SEARCH · PubMed Health

Results for “spatial dependency”

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 505 records · Page 28Linked to original sources

PubMed2026

Lipid metabolism is a key central, systemic and gut microbial feature of the decline in rat hippocampal function during middle age.

Middle age is emerging as a turning point in brain ageing, prognostic of future cognitive health and amenable to intervention. Metabolic and proteomic differences during this period are not yet fully understood and may potentially influence functions of the hippocampus, a brain area that regulates memory and anxiety. While the gut microbiota is implicated in brain ageing, the relationship between the gut microbiota, the metabolic state, and hippocampal proteome in middle age has not been investigated. We hypothesise that peripheral metabolic or protein features are associated with hippocampal vulnerability in middle age. Therefore, young adult and middle-aged rats were assessed for behavioural, proteomic, metabolic, and gut microbiota differences. Proteomic profiling of the hippocampus revealed differential expression of proteins indicative of altered synaptic signalling. Concurrently, adult hippocampal neurogenesis was decreased in middle age. Hippocampal microglia exhibited a lipid rich, inflammatory phenotype in middle age which correlated with poorer memory performance. CSF and serum proteomic and metabolomic analyses identified dysregulated lipid-related pathways potentially contributing to hippocampal vulnerability in middle age. Furthermore, 16S rRNA sequencing revealed reduced abundance of bacteria involved in lipid metabolism regulation. However, faecal microbiota transfer from young to middle aged rats was not sufficient to robustly improve hippocampus-dependent spatial memory. Together, these findings highlight dysfunctional lipid metabolism as a key feature of middle age that may contribute to decline in hippocampal function. Given that the scope for intervention is limited during older age, targeting biomarkers involved in metabolic and lipid homeostasis may be pivotal for the development of pharmacological or lifestyle-based interventions during middle age which could ultimately delay future cognitive ageing.

Animals↗

PubMed2006

Application of k-space energy spectrum analysis to susceptibility field mapping and distortion correction in gradient-echo EPI.

Echo-planar imaging (EPI) is widely used in functional MRI studies. It is well known that EPI quality is usually degraded by geometric distortions, when there exist susceptibility field inhomogeneities. EPI distortions may be corrected if the field maps are available. It is possible to estimate the susceptibility field gradients from the phase reconstruction of a single-TE EPI image, after a successful phase-unwrapping procedure. However, in regions affected by pronounced field gradients, the phase-unwrapping of a single-TE image may fail, and therefore the estimated field maps may be incorrect. It has been reported that the field inhomogeneity may be calculated more reliably from T2*-weighted images corresponding to multiple TEs. However, the multi-TE MRI field mapping increases the scan time. Furthermore, the measured field maps may be invalid if the subject's position changes during dynamic scans. To overcome the limitations in conventional field mapping approaches, a novel k-space energy spectrum analysis algorithm is developed, which quantifies the spatially dependent echo-shifting effect and the susceptibility field gradients directly from the k-space data of single-TE gradient-echo EPI. Using the k-space energy spectrum analysis, susceptibility field gradients can be reliably measured without phase-unwrapping, and EPI distortions can be corrected without extra field mapping scans or pulse sequence modification. The reported technique can be used to retrospectively improve the image quality of the previously acquired EPI and functional MRI data, provided that the complex-domain k-space data are still available.

Artifacts↗

PubMed2007

Hypobaric hypoxia-induced dendritic atrophy of hippocampal neurons is associated with cognitive impairment in adult rats.

