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PubMed1995

Concentrations and spatial distribution of trace metals in mangrove sediments from the Brisbane River, Australia.

Fifty surface sediment samples were collected from 12 transects through a mangrove woodland near the mouth of the Brisbane River, Australia. Samples were analyzed for 10 trace metals; measured concentrations (microg g(-1)) were: Cu 3.1-34.1; Pb 7.7-84.7; Zn 40.8-144.0; Ag nd-2.8; Cr 7.6-116.8; V 16.4-57.9; Mo 7.6-82.6; Ni 2.4-57.6; As nd-13.0; and Cd nd-2.0. Coefficients of variation ranged from 22-153%. Sediments were moderately contaminated, with Mo being enriched by up to thirty times background levels. Spatial variation was considerable, and canonical trend analysis showed concentrations of all metals but Ag and Cr tended to increase downshore, suggesting tidal deposition was important in determining metal concentrations. Silver and Cr concentrations appeared to be a consequence of contamination from landward point sources. An investigation of different random sampling intensities showed the spatial variation required that many samples be taken for a satisfactory assessment of metal burdens. Even so, mean values were not a good indication of the possible environmental effects of the metal contamination. Since metal concentrations increase downshore in inter-tidal environments, a more appropriate sampling strategy would be to take a downshore series of samples normal to the shoreline.

Journal Article↗
PubMed2006

Optimal design of spatial distribution networks.

We consider the problem of constructing facilities such as hospitals, airports, or malls in a country with a nonuniform population density, such that the average distance from a person's home to the nearest facility is minimized. We review some previous approximate treatments of this problem that indicate that the optimal distribution of facilities should have a density that increases with population density, but does so slower than linearly, as the two-thirds power. We confirm this result numerically for the particular case of the United States with recent population data using two independent methods, one a straightforward regression analysis, the other based on density-dependent map projections. We also consider strategies for linking the facilities to form a spatial network, such as a network of flights between airports, so that the combined cost of maintenance of and travel on the network is minimized. We show specific examples of such optimal networks for the case of the United States.

Journal Article↗
PubMed1990

Functional implications of the unusual spatial distribution of a minor alpha-tubulin isotype in Drosophila: a common thread among chordotonal ligaments, developing muscle, and testis cyst cells.

Three of the four alpha-tubulin genes in Drosophila melanogaster are temporally regulated. mRNA from one of these genes, alpha 85E-tubulin, first appears in 6- to 8-hr embryos and persists, with marked fluctuations, through the end of pupal development. In adults, alpha 85E mRNA has been unequivocally identified only in testes. In the present study, isotype-specific antibodies have been used to localize alpha 85E tubulin protein in whole tissues. The results demonstrate a spatially restricted expression pattern of the alpha 85E gene that includes tissues of both ectodermal and mesodermal origins. Specifically, embryonic accumulation of alpha 85E tubulin is limited to support cells of chordotonal organs and the developing musculature of the viscera and body wall. In late third instar larvae, chordotonal organs and a subset of larval nerves, but not muscle, stain with anti-alpha 85E. The timing of protein accumulation during pupal development suggests that alpha 85E tubulin is involved in the construction of the adult as well as the larval musculature. In testis, only the somatically derived cyst cells that surround developing spermatid bundles accumulate alpha 85E-tubulin. The cell types that express alpha 85E share a requirement for extensive cell shape changes during development, suggesting that this minor alpha-tubulin may have distinct functional properties.

Animals↗
PubMed2006

Effects of concurrent random-time schedules on the spatial distribution of behavior in rats.

We evaluated the effects of concurrent random temporal contingencies of water delivery on the location of rats' behavior. In this experiment, two concurrent random-time schedules delivered water in two dispensers that were located at opposite ends of the chamber, and provided complementary frequencies of water deliveries while the overall number of deliveries stayed constant. Time allocation in the areas adjacent to each water dispenser covaried with water deliveries, although no proportional relation was found. Rats showed a preference for the area where water was initially presented. The results point to the importance of examining spatial properties and patterning of behavior.

Animals↗
PubMed1996

Spatial distribution of acid-volatile sulphide concentration and metal bioavailability in mangrove sediments from the Brisbane River, Australia.

