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Results for “spatial dependency”

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PubMed1991

One-way blocks in cardiac tissue: a mechanism for propagation failure in Purkinje fibres.

The concept of a one-way block, arising from a region of depressed tissue, has remained central to theories for cardiac arrhythmias. We show that both the geometry of a depressed region and spatial heterogeneities in depression are key factors for inducing such a block. By using an asymptotic approximation, known as the eikonal equation, to model qualitatively the movement of a depolarization wave-front down a Purkinje fibre bundle, we show how a one-way block in conduction may result from asymmetric constriction in the width of a depressed bundle. We demonstrate that this theory is valid for biologically relevant parameters and simulate a one-way block by numerically solving the eikonal approximation. We consider the case of non-uniform depression, where the planar travelling wave speed is spatially dependent. Here, numerical simulations indicate that such a spatial dependency may, in itself, be sufficient to produce a one-way block.

Animals↗

PubMed2004

Characterization of land contaminated by past heavy metal mining using geostatistical methods.

Aspects of mining operations, including production, processing, transportation, and, finally, abandoned mine workings, can undoubtedly cause varying degrees of environmental pollution and contamination. Characterization of the spatial distribution of pollutants in contaminated soil is important for risk assessment and soil remediation. This study has been carried out using both conventional statistics and geostatistical methods which are useful for site assessment, characterization, and monitoring situations where data are collected spatially. These methods are particularly suited to cases where spatial dependence structure and contour maps of contaminant concentration or other variables are needed. The aims of this paper were to determine the extent and severity of the pollution levels on land contaminated by past mining activity through using geostatistical techniques, assess sampling strategy, and interpret data for potentially contaminated land. In the study, a total of 329 soil samples was collected at 1-, 5-, and 10-m regular grid intervals of 100 x 1 00-m area of Carsington Pasture in the UK and analyzed using an automatic X-ray fluorescence machine for a large number of elements. Only a few elements (Cu, Pb, Zn, and Ba) were subsequently studied in detail because of their association with the mining operation. Directional and omnidirectional experimental semivariograms of elements Cu, Pb, Zn, and Ba for 10-m grid intervals, pooled data of 10- and 5-m grid intervals (S10+5), and pooled data of 10-, 5-, and 1-m grid interval (S10+5+1) showed that neither geometric nor zonal anisotropy exits in the data. The most evident spatial dependence structure of the continuity for omnidirectional experimental semivariogram, characterized by a spherical model, of each element was obtained for the pooled data set of S10+5. Pooling 1-m grid data to S10+5 shaded the spatial dependence structure. The spherical model adopted for each element embodied no nugget effect, indicating that mineralization did not occur as nuggets in the study area, but it did show different ranges of influence and sill values. Spherical semivariogram model of each element for the pooled data set of S10+5 passed the cross validation tests. Graphs of cross validation test results proved that the isotropic spherical model, with its parameters and kriging search parameters, is representative for the area under study. The values of 10,000 grid nodes established regularly over the study area were estimated by kriging interpolation technique to smooth out the contours. Kriged maps for the Cu, Pb, Zn, and Ba elements were plotted. Anomalies such as bull's-eyes and drift did not observed in the maps generated. Contour maps of estimated element concentrations revealed the extent and severity of contamination. Spatial distribution maps of the elements might be used in the remediation studies and help decision-makers and others involved in the abandoned heavy metal mining site in the UK.

Data Interpretation, Statistical↗

PubMed1992

Stimulus mislocalization depends on spatial frequency.

It was previously reported that briefly presented peripheral stimuli are perceived closer to fixation than continuously presented stimuli at the same eccentricity; this effect has, however, not proved consistently replicable. In this study it was investigated whether the misperception of location might depend upon the spatial frequency content of the stimulus. Spatial-frequency-filtered vertical bars were displayed briefly and their locations were judged relative to continuously visible comparison spots. For monocular stimuli, a significant foveopetal mislocalization of the bar was obtained that increased in size as spatial frequency was lowered. Even larger mislocalizations were obtained for dichoptically presented horizontally disparate pairs of bars, and this effect also increased at low spatial frequencies. Possible underlying mechanisms are discussed, and spatial frequency is suggested to have been the confounding factor in previous studies.

Eye Movements↗

PubMed1991

Motion detection is dependent on spatial frequency not size.

