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Results for “Contrastive learning”

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At least 1,027 records · Page 57Linked to original sources

PubMed2000

Deficits in memory and motor performance in synaptotagmin IV mutant mice.

Synaptotagmin (Syt) IV is a synaptic vesicle protein. Syt IV expression is induced in the rat hippocampus after systemic kainic acid treatment. To examine the functional role of this protein in vivo, we derived Syt IV null [Syt IV(-/-)] mutant mice. Studies with the rotorod revealed that the Syt IV mutants have impaired motor coordination, a result consistent with constitutive Syt IV expression in the cerebellum. Because Syt IV is thought to modulate synaptic function, we also have examined Syt IV mutant mice in learning and memory tests. Our studies show that the Syt IV mutation disrupts contextual fear conditioning, a learning task sensitive to hippocampal and amygdala lesions. In contrast, cued fear conditioning is normal in the Syt IV mutants, suggesting that this mutation did not disrupt amygdala function. Conditioned taste aversion, which also depends on the amygdala, is normal in the Syt IV mutants. Consistent with the idea that the Syt IV mutation preferentially affects hippocampal function, Syt IV mutant mice also display impaired social transmission of food preference. These studies demonstrate that Syt IV is critical for brain function and suggest that the Syt IV mutation affects hippocampal-dependent learning and memory, as well as motor coordination.

Acoustic Stimulation↗

PubMed1997

Neonatal vs. adult unilateral hippocampal lesions: differential alterations in contralateral hippocampal theta rhythm.

Subcortical damage often has more severe consequences in neonates than in adults. For example, unilateral hippocampal lesions in adult rats typically lead to transient memory deficits, whereas neonatal lesions cause lasting learning impairment. We hypothesized that the defects triggered by unilateral damage may include synaptic dysfunction in the contralateral hippocampus. Consequently, we examined the hippocampal theta rhythm, an EEG pattern thought to be associated with learning. Initial comparisons between intact and lesioned rats revealed no obvious differences in basal theta rhythm properties. However, manipulations of ascending brainstem projections to hippocampus with drugs specific for serotonergic, noradrenergic and cholinergic receptors uncovered differences. Antagonism of 5-HT3 receptors known to promote learning significantly increased theta frequency in controls and adult lesioned rats, but not after neonatal damage. In contrast, blockade of noradrenergic-alpha2 receptors had no effect. Antagonism of cholinergic receptors which typically impairs learning disrupted theta and caused irregular, high-amplitude activity that was significantly more pronounced in the lesioned groups. A final approach involved pharmacological facilitation of AMPA receptor-mediated currents, using a drug which enhances memory. This treatment significantly enhanced theta frequency in controls and animals lesioned as adults. In contrast, it failed to do so in rats lesioned at birth. These observations suggest that latent dysfunction in contralateral hippocampal physiology may contribute to the lasting memory deficits seen after unilateral hippocampal lesion in neonates.

Adrenergic alpha-Antagonists↗

PubMed2005

Learning and memory in aging combat veterans with PTSD.

The California Verbal Learning Test (CVLT) was administered to examine learning and memory performance in aging combat veterans with (n = 30) and without PTSD (n = 20), and veterans unexposed to combat (n = 15). Combat veterans with PTSD (PTSD+) showed many impairments compared to non-exposed veterans, but only long-delay free recall consistently discriminated the PTSD+ group from combat-exposed subjects without PTSD (PTSD-), when data were corrected for subscale scores on the WAIS (Vocabulary, Block Design). Alterations in total learning were associated with PTSD when controlling for substance abuse and depression. Two contrast measures, proactive interference and recognition hits, distinguished combat from noncombat veterans, and may be related to trauma exposure. Impairments in total learning are similar to what has been observed in Holocaust survivors. However, increased severity of rapid forgetting may be a specific alteration in older combat veterans, likely reflecting aspects of both combat exposure and aging.

Aged↗

PubMed2006

Stress-induced alterations in prefrontal cortical dendritic morphology predict selective impairments in perceptual attentional set-shifting.

