Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “representation learning”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

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

At least 1,531 records · Page 85Linked to original sources
PubMed1994

Studies on the formation of perceptually based basic-level categories in young infants.

A series of experiments examined the abilities of 3- and 4-month-old infants to form categorical representations to exemplars of natural kinds--cats and horses. These experiments also permitted assessment of the relative exclusivity of these representations--the extent to which they exclude exemplars from contrasting basic-level categories from the same superordinate category. We found that categorical representations could be formed for horses that excluded cats, zebras, and giraffes, and for cats that excluded horses and tigers but not female lions. Lions were, however, excluded from the representations of cats in 6- and 7-month-old infants. Evidence was also obtained for 2 a priori preferences for members of one category over another. The discussion examined the roles of similarity between the exemplars of the contrasting categories and those of the tested category on estimates of exclusivity and of spontaneous preferences on the formation and exclusivity of categorical representations.

Choice Behavior↗
PubMed1997

[Retrograde memory after rupture of aneurysms of the anterior communicating artery].

The aim of this study was to investigate deficits of retrograde memory, semantic, autobiographical, and for famous events, associated with prefrontal, cingulate and subcortical lesions resulting from anterior communicating artery rupture. Analyses were performed during the secondary phase post-stroke in 16 patients, and performances were compared to those of an equivalent number of matched control subjects. Semantic investigations revealed a significant deficit in each task using evocation, more especially categorical and literal evocations, and the verbal subtests of the WAIS-R: vocabulary, information, comprehension, and similarities. Furthermore, the capacity to categories was preserved. The Crovitz paradigm, which evaluated the autobiographical memory showed a severe deficit in the evocation of events associated with a precise context in place and moreover in time, with a clear tendency to produce semantic responses, but without significant increase in confabulations. The questionnaire on famous events (1936-1985) did not document deficit in recognition and recall. Furthermore, the patients disorder was more severe in learning new information. Memory disorders were best explained by the severity of lesions in the medio-basal frontal and cingulate cortices, but also by the subcortical injury. Significant correlations were observed between the retrograde memory performance and "frontal" tasks, more especially the WCST; however, similar relations were also documented between learning new information and "frontal" performance. These data suggest that retrograde amnesia results from a selective impairment in accessing old memory representations, and that cognitive processes more specifically altered have tight relations with the capacity to organize the search and to shift.

Adult↗
PubMed2002

Human brain plasticity: evidence from sensory deprivation and altered language experience.