Simulated hypobaric hypoxia (HBH), resembling high altitude hypoxia severely affects the CNS and results in several physiological changes. The hippocampus is closely associated with learning and memory and an insult to this region affects cognition. Previous studies suggest that rapid or prolonged exposures to HBH are associated with psychomotor and cognitive impairments. The defense personnel, mountain climbers and rescue teams are exposed to such harsh environment and thus it demands a systematic study emphasizing the subtle effects of such extreme environments on cognitive function. Accordingly, this study evaluated the effect of hypobaric hypoxia on structural changes in the principal neurons of the hippocampus and learning in eight-arm radial maze. Adult male Wistar rats, subjected to simulated hypobaric hypoxia equivalent to an altitude of 6000 m for a period of 2 or 7 days, in a hypoxic chamber served as hypoxic group (HY). Rats housed in a similar chamber for the same period of time, without hypoxic exposure served as sham control (SC), while normal control (NC) group of rats were housed in standard laboratory conditions. The dendritic morphology of neurons in cornu ammonis region 1 (CA1) and cornu ammonis region 3 (CA3) was studied in Golgi-impregnated hippocampal sections. Exposure for 2 days to hypobaric hypoxia had minimal deleterious effects on the CA1 pyramidal neurons, while exposure for 7 days resulted in a significant decrease in the number of branching points, intersections and dendritic length. Unlike the CA1 pyramidal neurons, the CA3 neurons exhibited dendritic atrophy following both 2 and 7 days of hypoxic exposure. Further, hippocampal-dependent spatial learning was affected marginally following 2 day exposure, while 7 day exposure severely affected learning of the partially baited radial arm maze task. Our study suggests that dendritic atrophy in the hippocampus on exposure to HBH could be one of the bases for the cognitive deficits exhibited under such conditions.

Animals↗

PubMed2004

Spatial heterogeneity of the risk of BSE in France following the ban of meat and bone meal in cattle feed.

In France, meat-and-bone meal (MBM) has been prohibited for cattle feeding since 1990, but bovine spongiform encephalopathy (BSE) cases, called 'NAIF', appeared in animals born after this feed ban. Furthermore, in 1996 a new measure was taken: removal of cadavers and specified risk materials (SRM) from the processing of MBM dedicated to animal feed. Nevertheless, BSE cases (called 'super-NAIF') appeared in cattle born after this measure was in force. We analysed the spatial distribution of 445 'NAIF' and 58 'super-NAIF' cases detected in France from July 1, 2001 to July 31, 2003. The detection of BSE was based both on the mandatory reporting system (MRS) and the systematic test screening of cattle at the abattoir and at the fallen-animal plant with rapid tests. The background population was based on the adult-cow census. The disease mapping of the BSE risk was based on the standardised incidence ratio (stochastic Poisson process). A spatial component, which takes into account the spatial dependence between the geographical units by a notion of adjacency was used to eliminate the over-dispersion in the risk assessment. The geographical units were defined by hexagons with a side of 23km (France had 1264 hexagons). The parameters were estimated by a Metropolis Gibbs sampling algorithm using the Markov-chain Monte Carlo methods. The BSE cases were not randomly distributed. Furthermore, the areas at risk for the 'super-NAIF' matched part of the areas at risk for the 'NAIF' cases-which suggests that it might be a common source of contamination.

Animal Feed↗

PubMed2006

The influence of a city on element contents of a terrestrial moss (Hylocomium splendens).

Forty terrestrial moss (Hylocomium splendens) samples were collected along a 120-km-long south-north transect running through Norway's largest city Oslo. Concentrations of 29 chemical elements (Ag, Al, Au, Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, Hg, K, La, Mg, Mn, Mo, Na, Ni, P, Pb, Pt, S, Sb, Sr, Th, Ti, and Zn) and values for loss on ignition (475 degrees C) are reported. Silver (Ag), Al, Au, Bi, Cd, Co, Cr, Cu, Fe, Mo, Ni, Pb, Pt, Sb, Th, Ti, and Zn all show a characteristic Oslo peak when element concentrations are plotted against location of the sample site along the transect. Gold (Au) and Pt show the greatest relative enrichment of all elements in the city (ca. 10x "background"). Titanium (Ti), which is related to local minerogenic dust rather than anthropogenic emissions, shows a significant peak in Oslo. Loss on ignition, a measure of the amount of organic material in a sample, shows a negative peak in Oslo and at sites close to a known dust source. Input of fine dust thus appears to dominate many of the observed element concentrations in moss. The concentrations of Na are clearly influenced by the input of marine aerosols and show decreasing concentrations from south (near Oslo Fjord) to north (inland). The major plant nutrients Ca, K, Mg, P and S, as well as Hg, are the few elements displaying no spatial dependency along the transect. Element concentrations reach background variation levels at a distance of 20-40 km from the city centre.