Stressful life events have been implicated clinically in the pathogenesis of mental illness, but the neural substrates that may account for this observation remain poorly understood. Attentional impairments symptomatic of these psychiatric conditions are associated with structural and functional abnormalities in a network of prefrontal cortical structures. Here, we examine whether chronic stress-induced dendritic alterations in the medial prefrontal cortex (mPFC) and orbital frontal cortex (OFC) underlie impairments in the behaviors that they subserve. After 21 d of repeated restraint stress, rats were tested on a perceptual attentional set-shifting task, which yields dissociable measures of reversal learning and attentional set-shifting, functions that are mediated by the OFC and mPFC, respectively. Intracellular iontophoretic injections of Lucifer yellow were performed in a subset of these rats to examine dendritic morphology in layer II/III pyramidal cells of the mPFC and lateral OFC. Chronic stress induced a selective impairment in attentional set-shifting and a corresponding retraction (20%) of apical dendritic arbors in the mPFC. In stressed rats, but not in controls, decreased dendritic arborization in the mPFC predicted impaired attentional set-shifting performance. In contrast, stress was not found to adversely affect reversal learning or dendritic morphology in the lateral OFC. Instead, apical dendritic arborization in the OFC was increased by 43%. This study provides the first direct evidence that dendritic remodeling in the prefrontal cortex may underlie the functional deficits in attentional control that are symptomatic of stress-related mental illnesses.

Animals↗

PubMed1997

A role for the Ras signalling pathway in synaptic transmission and long-term memory.

Members of the Ras subfamily of small guanine-nucleotide-binding proteins are essential for controlling normal and malignant cell proliferation as well as cell differentiation. The neuronal-specific guanine-nucleotide-exchange factor, Ras-GRF/CDC25Mm, induces Ras signalling in response to Ca2+ influx and activation of G-protein-coupled receptors in vitro, suggesting that it plays a role in neurotransmission and plasticity in vivo. Here we report that mice lacking Ras-GRF are impaired in the process of memory consolidation, as revealed by emotional conditioning tasks that require the function of the amygdala; learning and short-term memory are intact. Electrophysiological measurements in the basolateral amygdala reveal that long-term plasticity is abnormal in mutant mice. In contrast, Ras-GRF mutants do not reveal major deficits in spatial learning tasks such as the Morris water maze, a test that requires hippocampal function. Consistent with apparently normal hippocampal functions, Ras-GRF mutants show normal NMDA (N-methyl-D-aspartate) receptor-dependent long-term potentiation in this structure. These results implicate Ras-GRF signalling via the Ras/MAP kinase pathway in synaptic events leading to formation of long-term memories.

3T3 Cells↗

PubMed2006

A fast learning algorithm for deep belief nets.

We show how to use "complementary priors" to eliminate the explaining-away effects that make inference difficult in densely connected belief nets that have many hidden layers. Using complementary priors, we derive a fast, greedy algorithm that can learn deep, directed belief networks one layer at a time, provided the top two layers form an undirected associative memory. The fast, greedy algorithm is used to initialize a slower learning procedure that fine-tunes the weights using a contrastive version of the wake-sleep algorithm. After fine-tuning, a network with three hidden layers forms a very good generative model of the joint distribution of handwritten digit images and their labels. This generative model gives better digit classification than the best discriminative learning algorithms. The low-dimensional manifolds on which the digits lie are modeled by long ravines in the free-energy landscape of the top-level associative memory, and it is easy to explore these ravines by using the directed connections to display what the associative memory has in mind.

Algorithms↗

PubMed2004

Timing of brain-derived neurotrophic factor exposure affects life expectancy of new neurons.

The high vocal center (HVC) of adult male canaries, Serinus canaria, is necessary for the production of learned song. New neurons are added to HVC every day, where they replace older neurons that have died, but the length of their survival depends on the time of year when they are born. A great number of HVC neurons born in the fall, when adult canaries learn a new song, are still present 8 mo later, when this song is used during the breeding season. By contrast, most of the neurons born in HVC in the spring, when little song learning takes place, disappear much sooner. Here we show that infusion of brain-derived neurotrophic factor into HVC during days 14-20 after new HVC neurons are born in the spring confers on them a life expectancy comparable to that of fall-born neurons; this extension on life is not seen when infusion occurs 10 days earlier or later. We suggest that there is, in the adult HVC, a subset of neurons whose life expectancy is determined by brain-derived neurotrophic factor during a sensitive period soon after these neurons reach destination and start forming connections.