Acid-volatile sulphide (AVS) was measured at regular positions along eight transects through a mangrove forest in the Brisbane River, Queensland, Australia. Concentrations ranged from 0.33 to 22.61 micromol S g(-1) sediment dry weight. There was no correlation between AVS concentration and the proportion of clay-sand in the sediment, but sediments with high AVS concentrations tended to contain more water (rs=0.43; p=0.01). AVS concentrations were used to assess the potential bioavailability of the sediment heavy metal burden. The spatial variability of potential bioavailability was high and depended to a great extent on which metals were considered as part of the AVS complexing system. It is suggested seasonal variations would further increase the observed variability in bioavailability. This variation should be taken into account when monitoring and assessing long-term trends in sediment toxicity.

Journal Article↗
PubMed1991

Food caching in captive coyotes: stereotypy of action sequence and spatial distribution of cache sites.

This paper describes some aspects of the food caching behaviour of four captive coyotes. Detailed observations of the actions used by coyotes to cache food revealed them to be strikingly similar to those previously described for timber wolves. The similarities included the identity of the movements used, their temporal sequencing, and their susceptibility to interruption. This suggests that there exists a stereotypy across canids in the action sequences used in caching. Second, an examination of the distribution of cache sites revealed that each coyote scattered cache sites widely within a wooded region of their enclosure and preferentially in terrains close to exposed tree roots.

Animals↗
PubMed1990

Spatial distribution of neural activity evoked by electrical stimulation of the cochlea.

Activity in the central auditory system was mapped with 2-deoxyglucose (2-DG) autoradiography, using either pure tones or electrical stimulation of the normal cochlea. Electrical stimulation with both monopolar (distant reference electrode) and bipolar prostheses near threshold increased 2-DG uptake in auditory nuclei in a manner similar to that seen with a pure tone: increased 2-DG uptake was restricted to a small frequency region of brainstem and mid-brain auditory nuclei. The position of this area was related to the cochlear location of the prosthesis. At higher current amplitudes only the bipolar prosthesis retained spatial restriction of evoked neural activity, while stimulation through a monopolar prosthesis produced evoked activity in all frequency regions of auditory nuclei, and in non-auditory nuclei. Activation of non-auditory structures was consistent with spread of current through the brainstem, rather than activation of peripheral nerves. At all current amplitudes, a monopolar prosthesis evoked higher levels of 2-DG uptake than a bipolar prosthesis. The results suggest that while a bipolar prosthesis provides greater spatial restriction of evoked neural activity and a greater dynamic range, a monopolar prosthesis produces higher levels of evoked activity.

Acoustic Stimulation↗
PubMed1997

Regulation of Growth Anisotropy in Well-Watered and Water-Stressed Maize Roots (I. Spatial Distribution of Longitudinal, Radial, and Tangential Expansion Rates).

As a system to study the regulation of growth anisotropy, we studied thinning of the primary root of maize (Zea mays L.) occurring developmentally or induced by water stress. Seedlings were transplanted into vermiculite at a water potential of approximately -0.03 MPa (well-watered) or -1.6 MPa (water-stressed). The diameter of roots in both treatments decreased with time after transplanting; the water-stressed roots became substantially thinner than well-watered roots at steady state, showing that root thinning is a genuine response to water stress. To analyze the thinning responses we quantified cell numbers and the spatial profiles of longitudinal, radial, and tangential expansion rates separately for the cortex and stele. The results showed that there was no zone of isotropic expansion and the degree of anisotropy varied greatly with position and treatment. Thinning over time in well-watered roots was caused by rates of radial and tangential expansion being too low to maintain the shape of the root. In response to low water potential, cell number in both tissues was unchanged radially but increased tangentially, which shows that thinning was caused wholly by reduced cell expansion. Water stress substantially decreased rates of tangential and radial expansion in both the stele and cortex, but only in the apical 5 mm of the root; basal to this, rates were similar in well-watered and water-stressed roots. By contrast, as in previous studies, longitudinal expansion was identical between the treatments in the apical 3 mm but in water-stressed roots was inhibited at more basal locations. The results show that expansion in longitudinal and radial directions can be regulated independently.

