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At least 1,153 records · Page 64Linked to original sources
PubMed2005

An investigation of district spatial variations of childhood diarrhoea and fever morbidity in Malawi.

Although diarrhoea and malaria are among the leading causes of child mortality and morbidity in Sub-Saharan Africa, few detailed studies have examined the patterns and determinants of these ailments in the most affected communities. In this paper, we investigate the spatial distribution of observed diarrhoea and fever prevalence in Malawi using individual data for 10,185 children from the 2000 Malawi Demographic and Health survey. We highlight inequalities in child health by mapping the residual district spatial effects using a geo-additive probit model that simultaneously controls for spatial dependence in the data and potential nonlinear effects of covariates. The residual spatial effects were modelled via a Bayesian approach. For both ailments, we were able to identify a distinct district pattern of childhood morbidity. In particular, the results suggest that children living in the capital city are less affected by fever, although this is not true for diarrhoea, where some urban agglomerations are associated with a higher childhood morbidity risk. The spatial patterns emphasize the role of remoteness as well as climatic, environmental, and geographic factors on morbidity. The fixed effects show that for diarrhoea, the risk of child morbidity appears to be lower among infants who are exclusively breastfed than among those who are mixed-fed. However, exclusive breastfeeding was not found to have a protective effect on fever. An important socio-economic factor for both diarrhoea and fever morbidity was parental education, especially maternal educational attainment. Diarrhoea and fever were both observed to show an interesting association with child's age. We were able to discern the continuous worsening of the child morbidity up to 8-12 months of age. This deterioration set in right after birth and continues, more or less linearly until 8-12 months, before beginning to decline thereafter. Independent of other factors, a separate spatial process produces district inequalities in child's health.

Breast Feeding↗
PubMed2004

Stabilizing effects in spatial parasitoid-host and predator-prey models: a review.

We review the literature on spatial host-parasitoid and predator-prey models. Dispersal on its own is not stabilizing and can destabilize a stable local equilibrium. We identify three mechanisms whereby limited dispersal of hosts and parasitoids combined with other features, such as spatial and temporal heterogeneity, can promote increased persistence and stability. The first mechanisms, "statistical stabilization", is simply the statistical effect that summing a number of out-of-phase population trajectories results in a relatively constant total population density. The second mechanism involves decoupling of immigration from local density, such that limited dispersal between asynchronous patches results in an effect that mimics density-dependence at the local patch level. The third mechanism involves altering spatially averaged parameter values resulting from spatial heterogeneity in density combined with non-linear responses to density. Persistence in spatially explicit models with local dispersal frequently associated with self-organized spatial patterning.

Animals↗
PubMed2006

Spatially structured superinfection and the evolution of disease virulence.

When pathogen strains differing in virulence compete for hosts, spatial structuring of disease transmission can govern both evolved levels of virulence and patterns in strain coexistence. We develop a spatially detailed model of superinfection, a form of contest competition between pathogen strains; the probability of superinfection depends explicitly on the difference in levels of virulence. We apply methods of adaptive dynamics to address the interplay of spatial dynamics and evolution. The mean-field approximation predicts evolution to criticality; any small increase in virulence capable of dynamical persistence is favored. Both pair approximation and simulation of the detailed model indicate that spatial structure constrains disease virulence. Increased spatial clustering reduces the maximal virulence capable of single-strain persistence and, more importantly, reduces the convergent-stable virulence level under strain competition. The spatially detailed model predicts that increasing the probability of superinfection, for given difference in virulence, increases the likelihood of between-strain coexistence. When strains differing in virulence can coexist ecologically, our results may suggest policies for managing diseases with localized transmission. Comparing equilibrium densities from the pair approximation, we find that introducing a more virulent strain into a host population infected by a less virulent strain can sometimes reduce total host mortality and increase global host density.

Adaptation, Physiological↗
PubMed2006

Optimal movement between patches under incomplete information about the spatial distribution of food items.

