Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “spatial”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

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

At least 1,693 records · Page 94Linked to original sources
PubMed2006

Effect of levetiracetam on visual-spatial memory following status epilepticus.

Status epilepticus (SE) is often followed by severe cognitive impairment, including memory impairment. Previous studies have shown that SE is associated with impairment of single cells in the hippocampus that fire action potentials when the animal is in a specific location in space, the so-called place cells, and that place cell function correlates well with performance in tasks of visual-spatial memory. Place cell patterns therefore appear to be an excellent measure of spatial memory and may serve as a tool to assess seizure-induced impairment in memory. In this study we determined the relationship between visual-spatial memory and place cell function following SE. In addition, we determined if levetiracetam (LEV), an antiepileptic drug with a novel mechanism of action, can improve cognitive function and place cell firing patterns when administered following SE. SE was induced in adult male rats which were then randomized to post-SE treatment with LEV or normal saline (NS) treatment for 14 days. Non-SE control rats also were randomized to LEV or NS. Following discontinuation of LEV rats were tested for visual-spatial memory in the Morris water-maze and then underwent unit recording in the CA1 region of the hippocampus. Brains were then evaluated for cell loss and mossy fiber sprouting. SE was associated with severely impaired performance in the water-maze with SE rats demonstrating no learning over four days of testing. Paralleling this memory deficit was a marked disturbance in firing patterns of pyramidal neurons in CA1. Non-SE rats learned quickly over four days of water-maze testing and had normal pyramidal cell firing patterns. LEV had no major effects on water-maze performance or place cell function. Histopathological examination of the brains showed severe cell loss in CA1 in all of the SE rats with lesser degrees of injury in CA3 and the hilus. LEV treatment resulted in less histological damage in the hippocampus but had no effect on visual-spatial function or place cell physiology in either control or SE rats.

Animals↗
PubMed2004

Spatial distribution of acoustic and elastic properties of human femoral cortical bone.

The purpose of this study is to quantify the spatial distribution of acoustic velocities and elastic properties (elastic constants) on Human femoral cortical bone. Four cross sections (average thickness of 2.09+/-0.27 mm) have been cut transversally between 40% and 70% of the total length and between them parallelepiped samples in each quadrant have been cut. Ultrasonic technique in transmission with immersion focused transducers at 5 MHz and contact transducers 2.25 MHz were used on the cross sections and parallelepiped samples, respectively. The first technique allows relative spatial distribution of velocities to be obtained, meanwhile the second technique allows the direct assessment of elastic constants. For both techniques, bulk velocities were found to be lower at the posterior side with an increase along the length (from 40% to 70% total length) (p < 0.05). Densities and elastic constants show equivalent pattern of variation. These variations are mainly due the cortical porosity related to vascularisation environment. The spatial distribution of velocities exhibits significant radial variation from the endosteal to the periosteal region. This is in agreement with variation of the porosity at that location. Same range of velocities was obtained with both techniques. The range of longitudinal velocities values varies from 3548 to 3967 m/s and between 18.5 and 33.1 GPa for the bulk velocities and axial elastic constants, respectively. Our results are within the range with those found in the literature. However, it must be noted that the range of acoustic and elastic properties variation is concerning the same bone. So, our new results show the ability of the technique to quantify accurately local variation of acoustic and elastic properties (within the section and along the length) of human cortical bone. Furthermore, our immersion technique could be used to assess the spatial distribution of the elastic constants with the knowledge of spatial distribution of densities.

Acoustics↗
PubMed2006

Corneal-thickness spatial profile and corneal-volume distribution: tomographic indices to detect keratoconus.

