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At least 199 records · Page 11Linked to original sources
PubMed2004

The nature of traumatic memories: a 4-T FMRI functional connectivity analysis.

OBJECTIVE: This study used functional connectivity analyses to assess interregional brain activity correlations during the recall of traumatic memories in traumatized subjects with and without posttraumatic stress disorder (PTSD). METHOD: Both 4-T functional magnetic resonance imaging (fMRI) and functional connectivity analyses were used to assess interregional brain activity correlations during script-driven symptom provocation in traumatized subjects with (N=11) and without (N=13) PTSD. Functional connectivity analyses were carried out by using data for brain regions activated in both the PTSD group and the comparison group. The use of functional connectivity analyses in addition to subtraction analyses allowed assessment of specific brain regions involved in the recall of traumatic events and of the neuronal networks underlying the recall of such events. RESULTS: Significant between-group differences in functional connectivity were found. Comparison of connectivity maps at coordinates x=2, y=20, z=36 (right anterior cingulate gyrus) for the two groups showed that the subjects without PTSD had greater correlation than the PTSD subjects in the left superior frontal gyrus (Brodmann's area 9), left anterior cingulate gyrus (Brodmann's area 32), left striatum (caudate), left parietal lobe (Brodmann's areas 40 and 43), and left insula (Brodmann's area 13). In contrast, the PTSD subjects showed greater correlation than the subjects without PTSD in the right posterior cingulate gyrus (Brodmann's area 29), right caudate, right parietal lobe (Brodmann's areas 7 and 40), and right occipital lobe (Brodmann's area 19). CONCLUSIONS: The differences in brain connectivity between PTSD and comparison subjects may account for the nonverbal nature of traumatic memory recall in PTSD subjects, compared to a more verbal pattern of traumatic memory recall in comparison subjects.

Brain↗
PubMed2002

Neural network pruning with Tukey-Kramer multiple comparison procedure.

Reducing a neural network's complexity improves the ability of the network to generalize future examples. Like an overfitted regression function, neural networks may miss their target because of the excessive degrees of freedom stored up in unnecessary parameters. Over the past decade, the subject of pruning networks produced nonstatistical algorithms like Skeletonization, Optimal Brain Damage, and Optimal Brain Surgeon as methods to remove connections with the least salience. The method proposed here uses the bootstrap algorithm to estimate the distribution of the model parameter saliences. Statistical multiple comparison procedures are then used to make pruning decisions. We show this method compares well with Optimal Brain Surgeon in terms of ability to prune and the resulting network performance.

Algorithms↗
PubMed2005

Neuronal network formation in human cerebral cortex.

Knowledge of the development of structural and functional connectivity in the human brain is of great fundamental and practical importance, but is largely lacking. In this review qualitative and quantitative data are presented on the formation of dendrites, axons and synapses in different regions of the human cerebral cortex from prenatal life until adulthood. This information is compiled to provide baseline information for comparison of similar data derived from postmortem brains of persons with developmental brain disorders. In addition, some data are provided on the influence of the sensory environment on cortical network formation in animals.

Cerebral Cortex↗
PubMed2005

Cerebral mechanisms of prosodic sensory integration using low-frequency bands of connected speech.

Even if speech perception has been reported to involve both left and right hemispheres, converging data have posited the existence of a functional asymmetry at the level of secondary auditory cortices. Using fMRI in 12 right-handed French men listening passively to long connected speech stimuli, we addressed the question of neuronal networks involved in the integration of low frequency bands of speech by comparing 1) differences in brain activity in two listening conditions (FN, NF) differing in the integration of pitch modulations (in FN, low frequencies, obtained by a low-pass filter, are addressed to the left ear while the whole acoustic message is simultaneously addressed to the right ear, NF being the reverse position); 2) differences in brain activity induced by high and low degrees of prosodic expression (expressive vs. flat); and 3) effects of the same connected speech stimulus in the two listening conditions. Each stimulus induced a specific cerebral network, the flat one weakening activations which were mainly reduced to the bilateral STG for both listening conditions. In the expressive condition, the specific sensory integration FN results in an increase of the articulatory loop and new recruitments such as right BA6-44, left BA39-40, the left posterior insula and the bilateral BA30. This finding may be accounted for by the existence of temporal windows differing both in length and in acoustic cues decoding, strengthening the "asymmetric sampling in time" hypothesis posited by Poeppel (Speech Commun 2003; 41:245-255). Such an improvement of prosodic integration could find applications in the rehabilitation of some speech disturbances.

