 3D concealment is derived from the physics of light and informed by perceptual psychology: we simulate a 3D world that incorporates naturalistic lighting environments. The model allows us to predict countershading coloration for terrestrial environments, for any body shape and a wide range of ecologically relevant parameters. The approach can be generalized to any light distribution, including those underwater.

A comparison between preference judgments of curvature and 2 sharpness in architectural façades 3 4 Word count: 5.976 (including bibliography: 7.547) Curvature and sharpness in architectural façades 6 A comparison between preference judgments of curvature and 7 sharpness in architectural façades 8 Can curvature drive preference for architectural façades and their perceived 9 familiarity, complexity, stability or approachability? In this study we aimed to 10 investigate if the well-known preference for curvature can be extended to the 11 architectural domain. We generated four different versions of the same reference 12 building, varying only the amount of curvature of the façade. Twenty-four 13 participants 1) made a preference forced-choice task between pairs of stimuli; 2) 14 ranked all stimuli from the most to the least preferred; 3) evaluated each stimulus 15 on different psychological variables. Multidimensional scaling on forced choices 16 showed that the curved façade was the most preferred. Multidimensional 17 unfolding on the ranking task showed that the majority expressed higher 18 preferences for the curved facades compared to sharp-angled and rectilinear ones. 19Ratings on different psychological variables gave supporting evidence for 20 curvature significantly influencing liking and approaching judgments. We then 21 processed the stimuli with a dynamical model of the visual cortex and a model 22 that characterises discomfort in terms of adherence to the statistics of natural 23 images. Results from these image analyses matched behavioural data. We discuss 24 the implications of the findings on our understanding of human preferences, 25 which are intrinsically dynamic and influenced by context and experience. 26

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An apparent and common feature of aposematic patterns is that they contain a high level of achromatic (luminance) contrast, for example, many warning signals combine black spots and stripes with a lighter colour such as yellow. However, the potential importance of achromatic contrast, as distinct from colour contrast, in reducing predation has been largely overlooked. Here, using domestic chicks as a model predator, we manipulated the degree of achromatic contrast in warning patterns to test if high luminance contrast in aposematic signals is important for deterring naïve predators. We found that the chicks were less likely to approach and eat prey with high contrast compared to low contrast patterns. These findings suggest that aposematic prey patterns with a high luminance contrast can benefit from increased survival through eliciting unlearned biases in naïve avian predators. Our work also highlights the importance of considering luminance contrast in future work investigating why aposematic patterns take the particular forms that they do.