If the food distribution contains spatial pattern, the food density in a particular patch provides a forager with information about nearby patches. Foragers might use this information to exploit patchily distributed resources profitably. We model the decision on how far to move to the next patch in linear environments with different spatial patterns in the food distribution (clumped, random, and regular) for foragers that differ in their degree of information. An ignorant forager is uninformed and therefore always moves to the nearest patch (be it empty or filled). In contrast, a prescient forager is fully informed and only exploits filled patches, skipping all empty patches. A Bayesian assessor has prior knowledge about the content of patches (i.e. it knows the characteristics of the spatial pattern) and may skip neighbouring patches accordingly by moving to the patch where the highest gain rate is expected. In most clumped and regular distributions there is a benefit of assessment, i.e. Bayesian assessors achieve substantially higher long-term gain rates than ignorant foragers. However, this is not the case in distributions with less strong spatial pattern, despite the fact that there is a large potential benefit from a sophisticated movement rule (i.e. a large penalty of ignorance). Bayesian assessors do also not achieve substantially higher gain rates in environments that are relatively rich or poor in food. These results underline that an incompletely informed forager that is sensitive to spatial pattern should not always respond to existing pattern. Furthermore, we show that an assessing forager can enhance its long-term gain rate in highly clumped and some specific near-regular food distributions, by sampling the environment in slightly larger spatial units.

Animals↗
PubMed2005

The spatial frequency and orientation selectivity of the mechanisms that extract motion-defined contours.

The human visual system can undertake a specialized form of motion integration, one that enables the presence of extended spatial contours to be disambiguated from their backgrounds. We have shown previously that the visual system can selectively integrate local motion signals when their directions are along spatial contours and its efficiency is inversely related to the curvature of the contour involved (Ledgeway, T., & Hess, R. F. (2002). Vision Research, 42, 653-659). This integration primarily involves the direction, rather than the speed, of local motion signals. In the present study, we sought to investigate both the spatial frequency and orientation tuning of this specialized contour integration process, using a path detection paradigm. The results show that the tuning for spatial frequency is very broad, in line with previous studies that have examined this issue. In contrast, the orientation selectivity of the mechanism mediating contour extraction under these conditions is relatively narrowband. Thus, spatial frequency but not orientation pooling appears to take place prior to the extraction of motion-defined contours, a situation that is different from that previously shown for spatial contours composed of static, oriented elements.

Form Perception↗
PubMed2004

A spatial and temporal analysis of safety-belt usage and safety-belt laws.

Safety-belt usage has increased significantly in the US since the introduction of mandatory safety-belt usage laws in the 1980s. This paper analyzes the impact of these laws on increasing safety-belt usage while controlling for other state-specific variables. A fixed effects cross-sectional time-series analyses shows the relative significance of various state-level attributes in explaining safety-belt usage, including whether or not primary or secondary safety-belt laws have been passed. To further explore these relationships we employ spatial analyses techniques and find spatial autocorrelation in the data. Spatial correlation also exhibits a clear east-west direction. When the analyses is further corrected for temporal autocorrelation we find that the spatial autocorrelation is greatly diminished and that many variables lose their statistical significance, though safety-belt laws are still statistically significant. Results suggest that for this data, it is critical to control for temporal autocorrelation while spatial autocorrelation is less important. We also find that our spatial analyses does provide interesting information on similarities between various regions on the effectiveness of safety-belt laws.

Adolescent↗
PubMed2003

On spatial response code activation in a Simon task.

The Simon effect refers to the performance advantage for trials where the task-irrelevant location of a target spatially corresponds with the location of the response. It is thought that the irrelevant spatial code of the target facilitates responding by automatically pre-activating the spatially corresponding response code. This spatial code is thought to passively decay shortly after its activation. In this investigation, the response was selected according to the identity of a central cue. The selected response was executed or withheld depending the identity (Experiment 1) or the presence (Experiment 2) of the target. Varying the stimulus-onset asynchrony (SOA), between the central response cue and the peripheral target, allowed for a time-course analysis of the Simon effect. The results of two experiments provided no indication that the activation level of the irrelevant spatial code decayed while the relevant response was prepared. Although reaction times increased as the SOA decreased, the Simon effect was additive with SOA, suggesting that the automatic activation of the task-irrelevant spatial code was delayed until the task-relevant response code was mostly prepared, perhaps due to the capacity limitations of response selection.