Acoustic Stimulation↗
PubMed2001

[Cellular and molecular mechanisms of memory].

There has been nearly a century of interest in the idea that information is encoded in the brain as specific spatio-temporal patterns of activity in distributed networks and stored as changes in the efficacy of synaptic connections on neurons that are activated during learning. The discovery and detailed report of the phenomenon generally known as long-term potentiation opened a new chapter in the study of synaptic plasticity in the vertebrate brain, and this form of synaptic plasticity has now become the dominant model in the search for the cellular bases of learning and memory. To date, the key events in the cellular and molecular mechanisms underlying synaptic plasticity are starting to be identified. They require the activation of specific receptors and of several molecular cascades to convert extracellular signals into persistent functional changes in neuronal connectivity. Accumulating evidence suggests that the rapid activation of the genetic machinery is a key mechanism underlying the enduring modification of neural networks required for the laying down of memory. The recent developments in the search for the cellular and molecular mechanisms of memory storage are reviewed.

Gene Expression Regulation↗
PubMed1997

A network of tufted layer 5 pyramidal neurons.

Tufted layer 5 (TL5) pyramidal neurons are important projection neurons from the cerebral cortex to subcortical areas. Recent and ongoing experiments aimed at understanding the computational analysis performed by a network of synaptically connected TL5 neurons are reviewed here. The experiments employed dual and triple whole-cell patch clamp recordings from visually identified and preselected neurons in brain slices of somatosensory cortex of young (14- to 16-day-old) rats. These studies suggest that a local network of TL5 neurons within a cortical module of diameter 300 microns consists of a few hundred neurons that are extensively inter-connected with reciprocal feedback from at least first-, second- and third-order target neurons. A statistical analysis of synaptic innervation suggests that this recurrent network is not randomly arranged and hence each neuron could be functionally unique. Synaptic transmission between these neurons is characterized by use-dependent synaptic depression which confers novel properties to this recurrent network of neurons. First, a range of rates of depression for different synaptic connections enable each TL5 neuron to receive a unique mixture of information about the average firing rates and the temporally correlated action potential (AP) activity in the population of presynaptic TL5 neurons. Second, each AP generated by any neuron in the network induces a change (defined as an iteration step) in the functional coupling of the neurons in the network (defined as network configuration). It is proposed that the network configuration is iterated during a stimulus to achieve an optimally orchestrated network response. Hebbian, anti-Hebbian and neuromodulatory-induced modifications of neurotransmitter release probability change the rates of synaptic depression and thereby alter the iteration step size. These data may be important to understand the dynamics of electrical activity within the network.

Animals↗
PubMed2004

Neurotrophic effects of antidepressant drugs.

The past decade has witnessed a growing interest in the trophic effects of antidepressant drugs. Antidepressants stimulate the production and signaling of plasticity-related proteins such as neurotrophins and cAMP response element binding protein, and neurotrophin signaling appears to be both sufficient and necessary for antidepressant action in rodents. Furthermore, several different antidepressant treatments increase neurogenesis in rodent hippocampus and this effect correlates with the behavioral effects produced by these drugs. These data suggest that antidepressants facilitate activity-dependent selection of functional synaptic connections in brain and, through their neurotrophic effects, improve information processing within neuronal networks compromised in mood disorders.

Animals↗
PubMed1983

Changing images of the brain.

