PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “Contrastive learning”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 541 records · Page 30Linked to original sources
PubMed2000

Feedback Delays: How Can Decision Makers Learn Not to Buy a New Car Every Time the Garage Is Empty?

Decision makers in dynamic environments (e.g., stock trading, inventory control, and firefighting) learn poorly in experiments where feedback about the outcomes of their actions is delayed. In searching for ways to mitigate these effects, this paper presents two computational models of learning with feedback delays and contrasts them against human decision-makers' performance. The no-memory model hypothesizes that decision makers always perceive feedback as immediate. The with-memory model hypothesizes that, over time, decision makers are able to develop internal representations of the task that help them to perform with delayed feedback. As borne out by human subjects, both models predict that a display of past history improves learning with delay and that increasing delay increasingly degrades performance. Even though the length of training in this task exceeds that used in many laboratory-based dynamic tasks, neither the two models nor the subjects are able to effectively learn without decision aids when faced with feedback delays. When given an amount of training that more closely approximates that provided in functioning dynamic environments, the with-memory model predicts that human decision makers may learn without decision aids over the long term if feedback delays are simple. These results raise several issues for continued theoretical investigation as well as potential suggestions for training and supporting decision makers in dynamic environments with feedback delays. Copyright 2000 Academic Press.

Journal Article↗
PubMed2001

Target spectral, dynamic spectral, and duration cues in infant perception of German vowels.

Previous studies of vowel perception have shown that adult speakers of American English and of North German identify native vowels by exploiting at least three types of acoustic information contained in consonant-vowel-consonant (CVC) syllables: target spectral information reflecting the articulatory target of the vowel, dynamic spectral information reflecting CV- and -VC coarticulation, and duration information. The present study examined the contribution of each of these three types of information to vowel perception in prelingual infants and adults using a discrimination task. Experiment 1 examined German adults' discrimination of four German vowel contrasts (see text), originally produced in /dVt/ syllables, in eight experimental conditions in which the type of vowel information was manipulated. Experiment 2 examined German-learning infants' discrimination of the same vowel contrasts using a comparable procedure. The results show that German adults and German-learning infants appear able to use either dynamic spectral information or target spectral information to discriminate contrasting vowels. With respect to duration information, the removal of this cue selectively affected the discriminability of two of the vowel contrasts for adults. However, for infants, removal of contrastive duration information had a larger effect on the discrimination of all contrasts tested.

Adult↗
PubMed2005

Effects of sublethal doses of fipronil on the behavior of the honeybee (Apis mellifera).

Fipronil is a phenylpyrazole insecticide introduced for pest control, but it can also affect non-target insects such as honeybees. In insects, fipronil is known to block GABA receptors and to inhibit ionotropic glutamate-gated chloride channels, but the behavioral effects of low doses are not yet fully understood. We have studied the effect of sublethal doses of fipronil on the behavior of the honeybee (Apis mellifera) under controlled laboratory conditions. The drug was either administered orally or applied topically on the thorax. A significant reduction of sucrose sensitivity was observed for the dose of 1 ng/bee 1 h after a thoracic application. No significant effect on sucrose sensitivity was obtained with acute oral treatment. A lower dose of fipronil (0.5 ng/bee applied topically) impaired the olfactory learning of the honeybees. By contrast, locomotor activity was not affected. Our results suggest a particular vulnerability of the olfactory memory processes and sucrose perception to sublethal doses of fipronil in the honeybee.

Animals↗
PubMed2000

Fmr1 knockout mouse has a distinctive strain-specific learning impairment.

The Fmr1 gene knockout mouse is a model for the human Fragile X mental retardation syndrome. Fmr1 knockout mice with a C57BL/6-129/OlaHsd hybrid background have been reported to have only a very mild deficiency in learning the Morris water maze task. We compared the effect of this knockout mutation on learning in mice with either an FVB/N-129/OlaHsd hybrid background or a C57BL/6 background. When FVB-129 mice were tested in a cross-shaped water maze task, the knockout mice showed a pronounced deficiency in their ability to learn the position of a hidden escape platform in comparison to normal littermates. In contrast, knockout mice with a C57BL/6 background learned the maze just as well as their normal littermates. Fear conditioning did not reveal differences between knockout and normal mice in either background. These results show that silencing the Fmr1 gene clearly interfered with learning a specific visuospatial task in FVB/N-129 hybrid mice but not in C57BL/6 mice. The strain dependence may model the influence of genetic background in the human Fragile X syndrome.

