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At least 1,693 records · Page 94Linked to original sources
PubMed2003

Functional imaging of perceptual learning in human primary and secondary somatosensory cortex.

Cellular mechanisms underlying synaptic plasticity are in line with the Hebbian concept. In contrast, data linking Hebbian learning to altered perception are rare. Combining functional magnetic resonance imaging with psychophysical tests, we studied cortical reorganization in primary and secondary somatosensory cortex (SI and SII) and the resulting changes of tactile perception before and after tactile coactivation, a simple type of Hebbian learning. Coactivation on the right index finger (IF) for 3 hr lowered its spatial discrimination threshold. In parallel, blood-oxygen level-dependent (BOLD) signals from the right IF representation in SI and SII enlarged. The individual threshold reduction was linearly correlated with the enlargement in SI, implying a close relation between altered discrimination and cortical reorganization. Controls consisting of a single-site stimulation did not affect thresholds and cortical maps. Accordingly, changes within distributed cortical networks based on Hebbian mechanisms alter the individual percept.

Adult↗
PubMed2003

Phonological neighbourhoods in the developing lexicon.

Structural analyses of developing lexicons have provided evidence for both children's holistic lexical representations and sensitivity to phonetic segments. In the present investigation, neighbourhood analyses of two children's (age 3;6) expressive lexicons, maternal input, and an adult lexicon were conducted. In addition to raw counts and frequency-weighted counts, neighbourhood size was calculated as the proportion of the lexicon to which each target word is similar, to normalize for vocabulary size differences. These analyses revealed that children's lexicons contain more similar sounding words than previous analyses indicated. Further, neighbourhoods appear denser earlier in development relative to vocabulary size, presumably because children first learn words with more frequent sounds and sound combinations. Neighbourhood density as a proportion of the size of the lexicon then decreases over development as children acquire words with less frequent sounds and sound combinations. These findings suggest that positing fundamentally different lexical representations for children may be premature.

Child↗
PubMed2004

Neural mechanisms of visual associative processing.

This is a review of our work on multiple microelectrode recordings from the visual cortex of monkeys and subdural recordings from humans--related to the potential underlying neural mechanisms. The former hypothesis of object representation by synchronization in visual cortex (or more generally: of flexible associative processing) has been supported by our recent experiments in monkeys. They demonstrated local synchrony among rhythmic or stochastic gamma-activities (30-90 Hz) and perceptual modulation, according to the rules of figure-ground segregation. However, gamma-synchrony in primary visual cortex is restricted to few millimeters, challenging the synchronization hypothesis for larger cortical object representations. We found that the spatial restriction is due to gamma-waves, traveling in random directions across the object representations. It will be argued that phase continuity of these waves can support the coding of object continuity. Based on models with spiking neurons, potentially underlying neural mechanisms are proposed: (i) Fast inhibitory feedback loops can generate locally synchronized gamma-activities; (ii) Hebbian learning of lateral and feed forward connections with distance-dependent delays can explain the stabilization of cortical retinotopy, the limited size of synchronization, the occurrence of gamma-waves, and the larger receptive fields at successive levels; (iii) slow inhibitory feedback can support figure-ground segregation; (iv) temporal dispersion in far projections destroys coherence of fast signals but preserves slow amplitude modulations. In conclusion, it is proposed that the hypothesis of flexible associative processing by gamma-synchronization, including coherent representations of visual objects, has to be extended to more general forms of signal coupling.

Animals↗
PubMed2006

Actions and consequences in bimanual interaction are represented in different coordinate systems.

Moving one part of the body can generate interaction forces that tend to destabilize other parts of the body. However, stability is maintained by mechanisms that predict and actively oppose these interaction forces. When our body or environment changes, these anticipatory mechanisms adapt so as to continue to produce accurate predictions. In this study, we examine the acquisition of a novel predictive coordination between the arms, in a situation in which a force is produced on one hand as a consequence of the action of the other hand. Specifically, a force was applied to the left hand that depended on the velocity of the right hand. With practice, subjects learned to stabilize the perturbed left arm during right-arm movements by predicting and actively opposing the externally applied forces. After adaptation, we examined how learning generalized to a new joint configuration of the right or left arm to investigate the coordinate systems in which the internal transformation from movement to force is represented. This revealed a dissociation between the representation of the action of the right arm and the representation of its consequence, that is the force on the left arm. The movement is represented in extrinsic coordinates related to the velocity of the hand, whereas the force resulting from the movement is represented in a joint-based intrinsic coordinate system.

