首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2626篇
  免费   154篇
  2021年   21篇
  2020年   42篇
  2019年   49篇
  2018年   80篇
  2017年   61篇
  2016年   77篇
  2015年   48篇
  2014年   67篇
  2013年   294篇
  2012年   114篇
  2011年   108篇
  2010年   65篇
  2009年   64篇
  2008年   130篇
  2007年   115篇
  2006年   108篇
  2005年   75篇
  2004年   73篇
  2003年   65篇
  2002年   69篇
  2001年   61篇
  2000年   59篇
  1999年   56篇
  1998年   36篇
  1997年   23篇
  1996年   43篇
  1995年   35篇
  1994年   26篇
  1993年   30篇
  1992年   42篇
  1991年   28篇
  1990年   29篇
  1989年   30篇
  1988年   29篇
  1987年   28篇
  1986年   29篇
  1985年   32篇
  1984年   26篇
  1983年   17篇
  1982年   28篇
  1981年   27篇
  1980年   22篇
  1979年   20篇
  1978年   33篇
  1977年   27篇
  1976年   19篇
  1975年   23篇
  1974年   21篇
  1971年   18篇
  1968年   23篇
排序方式: 共有2780条查询结果,搜索用时 31 毫秒
961.
Background In addition to the core symptoms, children with Pervasive Developmental Disorders (PDD) often exhibit other problem behaviors such as aggression, hyperactivity, and anxiety, which can contribute to overall impairment and, therefore, become the focus of clinical attention. Limited data are available on the prevalence of anxiety in these children. We examined frequency and correlates of parent-rated anxiety symptoms in a large sample of children with PDD. Methods The goals of this study were to examine the frequency and correlates of parent-rated anxiety symptoms in a sample of 171 medication-free children with PDD who participated in two NIH-funded medication trials. Twenty items of the Child and Adolescent Symptom Inventory (CASI) were used to measure anxiety. Results Forty three percent of the total sample met screening cut-off criteria for at least one anxiety disorder. Higher levels of anxiety on the 20-item CASI scale were associated with higher IQ, the presence of functional language use, and with higher levels of stereotyped behaviors. In children with higher IQ, anxiety was also associated with greater impairment in social reciprocity. Conclusion Anxiety is common in PDD and warrants consideration in clinical evaluation and treatment planning. This study suggests that parent ratings could be a useful source of information about anxiety symptoms in this population. Some anxiety symptoms such as phobic and social anxiety may be closer to core symptoms of PDD. Further efforts to validate tools to ascertain anxiety are needed, as are studies to empirically test approaches to treat anxiety in PDD.  相似文献   
962.
The roles of social support and coping as intervening processes between exposure to community violence and internalizing symptoms were examined longitudinally among a community sample of 667 middle school students in the inner city. After controlling for potential confounders (e.g., social desirability, victimization and witnessing of family violence, guardian's psychological symptomatology), internalizing symptoms at Year 2 were predicted by hypothesized changes over 1 year, such that increased community violence exposure, decreased guardian and peer support, and increased use of defensive and confrontational behavioral coping were related to more internalizing symptoms of anxiety, depression and PTSD, although some of these relations varied by gender. The relations between internalizing symptoms at Year 3 and increased changes in exposure to community violence over 2 years were moderated by social support and/or coping, such that decreased guardian support and increased use of defensive and confrontational coping were generally associated with more symptoms for boys exposed to community violence. Girls who witnessed increased community violence and who increased their use of defensive or confrontational coping experienced more internalizing symptoms. The findings underscore the importance of developmental and contextual considerations in the design and implementation of interventions.  相似文献   
963.