Animals↗
PubMed2004

The anatomy of semantic knowledge: medial vs. lateral temporal lobe.

Semantic knowledge (e.g., long-established knowledge about objects, facts, and word meanings) is known to be severely impaired by damage to the anterolateral temporal lobe. For example, patients with semantic dementia have prominent atrophy in anterolateral temporal cortex and also have significant damage within the medial aspect of the temporal lobe. However, there is uncertainty about the contribution of medial temporal lobe damage, including perirhinal cortex damage, to impaired semantic knowledge. Drawing largely on published material from multiple sources, we compared the performance of severely amnesic patients with large medial temporal lobe lesions and patients with semantic dementia on nine tests of semantic knowledge and two tests of new learning ability. On the tests of semantic knowledge, the amnesic patients performed markedly better than the patients with semantic dementia. By contrast, on the tests of new learning, the patients with semantic dementia performed markedly better than the amnesic patients. We conclude that medial temporal lobe damage impairs the formation of declarative memory, and that semantic knowledge is impaired to the extent that damage extends laterally in the temporal lobe. Reports that the extent of atrophy in perirhinal cortex correlated with the severity of impaired semantic knowledge may be understood by supposing that the extent of damage in many temporal lobe areas is intercorrelated in this progressive disease, and that the extent of atrophy in perirhinal cortex is a proxy for the overall severity of dementia.

Aged↗
PubMed2003

The role of perirhinal cortex in visual discrimination learning for visual secondary reinforcement in rats.

Perirhinal cortex in monkeys has been thought to be involved in visual associative learning. The authors examined rats' ability to make associations between visual stimuli in a visual secondary reinforcement task. Rats learned 2-choice visual discriminations for secondary visual reinforcement. They showed significant learning of discriminations before any primary reinforcement. Following bilateral perirhinal cortex lesions, rats continued to learn visual discriminations for visual secondary reinforcement at the same rate as before surgery. Thus, this study does not support a critical role of perirhinal cortex in learning for visual secondary reinforcement. Contrasting this result with other positive results, the authors suggest that the role of perirhinal cortex is in "within-object" associations and that it plays a much lesser role in stimulus-stimulus associations between objects.

Animals↗
PubMedDate not supplied

An age-related spatial learning deficit: choline uptake distinguishes "impaired" and "unimpaired" rats.

A functional decline in the hippocampal formation may underlie the emergence of spatial learning deficits in aged rodents. In this study, sodium-dependent high-affinity choline uptake (HACU) was used to monitor hippocampal function in response to training on a spatial task. The subjects were male Long-Evans rats at either 4 months or 22-24 months of age. Animals were trained to locate a camouflaged escape platform in the Morris water maze. Each animal that received place training had a yoked counterpart that was exposed to swimming in the maze but was not required to learn the task. Animals, both young and aged, were sacrificed after attaining a criterion performance. Relative to animals in the yoked condition, place training significantly reduced HACU in both the young rats and in a subpopulation of the aged animals that learned the task rapidly. In contrast, for aged rats that had an impaired rate of acquisition, no effect of place training on HACU was observed. These results provide evidence for a relationship between the behavioral capacities of aged rats and changes in the status of hippocampal function.

Aging↗
PubMed2000

Circuit-specific alterations in hippocampal synaptophysin immunoreactivity predict spatial learning impairment in aged rats.

The present study examined the long-standing concept that changes in hippocampal circuitry contribute to age-related learning impairment. Individual differences in spatial learning were documented in young and aged Long-Evans rats by using a hippocampal-dependent version of the Morris water maze. Postmortem analysis used a confocal laser-scanning microscopy method to quantify changes in immunofluorescence staining for the presynaptic vesicle glycoprotein, synaptophysin (SYN), in the principal relays of hippocampal circuitry. Comparisons based on chronological age alone failed to reveal a reliable difference in the intensity of SYN staining in any region that was examined. In contrast, aged subjects with spatial learning deficits displayed significant reductions in SYN immunoreactivity in CA3 lacunosum-moleculare (LM) relative to either young controls or age-matched rats with preserved learning. SYN intensity values for the latter groups were indistinguishable. In addition, individual differences in spatial learning capacity among the aged rats correlated with levels of SYN staining selectively in three regions: outer and middle portions of the dentate gyrus molecular layer and CA3-LM. The cross-sectional area of SYN labeling, by comparison, was not reliably affected in relation cognitive status. These findings are the first to demonstrate that a circuit-specific pattern of variability in the connectional organization of the hippocampus is coupled to individual differences in the cognitive outcome of normal aging. The regional specificity of these effects suggests that a decline in the fidelity of input to the hippocampus from the entorhinal cortex may play a critical role.

