PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “spatial localization”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 397 records · Page 22Linked to original sources
PubMed2000

Bubonic plague: a metapopulation model of a zoonosis.

Bubonic plague (Yersinia pestis) is generally thought of as a historical disease; however, it is still responsible for around 1000-3000 deaths each year worldwide. This paper expands the analysis of a model for bubonic plague that encompasses the disease dynamics in rat, flea and human populations. Some key variables of the deterministic model, including the force of infection to humans, are shown to be robust to changes in the basic parameters, although variation in the flea searching efficiency, and the movement rates of rats and fleas will be considered throughout the paper. The stochastic behaviour of the corresponding metapopulation model is discussed, with attention focused on the dynamics of rats and the force of infection at the local spatial scale. Short-lived local epidemics in rats govern the invasion of the disease and produce an irregular pattern of human cases similar to those observed. However, the endemic behaviour in a few rat subpopulations allows the disease to persist for many years. This spatial stochastic model is also used to identify the criteria for the spread to human populations in terms of the rat density. Finally, the full stochastic model is reduced to the form of a probabilistic cellular automaton, which allows the analysis of a large number of replicated epidemics in large populations. This simplified model enables us to analyse the spatial properties of rat epidemics and the effects of movement rates, and also to test whether the emergent metapopulation behaviour is a property of the local dynamics rather than the precise details of the model.

Animals↗
PubMed2002

The application of local measures of spatial autocorrelation for describing pattern in north Australian landscapes.

This paper tests the use of a spatial analysis technique, based on the calculation of local spatial autocorrelation, as a possible approach for modelling and quantifying structure in northern Australian savanna landscapes. Unlike many landscapes in the world, northern Australian savanna landscapes appear on the surface to be intact. They have not experienced the same large-scale land clearance and intensive land management as other landscapes across Australia. Despite this, natural resource managers are beginning to notice that processes are breaking down and declines in species are becoming more evident. With future declines of species looking more imminent it is particularly important that models are available that can help to assess landscape health, and quantify any structural change that takes place. GIS and landscape ecology provide a useful way of describing landscapes both spatially and temporally and have proved to be particularly useful for understanding vegetation structure or pattern in landscapes across the world. There are many measures that examine spatial structure in the landscape and most of these are now available in a GIS environment (e.g. FRAGSTATS* ARC, r.le, and Patch Analyst). All these methods depend on a landscape described in terms of patches, corridors and matrix. However, since landscapes in northern Australia appear to be relatively intact they tend to exist as surfaces of continuous variation rather than in clearly defined homogeneous units. As a result they cannot be easily described using entity-based models requiring patches and other essentially cartographic approaches. This means that more appropriate methods need to be developed and explored. The approach examined in this paper enables clustering and local pattern in the data to be identified and forms a generic method for conceptualising the landscape structure where patches are not obvious and where boundaries between landscape features are difficult to determine. Two sites are examined using this approach. They have been exposed to different degrees of disturbance by fire and grazing. The results show that savanna landscapes are very complex and that even where there is a high degree of disturbance the landscape is still relatively heterogeneous. This means that treating savanna landscapes as being made up of homogeneous units can limit analysis of pattern, as it can over simplify the structure present, and that methods such as the autocorrelation approach are useful tools for quantifying the variable nature of these landscapes.

Australia↗
PubMed1982

Spatial organization of neurochemically classified interneurons of the goldfish retina-I. Local patterns.

Certain interneurons in the goldfish retina are uniquely specifiable by their abilities to take up exogenously supplied [3H]gamma-aminobutyric acid (GABA), [3H]glycine, [3H]dopamine of [3H]serotonin al low micromolar concentrations. Each ligand labels one or two unique populations of interneurons yielding six cell types characterized by soma location, soma size, level and form of dendritic arborization, and local spatial patterning. This report summarizes these qualitative features and provides quantitative evidence on the size and spatial distributions of : (1) GABA-ergic horizontal cells, (2) GABA-ergic amacrine cells, (3) glycinergic amacrine cells; (4) glycinergic interplexiform cells; (5) dopaminergic interplexiform cells; and (6) indoleaminergic amacrine cells.

