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21.
Hebb's principal theoretical propositions, the cell assembly and the nature of synaptic change, were generated at a time when the focus of work in behavioural neuroscience was directed at understanding issues such as the principles governing the behaviour of animals in neuropsychological studies of learning and memory and the role of drives in the control of behaviours like sex and feeding and drinking. It was not until attention shifted to understanding the neural underpinnings of learning and memory that Hebb's propositions had an impact on behavioural neuroscience as they provided a simple, and testable, mechanism for synaptic plasticity observed both in learning and in other forms of experience-dependent neural change. But much of the field remains interested in other issues such as sensation and perception, motivation, attention, and so on, and to date, Hebb's propositions have had little impact. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
22.
Development and implementation of a NURBS curve motion interpolator   总被引:4,自引:0,他引:4  
This paper deals with the issues of development and implementation of a real-time NURBS interpolator for a six-axis robot. Using an open-architecture controller system as a testbed, a real-time NURBS curve interpolator was developed, implemented and tested. Sample runs were conducted with the resulting trajectories measured in real-time during robot motion. The resulting trajectories are analyzed, discussed and compared with those from a commonly used point-to-point approximation technique. The real-time NURBS curve interpolator's feasibility, advantages and related issues are also discussed.  相似文献   
23.
The potential associated with quantum well (QW) structures is usually assumed to be a step function; this implies a compositional abruptness at each interface. But abrupt interfaces do not occur in practice, especially if one of the atoms segregates during growth. This leads to asymmetries in the QW potentials which could radically affect device performance.  相似文献   
24.
This study examines the presence of dental caries in a large sample of adult skeletons from the 19th century cemetery of St. Thomas' Anglican Church in Belleville, Ontario. The cemetery was used from 1821 to 1874. Caries prevalence and frequencies of diseased and missing teeth were calculated both by observing summary statistics of individual rates and by the total sample of teeth. Postmortem tooth loss is low in this sample and antemortem tooth loss is highest in first mandibular molars, all other molars and then premolars. Age at death, but not sex, was found to be significantly related to the overall Caries Rate while both age and sex were significantly associated with the Diseased-Missing Index. The increase in diseased and missing teeth in older individuals is expected while the sex difference is not explained by simple dietary factors. When compared to reports on British and American samples, caries and antemortem tooth loss in the St. Thomas' sample is most similar to a pre-1850 British group and higher than American samples. Although there is undoubtedly a complex of factors contributing to caries prevalence in this sample, more data are required from large historic samples, particularly from the American northeast and late 19th century Britain, to have a clearer understanding of the influence of diet, cultural, and environmental factors affecting caries rates in historic populations.  相似文献   
25.
This paper explores current applications for handheld devices and questions which of these make full use of the unique attributes of handheld devices in order to facilitate learning in a pedagogically sensible manner. In order to do so, the paper presents a functional framework which analyses handheld application in relation to their use as well as the pedagogical underpinning, if any, that informs their development. Our framework currently consists of seven distinct categories of application, which we term: administrative, referential, interactive, microworld, data collection, location aware and collaborative. We argue that three categories, namely data collection, location aware and collaborative, are particularly suited to learning with handheld devices when they are informed by collaborative, contextual and constructionist learning theories. Furthermore, we contend that applications of the type just outlined deserve further research since they are not attempting to replicate, or even augment, existing learning scenarios but rather they try to create new learning opportunities which would not be possible without (mobile) technology. Finally, the authors introduce a number of applications and learning scenarios that they have developed for handheld devices and explain their position within the framework.  相似文献   
26.
