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PubMed1997

Young chickens learn to localize the centre of a spatial environment.

Young chickens were trained to find food by ground-scratching in the centre of a closed uniform arena and were then tested in arenas of similar areas but of different shapes. Chickens showed localized searching behaviour in the square-shaped arena, and maintained this behaviour when placed in a circular or triangular (both equilateral and isosceles) arena. With a rectangular-shaped arena, obtained by doubling the original square-shaped one, chickens showed more dispersed searching along the major axis, but searching tended to be concentrated around the centres of the composing squares and around the centre of the rectangle itself. When trained in a square- or triangle-shaped arena and then tested in an arena of the same shape but a larger area, chickens displayed searching behaviour at two different distances from the wall of the arena, one corresponding to the correct distance (i.e. centre) in the smaller (training) arena, the other to the actual centre of the test arena. On the other hand, in a circular arena, chickens searched mainly at a distance midway between the radius of the small (training) and of the large (testing) circular arena. These results suggest that, during training, chickens encoded information on both the absolute and the relative distance of the food from the walls of the arena, the latter information being more accurate when the arena displayed identifiable features such as corners.

Animals↗
PubMed2003

Mathematical model of the spatio-temporal dynamics of second messengers in visual transduction.

A model describing the role of transversal and longitudinal diffusion of cGMP and Ca(2+) in signaling in the rod outer segment of vertebrates is developed. Utilizing a novel notion of surface-volume reaction and the mathematical theories of homogenization and concentrated capacity, the diffusion of cGMP and Ca(2+) in the inter-disc spaces is shown to be reducible to a one-parameter family of diffusion processes taking place on a single rod cross section; whereas the diffusion in the outer shell is shown to be reducible to a diffusion on a cylindrical surface. Moreover, the exterior flux of the former serves as a source term for the latter, alleviating the assumption of a well-stirred cytosol. A previous model of visual transduction that assumes a well-stirred rod outer segment cytosol (and thus contains no spatial information) can be recovered from this model by imposing a "bulk" assumption. The model shows that upon activation of a single rhodopsin, cGMP changes are local, and exhibit both a longitudinal and a transversal component. Consequently, membrane current is also highly localized. The spatial spread of the single photon response along the longitudinal axis of the outer segment is predicted to be 3-5 microm, consistent with experimental data. This approach represents a tool to analyze point-wise signaling dynamics without requiring averaging over the entire cell by global Michaelis-Menten kinetics.

Animals↗
PubMed2003

Metapopulation-level adaptation of insect host plant preference and extinction-colonization dynamics in heterogeneous landscapes.

Species living in highly fragmented landscapes typically occur as metapopulations with frequent turnover of local populations. The turnover rate depends on population sizes and connectivities, but it may also depend on the phenotypic and genotypic composition of populations. The Glanville fritillary butterfly (Melitaea cinxia) in Finland uses two host plant species, which show variation in their relative abundances at two spatial scales: locally among individual habitat patches and regionally among networks of patches. Female butterflies in turn exhibit spatial variation in genetically determined host plant preference within and among patch networks. Emigration, immigration and establishment of new populations have all been shown to be strongly influenced by the match between the host plant composition of otherwise suitable habitat patches and the host plant preference of migrating butterflies. The evolutionary consequences of such biased migration and colonization with respect to butterfly phenotypes might differ depending on spatial configuration and plant species composition of the patches in heterogeneous patch networks. Using a spatially realistic individual-based model we show that the model-predicted evolution of host plant preference due to biased migration explains a significant amount of the observed variation in host plant use among metapopulations living in dissimilar networks. This example illustrates how the ecological extinction-colonization dynamics may be linked with the evolutionary dynamics of life history traits in metapopulations.

Adaptation, Physiological↗
PubMed1991

Vectorial sequence of mineralization in the turkey leg tendon determined by electron microscopic imaging.

Turkey leg tendons were used as a model tissue to study the spatial and temporal relationships of mineral deposition between matrix vesicles and collagen fibrils by various electron microscopic techniques--bright field, selected-area dark field (SADF), and electron spectroscopic imaging (ESI). These latter imaging techniques enabled the direct localization and spatial distributions of both apatite crystals and atomic elements (Ca, P) within matrix vesicles and collagen. In longitudinal planes of section, a consistent vectorial gradient of mineralization was observed which started with the first localization of apatite mineral in matrix vesicles; with further development, the mineral spread from the vesicle to the extravesicular interstices and then into the adjacent collagen fibrils. Once intrafibrillar, the mineral was observed to advance both laterally and axially. The association of vesicle/collagen mineral was examined by ESI analysis of Ca and P elemental maps and appeared as a continuum between the vesicles and the adjacent collagen fibrils. Similarly, an intimate spatial relationship was observed between the mineral of vesicles and collagen in transversely cut sections of tendon. The sequential development of this mineralized matrix is discussed in light of matrix vesicle/collagen interactions.