The results from the language studies taken as a whole point to different developmental time courses and developmental vulnerabilities of aspects of grammatical and semantic/lexical processing. They thus provide support for conceptions of language that distinguish these subprocesses within language. Similarly, following auditory deprivation, processes associated with the dorsal visual pathway were more altered than were functions associated with the ventral pathway, providing support for conceptions of visual system organization that distinguish functions along these lines. Could the effects observed in blind and deaf adults be accounted for, at least in part, by the redundant connectivity of the immature human brain? One way we tested this hypothesis was to study the differentiation of visual and auditory sensory responses in normal development (Neville, 1995). In normal adults, auditory stimuli elicit ERP responses that are large over temporal brain regions but small or absent over occipital regions. By contrast, in 6-month-old children we observed that auditory ERPs are equally large over temporal and visual brain regions, consistent with the idea that there is less specificity and more redundancy of connections between the auditory and visual cortex at this time. Between 6 and 36 months, however, we observed a gradual decrease in the amplitude of the auditory ERP over visual areas, while the amplitude over the temporal areas was unchanged. These results suggest that early in human development, there exists a redundancy of connections between auditory and visual areas and that this overlap gradually decreases after birth. This loss of redundancy may be a boundary condition that determines when sensory deprivation can result in alterations in the organization of remaining sensory systems. The considerable variability in timing of sensitive periods may also be in part due to temporal differences in the occurrence of redundancy within different systems. Ongoing studies of infants and children employing different types of stimuli will test for the specificity of these effects (Mitchell et al., 1999). Differences in the degree of plasticity may also be due to differences in the overall level of redundant connectivity within different systems. For example, it may be that aspects of sensory systems that are specialized for high spatial acuity (e.g., central vision and central audition) exhibit fewer developmental redundancies, decreased modifiability and more specificity than those displaying less acuity and precision (e.g., peripheral representations within vision and audition). There is some evidence for this hypothesis within the visual system (Chalupa and Dreher, 1991). In addition, there may be molecular differences between systems displaying different levels and patterns of experience-dependent plasticity. It is of interest that all levels of the dorsal pathway of the visual system, which in the studies reviewed here shows a high level of modifiability, displays strong immunoreactivity for the monoclonal antibody CAT 301 in macaque monkeys (DeYoe et al., 1990). By contrast there is very little labeling within the ventral visual pathway. Moreover, the expression of CAT 301 immunoreactivity shows marked experience-dependent plasticity, suggesting it may play a role in the guidance and/or stabilization of synaptic structure (Sur et al., 1988). Further research along these lines within the auditory system and in animal models of sensory deprivation and other developmental disorders may elucidate the role of specific molecular factors in the developmental plasticity of different neural systems. A related, more general hypothesis that may account for the different patterns of plasticity within both vision and language is that systems employing fundamentally different learning mechanisms (perhaps mediated by different anatomical and molecular substrates) display different patterns of developmental plasticity. It may be that systems that display experience-dependent change throughout life, including the topography of sensory maps (Merzenich et al., 1988; Gilbert, 1995; Kaas, 1995), lexical acquisition (i.e. object-word associations), and the establishment of form, face, and object representations (i.e., ventral pathway functions) rely upon very general, associative learning mechanisms that permit learning and adaptation throughout life. By contrast, systems that are important for computing dynamically shifting relations among locations, objects and events (including the dorsal visual pathway and the systems of the brain that mediate grammar) appear dependent on and modifiable by experience primarily during more limited periods in development. This could account for both the greater developmental deficits and enhancements of dorsal pathway function following various developmental anomalies and for the greater effects of altered language experience on grammatical functions. Further research is necessary to characterize systems that become constrained in this way and those that can be modified throughout life. This type of developmental evidence can contribute to fundamental descriptions of the architecture of different cognitive systems and can guide future studies of the cellular and molecular mechanisms important in neuroplasticity. Additionally, in the long run, they may contribute to the design of educational and habilitative programs for both normally and abnormally developing children.

Adaptation, Physiological↗
PubMed2006

Learning-related development of context-specific neuronal responses to places and events: the hippocampal role in context processing.

Contextual information plays a key role in learning and memory. Learned information becomes associated with the context such that the context can cue the relevant memories and behaviors. An extensive literature involving experimental brain lesions has implicated the hippocampus in context processing. However, the neurophysiological mechanisms of context coding are not known. Although "context" has typically been defined in terms of the background cues, recent studies indicate that hippocampal neurons are sensitive to subtle changes in task demands, even in an unchanging environment. Thus, the context may also include non-environmental features of a learning situation. In the present study, hippocampal neuronal activity was recorded while rats learned to approach different reward locations in two contexts. Because all of the training took place in the same environment, the contexts were defined by the task demands rather than by environmental stimuli. Learning to differentiate two such contexts was associated with the development of highly context-specific neuronal firing patterns. These included different place fields in pyramidal neurons and different event (e.g., reward) responses in pyramidal and interneurons. The differential firing patterns did not develop in a control condition that did not involve a context manipulation. The context-specific firing patterns could modulate activity in extrahippocampal structures to prime context-appropriate behavioral responses and memories. These results provide direct support for a context processing role of the hippocampus and suggest that the hippocampus contributes contextual representations to episodic memories.

Action Potentials↗
PubMed1998

Generalization and exclusive allocation of credit in unsupervised category learning.

A new way of measuring generalization in unsupervised learning is presented. The measure is based on an exclusive allocation, or credit assignment, criterion. In a classifier that satisfies the criterion, input patterns are parsed so that the credit for each input feature is assigned exclusively to one of multiple, possibly overlapping, output categories. Such a classifier achieves context-sensitive, global representations of pattern data. Two additional constraints, sequence masking and uncertainty multiplexing, are described; these can be used to refine the measure of generalization. The generalization performance of EXIN networks, winner-take-all competitive learning networks, linear decorrelator networks, and Nigrin's SONNET-2 network are compared.

