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At least 1,729 records · Page 96Linked to original sources
PubMed1999

Function of proline residues of MotA in torque generation by the flagellar motor of Escherichia coli.

Bacterial flagellar motors obtain energy for rotation from the membrane gradient of protons or, in some species, sodium ions. The molecular mechanism of flagellar rotation is not understood. MotA and MotB are integral membrane proteins that function in proton conduction and are believed to form the stator of the motor. Previous mutational studies identified two conserved proline residues in MotA (Pro 173 and Pro 222 in the protein from Escherichia coli) and a conserved aspartic acid residue in MotB (Asp 32) that are important for function. Asp 32 of MotB probably forms part of the proton path through the motor. To learn more about the roles of the conserved proline residues of MotA, we examined motor function in Pro 173 and Pro 222 mutants, making measurements of torque at high load, speed at low and intermediate loads, and solvent-isotope effects (D2O versus H2O). Proton conduction by wild-type and mutant MotA-MotB channels was also assayed, by a growth defect that occurs upon overexpression. Several different mutations of Pro 173 reduced the torque of the motor under high load, and a few prevented motor rotation but still allowed proton flow through the MotA-MotB channels. These and other properties of the mutants suggest that Pro 173 has a pivotal role in coupling proton flow to motor rotation and is positioned in the channel near Asp 32 of MotB. Replacements of Pro 222 abolished function in all assays and were strongly dominant. Certain Pro 222 mutant proteins prevented swimming almost completely when expressed at moderate levels in wild-type cells. This dominance might be caused by rotor-stator jamming, because it was weaker when FliG carried a mutation believed to increase rotor-stator clearance. We propose a mechanism for torque generation, in which specific functions are suggested for the proline residues of MotA and Asp32 of MotB.

Alleles↗
PubMed2005

Nonlinear and noisy extension of independent component analysis: theory and its application to a pitch sensation model.

In this letter, we propose a noisy nonlinear version of independent component analysis (ICA). Assuming that the probability density function (p. d. f.) of sources is known, a learning rule is derived based on maximum likelihood estimation (MLE). Our model involves some algorithms of noisy linear ICA (e. g., Bermond & Cardoso, 1999) or noise-free nonlinear ICA (e. g., Lee, Koehler, & Orglmeister, 1997) as special cases. Especially when the nonlinear function is linear, the learning rule derived as a generalized expectation-maximization algorithm has a similar form to the noisy ICA algorithm previously presented by Douglas, Cichocki, and Amari (1998). Moreover, our learning rule becomes identical to the standard noise-free linear ICA algorithm in the noiseless limit, while existing MLE-based noisy ICA algorithms do not rigorously include the noise-free ICA. We trained our noisy nonlinear ICA by using acoustic signals such as speech and music. The model after learning successfully simulates virtual pitch phenomena, and the existence region of virtual pitch is qualitatively similar to that observed in a psychoacoustic experiment. Although a linear transformation hypothesized in the central auditory system can account for the pitch sensation, our model suggests that the linear transformation can be acquired through learning from actual acoustic signals. Since our model includes a cepstrum analysis in a special case, it is expected to provide a useful feature extraction method that has often been given by the cepstrum analysis.

Algorithms↗
PubMed2006

Control of attention and gaze in complex environments.

In natural behavior, fixation patterns are tightly linked to the ongoing task. However, a critical problem for task-driven systems is dealing with unexpected stimuli. We studied the effect of unexpected potential collisions with pedestrians on the distribution of gaze of subjects walking in a virtual environment. Pedestrians on a collision course with the subject were surprisingly ineffective in evoking fixations, especially when subjects were additionally occupied with another task, suggesting that potential collisions do not automatically attract attention. However, prior fixations on pedestrians did increase collision detection performance. Additionally, the detection of potential collisions led to a short-term change in the strategy of looking at subsequent pedestrians. The overall pattern of results is consistent with the hypothesis that subjects typically rely on mechanisms that are initiated top-down to detect unexpected events such as potential collisions. For this to be effective, subjects must learn an appropriate schedule for initiating search through experience with the probabilities of environmental events.

Adult↗
PubMed1999

Interaction of cortex and hippocampus in a model of amnesia and semantic dementia.

We describe a systems-level computational model, called TraceLink, that can explain the major characteristics of the neuropsychology of amnesia and of semantic dementia, a recently discovered syndrome in which there is a progressive loss of semantic memory. It also approximates the normal forgetting curve and presents an explanation of why spaced learning is more efficient than massed learning. A central assumption is that consolidation of memory takes place, probably during dream sleep. The model consists of three systems: trace system (certain parts of the neocortex), link system (includes the hippocampus), and the modulatory system (includes certain basal forebrain nuclei). Lesioning each of these causes a characteristic form of amnesia or semantic dementia. Lesioning the modulatory system causes anterograde amnesia only. Lesioning the link system causes a correlated degree of retrograde and anterograde amnesia. Retrograde amnesia shows the characteristic Ribot curve with relative sparing of remote memories but loss of recent ones. Lesioning the trace system causes semantic dementia. We also review the main sources of constraints for the model and discuss its status and function as well as its falsifiability.