Animals↗
PubMed1998

Two-dimensional Gabor-type receptive field as derived by mutual information maximization.

Two-dimensional receptive fields are investigated from an information theoretic viewpoint. It is known that the spatially localized and orientation- and spatial-frequency-tuned receptive fields of simple cells in the visual cortex are well described by Gabor functions. This paper shows that the Gabor functions are derived as solutions for a certain mutual-information maximization problem. This means that, in a low signal-to-noise ratio limit, the Gabor-type receptive field can extract the maximum information from input local images with regard to their categories. Accordingly, this suggests that the receptive fields of simple cells are optimally designed from an information theoretic viewpoint.

Journal Article↗
PubMed2006

Bumblebees experience landscapes at different spatial scales: possible implications for coexistence.

Coexistence in bumblebee communities has largely been investigated at local spatial scales. However, local resource partitioning does not fully explain the species diversity of bumblebee communities. Theoretical studies provide new evidence that partitioning of space can promote species coexistence, when species interact with their environment at different spatial scales. If bumblebee species possess specific foraging ranges, different spatial resource utilisation patterns might operate as an additional mechanism of coexistence in bumblebee communities. We investigated the effects of the landscape-wide availability of different resources (mass flowering crops and semi-natural habitats) on the local densities of four bumblebee species at 12 spatial scales (landscape sectors with 250-3,000 m radius) to indirectly identify the spatial scales at which the bumblebees perceive their environment. The densities of all bumblebee species were enhanced in landscapes with high proportions of mass flowering crops (mainly oilseed rape). We found the strongest effects for Bombus terrestris agg. and Bombus lapidarius at large spatial scales, implying foraging distances of 3,000 and 2,750 m, respectively. The densities of Bombus pascuorum were most strongly influenced at a medium spatial scale (1,000 m), and of Bombus pratorum (with marginal significance) at a small spatial scale (250 m). The estimated foraging ranges tended to be related to body and colony sizes, indicating that larger species travel over larger distances than smaller species, presumably enabling them to build up larger colonies through a better exploitation of food resources. We conclude that coexistence in bumblebee communities could potentially be mediated by species-specific differences in the spatial resource utilisation patterns, which should be considered in conservation schemes.

Animals↗
PubMed1972

The caudate nucleus egocentric localization system.

It is hypothesized that the motor and spatial functions ascribed to the caudate nucleus are actually different aspects of a single system for spatial localization. Within this system movement programs are the code for spatial locations, i.e. the position of any point in space is defined by the head/eye movement necessary to bring that point into focus. This is an egocentric system; points in space are defined solely in terms of their distance and direction from the observer. The experimental finding that caudate lesions in rats impair their performance in an egocentric localization task, but not in another, equally difficult spatial task, demonstrates a caudate role in this system. Since, in an egocentric system, positions in space are defined relative to the observer, every movement of the observer must be accompanied by a compensatory updating of the internal representation of any given point. Studies with Huntington's Chorea patients are consistent with a caudate involvement in this feature of the system also. Electrophysiological experiments with cats indicate that vestibular information, which can be used in the compensatory updating process, is available to the caudate. It is suggested that this system is used by normal animals in delayed response performance.

Animals↗
PubMed1997

Retention of local information in generation of subjective contours.

Temporal integration characteristics of subjective contour perception was investigated, using the sequential presentation of two pairs of disks with a sector removed. In the first experiment, by matching the contrast of a "real" stimulus, the perceived contrast of the subjective contours was measured as a function of the stimulus onset asynchrony (SOA) between the two pairs of the disks. With increasing SOA, the perceived contrast decreased gradually and levelled off at the SOA of ca 372 msec (1 SD = 119 msec). In the second experiment, the perceived contrast of the inducing disks was measured as a function of SOA using the matching method. The time limit of the additive effect for the contrast perception of the inducing disks was much shorter than that for subjective contours; the critical SOA was ca 65 msec (1 SD = 36 msec). The remarkable difference of the integration time was explained by a hierarchical process; the local spatial filtering, the retention of local information, and the completion of gaps by multiplicative or AND operation.

