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Results for “functional network connectivity”

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PubMed2005

The self-trapping attractor neural network--part II: properties of a sparsely connected model storing multiple memories.

In a previous paper, the self-trapping network (STN) was introduced as more biologically realistic than attractor neural networks (ANNs) based on the Ising model. This paper extends the previous analysis of a one-dimensional (1-D) STN storing a single memory to a model that stores multiple memories and that possesses generalized sparse connnectivity. The energy, Lyapunov function, and partition function derived for the 1-D model are generalized to the case of an attractor network with only near-neighbor synapses, coupled to a system that computes memory overlaps. Simulations reveal that 1) the STN dramatically reduces intra-ANN connectivity without severly affecting the size of basins of attraction, with fast self-trapping able to sustain attractors even in the absence of intra-ANN synapses; 2) the basins of attraction can be controlled by a single free parameter, providing natural attention-like effects; 3) the same parameter determines the memory capacity of the network, and the latter is much less dependent than a standard ANN on the noise level of the system; 4) the STN serves as a useful memory for some correlated memory patterns for which the standard ANN totally fails; 5) the STN can store a large number of sparse patterns; and 6) a Monte Carlo procedure, a competitive neural network, and binary neurons with thresholds can be used to induce self-trapping.

Algorithms↗

PubMed2005

Number and length of attractors in a critical Kauffman model with connectivity one.

The Kauffman model describes a system of randomly connected nodes with dynamics based on Boolean update functions. Though it is a simple model, it exhibits very complex behavior for "critical" parameter values at the boundary between a frozen and a disordered phase, and is therefore used for studies of real network problems. We prove here that the mean number and mean length of attractors in critical random Boolean networks with connectivity one both increase faster than any power law with network size. We derive these results by generating the networks through a growth process and by calculating lower bounds.

Economics↗

PubMed2006

Microscopic structure and elasticity of weakly aggregated colloidal gels.

We directly probe the microscopic structure, connectivity, and elasticity of colloidal gels using confocal microscopy. We show that the gel is a random network of one-dimensional chains of particles. By measuring thermal fluctuations, we determine the effective spring constant between pairs of particles as a function of separation; this is in agreement with the theory for fractal chains. Long-range attractions between particles lead to freely rotating bonds, and the gel is stabilized by multiple connections among the chains. By contrast, short-range attractions lead to bonds that resist bending, with dramatically suppressed formation of loops of particles.

Journal Article↗

PubMed1977

Morphology of the intrarenal lymphatic system. Capsular and hilar communications.

Recent functional evidence has indicated that ureteric obstruction for three or more days in dogs causes a diversion of renal lymph from the hilar to the capsular system. The present study was concerned with the structural correlate of this functional evidence, having special reference to communications between the two systems. Within three days of ureteric occlusion the capsular lymphatic system became dilated. On histological examination two types of tributaries were found. One (termed a perforating lymphatic) served as a primary pathway for superficial cortical lymph from the subcapsular plexus, and penetrated the capsule either alone or in company with a small vein. The other (termed a communicating lymphatic) was closely associated with the occasional penetrating interlobular blood vessel which traversed the capsule to ramify in the perirenal tissue. Approximately 60% of the penetrating arteries in eight dog kidneys had associated communicating lymphatics. On the renal surface the perforating and communicating lymphatics formed the primary collecting vessels of the capsular system. Within the renal substance the communicating lymphatics were directly continuous with lymphatics which surrounded the interlobular blood vessels and which have been shown to drain into the hilar network. Thus the communicating lymphatics formed direct connections between the hilar and capsular systems. It was concluded that these communications, although dilated by hydronephrosis, existed under control conditions. Functionally they are probably of importance only under special circumstances when intrarenal lymph may be diverted from one to the other system.

Animals↗

PubMed2001

The synaptic organization of VMH neurons that mediate the effects of estrogen on sexual behavior.

