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At least 289 records · Page 16Linked to original sources
PubMed2006

High dynamic range image rendering with a Retinex-based adaptive filter.

We propose a new method to render high dynamic range images that models global and local adaptation of the human visual system. Our method is based on the center-surround Retinex model. The novelties of our method is first to use an adaptive filter, whose shape follows the image high-contrast edges, thus reducing halo artifacts common to other methods. Second, only the luminance channel is processed, which is defined by the first component of a principal component analysis. Principal component analysis provides orthogonality between channels and thus reduces the chromatic changes caused by the modification of luminance. We show that our method efficiently renders high dynamic range images and we compare our results with the current state of the art.

Algorithms↗
PubMed2002

Analysis of an adaptive strain estimation technique in elastography.

Elastography is based on the estimation of strain due to tissue compression or expansion. Conventional elastography involves computing strain as the gradient of the displacement (time-delay) estimates between gated pre- and postcompression signals. Uniform temporal stretching of the postcompression signals has been used to reduce the echo-signal decorrelation noise. However, a uniform stretch of the entire postcompression signal is not optimal in the presence of strain contrast in the tissue and could result in loss of contrast in the elastogram. This has prompted the use of local adaptive stretching techniques. Several adaptive strain estimation techniques using wavelets, local stretching and iterative strain estimation have been proposed. Yet, a quantitative analysis of the improvement in quality of the strain estimates overconventional strain estimation techniques has not been reported. We propose a two-stage adaptive strain estimation technique and perform a quantitative comparison with the conventional strain estimation techniques in elastography. In this technique, initial displacement and strain estimates using global stretching are computed, filtered and then used to locally shift and stretch the postcompression signal. This is followed by a correlation of the shifted and stretched postcompression signal with the precompression signal to estimate the local displacements and hence the local strains. As proof of principle, this adaptive stretching technique was tested using simulated and experimental data.

Elasticity↗
PubMed2004

Evolutionary potential of Chamaecrista fasciculata in relation to climate change. I. Clinal patterns of selection along an environmental gradient in the great plains.

Climate change will alter natural selection on native plant populations. Little information is available to predict how selection will change in the future and how populations will respond. Insight can be obtained by comparing selection regimes in current environments to selection regimes in environments similar to those predicted for the future. To mimic predicted temporal change in climate, three natural populations of the annual legume Chamaecrista fasciculata were sampled from a climate gradient in the Great Plains and progeny of formal crosses were reciprocally planted back into common gardens across this climate gradient. In each garden, native populations produced significantly more seed than the other populations, providing strong evidence of local adaptation. Phenotypic selection analysis conducted by site showed that plants with slower reproductive development, more leaves, and thicker leaves were favored in the most southern garden. Evidence of clinal variation in selection regimes was also found; selection coefficients were ordered according to the latitude of the common gardens. The adaptive value of native traits was indicated by selection toward the mean of local populations. Repeated clinal patterns in linear and nonlinear selection coefficients among populations and within and between sites were found. To the extent that temporal change in climate into the future will parallel the differences in selection across this spatial gradient, this study suggests that selection regimes will be displaced northward and different trait values will be favored in natural populations.

Adaptation, Physiological↗
PubMed1997

What are hospitals doing about clinical guidelines?