Contrast Sensitivity↗
PubMed1994

Localization of element clusters by the human visual system.

A spatial localization task was used to determine the accuracy of localization of random element clusters about their centroid. The random element clusters were generated within a circular region and the observers' task was to decide whether the cluster lay to the right or left of a reference line, defined by two vertically separated reference elements. Both the reference elements and the cluster elements themselves were comprised of spatially narrowband stimuli (gabor patches), and were presented at a constant suprathreshold level. The thresholds for localization of the element cluster were measured with varying element number, and for a number of different sizes of circular region and element. The relationship between localization threshold and element number was not monotonic. Thresholds were found to rise with increasing element number up to about six elements within a region, and to then fall with further increase in element number. Asymptotic thresholds at high element number were indistinguishable from those obtained for filled circles of the same size. The results do not conform to any one of a number of models based on a centroid analysis alone. By manipulating the spatial and orientational properties of the elements comprising the cluster to be localized it was determined that the more central mechanism underlying localization receive input from different spatial and orientation filters.

Contrast Sensitivity↗
PubMed1986

Altered excitability of goldfish Mauthner cell following axotomy. II. Localization and ionic basis.

The ionic basis and spatial localizations of spike generation were examined in normal and axotomized goldfish Mauthner (M-) cells using intra- and extracellular recordings and pharmacological manipulation of ionic conductances, including localized iontophoretic drug applications. Tetrodotoxin (TTX) abolished both the initial segment (IS) spike in normal cells and the larger, two-component action potential in axotomized cells, whereas calcium (Ca2+) blockers did not. Thus, sodium (Na+) appears to be the major inward current carrier in both cases. A shoulder or plateau following the fast-rising Na+-dependent action potential was unmasked in both normal and axotomized M-cells by intracellular injections of tetraethylammonium (TEA), either alone or in conjunction with 4-aminopyridine (4-AP) or cesium (Cs+). This plateau potential was abolished by superfusing with saline containing the Ca2 antagonists, Co2+, Mn2+, or Cd2+. However, barium (Ba2+), which normally substitutes for Ca2+ and also blocks K+ conductances, did not produce a plateau spike, and no action potentials could be evoked in the presence of TTX. Simultaneous extra- and intracellular recordings from the soma and lateral dendrite revealed that both the full-sized axotomized spike and its individual labile components were always maximal at the soma. These data support the earlier suggestion that the axotomy-induced electrogenicity is primarily localized to that region. Iontophoretic application of TTX inside the axon cap, a distinctive neuropil surrounding the initial segment and the axon hillock and circumscribed by a glial border, and at various positions along the lateral dendrite confirmed the Na+-dependency of the action potentials recorded in normal and axotomized cells and further demonstrated that the soma generates the additional spike component in the latter. The results suggest that axotomy causes a persistent change in voltage-gated Na+ channel distribution in the M-cell, with Na+ channels appearing or becoming more numerous in the soma while becoming less concentrated in the initial segment-axon hillock. Possible related shifts in other voltage-dependent conductances are also discussed. Finally, these are the first detailed studies of the ionic basis of axotomy-induced electrogenicity in a vertebrate neuron, central or peripheral, and the similarity to the results obtained with invertebrate neurons suggests common mechanisms underlying the axon reaction.

Action Potentials↗
PubMed1992

[Errors of monocular localization in strabismic amblyopia. Two-dimensional distortion].

