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At least 469 records · Page 26Linked to original sources
PubMed2004

[Molecular mechanisms of neuronal connective tissue genesis during the course of cerebral cortex development].

The cerebral cortex constitutes one of the most complex structures in our brain. In correlation with its elaborate functions, it is characterized by the great complexity of its neuronal connections, but the mechanisms responsible for the generation of these connections remain poorly known. We have recently initiated the characterization of a new multigenic family of axon guidance factors, the ephrin/Eph gene family, during the development of neuronal connections in the mouse cortex. Combining expression studies, in vitro guidance essays, and in vivo analysis of mutant mice, enabled us to demonstrate the critical role of ephrin/Eph genes in the development of cortical networks. Mutant mice for ephrin/Eph genes display a topographic distortion of their cortical somatosensory map, as well as ectopic projections from the motor thalamus to the somatosensory cortex. The identification of factors like ephrins, capable to (re)specify the pattern of neuronal connections, has implications for our understanding of pathological brain development (epilepsy, abnormal movements, psychiatric diseases), and in the perspective of the rational design of cell therapies of neurodegenerative diseases.

Animals↗
PubMedDate not supplied

[Functional reorganizations of the neuronal structures of the cat associative cortex after total sectioning of the corpus callosum].

In given work results of researches functional organization of neural networks at associative cortex level in intact and after total corpus callosum section (Cc sec) of cat's brain. In result of the given researches is established, that after total Cc sec the brain is exposed significant morphological and functional changes. Is shown, that Cc sec is accompanied by infringements in act of reception of food and head orientation of the animal in space. A configuration and characteristics varies conducting ways, opportunity of an exchange the information between is lost by hemispheres, and accordingly, to receive the information from ipsilateral hemifield of vision. The structural and functional characteristics vary of neural structures of both hemispheres: asymmetry of hemispheres in size disappears, orientational and directional characteristics and decreases number of subzones in receptive fields (RF). In RF of this area occur extensive inhibitory zones. Arrangements RF in vision field main borrows contralateral hemifield of vision, in intact a brain large part RF pass in ipsilateral hemifield of vision. Thus in result of these researches is established, that the majority of neural functions of left and right hemispheres at level associative cortex, revealed on intact brain are formed for account transcallosal of connections. In conditions of isolated functioning both hemispheres there are the changes as in ways of the information processing, and in complementary ways of display of space. In result callosotomy of neuron networks in both hemispheres undergo essential changes: the hemispheres lose ability to integrate the set of the parties of subjects, disturbed an opportunity of invariant the descriptions of images, hence the description of the space information occurs on separate attributes of this information.

Animals↗
PubMed2005

The plastic human brain cortex.

Plasticity is an intrinsic property of the human brain and represents evolution's invention to enable the nervous system to escape the restrictions of its own genome and thus adapt to environmental pressures, physiologic changes, and experiences. Dynamic shifts in the strength of preexisting connections across distributed neural networks, changes in task-related cortico-cortical and cortico-subcortical coherence and modifications of the mapping between behavior and neural activity take place in response to changes in afferent input or efferent demand. Such rapid, ongoing changes may be followed by the establishment of new connections through dendritic growth and arborization. However, they harbor the danger that the evolving pattern of neural activation may in itself lead to abnormal behavior. Plasticity is the mechanism for development and learning, as much as a cause of pathology. The challenge we face is to learn enough about the mechanisms of plasticity to modulate them to achieve the best behavioral outcome for a given subject.

Blindness↗
PubMed2004

"Dynamic" connectivity in neural systems: theoretical and empirical considerations.

The study of functional interdependences between brain regions is a rapidly growing focus of neuroscience research. This endeavor has been greatly facilitated by the appearance of a number of innovative methodologies for the examination of neurophysiological and neuroimaging data. The aim of this article is to present an overview of dynamical measures of interdependence and contrast these with statistical measures that have been more widely employed. We first review the motivation, conceptual basis, and experimental approach of dynamical measures of interdependence and their application to the study of neural systems. A consideration of boot-strap "surrogate data" techniques, which facilitate hypothesis testing of dynamical measures, is then used to clarify the difference between dynamical and statistical measures of interdependence. An overview of some of the most active research areas such as the study of the "synchronization manifold," dynamical interdependence in neurophysiology data and the putative role of nonlinear desynchronization is then given. We conclude by suggesting that techniques based on dynamical interdependence--or "dynamical connectivity"--show significant potential for extracting meaningful information from functional neuroimaging data.