Humans↗
PubMed1998

Spatial sensing of stimulus gradients can be superior to temporal sensing for free-swimming bacteria.

Predictions of the minimal size an organism must have to swim along stimulus gradients were used to compare the relative advantages of sensory systems employing spatial (simultaneous) and temporal (sequential) gradient detection mechanisms for small free-swimming bacteria, leading to the following conclusions: 1) there are environmental conditions where spatial detection mechanisms can function for smaller organisms than can temporal mechanisms, 2) temporal mechanisms are superior (have a smaller size limit) for the difficult conditions of low concentration and shallow gradients, but 3) observed bacterial chemotaxis occurs mostly under conditions where spatial mechanisms have a smaller size limit, and 4) relevant conditions in the natural environment favor temporal mechanisms in some cases and spatial mechanisms in others. Thus, sensory ecology considerations do not preclude free-swimming bacteria from employing spatial detection mechanisms, as has been thought, and microbiologists should be on the lookout for them. If spatial mechanisms do not occur, the explanation should be sought elsewhere.

Bacterial Physiological Phenomena↗
PubMed1998

Sex differences in spatial learning and prefrontal and parietal cortical dendritic morphology in the meadow vole, Microtus pennsylvanicus.

The prefrontal and parietal cortex has been implicated in the mediation of spatially related behaviors in male and female laboratory rats. Meadow voles, Microtus pennsylvanicus, are diurnally-crepuscularly active microtine rodents that exhibit a variety of sexually dimorphic spatially associated behaviors in both the laboratory and wild. In the present study we examined both the spatial Morris water maze performance and dendritic architecture and branching of neuronal cells in the prefrontal and parietal cortex of reproductive male and female meadow voles. Males learned the location of the hidden platform in the water task faster than estrous females and on probe trials they spent more time in the previously correct quadrant than females. Dendritic analysis with Golgi-Cox stained sections showed that male voles had significantly more dendritic arborization in the medial prefrontal and parietal cortex than females. These sex differences in both spatial navigation ability and in neural structures related to spatial navigation in meadow voles suggest that the size of neural areas might be shaped by ecological pressures associated with sexually dimorphic spatial behaviors.

Animals↗
PubMed1977

Failure to confirm a spatial ability impairment in persons with evidence of right hemisphere speech capability.

Levy hypothesized that persons lacking strict left hemispheric dominance for language processing should be impaired in spatial processing ability. This leads to the expectation that left-handers, because they are more often not strictly left hemisphere dominant for language, would, as a group, be impaired in spatial ability relative to right-handers. Evidence regarding this hypothesis has been inconsistent. The present study systematically examined the effects of handedness, sex, and hemispheric language dominance on verbal and spatial ability in 80 right-handed and 71 left-handed university students. Males were found to be superior to females in all handedness-hemispheric language dominance groups on the spatial ability measure. No effects of handedness or hemispheric lanaguage dominance nor interactions of these factors obtained for either verbal or spatial ability measures. It was concluded that the hypothesis of verbal-spatial processing incompatibility within the same hemisphere was not supported.

Dominance, Cerebral↗
PubMed2003

Prevention of age-related spatial memory deficits in a transgenic mouse model of Alzheimer's disease by chronic Ginkgo biloba treatment.

Alzheimer's disease (AD) is characterized by cognitive decline and deposition of beta-amyloid (Abeta) plaques in cortex and hippocampus. A transgenic mouse AD model (Tg2576) that overexpresses a mutant form of human Abeta precursor protein exhibits age-related cognitive deficits, Abeta plaque deposition, and oxidative damage in the brain. We tested the ability of Ginkgo biloba, a flavonoid-rich antioxidant, to antagonize the age-related behavioral impairment and neuropathology exhibited by Tg2576 mice. At 8 months of age, 16 female Tg2576 and 15 female wild-type (wt) littermate mice were given ad lib access to tap water or Ginkgo biloba (70 mg/kg/day in water). After 6 months of treatment, all mice received Morris water maze training (4 trials/day for 10 days) to assess hippocampal dependent spatial learning. All mice received a 60-s probe test of spatial memory retention 24 h after the 40th trial. Untreated Tg2576 mice exhibited a spatial learning impairment, relative to wt mice, while Ginkgo biloba-treated Tg2576 mice exhibited spatial memory retention comparable to wt during the probe test. Spatial learning was not different between Ginkgo biloba-treated and untreated wt mice. There were no group differences in learning to swim to a visible platform. Soluble Abeta and hippocampal Abeta plaque burden did not differ between the Tg2576 groups. Brain levels of protein carbonyls were paradoxically elevated in Ginkgo biloba-treated mice. These data indicate that chronic Ginkgo biloba treatment can block an age-dependent decline in spatial cognition without altering Abeta levels and without suppressing protein oxidation in a transgenic mouse model of AD.