PURPOSE: To evaluate whether the corneal-thickness spatial profile and corneal-volume distribution differentiate keratoconic corneas from normal corneas using new tomography parameters. SETTING: Subspecialty cornea and refractive practice, Fluminense Federal University, Rio de Janeiro, Brazil. METHODS: Forty-six eyes diagnosed with mild to moderate keratoconus and 364 normal eyes were studied by the Pentacam Comprehensive Eye Scanner. Corneal thickness at the thinnest point and the averages of the points on 22 imaginary circles centered on the thinnest point with increased diameters at 0.4 mm steps were calculated to create a corneal-thickness spatial profile. Corneal volume was calculated within diameters from 1.0 to 7.0 mm with 0.5 mm steps centered on the thinnest point to create the corneal-volume distribution. The percentage increase in thickness and the percentage increase in volume were calculated for each position of the corneal-thickness spatial profile and corneal-volume distribution from their first value. Statistical analysis was done using the Wilcoxon 2-independent-sample test to compare mean levels using S-Plus-4.0 software (MathSoft) and a normal linear model under a Bayesian frame for estimating the mean variation in thickness and volume using the BUGS 0.6 package. RESULTS: Statistically significant differences were observed between the groups (P<.05) in all positions of corneal-thickness spatial profile and corneal-volume distribution and in the percentage increase in thickness and percentage increase in volume between 3.5 mm and 7.0 mm diameters. CONCLUSIONS: Corneal-thickness spatial profile, corneal-volume distribution, percentage increase in thickness, and percentage increase in volume were different between keratoconic corneas and normal corneas and could serve as indices to diagnose keratoconus and screen refractive candidates. Further studies are necessary to evaluate whether these tomographic indices are more sensitive and specific than the classic Placido-based topography.

Cornea↗
PubMed2005

Method for determining the spatial position of the shoulder with ultrasound-based motion analyzer.

Several methods have been developed recently for the analysis of the spatial motion of the scapula and the arm, whereby the spatial position of shoulder bones is determined in static conditions by interrupting motion. The authors have developed a 3D motion analysis method recording scapular motion in progress with appropriate accuracy in the course of arm movements of various degrees. The objective of this study is to explore the applicability of the method developed, as well as to compare it with and verify it by other methods developed earlier. The position and displacements of shoulder bones were determined on 30 shoulders of 15 healthy people. The newly developed measurement method is based on the mechanical basic principle stating that the position and motion of a rigid body -- in this case, the bones (segments) forming the shoulder joint -- can be calculated at any moment from the spatial coordinates of three points of a segment and any changes thereof in the course of motion. Ultrasound-based triplets providing the three points (fundamental points) by a segment as required for measurement were fixed on the sternum (modeling the trunk), the clavicle, the acromion (modeling the scapula), the upper arm, and the lower arm. The position of the sixteen anatomical points involved in the study were determined by an ultrasound-based pointer in the local coordinate system specified by the fundamental points before starting measurements. The ZEBRIS ultrasound-based motion analysis system was used for measuring the spatial coordinates of triplets in the course of continuous motion. The spatial coordinates of the designated anatomical points can be calculated by the method of triangulation. The method was calibrated by a ZEBRIS mapping (3DCAD) software commercially available, and the measurement error rate of the method was determined by statistical calculations. On the basis of calibration and error calculations it could be established that the accuracy and the reproducibility of the method were appropriate, in accordance with the limit values to be found in the literature.

Acromion↗
PubMed2006

Using two-dimensional spatial information in decomposition of surface EMG signals.

Recently, high-density surface EMG electrode grids and multi-channel amplifiers became available for non-invasive recording of human motor units (MUs). We present a way to decompose surface EMG signals into MU firing patterns, whereby we concentrate on the importance of two-dimensional spatial differences between the MU action potentials (MUAPs). Our method is exemplified with high-density EMG data from the vastus lateralis muscle of a single subject. Bipolar and Laplacian spatial filtering was applied to the monopolar raw signals. From the single recording in this subject six different simultaneously active MUs could be distinguished using the spatial differences between MUAPs in the direction perpendicular to the muscle fiber direction. After spike-triggered averaging, 125-channel two-dimensional MUAP templates were obtained. Template-matching allowed tracking of all MU firings. The impact of spatial information was measured by using subsets of the MUAP templates, either in parallel or perpendicular to the muscle fiber direction. The use of one-dimensional spatial information perpendicular to the muscle fiber direction was superior to the use of a linear array electrode in the longitudinal direction. However, to detect the firing events of the MUs with a high accuracy, as needed for instance for estimation of firing synchrony, two-dimensional information from the complete grid electrode appears essential.

Action Potentials↗
PubMed2006

Linking linear programming and spatial simulation models to predict landscape effects of forest management alternatives.

