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At least 433 records · Page 24Linked to original sources
PubMed1995

Mammalian rod terminal: architecture of a binary synapse.

The mammalian rod synapse transmits a binary signal (one photon or none) using tonic, rapid exocytosis. We constructed a quantitative, physical model of the synapse. Presynaptically, a single, linear active zone provides docking sites for approximately 130 vesicles, and a "ribbon" anchored to the active zone provides a depot for approximately 640 vesicles. Postsynaptically, 4 processes invaginate the terminal: 2 (known to have low affinity glutamate receptors) lie near the active zone (16 nm), and 2 (known to have high affinity glutamate receptors) lie at a distance (130-640 nm). The presynaptic structure seems designed to minimize fluctuations in tonic rate owing to empty docking sites, whereas the postsynaptic geometry may permit 1 vesicle to evoke an all-or-none response at all 4 postsynaptic processes.

Animals↗
PubMed1996

The role of inhibition in the hierarchical gating of executed and imagined movements.

A theory is presented concerning the neuronal mechanisms which may underlie the organisation of imagined versus executed movements. A review is first presented of previous theoretical and experimental evidence suggesting that the brain can use the same mechanisms for the imagination and the execution of movement. In particular the fact that adaptation of the vestibulo-ocular reflex can be obtained by pure mental effort and not solely by conflicting visual and vestibular cues has been suggestive of the fact that the brain could internally simulate conflicts and use the same adaptive mechanisms used when actual sensory cues were in conflict. The saccadic system is taken as a good model for the study of this question because the mechanisms which underlie saccade generation are now partially understood at different levels from the brain stem to the cortex. The central idea of the theory is based upon the fact that, in parallel with the excitatory mechanisms underlying saccade generation, several inhibitory mechanisms in cascade allow the selective modulation and blockage of saccades. Synaptic inhibition is therefore supposed to play a major role in a hierarchical selective gating of saccade execution not at one but at several levels allowing a variety of different types of "imagined movements' some involving only the higher levels some in which the execution is only blocked at the very immediate premotor level. But in all cases the theory proposes that imagination and execution have many mechanisms in common. PET data showing that indeed the same structures are activated in both types of movements support this idea although the final answer will have to be brought by neuronal data.

Adaptation, Physiological↗
PubMed1994

High-specificity neurological localization using a connectionist model.

Most previous connectionist models for diagnosis have been developed using error backpropagation. While these systems function reasonably well, they have been limited by their need for a large database of test cases, to situations where a single disorder is present, and by the large number of connections required between fully-connected sets of processing units. Here we describe a recently developed connectionist model that overcomes these limitations. This approach can reuse existing causal knowledge bases, works well in situations where multiple disorders can occur simultaneously, and does not require fully-connected sets of processing units. We demonstrate that the accuracy of this model is comparable to that of more conventional AI programs using the same knowledge base in determining precisely the site of brain damage in a group of 50 stroke patients. These results support the conclusion that connectionist models can effectively use pre-existing causal knowledge bases from AI systems, and that they can function accurately when handling actual clinical problems.

Artificial Intelligence↗
PubMed1993

Learning-induced changes of auditory receptive fields.

Classical conditioning specifically modifies receptive fields in primary and secondary auditory cortical areas to favor the frequency of a tone signal over other frequencies, including tuning shifts toward, or to, this frequency. This plasticity of receptive fields is associative and highly specific, can develop very rapidly, can be expressed under anesthesia and can be maintained indefinitely. Muscarinic receptors in the cortex may be involved. Non-lemniscal thalamic nuclei also develop receptive field plasticity that may contribute to cortical plasticity.

Animals↗
PubMed1993

Visual pathways supporting perception and action in the primate cerebral cortex.

Behavioral and electrophysiological evidence suggests a new interpretation of the division of labor between the ventral and dorsal streams of visual processing in primate cerebral cortex. It is suggested that the ventral stream mediates the perception of objects while the dorsal stream mediates the on-line control of skilled actions directed at those objects.

Animals↗
PubMed1994

Learning and representation in speech and language.

Infants learn language with remarkable speed. By the end of their second year they speak in sentences with an 'accent' typical of a native speaker. How does an individual acquire a specific language? While acknowledging the biological preparation for language, this review focuses on the effects of early language experience on infants' perceptual and perceptual-motor systems. The data show that by the time infants begin to master the higher levels of language--sound-meaning correspondences, contrastive phonology, and grammatical rules--their perceptual and perceptual-motor systems are already tuned to a specific language. The consequences of this are described in a developmental theory at the phonetic level that holds promise for higher levels of language.

Humans↗
PubMed2004

Length effect in reading and lexical decision: evidence from skilled readers and a developmental dyslexic participant.