Animals↗

PubMed2004

Role of the basal ganglia in category learning: how do patients with Parkinson's disease learn?

The purpose of the present study was to gain a deeper understanding of the role of the basal ganglia in learning and memory by examining learning strategies among patients with basal ganglia dysfunction. Using a probabilistic category learning task (the "weather prediction" task) previously shown to be sensitive to basal ganglia function, the authors examined patterns of performance during learning and used mathematical models to capture different learning strategies. Results showed that patients with Parkinson's disease exhibit different patterns of strategy use. Specifically, most controls initially used a simple, but suboptimal, strategy that focused on single-cue-outcome associations; eventually, however, most controls adopted a more complex, optimal learning strategy, integrating single-cue associations to predict outcomes for multiple-cue stimuli. In contrast, the majority of individuals with Parkinson's disease continued to rely on simple single-cue learning strategies throughout the experiment.

Aged↗

PubMed1998

Pharmacological specialization of learned auditory responses in the inferior colliculus of the barn owl.

Neural tuning for interaural time difference (ITD) in the optic tectum of the owl is calibrated by experience-dependent plasticity occurring in the external nucleus of the inferior colliculus (ICX). When juvenile owls are subjected to a sustained lateral displacement of the visual field by wearing prismatic spectacles, the ITD tuning of ICX neurons becomes systematically altered; ICX neurons acquire novel auditory responses, termed "learned responses," to ITD values outside their normal, pre-existing tuning range. In this study, we compared the glutamatergic pharmacology of learned responses with that of normal responses expressed by the same ICX neurons. Measurements were made in the ICX using iontophoretic application of glutamate receptor antagonists. We found that in early stages of ITD tuning adjustment, soon after learned responses had been induced by experience-dependent processes, the NMDA receptor antagonist D, L-2-amino-5-phosphonopentanoic acid (AP-5) preferentially blocked the expression of learned responses of many ICX neurons compared with that of normal responses of the same neurons. In contrast, the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocked learned and normal responses equally. After long periods of prism experience, preferential blockade of learned responses by AP-5 was no longer observed. These results indicate that NMDA receptors play a preferential role in the expression of learned responses soon after these responses have been induced by experience-dependent processes, whereas later in development or with additional prism experience (we cannot distinguish which), the differential NMDA receptor-mediated component of these responses disappears. This pharmacological progression resembles the changes that occur during maturation of glutamatergic synaptic currents during early development.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMedDate not supplied

Intravenous l-DOPA plus carbidopa in depressed patients: average evoked response, learning, and behavioral changes.

The peripheral decarboxylase inhibitor carbidopa (L-alphamethyl-dopa-hydrazine) allowed safe intravenous administration of L-DOPA in amounts sufficient to alter cortical average evoked response (AER) and learning function in 13 depressed patients. The apparently rapid conversion of L-DOPA to dopamine, as reported from studies in animals, is consistent with the 20-30 min onset of effects seen in our study. Unipolar and bipolar depressed patients responded differently to the alterations in brain biogenic amines and also to the nonspecific stress of the experiment. Intravenous L-DOPA given acutely had effects on the AER that were similar to those documented with oral dopa given chronically--an augmentation of amplitude--intensity slopes in unipolar patients and a relative reduction of slopes in bipolar patients. In contrast, intravenous L-DOPA did not enhance verbal learning as did chronic oral treatment, but rather was associated with reduced learning compared with placebo infusions. Different neurochemical changes following L-DOPA given in single intravenous doses may be responsible for the different learning and behavioral changes form those found previously with oral dopa administered chronically.

Administration, Oral↗

PubMed1982

Specific and non-specific effects of neuro-excitatory amino acids on learning and other behaviours in the chicken.

Intracranial administration of glutamate to the chicken in the first week of life subsequently retards visual discrimination learning. After this susceptible period in the first week, the brain can more effectively metabolize or extrude a loading dose of glutamate. The potent glutamate and aspartate agonists, kainic acid and N-methylaspartate, also retard learning, but alpha-methylaspartate, which is electrophysiologically inactive, does not. Kainic acid also shares glutamate's requirement for interaction with patterned visual input if it is to retard visual learning. Since the amino acid putative neurotransmitters, gamma-aminobutyric acid, taurine and aspartate, also retard visual learning, this effect on behaviour is non-specific. In contrast, glutamate administered to the left hemisphere has a specific effect in elevating attack and copulation, mimicked only by kainic acid and very high doses of N-methylaspartate, but not by the other amino acid putative neurotransmitters. This action of glutamate also differs from its retardation of learning in that is has no requirement for interaction with patterned visual input.