Adaptation, Physiological↗
PubMed2005

Excitotoxic lesions of the entorhinal cortex leave gustatory within-event learning intact.

The ability of rats with ibotenate lesions of the entorhinal cortex to form memories for events was assessed by using a gustatory within-event learning procedure. Rats first received exposure to 2 events, AX and BY, each composed of a pair of flavors. Following this exposure period, Flavor X alone was paired with the delivery of lithium chloride. Lesioned and control rats showed a greater aversion to A than to B and to AX than to BX. These results challenge theories that suppose that the entorhinal cortex plays a general role in forming representations of patterns of stimulation.

Analysis of Variance↗
PubMed2006

Dissociation of the roles of NMDA receptor and hippocampus in rats' spatial learning: the effects of environmental familiarity and task familiarity.

In this study, we compared the effects of MK-801 and hippocampal lesions on re-training of Morris water maze place task in familiar and novel environments in rats. In Experiment 1, rats were pre-trained with the place task. After acquiring the task, rats were re-trained with the same task in a familiar environment following MK-801 injection, and were then trained with the same task in a novel environment following MK-801 injection. In the familiar environment, MK-801 had no effect, but in the novel environment performance was impaired. In Experiment 2, after the place task training, the hippocampus was lesioned, and rats were re-trained with the same task in the familiar environment then retrained again in the novel environment. Rats showed severe impairment in both environments. These two experiments suggest different functions for NMDA receptors and the hippocampus. The results of Experiment 1 showed that NMDA receptors are not required for utilizing spatial representations but they play an important role in the construction of spatial representations. The results of Experiment 2 show that the hippocampus is necessary for both the utilization of spatial representations already formed and the formation of new spatial representations.

Animals↗
PubMed1993

Specific language impairment in children: research findings and their therapeutic implications.

This paper reports the findings from a research project investigating a subgroup of specifically language-impaired (SLI) children. The subgroup of SLI children consists of those characterised by persisting grammatical deficits in comprehension and expression of language. The paper summarises the findings in order to highlight the therapeutic implications from the investigations. The main focus of the investigations was to characterise the SLI children's grammatical knowledge of sentence comprehension, specifically their ability to learn the semantic and syntactic properties of verbs. In addition, an investigation of verbal short-term memory (STM) was carried out, and an analysis was undertaken of the expressive morpho-grammatical characteristics of the children. The investigations revealed that the SLI children did not differ in their STM abilities from children carefully matched on language abilities. Thus, the data do not provide support for therapy directed at increasing auditory memory with an aim of improving expression or comprehension of sentences. It is hypothesised that the SLI children have a deficit in syntactic representations and are unable to specify the structural relationships between constituents in syntax. The implications of the study are that this subgroup of SLI children may be unable to use syntactic cues to help learn semantic properties of verbs, but semantic cues may facilitate learning the syntactic properties of verbs.

Child↗
PubMed1991

Knowledge acquisition and information searching strategies in diagnostic decision making.

The effect of three learning procedures on diagnostic information searching strategies was investigated. Undergraduate students acquired an artificial knowledge domain through either a Taxonomic-, Inductive-, or Case-oriented knowledge acquisition procedure. The use of the competing-hypotheses heuristic as a practical strategy to guide decision making in the face of uncertainty, was compared among the three learning conditions and between each condition and a non-learning control group using the method developed by Kern and Doherty (1982) and Wolf et al. (1985). The small instruction intervention had an effect on the diagnostic nature of subjects' information-searching strategies. Subjects in the Inductive-learning condition exhibited a stronger tendency to seek diagnostically worthless information than the other learning conditions. The outcomes are considered relevant to the nature of expertise in decision making, and to the effect of instruction methodologies on the knowledge representations available to diagnostic classification and decision-making processes.