Computer Simulation↗
PubMed2006

The ERP old-new effect: A useful indicator in studying the effects of sleep on memory retrieval processes.

STUDY OBJECTIVES: To verify that the classic "Old/New" memory effect can be detected after a long delay, and to investigate the differential influence of declarative memory processes after normal sleep and daytime wake. DESIGN: The protocol is a variation of a more traditional study-recognition test used in event-related potential (ERP) studies in which sleep or wake is inserted between the learning and recognition session in order to verify the existence of the Old/New effect (ie, positive shift that occurs when stimuli are repeated). ERPs were recorded during the recognition-test session. The protocol was based on early work that compared the effect of sleep on memory without recording sleep. SETTING: Data collection occurred in the outpatient sleep laboratory. PATIENTS OR PARTICIPANTS: Results from 13 subjects (6 men) aged between 21 and 39 years. MEASUREMENTS AND RESULTS: The subjects performed the recognition memory test after sleep and daytime wake periods. More-accurate performance for the old (studied) stimuli occurred after the sleep session. Analysis of variance on correctly answered reaction times revealed a significant effect of condition (old/new) with no difference across session. A repeated-measure analysis revealed differences in "Old/New" effect, whereby the amplitude difference between the old and new items was larger after sleep than after wake. CONCLUSIONS: This effect of sleep was found in early frontal and later posterior ERP components, processes that represent strategic, contextual processing and facilitation of episodic memory. Memory representation was not different across sessions. These findings suggest that sleep and wake facilitate 2 components of memory unequally, ie, episodic recognition and memory representation functioning.

Adult↗
PubMed2005

Toddlers recognize verbs in novel situations and sentences.

Toddlers' (MA=22 and 27 months) ability to extend newly taught verbs to new situational and sentential contexts was investigated. Children were interactively taught two novel verbs, presented in only the transitive frame (e.g. You're lorping the ball), in a playroom setting. They then viewed the verb actions presented on side-by-side monitors and were asked to distinguish the verbs in three test frames (transitive, intransitive, neutral (e.g. lorping)). Both groups demonstrated learning of the verbs in the new situation and generalized the verbs to the intransitive sentence frame, but neither generalized reliably to the neutral frame. We conclude that even 22-month-olds demonstrate considerable context independence in their verb representations.

Analysis of Variance↗
PubMedDate not supplied

Is the early visual system optimised to be energy efficient?

This paper demonstrates that a representation which balances natural image encoding with metabolic energy efficiency shows many similarities to the neural organisation observed in the early visual system. A simple linear model was constructed that learned receptive fields by optimally balancing information coding with metabolic expense for an entire visual field in a 2-stage visual system. The input to the model consists of a space variant retinal array of photoreceptors. Natural images were then encoded through a bottleneck such as the retinal ganglion cells that form the optic nerve. The natural images represented by the activity of retinal ganglion cells were then encoded by many more 'cortical' cells in a divergent representation. Qualitatively, the system learnt by optimising information coding and energy expenditure and matched (1) the centre surround organisation of retinal ganglion cells; (2) the Gabor-like organisation of cortical simple cells; (3) higher densities of receptive fields in the fovea decreasing in the periphery; (4) smaller receptive fields in the fovea increasing in size in the periphery; (5) spacing ratios of retinal cells; and (6) aspect ratios of cortical receptive fields. Quantitatively, however, there are small but significant discrepancies between density slopes which may be accounted for by taking optic blur and fixation induced image statistics into account. In addition, the model cortical receptive fields are more broadly tuned than biological cortical neurons; this may be accounted for by the computational limitation of modelling a relatively low number of neurons. This paper shows that retinal receptive field properties can be understood in terms of balancing coding with synaptic energy expenditure and cortical receptive fields with firing rate energy expenditure, and provides a sound biological explanation of why 'sparse' distributions are beneficial.

Animals↗
PubMed2005

Object, space, and object-space representations in the primate hippocampus.