Aggression↗

PubMed2006

First-year medical students prefer multiple learning styles.

Students have preferences for the ways in which they receive information. The visual, auditory, reading/writing, kinesthetic (VARK) questionnaire identifies student's preferences for particular modes of information presentation. We administered the VARK questionnaire to our first-year medical students, and 166 of 250 students (66%) returned the completed questionnaire. Only 36.1% of the students preferred a single mode of information presentation. Among these students, 5.4% preferred visual (learning from graphs, charts, and flow diagrams), 4.8% preferred auditory (learning from speech), 7.8% preferred printed words (learning from reading and writing), and 18.1% preferred using all their senses (kinesthetics: learning from touch, hearing, smell, taste, and sight). In contrast, most students (63.8%) preferred multiple modes [2 modes (24.5%), 3 modes (32.1%), or 4 modes (43.4%)] of information presentation. Knowing the students preferred modes can 1) help provide instruction tailored to the student's individual preference, 2) overcome the predisposition to treat all students in a similar way, and 3) motivate teachers to move from their preferred mode(s) to using others.

Humans↗

PubMed2002

Context-enabled learning in the human visual system.

Training was found to improve the performance of humans on a variety of visual perceptual tasks. However, the ability to detect small changes in the contrast of simple visual stimuli could not be improved by repetition. Here we show that the performance of this basic task could be modified after the discrimination of the stimulus contrast was practised in the presence of similar laterally placed stimuli, suggesting a change in the local neuronal circuit involved in the task. On the basis of a combination of hebbian and anti-hebbian synaptic learning rules compatible with our results, we propose a mechanism of plasticity in the visual cortex that is enabled by a change in the context.

Adolescent↗

PubMed1991

Effects of NMDA receptor antagonists and sigma ligands on the acquisition of conditioned fear in mice.

Recent studies have shown that several compounds known to act as competitive or non-competitive antagonists of NMDA receptors can disrupt learning in rodents. The present study was carried out to investigate the effects of a range of NMDA antagonists, acting at several sites in the NMDA receptor complex, on the acquisition of learned fear in mice. Dose-related disruptions of learning were produced by the non-competitive antagonists phencyclidine, dizocilpine, dextromethorphan and (+) and (-)N-allylnormetazocine. The (+) enantiomer of N-allylnormetazocine was approximately twice as potent as the (-) enantiomer. The competitive NMDA receptor antagonist, CGS 19755, also blocked the acquisition of learned fear as did the non-specific glutamate antagonist riluzole. In contrast, the anti-ischaemic drugs ifenprodil and SL 82.0715, which probably act as NMDA antagonists through an effect on the polyamine site, had no effect on learning up to doses which substantially reduced locomotion. The sigma receptor ligand DTG was also inactive. These results confirm that both competitive and non-competitive NMDA antagonists disrupt learning but indicate that the extent to which such an effect is observed may depend on the site at which the compounds act within the receptor complex. Activity at sigma receptors is unrelated to the effect on learning.

Animals↗

PubMed2003

Perceptual learning in speech.

This study demonstrates that listeners use lexical knowledge in perceptual learning of speech sounds. Dutch listeners first made lexical decisions on Dutch words and nonwords. The final fricative of 20 critical words had been replaced by an ambiguous sound, between [f] and [s]. One group of listeners heard ambiguous [f]-final words (e.g., [WItlo?], from witlof, chicory) and unambiguous [s]-final words (e.g., naaldbos, pine forest). Another group heard the reverse (e.g., ambiguous [na:ldbo?], unambiguous witlof). Listeners who had heard [?] in [f]-final words were subsequently more likely to categorize ambiguous sounds on an [f]-[s] continuum as [f] than those who heard [?] in [s]-final words. Control conditions ruled out alternative explanations based on selective adaptation and contrast. Lexical information can thus be used to train categorization of speech. This use of lexical information differs from the on-line lexical feedback embodied in interactive models of speech perception. In contrast to on-line feedback, lexical feedback for learning is of benefit to spoken word recognition (e.g., in adapting to a newly encountered dialect).