Diagnosis↗
PubMed1987

Circuitry of the frontal association cortex and its relevance to dementia.

The prefrontal cortex reaches its peak size and complexity in the human brain where it occupies more than a quarter of the cerebral cortical surface. This paper reviews studies on that portion of the prefrontal cortex that is buried in and around the principal sulcus of macaque monkeys and corresponds to Brodmann's area 46 in man. Neuropsychological research as well as neurophysiology and 2-deoxyglucose metabolic mapping indicate that the principal sulcus is essential for regulation of motor behavior by internalized representations of visuo-spatial events. Conversely, the prefrontal cortex is unnecessary for behavior regulated by external stimuli, as is the case with many associative learning and recognition memory processes. Research over the past decade suggests that the principal sulcus accomplishes its regulatory functions by its interconnections with (1) the posterior parietal cortex which provides it with access to visuo-spatial data, (2) the parahippocampal gyrus and subiculum which allows information to be held 'on line' and deposited in long-term storage, and (3) motor centers such as the basal ganglia, deep layers of the superior colliculus and several premotor areas that control head, eye and hand movements. In addition, modulatory influences on prefrontal functions are exerted by (4) dopamine-, norepinephrine- and serotonin-containing fiber systems that originate in the brain stem and innervate the prefrontal cortex in a selective manner. Comparison between syndromes present in patients with a variety of diagnoses and human and nonhuman subjects with prefrontal injuries provides suggestive evidence that prefrontal dysfunction may underly the disordered thinking and abnormal social and affective responses found in many of these diseases. Accordingly, knowledge of the neural mechanisms underlying cognitive processing in nonhuman primates should be helpful in the analysis of the 'neurology' of many neurological and psychiatric illnesses.

Animals↗
PubMed2005

Effects of syllable frequency in speech production.

In the speech production model proposed by [Levelt, W. J. M., Roelofs, A., Meyer, A. S. (1999). A theory of lexical access in speech production. Behavioral and Brain Sciences, 22, pp. 1-75.], syllables play a crucial role at the interface of phonological and phonetic encoding. At this interface, abstract phonological syllables are translated into phonetic syllables. It is assumed that this translation process is mediated by a so-called Mental Syllabary. Rather than constructing the motor programs for each syllable on-line, the mental syllabary is hypothesized to provide pre-compiled gestural scores for the articulators. In order to find evidence for such a repository, we investigated syllable-frequency effects: If the mental syllabary consists of retrievable representations corresponding to syllables, then the retrieval process should be sensitive to frequency differences. In a series of experiments using a symbol-position association learning task, we tested whether high-frequency syllables are retrieved and produced faster compared to low-frequency syllables. We found significant syllable frequency effects with monosyllabic pseudo-words and disyllabic pseudo-words in which the first syllable bore the frequency manipulation; no effect was found when the frequency manipulation was on the second syllable. The implications of these results for the theory of word form encoding at the interface of phonological and phonetic encoding; especially with respect to the access mechanisms to the mental syllabary in the speech production model by (Levelt et al.) are discussed.

Humans↗
PubMed2005

[A clinical study of the number processing system: decimal size effects on reading numbers in patients with left parieto-occipital gliomas].