The maximum displacement for the detection of apparent motion is measured using stimuli made up of micro-patterns randomly distributed across the stimulus field. The micro-patterns were Gabor functions or half-wave rectified Gabor functions. Dmax is shown to be dependent of the spatial frequency content of the stimulus, and to be largely independent of the size of the stimulus elements or the number of them in the stimulus field (within limits). Evidence is shown for Dmax to be considered a measure of motion detection that reveals properties of low level underlying motion mechanisms.

Contrast Sensitivity↗

PubMed2004

BOLD repetition decreases in object-responsive ventral visual areas depend on spatial attention.

Functional imaging studies of priming-related repetition phenomena have become widely used to study neural object representation. Although blood oxygenation level-dependent (BOLD) repetition decreases can sometimes be observed without awareness of repetition, any role for spatial attention in BOLD repetition effects remains largely unknown. We used fMRI in 13 healthy subjects to test whether BOLD repetition decreases for repeated objects in ventral visual cortices depend on allocation of spatial attention to the prime. Subjects performed a size-judgment task on a probe object that had been attended or ignored in a preceding prime display of 2 lateralized objects. Reaction times showed faster responses when the probe was the same object as the attended prime, independent of the view tested (identical vs. mirror image). No behavioral effect was evident from unattended primes. BOLD repetition decreases for attended primes were found in lateral occipital and fusiform regions bilaterally, which generalized across identical and mirror-image repeats. No repetition decreases were observed for ignored primes. Our results suggest a critical role for attention in achieving visual representations of objects that lead to both BOLD signal decreases and behavioral priming on repeated presentation.

Adult↗

PubMed2004

Hierarchical Bayesian modeling in dichotomous processes in the presence of nonresponse.

Sampling units that do not answer a survey may dramatically affect the estimation results of interest. The response may even be conditional on the outcome of interest in the survey. If estimates are found using only those who responded, the estimate may be biased, known as nonresponse bias. We are interested in finding estimates of success rates from a survey. We begin by looking at two current Bayesian approaches to treating nonresponse in a hierarchical model. However, these approaches do not consider possible spatial correlations between domains for either success rate or response rate. We build a Bayesian hierarchical spatial model to explicitly estimate the success rate, response rate given success, and response rate given failure. The success rates in the domains of the survey are allowed to be spatially correlated. We also allow spatial dependence between domains in both response rate given success and response rate given failure. Spatial dependence is induced by a common latent spatial structure between the two conditional response rates. We use the 1998 Missouri Turkey Hunting Survey to illustrate this methodology. We find significant spatial correlation in the success rates and incorporating nonrespondents has an impact on the success rate estimates.

Animals↗

PubMed1986

Sustained and transient temporal integration functions depend on spatial frequency, not grating area.

Contrast sensitivity for sinusoidal gratings was measured by a two-alternative forced-choice method and plotted as a function of duration. Gratings of large or small area and low or high spatial frequency (0.75 or 8.25 c/deg) were used. The temporal integration curve was much flatter in the low spatial frequency conditions than in the high frequency conditions, but its shape did not depend on the area of the grating. The results support the established view that the visual system exhibits transient temporal properties at low spatial frequencies and sustained properties at higher frequencies, but disconfirm the recent proposal that temporal integration depends on the area of the grating (expressed in square-cycles) rather than its spatial frequency.

Fovea Centralis↗

PubMed1999

Detection of temporal structure depends on spatial structure.

Observers can more easily detect correlated patterns of temporal contrast modulation within hybrid visual images composed of two components when those components are drawn from the same original picture (Blake, R., & Yang, Y. (1997). Proceedings of the National Academy of Science, 94, 7115-7119). To learn whether spatial phase is a mediating variable, we measured thresholds for detection of contrast modulation over time among component gratings while manipulating spatial phase among those components. In Experiment 1, observers more easily detected correlated contrast modulation when two component gratings were aligned in peaks-subtract phase. Experiment 2 showed that this phase-dependent detectability of synchronized contrast modulation is mediated by the phase-dependent, non-linear interaction among spatial frequency channels. The rigorous evaluation of several a priori reasonable hypotheses indicates that the phase-dependent detectability is not based on local spatial features such as local luminance, contrast or luminance gradient. Taken together, our results indicate that the spatial phase relationship and the temporal correlation of contrast modulation of two component gratings are both important for triggering facilitatory interaction between neural analyzers tuned to those gratings.

