Brightness versus apparent contrast. 3: Blurred disks and concentric cosine gratings.

Matching either the centre-brightness or the apparent contrast of incremental 1-deg disks, with varying edge-sharpness, yields quite different results. These suggest that the maximum brightness gradient at the edge determines apparent-contrast perception. However, no significant differences are found in matching the brightness maxima, the brightness minima, or the apparent contrast of concentric cosine gratings with varying spatial frequency. This means that spatial frequency influences both brightness extremes in the same way, and that apparent contrast is a function of the difference between the brightness extremes. The grating results therefore deviate from the results obtained with sharp as well as blurred disks. These contradictions complicate the construction of a model which unifies suprathreshold disk and grating perception.     

Brightness versus apparent contrast. 1: Incremental and decremental disks with varying diameter.

This study describes the matching of the brightness and of the apparent contrast of foveal disks, presented as an increment or decrement with varying diameter against a 300 cd.m-2 background. If the brightness in the centre of the disks is matched with a constant reference brightness, the well-known spatial Broca-Sulzer phenomenon is obtained. This effect is not found if the apparent contrast of the disks is matched instead. All matching results and detection thresholds indicate that luminance increments and decrements are processed asymmetrically by the visual system: for decrements the threshold curve and curves of isobrightness as well as apparent iso-contrast are all shifted towards larger disk radii relative to those for increments. Two nonlinear single-channel models are tested against the data.     

Brightness contrast effects in monkey lateral geniculate nucleus.

Brightness contrast effects shown by single cells in the macaque's lateral geniculate nucleus were studied with black and white lines of various widths, consisting of either: (1) "simultaneous contrast" stimuli in which the line was produced by luminance changes in the flanking areas or (2) "successive contrast" stimuli in which the line itself changed in luminance. Line widths that gave optimal responses and response magnitudes themselves were similar for the two types of stimulus, except for the widest lines used (2 degrees). Thus, simultaneous brightness contrast is a primary determinant of the response of primate LGN cells but only within 2 degrees of the center of the receptive field. Neural processing up to this level cannot therefore explain the long distance effects of simultaneous brightness contrast in human perception.     

Brightness contrast inhibits color induction: evidence for a new kind of color theory

A gray region can be made to look colored by a colored surround. This phenomenon, chromatic induction, depends on color differences around the boundary of the region. We performed experiments on chromatic induction with small, initially achromatic, targets on nine different colored surrounds ranging in color from blue to red. Using scaling of saturation as our measure of perceived color strength, we found that chromatic induction is at its maximum when the brightness contrast at the boundary between target and surroundings is minimal. This implies that the neural mechanism in the cerebral cortex that mediates the appearance of brightness at a boundary inhibits the activity of chromatic mechanisms at that same boundary. Observers matched the apparent brightness and luminance of each of the colored surrounds. For surround colors where brightness and luminance matches differ, brightness contrast, not luminance contrast, controls chromatic induction. These new findings, taken together with other evidence, require a new theory of color appearance that includes mutually inhibitory interactions between color and brightness mechanisms that are sensing color and brightness contrast at visual boundaries.     

True versus apparent malaria infection prevalence: the contribution of a Bayesian approach

Aims

To present a new approach for estimating the “true prevalence” of malaria and apply it to datasets from Peru, Vietnam, and Cambodia.

Methods

Bayesian models were developed for estimating both the malaria prevalence using different diagnostic tests (microscopy, PCR & ELISA), without the need of a gold standard, and the tests'' characteristics. Several sources of information, i.e. data, expert opinions and other sources of knowledge can be integrated into the model. This approach resulting in an optimal and harmonized estimate of malaria infection prevalence, with no conflict between the different sources of information, was tested on data from Peru, Vietnam and Cambodia.

Results

Malaria sero-prevalence was relatively low in all sites, with ELISA showing the highest estimates. The sensitivity of microscopy and ELISA were statistically lower in Vietnam than in the other sites. Similarly, the specificities of microscopy, ELISA and PCR were significantly lower in Vietnam than in the other sites. In Vietnam and Peru, microscopy was closer to the “true” estimate than the other 2 tests while as expected ELISA, with its lower specificity, usually overestimated the prevalence.

Conclusions

Bayesian methods are useful for analyzing prevalence results when no gold standard diagnostic test is available. Though some results are expected, e.g. PCR more sensitive than microscopy, a standardized and context-independent quantification of the diagnostic tests'' characteristics (sensitivity and specificity) and the underlying malaria prevalence may be useful for comparing different sites. Indeed, the use of a single diagnostic technique could strongly bias the prevalence estimation. This limitation can be circumvented by using a Bayesian framework taking into account the imperfect characteristics of the currently available diagnostic tests. As discussed in the paper, this approach may further support global malaria burden estimation initiatives.     