An increasing number of studies are focusing on the anatomo-functional organisation of number processing and some cognitive models have been recently developed. Nevertheless, relationships between areas implicated in number processing, and language areas and circuits remain unclear. Recently, Dehaene and Cohen, in their "triple-code model of number processing", (Dehaene and Cohen, 1995) distinguished two alternative number representation and processing systems: one depending on verbal processes, the other representing a quantity manipulation. According to this model, the retrieval of "arithmetical facts" (AF), learned by rote at school and memorised in a verbal form (such as the multiplication table or simple addition problems) can be considered as a verbal automatism; conversely, subtraction problems, which require mental manipulation of the quantities, represent an abstract, semantic elaboration: "Actual Calculation" (AC). The anatomical correlate of the retrieval of AF (depending on automatic verbal associations) seems to correspond to the left-hemispheric perisylvian areas, while impairment of the actual calculation (AC) depends on the intraparietal region, particularly in the left dominant hemisphere. The present study describes the neuropsychological assessment of three patients, tested after surgery for left parieto-occipital tumors. Two of them were affected by an anaplasic glioma, the third by a low-grade glioma. The cognitive evaluation included: words of Rey, numeral (directed and reversed) span, reading of "simple" numbers (from 1 to 10) and of "complex" numbers (many decimals), writing (dictation) and reading a standard text, finger denomination and right-left distinction. All patients showed language disturbances, dysgraphia and severe dyslexia. In reading numbers, we identified two types of errors: lexical and syntactic. "Lexical errors" consisted in a wrong choice among words in the number's lexicon. For instance, all patients made errors in reading "complex" numbers composed by many decimals, switching single numbers but respecting the decimal size and the structure of the whole number (such as 69107 instead of 68107). On the other hand, only one patient committed syntactic errors, misunderstanding the decimal size and the structure of the number. We considered lexical errors as verbal errors, and syntactic errors as semantic errors, affecting the notion of quantity. We tried to explain verbal disturbances as well as lexical errors as a consequence of lesion of the left-hemispheric perisylvian areas, while syntactic errors as a consequence of impairment of the intraparietal region.

Adult↗
PubMed1989

Getting in touch with pictures and words: educational strategies for the blind.

Tactual processing of information is of much more significance for blind than sighted persons, for whom it is a less dominant channel of communication. However, it has received much less attention than verbal or visual processing of information. In this article, the main outcomes of a series of experiments with congenitally blind children are described, with their performance on a series of spatial reasoning and shape recognition tasks compared to that of blindfolded, partially sighted controls. Results suggest that congenitally blind subjects perform as well as, if not better than, blindfolded subjects on simple 2-dimensional tactual processing tasks, but less well on more complex tasks requiring them to store, compare and label objects. This is thought to reflect their poorer sensory coding abilities and their more limited experience of pictures and representations of objects. While other work suggests that blind children can be trained to enhance these skills, the fact that they appear to utilise different learning strategies should be taken into consideration in educational practice and explored further in future research.

Blindness↗
PubMedDate not supplied

Memory reactivation and consolidation during sleep.

Do our memories remain static during sleep, or do they change? We argue here that memory change is not only a natural result of sleep cognition, but further, that such change constitutes a fundamental characteristic of declarative memories. In general, declarative memories change due to retrieval events at various times after initial learning and due to the formation and elaboration of associations with other memories, including memories formed after the initial learning episode. We propose that declarative memories change both during waking and during sleep, and that such change contributes to enhancing binding of the distinct representational components of some memories, and thus to a gradual process of cross-cortical consolidation. As a result of this special form of consolidation, declarative memories can become more cohesive and also more thoroughly integrated with other stored information. Further benefits of this memory reprocessing can include developing complex networks of interrelated memories, aligning memories with long-term strategies and goals, and generating insights based on novel combinations of memory fragments. A variety of research findings are consistent with the hypothesis that cross-cortical consolidation can progress during sleep, although further support is needed, and we suggest some potentially fruitful research directions. Determining how processing during sleep can facilitate memory storage will be an exciting focus of research in the coming years.

Animals↗
PubMedDate not supplied

Declarative memory consolidation: mechanisms acting during human sleep.