Forest management planners require analytical tools to assess the effects of alternative strategies on the sometimes disparate benefits from forests such as timber production and wildlife habitat. We assessed the spatial patterns of alternative management strategies by linking two models that were developed for different purposes. We used a linear programming model (Spectrum) to optimize timber harvest schedules, then a simulation model (HARVEST) to project those schedules in a spatially explicit way and produce maps from which the spatial pattern of habitat could be calculated. We demonstrated the power of this approach by evaluating alternative plans developed for a national forest plan revision in Wisconsin, USA. The amount of forest interior habitat was inversely related to the amount of timber cut, and increased under the alternatives compared to the current plan. The amount of edge habitat was positively related to the amount of timber cut, and increased under all alternatives. The amount of mature northern hardwood interior and edge habitat increased for all alternatives, but mature pine habitat area varied. Mature age classes of all forest types increased, and young classes decreased under all alternatives. The average size of patches (defined by age class) generally decreased. These results are consistent with the design goals of each of the alternatives, but reveal that the spatial differences among the alternatives are modest. These complementary models are valuable for quantifying and comparing the spatial effects of alternative management strategies.

Computer Simulation↗
PubMed2005

Spatially resolved emulsion droplet sizing using inverse Abel transforms.

Pulsed field gradient (PFG) nuclear magnetic resonance (NMR) is well established as a tool for determining emulsion droplet-size distributions via measurement of restricted self-diffusion. Most measurements made to date have not been spatially resolved, but have measured an average size distribution for a certain volume of emulsion. This paper demonstrates a rapid method of performing spatially resolved, restricted diffusion measurements, which enables emulsion droplet sizing to be spatially resolved as a function of radius in cylindrical geometries or pipes. This is achieved by the use of an Abel transform. The technique is demonstrated in various annular systems containing two emulsions, with different droplet-size distributions, and/or a pure fluid. It is also shown that by modifying the pulse sequence by the inclusion of flow-compensating magnetic field gradients, the technique can measure spatially resolved droplet-size distributions in flowing emulsions, with potential applications in spatially resolved on-line droplet-size analysis.

Journal Article↗
PubMed2005

Spatial dynamics of invasion: the geometry of introduced species.

Many exotic species combine low probability of establishment at each introduction with rapid population growth once introduction does succeed. To analyse this phenomenon, we note that invaders often cluster spatially when rare, and consequently an introduced exotic's population dynamics should depend on locally structured interactions. Ecological theory for spatially structured invasion relies on deterministic approximations, and determinism does not address the observed uncertainty of the exotic-introduction process. We take a new approach to the population dynamics of invasion and, by extension, to the general question of invasibility in any spatial ecology. We apply the physical theory for nucleation of spatial systems to a lattice-based model of competition between plant species, a resident and an invader, and the analysis reaches conclusions that differ qualitatively from the standard ecological theories. Nucleation theory distinguishes between dynamics of single- and multi-cluster invasion. Low introduction rates and small system size produce single-cluster dynamics, where success or failure of introduction is inherently stochastic. Single-cluster invasion occurs only if the cluster reaches a critical size, typically preceded by a number of failed attempts. For this case, we identify the functional form of the probability distribution of time elapsing until invasion succeeds. Although multi-cluster invasion for sufficiently large systems exhibits spatial averaging and almost-deterministic dynamics of the global densities, an analytical approximation from nucleation theory, known as Avrami's law, describes our simulation results far better than standard ecological approximations.

Animal Migration↗
PubMed2005

The evolution of a Müllerian mimic in a spatially distributed community.