Contrast Sensitivity↗

PubMed1993

Thresholds and stability analysis of models for the spatial spread of a fatal disease.

A simple two-class (susceptibles and infectives) model describing the dynamics of a fatal disease in a variable-size population is presented and analysed. Spatial dependence is introduced into the model by considering two different mechanisms for the geographic spread of the disease: nonlocal interaction between susceptibles and infectives, and migratory spread of the animals. The steady states and their stability for these spatially dependent models are deduced; no spatially heterogeneous steady states were possible. For nonlocal interaction, there were two spatially uniform steady states: the trivial state (no infectives or susceptibles), which was unstable, and the endemic state (constant proportion of the population infected), which was locally asymptotically stable. With migratory spread, the number of spatially uniform steady states was dependent on the boundary conditions imposed. With hostile (Dirichlet) boundary conditions, only the trivial steady state was possible and its local stability found to depend on the rate of diffusion of the total population. With no-flux (Neumann) boundary conditions, the steady states are the trivial and endemic states; these were unstable and locally asymptotically stable, respectively.

Animals↗

PubMed2005

Spatial heterogeneity of DTPA-extractable zinc in cultivated soils induced by city pollution and land use.

The spatial heterogeneity of DTPA-extractable zinc in the cultivated soils of Shenyang suburbs in Liaoning Province of China was investigated, and its map was drawn by the methods of geostatistics combined with geographic information system. The data of soil DTPA-extractable zinc fitted normal distribution after logarithm transformation, and its semivariogram fitted a spherical model. The semivariogram indicated that the spatial dependence of soil DTPA-extractable zinc content was moderate, with the spatial dependence range of 1.69 km and the fractal dimension of 1.96. Stochastic factors contributed to 49.9% of the spatial variability, while structural factors contributed to 50.1% of it. The spatial heterogeneity of soil DTPA-extractable zinc shown by a kriged interpolation map was deeply influenced by stochastic factors such as city pollution, land use pattern and crop distributions. For example, the average content of Zn in vegetable garden soils was 2.5-4 times as much as in their originated soils, and was lower in paddy soils than in their originated soils. The areas with a higher content of soil DTPA-extractable zinc appeared in the near suburbs and the riverside along Hunhe River and the wastewater drainage of Xihe River, and the extremely high values in the near suburb of the city's residential area were a striking feature, indicating the key role of city pollution in the spatial heterogeneity of soil DTPA-extractable zinc. When recorded in the form of a soil pollution map, the results of such a survey make it possible to identify the unusually polluted areas, and to provide more information for precise agriculture and environmental control.

Agriculture↗

PubMed2000

Geostatistical analysis of the distribution of NH(4)(+) and NO(2)(-)-oxidizing bacteria and serotypes at the millimeter scale along a soil transect.

Soil is known to be heterogeneous for different activities at several spatial scales. Most studies have focused on macro- and meso-scales but micro-scales are rarely addressed. Hence, the spatial structure of NH(4)(+)- and NO(2)(-)-oxidizers and of various serotypes of the latter was studied along two transects of approximately 10 cm, with two micro-samples taken from each millimeter. The presence of NH(4)(+)- and NO(2)(-)-oxidizers in a micro-sample was detected using colorimetric tests for the presence or absence of NO(2)(-) in cultures of the micro-samples. Geostatistics was used to determine the range of spatial influence of the bacterial types. For both types, semi-variograms indicated a non-random spatial pattern. The spatial dependence ranged from 2 to 4 mm for NO(2)(-)- and NH(4)(+)-oxidizers respectively, and the two bacterial types were not independently spatially located. Among the six serotypes of NO(2)(-)-oxidizers, only one exhibited a spatial dependence. The existence of a spatial structure at the millimeter scale suggests that micro-scale sampling should be employed for soil studies. Therby, data on bacterial populations and activities can be referred to a spatial scale which is meaningful to these organisms.

Journal Article↗

PubMed2003

Generalized reconstruction of phase contrast MRI: analysis and correction of the effect of gradient field distortions.