Brain↗
PubMed2005

Visuo-motor pathways in humans revealed by event-related fMRI.

Whether different brain networks are involved in generating unimanual responses to a simple visual stimulus presented in the ipsilateral versus contralateral hemifield remains a controversial issue. Visuo-motor routing was investigated with event-related functional magnetic resonance imaging (fMRI) using the Poffenberger reaction time task. A 2 hemifield x 2 response hand design generated the "crossed" and "uncrossed" conditions, describing the spatial relation between these factors. Both conditions, with responses executed by the left or right hand, showed a similar spatial pattern of activated areas, including striate and extrastriate areas bilaterally, SMA, and M1 contralateral to the responding hand. These results demonstrated that visual information is processed bilaterally in striate and extrastriate visual areas, even in the "uncrossed" condition. Additional analyses based on sorting data according to subjects' reaction times revealed differential crossed versus uncrossed activity only for the slowest trials, with response strength in infero-temporal cortices significantly correlating with crossed-uncrossed differences (CUD) in reaction times. Collectively, the data favor a parallel, distributed model of brain activation. The presence of interhemispheric interactions and its consequent bilateral activity is not determined by the crossed anatomic projections of the primary visual and motor pathways. Distinct visuo-motor networks need not be engaged to mediate behavioral responses for the crossed visual field/response hand condition. While anatomical connectivity heavily influences the spatial pattern of activated visuo-motor pathways, behavioral and functional parameters appear to also affect the strength and dynamics of responses within these pathways.

Adult↗
PubMed2003

Network capacity analysis for latent attractor computation.

Attractor networks have been one of the most successful paradigms in neural computation, and have been used as models of computation in the nervous system. Recently, we proposed a paradigm called 'latent attractors' where attractors embedded in a recurrent network via Hebbian learning are used to channel network response to external input rather than becoming manifest themselves. This allows the network to generate context-sensitive internal codes in complex situations. Latent attractors are particularly helpful in explaining computations within the hippocampus--a brain region of fundamental significance for memory and spatial learning. Latent attractor networks are a special case of associative memory networks. The model studied here consists of a two-layer recurrent network with attractors stored in the recurrent connections using a clipped Hebbian learning rule. The firing in both layers is competitive--K winners take all firing. The number of neurons allowed to fire, K, is smaller than the size of the active set of the stored attractors. The performance of latent attractor networks depends on the number of such attractors that a network can sustain. In this paper, we use signal-to-noise methods developed for standard associative memory networks to do a theoretical and computational analysis of the capacity and dynamics of latent attractor networks. This is an important first step in making latent attractors a viable tool in the repertoire of neural computation. The method developed here leads to numerical estimates of capacity limits and dynamics of latent attractor networks. The technique represents a general approach to analyse standard associative memory networks with competitive firing. The theoretical analysis is based on estimates of the dendritic sum distributions using Gaussian approximation. Because of the competitive firing property, the capacity results are estimated only numerically by iteratively computing the probability of erroneous firings. The analysis contains two cases: the simple case analysis which accounts for the correlations between weights due to shared patterns and the detailed case analysis which includes also the temporal correlations between the network's present and previous state. The latter case predicts better the dynamics of the network state for non-zero initial spurious firing. The theoretical analysis also shows the influence of the main parameters of the model on the storage capacity.

Animals↗
PubMed2001

Electrical activity and development of neural circuits.

A distinct feature of the nervous system is the intricate network of synaptic connections among neurons of diverse phenotypes. Although initial connections are formed largely through molecular mechanisms that depend on intrinsic developmental programs, spontaneous and experience-driven electrical activities in the developing brain exert critical epigenetic influence on synaptic maturation and refinement of neural circuits. Selective findings discussed here illustrate some of our current understanding of the effects of electrical activity on circuit development and highlight areas that await further study.

Action Potentials↗
PubMed2003

Three-dimensional structure and survival of newly formed blood vessels after focal cerebral ischemia.