Animals↗
PubMed2006

Bilateral spatial filtering: refining methods for localizing brain activation in the presence of parenchymal abnormalities.

Functional MRI (fMRI) is an important tool for pre-surgical localization of eloquent cortex prior to resection of brain lesions. To increase the inherently low activation signal to noise ratio, fMRI pre-processing steps often include spatial smoothing. However, the effects of smoothing in the presence of brain lesions have not been studied. We have adapted the widely used method of Gaussian spatial filtering to include an "edge stopping" function. This method, termed bilateral filtering, minimizes blurring of apparent brain activity across anatomic boundaries and into regions of non-activation. fMRI data were acquired in a patient with a known low grade glioma during a blocked finger-tapping paradigm. Simulated activity was superimposed on baseline images of non-activated brain using the same paradigm, with additive signal equal to 1, 3, and 5% of the mean physiologic background. Comparison of Gaussian and bilateral filtering suggests that the modified technique more accurately locates brain activation and increases the significance of activation bordering sharp transitions. Thus, spatial pre-processing with a bilateral filter may be particularly useful in the pre-operative assessment of brain lesions.

Brain↗
PubMed2005

Microscopic theory of linear light scattering from mesoscopic media and in near-field optics.

On the basis of quantum mechanical response theory a microscopic propagator theory of linear light scattering from mesoscopic systems is presented. The central integral equation problem is transferred to a matrix equation problem by discretization in transitions between pairs of (many-body) energy eigenstates. The local-field calculation which appears from this approach is valid down to the microscopic region. Previous theories based on the (macroscopic) dielectric constant concept make use of spatial (geometrical) discretization and cannot in general be trusted on the mesoscopic length scale. The present theory can be applied to light scattering studies in near-field optics. After a brief discussion of the macroscopic integral equation problem a microscopic potential description of the scattering process is established. In combination with the use of microscopic electromagnetic propagators the formalism allows one to make contact to the macroscopic theory of light scattering and to the spatial photon localization problem. The quantum structure of the microscopic conductivity response tensor enables one to establish a clear physical picture of the origin of local-field phenomena in mesoscopic and near-field optics. The Huygens scalar propagator formalism is revisited and its generality in microscopic physics pointed out.

Journal Article↗
PubMed1999

Aedes aegypti (L) in Port Blair, Andaman and Nicobar islands-distribution and larval ecology.

A comprehensive survey was carried out in the Port Blair town in Andaman and Nicobar islands, in order to study the distribution and relative prevalence of Aedes aegypti during the monsoon (July'1997-October'1997) season using larval survey and adult collection methods. Ae. aegypti was found in all 21 localities surveyed. Spatial variations in distribution was evident which was closely related to population density. The nature of the larval habitats was observed to be similar in all the localities. For both outdoor and in-door containers, breeding preference ratio was highest for mud/brick/cement containers, followed by metal and plastic containers. These findings are correlated with water storage habits of the residents in the localities surveyed.

Aedes↗
PubMed2005

Seed, dispersal, microsite, habitat and recruitment limitation: identification of terms and concepts in studies of limitations.

Recently, there is an increase in number of studies concerned with the effect of various types of limitations on species local population size and distribution pattern at the landscape scale. The terminology used to describe these limitations is, however, very inconsistent. Since the different terms often appear in conclusions of the papers, the inconsistency in their use obscures the message of these papers. In this study, we review the current uses of these terms, identify the basic concepts involved in the discussion of a limitation and link the concepts with the currently used terms. Finally, we discuss the experimental approaches used to assess the different types of limitations. We differentiated four basic concepts resulting from the combination of limitation by environment versus ability to grow and spread, and two spatial scales (local and regional scale). The two concepts at each spatial scale are expected to form a gradient of all possible combinations of the two respective types of limitations. In the considerations of various experimental approaches used to assess these limitations, we conclude that sowing experiments, i.e. seed addition into existing populations or seed introduction into unoccupied habitats, are the only reliable types of evidence for the different types of limitations.

Biodiversity↗
PubMed2005

Edges and bars: where do people see features in 1-D images?