The effects of prenatal choline availability on Pavlovian conditioning were assessed in adult male rats (3–4 mo). Neither supplementation nor deprivation of prenatal choline affected the acquisition and extinction of simple Pavlovian conditioned excitation, or the acquisition and retardation of conditioned inhibition. However, prenatal choline availability significantly altered the contextual control of these learned behaviors. Both control and choline-deprived rats exhibited context specificity of conditioned excitation as exhibited by a loss in responding when tested in an alternate context after conditioning; in contrast, choline-supplemented rats showed no such effect. When switched to a different context following extinction, however, both choline-supplemented and control rats showed substantial contextual control of responding, whereas choline-deficient rats did not. These data support the view that configural associations that rely on hippocampal function are selectively sensitive to prenatal manipulations of dietary choline during prenatal development.There is increasing evidence that variations in maternal dietary choline intake during the second half of pregnancy cause structural, biochemical, and physiological changes in basal forebrain neurons and their projections to the hippocampal complex as well as long-term cognitive changes in the offspring (e.g., Meck and Williams 2003; McCann et al. 2006; Meck et al. 2008). We know, for instance, that the adult offspring of pregnant rats supplemented with 4.5 times the amount of choline in the standard laboratory diet display improved memory capacity and precision on the radial-arm maze (e.g., Meck et al. 1988, 1989; Meck and Williams 1997b, 1999; Tees 1999a), Morris water maze (e.g., Tees 1999b; Tees and Mohammadi 1999; Yang et al. 2000; Brandner 2002), as well as facilitation of sustained attention and interval timing (e.g., Meck and Williams 1997a, c; Mohler et al. 2001; Cheng et al. 2006, 2008a, b; Cheng and Meck 2007) compared with offspring of dams fed a standard diet. Choline deficiency during the same developmental time frame, embryonic days (ED) 12–17, results in impaired performance on some, but not all, of these behavioral measures (e.g., Meck and Williams 1999, 2003). Furthermore, perinatal choline supplementation can alter behavior following a variety of developmental disorders, including the alleviation of abnormalities associated with fetal alcohol syndrome in rats (Thomas et al. 2000, 2004, 2007; Wagner and Hunt 2006), attenuation of some of the motor deficits observed in a Mecp21lox mouse model of Rett syndrome (Nag and Berger-Sweeney 2007), and the improvement of sensory gating in a DBA/2 mouse model of schizophrenia that exhibits reduced numbers of hippocampal a7 nicotinic receptors (Stevens et al. 2008).These choline-induced alterations in cognitive function are accompanied by changes in the size and shape of basal forebrain cholinergic neurons (e.g., Williams et al. 1998; McKeon-O’Malley et al. 2003); modifications in acetylcholine turnover and choline transporter expression in the septum and hippocampus (Cermak et al. 1999; Mellott et al. 2007b); modulation of hippocampal neurogenesis, gene expression, phospholipase D activity, NGF levels, and MAPK and CREB activation (e.g., Holler et al. 1996; Sandstrom et al. 2002; Mellott et al. 2004, 2007a; Glenn et al. 2007); changes in dendritic fields and spine density in CA1 and dentate gyrus (DG) regions of the hippocampus (Meck et al. 2008); as well as modification of the neuropathological response to status epilepticus (e.g., Holmes et al. 2002; Wong-Goodrich et al. 2008a) and thresholds for eliciting long-term potentiation (LTP) in the hippocampus (Pyapali et al. 1998; Jones III et al. 1999). Together, these findings suggest that alterations in choline availability during early development may have specific impact on the ontogeny and later functioning of basal forebrain cholinergic neurons as well as efferent neurons involved in hippocampal LTP (Montoya et al. 2000). These findings also predict that behaviors that rely on the hippocampus are likely to be most affected by this dietary manipulation.Although choline is well known as the precursor for the neurotransmitter acetylcholine, it may be especially crucial to young or developing mammals for a number of other reasons (see Blusztajn and Wurtman 1983; Blusztajn 1998; Zeisel 2000, 2004, 2005). It is the precursor of certain phospholipids (e.g., phosphatidylcholine, sphingomyelin, and plasmenylcholine), which constitute the bulk of phospholipids in all biological membranes. Thus, there may be a particularly high demand for choline during prenatal and neonatal periods associated with rapid neurogenesis and synaptogenesis. Choline can also be enzymatically oxidized to betaine (mostly in peripheral tissues) and the methyl groups of betaine can then be used to resynthesize methionine from homocysteine. Changes in methionine availability alter the methylation of regulatory sequences of genes and of histones, leading to alterations in the patterns