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PubMed2006

Nonlinear dynamics and pattern bifurcations in a model for vegetation stripes in semi-arid environments.

In many semi-arid environments, vegetation is self-organised into spatial patterns. The most striking examples of this are on gentle slopes, where striped patterns are typical, running parallel to the contours. Previously, Klausmeier [1999. Regular and irregular patterns in semiarid vegetation. Science 284, 1826-1828.] has proposed a model for vegetation stripes based on competition for water. Here, we present a detailed study of the patterned solutions in the full nonlinear model, using numerical bifurcation analysis of both the pattern odes and the model pdes. We show that patterns exist for a wide range of rainfall levels, and in particular for much lower rainfall than have been considered by previous authors. Moreover, we show that for many rainfall levels, patterns with a variety of different wavelengths are stable, with mode selection dependent on initial conditions. This raises the possibility of hysteresis, and in numerical solutions of the model we show that pattern selection depends on rainfall history in a relatively simple way.

Demography↗
PubMed2000

Exact banded patterns from a Doi-Marrucci-Greco model of nematic liquid crystal polymers.

Elements of pattern formation in nematic liquid crystal polymers are presented using the Doi-Marrucci-Greco (DMG) moment-averaged theory. The theory yields a full tensor orientation equation, accounting for excluded-volume and distortional elasticity potentials, with rotational molecular diffusion. Spinodal decomposition associated with unstable homogeneous phases is described first by way of an exact solution of the linearized DMG model. A variety of uniaxial and biaxial banded spatial patterns are then explicitly constructed from the DMG model. Exact solutions are given that possess order parameter spatial variations as well as solutions whose banded intensity patterns arise from sinuous director heterogeneity. These constructions pose as analytical models for banded structures observed during and after cessation of simple shear or elongation.

Journal Article↗
PubMed1999

Preserved negative patterning and impaired spatial learning in pigeons (Columba livia) with lesions of the hippocampus.

Pigeons (Columba livia) with bilateral electrolytic lesions of the hippocampus and area parahippocampalis were compared with control pigeons on 2 tasks: negative patterning and delayed spatial alternation. Negative patterning demands configural stimulus representations for its successful solution. The only effect of hippocampal lesions on this task was an increased response rate to the rewarded stimuli. On the delayed spatial alternation task, hippocampal birds showed deficits relative to controls. Differences in the results of prior studies on negative patterning appear to be due to different response requirements to the nonreinforced stimuli. These results are consistent with prior work with rats and suggest that the avian hippocampus is essential for spatial memory and response inhibition but is not involved in configural learning.

Animals↗
PubMed1991

Mouse major histocompatibility complex and lung development: haplotype variation, H-2 immunolocalization, and progressive maturation.

The association of the mouse major histocompatibility complex (H-2), lung maturation, and corticosteroid responsiveness has recently been demonstrated in congenic B10 (H-2b) and B10.A (H-2a) mice (Hu et al.: American Journal of Medical Genetics 35:126-131, 1990). We have investigated additional haplotypes [B10.BR (H-2k) and B10.D2 (H-2d)] to confirm that there is a strong association between H-2 haplotype variation and the degree of pulmonary maturation. Lungs of B10.D2, B10.BR, B10, and B10.A congenic mice achieve haplotypic specific maturation: B10.D2 lungs greater than B10 lungs = B10.BR lung greater than B10.A lungs. It appears that the expression of these developmental potentials is under corticosteroid regulation. Further, to test the hypothesis that H-2 antigens would be expressed earlier in embryonic lungs with the H-2b (B10) or H-2k (B10.BR) haplotype than with the H-2a (B10.A) haplotype, we investigated the spatiotemporal patterns of H-2 antigen localization in B10, B10.BR, and B10.A congenic mouse strains with and without corticosteroid treatment. The spatial patterns of H-2 antigen localization was similar in the B10, B10.BR, and B10.A mouse lungs; however, these patterns appeared earlier in both untreated and treated B10 and B10.BR mice as compared with untreated B10.A mice, suggesting an H-2 haplotype associated rate of pulmonary maturation. Following corticosteroid treatment, all congenic strains had a temporally comparable spatial distribution of H-2 antigens. Our results provide preliminary evidence suggesting that both a lung "developmental gene(s)" and a "glucocorticoid responsiveness gene(s)" are most likely outside the K-D subregions of the H-2 complex. A model of the H-2 regulation of lung maturation and corticosteroid responsiveness is discussed.