Cone contrast computations: physical versus perceived background and colour constancy

The light reflected from an object depends both on the object's surface and on the illuminant. Visual systems attempt to resolve this intrinsic ambiguity by comparing the light reflected from the object with respect to the background by computing the difference between the object-background light sampled by three cones. The cone-contrasts for the sample-background stimulus under the test illumination should correspond to the cone-contrasts for samples matched in appearance under the standard background (C). The validity of this cone-contrast rule (Whittle, 2003) and its possible link with stability of perceived colour was studied here using six test illuminants. A successive asymmetric colour-matching task with 40 simulated Munsell samples (value 7, chroma 4) on a neutral background (N7) was used. The subject adjusted the sample under standard illuminant C to match the colour appearance of the test sample under one of the test illuminants. Brunswik Ratio (BR) was used as an index of stability of colour appearance. When computed with respect to the reference illuminant C, the cone contrast rule was violated (particularly for S-cone-contrast). However, if a new reference point based on the perceived colour of the neutral background under the test illumination was used, the cone contrast rule applied. That is, when cone contrasts of the matching samples are computed with respect to this perceived background, they correspond to cone contrasts of the test stimuli. This represents a form of discounting the illuminant for the purpose of determining an object's cone-contrast against the background, which does not vary with background illumination. These cone contrasts, however, do not provide any information about the colour appearance of objects under particular viewing conditions, unless calibrated against a standard by allowing subjects to learn particular colours.     

Systemic organization of functional interhemisphere asymmetry. Mirror asymmetry

The systemic organization of hemispherical asymmetry at the behavioral, neurophysiological, and biochemical levels was studied. An interconnection and hierarchic coordination of the parameters of different levels were revealed, which are confirmed by multiple correlations between them. An interrelation between the processes of peroxidation of brain membrane lipids, the level of direct current potential, and the behavior of animals was shown. A significant difference in correlations between right-hander and left-hander animals was found.     

Non-consecutive versus consecutive footstrikes as an equivalent method of assessing gait asymmetry

Asymmetry of gait is often studied to characterize populations and assess the efficacy of treatment protocols. However, despite the continuous nature of gait, many studies have made comparisons between data from non-consecutive footstrikes. This is typically considered a limitation of these studies. However, if gait characteristics are sufficiently repeatable within a side, consecutive footstrikes may not be necessary to properly describe the asymmetry between sides. Therefore, one purpose of this study was to compare asymmetry values calculated from consecutive and non-consecutive footstrikes. Additionally, the variability of gait within and between sides was compared to assess the repeatability and distinctiveness of the characteristics on each side. The results suggest that kinetic and kinematic asymmetry can be assessed from either consecutive or non-consecutive footstrikes. Further, the patterns of movement tend to be sufficiently consistent within a side, such that the variability within a side is much lower than the variability between sides. However, there may be some variables, or populations, that exhibit high within-side variability. Several trials of consecutive footstrikes may be a better way to characterize asymmetry of those variables.     

Mixture analysis of asymmetry: modelling directional asymmetry, antisymmetry and heterogeneity in fluctuating asymmetry

The occurrence of different forms of asymmetry complicates the analysis and interpretation of patterns in asymmetry. Furthermore, between-individual heterogeneity in developmental stability (DS) and thus fluctuating asymmetry (FA), is required to find relationships between DS and other factors. Separating directional asymmetry (DA) and antisymmetry (AS) from real FA and understanding between-individual heterogeneity in FA is therefore crucial in the analysis and interpretation of patterns in asymmetry. In this paper we introduce and explore mixture analysis to (i) identify FA, DA and AS from the distribution of the signed asymmetry, and (ii) to model and quantify between-individual heterogeneity in developmental stability and FA. In addition, we expand mixtures to the estimation of the proportion of variation in the unsigned FA that can be attributed to between-individual heterogeneity in the presumed underlying developmental stability (the so-called hypothetical repeatability). Finally, we construct weighted normal probability plots to investigate the assumption of underlying normality of the different components. We specifically show that (i) model selection based on the likelihood ratio test has the potential to yield models that incorporate nearly all heterogeneity in FA; (ii) mixtures appear to be a powerful and sensitive statistical technique to identify the different forms of asymmetry; (iii) restricted measurement accuracy and the occurrence of many zero observations results in an overestimation of the hypothetical repeatability on the basis of the model parameters; and (iv) as judged from the high correlation coefficients of the normal probability plots, the underlying normality assumption appears to hold for the empirical data we analysed. In conclusion, mixtures provide a useful statistical tool to study patterns in asymmetry.     

Neuronal membrane lipid asymmetry.

The outer surface of intact synaptosomes was covalently labelled with trinitrobenzenesulfonic acid prior to isolation of the synaptic plasma membrane. Analysis of the membrane lipid demonstrated an asymmetric distribution of phospholipids across the synaptosomal plasma membrane. In addition, the fatty acyl composition of phosphatidylethanolamine from this neuronal membrane fraction was also distributed asymmetrically. The data are consistent with a $\text{de novo}$ generation of phospholipid asymmetry independent of serum lipid exchange processes. This structural asymmetry may have important consequences for neurotransmission.     

