全文获取类型
收费全文 | 889篇 |
免费 | 24篇 |
国内免费 | 40篇 |
专业分类
953篇 |
出版年
2023年 | 11篇 |
2022年 | 7篇 |
2021年 | 8篇 |
2020年 | 6篇 |
2019年 | 18篇 |
2018年 | 7篇 |
2017年 | 12篇 |
2016年 | 15篇 |
2015年 | 13篇 |
2014年 | 27篇 |
2013年 | 40篇 |
2012年 | 22篇 |
2011年 | 51篇 |
2010年 | 46篇 |
2009年 | 45篇 |
2008年 | 44篇 |
2007年 | 41篇 |
2006年 | 54篇 |
2005年 | 28篇 |
2004年 | 34篇 |
2003年 | 27篇 |
2002年 | 10篇 |
2001年 | 12篇 |
2000年 | 15篇 |
1999年 | 31篇 |
1998年 | 20篇 |
1997年 | 23篇 |
1996年 | 14篇 |
1995年 | 18篇 |
1994年 | 21篇 |
1993年 | 10篇 |
1992年 | 17篇 |
1991年 | 17篇 |
1990年 | 24篇 |
1989年 | 13篇 |
1988年 | 12篇 |
1987年 | 16篇 |
1986年 | 18篇 |
1985年 | 6篇 |
1984年 | 19篇 |
1982年 | 13篇 |
1981年 | 7篇 |
1980年 | 8篇 |
1979年 | 14篇 |
1978年 | 8篇 |
1977年 | 7篇 |
1976年 | 2篇 |
1974年 | 3篇 |
1973年 | 7篇 |
1972年 | 4篇 |
排序方式: 共有953条查询结果,搜索用时 15 毫秒
121.
Zhe Chen Jianting Cao Yang Cao Yue Zhang Fanji Gu Guoxian Zhu Zhen Hong Bin Wang Andrzej Cichocki 《Cognitive neurodynamics》2008,2(3):257-271
Electroencephalogram (EEG) is often used in the confirmatory test for brain death diagnosis in clinical practice. Because
EEG recording and monitoring is relatively safe for the patients in deep coma, it is believed to be valuable for either reducing
the risk of brain death diagnosis (while comparing other tests such as the apnea) or preventing mistaken diagnosis. The objective
of this paper is to study several statistical methods for quantitative EEG analysis in order to help bedside or ambulatory
monitoring or diagnosis. We apply signal processing and quantitative statistical analysis for the EEG recordings of 32 adult
patients. For EEG signal processing, independent component analysis (ICA) was applied to separate the independent source components,
followed by Fourier and time-frequency analysis. For quantitative EEG analysis, we apply several statistical complexity measures
to the EEG signals and evaluate the differences between two groups of patients: the subjects in deep coma, and the subjects
who were categorized as brain death. We report statistically significant differences of quantitative statistics with real-life
EEG recordings in such a clinical study, and we also present interpretation and discussions on the preliminary experimental
results.
相似文献
Zhe ChenEmail: |
122.
123.
Uses and Conservation of Plant Species in a National Park—A Case Study of Ben En, Vietnam. This paper surveys the use of wild and cultivated plants by local people in Ben En National Park, Vietnam, and analyzes its
impact on the conservation status of some of the utilized species. A total of 208 species used for a range of nonmedicinal
purposes are listed. See Hoang et al. (2008a) for 230 medicinal plants used in the park. Most species are used for food. The use of plants contributes very significantly
to the livelihood of local people in the park, but the current use patterns are not sustainable and would lead to local extinction
of rare and endangered species if no additional conservation measures are introduced. Men collect nonmedicinal plants more
often than women. A total of 38 useful plant species are commercialized, and contribute 12% of the average income of individual
households. Bamboo shoots of Schizostachyum funghomii (Poaceae) are the most important for income generation. The monetary equivalent of noncommercialized useful plants probably
far exceeds the value of the traded plant products. Plant use is independent of the ethnicity of the different populations
living in the park. Larger households make use of a greater variety of useful plant species than small families. Abundant
species in the forest have a higher use index (UI) than less common species. Out of the 208 useful species, as many as 27
were found to be endangered locally, many more than the 11 or 8 endangered species included in national or global red lists.
Currently, useful plants, especially important timber trees, are more abundant in the less disturbed parts of the park, far
away from the villages, indicating the pressures of illegal logging and harvesting near villages on the ecosystems. 相似文献
124.
A. Schmidt B. Ronacher R. M. Hennig 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》2008,194(3):221-233
Acoustic signals consist of pressure changes over time and can thus be analyzed in the frequency- or in the time-domain. With
behavioural experiments we investigated which frequency components (FC) are necessary for the recognition of the periodic
envelope of the conspecific song by females of the grasshopper Chorthippus biguttulus. Further, we determined up to which frequency component phase information is required which would indicate processing in
the time domain. Responses of females revealed that signals composed of FC between 10 and 50 Hz are sufficient for recognition
of the song envelope. A systematic reduction in the number of FC showed that no single frequency component was required; signals
without the fundamental frequency were still highly attractive and only three FC may be sufficient for song recognition. Phase
changes for frequencies up to 40 Hz strongly changed the attractiveness of song signals but only little at 50 Hz. Females
were also tested with rectangular signals in which pause duration was varied. Evidently, and despite the high attractiveness
of song signals with a “missing fundamental”, females evaluated the attractiveness of signals in the time-domain, since the
selectivity for pause duration predicted the responses to signals composed from FC well. 相似文献
125.