To characterize gradient field nonuniformity and its effect on velocity encoding in phase contrast (PC) MRI, a generalized model that describes this phenomenon and enables the accurate reconstruction of velocities is presented. In addition to considerable geometric distortions, inhomogeneous gradient fields can introduce deviations from the nominal gradient strength and orientation, and therefore spatially-dependent first gradient moments. Resulting errors in the measured phase shifts used for velocity encoding can therefore cause significant deviations in velocity quantification. The true magnitude and direction of the underlying velocities can be recovered from the phase difference images by a generalized PC velocity reconstruction, which requires the acquisition of full three-directional velocity information. The generalized reconstruction of velocities is applied using a matrix formalism that includes relative gradient field deviations derived from a theoretical model of local gradient field nonuniformity. In addition, an approximate solution for the correction of one-directional velocity encoding is given. Depending on the spatial location of the velocity measurements, errors in velocity magnitude can be as high as 60%, while errors in the velocity encoding direction can be up to 45 degrees. Results of phantom measurements demonstrate that effects of gradient field nonuniformity on PC-MRI can be corrected with the proposed method.

Blood Flow Velocity↗

PubMed2004

Spatial analysis of the distribution of intestinal nematode infections in Uganda.

The spatial epidemiology of intestinal nematodes in Uganda was investigated using generalized additive models and geostatistical methods. The prevalence of Ascaris lumbricoides and Trichuris trichiura was unevenly distributed in the country with prevalence greatest in southwest Uganda whereas hookworm was more homogeneously distributed. A. lumbricoides and T. Trichiura prevalence were nonlinearly related to satellite sensor-based estimates of land surface temperature; hookworm was nonlinearly associated with rainfall. Semivariogram analysis indicated that T. trichiura prevalence exhibited no spatial structure and that A. lumbricoides exhibited some spatial dependency at small spatial distances, once large-scale, mainly environmental, trends had been removed. In contrast, there was much more spatial structure in hookworm prevalence although the underlying factors are at present unclear. The implications of the results are discussed in relation to parasite spatial epidemiology and the prediction of infection distributions.

Adolescent↗

PubMed2004

Background gradient suppression in stimulated echo NMR diffusion studies using magic pulsed field gradient ratios.

By evaluating the spin echo attenuation for a generalized 13-interval PFG NMR sequence consisting of pulsed field gradients with four different effective intensities (F(p/r) and G(p/r)), magic pulsed field gradient (MPFG) ratios for the prepare (G(p)/F(p)) and the read (G(r)/F(r)) interval are derived, which suppress the cross term between background field gradients and the pulsed field gradients even in the cases where the background field gradients may change during the z-store interval of the pulse sequence. These MPFG ratios depend only on the timing of the pulsed gradients in the pulse sequence and allow a convenient experimental approach to background gradient suppression in NMR diffusion studies with heterogeneous systems, where the local properties of the (internal) background gradients are often unknown. If the pulsed field gradients are centered in the tau-intervals between the pi and pi/2 rf pulses, these two MPFG ratios coincide to eta=G(p/r)/F(p/r)=1-8/[1+(1/3)(delta/tau)(2)]. Since the width of the pulsed field gradients (delta) is bounded by 0< or =delta< or =tau, eta can only be in the range of 5< or =-eta< or =7. The predicted suppression of the unwanted cross terms is demonstrated experimentally using time-dependent external gradients which are controlled in the NMR experiment as well as spatially dependent internal background gradients generated by the magnetic properties of the sample itself. The theoretical and experimental results confirm and extend the approach of Sun et al. (J. Magn. Reson. 161 (2003) 168), who recently introduced a 13-interval type PFG NMR sequence with two asymmetric pulsed magnetic field gradients suitable to suppress unwanted cross terms with spatially dependent background field gradients.

Diffusion↗

PubMed2004

Effects of land management change on spatial variability of organic matter and nutrients in paddy field: a case study of Pinghu, China.

The Household Responsibility System initiated in the late 1970s in China has brought a profound change to its rural economy. The shift from the collective farming system to individual family farms has changed land management. The change, including fertilization and crop systems, may have significant effects on soil quality and agro-environmental sustainability. However, very little research is being carried out on the impact of reformed land tenure systems on the spatial variability of soil nutrients. In this study, geostatistics was applied to analyze changes in the spatial variability of soil organic matter and nutrients in paddy fields in Pinghu County, China after 20 years of land management change. In 1984 and 2002, 617 and 131 locations were selected, respectively, for collecting surface soil samples to analyze soil organic matter (SOM), total nitrogen (TN), available phosphorus (AP), and available potassium (AK). From 1984 to 2002, variability of the SOM and TN changed from strongly to moderately spatial-dependent, and the variability of AP remained weakly dependent on space, but that of the AK changed from moderately to weakly spatial-dependent. That the trend of the variability of four soil properties in 2002 became weaker than that in 1984 showed that the extrinsic factors (soil management practices, such as fertilization) weakened the effect by intrinsic factors (soil formation factors, such as soil parent materials) owing to a long period of land management change. The temporal geographic maps of the SOM and nutrients spatial distributions suggested that the concentrations of the SOM and nutrients had been changed to different extents during the period. Significant increase in AP and decrease in AK were noted. The changes were likely due to the imbalance between N, P, and K fertilizers and increased grain yield.

