Paul George Lovell

Paul George Lovell
Abertay University | UAD · Social and Health Sciences, Division of Psychology

BSc, MSc, PhD

About

76
Publications
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Publications

Publications (76)
Preprint
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Animal warning signals show remarkable diversity, yet subjectively appear to share visual features that make defended prey stand out and look different from more cryptic palatable species. Here we develop and apply a computational model that emulates avian visual processing of pattern and colour to Lepidopteran wing patterns to show that warning si...
Article
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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...
Article
Full-text available
Disruptive colouration (DC) is a form of camouflage comprised of areas of pigmentation across a target’s surface that form false edges, which are said to impede detection by disguising the outline of the target. In nature, many species with DC also exhibit edge enhancement (EE); light areas have lighter edges and dark areas have darker edges. EE DC...
Article
Full-text available
Disruptive camouflage features contrasting areas of pigmentation across the animals' surface that form false edges which disguise the shape of the body and impede detection. In many taxa these false edges feature local contrast enhancement or edge enhancement, light areas have lighter edges and dark areas have darker edges. This additional quality...
Article
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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 hen...
Article
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We report a confusing stimulus which demonstrates the power of local interpretation of three-dimensional structure to disrupt a coherent global perception.
Article
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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 th...
Article
Full-text available
Disruptive colouration is a visual camouflage composed of false edges and boundaries. Many disruptively camouflaged animals feature enhanced edges; light patches are surrounded by a lighter outline and/or a dark patches are surrounded by a darker outline. This camouflage is particularly common in amphibians, reptiles and lepidopterans. We explored...
Article
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...
Article
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No single hypothesis is likely to explain the diversity in eggshell coloration and patterning across birds, suggesting that eggshell appearance is most likely to have evolved to fulfill many nonexclusive functions. By controlling for nonindependent phylogenetic associations between related species, we describe this diversity using museum eggshells...
Article
Countershading (darker coloration on the part of the body exposed to a greater light intensity) is widespread in the animal kingdom and has received constant attention by evolutionary biologists since the 19th century. It may serve several different functions, camouflage being one. Here, the gradient of coloration on the body compensates the gradie...
Article
One of the most widespread patterns of colouration in the animal kingdom is countershading, a gradation of colour in which body parts that face a higher light intensity are darker. Countershading may help counterbalance the shadowing created by directional light, and, hence, reduce 3D object recognition via shape-from-shading. There is evidence tha...
Data
Appendix S1. Independence of the results on the choice of reference orientation. Appendix S2. Compatibility of the three selective pressures for animals limited to horizontal orientations. Appendix S3. Studies showing non‐random orientation with respect to the sun.
Article
Egg camouflage may explain the adaptive significance of avian eggshell pigmentation in ground-nesting species. Eggshell maculation (spots) is predominantly due to protoporphyrin, but both biliverdin (antioxidant) and protoporphyrin (pro-oxidant) may be present in spotted eggshells. Because of their role in oxidative stress, the deposition of eggshe...
Article
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, countershad...
Article
Full-text available
Human ovulation is not advertised, as it is in several primate species, by conspicuous sexual swellings. However, there is increasing evidence that the attractiveness of women's body odor, voice, and facial appearance peak during the fertile phase of their ovulatory cycle. Cycle effects on facial attractiveness may be underpinned by changes in faci...
Article
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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...
Article
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Shape-from-shading is a ubiquitous cue informing object identification and depth judgements. Cast-shadows contribute towards these judgements (see Mammassian, Knill and Kersten, 1998). A number of studies have reported that search-times for inconsistent shadows vary according to whether the scene is presented as-if illuminated from above or below....
Article
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Animal camouflage is a paradigmatic example of evolution by natural selection, and can only be understood in terms of adaptation to deceive the brains of potential predators and to the environment. Counter-shading is a very common pattern of coloration in the animal kingdom. Countershaded animals are darker on the side that faces a greater light in...
Article
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Reproduction is a critical period for birds as they have to cope with many stressful events. One consequence of an acute exposure to stress is the release of corticosterone, the avian stress hormone. Prolonged stress can have negative impacts on the immune system, resulting in, for example, increased oxidative stress. Through maternal effects, fema...