In order to obtain a bonded wood product from the tree, wood must be subjected to a series of processes. These include, among others, vital processes like drying of the green wood and machining of the surfaces of the dried wood in preparation for bonding. However, when wood is machined with dull blades, thermal degradation of the wood frequently occurs. The effects of kiln drying and thermal degradation of wood on the performance of the adhesive joints subsequently formed from such wood were investigated. Tapered double cantilever beam (TDCB) test specimens obtained from hard maple and phenol–resorcinol–formaldehyde (PRF) adhesives were used in the investigations. The enhancement of fracture energy due to the reduction in the moisture content of wood, the improved performance of adhesive joints due to rejointing of wood surfaces before bonding, and the reduction of adhesive joint strength resulting from thermal degradation of wood are presented and discussed.  相似文献   
27.
28.
Proteins denature at low pH because of intramolecular electrostatic repulsions. The addition of salt partially overcomes this repulsion for some proteins, yielding a collapsed conformation called the A-state. A-states have characteristics expected for the molten globule, a notional kinetic protein folding intermediate. Here we show that the addition of neutral sugars to solutions of acid-denatured equine ferricytochrome c induces formation of the A-state in the absence of added salt. We characterized the structure and stability of the sugar-induced A-state with circular dichroism spectropolarimetry (CD) and NMR-monitored hydrogen-deuterium exchange experiments. We also examined the stability of the sugar-induced A-state as a function of sugar size and concentration. The results are interpreted using several models and we conclude that the stabilizing effect is consistent with increased steric repulsion between the protein and the sugar solutions.  相似文献   
29.
Lactate dehydrogenases are of considerable interest as stereospecificcatalysts in the chemical preparation of enantiomerically pure-hydroxyacid synthons. For such applications in synthetic organicchemistry it would be desirable to have enzymes which tolerateelevated temperatures for prolonged reaction times, to increaseproductivity and to extend then applicability to poor substrates.Here, two examples are reported of significant thermostabilizations,induced by sitedirected mutagenesis, of an already thermostableprotein, the L-lactate dehydrogenase (EC 1.1.1.27 [EC] , 35 kDa permonomer subunit) from Bacillus stearothermophilus. Thermal inactivationof this enzyme is accompanied by irreversible unfolding of thenative protein structure. The replacement of Argl71 by Tyr stabilizesthe enzyme against thermal inactivation and unfolding. Thisstabilizing effect appears to be based on improved interactionsbetween the subunits in the core of the active dimeric or tetramericforms of the enzyme. The thermal stability of L-lactate dehydrogenasevariants with an active site Arg residue, either in the 171(wild-type) or in the 102 position, is further increased bysulfate ions. The two stabilizing effects are additive, as foundfor the Argl71Tyr/ Gln1O2Arg double mutant, for which the stabilityof the protein in 100 mM sulfate solution reaches that of L-lactatedehydrogenases from extreme thermophiles. All mutant proteinsretain significant catalytic activity, both in the presenceand absence of stnhilfoing salts, and are viable catalysts inpreparative scale reactions.  相似文献   
30.
Databases developed independently in a common open distributed environment may be heterogeneous with respect to both data schema and the embedded semantics. Managing schema and semantic heterogeneities brings considerable challenges to learning from distributed data and to support applications involving cooperation between different organisations. In this paper, we are concerned mainly with heterogeneous databases that hold aggregates on a set of attributes, which are often the result of materialised views of native large-scale distributed databases. A model-based clustering algorithm is proposed to construct a mixture model where each component corresponds to a cluster which is used to capture the contextual heterogeneity among databases from different populations. Schema heterogeneity, which can be recast as incomplete information, is handled within the clustering process using Expectation-Maximisation estimation and integration is carried out within a clustering iteration. Our proposed algorithm resolves the schema heterogeneity as part of the clustering process, thus avoiding transformation of the data into a unified schema. Results of algorithm evaluation on classification, scalability and reliability, using both real and synthetic data, demonstrate that our algorithm can achieve good performance by incorporating all of the information from available heterogeneous data. Our clustering approach has great potential for scalable knowledge discovery from semantically heterogeneous databases and for applications in an open distributed environment, such as the Semantic Web.  相似文献   
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