Adsorption↗

PubMed2004

Pulse laser acoustics for the characterization of inhomogeneities at interfaces of microstructures.

Interfaces between neighbouring materials are often subjected to diffusion processes which cause layers having gradually varying mechanical properties--like densities, Young's moduli or shear moduli--perpendicular to the surface or interface. In this investigation particular interest is drawn on the question how the propagation characteristics of bulk acoustic waves are affected by diffusion layers. The reflection and transmission behavior of bulk acoustic waves encountering a continuum having a spatially dependent sound velocity is discussed based on numerical simulations as well as on experimental verifications. The simulated results are part of an on-going project in which material properties of MEMS devices are investigated by short pulse laser acoustic methods. Mechanical waves are excited and detected thermoelastically using laser pulses of 70 fs duration. For metals this leads to wavelengths of 10-20 nm and the corresponding frequencies amount to 0.3-0.6 THz. In contrast to previous work done in this field in which diffusion effects are generally considered as undesirable phenomena, the deliberate realization of microstructures having well defined gradually varying material properties in one or more dimensions represents a goal of this investigation. For metallic thin film multilayers thermally induced diffusion processes have shown to be an easy and reliable technique for the realization of layered structures having continuously varying mechanical properties within several 10 nm. Among the experimental methods suitable for the in-depth profiling of submicron metallic thin films providing resolutions of several nanometers, are short pulse laser acoustic methods, Rutherford backscattering spectroscopy (RBS), and glow discharge optical emission spectroscopy (GDOES). Short pulse laser acoustic methods and RBS have the advantage to be nondestructive. The short pulse laser acoustic method is described in detail and RBS measurements are presented for verification purposes. Finally potential engineering applications like micro-machined spectrum analyzers, acoustic isolation layers, and band pass filters, operating at very high frequencies are presented.

Journal Article↗

PubMed2004

Tuning properties of radial phantom motion aftereffects.

Motion aftereffects are normally tested in regions of the visual field that have been directly exposed to motion (local or concrete MAEs). We compared concrete MAEs with remote or phantom MAEs, in which motion is perceived in regions not previously adapted to motion. Our aim was to study the spatial dependencies and spatiotemporal tuning of phantom MAEs generated by radially expanding stimuli. For concrete and phantom MAEs, peripheral stimuli generated stronger aftereffects than central stimuli. Concrete MAEs display temporal frequency tuning, while phantom MAEs do not show categorical temporal frequency or velocity tuning. We found that subjects may use different response strategies to determine motion direction when presented with different stimulus sizes. In some subjects, as adapting stimulus size increased, phantom MAE strength increased while the concrete MAE strength decreased; in other subjects, the opposite effects were observed. We hypothesise that these opposing findings reflect interplay between the adaptation of global motion sensors and local motion sensors with inhibitory interconnections.

Adaptation, Ocular↗

PubMed1997

Estimating the electrostatic potential at the acetylcholine receptor agonist site using power saturation EPR.

Continuous wave EPR power saturation was used to measure electrostatic potentials at spin-labeled sites. Membrane surface potentials were estimated by power saturating the EPR spectrum of a membrane bound 14N spin-labeled amphiphile in the presence of a neutral or positively charged 15N labeled aqueous spin probe. The potentials that are measured are in good agreement with other probe measurements and with the predictions of the Gouy-Chapman-Stern theory, indicating that this is a valid approach to determine electrostatic potentials. A spin-labeled affinity probe based on maleimidobenzyltrimethylammonium was synthesized and could be derivatized to a sulfhydryl near either agonist site on the nicotinic acetylcholine receptor. The amplitudes of motion of the spin-probe on the ns time scale are significantly different when the two labeled sites are compared, and the probe is more restricted in its motion when attached to the more easily labeled site. When attached to this agonist site, power saturation EPR yields an electrostatic potential of -15 mV. Two other sulfhydryl-specific probes were used to label this site in reconstituted receptor containing membranes. These probes show less contact with the receptor and reduced electrostatic potentials, indicating that there is a strong spatial dependence to the potential at the agonist site. This work demonstrates that power saturation EPR provides a general method that can be used to estimate electrostatic potentials at any specifically spin-labeled macromolecular site.