OBJECTIVES: To assess the attitudes of senior hospital staff towards clinical guidelines, and to ascertain the perceived extent and benefits of their local use; to identify those hospitals with current or planned future written strategies for the systematic development of clinical guidelines, and the staff responsible for leading them; and to establish the essential elements of existing strategies, and the methods used to ensure the proper development, dissemination, implementation, and evaluation of local guidelines. DESIGN: Cross sectional survey. PARTICIPANTS: Senior staff of 270 acute hospitals in the United Kingdom (response rate 202/270 (75%)) in 1995. RESULTS: 197/199 (99%) of respondents thought that clinical guidelines were a good idea, and 176/196 (90%) were aware of some guideline activity occurring within their hospitals. The most important benefits of local guideline activity were increased healthcare efficiency and effectiveness, greater consistency of treatment, and team building. 174/194 (90%) of respondents were in favour of the development of a readily accessible national repository of evidence-based clinical guidelines. 38/201 (19%) of respondents had a clinical guidelines strategy and a further 91/201 (45%) said that they had plans to develop one in the near future. The need to improve clinical outcomes was most often reported as the reason for developing a strategy. Medical directors most commonly had formal responsibility to lead the strategy, but someone without formal responsibility ran the operation in half the hospitals. Only 18/36 (50%) of strategies gave advice on the development of guidelines; and only a few strategies made explicit statements on which clinical services to target for guideline development, or the methods to be used for their validation and promotion. Some strategies lacked explicit statements on methods to monitor adherence, routine review, and update of guidelines. Internal literature searches (29/31 (94%)), the use of national guidelines (29/32 (91%)), local consensus conferences (28/32 (88%)), and peer group review (21/24 (88%)) were the most popular methods of validation used in hospitals with a strategy. Methods used to promote the dissemination, implementation, and evaluation of clinical guidelines included clinical audit (31/32 (97%)), peer review (25/30 (83%)), continuing education (23/29 (79%)), targeting of opinion leaders (17/26 (65%)), use of structured case notes (14/31 (45%)), patient mediated interventions (9/26 (35%)), and patient specific reminders (8/26 (31%)). CONCLUSIONS: Most senior hospital staff have a favourable attitude towards clinical guidelines. Most hospitals are undertaking some guideline activity, but few seem to be doing so within a locally agreed hospital wide strategy in which guideline development, dissemination, implementation, and evaluation are systematically considered. Many of the current methods used to validate guidelines locally are inadequate. Evidence-based clinical guidelines should be developed nationally, leaving hospitals to focus their energies on the local adaptation, dissemination, implementation, and evaluation of such guidelines. Only in this way will local guidelines achieve their full potential to improve clinical care and patient outcomes.

Attitude of Health Personnel↗
PubMed2005

Stress-induced variation in evolution: from behavioural plasticity to genetic assimilation.

Extreme environments are closely associated with phenotypic evolution, yet the mechanisms behind this relationship are poorly understood. Several themes and approaches in recent studies significantly further our understanding of the importance that stress-induced variation plays in evolution. First, stressful environments modify (and often reduce) the integration of neuroendocrinological, morphological and behavioural regulatory systems. Second, such reduced integration and subsequent accommodation of stress-induced variation by developmental systems enables organismal 'memory' of a stressful event as well as phenotypic and genetic assimilation of the response to a stressor. Third, in complex functional systems, a stress-induced increase in phenotypic and genetic variance is often directional, channelled by existing ontogenetic pathways. This accounts for similarity among individuals in stress-induced changes and thus significantly facilitates the rate of adaptive evolution. Fourth, accumulation of phenotypically neutral genetic variation might be a common property of locally adapted and complex organismal systems, and extreme environments facilitate the phenotypic expression of this variance. Finally, stress-induced effects and stress-resistance strategies often persist for several generations through maternal, ecological and cultural inheritance. These transgenerational effects, along with both the complexity of developmental systems and stressor recurrence, might facilitate genetic assimilation of stress-induced effects. Accumulation of phenotypically neutral genetic variance by developmental systems and phenotypic accommodation of stress-induced effects, together with the inheritance of stress-induced modifications, ensure the evolutionary persistence of stress-response strategies and provide a link between individual adaptability and evolutionary adaptation.

Acclimatization↗
PubMed1985

Comparison of signal and adaptive sensitivity profiles of the surround mechanism of cat retinal ganglion cells.

Signal and adaptive sensitivity profiles of the surrounds of cat retinal ganglion cells were determined by varying the position of concentric annuli whose outside and inside diameters varied but whose total area remained constant. Signal sensitivity profiles were determined by adjusting the luminances of these annuli so as to produce a weak suprathreshold response of constant magnitude and time course. Adaptive sensitivity profiles were determined by varying the luminances of concentric equal-area, unmodulated annuli until the response to a temporally modulated annulus attained a criterion level. The results provided evidence that the retinal region over which the surround mechanism of an X cell pools adaptive information and pools signals are the same, and that the distribution of adaptive and signal sensitivities within these regions is similar. A small number of X cells showed local adaptation effects. The adaptive pooling area appeared to be smaller than the signal pooling area for Y cells.

