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.
INTRODUCTION 1599 I. THE DAWNING OF A NEW CIVIL RIGHTS ERA 1607 A. WORLD WAR II AND THE MOBILIZATION OF AFRICAN AMERICANS ..... 1607 B. THE GROWTH OF THE CIVIL RIGHTS INFRASTRUCTURE 1609 II. CIVIL RIGHTS ENFORCEMENT CHALLENGES UNDER TRUMAN 1610 A. TRUMAN’S FIRST YEAR IN OFFICE 1610 B. 1946: RACIAL VIOLENCE AND CALLS FOR JUSTICE 1612 III. THE PRESIDENT’S COMMITTEE ON CIVIL RIGHTS 1618 A. UNDER PRESSURE: TRUMAN CREATES A COMMITTEE 1618 B. CIVIL RIGHTS ENFORCEMENT POLICIES: PROPOSALS FOR REFORM....... 1625 C. THE REPORT OF THE PRESIDENT’S COMMITTEE ON CIVIL RIGHTS ........ 1635 IV. REACTION AND REVOLT: TRUMAN’S CIVIL RIGHTS AGENDA 1638 A. THE NEW AMICUS POLICY 1638 B. THE PCCR REPORT’S PUBLIC RECEPTION 1639 C. THE 1948 ELECTION 1644 D. STALEMATE IN CONGRESS 1647 E. TRUMAN’S JUDICIAL POLICY 1650 F. THE FATE OF CIVIL RIGHTS REFORM 1650 CONCLUSION 1655
Abstract 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 these naturalistic differences between image pairs. We model the action-potential firing rates of millions of neurons, having linear and non-linear summation behaviour closely modelled on real V1 neurons. The numerical parameters of the model's sigmoidal transducer function are set by optimising the same model to experiments on contrast discrimination (contrast 'dippers') on monochrome photographs of natural scenes. The model, optimised on a stimulus-intensity domain in an experiment reminiscent of the Weber–Fechner relation, then produces tolerable predictions of the ratings for most kinds of naturalistic image change. Importantly, rating rises roughly linearly with the model's numerical output, which represents differences in neuronal firing rate in response to the two images under comparison; this implies that rating is proportional to the neuronal response.
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, and whether this variability is associated with either eggshell appearance and/or species life-history traits, remains poorly understood. We quantified the concentrations of the two key eggshell pigments, protoporphyrin IX and biliverdin, from a diverse sample of eggshells stored at the Natural History Museum, Tring, UK. We explicitly tested how these two pigments are associated with physical measures of eggshell coloration and whether the pigment concentrations and colour diversity co-vary with phylogenetic affiliations among species. We also tested a series of comparative hypotheses regarding the association between the concentrations of the two pigments and specific life-history and breeding ecology traits. Across species, the average concentrations of protoporphyrin and biliverdin were positively correlated, and both strongly co-varied with phylogenetic relatedness. Controlling for phylogeny, protoporphyrin concentration was associated with a higher likelihood of cavity nesting and ground nesting, whereas biliverdin concentration was associated with a higher likelihood of non-cavity nesting habit and bi-parental provisioning. Although unlikely to be explained by a single function, the breeding ecology and life history-dependence of eggshell pigment concentrations in these comparative analyses implies that related species share pigment strategies, and that those strategies relate to broad adaptive roles in the evolution of variation in avian eggshell coloration and its underlying mechanisms.
We measured the spatiotemporal chromatic properties of the natural world using a high speed calibrated digital video camera. Our video clips, each lasting 10 seconds and gathered at 200 Hz with a stationary camera, featured a wide variety of scenes, ranging from temporal texture (such as grass blowing in the wind and waves breaking on the sea) to meaningful spatiotemporal structure (such as people communicating using British Sign Language). The raw video output was calibrated and combined to closely approximate the human luminance, red-green and blue-yellow channels (Lovell et al. 2004). By analysing the videos using the power spectrum of the 3D FFT transform, we characterised the natural world as conveyed to the visual cortex. Examination of spatial characteristics showed that the amplitudes of the various spatial frequencies are, as expected, well characterised by a 1/fn relationship with n close to 1 for the luminance channel. In the temporal domain, the overall statistics follow a 1/ωn pattern (where ω denotes temporal frequency) with values of n substantially less than 1 for all three channels. However, when examined on a video-by-video basis a markedly different temporal structure can be observed (e.g. peaks in the temporal spectrum for waves in a river at 6Hz). We note that such peaks are invariant to viewing distance and we propose that vision may use this invariant structure to extract temporal gist from a scene. The spatiotemporal sensitivities of visual organisms may well be driven by a need to capture such information optimally.
We report a confusing stimulus which demonstrates the power of local interpretation of three-dimensional structure to disrupt a coherent global perception.
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 improved lightness constancy under binocular, bi-ocular (identical images to each eye), and monocular viewing. Stimuli consisted of a diffusely illuminated Lambertian grey box, containing a single target: a faceted rectangular block that could be presented at a range of depths within the box. Stimuli were presented stereoscopically (to provide full binocular cues), monocularly, and bi-ocularly. In the experiment, participants were asked to select one of a number of blocks, presented below the box, that was of the same material as the target block. Four groups of participants received differing training regimes. Active training involved moving a block of fixed reflectance within the box along a user-defined path (by pressing keyboard buttons). Passive training was the same as active, except block movements were pre-recorded. One group received both active and passive training. No-training used a static presentation of blocks to indicate depth. We found that stereoscopic presentation produced the best lightness constancy. Training regimes delivered a performance difference for stereoscopic, monocular, and bi-ocular presentations, with passive and active training being equally effective. With some training regimes participants took longer to achieve their best performance.
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 compared DLB, PCA, and healthy control participants, in four visual tasks designed to measure the efficiency of the visual system at different levels of processing. In ascending order of complexity there were tasks of visual acuity, line orientation, contour integration, and rotated object comparison. DLB patients did not differ from controls in low level visual functions of visual acuity and line orientation, suggesting that early visual processing areas including V1 were relatively preserved, consistent with pathology evidence (Yamamoto et al., 2006). However, higher level visual functions of contour integration, mediated by extrastriatal areas, and the most complex task of object rotation, relying on processing within inferior temporal (IT), parietal, and frontal cortices, were impaired in DLB. In contrast, PCA patients were impaired in all tasks, consistent with evidence of widespread pathology within occipital and parietal areas in PCA. The latter suggests that both lower and higher level visual impairments contribute to the complex visual symptoms associated with PCA.
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 disorientation. Dementia with Lewy bodies (DLB) is, similar to PCA, also characterised by profound impairments in visual perception. However, little is known about the nature underlying these complex visual deficits associated with both conditions. To investigate the nature of visual impairments, the present study compared PCA, DLB, and healthy control participants in visual tasks designed to measure the efficiency of the visual system at different levels of processing. In ascending order of complexity there were tasks of visual acuity, line orientation, contour integration, and rotated object comparison. DLB patients did not differ from controls in visual acuity and line orientation, but were impaired in contour integration and object comparison. PCA patients were impaired in all tasks. In PCA all processing stages were affected, whereas DLB was only associated with deficits in contour integration and object comparison. We conclude that low level impairments affecting processing stages before the dorsal-ventral distinction contribute to visual deficits in PCA. In addition our results suggest that deficits in feature integration thresholds may contribute to object recognition impairments in DLB.
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