Albright S Chen B Holbrook K Jain NU 《Biochemical and biophysical research communications》2008,368(2):231-237
CD14 functions as a key pattern recognition receptor for a diverse array of Gram-negative and Gram-positive cell-wall components in the host innate immune response by binding to pathogen-associated molecular patterns (PAMPs) at partially overlapping binding site(s). To determine the potential contribution of CD14 residues in this pattern recognition, we have examined using solution NMR spectroscopy, the binding of three different endotoxin ligands, lipopolysaccharide, lipoteichoic acid, and a PGN-derived compound, muramyl dipeptide to a 15N isotopically labeled 152-residue N-terminal fragment of sCD14 expressed in Pichia pastoris. Mapping of NMR spectral changes upon addition of ligands revealed that the pattern of residues affected by binding of each ligand is partially similar and partially different. This first direct structural observation of the ability of specific residue combinations of CD14 to differentially affect endotoxin binding may help explain the broad specificity of CD14 in ligand recognition and provide a structural basis for pattern recognition. Another interesting finding from the observed spectral changes is that the mode of binding may be dynamically modulated and could provide a mechanism for binding endotoxins with structural diversity through a common binding site. 相似文献
126.
The Florida Everglades freshwater landscape exhibits a distribution of islands covered by woody vegetation and bordered by marshes and wet prairies. Known as “tree islands”, these ecogeomorphic features can be found in few other low gradient, nutrient limited freshwater wetlands. In the last few decades, however, a large percentage of tree islands have either shrank or disappeared in apparent response to altered water depths and other stressors associated with human impacts on the Everglades. Because the processes determining the formation and spatial organization of tree islands remain poorly understood, it is still unclear what controls the sensitivity of these landscapes to altered conditions. We hypothesize that positive feedbacks between woody plants and soil accretion are crucial to emergence and decline of tree islands. Likewise, positive feedbacks between phosphorus (P) accumulation and trees explain the P enrichment commonly observed in tree island soils. Here, we develop a spatially-explicit model of tree island formation and evolution, which accounts for these positive feedbacks (facilitation) as well as for long range competition and fire dynamics. It is found that tree island patterns form within a range of parameter values consistent with field data. Simulated impacts of reduced water levels, increased intensity of drought, and increased frequency of dry season/soil consuming fires on these feedback mechanisms result in the decline and disappearance of tree islands on the landscape. 相似文献
127.
Indicators of landscape condition should be selected based on their sensitivity to environmental changes and their capacity to provide early warning detection of those changes. We assessed the performance of a suite of spatial-pattern metrics selected to quantify the condition of the ridge-slough landscape in the Everglades (South Florida, USA). Spatial pattern metrics (n = 14) that describe landscape composition, geometry and hydrologic connectivity were enumerated from vegetation maps of twenty-five 2 × 2 km primary sampling units (PSUs) that span a gradient of hydrologic and ecological condition across the greater Everglades ecosystem. Metrics were assessed in comparison with field measurements from each PSU of landscape condition obtained from regional surveys of soil elevation, which have previously been shown to capture dramatic differences between conserved and degraded locations. Elevation-based measures of landscape condition included soil elevation bi-modality (BISE), a binary measure of landscape condition, and also the standard deviation of soil elevation (SDSE), a continuous measure of condition. Metric performance was assessed based on the strength (sensitivity) and shape (leading vs. lagging) of the relationship between spatial pattern metrics and these elevation-based measures. We observed significant logistic regression slopes with BISE for only 4 metrics (slough width, ridge density, directional connectivity index – DCI, and least flow cost – LFC). More significant relationships (n = 8 metrics) were observed with SDSE, with the strongest associations for slough density, mean ridge width, and the average length of straight flow, as well as for a suite of hydrologic connectivity metrics (DCI, LFC and landscape discharge competence – LDC). Leading vs. lagging performance, inferred from the curvature of the association obtained from the exponent of fitted power functions, suggest that only DCI was a leading metric of the loss of soil elevation variation; most metrics were indeterminate, though some were clearly lagging. Our findings support the contention that soil elevation changes from altered peat accretion dynamics precede changes in landscape pattern, and offer insights that will enable efficient monitoring of the ridge-slough landscape as part of the ongoing Everglades restoration effort. 相似文献
128.
129.
Spatial distributions of individuals may be considered in two ways: Firstly, individuals are situated on a continuum. Secondly, individuals are situated on discrete units, for example, insect eggs on plants. The aim of this paper is to show that sometimes, in the latter case, sampling methods based on infestation runs can be of interest in estimating population density. Analytical results are obtained under complete spatial randomness hypothesis and alternative hypotheses. Sampling procedures with limited cost are discussed and the European corn borer (ECB) case is mainly considered. 相似文献
130.