Estrogen acts in the hypothalamic ventromedial nucleus (VMH) of female rats to promote sexual behavior, as typified by the lordosis response. Morphological changes in the VMH, such as increased synaptic profiles and increased dendritic spines, suggest that estrogen may modulate behavior by altering VMH synaptic organization. To understand the significance of these changes, this laboratory has been investigating the functional classes of lordosis-relevant neurons and their local connectivity. A neurotropic virus, pseudorabies virus (PRV), was used to transneuronally label the CNS network that controls the lordosis-producing muscles. When PRV was placed in the lumbar epaxial muscles, it was sequentially detected in the lumbar ventral horn, the medullary reticular formation, the periaqueductal gray, and finally the VMH. Subsequent analysis showed that the population of VMH neurons that were initially infected with PRV largely resided beyond the cluster of estrogen receptor-containing neurons. In a separate study, VMH neurons were visualized with Lucifer yellow, and their morphology was analyzed using confocal microscopy. Such analysis confirmed that estrogen treatment increased dendritic spines in the VMH. The particular VMH neurons in this study did not express nuclear estrogen receptor, which suggests that estrogen can increase spine density indirectly. These results represent initial steps toward unraveling the local circuit that mediates estrogenic action on a specific reproductive behavior.

Animals↗

PubMed1985

Interhemispheric influences on area 19 of the cat.

Anatomical studies have shown an extensive network of homotopic and heterotopic interhemispheric connections in area 19 of the cat visual cortex (Segraves and Rosenquist 1982a; 1982b). We have investigated their functional organization by recording visual responses in area 19 of cats following a midsagittal section of the optic chiasm. This operation interrupts all crossed optic fibers coming both from the nasal and the temporal retinae; as a result, each hemisphere receives optic fibers only from the lateral hemiretina of the ipsilateral eye which conveys information from the contralateral visual field. Visual information transmitted to the same hemisphere from the contralateral retina and the ipsilateral visual field must be attributed to an indirect, interhemispheric pathway. We found that a rather high proportion of neurons (31.8%) in area 19 of seven split-chiasm cats responded to visual stimuli presented to the contralateral eye. All neurons receiving this interhemispheric activation were also driven by the ipsilateral eye via an intrahemispheric pathway. The property of binocularity was significantly related to the visuotopic map in that both receptive fields of each binocular neuron adjoined or were in the immediate vicinity of the vertical meridian. Due to the small size of receptive fields in area 19, the contribution of the interhemispheric pathway to the representation of the visual field is rather limited and it is certainly less extensive than that predicted by anatomical studies. The representation of the ipsilateral visual field in area 19 of intact cats, as assessed electrophysiologically, was comparable to that found in split-chiasm cats. Recordings in areas 17-18 of split-chiasm cats showed that the visual field represented through the corpus callosum in these visual areas is certainly not less and probably more, extensive than that found in area 19. The results support the conclusion that the relation to the vertical meridian and the receptive field size can explain the organization of the interhemispheric connections in the visual areas studied so far.

Animals↗

PubMed2005

Wavelet neural network classification of EEG signals by using AR model with MLE preprocessing.

Since EEG is one of the most important sources of information in therapy of epilepsy, several researchers tried to address the issue of decision support for such a data. In this paper, we introduce two fundamentally different approaches for designing classification models (classifiers); the traditional statistical method based on logistic regression and the emerging computationally powerful techniques based on artificial neural networks (ANNs). Logistic regression as well as feedforward error backpropagation artificial neural networks (FEBANN) and wavelet neural networks (WNN) based classifiers were developed and compared in relation to their accuracy in classification of EEG signals. In these methods we used FFT and autoregressive (AR) model by using maximum likelihood estimation (MLE) of EEG signals as an input to classification system with two discrete outputs: epileptic seizure or nonepileptic seizure. By identifying features in the signal we want to provide an automatic system that will support a physician in the diagnosing process. By applying AR with MLE in connection with WNN, we obtained novel and reliable classifier architecture. The network is constructed by the error backpropagation neural network using Morlet mother wavelet basic function as node activation function. The comparisons between the developed classifiers were primarily based on analysis of the receiver operating characteristic (ROC) curves as well as a number of scalar performance measures pertaining to the classification. The WNN-based classifier outperformed the FEBANN and logistic regression based counterpart. Within the same group, the WNN-based classifier was more accurate than the FEBANN-based classifier, and the logistic regression-based classifier.