Humans↗

PubMed1978

The role of frontal and parietal cortex in cognitive processing: tests of spatial and sequence functions.

Normal monkeys and monkeys with resection of anterior frontal or posterior parietal cortex were trained to press a panel next to a green panel as a test of extrapersonal spatial orientation and to press a panel next to their own prior press as a test of personal spatial orientation. All monkeys also learned two sets of sequence problems in which the solutions were made independent of spatial location by randomly shifting the locations of the stimuli after each response within a trial. The Parietal Group was significantly impaired on the extrapersonal 'next-to' task but not the more difficult personal 'next-to' task. The Frontal Group was impaired on both the personal and the extrapersonal 'next-to' tasks but only when the relevant cues shifted spatial locations from trial to trial. The performance of the Parietal Group completely overlapped that of the Normal Group on the sequence problems regardless of the level of testing sophistication the monkeys had attained. In contrast, the Frontal Group demonstrated a significant impairment in learning sequences but only when the monkeys were naive. Once they became sophisticated they learned each sequence at a normal rate. Their poor performance was attributed to the lack of stability in the spatial location of the stimuli. The data support the view that a distinction between personal and extrapersonal spatial orientation is relevant to posterior parietal function but indicate that neither sequencing per se nor personal spatial orientation or spatial memory per se is dependent on intact frontal functioning. Rather, the frontal cortex is involved with a higher-order control essential to allow the monkey to perceive the reliable aspects of stimuli contained in a stimulus context full of unreliable noise and to further allow for flexible response pattern appropriate to the demands of a variable context.

Animals↗

PubMed1989

Reduction of demand characteristics in the measurement of certainty during modeled conservation.

The purpose of the present experiment was to document the cognitive processes mediating the observational learning of conservation. Two formats for measuring certainty were contrasted among nonconservers assigned to one of give groups. Under a public format, two groups used a certainty rating scale and showed the experimenter how sure they were about the correctness of a peer model's statements. Under a private format, two groups employed the same scale but were assured that their rating was not known by the experimenter. Within each format, one group was presented with the model's conservation answers and one group with the model's opinions on arbitrary matters. Conservation was demonstrated to the fifth group which did not perform the certainty assessment. All conservation groups were post-tested immediately after modeling and 6 weeks later and all groups displayed equivalent stable learning. Whereas public certainty was constantly high, private certainty increased as the model proceeded through conservation exemplification, in accordance with a hypothesis of sequential imitation and comprehension processes. Private certainty was also higher for conservation than for opinion, although not on the first two items. Public certainty was high regardless of modeled content, and public rating latency was always found to be shorter. These results strongly suggested the presence of demand characteristics for high undifferentiated ratings under the public format of certainty appraisal. This contamination prevented valid monitoring of the course of cognitive rule processing.

Attention↗

PubMed1980

Failure of rats to escape from a potentially lethal microwave field.

Ocularly pigmented rats, all mature females of the Long-Evans strain, were repeatedly presented an opportunity to escape from an intense 918-MHz field (whole-body dose rate = 60 mW/g) to a field of lower intensity (40, 30, 20, or 2 mW/g) by performing a simple locomotor response. Other rats could escape 800-microamperemeter faradic shock to the feet and tail by performing the same response in the same milieu, a multimode cavity. None of 20 irradiated rats learned to associate entry into a visually well-demarcated area of the cavity with immediate reduction of dose rate, in spite of field-induced elevations of body temperature to levels that exceeded 41 degrees C and would have been lethal but for a limit on durations of irradiation. In contrast, all of ten rats motivated by faradic shock rapidly learned to escape. The failure of escape learning by irradiated animals probably arose from deficiencies of motivation and, especially, sensory feedback. Whole-body hyperthermia induced by a multipath field may lack the painful or directional sensory properties that optimally promote the motive to escape. Moreover, a decline of body temperature after an escape-response-contingent reduction of field strength will be relatively slow because of the large thermal time constants of mammalian tissues. Without timely sensory feedback, which is an essential element of negative reinforcement, stimulus-response associability would be imparied, which could retard or preclude learning of an escape response.

Animals↗