Amnesia↗
PubMed1998

Modelling medical decisions in DynaMoL: a new general framework of dynamic decision analysis.

Dynamic decision analysis concerns decision problems in which both time and uncertainty are explicitly considered. We present a new dynamic decision analysis framework, called DynamoL, that supports graphical presentation of the decision factors in multiple perspectives. To alleviate the difficulty in assessing conditional probabilities over time in dynamic decision models, DynaMoL incorporates a Bayesian learning system to automatically learn the probabilistic parameters from large medical databases. We describe the DynaMoL modeling and learning architecture through a medical decision problem on the optimal follow-up schedule for patients after curative colorectal cancer surgery. We also show that the modeling experience and results indicate practical promise for the framework.

Artificial Intelligence↗
PubMedDate not supplied

[Effect of piracetam on the conditional reflex memory under probable reinforcement conditions].

The effect of piracetam on the development of a two-link system of feed-procuring conditioned reflexes was studied in rats with probability reinforcement. The experiments showed that piracetam improves both the current learning process and the memory track (previous experience) reproduction. In addition, the drug decreases the role of the probabilic character of reinforcement, thus facilitating the synthesis of two reflexes in a united functional complex.

Animals↗
PubMed1992

[The provision of hearing aids to young children with congenital hearing impairment].

Early screening for hearing impairment in children is generally advised to prevent sensory deprivation and speech, language and learning problems. In the present study, the age at which 176 probably congenitally hearing impaired infants were referred to the Audiological Centre in Nijmegen between 1983 and 1990 was examined. The mean age of fitting the hearing aids was 35 months. This was about 2 years after the auditory screening test (Ewing test) as performed in most baby health care centres. In general, if a screening test is positive, the child will be referred to the general practitioner, who can refer the child to either a general otolaryngologist or a ENT clinic with a (paedo-) audiological centre. Hearing aids can only be provided by an audiological centre. On average, hearing-impaired children who were directly referred to our clinic/audiological centre were provided with hearing aids 13 months earlier than children initially referred to a general otolaryngologist. It is concluded that young children who fail an auditory screening test should be thoroughly tested audiometrically after an otoscopic examination.

Age Factors↗
PubMed1990

A new physiological concept on cerebellum.

A general functional model of the cerebellum is proposed based on recent anatomical physiological findings. A cerebellar microcomplex composed of a cortical microzone and a small cell group in a cerebellar or vestibular nucleus acts as an adaptive controller based on the synaptic plasticity of the long-term depression type in Purkinje cells, which is effected by control error signals of climbing fibers. Microcomplexes will be inserted in reflex arcs, command systems of voluntary motor control, and probably even cortical systems performing certain mental activities, providing adaptive-learning capabilities to these systems.

Cerebellum↗
PubMed1995

Finding information in the medical literature.

There is such an enormous amount of information available to ophthalmic nurses, it is often intimidating to find the single item of information needed. Informal, word-of-mouth information sources are convenient but may also be incorrect or incomplete. There are a variety of formal information sources: dictionaries, encyclopedias, handbooks, directories, textbooks, annuals, and journals articles. Journal articles are probably the most important literature in healthcare because of their currency. Learning to use the medical library effectively and using the services of a medical librarian can help one take advantage of this vast amount of information.

Abstracting and Indexing↗
PubMed2005

A trained spin-glass model for grouping of image primitives.

A method is presented that uses grouping to improve local classification of image primitives. The grouping process is based upon a spin-glass system, where the image primitives are treated as possessing a spin. The system is subject to an energy functional consisting of a local and a bilocal part, allowing interaction between the image primitives. Instead of defining the state of lowest energy as the grouping result, the mean state of the system is taken. In this way, instabilities caused by multiple minima in the energy are being avoided. The means of the spins are taken as the a posteriori probabilities for the grouping result. In the paper, it is shown how the energy functional can be learned from example data. The energy functional is defined in such a way that, in case of no interactions between the elements, the means of the spins equal the a priori local probabilities. The grouping process enables the fusion of the a priori local and bilocal probabilities into the a posteriori probabilities. The method is illustrated both on grouping of line elements in synthetic images and on vessel detection in retinal fundus images.

Algorithms↗
PubMed2004

Development, direction, and damage limitation: social learning in domestic fowl.