Journal Article↗
PubMed2006

A general population-genetic model for the production by population structure of spurious genotype-phenotype associations in discrete, admixed or spatially distributed populations.

In linkage disequilibrium mapping of genetic variants causally associated with phenotypes, spurious associations can potentially be generated by any of a variety of types of population structure. However, mathematical theory of the production of spurious associations has largely been restricted to population structure models that involve the sampling of individuals from a collection of discrete subpopulations. Here, we introduce a general model of spurious association in structured populations, appropriate whether the population structure involves discrete groups, admixture among such groups, or continuous variation across space. Under the assumptions of the model, we find that a single common principle--applicable to both the discrete and admixed settings as well as to spatial populations--gives a necessary and sufficient condition for the occurrence of spurious associations. Using a mathematical connection between the discrete and admixed cases, we show that in admixed populations, spurious associations are less severe than in corresponding mixtures of discrete subpopulations, especially when the variance of admixture across individuals is small. This observation, together with the results of simulations that examine the relative influences of various model parameters, has important implications for the design and analysis of genetic association studies in structured populations.

Computer Simulation↗
PubMed2004

In vivo investigation of the transcription, processing, endonucleolytic activity, and functional relevance of the spatial distribution of a plant miRNA.

We show, with miR171, that plant miRNA genes are modular independent transcription units in which the fold-back pre-miRNA is sufficient for miRNA processing, and that the upstream region contains highly specific promoter elements. Processing depends on flanking sequences within the miRNA stem-loop precursor rather than the miRNA sequence itself, and mutations affecting target pairing at the center and 5' but not 3' region of the miRNA compromise its function in vivo. Inactivation of the SDE1 RNA-dependent-RNA-polymerase was mandatory for accurate representation of miRNA activity by sensor constructs in Arabidopsis. Work in sde1 background revealed a near-perfect spatial overlap between the patterns of miR171 transcription and activity, supporting the idea that plant miRNAs enable cell differentiation.

Arabidopsis↗
PubMed1983

Spatial distribution of Antennapedia transcripts during Drosophila development.

We have localized transcripts specified by the homeotic Antennapedia (Antp) locus within serial tissue sections of Drosophila embryos and larvae by in situ hybridization. As a hybridization probe we used a 2.2-kb cDNA sequence (903) which is complementary to at least four non-contiguous chromosomal DNA regions within a span of 100 kb derived from the Antp+ locus. The tritiated probe was directly hybridized to frozen tissue sections of wild-type embryos and larvae. Hybridization was first detected to the progenitors of the thoracic segments during the cellularization of the syncytial blastoderm, when embryonic cells first become determined to form particular adult segments. At later embryonic stages the ventral nervous system also becomes labeled in a spatially-restricted manner. Initially, accumulation of transcripts is detected in all thoracic and abdominal ganglia of the ventral cord. Subsequently, the highest concentration of transcripts is detected in the ganglion cells of the mesothorax. Proper development of this segment is known to be affected in individuals homozygous for putative Antp null alleles. In third-instar larvae, hybridization to the different imaginal disks of all three thoracic segments is observed, localized mainly to those regions that will form proximal, cuticular portions of the respective segments. These results are discussed with respect to the role of homeotic genes in the specification of particular anatomical segments.

Animals↗
PubMedDate not supplied

Vision in the southern hemisphere lamprey Mordacia mordax: spatial distribution, spectral absorption characteristics, and optical sensitivity of a single class of retinal photoreceptor.

The dorso-laterally located eyes of the southern hemisphere lamprey Mordacia mordax (Agnatha) contain a single morphological type of retinal photoreceptor, which possesses ultrastructural characteristics of both rods and cones. This photoreceptor has a large refractile ellipsosome in the inner segment and a long cylindrical outer segment surrounded by a retinal pigment epithelium that contains two types of tapetal reflectors. The photoreceptors form a hexagonal array and attain their peak density (33,200 receptors/mm2) in the ventro-temporal retina. Using the size and spacing of the photoreceptors and direct measures of aperture size and eye dimensions, the peak spatial resolving power and optical sensitivity are estimated to be 1.7 cycles deg-1 (minimum separable angle of 34'7'') and 0.64 microm2 steradian (white light) and 1.38 microm2 steradian (preferred wavelength or lambdamax), respectively. Microspectrophotometry reveals that the visual pigment located within the outer segment is a rhodopsin with a wavelength of maximum absorbance (lambdamax) at 514 nm. The ellipsosome has very low absorptance (<0.05) across the measured spectrum (350-750 nm) and probably does not act as a spectral filter. In contrast to all other lampreys studied, the optimized receptor packing, the large width of the ellipsosome-bearing inner segment, together with the presence of a retinal tapetum in the photophobic Mordacia, all represent adaptations for low light vision and optimizing photon capture.