Binding Sites↗

PubMed1978

Quasi-elastic light scattering from migrating chemotactic bands of Escherichia coli.

We report the observation of migrating chemotactic bands of Escherichia coli in a buffer solution. The temporal development of the bacterial density profile is observed by the scattered light intensity as the band migrates through a stationary laser beam. We have made a preliminary analysis of the observed band profile with help of the Keller-Segel theory. The model accounts for only some aspects of the observed time evolution of the density profile. The microscopic motility characteristics of the E. coli in the band are simultaneously studied by photon correlation. The measured correlation functions are analyzed to obtain the spatial dependence of the half-width within the band. A simple analytical model is proposed to account for the contribution of the twiddle motion to the correlation function. By analyzing the correlation function as a superposition of straight-line and twiddle motions, we obtain a satisfactory agreement between the theory and the measured angular dependence of the line shape. As a consequence we are able to extract a parameter beta, which measures the average fraction of twiddling bacteria in the center of the band at a given time.

Cell Movement↗

PubMed1980

Surface-induced lamellar orientation of multilayer membrane arrays. Theoretical analysis and a new method with application to purple membrane fragments.

The orientation of membrane fragments into a lamellar array by a flat surface is analyzed. This analysis includes processes such as centrifugation and drying and physical effects due to membrane fragment steric interactions, finite size, elasticity, and thermal fluctuations. Several model calculations of optimal orientational order in multilayer membrane arrays are presented. The predictions of a smectic A model agree quantitatively with the measured spatial dependence of the fluctuations in layer orientation in a multilamellar arrays. A new technique, based in part on this analysis, for the preparation of well-oriented multilamellar arrays of natural and artificial membranes, isopotential spin-dry centrifugation, is described. The method involves the use of specially designed inserts for the buckets of a standard vacuum ultracentrifuge. The membrane fragments to be oriented are sedimented from solution or suspension onto a substrate of a convenient material which forms a gravitational isopotential surface at high g. Sedimentation is accompanied by removal of the suspending medium at high g to produce oriented films with a selected degree of solvation. In addition, a method is described whereby small solute molecules can be maintained in constant concentration with the membrane fragments during this process. Initial application of the method to the orientation of purple membrane fragments is described. The degree of orientation obtained in this system is evaluated using freeze-fracture and scanning electron microscopy, optical birefringence, linear dichroism, and microscopy.

Bacteriorhodopsins↗

PubMed1989

Current injection into a two-dimensional anisotropic bidomain.

A two-dimensional sheet of anisotropic cardiac tissue is represented with the bidomain model, and the finite element method is used to solve the bidomain equations. When the anisotropy ratios of the intracellular and extracellular spaces are not equal, the injection of current into the tissue induces a transmembrane potential that has a complicated spatial dependence, including adjacent regions of depolarized and hyperpolarized tissue. This behavior may have important implications for the electrical stimulation of cardiac tissue and for defibrillation.

Animals↗

PubMed1997

Stochastic versions of the Hodgkin-Huxley equations.

A Hodgkin-Huxley model algorithm for the numerical simulation of noise in neurons is contracted from a master equation description (cellular automoton) into a Langevin description. This reduction reduces the time required for a simulation by about two orders of magnitude. Earlier work is summarized, condensed, and made explicit to make the algorithm transparent and facilitate applications. Two approximate treatments are reported. An extension of this approach is presented that includes spatial dependence and the propagation of a noisy action potential along an axon.