Penumbra tissue becomes highly angiogenic after ischaemic stroke in man, and the re-establishment of a functional vasculature might be beneficial to patients. Unilateral ischaemia was induced in male Sprague-Dawley rats by permanent occlusion of the distal left middle cerebral artery (MCAO). Animals with stroke were kept alive for 1, 7, 14, 21 or 28 days after which time they were terminally anaesthetized. Vascular casts of infarcted areas, analyzed by scanning electron microscopy demonstrated that radially arranged neocortical arterioles and venules lost their regular patterns within one day of occlusion, and soon afterwards started to form a very dense network of anastomosing microvessels. At 1 week, vascular budding was visible at many sites. The smallest microvessels (4-10 microm) formed connections with the surrounding proliferating vessels similar to those in the normal brain. Survival of microvascular endothelial cells (ECs) was studied by double labeling of tissue sections using immunohistochemistry and antibodies to caspase-3, and TUNEL staining for apoptotic cells. ECs demonstrated intensive staining for caspase-3 and also staining by TUNEL, particularly near the infarct border, 14 days post-MCAO. These data support the hypothesis that growing blood vessels in ischaemic tissue form new connections, the pattern of which is similar to that in normal rat brain, but different to those formed in growing tumours. This normal growth pattern might be essential in future therapies involving induction of vascularization and neuroprotection to enhance long-term survival of the penumbra.

Animals↗
PubMed1998

Changing central nervous system control following intercostal nerve transfer.

OBJECT: The goal of this study was to find which central nervous system (CNS) pathways are involved in volitional control over reinnervated biceps or pectoral muscles. METHODS: Intercostal nerves (ICNs) were coapted to the musculocutaneous nerve (MCN) or the medial pectoral nerve (MPN) in 23 patients with root avulsions of the brachial plexus to restore biceps or pectoral muscle function. The facilitatory effects of respiration and voluntary contraction on cortical motor-evoked potentials of biceps or pectoral muscles were used to study CNS control over the reinnervated muscles. The time course of the facilitatory effect of respiration and voluntary contraction differed significantly. In the end stage of nerve regeneration, the facilitatory effect of voluntary contraction was significantly larger than that of respiration, indicating that the CNS control network over the muscle comes to resemble that of the recipient nerve (MCN or MPN) rather than that of the donor nerve (ICN). CONCLUSIONS: The strengthening of previously subthreshold synaptic connections in a CNS network connecting ICN to MCN or MPN neurons may underlie changing excitability.

Adult↗
PubMed2004

Simulated apoptosis/neurogenesis regulates learning and memory capabilities of adaptive neural networks.

Characterization of neuronal death and neurogenesis in the adult brain of birds, humans, and other mammals raises the possibility that neuronal turnover represents a special form of neuroplasticity associated with stress responses, cognition, and the pathophysiology and treatment of psychiatric disorders. Multilayer neural network models capable of learning alphabetic character representations via incremental synaptic connection strength changes were used to assess additional learning and memory effects incurred by simulation of coordinated apoptotic and neurogenic events in the middle layer. Using a consistent incremental learning capability across all neurons and experimental conditions, increasing the number of middle layer neurons undergoing turnover increased network learning capacity for new information, and increased forgetting of old information. Simulations also showed that specific patterns of neural turnover based on individual neuronal connection characteristics, or the temporal-spatial pattern of neurons chosen for turnover during new learning impacts new learning performance. These simulations predict that apoptotic and neurogenic events could act together to produce specific learning and memory effects beyond those provided by ongoing mechanisms of connection plasticity in neuronal populations. Regulation of rates as well as patterns of neuronal turnover may serve an important function in tuning the informatic properties of plastic networks according to novel informational demands. Analogous regulation in the hippocampus may provide for adaptive cognitive and emotional responses to novel and stressful contexts, or operate suboptimally as a basis for psychiatric disorders. The implications of these elementary simulations for future biological and neural modeling research on apoptosis and neurogenesis are discussed.

Animals↗
PubMed1996

Computational study on the neural mechanism of sequential pattern memory.