Animals↗
PubMed1992

Deblurring of 3-dimensional patterns of evoked rat cerebellar cortical activity: a study using voltage-sensitive dyes and optical sectioning.

One of the benefits of imaging neuronal activity is the capability of resolving spatial patterns in the x-y plane. With optical sectioning microscopy, the 3-dimensional (3-D) structure may also be studied without physical deformation by serially moving the focal plane of the microscope through the volume of interest along the focal axis. However, each image is blurred by contributions from neighboring planes. This degradation is most severe for low numerical aperture lenses and large amounts of defocus. In this study, an image restoration method using the optical properties of an aberration-free, defocused optical system has been developed for improving optical signals from voltage-sensitive dyes. Deblurring based on the optical transfer function (OTF) of the system was applied on two test sets of serially sectioned images: (1) fluorescent beads and (2) in vivo rat cerebellar cortex stained with the voltage-sensitive dye RH795. This method was shown to reduce significantly the out-of-focus contribution to the images, improving the spatial resolution not only in the x-y plane, but also the z axis. The algorithms were then applied to optical signals obtained by stimulation of the cerebellar surface. Optical signals having a distinct beam-like pattern were evoked and recorded over depths ranging from 0 to 300 microns prior to deblurring. Application of the deblurring algorithm reduced the depth of cerebellar cortex over which the optical signals were observed. In agreement, field potential recordings of the evoked parallel fiber volley and post-synaptic components were restricted to a narrow range of depths similar to the deblurred optical images. Removal of out-of-focus information is an essential step in the serial sectioning of central nervous system structures for neuronal imaging and 3-D reconstruction.

Algorithms↗
PubMed1988

Redistribution of the elderly population in Poland: regional and rural-urban dimensions.

"The present paper looks into spatial patterns of the elderly population and the recent configurations of elderly migration in Poland....Compared with other countries in Europe, the ageing of the population of Poland is neither very advanced nor particularly rapid. However, spatial concentration of the elderly population, as well as specific patterns of its redistribution, generate a number of policy issues on the regional and local level. Such problems range from the provision of specialized services to the maintenance of housing and the utilization of farmland."

Adult↗
PubMed1990

Different patterns of transcription from the two Antennapedia promoters during Drosophila embryogenesis.

The homeotic genes of Drosophila control the differentiation of segments during development. Mutations in these genes cause one or more segments to develop structures normally found elsewhere in the organism. Several studies have shown that the spatial patterns of homeotic gene transcription are highly complex, and that these precise patterns of transcription are critical to normal development. The homeotic gene Antennapedia (Antp), a member of the Antennapedia Complex, is required for the correct differentiation of thoracic segments in both embryos and adults. The patterns of total Antp transcript and protein accumulation have been described in detail, but the contribution of each promoter to the overall pattern in embryos has not been reported. We have examined in detail the spatial distribution of transcripts from each of the Antp promoters in both embryo sections and whole embryos by in situ hybridization using promoter-specific probes. We show that the transcripts from each of the two promoters accumulate in distinct, but overlapping patterns during embryogenesis. The results demonstrate that the two Antp promoters are differentially regulated in embryos and provide a basis for examining the regulation of the two promoters and characterizing more fully the function of Antp during embryogenesis. In addition, we have examined the regulation of each of the Antp promoters by genes of the bithorax complex (BX-C). We show that in BX-C- embryos both promoters are derepressed in the abdomen.

Animals↗
PubMed2005

Initial results for urban metal distributions in house dusts of Syracuse, New York, USA.

A program of house dust sample collection and analysis has begun in Syracuse, New York, USA, in order to determine the feasibility of a geography-based exposure assessment for urban metals. The sampling program, and the protocols it employs, is described for two different types of wipe media, Ghost Wipes and Whatman Filters. Preliminary results show that strong spatial patterns of floor dust loading (mg dust per square foot) can be observed for data aggregated at a spatial scale of about 1600 m (approximately 2.5 km2). Floor dust metal concentrations were similar to those found in other urban environments, with some regional variation. The median floor dust Pb concentration was approximately 108 mg x kg(-1) for this initial data set of approximately 264 sampled residential locations, and varied from 50 to 1100 mg Pb x kg(-1).