Aging↗
PubMed2003

Using network models to approximate spatial point-process models.

Spatial effects are fundamental to ecological and epidemiological systems, yet the incorporation of space into models is potentially complex. Fixed-edge network models (i.e. networks where each edge has the same fixed strength of interaction) are widely used to study spatial processes but they make simplistic assumptions about spatial scale and structure. Furthermore, it can be difficult to parameterize such models with empirical data. By comparison, spatial point-process models are often more realistic than fixed-edge network models, but are also more difficult to analyze. Here we develop a moment closure technique that allows us to define a fixed-edge network model which predicts the prevalence and rate of epidemic spread of a continuous spatial point-process epidemic model. This approach provides a systematic method for accurate parameterization of network models using data from continuously distributed populations (such as data on dispersal kernels). Insofar as point-process models are accurate representations of real spatial biological systems, our example also supports the view that network models are realistic representations of space.

Animals↗
PubMed2000

Galanin and spatial learning in the rat. Evidence for a differential role for galanin in subregions of the hippocampal formation.

Anatomical, neurochemical and behavioural evidence support a role for galanin in hippocampally mediated functions such as spatial learning and memory. To obtain more precise information on this role, galanin (3 nmol/rat) was infused via bilateral chronic cannulae into different areas of the hippocampal formation which are characterized by different galanin receptor subtypes and also by different galanin innervation patterns. The effects of infused galanin on spatial learning were examined in the Morris swim maze. Infusions of galanin into both the dorsal and ventral dentate gyrus, which mainly contain GAL-R2 receptor mRNA and a high degree of galanin-noradrenaline coexistence, significantly retarded spatial acquisition without affecting swim speed or performance in the visible platform test. This spatial learning deficit was fully blocked by pretreatment with the non-selective galanin antagonist M35. Analysis of retention performance suggested that the major effect of intrahippocampal galanin is mediated via a specific disruption of acquisition mechanisms of importance for performance in the probe trial. Galanin infused into the ventral CA1 (a mainly GAL-R1 receptor mRNA expressing region) or into anterior, ventral CA3 regions did not produce any deficits in spatial learning compared to control animals. These results suggest that galanin mediates its action on spatial learning mainly through the GAL-R2 receptor subtype in areas where most of the galanin is present in noradrenergic terminals. A possible role for the GAL-R1 receptor subtype in cognition in the dorsal and ventral hippocampus remains to be defined. The results suggest a differential functional role for galanin and galanin receptor subtypes within subregions of the hippocampal formation.

Animals↗
PubMed2001

Spatial working memory deficit in unilateral neglect.

Based on the similarity of brain areas lesioned in neglect and those activated by spatial working memory (WM) tasks in normals, we hypothesized that neglect may involve spatial WM impairments. A left neglect patient with right inferior frontal and basal ganglia damage performed cancellation tasks, making either highly visible marks (to provide a reminder of visited items), or invisible marks (so only spatial WM could represent cancelled items). Invisible marks led to repeated cancellations for targets that differed only in location, but not for targets with memorable unique identities, suggesting a deficit of spatial WM, with non-spatial WM spared. Neglect was greater for cancellation with invisible marks, consistent with a role for deficient spatial WM in cancellation deficits, but contrary to account solely in terms of attention capture by salient visible marks made in ipsilesional space.

Attention↗
PubMed2002

Lateralized spatial processes and their lexical implications.