Article
Camouflage is conferred by background matching and disruption, which are both affected by microhabitat [1]. However, microhabitat selection that enhances camouflage has only been demonstrated in species with discrete phenotypic morphs [2, 3]. For most animals, phenotypic variation is continuous [4, 5]; here we explore whether such individuals can s...
Article
With sponsorship from Network Rail, Human Engineering Limited and the University of Bristol conducted a programme of work which aimed to assess the feasibility of, and subsequently develop, a visibility assessment tool to support Network Rail's sign and signal sighting work on the railway. This paper outlines the development of this visibility tool...
Article
Studies investigating cue-combinations between binocular disparity and shading tend to adopt either forced-choice or method-of-adjustment paradigms. The results of such studies are constrained by the limitations and demand characteristics of each approach. For example, forced-choice paradigms do not allow perceptual biases to be explored; method of...
Article
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The colourful appearance of bird eggshells has long fascinated biologists and considerable research effort has focused on the structure and biochemistry of the avian eggshell matrix. The presence of tetrapyrrole pigments was identified nearly a century ago. Surprisingly, how the concentrations of avian eggshell pigments vary among related species,...
Article
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Attempts to measure cue combination at very brief stimulus durations are rare. Previous studies with static (2 s duration) stimuli have demonstrated efficient (maximum likelihood estimate—MLE) cue combination between shading and binocular disparity (SVG 2011, JOV 2012). Here, we present a novel methodology and additional experimental data, which en...
Article
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Lightness constancy refers to the ability to estimate an object's lightness (ie, surface reflectance), regardless of variations in the light being reflected from the object—for example, when the illumination changes or the object moves. Buckley et al (1994) observed that binocular cues improve lightness constancy. Here we explored how training impr...
Article
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We explore the relative utility of shape from shading and binocular disparity for depth perception. Ray-traced images either featured a smooth surface illuminated from above (shading-only) or were defined by small dots (disparity-only). Observers judged which of a pair of smoothly curved convex objects had most depth. The shading cue was around hal...
Article
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The diversity of eggshell colours and patterning across different birds is a fascinating example of the extended avian phenotype. The avian egg's background colours range from light green/yellow to brilliant blue and intense maroon, and with the scrawling patterns comparable to the assortment of eggs found in an Easter Bunny's basket. Here we brief...
Article
Many cues have been shown to be optimally combined for depth perception, including, in our own hands, realistically rendered shading and binocular disparity. Almost no studies, however, have explored the timing of cue combination. We therefore do not know whether particular cues are processed faster than others when both are present in a stimulus....
Article
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Cue-conflict studies have been used with considerable success to examine the mechanisms underlying the integration of different sensory cues. Our recent studies have used these methods to examine the combination of stereo-disparity and shading cues in realistically rendered stimuli. The stimuli featured circular convexities with cyclopean shading a...
Article
We are studying how people perceive naturalistic suprathreshold changes in the colour, size, shape or location of items in images of natural scenes, using magnitude estimation ratings to characterise the sizes of the perceived changes in coloured photographs. We have implemented a computational model that tries to explain observers' ratings of thes...
Article
Aim. We measured psychophysical performance of human observers for discrimination of monochrome natural images that could be either blended one into the other (Tolhurst & Tadmor, 2000) or morphed (Parraga et al, 2000) . We propose a visual-cortex based model to predict such performance. The model consists of multiple narrowband spatial filters (Cam...
Article
Simple everyday tasks, such as visual search, require a visual system that is sensitive to differences. Here we report how observers perceive changes in natural image stimuli, and what happens if objects change color, position, or identity-i.e., when the external scene changes in a naturalistic manner. We investigated whether a V1-based difference-...
Article
Quick's vector magnitude model (1974, Kybernetik 16, 65–67) suggests that the detectability of a compound visual stimulus can be estimated by nonlinearly summing the detectability of the component elements. We investigated whether this could be extended to suprathreshold tasks using complex natural images. In three discrimination experiments, obser...
Article
How do effects of crowding manifest themselves when viewing elements of natural scenes? We studied the effects of crowding in central and peripheral vision using suprathreshold discrimination experiments. Observers rated the differences between two 5.2-deg patches of natural images that were presented alone or amongst four flankers. In the central...
Article
Evidence from schematic stimuli (Rensink and Cavanagh, 2004 Perception) suggests that the detection of discrepant shadows in a visual search task is impaired in upright, but not inverted, images - hypothetically due to the discounting of shadows. The current study investigates this phenomenon with real objects and their shadows. Sixty pebbles were...