Air Pollution, Indoor↗
PubMed1994

Temporal integration of random dot apparent motion information in human central vision.

Human motion perception is assumed to be functionally described by an array of bi-local detectors feeding later, higher order computational stages. Using this model as a guide, improvement of spatio-temporal displacement sensitivity by temporal integration (summation) was measured in human central vision using random dot pattern apparent-motion stimuli. Our results agree with previous experiments with regard to improvement of maximum perceivable spatial displacement but show that contrary to previous results the minimum perceivable spatial displacement can be improved in a similar manner. Furthermore, stimulus duration is a more accurate predictor of sensitivity than the number of frames in the stimulus over a wide range of stimulus parameter values. Finally, our results indicate that temporal tuning of motion detectors is inversely related to the size of the spatial pattern displacement.

Contrast Sensitivity↗
PubMed1985

Patterns of exploration in rats distinguish lisuride from lysergic acid diethylamide.

In order to further validate a previously proposed animal model of the effects of LSD in humans, doses of 5, 15, 30 and 60 micrograms/kg lisuride (a non-hallucinogenic congener of LSD) were studied using a behavioral pattern monitor (BPM). The BPM provided both quantitative measures of crossovers, rearings, and holepokes and qualitative measures of spatial patterns of locomotion. A holeboard chamber connected to a homecage provided two test situations. Rats were tested either with (free exploration) or without access to the homecage (forced exploration). In both situations, lisuride exhibited a biphasic dose-response curve for horizontal locomotion (low dose suppression and high dose enhancement), while rearing was significantly reduced at all doses. Lisuride also produced a dose-dependent increase in the perseverative quality of locomotor patterns. A comparison of these results with our previous studies with lysergic acid diethalmide (LSD) indicate that, with the exception of rearings, lisuride fails to mimic LSD's characteristic effects on exploratory activity. Rather, lisuride exhibited many similarities to the dopamine angonist apomorphine.

Animals↗
PubMed1989

An autoradiographic analysis of neurogenesis in the chick retina in vitro and in vivo.

Patterns of [3H]thymidine incorporation during neurogenesis of the embryonic chick retina have been compared in vitro and in ovo. Pieces of posterior, undifferentiated retinas were dissected from embryos on day 6 of incubation (E6) and cultured in the presence of [3H]thymidine. Label was added to the medium for 3 h on day 1, 2, 3 or 4 in culture. The retinas were fixed on the fifth day, embedded in epon, sectioned and processed for autoradiography. In parallel experiments, in ovo injections were made on embryonic day 6, 7, 8 or 9 (E6-E9). On E12 the embryos were fixed and a piece of the posterior retina from each eye was dissected and processed for autoradiography as above. Results show that the retinal explants develop well in culture and all of the layers of the neural retina differentiate. However, the cultured retinas are thinner than those grown in ovo. [3H]Thymidine labeling indicates that nearly all retinal neurons undergo their final mitotic divisions between E6 and E9. In addition the patterns of labeling in culture are similar to those in ovo. Most neurons, including the majority of cells in the ganglion cell layer and outer nuclear layer, are labeled on the first three days in culture and in E6-E7 embryos, while labeled cells are restricted to the inner nuclear layer in older specimens. Counts of labeled and unlabeled neurons in the ganglion cell layer suggest that the temporal pattern of neurogenesis in culture lags behind that in the embryo by about one day but that the spatial patterns of cell migration are the same.

Animals↗
PubMed2002

Towards a global historical emission inventory for selected PCB congeners--a mass balance approach. 1. Global production and consumption.

Information on the historical global production and consumption of polychlorinated biphenyls (PCBs) is urgently needed for estimating PCB fluxes to the environment and for interpreting global contamination patterns by these pollutants. This study presents the methodology, principal uncertainties and selected results from an inventory, aiming to quantify the global production and consumption of total PCBs as well as 22 PCB congeners. The available data on the historical production of PCBs and the chemical composition of various technical mixtures have been compiled from the literature. For some producers with less detailed information, the production of individual PCB constituents has been estimated to derive a global estimate for individual homologues and selected congeners. Information on imports, exports and consumption, as well as restrictions on production and imports, has further been compiled for individual countries. These data, along with assumptions on the trade between countries and regions, have been utilised to derive an estimate of the global historical consumption pattern. Although there are substantial uncertainties involved in these estimates, important aspects governing the large scale temporal and spatial patterns are most likely captured in these estimates. In particular, the information on imports and exports for the principal users of PCBs around the time of peak production is considered to be fairly reliable. The estimates account for a reported historical global production of approximately 1.3 million t PCBs, more than 70% of which are tri-, tetra- and pentachlorinated biphenyls. The results further suggest that almost 97% of the global historical use of PCBs have occurred in the Northern Hemisphere.