of gene expression (e.g., Waterland and Michels 2007; Nafee et al. 2008). Choline is also the precursor of two signaling molecules, platelet-activating factor, and sphingosylphosphorylcholine. Changes in choline availability may also alter membrane synthesis, methylation, and signaling broadly throughout the brain and periphery as well as more restricted effects on cholinergic neuronal pathways (e.g., Zeisel and Blusztajn 1994; Meck and Williams 2003).One common distinction in the Pavlovian-conditioning literature is between tasks that are sensitive to manipulation of the hippocampal formation from those that are not (e.g., Ross et al. 1984; Meck 1988; Schmajuk and Buhusi 1997; Holland et al. 1999). For example, simple excitatory Pavlovian conditioning is typically found to be unaffected by lesions of the hippocampus, while conditional discriminations in which animals must rely on combinations of predictive cues to respond correctly are disrupted by hippocampal damage (e.g., Jarrard and Davidson 1990, 1991). If prenatal choline availability is altering the development of cholinergic neurons in the basal forebrain that project to the hippocampus (see Meck and Williams 2003), our dietary manipulation might only be expected to affect conditioning tasks that require hippocampal involvement, not relatively simple tasks such as excitatory conditioning which do not rely on the hippocampus (e.g., Green and Woodruff-Pak 2000).In the current series of experiments, we examined the effects of prenatal choline supplementation and deficiency using a series of appetitive Pavlovian-conditioning tasks, all of which require associative learning. Our rationale was to determine whether variations in choline availability during prenatal development altered the learning of a simple association between the conditioned (CS) and unconditioned (US) stimuli (e.g., noise → food sequence), or if the dietary manipulation primarily affected conditioning tasks that require more complex relational processing and intact septal-hippocampal function (e.g., context A = tone → food; context B = noise → no food).In order to assess the importance of prenatal choline availability on associative learning, we investigated basic aspects of appetitive Pavlovian conditioning, i.e., conditioned excitation and extinction (e.g., Pavlov 1927) in experiment 1. In this paradigm, rats first receive repeated trials in which the CS occurs just before the presentation of the US, i.e., in a noise → food sequence. During this initial phase of training, the rat develops an increasing tendency to perform the conditioned response (CR) in the presence of the CS indicating that it expects the occurrence of the US. Typically, the probability of the CR increases in a negatively accelerating fashion until it reaches an asymptotic level. If the CS is then repeatedly presented in the absence of the reinforcing US (i.e., noise → no food), then the CR gradually declines; this is referred to as extinction of the CR (Gallistel and Gibbon 2000).One behavioral phenomenon that has been shown to be sensitive to hippocampal manipulation is the discriminative use of contextual cues to control conditioned responding (e.g., Holland and Bouton 1999). Typically, when CS-US pairings occur in one training environment or context, there is a small loss of responding to the CS if it is subsequently presented to the animal in the presence of a different set of contextual cues (e.g., Lovibond et al. 1984; Hall and Honey 1990; Honey et al. 1990; Kaye and Mackintosh 1990). However, this typical decrement in responding with a context switch is not observed in rats with electrolytic or aspiration lesions of the hippocampus (e.g., Good et al. 1997).In order to assess the effects of prenatal choline availability on contextual control of conditioned responding, we employed a renewal design (e.g., Bouton and Bolles 1979) in experiment 2. In this design, rats receive conditioning in one physical context (context A) prior to extinction in either the same context or a context different from that in which they received the initial CS-US pairings (context B). Finally, all of the rats are retested in the original conditioning context (i.e., context A). Bouton and colleagues (e.g., Bouton and Bolles 1979; Frohardt et al. 2000) have found that when rats that are conditioned in context A followed by extinction training in context B are later returned to the original training context for the final testing phase, they show a substantial recovery of the initial CR. Presumably, stimuli contained within the original training context act as reminder cues in this ABA condition, retrieving the memory for the initial acquisition phase (A) of the experiment during the final test phase as opposed to the more recent extinction phase (B). Rats in the AAA condition have no effective cues to discriminate the different phases of the experiment and as a consequence cannot selectively retrieve a specific memory from the sequence. In contrast, test session responding for the ABA condition should be more similar to the low levels