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Results for “spatial dependency”

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PubMed1988

On estimating the number density of random scatterers from backscattered acoustic signals.

When an acoustic pulse interacts with an inhomogeneous, attenuating medium, the backscattered signals exhibit random fluctuations which are correlated with the physical properties of the medium. This paper proposes a robust model for characterizing the statistical nature of these backscattered signals. This model takes into account frequency-dependent attenuation, spatially varying media statistics, arbitrary beam geometries, and arbitrary pulse shapes. Based on this model, statistical estimation schemes are proposed for estimating both the attenuation coefficient and scatterer number density of the medium. Using appropriate simplifying assumptions, it is shown that this model is consistent with attenuation estimation algorithms currently used for ultrasonic tissue characterization. A statistical approach for estimating the number density of scatterers is described and its theoretical performance is evaluated. The algorithm for estimating the scatterer number density incorporates measurements of both the statistical moments of the backscattered signals and the point spread function of the acoustic system. The number density algorithm has been applied to simulated waveforms, waveforms obtained from ultrasonic phantoms with known number densities, and in vitro mammalian tissues. There is an excellent agreement between theoretical, simulation, and experimental results. The application of this technique to ultrasonic tissue characterization is also discussed.

Algorithms↗

PubMed1993

Monkey hippocampal neuron responses related to spatial and non-spatial influence.

Activity in the monkey hippocampal formation (HF) was analyzed during movement guided by auditory and visual cues. The monkey could control a motorized cab by pressing the proper 1 of 4 available levers within a 2.5 x 2.5 m field. Of 238 HF neurons analyzed, responses of 79 were place related and responses of 110 were task related. Of the task-related neurons, 33 were also place related, and responded to task cues only where place-related responses occurred. The results suggest HF gating of incoming non-spatial information, depending on information about location.

Acoustic Stimulation↗

PubMed1996

Role of nitric oxide and cyclic GMP in the dizocilpine-induced impairment of spontaneous alternation behavior in mice.

The activation of N-methyl-D-aspartate receptors induces the synthesis of nitric oxide, which activates soluble guanylate cyclase and leads to the formation of cyclic GMP in the brain. The inhibition of nitric oxide production, as well as the blockade of N-methyl-D-aspartate receptors, has been reported to prevent the induction of hippocampal long-term potentiation and learning and memory formation in vivo, although the effects of inhibitors of nitric oxide synthase are still controversial. We investigated the putative role of nitric oxide and cyclic GMP in dizocilpine-induced memory impairment in mice. The nitric oxide synthase inhibitors, NG-nitro-L-arginine methyl ester and 7-nitro indazole, as well as dizocilpine, a non-competitive N-methyl-D-aspartate receptor antagonist, dose-dependently impaired spatial working memory in mice, assessed by their spontaneous alternation behavior in a Y-maze. The inhibitory effects of both NG-nitro-L-arginine methyl ester and dizocilpine on their behavior were completely reversed by 8-bromo-cyclic GMP. Cyclic GMP levels in the cerebellum were reduced by treatment with dizocilpine. NG-Nitro-L-arginine methyl ester and 7-nitro indazole reduced cyclic GMP levels in the cerebral cortex/hippocampus and cerebellum, and the suppressive effect of NG-nitro-L-arginine methyl ester on cyclic GMP levels in the cerebral cortex/hippocampus was reversed by co-treatment with L-arginine. Cyclic AMP levels in the brain were not affected by treatment with either dizocilpine, NG-nitro-L-arginine methyl ester, or 7-nitro indazole. Neither NG-nitro-L-arginine methyl ester nor L-arginine had any effect on monoamine and acetylcholine metabolism in the brain. These results suggest that the reduction in nitric oxide/cyclic GMP production in the brain may be responsible for dizocilpine-induced impairment of spontaneous alternation behavior in a Y-maze.

Animals↗

PubMed1992

Quantitative NMR imaging of multiphase flow in porous media.

Quantitative description of multiphase flow in porous media and local saturation distributions at steady states are of fundamental importance for petroleum recovery. The use of MRI provides an unprecedented means for obtaining such information. In this paper, profile imaging techniques for quantitative evaluation of fluid saturations during flow experiments in porous media are developed. The procedures for overcoming problems arising from very short, fluid saturation-dependent and spatial variation of T2, which are common in porous media, were addressed. The general methods developed should also be applicable to similar inhomogeneous biological systems. Experimental NMR imaging measurements of two-phase displacement were conducted in several limestones and sandstones representing various different types of pore structures, including a macroscopically homogeneous structure, a laminated structure, and a sample that exhibits porosity at different scales. The advantages of using each different type of profile imaging sequence to investigate flow in different types of porous structures are demonstrated. Images showing many features of multiphase flow, including nonuniform flow through different bedding structures, are obtained during the flow experiments. The use of profile images for obtaining many important petrophysical properties, such as permeability, porosity, saturation, and pore structural information, is discussed.