Adaptation, Physiological↗
PubMed2006

Limits to evolution at range margins: when and why does adaptation fail?

What stops populations expanding into new territory beyond the edge of a range margin? Recent models addressing this problem have brought together population genetics and population ecology, and some have included interactions among species at range edges. Here, we review these models of adaptation at environmental or parapatric margins, and discuss the contrasting effects of migration in either swamping local adaptation, or supplying the genetic variation that is necessary for adaptation to continue. We illustrate how studying adaptation at range margins (both with and without hybridization) can provide insight into the genetic and ecological factors that limit evolution more generally, especially in response to current rates of environmental change.

Adaptation, Biological↗
PubMed2005

Remodeling of blood vessels: responses of diameter and wall thickness to hemodynamic and metabolic stimuli.

Vascular functions, including tissue perfusion and peripheral resistance, reflect continuous structural adaptation (remodeling) of blood vessels in response to several stimuli. Here, a theoretical model is presented that relates the structural and functional properties of microvascular networks to the adaptive responses of individual segments to hemodynamic and metabolic stimuli. All vessels are assumed to respond, according to a common set of adaptation rules, to changes in wall shear stress, circumferential wall stress, and tissue metabolic status (indicated by partial pressure of oxygen). An increase in vessel diameter with increasing wall shear stress and an increase in wall mass with increased circumferential stress are needed to ensure stable vascular adaptation. The model allows quantitative predictions of the effects of changes in systemic hemodynamic conditions or local adaptation characteristics on vessel structure and on peripheral resistance. Predicted effects of driving pressure on the ratio of wall thickness to vessel diameter are consistent with experimental observations. In addition, peripheral resistance increases by approximately 65% for an increase in driving pressure from 50 to 150 mm Hg. Peripheral resistance is predicted to be markedly increased in response to a decrease in vascular sensitivity to wall shear stress, and to be decreased in response to increased tissue metabolic demand. This theoretical approach provides a framework for integrating available information on structural remodeling in the vascular system and predicting responses to changing conditions or altered vascular reactivity, as may occur in hypertension.

Adaptation, Physiological↗
PubMed1999

Do parasites live in extreme environments? Constructing hostile niches and living in them.

We develop the hypothesis that parasites do not invade extreme environments, i.e. hostile hosts, but rather 'create' them. We argue that parasites may have driven the evolution of the constitutive and adaptive immune system. This leads to several implications. First, parasites respond to 'genes to kill' by 'genes to survive' and this triggers an indefinite selection of measures and counter-measures. Second, these coevolutionary arms races may lead to local adaptation, in which parasite populations perform better on local hosts. Third, the evolution of the immune system, whose responses are predictable, may allow parasites to specialize, to evade and even to manipulate. Finally we show that the correlations between the increase in the antibody repertoire, the expansion of MHC loci and parasite pressures support our hypothesis that both host complexity and parasite pressures can be invoked to explain the diversity of antibodies, T-receptors and MHC molecules.

Adaptation, Physiological↗
PubMed1982

Localized effects of spatial frequency adaptation.

The spatially localized threshold-elevation aftereffect of spatial-frequency adaptation was measured by using localized, aperiodic test patterns that have bandpass Fourier transforms. At a given retinal location, the threshold-elevation curves are consistent with the fatigue of size-turned mechanisms with center-surround sensitivity profile. Only a few different sizes of such mechanisms were required to fit the local results. The local aftereffect was also measured as a function of eccentricity near the fovea. The results indicate that the threshold-elevation aftereffect of spatial-frequency adaptation is not spatially homogeneous.

Adaptation, Ocular↗
PubMed1993

Visible flicker from invisible patterns.