A number of experimental data have shown that naming latency increases with length for pseudo-words but not for frequent real words. Different interpretations have been proposed by current models of reading to account for such a length effect. The aim of the present study was to assess the impact of lexicality on length effect in both the reading and lexical decision tasks. For this purpose, skilled readers were asked to either name or make a lexical decision on words and pseudo-words differing in length from one to three syllables. Skilled readers' results show that length effect is modulated by lexicality in the reading task but no length effect was found in the lexical decision task. The tasks were further proposed to a well-compensated dyslexic participant who exhibited a visual attentional disorder in the absence of any associated phonological problems. A length effect on RTs was found for both words and pseudo-words in lexical decision but naming latencies were affected by length for the pseudo-words only. The present results largely conform to the predictions of the ACV98 model of reading. They are not compatible with the PDP models of reading and can only be partially accounted for by dual route models.

Adult↗
PubMed2006

Synaptic proteins associate with a sub-set of lipid rafts when isolated from nerve endings at physiological temperature.

Although the high presence of cholesterol in nerve terminals is well documented, specific roles of this lipid in transmitter release have remained elusive. Since cholesterol is a highly enriched component in the membrane microdomains known as lipid rafts, it is probable that these domains are very important in synaptic function. The extraction of lipid rafts using Brij 98 at 37 degrees C avoids the formation of nonspecific membrane aggregates at low temperature, allowing the isolation of more physiologically relevant lipid rafts. In the present work, we examine, by means of buoyancy analysis in sucrose gradients after solubilization of the membranes with Brij 98 or with Lubrol WX, the presence of proteins involved in exocytosis in detergent-resistant membranes (DRM) using rat brain synaptosomes as a neurological model. Significant proportions of the proteins tested in the present work, which are involved in neurotransmitter release, are found in Brij 98 raft fractions, demonstrating that significant pools of synaptic proteins are segregated in specific parts of the membrane at physiological temperature. On the other hand, Lubrol WX is unable to solubilize the major fraction of the proteins tested. Treatment of synaptosomes with methyl-beta-cyclodextrin (mbetaCD) causes alteration in the buoyancy properties of proteins initially present in Brij- as well as in Lubrol-resistant membranes, indicating the cholesterol-dependency of both kinds of microdomains. Finally, we detect the depolarization-induced enhancement of the cholesterol-dependent association of syntaxin 1 with Brij 98-rafts, under the same conditions in which prolonged neurotransmitter release is stimulated.

Animals↗
PubMed2004

Depression: a case of neuronal life and death?

Preclinical and clinical studies have demonstrated that stress or depression can lead to atrophy and cell loss in limbic brain structures that are critically involved in depression, including the hippocampus. Studies in experimental animals demonstrate that decreased birth of new neurons in adult hippocampus could contribute to this atrophy. In contrast, antidepressant treatment increases neurogenesis in the hippocampus of adult animals and blocks the effects of stress. Moreover, blockade of hippocampal neurogenesis blocks the actions of antidepressants in behavioral models of depression, demonstrating a direct link between behavior and new cell birth. This perspective reviews the literature in support of the hypothesis that altered birth of new neurons in the adult brain contributes to the etiology and treatment of depression and considers research strategies to test this hypothesis.

Animals↗
PubMed2004

A parameter control method in reinforcement learning to rapidly follow unexpected environmental changes.

In order to rapidly follow unexpected environmental changes, we propose a parameter control method in reinforcement learning that changes each of learning parameters in appropriate directions. We determine each appropriate direction on the basis of relationships between behaviors and neuromodulators by considering an emergency as a key word. Computer experiments show that the agents using our proposed method could rapidly respond to unexpected environmental changes, not depending on either two reinforcement learning algorithms (Q-learning and actor-critic (AC) architecture) or two learning problems (discontinuous and continuous state-action problems).

Adaptation, Physiological↗
PubMed2004

Spike trains in a stochastic Hodgkin-Huxley system.

We consider a standard Hodgkin-Huxley model neuron with a Gaussian white noise input current with drift parameter mu and variance parameter sigma(2). Partial differential equations of second order are obtained for the first two moments of the time taken to spike from (any) initial state, as functions of the initial values. The analytical theory for a 2-component (V,m) approximation is also considered. Let mu(c) (approximately 4.15) be the critical value of mu for firing when noise is absent. Large sample simulation results are obtained for mu mu(c), for many values of sigma between 0 and 25. For the time to spike, the 2-component approximation is accurate for all sigma when mu=10, for sigma>7 when mu=5 and only when sigma>15 when mu=2. When mu >mu(c), most paths show similar behavior and the moments exhibit smoothly changing behavior as sigma increases. Thus there are a different number of regimes depending on the magnitude of mu relative to mu(c): one when mu is small and when mu is large; but three when mu is close to and above mu(c). Both for the Hodgkin-Huxley (HH) system and the 2-component approximation, and regardless of the value of mu, the CV tends to about 1.3 at the largest value (25) of sigma considered. We also discuss in detail the problem of determining the interspike interval and give an accurate method for estimating this random variable by decomposing the interval into stochastic and almost deterministic components.

Action Potentials↗
PubMed2005

Indispensable role of quantum theory in the brain dynamics.