This review highlights two areas of particular interest in the study of social learning in fowl. First, the role of social learning in the development of feeding and foraging behavior in young chicks and older birds is described. The role of the hen as a demonstrator and possible teacher is considered, and the subsequent social influence of brood mates and other companions on food avoidance and food preference learning is discussed. Second, the way in which work on domestic fowl has contributed to an understanding of the importance of directed social learning is examined. The well-characterized hierarchical social organization of small chicken flocks has been used to design studies which demonstrate that the probability of social transmission is strongly influenced by social relationships between birds. The practical implications of understanding the role of social learning in the spread of injurious behaviors in this economically important species are briefly considered.

Adaptation, Psychological↗
PubMed2006

Prediction error as a linear function of reward probability is coded in human nucleus accumbens.

Reward probability has been shown to be coded by dopamine neurons in monkeys. Phasic neuronal activation not only increased linearly with reward probability upon expectation of reward, but also varied monotonically across the range of probabilities upon omission or receipt of rewards, therefore modeling discrepancies between expected and received rewards. Such a discrete coding of prediction error has been suggested to be one of the basic principles of learning. We used functional magnetic resonance imaging (fMRI) to show that the human dopamine system codes reward probability and prediction error in a similar way. We used a simple delayed incentive task with a discrete range of reward probabilities from 0%-100%. Activity in the nucleus accumbens of human subjects strongly resembled the phasic responses found in monkey neurons. First, during the expectation period of the task, the fMRI signal in the human nucleus accumbens (NAc) increased linearly with the probability of the reward. Second, during the outcome phase, activity in the NAc coded the prediction error as a linear function of reward probabilities. Third, we found that the Nac signal was correlated with individual differences in sensation seeking and novelty seeking, indicating a link between individual fMRI activation of the dopamine system in a probabilistic paradigm and personality traits previously suggested to be linked with reward processing. We therefore identify two different covariates that model activity in the Nac: specific properties of a psychological task and individual character traits.

Adult↗
PubMed2005

On adaptation, maximization, and reinforcement learning among cognitive strategies.

Analysis of binary choice behavior in iterated tasks with immediate feedback reveals robust deviations from maximization that can be described as indications of 3 effects: (a) a payoff variability effect, in which high payoff variability seems to move choice behavior toward random choice; (b) underweighting of rare events, in which alternatives that yield the best payoffs most of the time are attractive even when they are associated with a lower expected return; and (c) loss aversion, in which alternatives that minimize the probability of losses can be more attractive than those that maximize expected payoffs. The results are closer to probability matching than to maximization. Best approximation is provided with a model of reinforcement learning among cognitive strategies (RELACS). This model captures the 3 deviations, the learning curves, and the effect of information on uncertainty avoidance. It outperforms other models in fitting the data and in predicting behavior in other experiments.

Adaptation, Psychological↗
PubMed2002

Error analysis of the nine-word California Verbal Learning Test (CVLT-9) among older adults with and without dementia.

The nine-word California Verbal Learning Test (CVLT-9; Libon et al., 1996; Spreen & Strauss, 1998) is a verbal list learning task used to assess declarative memory impairment among dementia patients. The present study sought to investigate the neuro-cognitive mechanisms that underlie the production of intrusions and perseverations on the list A, free recall learning trials, and the false positive responses made on the delayed recognition condition. Patients with probable Alzheimer's disease (AD), Ischaemic Vascular Dementia associated with periventricular and deep white matter changes (IVD), and individuals without dementia (NC) were studied. Between-group analyses showed that AD participants produced more initial intrusion errors, and perseverated on those same intrusion errors across list A learning trials than IVD or NC participants. Also, as participants with dementia produced initial free recall intrusion errors, the semantic organization of their responses on the 'animal' word list generation task declined (Giovannetti-Carew, Lamar, Cloud, Grossman, & Libon, 1997). On the delayed recognition test condition, within-group analyses revealed that the IVD group endorsed more list B interference foils, than other errors. AD participants endorsed semantically related foils and list B interference foils. In addition, as participants with dementia endorsed more list B interference foils, more perseverations were produced on the Graphical Sequence Test - Dementia Version (Lamar et al., 1997). These results were interpreted within the context of the semantic knowledge, and executive functions deficits that typify AD and IVD, respectively.

Aged↗
PubMed1997

Probabilistic independence networks for hidden Markov probability models.