Axons↗

PubMed1998

Simulation of the fertilization Ca2+ wave in Xenopus laevis eggs.

In the preceding paper Fontanilla and Nuccitelli (Biophysical Journal 75:2079-2087 (1998)) present detailed measurements of the shape and speed of the fertilization Ca2+ wave in Xenopus laevis eggs. In order to help interpret their results, we develop here a computational technique based on the finite element method that allows us to carry out realistic simulations of the fertilization wave. Our simulations support the hypothesis that the physiological state of the mature egg is bistable, i.e., that its cytoplasm can accommodate two alternative physiological Ca2+ concentrations: a low concentration characteristic of the prefertilization state and a greatly elevated concentration characteristic of the state following the passage of the wave. We explore this hypothesis by assuming that the bistability is due to the release and re-uptake properties of the endoplasmic reticulum (ER) as determined by inositol trisphosphate (IP3) receptor/Ca2+ channels and sarcoendoplasmic reticulum calcium ATPase (SERCA) pumps. When combined with buffered diffusion of Ca2+ in the cytoplasm, our simulations show that inhomogeneities in the Ca2+ release properties near the plasma membrane are required to explain the temporal and spatial dependences of the shape and speed of these waves. Our results are consistent with an elevated IP3 concentration near the plasma membrane in the unfertilized egg that is augmented significantly near the site of fertilization. These gradients are essential in determining the concave shape of the Ca2+ fertilization wave front.

Animals↗

PubMed2002

Allopregnanolone inhibits learning in the Morris water maze.

The progesterone metabolite allopregnanolone (3alpha-OH-5alpha-pregnane-20-one) inhibits neural functions, enhancing the GABA induced GABA(A) receptor activation. This effect is benzodiazepine like and benzodiazepines are known to impair memory. Acute effects of allopregnanolone on the hippocampus dependent spatial learning in the Morris water maze have not been studied. Adult male Wistar rats where injected (i.v.) with allopregnanolone (2 mg/kg), or vehicle, daily for 11 days. At 8 or 20 min after each injection, studies of place navigation were performed in the Morris water maze. Allopregnanolone concentrations in plasma and in nine different brain areas where analyzed by radioimmunoassay. The latency to find the platform was increased 8 min after the allopregnanolone injection, while normal learning was seen after 20 min. Swim speed did not differ between groups. A higher number of rats were swimming close to the pool wall (thigmotaxis) in the 8 min allopregnanolone group compared to the other groups. Allopregnanolone concentrations in the brain tissue at 8 min were 1.5 to 2.5 times higher then at 20 min after the allopregnanolone injections. After vehicle injections the brain concentrations of allopregnanolone were at control levels. Plasma concentrations of allopregnanolone followed the same pattern as in the brain, with the exception of an increase 8 min after vehicle injections. The natural progesterone metabolite allopregnanolone can inhibit learning in the Morris water maze, an effect not caused by motor impairment. The learning impairment might be due to a combination of changed swimming behavior and difficulties in navigation.

Anesthetics↗

PubMed2000

The effects of blur and size on monocular and stereoscopic localization.

Monocular localization of non-abutting stimuli and stereoscopic localization of the same second-order targets are performed with the same precision (Wilcox, L.M. & Hess, R.F. (1996) Is the site of non-linear filtering in stereopsis before or after binocular combination? Vision Research, 36, 391-399). Further, both tasks show a similar dependence on the scale of the stimulus. Since prior studies used Gaussian-enveloped stimuli, modifications of stimulus scale produced concurrent changes in edge blur. The experiments reported here assess the relative contributions of size and blur to the observed dependence on envelope scale for both monocular localization and stereoacuity. Stereoacuity for first-order targets was found to be an order of magnitude better than stereoacuity for second-order targets and monocular acuity for both first- and second-order targets. Further, while first-order stereopsis was found to depend solely on blur, second-order stereoacuity and monocular acuity were affected by both size and blur. These results suggest that while stereoacuity for first-order stimuli may be determined by a correlative process limited by early additive noise, stereoacuity for second-order stimuli and monocular acuity for non-abutting targets are more likely limited by stimulus-dependent spatial subsampling.