Recently, Tegmark pointed out that the superposition of ion states involved in the superposition of firing and resting states of a neuron quickly decohere. It undoubtedly indicates that neural networks cannot work as quantum computers, or computers taking advantage of coherent states. Does it also mean that the brain can be modeled as a neural network obeying classical physics? Here we show that it does not mean that the brain can be modeled as a neural network obeying classical physics. A brand new perspective in research of neural networks from quantum theoretical aspect is presented.

Animals↗
PubMed2005

Does consciousness really collapse the wave function? A possible objective biophysical resolution of the measurement problem.

An analysis has been performed of the theories and postulates advanced by von Neumann, London and Bauer, and Wigner, concerning the role that consciousness might play in the collapse of the wave function, which has become known as the measurement problem. This reveals that an error may have been made by them in the area of biology and its interface with quantum mechanics when they called for the reduction of any superposition states in the brain through the mind or consciousness. Many years later Wigner changed his mind to reflect a simpler and more realistic objective position which appears to offer a way to resolve this issue. The argument is therefore made that the wave function of any superposed photon state or states is always objectively and stochastically changed within the complex architecture of the eye in a continuous linear process initially for most of the superposed photons, followed by a discontinuous nonlinear collapse process later for any remaining superposed photons, thereby guaranteeing that only final, measured information is presented to the brain, mind or consciousness. An experiment to be conducted in the near future may enable us to simultaneously resolve the measurement problem and also determine if the linear nature of quantum mechanics is violated by the perceptual process.

Animals↗
PubMed2005

Bio-computational model of object-recognition: quantum Hebbian processing with neurally shaped Gabor wavelets.

Theoretical and simulational evidence, as well as experimental indications, are accumulating that quantum associative memory and imaging are possible. We compare these data with biological evidence, since we find them to a significant extent compatible. This paper presents a computationally implementable integrative model of appearance-based viewpoint-invariant recognition of objects. The neuro-quantum hybrid model incorporates neural processing up to V1 and quantum associative processing in V1, achieving together an object-recognition result in V2 and ITC. Results of our simulation of the central quantum-like parts of the bio-model, receiving neurally pre-processed inputs, are presented. This part contains our original simulated storage by multiple quantum interference of image-encoding Gabor wavelets done in a Hebbian way, especially using the Griniasty et al. pose-sequence learning rule.

Humans↗
PubMed2006

When language breaks into pieces. A conflict between communication through isolated signals and language.

Here, we study a communication model where signals associate to stimuli. The model assumes that signals follow Zipf's law and the exponent of the law depends on a balance between maximizing the information transfer and saving the cost of signal use. We study the effect of tuning that balance on the structure of signal-stimulus associations. The model starts from two recent results. First, the exponent grows as the weight of information transfer increases. Second, a rudimentary form of language is obtained when the network of signal-stimulus associations is almost connected. Here, we show the existence of a sudden destruction of language once a critical balance is crossed. The model shows that maximizing the information transfer through isolated signals and language are in conflict. The model proposes a strong reason for not finding large exponents in complex communication systems: language is in danger. Besides, the findings suggest that human words may need to be ambiguous to keep language alive. Interestingly, the model predicts that large exponents should be associated to decreased synaptic density. It is not surprising that the largest exponents correspond to schizophrenic patients since, according to the spirit of Feinberg's hypothesis, i.e. decreased synaptic density may lead to schizophrenia. Our findings suggest that the exponent of Zipf's law is intimately related to language and that it could be used to detect anomalous structure and organization of the brain.

Animals↗
PubMed2006

Where and how could intentional programs be generated in the brain? A hypothetical model based on glial-neuronal interactions.

Based on glial-neuronal interaction a formalism (negative language) for the generation of intentional programs is proposed. An intentional program generates a specific multirelational structure in an inner or outer appropriate environment according to the principle of feasibility. After description of the glial spatio-temporal boundary-setting function in its interaction with the neuronal system, it is hypothesized that intentional programs may be generated in glial networks (syncytia) in line with the formalism of negative language. Gap junctions are interpreted as multirelational negation operators, generating cycles in a permutation system. These cycles could represent intentional programs that can either be realized or not in neuronal networks embodying a permutation system. The feasibility of these intentional programs is essentially dependent on appropriate environmental information. Since the realization of intentional programs in neuronal networks allows high degrees of freedom, the problem of free will is tackled, as well. Free will is defined as the subjective freedom to choose between the inner determination of intentional programs and the overdetermination of their feasibility in an appropriate environment. Finally, the possible implementation of the proposed brain model in robot brains is briefly discussed.

Cell Communication↗
PubMed2006

Small universal spiking neural P systems.

In search for small universal computing devices of various types, we consider here the case of spiking neural P systems (SN P systems), in two variants: as devices that compute functions and as devices that generate sets of numbers. We start with the first case and we produce a universal spiking neural P system with 84 neurons. If a slight generalization of the used rules is adopted, namely, we allow rules for producing simultaneously several spikes, then a considerable reduction, to 49 neurons, is obtained. For SN P systems used as generators of sets of numbers, we find a universal system with restricted rules having 76 neurons and one with extended rules having 50 neurons.

Algorithms↗