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Spatial Properties (spatial + property)
Selected AbstractsSpatial Properties of QT and the T-Wave MorphologyANNALS OF NONINVASIVE ELECTROCARDIOLOGY, Issue 3 2000Kaspar Lund M.S.S.E. Objective: To describe the relation between the QT interval and the T-wave morphology. Material and methods: Frank orthogonal leads X, Y, Z of one subject and resting 12-lead ECG of 40 subjects. QT was measured by the tangent method. The QT values are organized according to the anatomic orientation of the leads: I, -aVR, II, aVF, III, -aVL, -I, aVR, -II, -aVF, -III, aVL. and: V1, V2, V3, V4, V5, V6, -V1 -V2, -V3, -V4, -V5, -V6. The T-wave amplitudes and QT were categorized according to QT into four groups with increasing mean QT. Results: Kruskal-Wallis nonparametric test showed that the shortest and longest QT values are measured on the T wave with the smallest amplitudes (P < 0.001). Inspection of plots of QT and T waves reveals that the shortest and longest QT values are usually measured in leads with a small difference in orientation (neighbor leads). The mechanism behind these characteristics is mainly that the shortest and longest QT values are measured on T waves that are close to a lead orientation, whereas the T waves are flat or biphasic. We also observed an almost significant (P = 0.057) decrease in the T-wave amplitude with increasing dispersion. Conclusion: The relation between T-wave morphology and QT in the same cardiac plane is highly organized. The shortest and longest QT values are measured on the T wave with the smallest amplitudes (P < 0.001). [source] Learning associations between places and visual cues without learning to navigate: Neither fornix nor entorhinal cortex is requiredHIPPOCAMPUS, Issue 4 2003E.A. Gaffan Abstract Rats with fornix transection, or with cytotoxic retrohippocampal lesions that removed entorhinal cortex plus ventral subiculum, performed a task that permits incidental learning about either allocentric (Allo) or egocentric (Ego) spatial cues without the need to navigate by them. Rats learned eight visual discriminations among computer-displayed scenes in a Y-maze, using the constant-negative paradigm. Every discrimination problem included two familiar scenes (constants) and many less familiar scenes (variables). On each trial, the rats chose between a constant and a variable scene, with the choice of the variable rewarded. In six problems, the two constant scenes had correlated spatial properties, either Allo (each constant appeared always in the same maze arm) or Ego (each constant always appeared in a fixed direction from the start arm) or both (Allo + Ego). In two No-Cue (NC) problems, the two constants appeared in randomly determined arms and directions. Intact rats learn problems with an added Allo or Ego cue faster than NC problems; this facilitation provides indirect evidence that they learn the associations between scenes and spatial cues, even though that is not required for problem solution. Fornix and retrohippocampal-lesioned groups learned NC problems at a similar rate to sham-operated controls and showed as much facilitation of learning by added spatial cues as did the controls; therefore, both lesion groups must have encoded the spatial cues and have incidentally learned their associations with particular constant scenes. Similar facilitation was seen in subgroups that had short or long prior experience with the apparatus and task. Therefore, neither major hippocampal input,output system is crucial for learning about allocentric or egocentric cues in this paradigm, which does not require rats to control their choices or navigation directly by spatial cues. Hippocampus 2003;13:445,460. © 2003 Wiley-Liss, Inc. [source] Steady-state and transient thermal lens behavior in the dual beam Z-scan arrangementLASER PHYSICS LETTERS, Issue 6 2006D. Lorenc Abstract Output beam spatial properties of a dual wavelength CW DPSS laser have been studied using the thermal lens (photothermal) effect. In employing the Z-scan technique both the steady-state and time-dependent thermal lens signal as a function of sample position could be examined simultaneously. The influence of sample thickness and laser output power upon obtained spatial properties of incident beam is studied. It is shown, that both the steady state and the transient photothermal signal provide comparable results. (© 2006 by Astro, Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source] Into the twilight zone: the complexities of mesopic vision and luminous efficiencyOPHTHALMIC AND PHYSIOLOGICAL OPTICS, Issue 3 2006Andrew Stockman Abstract Of all the functions that define visual performance, the mesopic luminous efficiency function is probably the most complex and hardest to standardise or model. Complexities arise because of the substantial and often rapid visual changes that accompany the transition from scotopic to photopic vision. These are caused not only by the switch from rod to cone photoreceptors, but also by switches between different post-receptoral pathways through which the rod and cone signals are transmitted. In this review, we list several of the complexities of mesopic vision, such as rod,cone interactions, rod saturation, mixed photoreceptor spectral sensitivities, different rod and cone retinal distributions, and the changes in the spatial properties of the visual system as it changes from rod- to cone-mediated. Our main focus, however, is the enormous and often neglected temporal changes that occur in the mesopic range and their effect on luminous efficiency. Even before the transition from rod to cone vision is complete, a transition occurs within the rod system itself from a sluggish, sensitive post-receptoral pathway to a faster, less sensitive pathway. As a consequence of these complexities, any measure of mesopic performance will depend not only on the illumination level, but also on the spectral content of the stimuli used to probe performance, their retinal location, their spatial frequency content, and their temporal frequency content. All these should be considered when attempting to derive (or to apply) a luminous efficiency function for mesopic vision. [source] Chromatic and spatial properties of parvocellular cells in the lateral geniculate nucleus of the marmoset (Callithrix jacchus)THE JOURNAL OF PHYSIOLOGY, Issue 1 2004Esther M. Blessing The parvocellular (PC) division of the afferent visual pathway is considered to carry neuronal signals which underlie the red,green dimension of colour vision as well as high-resolution spatial vision. In order to understand the origin of these signals, and the way in which they are combined, the responses of PC cells in dichromatic (,red,green colour-blind') and trichromatic marmosets were compared. Visual stimuli included coloured and achromatic