Geological Phenomena↗

PubMed1996

The design and synthesis of substituted biphenyl libraries.

A novel scaffold system for the generation of diversity libraries has been designed which allows for rapid modification not only of functional groups, but their spatial arrangements as well. The biphenyl scaffold allows for display of three or four diverse functional groups in a wide variety of spatial arrangements depending on the substitution pattern selected. The libraries are generated by a combination of solution and solid-phase chemistries and are cleaved off the solid-support for screening.

Biphenyl Compounds↗

PubMed2005

Theoretical considerations for evaluating the degree of random-periodicity of radiographic noise.

Radiographic noise properties have been evaluated using the Wiener spectrum. However, this approach is not appropriate for periodic noise for two reasons. One is that it takes infinite values at the spatial frequencies of periodic noise. The other is that when adopting a numerical integration, it allows unstable values at each spatial frequency, depending on the integral region. Introducing three types of spectra (W(1), W(2), and W(3)) in connection with the Wiener spectrum, we propose a practical approach to evaluation of periodic noise. Radiographic images sometimes contain noise that is not totally random and not perfectly periodic. Therefore, using the Wiener spectrum and the W(1) spectrum, we also propose two factors for evaluation of the degree of random-periodicity of noise containing periodic signals.

Journal Article↗

PubMed2006

Imaging and speciation of trace elements in biological environment.

Mineral elements, often at the trace level, play a considerable role in physiology and pathology of biological systems. Metallogenomics, metalloproteomics, and metallomics are among the emerging disciplines which are critically dependent on spatially resolved concentration maps of trace elements in a cell or tissue, on information on chemical speciation, and on that on metal-binding coordination sites. The mini-review discusses recent progress in analytical techniques for element profiling on the genome scale, biological trace element imaging, and probing, identification and quantification of chemical species in the biological environment. Imaging of the element distribution in cells and tissue sections is becoming possible with sub-micrometer spatial resolution and picogram-level sensitivity owing to advances in laser ablation MS, ion beam and synchrotron radiation X-ray fluorescence microprobes. Progress in nanoflow chromatography and capillary electrophoresis coupled with element specific ICP MS and molecule-specific electrospray MS/MS and MALDI enables speciation of elements in microsamples in a complex biological environment. Laser ablation ICP MS, micro-SXRF, and micro-PIXE allow mapping of trace element distribution in 1D and 2D proteomics gels. The increasing sensitivity of EXAFS and XANES owing to the use of more intense synchrotron beams and efficient focusing optics provide information about oxidation state, fingerprint speciation of metal sites and metal-site structures.

Environmental Pollution↗

PubMed2006

Rater-dependent accuracy in predicting the spatial location of functional centers on anatomical MR images.

OBJECTIVES: The determination of eloquent cortex is essential when planning neurosurgical approaches to brain lesions. This study examined the abilities of medical personnel of various backgrounds to predict the location of functional cortex using anatomical information provided by MR imaging. PATIENTS AND METHODS: Neurosurgeons, neuroscientists, neuroradiologists, medical students and MR technologists viewed anatomical MR images acquired from patients with brain tumors and healthy controls. These five groups of raters were then asked to locate the primary motor hand, supplementary motor and primary auditory areas and their predictions were compared to fMRI data acquired from the same subjects. RESULTS: The overall mean distance from the center of the fMRI activation was 2.38 cm. The neuroscientists performed the best and MR technologists performed the worst (mean distance from center of 1.83 and 3.04 cm, respectively, p<0.05). The difference between patients and controls was not significant. The mean distance by ROI was primary motor hand 2.03 cm, auditory area 2.06 cm and supplementary motor area 3.18 cm (p<0.05). Raters also performed best in the medial-lateral direction, compared to superior-inferior and anterior-posterior directions (mean distances from center 0.42, 1.04 and 1.81 cm, respectively). Finally, the approximate minimum fields of view necessary to capture the entire fMRI activations using the raters' predictions ranged from 5 to 15 cm, or 3 to 12 cm larger than the fMRI activations. CONCLUSION: Medical personnel of various training perform poorly when using only anatomical information to predict the location of functional areas of cortex.

Adult↗

PubMed2006

Invasion of pests resistant to Bt toxins can lead to inherent non-uniqueness in genetically modified Bt-plant dynamics: mathematical modeling.