Aging↗
PubMed1988

[Effect of synthetic fragments of thymic peptide hormones on learning and memory processes].

Seven synthetic peptide fragments of thymic hormones were investigated using behavioral and electrophysiological methods. Splenopentin was shown to increase markedly learning capacities and memory. In contrast, alpha 1-thymosin fragment decreased them. Other peptides only slightly affected the capacities. Two active peptides seem to affect the level of activation of subcortical structures responsible for the excitation and inhibition.

Animals↗
PubMed1996

Polarization contrast vision in Octopus.

While the ability to analyze polarized light is widespread among animals, its contribution to form vision has not yet been documented. We tested the hypothesis that polarization vision can be used for object discrimination, by training octopuses to distinguish between targets on the basis of the presence or absence of a pattern produced by a 90 degrees polarization contrast within the target. Octopuses recognized a 90 degrees contrast pattern within a single target, when presented either on a horizontal/vertical axis or on a 45 degrees/135 degrees axis. They were able to transfer their learning to new situations and to detect a polarization contrast when the orientations of the e-vector of light passing through the target center and background differed by as little as 20 degrees. Polarization vision may provide information similar to that available from color vision and thus serve to enhance the detection and recognition of objects.

Animals↗
PubMed1988

Delta-9-tetrahydrocannabinol impairs visual recognition memory but not discrimination learning in rhesus monkeys.

The effects of orally administered delta-9-tetrahydrocannabinol (THC) were evaluated on two different learning abilities in monkeys. Visual recognition memory, known to depend on limbic system integrity, was tested by means of delayed nonmatching-to-sample and found to be significantly impaired by acute administration of 2 and 4 mg/kg THC given 1 or 2 h prior to testing. Performance was significantly impaired throughout a 21-day period of repeated administration of 4 mg/kg THC and also during a 3-5 day period that began 7-10 days after the last dose of THC. By contrast, 24-h concurrent discrimination learning, a task that monkeys with limbic lesions can perform normally, was not impaired by THC, even following doses as high as 16 mg/kg. These results suggest that THC interferes with recognition memory more than discrimination learning, possibly reflecting a selective action of THC on limbic mechanisms.

Administration, Oral↗
PubMed1995

Ensemble competitive learning neural networks with reduced input dimension.

Conventional neural networks utilize all the dimensions of the original input patterns for training and classification. However, a particular attribute of the input patterns does not necessarily contribute to classification and may even cause misclassification in certain cases. A new ensemble competitive learning method using the reduced input dimension is proposed. In contrast to the previous ensemble neural networks which adjust learning parameters, the proposed method takes advantage of the information in each dimension of the input patterns. Since the degree of contribution of each attribute to classification is not known beforehand, the different input data sets with one dimension reduced are presented to multiple neural networks. The classification information from each competitive learning neural network is then combined to make a final decision for classification. In order to improve classification accuracy, the ambiguous output neurons are eliminated which cannot be assigned to any class after training. We use three consensus schemes to judge the classification using ensemble neural networks. The experimental results with remote sensing and speech data indicate the improved performance of the proposed method.

Algorithms↗
PubMedDate not supplied

[Multimedia learning program for medical consultation].

To improve communicative competence during physicians' consultations, we developed a computer-assisted multimedia learning programme. This programme can be used in medical education as well as in continuing education for physicians. Dependent on individual skills and needs, the system, based on hypertext links, can be tailored to expand and improve communicative competence in a series of steps. The programme is based on a manual of medical interviewing and breaking bad news. The user gains information about theory and techniques of verbal intervention. He is trained to be able to reflectively observe and interactively intervene in video-taped and transcribed physician-patient dialogues. In a contrast-typological approach to model learning, in which negative and positive cases can be compared in a problem-based concept, alternative types of interventions can be recognised and evaluated. Finally, the learning goals in verbal intervention techniques should be subject of self-evaluation which can be objectified by further evaluations.

Communication↗
PubMedDate not supplied

Empowered learning on the inside: an externship experience.