Our concept of the structure and function of the normal and diseased brain has developed throughout the centuries. The first stage in the discovery of the brain stretched over three millennia, from the earliest descriptions by the Egyptians in the sixteenth century B.C. to the comprehensive anatomical treatise of Vesalius. The invention of the light microscope brought to the eye a previously invisible world, and heralded the beginnings of the systematic histological investigation of the immensely complex cellular networks of the brain. With the advent of electron microscopy, the organelles and connections of brain cells have been revealed, and the new era of molecular biology has begun. Neuropathology, which concerns itself with diseases of the brain, spinal cord, nerve and muscle, has enormously benefited from these developments to establish the morphological basis of diseases of the nervous system.

Anatomy↗
PubMed1996

Motion processing using asymmetric shunting lateral inhibitory networks.

Asymmetrically connected inhibitory shunting networks are believed to occur in many areas of the brain, including the visual system; they can explain many peripheral visual phenomena. This paper illustrates their usefulness in motion detection, and compares details of their performance with experimental results. Two stimuli are considered: drifting gratings and jumping bars. An elementary motion detector (EMD) is defined, consisting of a pair of opposite-directed asymmetric inhibitory networks. A frequency-space expansion (equivalent to a Volterra-Wiener expansion) is used to examine the EMD's response to a drifting grating. The EMD can be tuned to independent spatial and temporal frequencies: by summing the outputs of a number of EMDs, a velocity-tuned network is obtained. This velocity-tuned network is very well suited to certain field effect transistor (FET) implementations. Since the network is nonlinear, the response to a drifting grating does not fully characterize its behaviour. The response to a jumping bar stimulus is also examined. Two animals are considered: *Cats: the work of Emerson et al on the response of directionally selective (DS) complex cells in cat visual cortex to a white noise signal. *Flies: the work of Franceschini et al on the response of the fly H1 cell to stimulation of two neighbouring receptor cells, and that of Horridge and Marcelja on the fly H1 response to various jumping bar and edge stimuli. Agreement with all three of these experiments cannot be expected, because the results for the two animals are not consistent. There are two main differences: *Time development of the velocity-tuned signal: for the fly H1 cell this generally consists of two sharp (ON and OFF) responses to the second presentation of the bar signal, while for the cat DS cell a more slowly varying signal is observed. *Response to two presentations of the bar signal in the same position (i.e., a 'jumping bar' that jumps zero distance): for the fly H1 cell there is generally no response, while the cat DS cell responds significantly. The velocity-tuned network agrees well with the results for the cat DS cells.

Journal Article↗
PubMed2000

What to expect from MRI in the investigation of the central nervous system?

Functional magnetic resonance imaging (fMRI) has appeared as a new tool that is very powerful for cognitive neuroscience, offering the potential to look at the dynamics of cerebral processes underlying cognition, non-invasively and on an individual basis. Work remains to be done to optimize the technique and to better understand its basic mechanisms, but one may expect to build in a foreseeable future a functional list of the main brain cortical networks implicated in sensory-motor or cognitive processes. Still, the real understanding of brain function requires direct access to the functional unit consisting of the neuron, so that one may look at the transient temporal relationships that exist between largely distributed groups of hundreds or thousands of neurons. Furthermore, communication pathways between networks, which are carried by brain white matter, must be identified to establish connectivity maps at the individual scale, taking into account individual variability resulting from genetic factors and cerebral plasticity. In this respect, MRI of molecular diffusion is very sensitive to water molecular motion and, thus, to tissue dynamic microstructure, such as cell size and geometry. Preliminary data suggest that diffusion MRI visualizes dynamic tissue changes associated with large neuronal activation and space orientation of large bundles of myelinated axons in the white matter.

Animals↗
PubMed2004

Organization, development and function of complex brain networks.

Recent research has revealed general principles in the structural and functional organization of complex networks which are shared by various natural, social and technological systems. This review examines these principles as applied to the organization, development and function of complex brain networks. Specifically, we examine the structural properties of large-scale anatomical and functional brain networks and discuss how they might arise in the course of network growth and rewiring. Moreover, we examine the relationship between the structural substrate of neuroanatomy and more dynamic functional and effective connectivity patterns that underlie human cognition. We suggest that network analysis offers new fundamental insights into global and integrative aspects of brain function, including the origin of flexible and coherent cognitive states within the neural architecture.