Graphical techniques for modeling the dependencies of random variables have been explored in a variety of different areas, including statistics, statistical physics, artificial intelligence, speech recognition, image processing, and genetics. Formalisms for manipulating these models have been developed relatively independently in these research communities. In this paper we explore hidden Markov models (HMMs) and related structures within the general framework of probabilistic independence networks (PINs). The paper presents a self-contained review of the basic principles of PINs. It is shown that the well-known forward-backward (F-B) and Viterbi algorithms for HMMs are special cases of more general inference algorithms for arbitrary PINs. Furthermore, the existence of inference and estimation algorithms for more general graphical models provides a set of analysis tools for HMM practitioners who wish to explore a richer class of HMM structures. Examples of relatively complex models to handle sensor fusion and coarticulation in speech recognition are introduced and treated within the graphical model framework to illustrate the advantages of the general approach.

Algorithms↗
PubMed2005

Relevance vector machine and support vector machine classifier analysis of scanning laser polarimetry retinal nerve fiber layer measurements.

PURPOSE: To classify healthy and glaucomatous eyes using relevance vector machine (RVM) and support vector machine (SVM) learning classifiers trained on retinal nerve fiber layer (RNFL) thickness measurements obtained by scanning laser polarimetry (SLP). METHODS: Seventy-two eyes of 72 healthy control subjects (average age = 64.3 +/- 8.8 years, visual field mean deviation = -0.71 +/- 1.2 dB) and 92 eyes of 92 patients with glaucoma (average age = 66.9 +/- 8.9 years, visual field mean deviation = -5.32 +/- 4.0 dB) were imaged with SLP with variable corneal compensation (GDx VCC; Laser Diagnostic Technologies, San Diego, CA). RVM and SVM learning classifiers were trained and tested on SLP-determined RNFL thickness measurements from 14 standard parameters and 64 sectors (approximately 5.6 degrees each) obtained in the circumpapillary area under the instrument-defined measurement ellipse (total 78 parameters). Ten-fold cross-validation was used to train and test RVM and SVM classifiers on unique subsets of the full 164-eye data set and areas under the receiver operating characteristic (AUROC) curve for the classification of eyes in the test set were generated. AUROC curve results from RVM and SVM were compared to those for 14 SLP software-generated global and regional RNFL thickness parameters. Also reported was the AUROC curve for the GDx VCC software-generated nerve fiber indicator (NFI). RESULTS: The AUROC curves for RVM and SVM were 0.90 and 0.91, respectively, and increased to 0.93 and 0.94 when the training sets were optimized with sequential forward and backward selection (resulting in reduced dimensional data sets). AUROC curves for optimized RVM and SVM were significantly larger than those for all individual SLP parameters. The AUROC curve for the NFI was 0.87. CONCLUSIONS: Results from RVM and SVM trained on SLP RNFL thickness measurements are similar and provide accurate classification of glaucomatous and healthy eyes. RVM may be preferable to SVM, because it provides a Bayesian-derived probability of glaucoma as an output. These results suggest that these machine learning classifiers show good potential for glaucoma diagnosis.

Aged↗
PubMed1962

All-or-none versus incremental learning of errorless shock escapes by the rat.

Investigation showed that in a two-choice correction procedure the probability of an errorless response in those rats which have consistently made errors is not constant, as required by the all-or-none model, but increases with trials. Even an all-or-none model which provides for accidental error and accidental success is inadequate.

Animals↗
PubMed1998

Physiological, pathophysiological and therapeutic roles of 5-HT systems in learning and memory.

Multiple 5-hydroxytryptamine (5-HT) receptors have been identified (5-HT1A/1B/1D/1E/1F, 5-HT2A/2B/2C, 5-HT3A/3B, 5-HT4A/4B, 5-HT5A/5B, 5-HT6 and 5-HT7A/7B/7C/7D) and extensive evidence suggests that 5-HT receptors have a role in learning and memory. Indeed, available evidence strongly supports physiological, pathophysiological and therapeutic roles of 5-HT systems in cognitive processes, although the evidence seems incomplete. Indeed, there has been a clear tendency to use pre-learning administration most frequently, whereas post-learning and pre-retention administration protocols have been utilized in only a few studies, and probably this trend has led to missed relevant information. For instance, when pre- vs post-training administration of 5-HT1A agonist, 5-HT2 antagonists and 5-HT4 agonists have been compared contrasting findings were reported in aversive and appetitive learning tasks. Emerging evidence also indicates that 5-HT1A and 5-HT4 receptor agonists, as well as, 5-HT1A antagonists, 5-HT2 antagonists, 5-HT3 antagonists and 5-HT uptake inhibitors may have therapeutic utility in the treatment of Alzheimer's disease and amnesia. Inasmuch as the activation or blockade of diverse 5-HT receptors is able to modulate cognitive processes, and 5-HT uptake inhibition could have therapeutic applications in the treatment of cognitive disorders, it seems evident that the role of 5-HT in learning and memory is more complex than a simple imbalance. Consequently, the notion that activation of the 5-HT systems impairs performance, whereas reduced serotonergic function may facilitate learning, must be reconsidered.

Animals↗