Numerous lateralized tasks have been identified as involving spatial processing, but the extent to which they use the same spatial process is largely unknown. The present research investigated relationships among significant asymmetries from seven visuo-spatial and two verbal tasks, combining samples from previous factor analytic experiments with those from three new studies (combined N=789). All of the spatial intercorrelations were negligible (r<0.20), unlike previous outcomes finding robust correlations within certain clusters of verbal asymmetries. However, lateral differences involving spatial quantitative processing on the one hand and figure-ground separation on the other, showed significant but independent correlations with one from visual lexical processing (word recognition). The results support three major conclusions. First, there are at least several and possibly many lateralized spatial processes. Second, hemispheric processes follow a modular architecture in preference to a diffuse or parallel distributed architecture. Finally, the dissociated correlations are consistent with a visual lexical processing model having both occipital and parietal components, potentially reconciling a current controversy over the cerebral localization of language.

Adult↗
PubMed2003

On the hemispheric specialization for categorical and coordinate spatial relations: a review of the current evidence.

This article reviews current evidence on the hemispheric specialization hypothesis for two types of spatial relations representations; categorical versus coordinate [Psychol. Rev. 94 (1987) 148; J. Exp. Psychol.: Percept. Perform. 15 (1989) 723]. Categorical representations capture general properties of the spatial structure of a visual stimulus, without defining the exact metric properties. Coordinate representations specify precise spatial locations of objects or parts in terms of metric units. It is claimed that a hemispheric difference in contribution to the computation of both types of spatial relations representations exists, in which the left hemisphere is specialized for the computation of categorical spatial representations while the right hemisphere is specialized for the computation of coordinate ones. Several forms of research (experimental, computer simulations, patient studies and neuroimaging studies) are reviewed. In general, there is convergent evidence for a conceptual separation of coordinate and categorical processing, with strongest indications for a relative right hemisphere advantage in encoding coordinate spatial relations, and weaker support for left hemispheric categorical specialization. The pattern appears to be critically linked to receptive field properties of the two hemispheres and as such is modulated by certain elementary visual characteristics of the displayed stimuli.

Brain Diseases↗
PubMed2000

Cognitive and vestibulo-proprioceptive components of spatial ability in Parkinson's disease.

Visual-spatial deficits are often associated with Parkinson's Disease (PD). Recent theories suggest that frontal-basal ganglionic dysfunction affects cognition in PD. Although this hypothesis does not entirely explain spatial deficits in PD, the inappropriate utilization of cues associated with executive dysfunction may induce spatial deficits. Alternatively, the vestibular system is also involved in spatial cognition, and vestibular dysfunction may affect visual-spatial ability in PD. To test these hypotheses, we administered the Water Jar Test, while perturbing vestibulo-proprioceptive input. Non-demented PD patients were significantly less accurate than controls in judging horizontal, and appeared to inappropriately utilize cues. No group effect was found for head tilt. These findings suggest the visual-spatial difficulties seen in PD are related to executive dysfunction that is associated with a disruption of the frontal-basal ganglionic and frontal-parietal systems.

Basal Ganglia↗
PubMed1998

Prenatal exposure to ethanol disrupts spatial memory: effect of the training-testing delay period.

The present study investigated how variations in the period of delay between training and testing in the Morris water maze task affect the use of spatial memory in adult rats that were prenatally exposed to ethanol. Previous results utilizing the Morris water maze task have shown that prenatal, or early postnatal, exposure to ethanol produces deficits in the use of spatial memory, a type of memory that is dependent on an intact hippocampus. However, in these prior studies the delay period between the training of animals and the testing of spatial memory is typically fixed at only 1 day. In the current study, which utilized a revised training procedure within the Morris water maze task, the period of delay between training and testing was altered such that it was either 1 day or 3 days. Following the 3-day delay, different levels of prenatal exposure to ethanol impaired the use of spatial memory. In contrast, following the 1-day delay, prenatal exposure to ethanol failed to impair the use of spatial memory. The present study thus shows that prenatal exposure to ethanol differentially affects spatial memory in the Morris water maze task depending on the period of delay between training and testing.

Analysis of Variance↗