A fundamental question about the function of the primate including human hippocampus is whether object as well as allocentric spatial information is represented. Recordings were made from single hippocampal formation neurons while macaques performed an object-place memory task that required the monkeys to learn associations between objects and where they were shown in a room. Some neurons (10%) responded differently to different objects independently of location; other neurons (13%) responded to the spatial view independently of which object was present at the location; and some neurons (12%) responded to a combination of a particular object and the place where it was shown in the room. These results show that there are separate as well as combined representations of objects and their locations in space in the primate hippocampus. This is a property required in an episodic memory system, for which associations between objects and the places where they are seen are prototypical. The results thus provide an important advance by showing that a requirement for a human episodic memory system, separate and combined neuronal representations of objects and where they are seen "out there" in the environment, is present in the primate hippocampus.

Action Potentials↗
PubMed2005

Medical students' mindset for reflective learning: a revalidation study of the reflection-in-learning scale.

The aims of this paper are to examine the measurement properties of the Reflection-in-Learning Scale (RLS) and to identify whether there are relationships between RLS scores early in the medical program and outcomes of the students' academic activity later on. The 14-item RLS was administered to second-year students (N = 275) at start and at end of the third semester, after the students had reviewed their previous learning experience with the Course Valuing Inventory. The internal consistency, temporal stability and dimensionality of the RLS scores were investigated in relation to the start-end perspectives. Furthermore, a 2-year follow-up allowed the assessment of the relationships of third-term RLS scores with sixth-term measures of both academic achievement and diagnostic reasoning as appraised by the Diagnostic Thinking Inventory (DTI). Findings indicate that RLS data have acceptable unidimensionality and consistency of measurement, notwithstanding a significant individual-context interaction. Repeated measures revealed distinct patterns of RLS scores relating to perceived self-efficacy on the ability to reflect. Third-term RLS scores were significant, albeit weak, predictors of sixth-term cognitive achievement and DTI-related diagnostic reasoning ability. In conclusion, the results do not support a major explanatory role for RLS on knowledge representation. Nevertheless, the findings appear to substantiate the construct validity of this tool as an index of the students' frame of mind as regards reflective learning. They suggest that the RLS captures a self-regulation or cognitive housekeeping dimension of the students' reflective learning. The individual pattern of such (reflective) activity is likely to vary with specific learning conditions.

Adolescent↗
PubMed2003

Side-specific olfactory conditioning leads to more specific odor representation between sides but not within sides in the honeybee antennal lobes.

Honeybees can be trained to associate odorants to sucrose reward by conditioning the proboscis extension response. Using this paradigm, we have recently shown that bees can solve a side-specific task: they learn simultaneously to discriminate a reinforced odor A from a non-reinforced odor B at one antenna (A+B-) and the reversed problem at the other antenna (A-B+). Side-specific (A+B-/B+A-) conditioning is an interesting tool to measure neurophysiological changes due to olfactory learning because the same odorant is excitatory (CS+) on one brain side and inhibitory (CS-) on the opposite side. In the bee brain, the antennal lobe (AL) is the first olfactory relay where the olfactory memory is established. Using calcium imaging, we compared odor-evoked activity in the functional units, the glomeruli, of the two ALs, both in naive and conditioned individuals. Each odor evoked a different pattern of glomerular activity, which was symmetrical between sides and highly conserved among naive animals. In conditioned bees, response patterns were overall symmetrical but showed more active glomeruli and topical differences between sides. By representing odor vectors in a virtual olfactory space whose dimensions are the responses of 23 identified glomeruli, we found that distances between odor representations on each brain side were significantly higher in conditioned than in naive bees, but only for CS+ and CS-. However, the distance between CS+ and CS- representations was equal to that of naive individuals. Our work suggests that side-specific conditioning decorrelates odor representations between AL sides but not between CS+ and CS- within one AL.

Animals↗
PubMed1994

Absolute memory for musical pitch: evidence from the production of learned melodies.