Environmental Pollutants↗
PubMed2001

Analysis of temporal shear stress gradients during the onset phase of flow over a backward-facing step.

Endothelial cells in blood vessels are exposed to bloodflow and thus fluid shear stress. In arterial bifurcations and stenoses, disturbed flow causes zones of recirculation and stagnation, which are associated with both spatial and temporal gradients of shear stress. Such gradients have been linked to the generation of atherosclerotic plaques. For in-vitro studies of endothelial cell responses, the sudden-expansion flow chamber has been widely used and described. A two-dimensional numerical simulation of the onset phase of flow through the chamber was performed. The wall shear stress action on the bottom plate was computed as a function of time and distance from the sudden expansion. The results showed that depending on the time for the flow to be established, significant temporal gradients occurred close to the second stagnation point of flow. Slowly ramping the flow over 15 s instead of 200 ms reduces the temporal gradients by a factor of 300, while spatial gradients are reduced by 23 percent. Thus, the effects of spatial and temporal gradients can be observed separately. In experiments on endothelial cells, disturbed flow stimulated cell proliferation only when flow onset was sudden. The spatial patterns of proliferation rate match the exposure to temporal gradients. This study provides information on the dynamics of spatial and temporal gradients to which the cells are exposed in a sudden-expansion flow chamber and relates them to changes in the onset phase of flow.

Algorithms↗
PubMed2001

New double-tracer digital radiography for analysis of spatial and temporal myocardial flow heterogeneity.

A new high-resolution digital radiographic technique based on the deposition of (125)I- and (3)H-labeled desmethylimipramine (IDMI and HDMI, respectively) was developed for the assessment of spatial and temporal myocardial flow heterogeneity at a microvascular level. The density distributions of two tracers, or relative flow distributions, were determined by subtraction digital radiography using two imaging plates of different sensitivity. The regions resolved are comparable in size to vascular regulatory units (400 x 400 microm(2)). This method was applied to the measurement of within-layer myocardial flow distributions in Langendorff-perfused rabbit hearts. The validity of this method was confirmed by the strong correlation between regional densities of two tracers injected simultaneously (r = 0.89 +/- 0.03, n = 8). The temporal flow stability was evaluated by a 90-s continuous IDMI injection and subsequent bolus HDMI injection (n = 8). Regional densities of the two tracers were fairly correlated (r = 0.86 +/- 0.03), indicating that the spatial pattern of flow distribution was stable even at a microvascular level over a 90-s period. The effect of microsphere embolization on the flow distribution was also investigated by the sequential injections of IDMI, 15-microm microspheres, and HDMI at 20-s intervals (n = 8). Microembolization increased the coefficient of variation of tracer density from 19 to 25% (P < 0.05), whereas the regional densities of two tracers were still correlated substantially, as in the case of no embolization (r = 0.84 +/- 0.06). Thus the microsphere embolization enhanced flow heterogeneity with increasing flow differences between control high-flow and control low-flow regions but rather maintained the pattern of flow distribution. In conclusion, double-tracer digital radiography will be a promising method for the spatial and temporal myocardial flow analysis at microvascular levels.

Animals↗
PubMed2005

Role of GABAergic inhibition in shaping odor-evoked spatiotemporal patterns in the Drosophila antennal lobe.