Of late, an increasing number of studies have shown a strong relationship between sleep and memory. Here we summarize a series of our own studies in humans supporting a beneficial influence of slow-wave sleep (SWS) on declarative memory formation, and try to identify some mechanisms that might underlie this influence. Specifically, these experiments show that declarative memory benefits mainly from sleep periods dominated by SWS, whereas there is no consistent benefit of this memory from periods rich in rapid eye movement (REM) sleep. A main mechanism of declarative memory formation is believed to be the reactivation of newly acquired memory representations in hippocampal networks that stimulates a transfer and integration of these representations into neocortical neuronal networks. Consistent with this model, spindle activity and slow oscillation-related EEG coherence increase during early sleep after intense declarative learning in humans, signs that together point toward a neocortical reprocessing of the learned material. In addition, sleep seems to provide an optimal milieu for declarative memory reprocessing and consolidation by reducing cholinergic activation and the cortisol feedback to the hippocampus during SWS.

Acetylcholine↗
PubMed2006

Incremental nonlinear dimensionality reduction by manifold learning.

Understanding the structure of multidimensional patterns, especially in unsupervised cases, is of fundamental importance in data mining, pattern recognition, and machine learning. Several algorithms have been proposed to analyze the structure of high-dimensional data based on the notion of manifold learning. These algorithms have been used to extract the intrinsic characteristics of different types of high-dimensional data by performing nonlinear dimensionality reduction. Most of these algorithms operate in a "batch" mode and cannot be efficiently applied when data are collected sequentially. In this paper, we describe an incremental version of ISOMAP, one of the key manifold learning algorithms. Our experiments on synthetic data as well as real world images demonstrate that our modified algorithm can maintain an accurate low-dimensional representation of the data in an efficient manner.

Algorithms↗
PubMed1986

Auditory profile analysis of irregular sound spectra.

The discrimination of changes in the shapes of sound spectra is reported. The change was always an intensity increment to the 948-Hz component of a multitone complex. First, the ability of naive listeners to learn to discriminate a change in a "regular" background or reference spectrum (equal-level tones equally spaced in logarithmic frequency) was measured as a function of the number of trials. On the average, threshold improved about 10 dB over 3000 trials, with about 50% of the decrease in threshold occurring during the first 750 trials. In a subsequent series of experiments, the overall pattern of spectral shape of the background was varied randomly. Two kinds of perturbations in spectral shape were employed: Randomly choosing the frequencies of the reference spectra and randomly choosing the amplitudes of the components of the reference spectra. The experimental manipulations involved fixing the random spectra across a block of trials, varying the reference spectra from interval to interval of each trial, and providing extensive practice in discriminating specific randomly perturbed reference spectra. The results of the spectrum-learning and random perturbation experiments provide insight into the roles of critical band filtering, sensory variability, and short-term and long-term memory representations in auditory profile analysis. Further, the appropriateness of the generalization of a simple energy detection model is discussed.

Acoustic Stimulation↗
PubMed1997

Taste and olfactory processing in the brain and its relation to the control of eating.

Taste processing in primates through the nucleus of the solitary tract and the primary taste cortex is shown to represent the identity and intensity of taste inputs. In contrast, in the secondary taste cortex in the orbitofrontal area, single neurons respond to the taste of a food only if hunger is present. The neurons here reflect the reward value of taste. They show sensory-specific satiety. In addition to neurons with best responses to sweet, salt, bitter, and sour, there are separate representations of the taste of protein ("umami") and of astringency (e.g., tannic acid). In the orbitofrontal cortex, olfactory inputs converge onto neurons with taste inputs, forming representations of flavor. Neurons in this region may also respond to the sight of food and to its texture. The olfactory representation for some neurons reflects the taste association of odors; and olfactory sensory-specific satiety is represented in this part of the brain. Rapid learning of visual-to-taste associations is also a feature of the neural processing that occurs in the orbitofrontal cortex.

Animals↗
PubMed1995

[Organization and reorganization of the human cortex].

The potential mechanisms underlying recovery and reorganisation of the motor cortex are described on the basis of functional imaging studies. These were performed in normal subjects and patients over the past years by the Hammersmith group. The mechanisms of motor learning are discussed and altered patterns of activation after suffering from stroke, amyotrophic lateral sclerosis and amputation are described. The potential role of ispilateral cortical representation in the primary motor cortex, as well as the role of opercular premotor cortex, insular cortex and inferior parietal cortex are discussed.

Brain Diseases↗