Using a laser interferometer we can create grating patterns of high optical contrast (interference fringes) directly on the retina. With coarse fringe patterns, the alternating light and dark bars of the pattern can be seen, but the bars of the finest fringes are not subjectively resolved. We report here that when we rapidly modulate the contrast of a fine fringe pattern (keeping overall luminance constant), observers experience flicker, even if the fringes are too finely spaced to be perceived as a grating. For this flicker to be seen, the pattern needs to be resolvable by the photoreceptors themselves, but not necessarily by later stages of visual processing. It can be explained if, in man, signals associated with individual cone receptors do not depend linearly on light intensity, but instead are scaled by a fast sensitivity-regulating or light-adaptation mechanism. Contrast-modulation flicker is not demonstrable in rod vision; rod vision therefore lacks such a local adaptation process.

Contrast Sensitivity↗
PubMed2006

Rapid adaptive divergence of life-history traits in response to abiotic stress within a natural population of a parthenogenetic nematode.

Sexual reproduction is acknowledged to facilitate adaptation to novel environments while asexual eukaryotes are often regarded as having low adaptive potential. This view has been challenged in a number of studies, but the adaptive potential of asexual populations in the field is poorly documented. We investigated the response of natural populations of the parthenogenetic nematode Acrobeloides nanus to imposed divergent selective pressures. For this purpose, we employed a replicated evolution experiment in the field. After 20 years of evolution under abiotic stress and control conditions, life-history traits were assessed in reaction norm- and reciprocal transplant experiments. Both these experiments indicated adaptive divergence within the population of A. nanus. Namely, the transplant experiment demonstrated that in the stressed soil environment, body growth rate was more reduced in the nematodes originating from the control treatment. In the reaction norm experiment, survival and reproduction were higher under test conditions corresponding to the native environment of the nematodes. The differences in the analysed traits are discussed in the context of life-history theory. Overall, our results strongly support high adaptive potential of A. nanus and suggest that population structure and distribution of asexual species is shaped by local adaptation events.

Adaptation, Physiological↗
PubMed2003

A new accurate and precise 3-D segmentation method for skeletal structures in volumetric CT data.

We developed a highly automated three-dimensionally based method for the segmentation of bone in volumetric computed tomography (CT) datasets. The multistep approach starts with three-dimensional (3-D) region-growing using local adaptive thresholds followed by procedures to correct for remaining boundary discontinuities and a subsequent anatomically oriented boundary adjustment using local values of cortical bone density. We describe the details of our approach and show applications in the proximal femur, the knee, and the skull. The accuracy of the determination of geometrical parameters was analyzed using CT scans of the semi-anthropomorphic European spine phantom. Depending on the settings of the segmentation parameters cortical thickness could be determined with an accuracy corresponding to the side length of 1 to 2.5 voxels. The impact of noise on the segmentation was investigated by artificially adding noise to the CT data. An increase in noise by factors of two and five changed cortical thickness corresponding to the side length of one voxel. Intraoperator and interoperator precision was analyzed by repeated analysis of nine pelvic CT scans. Precision errors were smaller than 1% for trabecular and total volumes and smaller than 2% for cortical thickness. Intraoperator and interoperator precision errors were not significantly different. Our segmentation approach shows: 1) high accuracy and precision and is 2) robust to noise, 3) insensitive to user-defined thresholds, 4) highly automated and fast, and 5) easy to initialize.

Algorithms↗
PubMed2006

Child health in complex emergencies.

Coordinated and effective interventions are critical for relief efforts to be successful in addressing the health needs of children in situations of armed conflict, population displacement, and/or food insecurity. We reviewed published literature and surveyed international relief organizations engaged in child health activities in complex emergencies. Our aim was to identify research needs and improve guidelines for the care of children. Much of the literature details the burden of disease and the causes of morbidity and mortality; few interventional studies have been published. Surveys of international relief organizations showed that most use World Health Organization (WHO), United Nations Children's Fund (UNICEF), and ministry of health guidelines designed for use in stable situations. Organizations were least likely to have formal guidelines on the management of asphyxia, prematurity, and infection in neonates; diagnosis and management of children with human immunodeficiency virus (HIV) infection; active case-finding and treatment of tuberculosis; paediatric trauma; and the diagnosis and management of mental-health problems in children. Guidelines often are not adapted to the different types of health-care workers who provide care in complex emergencies. Evidence-based, locally adapted guidelines for the care of children in complex emergencies should be adopted by ministries of health, supported by WHO and UNICEF, and disseminated to international relief organizations to ensure appropriate, effective, and uniform care.