observed at the end of the initial extinction phase due to the availability of differential contextual cues. This renewal design is particularly useful in that it provides for the potential to observe treatment effects in both the extinction and the renewal test phases of the experiment. Specifically, either the loss of responding with a context switch during extinction or the response recovery in the renewal test (or both) may be affected by prenatal choline availability. More importantly, those two effects may be due to either the same mechanism (e.g., processing of contextual stimuli) or two different mechanisms (e.g., context conditioning and memory retrieval)—potentially resulting in nonlinear effects of prenatal choline availability across the two experimental phases.A second basic type of associative learning, conditioned inhibition, in which the animal learns to predict the absence of an important event, was described by Pavlov (1927). A typical conditioned-inhibition task consists of training with two types of intermixed trials: On reinforced trials, one CS is followed by reinforcement (e.g., noise → food). On other trials, the same CS is paired with a second stimulus in the absence of the reinforcement (i.e., light/noise → no food). It is presumed that under these training conditions animals learn that the noise predicts the occurrence of the food, while light, the “conditioned inhibitor,” comes to predict the absence of food. That is, light “inhibits” the learned response to noise alone.A relatively small number of studies have examined the neural substrates of inhibitory learning. Aspiration lesions of the hippocampus, for example, impaired a relatively complex phenomenon called “blocking” of excitatory conditioning, but not the learning of conditioned inhibition (e.g., Solomon 1977; Chan et al. 2001). These data suggest that the hippocampal complex is not required for learning conditioned inhibition. Thus, in order to further assess whether prenatal choline availability affects basic associative learning, experiment 3 was designed to evaluate conditioned inhibition in supplemented (SUP), deficient (DEF), and control (CON) rats. In this experiment, rats were given randomly mixed presentations of reinforced and nonreinforced trial types. As training proceeds, the rats should learn to respond more on reinforced trials than on nonreinforced trials. After acquisition of the discrimination, the rats were presented with a retardation test phase in which the inhibitory light CS was paired with food. Rescorla (1969) described this retardation test as one of the critical measures of conditioned inhibition. Presumably, if the CS is a true inhibitor and predicts the absence of reinforcement at the outset of the retardation test, then acquisition of conditioned responding to the cue should be relatively slow during this phase of testing. Consequently, tests of conditioned inhibition should distinguish among prenatal choline treatment groups if inhibitory mechanisms are strengthened or weakened by prenatal choline availability.  相似文献   
964.
The shell division of the nucleus accumbens receives noradrenergic input from neurons in the nucleus of the solitary tract (NTS) that transmit information regarding fluctuations in peripheral hormonal and autonomic activity. Accumbens shell neurons also receive converging inputs from limbic areas such as the hippocampus and amygdala that process newly acquired information. However, few studies have explored whether peripheral information regarding changes in emotional arousal contributes to memory processing in the accumbens. The beneficial effects on memory produced by emotional arousal and the corresponding activation of NTS neurons may be mediated through influences on neuronal activity in the accumbens shell during memory encoding. To explore this putative relationship, Experiment 1 examined interactions between the NTS and the accumbens shell in modulating memory for responses acquired after footshock training in a water-motivated inhibitory avoidance task. Memory for the noxious shock was significantly improved by posttraining excitation of noradrenergic NTS neurons. The enhanced retention produced by activating NTS neurons was attenuated by suppressing neuronal activity in the accumbens shell with bupivacaine (0.25%/0.5 microl). Experiment 2 examined the direct involvement of accumbens shell noradrenergic activation in the modulation of memory for psychologically arousing events such as a reduction in perceived reward value. Noradrenergic activation of the accumbens shell with phenylephrine (1.0 microg/0.5 microl) produced an enhancement in memory for the frustrating experience relative to control injections as evidenced by runway performance on an extended seven-day retention test. These findings demonstrate a functional relationship between NTS neurons and the accumbens shell in modulating memory following physiological arousal and identifies a role of norepinephrine in modulating synaptic activity in the accumbens shell to facilitate this process.  相似文献   
965.