gratings, and spatially uniform red and green lights presented at varying temporal phases and frequencies. The sensitivity of PC cells to red,green chromatic modulation was found to depend primarily on the spectral separation between the medium- and long-wavelength-sensitive cone pigments (20 or 7 nm) in the two trichromatic marmoset phenotypes studied. The temporal frequency dependence of chromatic sensitivity was consistent with centre,surround interactions. Some evidence for chromatic selectivity was seen in peripheral PC cells. The receptive field dimensions of parvocellular cells were similar in dichromatic and trichromatic animals, but the achromatic contrast sensitivity of cells was slightly higher (by about 30%) in dichromats than in trichromats. These data support the hypothesis that the primary role of the PC is to transmit high-acuity spatial signals, with red,green opponent signals appearing as an additional response dimension in trichromatic animals. [source] The dynamics of NAO teleconnection pattern growth and decayTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 589 2003Steven B. Feldstein Abstract This investigation performs both diagnostic analyses with NCEP/NCAR re-analysis data and forced, barotropic model calculations to examine the dynamical mechanisms associated with the growth and decay of the North Atlantic Oscillation (NAO) teleconnection pattern. The diagnostic calculations include projection and composite analyses of each term in the stream-function-tendency equation. The results of the analyses reveal a complete life cycle of growth and decay within approximately two weeks. The positive NAO phase is found to develop after anomalous wavetrain propagation across the North Pacific to the east coast of North America. This contrasts with the negative NAO phase which appeared to develop in situ. Both high-frequency (period <10 days) and low-frequency (period >10 days) transient eddy fluxes drive the NAO growth. After the NAO anomaly attains its maximum amplitude, the high-frequency transient eddy fluxes continue to drive the NAO anomaly in a manner that is consistent with a positive feedback process. The decay of the NAO occurs through both the divergence term and the low-frequency transient eddy fluxes. The temporal and spatial properties of the divergence term are found to be consistent with Ekman pumping. These results illustrate many important differences between the NAO and Pacific/North American (PNA) teleconnection patterns, perhaps most striking being that the NAO life cycle is dominated by nonlinear processes, whereas the PNA evolution is primarily linear, In addition, the relation between the NAO and the zonal index is discussed. Copyright © 2003 Royal Meteorological Society [source] Dynamic ecosystem models and the evaluation of ecosystem effects of fishing: can we make meaningful predictions?AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS, Issue 1 2003L.A. Robinson Abstract 1.Fishing is one of the most widespread anthropogenic impacts on marine ecosystems. In recent times, the development of measurable metrics of the resultant ecosystem effects, has become an important aspect of fisheries management. Ecosystem models are often advocated as tools for the evaluation of system effects, but the extent to which models are able to make meaningful predictions, has not yet been fully addressed. 2.In order to assess the suitability of models, to evaluate ecosystem effects of fisheries, the direct and indirect effects were catalogued. 3.From the literature, 33 applications of marine ecosystem models were identified for analysis of their ability to fully assess these catalogued effects. Analysis was possible for only 24 of the models due to poor documentation of the other 9. 4.Each model was examined for their inclusion of nine functional groups, deemed essential for the assessment of impacts of fishing on the whole ecosystem (e.g. detritus, marine mammals). The models were also assessed for their inclusion of several additional factors, either fundamental in the regulation of marine ecosystems (e.g. environmental forcing), or important in the classification of their role as a predictor of changes in ecological processes (e.g. simulation, spatial properties). 5.No model formulation provided coverage in all the areas necessary to cover the identified effects of fisheries. Eight models provided good coverage, nutrient dynamics and benthos were the least well represented aspects of the ecosystem. 6.The ECOPATH with Ecosim family of models, the European Regional Seas Ecosystem Model (ERSEM) and the Anderson & Ursin multispecies extension to the Beverton & Holt model all seem likely to yield good insights. 7.In further developing these models, however, consideration must be given to explicitly incorporate spatial factors and extrinsic forcing functions, such as climate. Copyright © 2002 John Wiley & Sons, Ltd. [source] Cluster Detection Based on Spatial Associations and Iterated Residuals in Generalized Linear Mixed ModelsBIOMETRICS, Issue 2 2009Tonglin Zhang Summary Spatial clustering is commonly modeled by a Bayesian method under the framework of generalized linear mixed effect models (GLMMs). Spatial clusters are commonly detected by a frequentist method through hypothesis testing. In this article, we provide a frequentist method for assessing spatial properties of GLMMs. We propose a strategy that detects spatial clusters through parameter estimates of spatial associations, and assesses spatial aspects of model improvement through iterated residuals. Simulations and a case study show that the proposed method is able to consistently and efficiently detect the locations and magnitudes of spatial clusters. [source] Mixing in time and space for lattice spin systems: A combinatorial viewRANDOM STRUCTURES AND ALGORITHMS, Issue 4 2004Martin Dyer The paper considers spin systems on the d -dimensional integer lattice ,d with nearest-neighbor interactions. A sharp equivalence is proved between decay with distance of spin correlations (a spatial property of the equilibrium state) and rapid mixing of the Glauber dynamics (a temporal property of a Markov chain Monte Carlo algorithm). Specifically, we show that if the mixing time of the Glauber dynamics is O(n log n) then spin correlations decay exponentially fast with distance. We also prove the converse implication for monotone systems, and for general systems we prove that exponential decay of correlations implies O(n log n) mixing time of a dynamics that updates sufficiently large blocks (rather than single sites). While the above equivalence was already known to hold in various forms, we give proofs that are purely combinatorial and avoid the functional analysis machinery employed in previous proofs. © 2004 Wiley Periodicals, Inc. Random Struct. Alg., 2004 [source] | ||||||||||||||||