Drosophila olfactory receptor neurons project to the antennal lobe, the insect analog of the mammalian olfactory bulb. GABAergic synaptic inhibition is thought to play a critical role in olfactory processing in the antennal lobe and olfactory bulb. However, the properties of GABAergic neurons and the cellular effects of GABA have not been described in Drosophila, an important model organism for olfaction research. We have used whole-cell patch-clamp recording, pharmacology, immunohistochemistry, and genetic markers to investigate how GABAergic inhibition affects olfactory processing in the Drosophila antennal lobe. We show that many axonless local neurons (LNs) in the adult antennal lobe are GABAergic. GABA hyperpolarizes antennal lobe projection neurons (PNs) via two distinct conductances, blocked by a GABAA- and GABAB-type antagonist, respectively. Whereas GABAA receptors shape PN odor responses during the early phase of odor responses, GABAB receptors mediate odor-evoked inhibition on longer time scales. The patterns of odor-evoked GABAB-mediated inhibition differ across glomeruli and across odors. Finally, we show that LNs display broad but diverse morphologies and odor preferences, suggesting a cellular basis for odor- and glomerulus-dependent patterns of inhibition. Together, these results are consistent with a model in which odors elicit stimulus-specific spatial patterns of GABA release, and as a result, GABAergic inhibition increases the degree of difference between the neural representations of different odors.

Animals↗
PubMed2004

Pattern formation during T-cell adhesion.

T cells form intriguing patterns during adhesion to antigen-presenting cells. The patterns are composed of two types of domains, which either contain short TCR/MHCp receptor-ligand complexes or the longer LFA-1/ICAM-1 complexes. The final pattern consists of a central TCR/MHCp domain surrounded by a ring-shaped LFA-1/ICAM-1 domain, whereas the characteristic pattern formed at intermediate times is inverted with TCR/MHCp complexes at the periphery of the contact zone and LFA-1/ICAM-1 complexes in the center. Several mechanisms have been proposed to explain the T-cell pattern formation. Whereas biologists have emphasized the role of active cytoskeletal processes, previous theoretical studies suggest that the pattern evolution may be caused by spontaneous self-assembly processes alone. Some of these studies focus on circularly symmetric patterns and propose a pivot mechanism for the formation of the intermediate inverted pattern. Here, we present a statistical-mechanical model which includes thermal fluctuations and the full range of spatial patterns. We confirm the observation that the intermediate inverted pattern may be formed by spontaneous self-assembly. However, we find a different self-assembly mechanism in which numerous TCR/MHCp microdomains initially nucleate throughout the contact zone. The diffusion of free receptors and ligands into the contact zone subsequently leads to faster growth of peripheral TCR/MHCp microdomains and to a closed ring for sufficiently large TCR/MHCp concentrations. At smaller TCR/MHCp concentrations, we observe a second regime of pattern formation with characteristic multifocal intermediates, which resemble patterns observed during adhesion of immature T cells or thymozytes. In contrast to other theoretical models, we find that the final T-cell pattern with a central TCR/MHCp domain is only obtained in the presence of active cytoskeletal transport processes.

Animals↗
PubMed1976

Social geography of suicidal behaviour in Hobart.

Psychological notions are required to explain geographical patterns. Mapping and statistical analysis for years 1971, 1973, 1974 and half of 1975 revealed that attempted suicide occurs commonly in the area surrounding the central business area of urban Hobart and also in the older housing department areas. Physical patterns and social structure are related in a tangible way for the expression of suicidal behaviour in a spatial pattern.

Australia↗
PubMed1997

Bones, feathers, teeth and coat markings: a unified model.

A first necessary criterion to be met by any model for pattern formation is that it must be able to reproduce the patterns it purports to model. The above examples show how simple ideas from self-organisation can produce spatial patterns of varying complexity that are consistent with those observed experimentally. Second, the model must be consistent with the results of experimental manipulation. Third, the model must make experimentally testable predictions. In this way, a mathematical model can help to elucidate the underlying biochemical and biophysical mechanisms of pattern formation. I have tried to illustrate these ideas with the above examples. These examples also show the breadth of patterning phenomena that can be captured by these models. In animal coat markings, the patterns are laid down simultaneously. In limb development, the skeletal elements are laid down sequentially. The application to feather germ formation illustrates complex sequential regular pattern formation, while the application to tooth primordium formation illustrates sequential irregular pattern formation. Many other patterning phenomena have been studied (see, for example, ref. 7). Pattern formation is one of the central issues in developmental biology and intense interdisciplinary research involving experimentalists and theoreticians is beginning to help us understand how this phenomenon occurs. A detailed understanding of normal development is a necessary first step to the understanding of abnormal development and, hopefully, will help medical science combat developmental defects.

Alligators and Crocodiles↗