Action Potentials↗

PubMed2006

Dynamic causal modeling: a generative model of slice timing in fMRI.

Dynamic causal modeling (DCM) of functional magnetic resonance imaging (fMRI) data allows one to make inferences about the architecture of distributed networks in the brain, in terms of effective connectivity. fMRI data are usually acquired using echo planar imaging (EPI). EPI sequences typically acquire slices at different times over a few seconds. DCM, in its original inception, was not informed about these slice timings and assumed that all slices were acquired simultaneously. It has been shown that DCM can cope with slice timing differences of up to 1 s. However, many fMRI studies employ a repetition time (TR) of 3 to 5 s, which precludes a straightforward DCM of these data. We show that this limitation can be overcome easily by including slice timing in the DCM. Using synthetic data we show that the extended DCM furnishes veridical posterior means, even if there are large slice-timing differences. Model comparisons show that, in general, the extended DCM out-performs the original model. We contrast the modeling of slice timing, in the context of DCM, with the less effective approach of 'slice-timing correction', prior to modeling. We apply our procedure to real data and show that slice timings are important parameters. We conclude that, generally, one should use DCM with slice timing.

Humans↗

PubMed1989

The brain and the immune system: a psychosomatic network.

The central nervous system (CNS) and the immune system, communicating through the neuroendocrine system, are closely involved in the individual's adaptation to the environment. The data from basic science research and clinical observations are overviewed, and more recent studies are summarized. As changes in immune function may mediate the effects of psychosocial factors in psychosomatic disorders, it is important for all biopsychosocially oriented physicians to understand the network that connects the central nervous and immune systems.

Brain↗

PubMed2003

To die or to sleep, perhaps to dream.

Establishing social contacts is the raison d'être of neurons throughout their entire life span. To form and retain functional connections, neuronal differentiation and death are ruthlessly regulated in development and kept strictly under control in post-mitotic systems. Derangements in neural networks affect neuronal populations at large. Therefore, failure to retain synaptic connectivity is linked to dysfunction and often followed by neuronal death. Loss of neurons is a predominant feature of neurodegenerative disease. Nevertheless, neuronal cell death is not an obligate requirement for neural dysfunction at the level of distributed circuits or local circuits. Although more or less wide spread neuronal loss can occur after acute insults such as brain ischemia or invasion of the brain by pathogens, neuronal death is a hallmark of end-stage neurodegenerative and psychiatric disease. The relative contributions made by loss of synaptic connectivity versus cell death for these diseases are still debated. Here these processes are discussed in relation to acute and chronic CNS disorders.

Apoptosis↗

PubMed2000

Single word reading in developmental stutterers and fluent speakers.

Ten fluent speakers and nine developmental stutterers read isolated nouns aloud in a delayed reading paradigm. Cortical activation sequences were mapped with a whole-head magnetoencephalography system. The stutterers were mostly fluent in this task. Although the overt performance was essentially identical in the two groups, the cortical activation patterns showed clear differences, both in the evoked responses, time-locked to word presentation and mouth movement onset, and in task-related suppression of 20-Hz oscillations. Within the first 400 ms after seeing the word, processing in fluent speakers advanced from the left inferior frontal cortex (articulatory programming) to the left lateral central sulcus and dorsal premotor cortex (motor preparation). This sequence was reversed in the stutterers, who showed an early left motor cortex activation followed by a delayed left inferior frontal signal. Stutterers thus appeared to initiate motor programmes before preparation of the articulatory code. During speech production, the right motor/premotor cortex generated consistent evoked activation in fluent speakers but was silent in stutterers. On the other hand, suppression of motor cortical 20-Hz rhythm, reflecting task-related neuronal processing, occurred bilaterally in both groups. Moreover, the suppression was right-hemisphere dominant in stutterers, as opposed to left-hemisphere dominant in fluent speakers. Accordingly, the right frontal cortex of stutterers was highly active during speech production but did not generate synchronous time-locked responses. The speech-related 20-Hz suppression concentrated in the mouth area in fluent speakers, but was evident in both the hand and mouth areas in stutterers. These findings may reflect imprecise functional connectivity within the right frontal cortex and incomplete segregation between the adjacent hand and mouth motor representations in stutterers during speech production. A network including the left inferior frontal cortex and the right motor/premotor cortex, likely to be relevant in merging linguistic and affective prosody with articulation during fluent speech, thus appears to be partly dysfunctional in developmental stutterers.