Spatial distortions, i.e. spatial localization errors, and precision of localization were measured monocularly in the central visual field of 7 normal observers, 17 strabismic amblyopes and 2 anisometropic amblyopes. The task of the subjects was to construct circles of 2 degrees, 4 degrees and 6 degrees radius around a fixation point, using the dominant and the amblyopic eye in turn. Normal observers set distances on the vertical meridian smaller than distances on the horizontal meridian. Anisometropic amblyopes showed localization errors and variances similar to those of normal observers. The amblyopic eyes of strabismic observers with a large angle strabismus and deep amblyopia showed significant, individually different localization errors correlating with the depth of amblyopia. Strabismics with microstrabismus exhibited parallels between monocular localization and dichoptic retinal correspondence.

Amblyopia↗
PubMed1996

Geometric determinants of the place fields of hippocampal neurons.

The human hippocampus has been implicated in memory, in particular episodic or declarative memory. In rats, hippocampal lesions cause selective spatial deficits, and hippocampal complex spike cells (place cells) exhibit spatially localized firing, suggesting a role in spatial memory, although broader functions have also been suggested. Here we report the identification of the environmental features controlling the location and shape of the receptive fields (place fields) of the place cells. This was done by recording from the same cell in four rectangular boxes that differed solely in the length of one or both sides. Most of our results are explained by a model in which the place field is formed by the summation of gaussian tuning curves, each oriented perpendicular to a box wall and peaked at a fixed distance from it.

Animals↗
PubMed2006

Long-range interactions and evolutionary stability in a predator-prey system.

Evolving ecosystems often are dominated by spatially local dynamics, but many also include long-range transport that mixes spatially separated groups. The existence of such mixing may be of critical importance since research shows spatial separation may be responsible for long-term stability of predator-prey systems. Complete mixing results in rapid global extinction, while spatial systems achive long term stability due to an inhomogeneous spatial pattern of local extinctions. We consider the robustness of a generic evolving predator-prey or host-pathogen model to long-range mixing and find a transition to global extinction at nontrivial values implying that even if significant mixing already exists, a small amount of additional mixing may cause extinction. Our results are relevant to the global mixing of species due to human intervention and to global transport of infectious disease.

Animals↗
PubMed2003

An adaptive spatial fuzzy clustering algorithm for 3-D MR image segmentation.

An adaptive spatial fuzzy c-means clustering algorithm is presented in this paper for the segmentation of three-dimensional (3-D) magnetic resonance (MR) images. The input images may be corrupted by noise and intensity nonuniformity (INU) artifact. The proposed algorithm takes into account the spatial continuity constraints by using a dissimilarity index that allows spatial interactions between image voxels. The local spatial continuity constraint reduces the noise effect and the classification ambiguity. The INU artifact is formulated as a multiplicative bias field affecting the true MR imaging signal. By modeling the log bias field as a stack of smoothing B-spline surfaces, with continuity enforced across slices, the computation of the 3-D bias field reduces to that of finding the B-spline coefficients, which can be obtained using a computationally efficient two-stage algorithm. The efficacy of the proposed algorithm is demonstrated by extensive segmentation experiments using both simulated and real MR images and by comparison with other published algorithms.

Algorithms↗
PubMed1999

An efficient MR phosphorous spectroscopic localization technique for studying ischemic heart.

To obtain the spatially resolved (31)P spectroscopic image from myocardium during an acute myocardium ischemia at a high signal-to-noise ratio (SNR) in a very limited time window, we have exploited the spatial variation of the radiofrequency (RF) field produced by a single loop transmit/receive (TR) RF coil along its axis for spatial discrimination. By incrementally lengthening the duration of a square RF excitation pulse, the positional information can be systematically encoded as harmonics of various orders in MR signal. In the in vivo open-chest animal experiment, this RF coil was surgically sutured onto the epicardial surface of the left ventricular (LV) wall over the region perfused by the left anterior descending coronary artery. Using only 17 encoding steps, we have obtained one-dimensional (31)P spectroscopic images from both a multiple-layer phosphor phantom and an in vivo LV myocardium. In the animal study, the cardiac gating is used with respiratory synchronization. The MR data were only collected during the end diastole phase of the cardiac cycle (cardiac and respiratory synchronized) with an effective sequence repetition time (TR) of 6 seconds (to ensure the complete relaxation of the phosphorous magnetization). The total acquisition time for a complete experiment is about 10 minutes. Prior to the CSI reconstruction process, the raw data matrix was zero-filled in the spatial dimension. The spatially resolved metabolite map exhibited all the metabolite peaks including creatine phosphate and adenosine triphosphate. At the layer of endocardium, two peaks corresponding to 2, 3-diphosphoglycerate, which is contained in the erythrocytes, were clearly seen in the LV wall. Also, the method allows compensation in both volume and coil sensitivity variations for the resulting spectra. All results have demonstrated that it is an efficient nuclear magnetic resonance method capable of obtaining high-quality (31)P spectroscopic images with both excellent spatial localization and SNR in the research of cardiac ischemia. J. Magn. Reson. Imaging 1999;10:892-898.