Depth Perception↗

PubMed1997

A comparison of saccadic and blink suppression in normal observers.

Recent research suggests that blink and saccadic suppression are produced by the same mechanism (Volkmann, 1986; Uchikawa & Sato, 1995; Ridder & Tomlinson, 1993, 1995). These studies demonstrated that blink and saccadic suppression have the same effect on various visual functions. However, none of these studies made a comparison of blink and saccadic suppression in the same individual. The purpose of this study was to compare the effects of blink and saccadic suppression on contrast sensitivity functions in the same subject. The effect of saccadic suppression on the contrast sensitivity function in three normal observers was determined. Employing a two-alternative, forced-choice technique, thresholds were measured for seven spatial frequencies. At each spatial frequency, the threshold was determined immediately following detection of a voluntary saccade. The magnitude of suppression was taken as the log ratio of the contrast sensitivities obtained while foveating the stimulus and those obtained during saccades. The magnitude of saccadic suppression was found to increase as the saccade amplitude increased and to be spatial-frequency dependent. Low spatial frequencies were suppressed more than high spatial frequencies. The blink suppression data have been measured previously (Ridder & Tomlinson, 1993). Saccadic and blink suppression were qualitatively similar. A vertical shift of the data brought the saccadic and blink suppression data into register. These results suggest that blink and saccadic suppression are produced by the same or similar mechanisms.

Blinking↗

PubMed2001

Inducible and reversible enhancement of learning, memory, and long-term potentiation by genetic inhibition of calcineurin.

The threshold for hippocampal-dependent synaptic plasticity and memory storage is thought to be determined by the balance between protein phosphorylation and dephosphorylation mediated by the kinase PKA and the phosphatase calcineurin. To establish whether endogenous calcineurin acts as an inhibitory constraint in this balance, we examined the effect of genetically inhibiting calcineurin on plasticity and memory. Using the doxycycline-dependent rtTA system to express a calcineurin inhibitor reversibly in the mouse brain, we find that the transient reduction of calcineurin activity facilitates LTP in vitro and in vivo. This facilitation is PKA dependent and persists over several days in vivo. It is accompanied by enhanced learning and strengthened short- and long-term memory in several hippocampal-dependent spatial and nonspatial tasks. The LTP and memory improvements are reversed fully by suppression of transgene expression. These results demonstrate that endogenous calcineurin constrains LTP and memory.

Animals↗

PubMed2001

Uncertainty analysis of the external gamma-dose rate due to the variability of the vertical distribution of 137Cs in the soil.

The external gamma-dose rate at 1 m height above a flat area due to the presence of fallout radiocesium in the soil is frequently calculated from the observed depth profile of the 137Cs activity as well as the soil mass per unit area. At a given site, these depth profiles may, however, vary considerably, thus introducing an uncertainty to the external gamma-dose calculated in this way. To assess this source of uncertainty for a typical grassland site, the activity of Chernobyl-derived 137Cs and the wet bulk density in the three upper soil layers at 100 plots in a 100 m x 100 m pasture were determined. Analysis of these data shows that the frequency distribution of the dose rates calculated from the corresponding depth profiles of all plots is similar to a log-normal distribution (mean 25 nGy h-1, median 22 nGy h-1, standard deviation 11 nGy h-1; range 1.6-56 nGy h-1). The various sources which contribute to the uncertainty of the dose rate are quantified. The semi-variogram indicates that any spatial dependence of the dose rates occurs on this pasture only over distances that are smaller than the shortest sampling interval (here about 10 m). It is estimated which errors have to be expected for the median dose rate when the depth profiles of 137Cs and of the wet bulk density are determined only for a small number of plots. It is preferable to calculate the mean dose rate as a mean from the n individual dose rates rather than from an averaged 137Cs depth profile of the n plots.

Agriculture↗