Genetically modified crops are effective pest management tools for worldwide growers. However, there is a concern that pests may develop resistance to Bt-toxins produced by genetically modified Bt-plants. We study the impact of the Bt-resistant pests on Bt-crops. Furthermore, the dynamics of the Bt-plant-Bt-susceptible insects-Bt-resistant insects system is analysed and it is shown that throughout the insect reproduction period the plant biomass dynamics resulting from invasion of Bt-resistant insects is non-unique. Namely, the chaotic attractor and the limit cycle, which are responsible for the plant and insect biomass dynamics, are shown to coexist. As a result, the Bt-plant-Bt-resistant insect system can manifest either chaotic or regular oscillations of plant and insect biomass depending on spatial patterns resulting from invasion of Bt resistant insects into the Bt plant-Bt susceptible insect system. We show that the non-uniqueness of the system dynamics under unfavorable environmental conditions, such as in the so-called zones of risky agriculture in many developing countries and industrialized countries, can lead to essential decrease in the plant biomass.

Animals↗

PubMed2005

Tactile functions after cerebral hemispherectomy.

Patients that were hemispherectomized due to brain lesions early in life sometimes have remarkably well-preserved tactile functions on their paretic body half. This has been attributed to developmental neuroplasticity. However, the tactile examinations generally have been fairly crude, and subtle deficits may not have been revealed. We investigated monofilament detection and three types of tactile directional sensibility in four hemispherectomized patients and six healthy controls. Patients were examined bilaterally on the face, forearm and lower leg. Normal subjects were examined unilaterally. Following each test of directional sensibility, subjects were asked to rate the intensity of the stimulation. On the nonparetic side, results were almost always in the normal range. On the paretic side, the patients' capacity for monofilament detection was less impaired than their directional sensibility. Despite the disturbed directional sensibility on their paretic side the patients rated tactile sensations evoked by the stimuli, on both their paretic and nonparetic body halves, as more intense than normals. Thus, mechanisms of plasticity seem adequate for tactile detection and intensity coding but not for more complex tactile functions such as directional sensibility. The reason for the high vulnerability of tactile directional sensibility may be that it depends on spatially and temporally precise afferent information processed in a distributed cortical network.

Adult↗

PubMed2005

Complementary memory systems: competition, cooperation and compensation.

Spatial navigation depends on dissociable memory systems that have distinct neural bases and employ different forms of representation. One system gradually acquires reliable sequences of responses to given situations (e.g. repeatedly following a fixed route), and depends on the striatum. The other develops flexible representations permitting novel responses (e.g. finding new shortcuts), and depends on the hippocampus. Voermans and colleagues explore the interaction between these two systems using functional neuroimaging and behavioural measures in a clinical population.

Animals↗

PubMed2006

Air-coupled ultrasonic investigation of multi-layered composite materials.

Air-coupled ultrasonics is fine alternative for the immersion testing technique. Usually a through transmission and a pitch-catch arrangement of ultrasonic transducers are used. The pitch-catch arrangement is very attractive for non-destructive testing and evaluation of materials, because it allows one-side access to the object. However, this technique has several disadvantages. It is sensitive to specularly reflected and edge waves. A spatial resolution depends on a distance between the transducers. A new method for detection and visualisation of inhomogeneities in composite materials using one-side access air-coupled ultrasonic measurement technique is described. Numerical predictions of Lamb wave interaction with a defect in a composite material are carried out and the interaction mechanism is explained. Experimental measurements are carried out with different arrangements of the transducers. The proposed method enables detect delamination and impact type defects in honeycomb materials.

Journal Article↗

PubMed2003

The effects of pitch tilt on postrotatory nystagmus in cats.

When the head is tilted away from the upright position immediately after termination of earth-vertical axis (EVA) head rotation at a constant velocity, the rotation axis of postrotatory nystagmus (PRN) is gradually directed toward gravity (spatial reorientation). During roll tilt, the extent of the axis shift varies among species. In cats, the reorientation is limited to about 30-45 degrees of head tilt. In the present study, we examined features of PRN during pitch tilt in cats. The animals were rotated about EVA at a constant velocity of 100 degrees /s for 120 s. Within 2 s after stopping EVA rotation, the animals were tilted toward the pitch plane by as much as +/-90 degrees (nose-down or nose-up) in steps of 15 degrees. Eye movements were measured with 3D magnetic search coils. The angle of the PRN plane and its slow phase eye velocity (SPV) were measured. The duration of PRN decreased as pitch tilt increased regardless of whether the direction was nose-up or nose-down. The mean time constant of SPV was significantly longer for nose-up than for nose-down. PRN maintained its horizontal orientation without any vertical drift during pitch tilt, but there was little reorientation toward gravity. That is, no significant torsional component of SPV emerged to reorient the eyes according to gravity. Our results indicate that spatial reorientation depends on head orientation. For the PRN responses after pitch tilts in cats, the interactions of semicircular canal and otolith inputs in the central vestibular system might be vestigial.