The brain stores various kinds of temporal sequences as long-term memories, such as motor sequences, episodes, and melodies. The present study aims at clarifying the general principle underlying such memories. For this purpose, the memory mechanism of sequential patterns is examined from the viewpoint of computational theory and neural network modeling, and a neural network model of sequential pattern memory based on a simple and reasonable principle is presented. Specifically, spatio-temporal patterns varying gradually with time are stably stored in a network consisting of pairs of excitatory and inhibitory cells with recurrent connections; such a pair can achieve non-monotonic input-output characteristics which are essential for smooth sequential recall. Storage is performed using a simple learning algorithm which is based on the covariance rule and requires only that the sequence be input several times and retrieval is highly tolerant to noise. It is thought that a similar principle is used in cerebral memory systems, and the relevance of this model to the brain is discussed. Also, possible roles of hippocampus and basal ganglia in memorizing sequences are suggested.

Algorithms↗
PubMed2004

Brain network interactions in auditory, visual and linguistic processing.

In the paper, we discuss the importance of network interactions between brain regions in mediating performance of sensorimotor and cognitive tasks, including those associated with language processing. Functional neuroimaging, especially PET and fMRI, provide data that are obtained essentially simultaneously from much of the brain, and thus are ideal for enabling one to assess interregional functional interactions. Two ways to use these types of data to assess network interactions are presented. First, using PET, we demonstrate that anterior and posterior perisylvian language areas have stronger functional connectivity during spontaneous narrative production than during other less linguistically demanding production tasks. Second, we show how one can use large-scale neural network modeling to relate neural activity to the hemodynamically-based data generated by fMRI and PET. We review two versions of a model of object processing - one for visual and one for auditory objects. The regions comprising the models include primary and secondary sensory cortex, association cortex in the temporal lobe, and prefrontal cortex. Each model incorporates specific assumptions about how neurons in each of these areas function, and how neurons in the different areas are interconnected with each other. Each model is able to perform a delayed match-to-sample task for simple objects (simple shapes for the visual model; tonal contours for the auditory model). We find that the simulated electrical activities in each region are similar to those observed in nonhuman primates performing analogous tasks, and the absolute values of the simulated integrated synaptic activity in each brain region match human fMRI/PET data. Thus, this type of modeling provides a way to understand the neural bases for the sensorimotor and cognitive tasks of interest.

Animals↗
PubMedDate not supplied

Brain size and number of neurons: an exercise in synthetic neuroanatomy.

Certain remarkable invariances have long been known in comparative neuroanatomy, such as the proportionality between neuronal density and the inverse of the cubic root of brain volume or that between the square root of brain weight and the cubic root of body weight. Very likely these quantitative relations reflect some general principles of the architecture of neuronal networks. Under the assumption that most of brain volume is due to fibers, we propose four abstract models: I, constant fiber length per neuron; II, fiber length proportionate to brain diameter; III, complete set of connections between all neurons; IV, complete set of connections between compartments each containing the square root of the total number of neurons. Model I conforms well to the cerebellar cortex. Model II yields the observed comparative invariances between number of neurons and brain size. Model III is totally unrealistic, while Model IV is compatible with the volume of the hemispheric white substance in different mammalian species.

Animals↗
PubMed1994

Location and anatomical connections of a paradoxical sleep induction site in the cat ventral pontine tegmentum.

The brainstem mechanisms for the generation of paradoxical sleep are under considerable debate. Previous experiments in cats have demonstrated that injections of the cholinergic agonist carbachol into the oral pontine tegmentum elicit paradoxical sleep behaviour and its polygraphic correlates. The different results on the pontine structures that mediate this effect do not agree. We report here that limited microinjections of a carbachol solution into the ventral part of the oral pontine reticular nucleus in the cat induce, with a short latency, a dramatic, long-lasting increase in paradoxical sleep. Moreover, neuronal tracing experiments show that this pontine site is connected with brain structures responsible for the different bioelectric events of paradoxical sleep. These two facts suggest that the ventral part of the oral pontine reticular nucleus is a nodal link in the neuronal network underlying paradoxical sleep mechanisms.

Animals↗
PubMed2004

Specific GABAA circuits for visual cortical plasticity.