Brain↗
PubMed2006

Quantitative evaluation of linear and nonlinear methods characterizing interdependencies between brain signals.

Brain functional connectivity can be characterized by the temporal evolution of correlation between signals recorded from spatially-distributed regions. It is aimed at explaining how different brain areas interact within networks involved during normal (as in cognitive tasks) or pathological (as in epilepsy) situations. Numerous techniques were introduced for assessing this connectivity. Recently, some efforts were made to compare methods performances but mainly qualitatively and for a special application. In this paper, we go further and propose a comprehensive comparison of different classes of methods (linear and nonlinear regressions, phase synchronization, and generalized synchronization) based on various simulation models. For this purpose, quantitative criteria are used: in addition to mean square error under null hypothesis (independence between two signals) and mean variance computed over all values of coupling degree in each model, we provide a criterion for comparing performances. Results show that the performances of the compared methods are highly dependent on the hypothesis regarding the underlying model for the generation of the signals. Moreover, none of them outperforms the others in all cases and the performance hierarchy is model dependent.

Brain↗
PubMed1993

Essential organization of the sympathetic nervous system.

The sympathetic nervous system consists of efferent neurones supplying the viscera. The cell bodies of preganglionic neurones are located in four areas in the thoracolumbar cord; however, the majority are found in the IML. Various tracing techniques have provided information concerning the location of the cell bodies of sympathetic preganglionic neurones projecting into various nerves and ganglia and regulating the adrenal gland, the kidney and the sympathetic supply to skeletal muscle. Numerous supraspinal neurones project to the neuropil surrounding sympathetic preganglionic neurones and may form synaptic contacts with these neurones. The areas of the brain that project to the IML appear to be part of a network of reciprocally connected supraspinal cell groups. Although much emphasis has been placed on the importance of the RVLM in the mediation of tonic and phasic inputs to sympathetic preganglionic neurones, it appears that other areas are of significant import; the RVLM should not be considered to be 'the vasomotor centre'. Spinal and cranial afferents influence the sympathetic nervous system. Baroreceptor afferents terminate in the NTS and may utilize an excitatory amino acid as their neurotransmitter. However, a number of neuropeptides are also associated with these afferents. Neurones within the NTS project to a number of brain stem areas thought to be involved in the regulation of sympathetic activity; consequently the baroreceptor reflex may be mediated over a number of parallel pathways involving both supraspinal and spinal sites of inhibition. Many neurotransmitters are thought to regulate the activity of sympathetic preganglionic neurons: monoamines, peptides and amino acids. Matching the chemical content of the cell bodies of neurones within a particular cell group with physiological characteristics is a challenging task; some barosensitive neurones of the RVLM do not appear to be adrenergic although they are in the midst of the C1 adrenergic cell group. Besides acetylcholine and noradrenaline, neurotransmission in the periphery appears to involve numerous peptides and ATP.

Animals↗
PubMed1999

Three-dimensional tracking of axonal projections in the brain by magnetic resonance imaging.

The relationship between brain structure and complex behavior is governed by large-scale neurocognitive networks. The availability of a noninvasive technique that can visualize the neuronal projections connecting the functional centers should therefore provide new keys to the understanding of brain function. By using high-resolution three-dimensional diffusion magnetic resonance imaging and a newly designed tracking approach, we show that neuronal pathways in the rat brain can be probed in situ. The results are validated through comparison with known anatomical locations of such fibers.

Animals↗
PubMed1996

Fibrin deposition in primary and metastatic human brain tumours.