Retina versus cortex; contrast adaptation in parallel visual pathways

Human vision adapts to the contrast of patterns by changing its sensitivity, but the origins of this perceptual adaptation have been disputed. In this issue of Neuron, Solomon et al. show that contrast adaptation in the primate arises mostly in the retina for the magnocellular pathway and mostly in the cortex for the parvocellular pathway. It appears that adaptation arises most strongly at sites that pool over many inputs.     

Wing shape versus asymmetry as an indicator of changing environmental conditions in insects

Abstract In insects, the fluctuating asymmetry of bilaterally symmetrical traits has been suggested as an indicator of environmental stress because asymmetry is expected to increase when stressful conditions disturb the normal development of organisms. However, the extensive literature on asymmetry–stress associations is indeterminate. Here we contrast changes in asymmetry with changes in an alternate stress indicator, the shape of insect wings. The development of wing shape involves numerous genes that act throughout egg-to-adult development, so stresses that act at a specific time could alter shape in specific ways. Shape changes, as measured by the Procrustes technique, were considered in five data sets: exposure of Drosophila melanogaster (Meigen) to multiple stresses involving ethanol, low nutrition and cold shocks; exposure of a chironomid ( Chironomus tepperi (Skuse)), a blowfly ( Lucilia cuprina (Wiedemann)), and lightbrown apple moth ( Epiphyas postvittana (Walker)) to pesticides; and development of C. tepperi under saline conditions. All these conditions influenced viability and development time. In none of these cases was a change in symmetry of wing size or wing shape detected. In contrast, in four of the five data sets there was a change in wing shape. These results suggest that wing shape may be altered more commonly by stress than trait asymmetry. Wing-shape monitoring may be useful in detecting stressful environmental conditions during development, at least under controlled conditions.     

Brightness preference in nocturnal and diurnal South American monkeys.

In a test situation where choice was not tied to reinforcement, both squirrel monkeys (Saimiri sciureus) and owl monkeys (Aotus trivirgatus) consistently chose to view the more luminant of a pair of lights. Over a two log unit range of luminance, the degree of preference in both species for the brighter of two lights was found to depend on their difference in luminance and not on their actual luminances.     

Vertebrate left-right asymmetry: old studies and new insights.

During vertebrate embryonic development, the organs of the chest and abdomen, heart, lung and gastrointestinal tract, acquire characteristic asymmetric positions with respect to the left-right body axis. In the beginning of the 20th century Hans Spemann and his co-workers described manipulations of amphibian embryos which resulted in inversion of organ laterality in a predictable manner. Hedwig Wilhelmi concluded from these experiments that determinants on the left side of the embryo specify laterality, and Meyer postulated that a mediator should transfer this positional information to the forming heart. In this review we discuss the classical experiments in the light of recent advances in the molecular understanding of left-right development, with a focus on the mediator role of the homeobox gene Pitx2.     

2-Deoxy-D-glucose accumulation in adipocytes: apparent transport discrimination between 2-deoxy-D-glucose and 3-O-methyl-D-glucose

The uphill accumulation of free 2-deoxy-D-glucose and 2-deoxy-D-glucose 6-phosphate in rat adipocytes was found not to affect the steady-state distribution of 3-O-methyl-D-glucose although both hexoses share a common transport pathway. This observation argues against a possible effect of 2-deoxy-D-glucose phosphate on the equilibrating nature of the carrier. The results are discussed in light of hypotheses advanced to explain free 2-deoxy-D-glucose accumulation in a variety of cells.     

Purification of mouse interleukin 2 to apparent homogeneity

A procedure has been developed for the rapid purification of mouse interleukin 2 (IL2) to apparent homogeneity, using gel filtration, anion exchange, hydrophobic chromatography, and reverse phase high pressure liquid chromatography (RP-HPLC). IL2 eluted at a high concentration of acetonitrile on HPLC (approximately 40%), well removed from other proteins. This protocol did not resolve isoelectric variant forms of IL2. Both the biological activity and protein migrated as a band of apparent molecular weight 22,000-23,000 on SDS-polyacrylamide gel electrophoresis. It had a high potency, producing 30% of the maximal response in T cell growth at a concentration of 2-4 X 10(-12) M. Mouse Il2 synthesized in a wheat germ cell-free translation system behaved similarly on RP-HPLC as the form secreted by EL4 cells. Thus, the hydrophobicity of mouse IL2, which facilitates its purification, is an intrinsic property of the protein, determined primarily by its amino acid sequence.     

Transmembrane asymmetry of vesicle lipids.

The role of fatty acyl chain unsaturation in promoting asymmetry in phospholipid vesicle bilayers was investigated in mixed lipid systems with differing acyl chains and a constant phosphatidylcholine headgroup. Ratios of outside to inside components were determined by nuclear magnetic resonance spectroscopy of 13C-enriched egg phosphatidylcholine. An asymmetry or disproportionation ratio is defined and used to express quantitatively how a mixture of two lipids distributes in the outer and inner vesicle surfaces. In mixed systems with 13C-enriched egg phosphatidylcholine as one component, increasing fatty acyl unsaturation in the other component results in an increasing preference of the unsaturated chains for the outer surface.