There have been two main approaches to feature detection in human and computer vision--based either on the luminance distribution and its spatial derivatives, or on the spatial distribution of local contrast energy. Thus, bars and edges might arise from peaks of luminance and luminance gradient respectively, or bars and edges might be found at peaks of local energy, where local phases are aligned across spatial frequency. This basic issue of definition is important because it guides more detailed models and interpretations of early vision. Which approach better describes the perceived positions of features in images? We used the class of 1-D images defined by Morrone and Burr in which the amplitude spectrum is that of a (partially blurred) square-wave and all Fourier components have a common phase. Observers used a cursor to mark where bars and edges were seen for different test phases (Experiment 1) or judged the spatial alignment of contours that had different phases (e.g. 0 degrees and 45 degrees ; Experiment 2). The feature positions defined by both tasks shifted systematically to the left or right according to the sign of the phase offset, increasing with the degree of blur. These shifts were well predicted by the location of luminance peaks (bars) and gradient peaks (edges), but not by energy peaks which (by design) predicted no shift at all. These results encourage models based on a Gaussian-derivative framework, but do not support the idea that human vision uses points of phase alignment to find local, first-order features. Nevertheless, we argue that both approaches are presently incomplete and a better understanding of early vision may combine insights from both.

Contrast Sensitivity↗
PubMed2001

Encoding of geometric and landmark information in the left and right hemispheres of the Avian Brain.

Chicks were trained binocularly to find food buried under sawdust in the center of a square enclosure. When tested in an enclosure made larger or smaller in size, binocular and left-eyed chicks searched mainly on the basis of relative distance of the food from the enclosure walls, whereas right-eyed chicks searched on the basis of absolute distance. Moreover, binocular and left-eyed chicks relied mainly on global spatial information (i.e., distances from the walls), whereas right-eyed chicks also used information provided by visual landmarks. These results suggest that the right hemisphere of the avian brain (fed mostly by the left eye) is primarily concerned with encoding of relational spatial information, whereas the left hemisphere (fed mainly by the right eye) is concerned with absolute metric information, possibly as part of an encoding strategy based primarily on local (both spatial and nonspatial) cues.

Animals↗
PubMed2005

A robust spatial filtering technique for multisource localization and geoacoustic inversion.

Geoacoustic inversion and source localization using beamformed data from a ship of opportunity has been demonstrated with a bottom-mounted array. An alternative approach, which lies within a class referred to as spatial filtering, transforms element level data into beam data, applies a bearing filter, and transforms back to element level data prior to performing inversions. Automation of this filtering approach is facilitated for broadband applications by restricting the inverse transform to the degrees of freedom of the array, i.e., the effective number of elements, for frequencies near or below the design frequency. A procedure is described for nonuniformly spaced elements that guarantees filter stability well above the design frequency. Monitoring energy conservation with respect to filter output confirms filter stability. Filter performance with both uniformly spaced and nonuniformly spaced array elements is discussed. Vertical (range and depth) and horizontal (range and bearing) ambiguity surfaces are constructed to examine filter performance. Examples that demonstrate this filtering technique with both synthetic data and real data are presented along with comparisons to inversion results using beamformed data. Examinations of cost functions calculated within a simulated annealing algorithm reveal the efficacy of the approach.

Journal Article↗
PubMed2002

Probing localized states with spectrally resolved speckle techniques.

We have studied the spatial extent of localized states in random media using speckle correlation techniques. Optical gain is introduced to a local region of a random medium to induce lasing of the localized states. The far-field speckle pattern of laser emission from a localized state gives its spatial field correlation function at the surface of the random medium. The envelope of the spatial field correlation function decays exponentially with the transverse coordinate on the surface. The decay length is independent of the pumping rate and the excitation area. We demonstrate that localized states exist in a disordered system that does not reach the localization threshold.

Journal Article↗
PubMed2005

fMRI localizer technique: efficient acquisition and functional properties of single retinotopic positions in the human visual cortex.

Current fMRI retinotopic mapping procedures often use checkerboard stimuli consisting of expanding rings and rotating wedges to measure the topography within human visual areas. Efficient procedures are well described in the literature. For many experimental paradigms, e.g., visuo-spatial attention paradigms, the identification of task-relevant positions is the only mandatory prerequisite. To define these specific "regions-of-interest" (ROIs), spatially defined localizers are used. A precise evaluation of localizer techniques in regard to efficient scanning time, optimal BOLD (blood oxygenic level dependent) response, as well as quantification of the resulting ROIs within each visual area (size, overlap, surround effects) has not been studied to date. Here, we suggest a mapping procedure designed to quantify spatial and functional properties of single positions at close proximity in multiple human visual areas. During a passive viewing task, various stimuli (e.g., checkerboards or colored objects) subtending 1.4 degrees of visual angle were presented at one out of four positions in a randomized block design. We measured the degree of overlap between positions at different hierarchical levels of the visual system (V1-V4v) and quantified modulatory effects on a specific position by stimulation at neighboring (1.7 degrees spacing) or distant positions (5.1 degrees or 8.5 degrees spacing). Within each visual area, "mexican-hat" distributions of local signal intensity changes, which describe a particular combination of facilitatory and suppressive effects, were found. Cubic fitting revealed the most localized tuning effect in V1, which gradually decreased throughout the higher visual areas. Colored objects were most efficient in localizing circumscribed retinotopic positions in both early and higher areas.