Animals↗
PubMed2001

Spatial distribution of EEG theta activity as a function of lifetime lead exposure in 9-year-old children.

The relationship between low-level childhood lead exposure and developmental retardation has been proposed but the existing evidence is weak. We examined the EEG of 42 children participating in the Mexico City Prospective Lead Study to determine if relative theta power and distribution across the scalp was related to history of lifetime lead exposure as measured by sequential blood lead concentration of the mother during pregnancy and the child after delivery. EEG was recorded from scalp electrodes placed according to the 10-20 system during eyes-closed. Theta activity (4-7 Hz) was filtered with a fast Fourier transform (FFT) and relative power calculated. The expected distribution of theta was found, with the greatest relative power centrally located and lesser amounts at frontal, occipital, and lateral derivations. Multiple regression models of theta at each electrode showed that increasing postnatal blood lead from 6 to 96 months was related to increasing relative theta power adjusted for age, sex and fetal suffering at delivery, in occipital derivations. The most significant increases in theta power were associated with blood lead levels (geometric mean = 10.3 microg/dl) measured between 54 and 72 months. Spatially weighted regression demonstrated that there was a significant antero-posterior gradient in lead-induced increase in relative theta power associated with postnatal blood lead levels at 54-72 months and 78-96 months. The greatest lead effect on both occipital relative theta power and the antero-posterior gradient of theta power was found with lead at an age during which relative theta power reaches its developmental maximum and starts to decrease. Results suggest that 54-72 months represent a critical period during which lead can exert lasting effects on the developmental pattern of theta activity. Occipital derivation of the largest effects of lead on theta activity may also be related to other lead-related developmental deficits.

Adult↗
PubMed2002

Spatial distribution of ventilation and perfusion in anesthetized dogs in lateral postures.

We aimed to assess the influence of lateral decubitus postures and positive end-expiratory pressure (PEEP) on the regional distribution of ventilation and perfusion. We measured regional ventilation (VA) and regional blood flow (Q) in six anesthetized, mechanically ventilated dogs in the left (LLD) and right lateral decubitus (RLD) postures with and without 10 cmH(2)O PEEP. Q was measured by use of intravenously injected 15-microm fluorescent microspheres, and VA was measured by aerosolized 1-microm fluorescent microspheres. Fluorescence was analyzed in lung pieces approximately 1.7 cm(3) in volume. Multiple linear regression analysis was used to evaluate three-dimensional spatial gradients of Q, VA, the ratio VA/Q, and regional PO(2) (Pr(O(2))) in both lungs. In the LLD posture, a gravity-dependent vertical gradient in Q was observed in both lungs in conjunction with a reduced blood flow and Pr(O(2)) to the dependent left lung. Change from the LLD to the RLD or 10 cmH(2)O PEEP increased local VA/Q and Pr(O(2)) in the left lung and minimized any role of hypoxia. The greatest reduction in individual lung volume occurred to the left lung in the LLD posture. We conclude that lung distortion caused by the weight of the heart and abdomen is greater in the LLD posture and influences both Q and VA, and ultimately gas exchange. In this respect, the smaller left lung was the most susceptible to impaired gas exchange in the LLD posture.

Aerosols↗
PubMed1998

Spatial distribution of potentiated synapses in hippocampus: dependence on cellular mechanisms and network properties.