Adolescent↗
PubMed2003

Boundary effect free and adaptive discrete signal sinc-interpolation algorithms for signal and image resampling.

The problem of digital signal and image resampling with discrete sinc interpolation is addressed. Discrete sine interpolation is theoretically the best one among the digital convolution-based signal resampling methods because it does not distort the signal as defined by its samples and is completely reversible. However, sinc interpolation is frequently not considered in applications because it suffers from boundary effects, tends to produce signal oscillations at the image edges, and has relatively high computational complexity when irregular signal resampling is required. A solution that enables the elimination of these limitations of the discrete sine interpolation is suggested. Two flexible and computationally efficient algorithms for boundary effects free and adaptive discrete sinc interpolation are presented: frame-wise (global) sine interpolation in the discrete cosine transform (DCT) domain and local adaptive sinc interpolation in the DCT domain of a sliding window. The latter offers options not available with other interpolation methods: interpolation with simultaneous signal restoration/enhancement and adaptive interpolation with super resolution.

Journal Article↗
PubMed2000

Risky driving behavior: a consequence of motion adaptation for visually guided motor action.

The authors examined the effect of adaptation to expansion on overtaking maneuvers in a driving simulator. Following driving on a straight empty road for 5 min, drivers initiated overtaking substantially later (220-510 ms) than comparable maneuvers made following viewing a static scene or following 5 min of curve driving. Following adaptation to contraction (produced by driving backward), observers initiated overtaking significantly sooner. The removal of the road texture significantly reduced the size of the adaptation effect. The authors propose that these changes in overtaking behavior are due to misestimation of the time headway produced by local adaptation of looming detectors that signal motion-in-depth for objects near the focus of expansion. This adaptation effect may increase the risk of rear-end collisions during highway driving.

Adaptation, Psychological↗
PubMed2003

A genome scan to detect candidate regions influenced by local natural selection in human populations.

As human populations dispersed throughout the world, they were subjected to new selective forces, which must have led to local adaptation via natural selection and hence altered patterns of genetic variation. Yet, there are very few examples known in which such local selection has clearly influenced human genetic variation. A potential approach for detecting local selection is to screen random loci across the genome; those loci that exhibit unusually large genetic distances between human populations are then potential markers of genomic regions under local selection. We investigated this approach by genotyping 332 short tandem repeat (STR) loci in Africans and Europeans and calculating the genetic differentiation for each locus. Patterns of genetic diversity at these loci were consistent with greater variation in Africa and with local selection operating on populations as they moved out of Africa. For 11 loci exhibiting the largest genetic differences, we genotyped an additional STR locus located nearby; the genetic distances for these nearby loci were significantly larger than average. These genomic regions therefore reproducibly exhibit larger genetic distances between populations than the "average" genomic region, consistent with local selection. Our results demonstrate that genome scans are a promising means of identifying candidate regions that have been subjected to local selection.

Genetics, Population↗
PubMed2006

Model-based analysis of dispersion curves using chirplets.

Time-frequency representations, like the spectrogram or the scalogram, are widely used to characterize dispersive waves. The resulting energy distributions, however, suffer from the uncertainty principle, which complicates the allocation of energy to individual propagation modes (especially when the dispersion curves of these modes are close to each other in the time-frequency domain). This research applies the chirplet as a tool to analyze dispersive wave signals based on a dispersion model. The chirplet transform, a generalization of both the wavelet and the short-time Fourier transform, enables the extraction of components of a signal with a particular instantaneous frequency and group delay. An adaptive algorithm identifies frequency regions for which quantitative statements can be made about an individual mode's energy, and employs chirplets (locally adapted to a dispersion curve model) to extract the (proportional) energy distribution of that single mode from a multimode dispersive wave signal. The effectiveness of this algorithm is demonstrated on a multimode synthetic Lamb wave signal for which the ground-truth energy distribution is known for each mode. Finally, the robustness of this algorithm is demonstrated on real, experimentally measured Lamb wave signals by an adaption of a correlation technique developed in previous research.

Journal Article↗