Agriculture↗

PubMed1997

Spatial Patterns of Rhizoplane Populations of Pseudomonas fluorescens.

Geostatistical analysis was used to compare rhizoplane colonization patterns of an antibiotic-producing biological control bacterium versus a non-antibiotic-producing mutant strain. Pea seeds were inoculated with Pseudomonas fluorescens 2-79RN(inf10) or P. fluorescens 2-79-B46 (a phenazine-deficient Tn5 mutant of P. fluorescens 2-79RN(inf10)) (10(sup8) CFU/pea), planted in sterile sand, and incubated at 20(deg)C. After 3 days, seedlings were prepared for scanning electron microscopy. Photomicrographs (x1,000) of the root surface were taken at the seed-root junction and at 0.5-cm intervals to the root tip. Bacterial counts on the root surface were made in 5- by 5-(mu)m sample units over an area which was 105 by 80 (mu)m. Coordinates and number of bacteria were recorded for each sample unit. Spatial statistics were calculated by covariance for the following directions: omnidirectional, 0, 45, 90, and 135(deg). The ranges of spatial influence and nugget (estimator of spatially dependent variation) were determined. For both P. fluorescens 2-79RN(inf10) and P. fluorescens 2-79-B46, spatial structure was evident along the entire root, particularly in the 0(deg) direction (along the root length) (e.g., range = 24 (mu)m, nugget = 0.52). The degree of spatial dependence observed indicated aggregation of bacterial cells. No differences were detected in the spatial patterns of colonies of P. fluorescens 2-79RN(inf10) and P. fluorescens 2-79-B46, indicating that the lack of phenazine production did not influence spatial patterns on the rhizoplane.

Journal Article↗

PubMed2003

Longitudinal axis of the hippocampus: both septal and temporal poles of the hippocampus support water maze spatial learning depending on the training protocol.

It has been suggested previously that 30% sparing of the hippocampus is enough to support spatial learning of a reference memory task in a water maze provided the spared tissue is located septally (Moser et al. 1995, Proc Natl Acad Sci USA 92:9697-9701). Therefore, the temporal hippocampus may not be involved in spatial memory. Place cells are also found in this part of the structure, and it has been suggested that these place cells have larger, less well-tuned place fields than are found in the septal hippocampus. We tested the possibility that the temporal hippocampus might be involved in spatial learning when the animals are required to distinguish between different contexts. Experiment 1 was a replication of the findings reported by Moser et al., using their protocol (8 trials/day, 6 days) and the groups with 20-40% hippocampus spared septally or temporally (volume assessed by quantitative volumetric techniques). In experiment 2, rats with also 20-40% sparing of the hippocampus either septally or temporally were trained in two water maze concurrently (four trials/day/water maze, 8 days). Rats with 20-40% hippocampus spared temporally were able to learn the two water maze tasks normally, and no difference was observed between rats with septal and temporal hippocampus spared across different measures of performance. In experiment 3, rats with 20-40% hippocampus spared septally or temporally were trained in one water maze as in experiment 1, but using a spaced training protocol similar to that of experiment 2 (four trials/day, 8 days). Rats with temporal hippocampus spared developed a preference for the training quadrant and acquired levels of performance indistinguishable from those of rats with septal hippocampus spared. The results suggest that the temporal hippocampus can support the learning of two, but also one, spatial water maze reference memory task, provided the training protocol is adequate.

Animals↗

PubMed2000

Monte Carlo calculations of the total-to-peak ratio in gamma-ray spectrometry

Monte Carlo calculations of the total-to-peak ratio for a coaxial closed end n-type gamma-ray detector were performed and the results compared to the measured values. Whereas reasonable agreement between the calculated and measured spatial dependencies was achieved, the calculation overestimates the absolute experimental values by about 10%. The spatial dependence of the total-to-peak ratio is discussed in terms of detector geometry.

Journal Article↗