Long-term potentiation (LTP) of synaptic transmission, studied intensively in reduced brain preparations such as hippocampal brain slices, is the leading candidate for the cellular/molecular basis of learning and memory. Serious consideration of LTP as underlying information storage in the intact brain, however, requires understanding how LTP can be induced selectively at specific synaptic sites in a neural system when the mechanisms underlying LTP are regulated by other structural and functional properties of the same neural system. In the studies reported here, we tested the hypothesis that different patterns of activity within the same population of entorhinal cortical afferents could lead to a selective potentiation of spatially distinct populations of synapses across different regions of the hippocampus, including those activated multisynaptically. We focused specifically on potentiation of direct, monosynaptic entorhinal input to dentate granule cells, which expresses an NMDA receptor-dependent LTP, and on potentiation of indirect, disynaptic entorhinal input to CA3 pyramidal cells, which is transmitted by the mossy fiber projection of dentate granule cells and expresses an NMDA receptor-independent LTP. The principal findings of these experiments show that lower stimulation frequencies (10-20 Hz) of entorhinal cortical axons selectively induce LTP of mossy fiber input to CA3 transsynaptically via excitation of dentate granule cells, and that patterns of stimulation of that mimic neuronal firing in the entorhinal cortex during endogenous theta rhythm (five-impulse bursts at 200 Hz, interburst intervals of 200 msec) induce LTP both monosynaptically for input to dentate granule cells and transsynaptically for mossy fiber input to CA3.

Animals↗
PubMedDate not supplied

Characterization of state transitions in spatially distributed, chaotic, nonlinear, dynamical systems in cerebral cortex.

The neurons of cerebral cortex are largely autonomous and generate activity that is manifested in trains of microscopic axonal action potentials. The neurons interact by sparse but numerous synaptic connections to generate macroscopic dendritic activity patterns that are observed in electroencephalographic (EEG) waves. The macroscopic patterns are constructed by the populations and they shape the output of cortical neurons in parallel arrays. Sensory cortexes receive sensory information in the form of microscopic action potentials, which induce state transitions in population dynamics. Each state transition transforms sensory information to perceptual meaning. The EEG reflects both kinds of activity. The sensory input is accessed by time ensemble averaging, whereas the perceptual output is found by spatial ensemble averaging. Spatial phase gradients in the EEG are useful for identifying EEG segments in a sequence of state transitions in response to sensory input. The rapidity and flexibility with which they take place give strong reason to postulate that the mechanism for the construction of these sequences of patterns is a dynamical system operating in a chaotic domain.

Action Potentials↗
PubMed2003

Advanced factor analysis of spatial distributions of PM2.5 in the eastern United States.

This work analyzes PM2.5 24-h average concentrations measured every third day at over 300 locations in the eastern United States during 2000. The non-negative factor analytic model, Positive Matrix Factorization, has been enhanced by modeling the dependence of PM2.5 concentrations on temperature, humidity, pressure, ozone concentrations, and wind velocity vectors. The model comprises 12 general factors, augmented by 5 urban-only factors intended to represent excess concentration present in urban locations only. The computed factor components or concentration fields are displayed as concentration maps, one for each factor, showing how much each factor contributes to the average concentration at each location. The factors are also displayed as flux maps that illustrate the spatial movement of PM2.5 aerosol, thus enabling one to pinpoint potential source areas of PM2.5. The quality of the results was investigated by examining how well the model reproduces especially high concentrations of PM2.5 on specific days at specific locations. Delimiting the spatial extent of all such factors that exhibit a clear regional maximum surrounded by an almost-zero outer domain lowered the uncertainty in the computed results.

Aerosols↗
PubMed2003

Spatial distribution and specification of mammalian replication origins during G1 phase.

We have examined the distribution of early replicating origins on stretched DNA fibers when nuclei from CHO cells synchronized at different times during G1 phase initiate DNA replication in Xenopus egg extracts. Origins were differentially labeled in vivo versus in vitro to allow a comparison of their relative positions and spacing. With nuclei isolated in the first hour of G1 phase, in vitro origins were distributed throughout a larger number of DNA fibers and did not coincide with in vivo origins. With nuclei isolated 1 h later, a similar total number of in vitro origins were clustered within a smaller number of DNA fibers but still did not coincide with in vivo origins. However, with nuclei isolated later in G1 phase, the positions of many in vitro origins coincided with in vivo origin sites without further change in origin number or density. These results highlight two distinct G1 steps that establish a spatial and temporal program for replication.

Animals↗