Animals↗
PubMed1996

Localizing quantal currents along frog neuromuscular junctions.

1. We spatially localized the origins of quantal currents by recording simultaneously with two intracellular electrodes and employing the prediction of the one-dimensional cable equations that the time integrals of the resulting voltage changes fall off exponentially with distance. 2. Miniature endplate potentials (MEPPs) were more frequent near the centre of the endplate. In contrast to some work using other methods, we did not find MEPPs originating at the margins of the endplate to be strikingly smaller. 3. Spontaneous MEPPs and uniquantal endplate potentials (EPPs) were released over the same length of endplate and with the same relative probabilities at different regions. 4. Nicotinic agonists decreased evoked quantal output, but did not change the length over which uniquantal EPPs were generated. We conclude they do not block nerve conduction in the terminals. 5. Data sets were obtained with an extracellular electrode and two intracellular electrodes. The extracellular electrode was invariably near the centre of the region in which congruous MEPPs appeared to be generated. However, the range in the calculated positions of the synchronous MEPPs was as long as 0.8 mm. Therefore, it may be possible that extracellular electrodes have a longer recording range than commonly assumed.

Animals↗
PubMed1995

Differences in influence between pitched-from-vertical lines and slanted-from-frontal horizontal lines on egocentric localization.

The visual field exerts powerful effects on egocentric spatial localization along both horizontal and vertical dimensions. Thus, (1) prism-produced visual pitch and visual slant generate similar mislocalizations of visually perceived eye level (VPEL) and visually perceived straight ahead (VPSA) and (2) in darkness curare-produced extraocular muscle paresis under eccentric gaze generates similar mislocalizations in VPEL and VPSA that are essentially eliminated by introducing a normal visual field. In the present experiments, however, a search for influences of real visual slant on VPSA to correspond to the influences of visual pitch on VPEL failed to find one. Although the elevation corresponding to VPEL changes linearly with the pitch of a visual field consisting of two isolated 66.5 degrees-long pitched-from-vertical lines, the corresponding manipulation of change in the slant of either a horizontal two-line or a horizontal four-line visual field on VPSA did not occur. The average slope of the VPEL-versus-pitch function across 5 subjects was +0.40 over a +/- 30 degrees pitch range, but was indistinguishable from 0.00 for the VPSA-versus-slant function over a +/- 30 degrees slant range. Possible contributions to the difference between susceptibility of VPEL and VPSA to visual influence from extraretinal eye position information, gravity, and several retinal gradients are discussed.

Adult↗
PubMed2000

Apparent position governs contour-element binding by the visual system.

An assumption inherent in many models of visual space is that the spatial coordinates of retinal cells implicitly give rise to the perceptual code for position. The results of the experiments reported here, in which it is shown that retinally non-veridical locations of contour elements are used by the visual system for contour-element binding, lend support to a different view. The visual system does not implicitly code position with reference to the labelled locations of retinal cells, but dynamically extracts spatial position from the aggregate result of local computations. These computations may include local spatial relationships between retinal cells, but are not confined to them; other computations, including position derived from local velocity cues, are combined to code the position of objects in the visual world.

Form Perception↗