Extravascular, intratumoral fibrin deposition is frequently observed within and around neoplastic tissue and has been implicated in various aspects of tumour growth. This is the first report on the presence and distribution of fibrinogen/fibrin in primary (14 glioblastomas) and metastatic (nine samples of lung cancer origin) human brain tumours detected by immunofluorescent techniques. All tissue samples showed specific staining for fibrinogen/fibrin. In glioblastomas fibrin deposits could be detected within and around tumour foci, while in metastatic brain tumours the tumour cell nodules were surrounded by fibrin deposits localized almost exclusively in the connective tissue compartment of tumours. Double-labelling reactions for von Willebrand factor and fibrinogen/fibrin has revealed that fibrin deposition occurred throughout the tumour stroma independently of tumour vasculature. The overlapping reactions for fibrinogen/fibrin and factor XIII subunit A, as well as the urea-insolubility of the deposits indicate the crosslinked, highly stabilized nature of fibrin both within and around tumours. Staining with Ki M7 monoclonal antibody specific for phagocytosing macrophages showed these cells to be scattered in the nonnecrotic areas in glioblastomas and to be accumulated at the interface of tumorous parenchyma and connective tissue in both primary and metastatic tumours. The close association between fibrin deposition and macrophage accumulation strongly suggests the active participation of tumour associated macrophages in the formation of stabilized intratumoral fibrin network in human brain neoplasms.

Blood Coagulation↗
PubMed1997

[High-field (3.OT) MR system: advantages in functional analysis].

In order to elucidate the precise mechanisms underlying higher brain function, investigations need to focus not only on functionality of neuronal networks but also the underlying connectivity. Newly developed magnetic resonance (MR) techniques offer revolutional, non-invasive tools for both functionality and connectivity analysis in humans, namely, BOLD (blood oxygen level dependent) functional magnetic resonance imaging (MRI) and three dimensional anisotropy contrast (3DAC) magnetic resonance axonograpy (MRX). The former technique makes use of changes in regional deoxyhemoglobin concentration associated with brain function, while the latter utilizes directional information of apparent diffusivity observed in axonal fibers. In these critically important but highly demanding imaging techniques, high-field (> or = 3.OT) MR systems possess significant advantages over conventional systems.

Axons↗
PubMed2006

Olfactory bulb networks revealed by lateral olfactory tract stimulation in the in vitro isolated guinea-pig brain.

Olfactory information processing is mediated by synaptic connections between the olfactory bulbs (OBs) and piriform-limbic cortices. Limited accessibility using common in vivo and in vitro preparations has hindered previous attempts to define these synaptic interactions. We utilized the isolated guinea-pig brain preparation to overcome these experimental limitations. Previous studies demonstrated extensive functional preservation in this preparation maintained in vitro by arterial perfusion. Field potential laminar profiles were performed with multi-channel probes in the OB following stimulation of both the lateral olfactory tract (LOT) and the anterior piriform cortex (APC). Current-source density analysis was carried out on laminar profiles to reconstruct current sinks/sources associated with intrinsic synaptic activities. LOT stimulation induced sequentially i) an antidromic population spike (at 2.66+/-0.39 ms) located in the mitral cell layer that was resistant to 100 Hz high-frequency stimulation (HFS) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (10 microM), ii) a component located in the external plexiform layer at 3.85+/-0.63 ms that was unaffected by HFS, iii) a large amplitude potential (peak amplitude at 5.84+/-0.58 ms) generated in the external plexiform layer, abolished by HFS and CNQX, but not by bicuculline (50 microM), iv) a late response (onset at 20.00+/-2.94 ms) abolished by CNQX and enhanced by bicuculline. Stimulation of the APC also induced a late potential abolished by HFS and CNQX. Both APC-evoked and late LOT-evoked responses were abolished by a transverse cut to separate OB from APC. These results demonstrate in an isolated mammalian brain preparation the presence of reciprocal synaptic interactions between the OB and piriform cortical structures.

2-Amino-5-phosphonovalerate↗
PubMed2003

Cell-cell signaling during synapse formation in the CNS.

Synapses join individual nerve cells into a functional network. Specific cell-cell signaling events regulate synapse formation during development and thereby generate a highly reproducible connectivity pattern. The accuracy of this process is fundamental for normal brain function, and aberrant connectivity leads to nervous system disorders. However, despite the overall precision with which neuronal circuits are formed, individual synapses and synaptic networks are also plastic and can readily adapt to external stimuli or perturbations. In recent studies, several trans-synaptic signaling systems have been identified that can mediate various aspects of synaptic differentiation in the central nervous system. It appears that these individual pathways functionally cooperate, thereby generating robustness and flexibility, which ensure normal nervous system function.

Animals↗