Adult↗
PubMed2004

Examining localized patterns of air quality perception in Texas: a spatial and statistical analysis.

Environmental and human health issues associated with outdoor air pollution, such as ozone, sulfur dioxide, and other pollutants in metropolitan regions, are an area of growing concern for both policy officials and the general public. Increasing attention from the news media, new health data, and public debate over the effectiveness of clean air regulations have raised the importance of air quality in the public consciousness. While public perceptions of air quality have been studied thoroughly dating back to the 1960s, little empirical research has been conducted to explain the spatial aspects of these perceptions, particularly at the local level. Although recent studies suggest characteristics of local setting are important in shaping perceptions of air quality, the roles of proximity, neighborhood characteristics, and location have not been clarified. This study seeks to improve understanding of the major factors shaping public perceptions of air quality by examining the spatial pattern of local risk perception, the role of socioeconomic characteristics in forming these perceptions, and the relationship between perceived and scientifically measured air pollution. First, we map the spatial pattern of local air quality perceptions using Geographic Information Systems (GIS) across the Dallas and Houston metropolitan areas. Next, we explain these perceptions through local contextual factors using both bivariate correlations and multivariate regression analysis. Results indicate that perceptions of air quality in the study areas are not significantly correlated with air quality based on readings of air monitoring stations. Instead, perceptions appear to be influenced by setting (urban vs. rural), state identification, access to information, and socioeconomic characteristics such as age, race, and political identification. We discuss the implications of the findings and provide direction on how further research can provide a deeper understanding of the local contextual factors influencing public perceptions.

Air Pollutants↗
PubMed1996

Spatial and temporal evaluation of locally induced skin trauma recorded with laser Doppler techniques.

The single-fiber laser Doppler flowmetry (SF-LDF) technique has been used to study perfusion within tissues. The insertion of the SF-LDF probe into human skin locally alters microcirculation for more than 70 min after positioning. In this study the SF-LDF and a newly developed laser Doppler imager (LDI) were used to describe the spatial distribution, the intensity, and the temporal course of the trauma following intracutaneous insertion of the probe. The LDI scanned a human skin area of 6 x 6 cm (36 x 36 pixels). A flexible SF-LDF probe was inserted intradermally in the same measuring region. Recordings were performed for 120 min with the two techniques simultaneously. With the SF-LDF probe maximal hyperemia was recorded after 5-30 min. After a gradual and slow decrease most of the subjects reached a steady-state level within 80-100 min. A 3-min tourniquet occlusion test was performed 120 min after SF probe positioning. A significant hyperemic response was recorded, indicating that almost no posttraumatic activity was present at this time. The area of increased perfusion recorded with the LDI had an oval to circular configuration with a maximal extension after 5-10 min (radius 1.5-2 cm, area 8.9 cm2). The zone of maximal hyperemia was distributed adjacent to the intradermal probe, gradually decreasing toward the periphery. No significant hyperemia was shown with LDI after 100 min. There was a concordance between the time courses of the hyperemic response obtained with the two techniques.

Adult↗
PubMed2006

The spatial distribution of atopic dermatitis cases in a population of insured Swedish dogs.

Canine atopic dermatitis (CAD) is manifested by pruritus and inflammation of the skin, and most affected dogs have specific IgE against environmental allergens. Our aim was to evaluate whether in Sweden the incidence of CAD varies spatially, and to investigate possible environmental causes of such variation in a longitudinal study of CAD incidence among insured Swedish dogs. The dataset consisted of >220,000 individuals which had been covered by an insurance plan between January 1, 1995 and December 31, 2002. Out of these, 1235 CAD cases were identified and matched to postal-code areas based upon the address of the owner. Environmental risk factors we considered included averages of long-term annual rainfall, and of January and July temperatures. Initial visualization of the incidence rate of CAD (cases/dog-years at risk) expressed as empirical Bayes smoothed spatial rates indicated geographic variation. Moran's I, adjusted for population at risk, revealed significant global clustering. Both the spatial scan statistic and a local indicator of spatial autocorrelation revealed a higher incidence of CAD in the major cities. In a Poisson-regression model (with a spatial covariance structure), the incidence of CAD increased with increasing human population density, increasing average annual rainfall, living in the southern half of Sweden, and having a veterinary dermatologist in the county.

Allergens↗