Animals↗

PubMed1985

Frequency content of the electrocardiogram. Spatial features and effects of myocardial infarction.

The frequency content of electrocardiographic signals has been used to define amplifier characteristics and, more recently, to detect myocardial pathology. It was the purpose of this study to evaluate the differences in frequency content of ECG signals recorded from different volume conductor surface sites and to determine the effects of coronary ligation on these spatial features. To do so, 15 isolated perfused rabbit hearts, enclosed within a precisely machined sphere, were studied; ten were examined prior to and 15 minutes after ligation of the anterior descending coronary artery. ECG signals, recorded from 32 surface electrodes, were subjected to Fourier analysis. Prior to occlusion, the frequency content of the different electrode signals varied widely, with a greater variance between electrodes than between hearts (p less than 0.01). This variation was related to a reproducible spatial effect, with records over the basal left ventricle having higher frequency components than waves from other sites. After litigation, ECG records from electrodes over electrodes over ischemic areas showed augmented R-wave amplitude and ST-segment elevation. This was associated with a spatial shift of the control high frequency peak to a site overlying the ischemic lesion. Thus the frequency content of ECG is spatially variable and hence lead-dependent, and this spatial variability is directly affected by acute myocardial infarction.

Animals↗

PubMedDate not supplied

Effects of the monoamine oxidase A inhibitor moclobemide on hippocampal plasticity in GR-impaired transgenic mice.

A reduction in glucocorticoid receptor (GR) function leads to hippocampus-dependent allocentric spatial learning deficits, altered novelty exploration and disrupted hippocampal long-term potentiation (LTP) in transgenic mice expressing a GR antisense construct. After continuous long-term treatment of these mice with moclobemide (a reversible inhibitor of monoamine oxidase A), spatial navigation performance but not accuracy improved during initial acquisition. These changes were associated with a shift of the threshold for the induction of hippocampal LTP at low stimulation frequencies. Moreover, novel object exploration increased in both control and transgenic animals following long-term treatment with moclobemide. These findings open the possibility that antidepressants might improve hippocampal function under conditions of impaired stress hormone regulation, and that these drugs might in part act through this mechanism to attenuate cognitive deficiency in disorders such as depression.

Animals↗

PubMed2000

Comparison of thresholds for high-speed drifting vernier and a matched temporal phase-discrimination task.

For rapidly translating targets, vernier thresholds correspond to millisecond asynchronies between targets. The 'temporal hypothesis' is that these thresholds reflect the limiting sensitivity of asynchrony detectors. Previous studies showed that temporal thresholds are generally higher than vernier thresholds, but failed to reject the 'temporal hypothesis' because stimuli had differing spatiotemporal characteristics, and temporal thresholds depend strongly on stimulus and task. Here we use matched grating stimuli to test - and reject - the temporal hypothesis. Expressed as asynchrony, temporal phase discrimination was typically 10-fold poorer than vernier thresholds, and differed in dependence on spatial frequency, temporal frequency, contrast, and susceptibility to stroboscopic masks.

Adult↗

PubMed2000

A contrast paradox in stereopsis, motion detection, and vernier acuity.

Stereoacuity improves with increasing contrast, unless the increase is monocular. In this case performance paradoxically suffers. This study examined whether this contrast paradox occurs for two other classes of visual judgment: two-frame motion and vernier acuity. We constructed three homologous tasks in which the two components of a gabor stimulus (stereo half-images, motion frames, vernier components) were either both high contrast, both low contrast, or mismatched. The contrast paradox was evident in all three tasks and showed a similar spatial frequency dependence. We suggest the contrast paradox results from the combination of mismatched signals by a single filter.

Contrast Sensitivity↗

PubMed2002

Orientation sensitivity of ganglion cells in primate retina.

The two-dimensional shape of the receptive field center of macaque retinal ganglion cells was determined by measuring responses to drifting sinusoidal gratings of different spatial frequency and orientation. The responses of most cells to high spatial frequencies depended on grating orientation, indicating that their centers were not circularly symmetric. In general, center shape was well described by an ellipse. The major axis of the ellipse tended to point towards the fovea or perpendicular to this. Parvocellular pathway cells had greater center ellipticity than magnocellular pathway cells; the median ratio of the major-to-minor axis was 1.72 and 1.38, respectively. Parvocellular pathway cells also had centers that were often bimodal in shape, suggesting that they received patchy cone/bipolar cell input. We conclude that most ganglion cells in primate retina have elongated receptive field centers and thus show orientation sensitivity.

Animals↗