Adult↗

PubMed2000

Endothelin-converting enzyme inhibitors: current status and perspectives.

Recent research has led to the discovery of potent selective or mixed endothelin-converting enzyme (ECE), ECE/neutral endopeptidase24.11 (NEP) and ECE/NEP24.11/angiotensin-converting enzyme (ACE) inhibitors. There is also increasing evidence, that the functions of the endothelin (ET), renin-angiotensin and NEP systems for the regulation of the cardiovascular homeostasis are connected by a complex regulation network. It will be the challenging task of future research with the newly available selective and mixed ECE-1 inhibitors to show whether the combined inhibition of more than one cardiovascular system is superior to selective inhibition.

Angiotensins↗

PubMed2006

L-Argininamidium bis(hydrogensquarate).

Cations and anions of the title compound {systematic name: 1-[4-(aminocarbonyl)butyl]guanidinium bis(hydrogensquarate)}, C6H17N5O2+.2C4HO4(-), are connected into a three-dimensional network by intermolecular N-H...O hydrogen bonds between the L-argininamidium ammonium, amide and guanidinium functions and the hydrogensquarate carbonyl O atoms. The independent hydrogensquarate monoanions are linked into dimers by pairs of O-H...O' hydrogen bonds.

Arginine↗

PubMed2005

Microarray analysis of androgen-regulated gene expression in testis: the use of the androgen-binding protein (ABP)-transgenic mouse as a model.

BACKGROUND: Spermatogenesis is an androgen-dependent process, yet the molecular mechanisms of androgens' actions in testis are poorly understood. Transgenic mice overexpressing rat androgen-binding protein (ABP) in their testes have reduced levels of intratesticular androgens and, as a result, show a progressive impairment of spermatogenesis. We used this model to characterize changes in global gene expression in testis in response to reduced bioavailability of androgens. METHODS: Total RNA was extracted from testes of 30-day old transgenic and wild-type control mice, converted to cRNA, labeled with biotin, and hybridized to oligonucleotide microarrays. Microarray results were confirmed by real-time reverse transcription polymerase chain reaction. RESULTS: Three-hundred-eighty-one genes (3.05% of all transcripts represented on the chips) were up-regulated and 198 genes (1.59%) were down-regulated by at least a factor of 2 in the androgen-deficient animals compared to controls. Genes encoding membrane proteins, intracellular signaling molecules, enzymes, proteins participating in the immune response, and those involved in cytoskeleton organization were significantly overrepresented in the up-regulated group. Among the down-regulated transcripts, those coding for extracellular proteins were overrepresented most dramatically, followed by those related to proteolysis, cell adhesion, immune response, and growth factor, cytokine, and ion channel activities. Transcripts with the greatest potential impact on cellular activities included several transcription factors, intracellular signal transducers, secreted signaling molecules and enzymes, and various cell surface molecules. Major nodes in the up-regulated network were IL-6, AGT, MYC, and A2M, those in the down-regulated network were IL-2, -4, and -10, MAPK8, SOCS1, and CREB1. CONCLUSION: Microarray analysis followed by gene ontology profiling and connectivity analysis identified several functional groups of genes and individual genes responding to sustained reduction of androgen levels in the mouse testis. These include genes whose products function as transcription factors, cell surface molecules including ion channels, extra- and intracellular signaling molecules, and secreted enzymes with the potential of regulating cell-to-cell attachment. The transcription factors CREB1 (down-regulated) and MYC (up-regulated) may mediate the most important initial phases of the testicular response to reduced levels of androgens. These results suggest specific avenues for further research that will lead to a better understanding of how androgens regulate spermatogenesis.