Strong positive density-dependence should lead to a loss of diversity, but warning-colour and Müllerian mimicry systems show extraordinary levels of diversity. Here, we propose an analytical model to explore the dynamics of two forms of a Müllerian mimic in a heterogeneous environment with two alternative model species. Two connected populations of a dimorphic, chemically defended mimic are allowed to evolve and disperse. The proportions of the respective model species vary spatially. We use a nonlinear approximation of Müller's number-dependent equations to model a situation where the mortality for either form of the mimic decreases hyberbolically when its local density increases. A first non-spatial analysis confirms that the positive density-dependence makes coexistence of mimetic forms unstable in a single isolated patch, but shows that mimicry of the rarer model can be stable once established. The two-patch analysis shows that when models have different abundance in different places, local mimetic diversity in the mimic is maintained only if spatial heterogeneity is strong, or, more interestingly, if the mimic is not too strongly distasteful. Therefore, mildly toxic species can become polymorphic in a wider range of ecological settings. Spatial dynamics thus reveal a region of Müllerian polymorphism separating classical Batesian polymorphism and Müllerian monomorphism along the mimic's palatability spectrum. Such polymorphism-palatability relationship in a spatial environment provides a parsimonious hypothesis accounting for the observed Müllerian polymorphism that does not require quasi-Batesian dynamics. While the stability of coexistence depends on all factors, only the migration rate and strength of selection appear to affect the level of diversity at the polymorphic equilibrium. Local adaptation is predicted in most polymorphic cases. These results are in very good accordance with recent empirical findings on the polymorphic butterflies Heliconius numata and H. cydno.

Adaptation, Physiological↗
PubMed2006

Bifurcations and phase transitions in spatially extended two-member hypercycles.

Mounting theoretical and experimental evidence indicates that the success of molecular replicators is strongly tied to the local nature of their interactions. Local dispersal in a given spatial domain, particularly on surfaces, might strongly enhance the growth and selection of fit molecules and their resistance to parasites. In this work the spatial dynamics of a simple hypercycle model consisting of two molecular species is analysed. In order to characterize it, both mean field models and stochastic, spatially explicit approaches are considered. The mean field approach predicts the presence of a saddle-node bifurcation separating a phase involving stable hypercycles from extinction, consistently with spatially explicit models, where an absorbing first-order phase transition is shown to exist and diffusion is explicitly introduced. The saddle-node bifurcation is shown to leave a ghost in the phase plane. A metapopulation-based model is also developed in order to account for the observed phases when both diffusion and reaction are considered. The role of information and diffusion as well as the relevance of these phases and the underlying spatial structures are discussed, and their potential implications for the evolution of early replicators are outlined.

Animals↗
PubMed2006

The role of trade-off shapes in the evolution of parasites in spatial host populations: an approximate analytical approach.

Given the substantial changes in mixing in many populations, there is considerable interest in the role that spatial structure can play in the evolution of disease. Here we examine the role of different trade-off shapes in the evolution of parasites in a spatially structured host population where infection can occur locally or globally. We develop an approximate adaptive dynamic analytical approach, to examine how the evolutionarily stable (ES) virulence depends not only on the fraction of global infection/transmission but also on the shape of the trade-off between transmission and virulence. Our analysis can successfully predict the ES virulence found previously by simulation of the full system. The analysis confirms that when there is a linear trade-off between transmission and virulence spatial structure may lead to an ES virulence that increases as the proportion of global transmission increases. However, we also show that the ESS disappears above a threshold level of global infection, leading to maximization. In addition just below this threshold, there is the possibility of evolutionary bi-stabilities. When we assume the realistic trade-off between transmission and virulence that results in an ESS in the classical mixed model, we find that spatial structure can increase or decrease the ES virulence. A relatively high proportion of local infection reduces virulence but intermediate levels can select for higher virulence. Our work not only emphasizes the importance of spatial structure to the evolution of parasites, but also makes it clear that situations between the local and the global need to be considered. We also emphasize the key role that the shape of trade-offs plays in evolutionary outcomes.

Adaptation, Physiological↗
PubMed2005

The spread of infectious diseases in spatially structured populations: an invasory pair approximation.

The invasion of new species and the spread of emergent infectious diseases in spatially structured populations has stimulated the study of explicit spatial models such as cellular automata, network models and lattice models. However, the analytic intractability of these models calls for the development of tractable mathematical approximations that can capture the dynamics of discrete, spatially-structured populations. Here we explore moment closure approximations for the invasion of an SIS epidemic on a regular lattice. We use moment closure methods to derive an expression for the basic reproductive number, R(0), in a lattice population. On lattices, R(0) should be bounded above by the number of neighbors per individual. However, we show that conventional pair approximations actually predict unbounded growth in R(0) with increasing transmission rates. To correct this problem, we propose an 'invasory' pair approximation which yields a relatively simple expression for R(0) that remains bounded above, and also predicts R(0) values from lattice model simulations more accurately than conventional pair and triple approximations. The invasory pair approximation is applicable to any spatial model, since it takes into account characteristics of invasions that are common to all spatially structured populations.