Evidence for the absolute nature of long-term auditory memory is provided by analyzing the production of familiar melodies. Additionally, a two-component theory of absolute pitch is presented, in which this rare ability is conceived as consisting of a more common ability, pitch memory, and a separate, less common ability, pitch labeling. Forty-six subjects sang two different popular songs, and their productions were compared with the actual pitches used in recordings of those songs. Forty percent of the subjects sang the correct pitch on at least one trial; 12% of the subjects hit the correct pitch on both trials, and 44% came within two semitones of the correct pitch on both trials. The results show a convergence with previous studies on the stability of auditory imagery and latent absolute pitch ability; the results further suggest that individuals might possess representations of pitch that are more stable and accurate than previously recognized.

Adolescent↗
PubMed2002

Old age impairs the use of arbitrary visual cues for predictive control of fingertip forces during grasp.

Old age impairs the ability to form new associations for declarative memory, but the ability to acquire and retain procedural memories remains relatively intact. Thus, it is unclear whether old age affects the ability to learn the visuomotor associations needed to set efficient fingertip forces for handling familiar objects. We studied the ability for human subjects to use visual cues (color) about the mechanical properties (texture or weight) of a grasped object to control fingertip forces during prehension. Old and young adults (mean age 77 years and 22 years, respectively) grasped and lifted an object that varied in texture at the gripped surfaces (experiment 1: sandpaper versus acetate surface materials) or weight (experiment 2: 200 g versus 400 g). The object was color-coded according to the mechanical property in the "visual cue" condition, and the mechanical property varied unpredictably across lifts in the "no visual cue" condition. In experiment 1 (texture), the young adults' grip force (force normal to the gripped surface) when the object lifted from the support surface was 24% smaller when the surfaces were color-coded. The old adults' grip force did not vary between the visual conditions despite their accurate reports of the grip surface colors prior to each lift. Comparable findings were obtained in experiment 2, when object weight was varied and peak grip force rate prior to object lift-off was measured. Furthermore old and young subjects alike used about 2 N of grip force when lifting the 200 g object in experiment 2. Therefore, the old adults' failure to adjust grip force when the color cue was present cannot be attributed to a general inability or unwillingness to use low grip force when handling objects. We conclude that old age affects the associative learning that links visual identification of an object with the fingertip forces for efficiently handling the object. In contrast, old and young subjects' grip force was influenced by the preceding lift, regardless of visual cues, which supports existing theories that multiple internal representations govern predictive control of fingertip forces during prehension.

Adult↗
PubMed1997

Functional stages in the formation of human long-term motor memory.

Previous research has demonstrated that the primate CNS has the ability to learn and store multiple and conflicting visuo-motor maps. Here we studied the ability of human subjects to learn to make reaching movements while interacting with one of two conflicting mechanical environments as produced by a robotic manipulandum. We demonstrate that two motor maps may be learned and retained, but only if the training sessions in the tasks are separated by an interval of approximately 5 hr. If the interval is shorter, learning of the second map begins with an internal model appropriate for the first task and performance in the second task is significantly impaired. Analysis of the after-effects suggests that with a short temporal distance, learning of the second task leads to an unlearning of the internal model for the first. With the longer temporal distance, learning of the second task starts with an unbiased internal model, and performance approaches that of naives. Furthermore, the memory of the consolidated skill lasts for at least 5 months after training. These results argue for a distinct change in the state of resistance of motor memory (to disruption) within a few hours after acquisition. We suggest that motor practice results in memories that have at least two functional components: soon after completion of practice, one component fades while another is strengthened. A further experiment suggests that the hypothetical first stage is not merely a gateway to long-term memory, but also temporary storage for items of information, whether new or old, for use in the near-term. Our results raise the possibility that there are distinct neuronal mechanisms for representation of the two functional stages of motor memory.

Adult↗
PubMed1994

Context, conditioning, and hippocampal rerepresentation in animal learning.

The researchers argue that a previous computational account of hippocampal region function in associative learning (M. Gluck & C. Myers, 1993) has emergent implications that accurately describe the role of the hippocampal region in contextual processing. This article unifies 2 seemingly conflicting views of contextual processing: It accords contextual cues no special representational status (e.g., R. Rescorla & A. Wagner, 1972), yet it also allows context to stand in a superordinate relationship to the cues it contains (e.g., L. Nadel & J. Willner, 1980). As a result, the account correctly expects that context can develop occasion-setting properties and that context shifts can weaken learned responses or attenuate latent inhibition. The article also explains data suggesting that hippocampal lesions reduce contextual sensitivity. It may help unify several previous theoretical accounts of the hippocampal region's role in contextual processing.