The purpose of this multiphasic qualitative research study was to obtain an in-depth understanding of how students experience a nonterminal summer externship program. Baccalaureate and associate degree nursing students participated in the six-week externship developed through a partnership between a regional hospital and a school of nursing in the southeastern United States. Data were collected before, during, immediately following, and six months after the externship experience using a combination of focus groups and interviews with student-preceptor dyads. Interviews were transcribed verbatim and analyzed using constant comparative methodology. Findings from the study indicated that the core process characterizing the experience was "empowered learning on the inside." In contrast to traditional instructor-led experiences, students were positioned on the inside and acculturated to the world of nursing. As insiders, students related to others and experienced nursing in new ways. Implications of the findings for nursing education are addressed.

Adult↗
PubMedDate not supplied

Prenatal stress in rats: effects on plasma corticosterone, hippocampal glucocorticoid receptors, and maze performance.

The present experiments were designed to investigate the effects of maternal stress on cognitive and endocrine parameters in the adult offspring. Pregnant rats were stressed daily during the last week of pregnancy (days 15-19) by restraint, and the performance of their offspring in the Morris water maze was recorded. Plasma corticosterone levels after swimming and the status of hippocampal glucocorticoid receptors (GRs) were determined. During acquisition of the task, prenatally stressed (PS) males - but not females - showed longer escape latencies than non-stressed controls when swimming in cold (10 degrees C) but not in warm (20 degrees C) water. This sex- and prenatal stress-specific difference was even more pronounced during reversal learning of the task. In contrast, PS females - but not males - had higher basal corticosterone levels and a lower density of hippocampal corticosteroid receptors than non-stressed controls. In all animals irrespective of treatment, swimming in the water maze causes an increase of corticosterone that was smaller on day 8 of swimming than on day 1. After swimming in cold water, the rise in corticosterone levels in females was steeper and returned faster to baseline values than after swimming in warm water. A similar pattern could be seen in PS females when compared to their non-stressed controls. The data suggest that prenatal stress impairs spatial learning in males but not in females. Basal and stress-induced increases in corticosterone levels, however, were altered in PS females and not in PS males; i.e., prenatal stress-induced changes in corticosterone secretion were not paralleled by prenatal stress-induced deficits in spatial learning.

Animals↗
PubMed1998

The development of features in object concepts.

According to one productive and influential approach to cognition, categorization, object recognition, and higher level cognitive processes operate on a set of fixed features, which are the output of lower level perceptual processes. In many situations, however, it is the higher level cognitive process being executed that influences the lower level features that are created. Rather than viewing the repertoire of features as being fixed by low-level processes, we present a theory in which people create features to subserve the representation and categorization of objects. Two types of category learning should be distinguished. Fixed space category learning occurs when new categorizations are representable with the available feature set. Flexible space category learning occurs when new categorizations cannot be represented with the features available. Whether fixed or flexible, learning depends on the featural contrasts and similarities between the new category to be represented and the individuals existing concepts. Fixed feature approaches face one of two problems with tasks that call for new features: If the fixed features are fairly high level and directly useful for categorization, then they will not be flexible enough to represent all objects that might be relevant for a new task. If the fixed features are small, subsymbolic fragments (such as pixels), then regularities at the level of the functional features required to accomplish categorizations will not be captured by these primitives. We present evidence of flexible perceptual changes arising from category learning and theoretical arguments for the importance of this flexibility. We describe conditions that promote feature creation and argue against interpreting them in terms of fixed features. Finally, we discuss the implications of functional features for object categorization, conceptual development, chunking, constructive induction, and formal models of dimensionality reduction.

Child↗
PubMed2001

Discriminating mild parkinsonism: methods for epidemiological research.

Methods for the efficient and accurate detection of parkinsonism are essential for epidemiological studies. We sought to determine whether parkinsonism could be detected by a neurologist from a videotaped assessment and whether neurobehavioral methods (motor, cognitive, and sensory) discriminated between patients with Parkinson's disease (PD) and controls. Fifteen patients with mild PD (Hoehn and Yahr I-III) were compared to 15 age-, sex-, and education- matched controls. Each participant underwent a videotaped neurological examination (based on the Unified Parkinson's Disease Rating Scale, UPDRS), administered by a trained technician, and reviewed by a neurologist, as well as a series of neurobehavioral tests. The neurologist identified PD patients with 86% sensitivity and 100% specificity. Among the neurobehavioral tests, finger tapping, combined with one or more among olfaction, visual contrast sensitivity, or Paired Associates Learning, correctly classified 90%, or more, of subjects. Individual psychological tests did not discriminate reliably between groups. We conclude that videotaped assessments of parkinsonism or objective tests of motor and sensory function can accurately detect patients with PD. Both approaches have potential for identifying PD cases, but the latter may be more efficient for screening.

Cognition Disorders↗