Animals↗
PubMed2003

Spatial working memory deficits in obsessive compulsive disorder are associated with excessive engagement of the medial frontal cortex.

Recent studies have shown that obsessive compulsive disorder (OCD) is associated with a specific deficit in spatial working memory, especially when task difficulty (i.e., working memory load) is high. It is not clear whether this deficit is associated with dysfunction of the brain system that subserves spatial working memory, or whether it is associated with a more generalized effect on executive functions. In contrast to studies in healthy volunteers and schizophrenia, spatial working memory in OCD has not been investigated before using functional neuroimaging techniques. We conducted a functional MRI study in 11 treatment-free female patients with OCD and 11 for sex-, age-, education-, and handedness pairwise-matched healthy controls in order to assess performance on a parametric spatial n-back task as well as the underlying neuronal substrate and its dynamics. Patients with OCD performed poorly at the highest level of task difficulty and engaged the same set of brain regions as the matched healthy controls. In this set, the effect of difficulty on magnitude of brain activity was the same in patients and in controls except for a region covering the anterior cingulate cortex. In this region activity was significantly elevated in patients with OCD at all levels of the parametric task. These findings do not provide evidence for a deficit of the spatial working memory system proper, but suggest that the abnormal performance pattern may be secondary to another aspect of executive dysfunctioning in OCD.

Adult↗
PubMed2006

Vector-based spatial-temporal minimum L1-norm solution for MEG.

Minimum L1-norm solutions have been used by many investigators to analyze MEG responses because they provide high spatial resolution images. However, conventional minimum L1-norm approaches suffer from instability in spatial construction, and poor smoothness of the reconstructed source time-courses. Activity commonly "jumps" from one grid point to (usually) the neighboring grid points. Equivalently, the time-course of one specific grid point can show substantial "spiky-looking" discontinuity. In the present study, we present a new vector-based spatial-temporal analysis using a L1-minimum-norm (VESTAL). This approach is based on a principle of MEG physics: the magnetic waveforms in sensor-space are linear functions of the source time-courses in the imaging-space. Our computer simulations showed that VESTAL provides good reconstruction of the source amplitude and orientation, with high stability and resolution in both the spatial and temporal domains. "Spiky-looking" discontinuity was not observed in the source time-courses. Importantly, the simulations also showed that VESTAL can resolve sources that are 100% correlated. We then examined the performance of VESTAL in the analysis of human median-nerve MEG responses. The results demonstrated that this method easily distinguishes sources very spatially close to each other, including individual primary somatosensory areas (BA 1, 2, 3b), primary motor area (BA 4), and other regions in the somatosensory system (e.g., BA 5, 7, SII, SMA, and temporal-parietal junction) with high temporal stability and resolution. VESTAL's potential for obtaining information on source extent was also examined.

Adult↗
PubMed2006

Representation of spatial goals in rat orbitofrontal cortex.

The orbitofrontal cortex (OFC) is thought to participate in making and evaluating goal-directed decisions. In rodents, spatial navigation is a major mode of goal-directed behavior, and anatomical and lesion studies implicate the OFC in spatial processing, but there is little direct evidence for coding of spatial or motor variables. Here, we recorded from ventrolateral and lateral OFC in an odor-cued two-alternative choice task requiring orientation and approach to spatial goal ports. In this context, over half of OFC neurons encoded choice direction or goal port location. A subset of neurons was jointly selective for the trial outcome and port location, information useful for the selection or evaluation of spatial goals. These observations show that the rodent OFC not only encodes information relating to general motivational significance, as shown previously, but also encodes spatiomotor variables needed to define specific behavioral goals and the locomotor actions required to attain them.

Action Potentials↗
PubMed2004

Functional MRI of visual-spatial processing in neurofibromatosis, type I.