Animals↗
PubMed1995

Dynamics of learning and recall at excitatory recurrent synapses and cholinergic modulation in rat hippocampal region CA3.

Hippocampal region CA3 contains strong recurrent excitation mediated by synapses of the longitudinal association fibers. These recurrent excitatory connections may play a dominant role in determining the information processing characteristics of this region. However, they result in feedback dynamics that may cause both runaway excitatory activity and runaway synaptic modification. Previous models of recurrent excitation have prevented unbounded activity using biologically unrealistic techniques. Here, the activation of feedback inhibition is shown to prevent unbounded activity, allowing stable activity states during recall and learning. In the model, cholinergic suppression of synaptic transmission at excitatory feedback synapses is shown to determine the extent to which activity depends upon new features of the afferent input versus components of previously stored representations. Experimental work in brain slice preparations of region CA3 demonstrates the cholinergic suppression of synaptic transmission in stratum radiatum, which contains synapses of the longitudinal association fibers.

Animals↗
PubMed1982

Unilateral removal of the second somatosensory projection cortex in the monkey: evidence for cerebral predominance?

The second somatosensory projection cortex (area SII) was removed unilaterally in 18 monkeys: alone, in 6; with bilateral removals of the supplementary motor area (SMA), in 4; with bilateral removals of anterior parietal association cortex, namely area PF (or 7b), in 4; and with neocortical commissure section, in 4. In half of these monkeys SII was removed from the hemisphere opposite to the preoperatively preferred hand, in half from the hemisphere on the same side as the preferred hand. Histological findings in all 18 monkeys indicated no consistent difference between the lesions (site, extent or depth) of the 9 monkeys with contralateral and the lesions of the 9 monkeys with ipsilateral removals of SII. These 18 monkeys, together with 6 additional unoperated control monkeys, were required to learn to make tactile discriminations, either with their preferred or with their nonpreferred hand. The discriminations were either between pairs of solid objects, or between graded differences in roughness or size. The monkeys with SII removal from the hemisphere contralateral to their preoperatively preferred hand were significantly impaired relative to the ipsilateral group or to the control group irrespective of the hand used (postoperatively preferred or nonpreferred hand). Such impairment was evidenced on the majority (but not all) of the tasks used in this study. In general, removal of SII from the ipsilateral hemisphere did not produce impairment, and bilateral removals of SMA combined with a unilateral removal of SII failed to give rise to additional impairment. However, bilateral removal of area PF, as also commissure section, did produce additional impairment, but only in animals of the ipsilateral group and when the postoperatively nonpreferred hand was required to be used. Certain of the behavior variations within groups can be explained in terms of weak preoperative hand preferences, or by reference to the histological findings for these animals. In general we conclude that the monkey has a degree of cerebral predominance of the hemisphere opposite to the preferred hand, possibly in relation to bilateral somatosensory representation.

Animals↗
PubMed2005

Bayesian network learning with feature abstraction for gene-drug dependency analysis.

Combined analysis of the microarray and drug-activity datasets has the potential of revealing valuable knowledge about various relations among gene expressions and drug activities in the malignant cell. In this paper, we apply Bayesian networks, a tool for compact representation of the joint probability distribution, to such analysis. For the alleviation of data dimensionality problem, the huge datasets were condensed using a feature abstraction technique. The proposed analysis method was applied to the NCI60 dataset (http://discover.nci.nih.gov) consisting of gene expression profiles and drug activity patterns on human cancer cell lines. The Bayesian networks, learned from the condensed dataset, identified most of the salient pairwise correlations and some known relationships among several features in the original dataset, confirming the effectiveness of the proposed feature abstraction method. Also, a survey of the recent literature confirms the several relationships appearing in the learned Bayesian network to be biologically meaningful.

Algorithms↗