The current study investigated whether, for spatial reference memory, age impacts (1) sensitivity to surgical ovarian hormone loss (Ovx), (2) response to estradiol therapy (ET), and (3) the relation between circulating estradiol levels and memory scores in ovary-intact sham and Ovx plus ET rats. Young, middle-aged and aged Fischer-344 rats received sham, Ovx or Ovx plus ET treatments, and were then tested on the Morris maze. After the last test trial, a probe trial was given whereby the platform was removed. Circulating estradiol levels were then determined and correlated with performance. In Study 1, Ovx facilitated learning on day one, but impaired performance after day one, in young rats. Ovx did not influence performance in middle-aged rats. In young and middle-aged Ovx rats, ET enhanced performance with higher exogenous estradiol levels correlating with better performance during testing and the probe trial. There was no relationship between endogenous estradiol levels and performance in sham young or middle-aged rats. Study 2 showed that, like middle-aged rats, aged rats were not impacted by Ovx. Further, for aged Ovx rats, the ET regimen that was beneficial at earlier ages was no longer effective during test trials, and had only minor benefits for platform localization as assessed by the probe trial. Collectively, the findings suggest that the effects of Ovx as well as responsivity to the currently utilized ET regimen changes with age. Further, there appears to be a distinction between sensitivity to Ovx and responsiveness to ET after Ovx for spatial reference memory performance.  相似文献   
966.
Cognitive models of clinical disorders conceptualise cognitive and behavioural safety-seeking behaviours as central to symptom persistence because they prevent disconfirmation of key maintaining beliefs. Despite growing evidence of the role of negative beliefs about intrusive memories in depression, it remains unclear why such beliefs persist. Accordingly, we examined whether safety behaviours in response to unhelpful beliefs about intrusive memories might play a role in their maintenance. Eighteen high dysphoric (i.e., BDI-II12) individuals who reported an intrusive negative autobiographical memory in the past week completed a battery of measures about their memory, associated negative beliefs and safety behaviours adopted in response to their beliefs. The most commonly endorsed beliefs reflected themes of wanting to control memories (e.g., ‘I should be able to rid my mind of this memory’) and self-deprecation about experiencing them (e.g., ‘Because I can’t control this memory, I am a weak person’). The beliefs prompted a range of safety behaviours, with cognitive distraction being the most common. The findings demonstrate that safety behaviours are common in response to maladaptive beliefs about intrusive memories. Treatment developments in this area are needed, and should incorporate strategies to challenge beliefs about memories, reduce the use of safety behaviours, and promote processing of intrusive memories.  相似文献   
967.
968.
The extent to which categorization of natural classes in animals reflects a generalization based on perceptual similarity versus an abstract conceptual representation remains unclear. Here, two experiments were conducted to identify the perceptual features used by orangutans when categorizing pictures. In Experiment 1, subjects were trained and tested for transfer on a concrete discrimination (gorillas or orangutans vs. other primates). Analysis of performance on both positive and negative exemplars revealed that performance was best on photos with faces, particularly close-ups. Moreover, error trials did not seem to reflect instances of mistaken identity, but rather, exemplars that may have been distracting for other reasons, such as novel coloration or morphology. In Experiment 2, photos were modified to test the effects of various features. Color modifications caused the biggest decrease in performance, and eye modifications also affected performance deleteriously. Therefore, two perceptual features, namely eye regions and color, played a key role in subjects' ability to categorize. However, performance based on an underlying concept cannot be ruled out, because both of these features are highly relevant in terms of defining category membership. Although a subset of features was identified as playing a key role in categorization, these features differed depending on whether feature-use was studied indirectly, as in Experiment 1, or directly, as in Experiment 2.  相似文献   
969.
This study investigated children's, adolescents', and adults' references to an actor's goals when explaining interpersonal actions. Participants were presented with eight brief stories containing a variety of social events and were asked to explain why the actor in each story performed the central action. Children, adolescents, and adults mentioned goals for most of the stories. Adults and adolescents mentioned psychological goals much more often than did children. Older children, adolescents, and adults mentioned complex psychological goals more often than did younger children. Younger children often mentioned instrumental or social goals. When making goal attributions, children, adolescents, and adults considered both the nature of the action and the social context in which it occurred. Participants mentioned psychological goals more often when explaining an antisocial action and social goals when explaining a prosocial action. Participants were also more likely to mention psychological goals when interpersonal events immediately preceding the central action were described. Thus, goals were offered as explanations for interpersonal actions by all age groups, but the type of goals mentioned varied by the age of the participant, the social context, and the valence of the action. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   
970.
Considered a naturally occurring process common to older adults, the life review becomes a more productive process through facilitation and development and application of effective prompts. The experiences of 6 older adults who participated in a facilitated life review process and the manner in which individualized prompts were developed are described.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号