Visual-spatial impairment and neuroanatomical abnormalities are considered hallmark features of neurofibromatosis, type I (NF-I). Numerous studies have demonstrated visual-spatial deficits in children with NF-I, but few relations between these deficits and neuroanatomical abnormalities have been identified. We compared the functional neuroanatomy of cerebral regions involved in the spatial transformation of alphanumeric stimuli in individuals with NF-I and healthy control participants using functional magnetic resonance imaging (fMRI). Given the prevalence of visual pathway abnormalities and visual-spatial deficits in NF-I, we hypothesized that less neuronal hemodynamic activity would be found in occipital and parietal cortices in this group compared with controls. However, NF-I participants relied to a greater degree than controls on posterior cortex (including occipital, parietal, and middle temporal cortices) relative to lateral and inferior frontal regions during visual-spatial analysis. This pattern was significantly related to their behavioral performance on the fMRI task, which in turn was also positively correlated with reading scores. These findings support evidence of frontal cortical anomalies in NF-I and may provide a pathophysiological basis for cognitive deficits in NF-I.

Adolescent↗
PubMed2004

Spatial-temporal anisometries following right parietal damage.

Patients with right parietal damage often have a lateralized deficit of spatial attention. In addition to a spatial deficit, such patients have also been reported to have a non-spatial deficit in temporal processing. Here, we tested the hypothesis that these spatial and temporal deficits might be linked if the right temporal-parietal cortex is important in integrating spatial and temporal attention. In AF, a patient with an acute right temporal-parietal stroke, we replicated previous observations showing that he was biased to judge ipsilesional stimuli as occurring before contralesional stimuli. More importantly, for vertically aligned stimuli, AF more accurately judged the temporal order of successive ipsilesional than contralesional stimuli. Furthermore, his contralesional performance improved with stimuli with larger vertical separations. Taken together, these findings provide additional evidence that right temporal-parietal damage produces a processing refractory period for stimuli in contralesional space that extends in both space and time. These findings are in agreement with other studies that suggest that the right temporal-parietal cortex is important in integrating the where and when of stimuli.

Adult↗
PubMed2005

Retrograde amnesia: neither partial nor complete hippocampal lesions in rats result in preferential sparing of remote spatial memory, even after reminding.

Many lesion experiments have provided evidence that the hippocampus plays a time-limited role in memory, consistent with the operation of a systems-level memory consolidation process during which lasting neocortical memory traces become established [see Squire, L. R., Clark, R. E., & Knowlton, B. J. (2001). Retrograde amnesia. Hippocampus 11, 50]. However, large lesions of the hippocampus at different time intervals after acquisition of a watermaze spatial reference memory task have consistently resulted in temporally ungraded retrograde amnesia [Bolhuis, J. J., Stewart, C. A., Forrest, E. M. (1994). Retrograde amnesia and memory reactivation in rats with ibotenate lesions to the hippocampus or subiculum. Quarterly Journal of Experimental Psychology 47B, 129; Mumby, D. G., Astur, R. S., Weisend, M. P., Sutherland, R. J. (1999). Retrograde amnesia and selective damage to the hippocampal formation: memory for places and object discriminations. Behavioural Brain Research 106, 97; Sutherland, R. J., Weisend, M. P., Mumby, D., Astur, R. S., Hanlon, F. M., et al. (2001). Retrograde amnesia after hippocampal damage: recent vs. remote memories in two tasks. Hippocampus 11, 27]. It is possible that spatial memories acquired during such a task remain permanently dependent on the hippocampus, that chance performance may reflect a failure to access memory traces that are initially unexpressed but still present, or that graded retrograde amnesia for spatial information might only be observed following partial hippocampal lesions. This study examined the retrograde memory impairments of rats that received either partial or complete lesions of the hippocampus either 1-2 days, or 6 weeks after training in a watermaze reference memory task. Memory retention was assessed using a novel 'reminding' procedure consisting of a series of rewarded probe trials, allowing the measurement of both free recall and memory reactivation. Rats with complete hippocampal lesions exhibited stable, temporally ungraded retrograde amnesia, and could not be reminded of the correct location. Partially lesioned rats could be reminded of a recently learned platform location, but no recovery of remote memory was observed. These results offer no support for hippocampus-dependent consolidation of allocentric spatial information, and suggest that the hippocampus can play a long-lasting role in spatial memory. The nature of this role--in the storage, retrieval, or expression of memory--is discussed.

Amnesia, Retrograde↗