Weak inhibition within visual cortex early in life prevents experience-dependent plasticity. Loss of responsiveness to an eye deprived of vision can be initiated prematurely by enhancing gamma-aminobutyric acid (GABA)-mediated transmission with benzodiazepines. Here, we use a mouse "knockin" mutation to alpha subunits that renders individual GABA type A (GABA(A)) receptors insensitive to diazepam to show that a particular inhibitory network controls expression of the critical period. Only alpha1-containing circuits were found to drive cortical plasticity, whereas alpha2-enriched connections separately regulated neuronal firing. This dissociation carries implications for models of brain development and the safe design of benzodiazepines for use in infants.

Animals↗
PubMed2000

[Aphasia research and speech localization in the brain].

Aphasia research has become an acknowledged branch of modern cognitive neuropsychology research whose aim is to explore more fully the structures of knowledge and of cerebral processes which might both be affected in patients with aphasia. Up to the second half of this century, a model based on a specific cerebral localisation of language processes had emerged based on brain localisation research by Broca and Wernicke (among others). New modern neuroimaging techniques, however, such as computed tomography and magnetic resonance imaging (MRI), but also functional imaging modalities such as positron emission tomography or functional MRI, have modified these concepts. It emerges that in comprehension as well as in production of language, not only a few well defined centres are responsible for the activity, but there is a synchronised activity in large neuronal networks connecting various regions located both in the cortex and in the deep subcortical structures; today, this activity can be demonstrated best in a non-invasive and reproducible way with functional MRI.

Aphasia↗
PubMed1997

Identification of rat brainstem multisynaptic connections to the oral motor nuclei using pseudorabies virus. III. Lingual muscle motor systems.

The present experiments complete our investigations of higher order afferent control of the orofacial muscles by examining the premotor systems controlling the lingual musculature. Pseudorabies virus (PRV) was injected into the extrinsic (protruders: genioglossus and geniohyoid; retractors: hyoglossus and styloglossus) and intrinsic tongue muscles in bilaterally sympathectomized rats. Injection volumes ranged from 1 to 12 microl with average titers of 4 x 10(8) pfu/ml and maximum survival times of 90 h. Consistent labeling patterns and distributions occurred across each of the individual muscles and between extrinsic and intrinsic muscle groups, as well as in comparison to the results from the previous masticatory and facial muscle experiments. Virus injections produced a predictable myotopic labeling pattern in the hypoglossal nucleus (Mo 12). Transneuronally labeled neurons occurred in regions known to project directly to Mo 12 motoneurons including the nucleus subcoeruleus, trigeminal sensory areas, parvicellular reticular formation, and the dorsal medullary reticular fields. Maximum survival times revealed more distant connections from medial and lateral reticular zones including the periaqueductal gray, dorsal raphe, laterodorsal and pedunculopontine tegmental areas, and substantia nigra in the midbrain, the gigantocellular region, pontine nucleus caudalis and ventralis, and lateral paragigantocellular region in the pons, and the nucleus of the solitary tract, paratrigeminal region, and paramedian field in the medulla. Thus, injections of PRV into the orofacial muscles revealed a complex, but remarkably uniform network of multisynaptic connections in the brainstem that control and coordinate the activity of the masticatory, facial, and lingual muscles.

Animals↗
PubMed2002

Design and performance of the UCLH mark 1b 64 channel electrical impedance tomography (EIT) system, optimized for imaging brain function.

The UCLH Mark 1b is a portable EIT system that can address up to 64 electrodes, which has been designed for imaging brain function with scalp electrodes. It employs a single impedance-measuring circuit and multiplexer so that electrode combinations may be addressed flexibly using software. It operates in the relatively low frequency band between 225 Hz and 77 kHz, as lower frequencies produce larger changes during brain activity, and has a videocassette-sized headbox on a lead 10 m long, connected to a base box the size of a video recorder, and notebook PC, so that recordings may be made in ambulant subjects. Its performance was assessed using a resistor-capacitor network, and two saline-filled tanks-a cylindrical Perspex one and a latex one which contained a human skull. System signal-to-noise ratio was better than 50 dB and the maximum reciprocity error less than 10% for most frequencies. The CMMR was better than 80 dB at 38 kHz and a sponge, 20 mm across, which caused a local 12% impedance increase, was correctly localized in images. This suggests that the system has adequate performance to image impedance changes of 5-50% known to occur in the brain during normal activity, epilepsy or stroke; clinical trials to image these conditions are in progress.

Brain↗