Article
“Visual difference predictors” (VDPs) are models which attempt to predict the extent to which people judge two natural scene images to be different from each other. A VDP generally samples scenes with arrays of localized, spatial-frequency and orientation-selective filters; where filters detect significant contrast differences a larger overall diff...
Article
Duncan and Humphreys (Psychological Review, 96(3), 1989) predicted that visual search efficiency would vary as a function of both target-distractor and distractor-distractor similarity. However, applying such concepts to search for targets in images containing complex, naturalistic, objects is made difficult because it is hard to quantify the degre...
Article
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Dementia with Lewy bodies (DLB) and Posterior Cortical Atrophy (PCA), the visual variant of Alzheimer's disease, are neurodegenerative diseases that present with progressive deterioration in visual perception. However, little is known about the precise nature underlying the complex visual deficits associated with both conditions. The present study...
Article
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Vision research has shown that perception of line orientation, in the fovea area, improves with line length (Westheimer & Ley, 1997). This suggests that the visual system may use spatial integration to improve perception of orientation. In the present experiments, we investigated the role of spatial integration in the perception of surface orientat...
Article
Background: Posterior Cortical Atrophy (PCA) is a rare neurodegenerative disease associated with Alzheimer's disease pathology that presents with progressive deterioration in visual perception. Patients with PCA present with complex visual impairments such as Balint syndrome, Gerstmann syndrome, simultanagnosia, neglect and topographical disorienta...
Article
Shadows may be "discounted" in human visual perception because they do not provide stable, lighting-invariant, information about the properties of objects in the environment. Using visual search, R. A. Rensink and P. Cavanagh (2004) found that search for an upright discrepant shadow was less efficient than for an inverted one. Here we replicate and...
Article
Full-text available
How does an animal conceal itself from visual detection by other animals? This review paper seeks to identify general principles that may apply in this broad area. It considers mechanisms of visual encoding, of grouping and object encoding, and of search. In most cases, the evidence base comes from studies of humans or species whose vision approxim...
Article
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Natural visual scenes are rich in information, and any neural system analysing them must piece together the many messages from large arrays of diverse feature detectors. It is known how threshold detection of compound visual stimuli (sinusoidal gratings) is determined by their components' thresholds. We investigate whether similar combination rules...
Data
Construction of stimuli; Observers; Reliability of rating measurements
Article
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How different are two images when viewed by a human observer? There is a class of computational models which attempt to predict perceived differences between subtly different images. These are derived from theoretical considerations of human vision and are mostly validated from psychophysical experiments on stimuli, such as sinusoidal gratings. We...
Article
Full-text available
When designing camouflage it is important to understand how the human visual system processes the information to discriminate the target from the background scene. A vision model has been developed to compare two images and detect differences in local contrast in each spatial frequency channel. Observer experiments are being undertaken to validate...
Article
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Illumination varies greatly both across parts of a natural scene and as a function of time, whereas the spectral reflectance function of surfaces remains more stable and is of much greater relevance when searching for specific targets. This study investigates the functional properties of postreceptoral opponent-channel responses, in particular rega...
Article
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Rayleigh scattering causes distant objects to appear bluer than their nearer counterparts. The phenomenon is exploited by landscape painters who add a blue tint to representations of more distant objects. Using calibrated cameras (Parraga, Troscianko and Tolhurst, 2002, Current Biology. 12, 483–487) we measured the chromatic properties of near and...
Article
Full-text available
Developmental stress has recently been shown to have adverse effects upon adult male song structure in birds, which may well act as an honest signal of male quality to discriminating females. However, it still remains to be shown if females can discriminate between the songs of stressed and non-stressed males. Here we use a novel experimental desig...
Conference Paper
Full-text available
How different are two images when viewed by a human observer? Such knowledge is needed in many situations including when one has to judge the degree to which a graphics representation may be similar to a high-quality photograph of the original scene. There is a class of computational models which attempt to predict such perceived differences. These...
Article
Visual context processing was examined in relation to schizotypy in a large nonclinical university population. Schizotypal status was assessed with the Schizotypal Personality Questionnaire (SPQ) [Schizophr. Bull. 17 (1991) 555]. Schizotypal (n=32) and non-schizotypal (n=37) subjects were tested on a contour integration task (where context processi...
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