Androgen-Binding Protein↗

PubMed2004

Stable isotope-mass spectrometric measurements of molecular fluxes in vivo: emerging applications in drug development.

Therapeutics require not only targets and chemical entities, but tools for measuring actions in vivo. Technologies for evaluating activities, filtering leads and predicting clinical response have lagged behind molecular discovery. 'Systems biology' has not provided systematic methods for predicting metabolic effects in complex systems. The flow of molecules through complex pathways, in contrast, reflects the connectivity relationships and emergent control features of fully assembled networks, but is a quantifiable therapeutic target. This strategy, which combines molecular specificity with intrinsic functional significance, requires different tools. Here, advances in critical pathway flux measurement utilizing stable isotopes and mass spectrometry are described. These include mass isotopomer analysis, heavy-water labeling techniques, secreted probes of intracellular processes, and analytic advances. Several fundamental advantages of kinetic measurements are demonstrated. Measurement of molecular fluxes represents a powerful addition to drug development technology.

Animals↗

PubMedDate not supplied

[The structure and connections of the basal ganglia. The striatum].

On the base of personal studies there were reviewed contemporary data about the internal organization of mammal striatum (n. caudatus, putamen): of their histological and histochemical differentiation, neuronal content, mediators of their neurons, and of the intrinsic and extrinsic connections of these structural elements: there were put forward some hypothesis about functional specificity of these elements and about the arrangement of striatal neuronal network.

Animals↗

PubMed2004

Laser microdissection of immunolabeled astrocytes allows quantification of astrocytic gene expression.

Astrocytes represent the major glial cell population within the central nervous system. In order to elucidate the function of astrocytes under physiological conditions and during the course of neurological disease, astrocytic gene expression profiling is necessary. However, since astrocytes form an intimately connected network with neurons and other cell types in the brain, gene expression analysis of astrocytes with a sufficient degree of cellular specificity is difficult. Here we are presenting a rapid and, thus, RNA preserving immunostaining protocol for the detection of astrocytes in rodent brain. This protocol can readily be combined with laser microdissection (Leica AS LMD platform) and quantitative RT-PCR (qPCR). Employing this method, we studied changes in glial fibrillary acidic protein (GFAP) expression in astrocytes of mouse entorhinal cortex following entorhinal cortex lesion. Using laser microdissection, astrocytes (n = 60) were collected in the tissue surrounding the lesion, the entorhinal cortex contralateral to the lesion, and in unlesioned control animals. Changes in GFAP mRNA were quantified using qPCR. GFAP mRNA levels were 82-fold higher in astrocytes of lesioned animals at the site of the lesion compared to GFAP mRNA levels in entorhinal cortex astrocytes of control mice. GFAP mRNA levels were only slightly elevated at the contralateral side (lesioned animals). This optimized protocol for immunolabeling and laser microdissection of astrocytes followed by qPCR allows quantification of astrocytic gene expression levels with a high degree of cellular specificity. It may similarly be employed in different settings where other cell types need to be identified and microdissected for gene expression profiling.

Animals↗

PubMed1978

Anistropic connectivity and cooperative phenomena as a basis for orientation sensitivity in the visual cortex.

A computer simulation model of the neural circuity underlying orientation sensitivity in cortical neurons is examined. The model consists of a network of 3000 neurons divided into two functionally distinct cell types: excitatory (E-cells) and inhibitory (I-cells). We demonstrate that both orientation sensitivity and shape selectivity can be accounted for by making the following assumptions: 1) thalamic afferents to a sheet of cortical neurons are retinotopically organized; 2) thalamic afferents come from a single neuron, or at most a few neurons, in the lateral geniculate nucleus; 3) cortical activity is cooperative, i.e. largely dependent on intracortical connections, some of which have anisotropies along directions parallel to the pial surface. Anisotropies are specified only by the distribution of cells which are postsynaptic to a particular neuron, without specifying the axonal or dendritic contributions. In this paper, orientation sensitivity arises through cooperative interactions among neurons having anisotropic excitatory, and isotropic inhibitory connections.

Action Potentials↗