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Distributed Network (distributed + network)
Selected AbstractsFunctional connectivity with the hippocampus during successful memory formationHIPPOCAMPUS, Issue 8 2005Charan Ranganath Abstract Although it is well established that the hippocampus is critical for episodic memory, little is known about how the hippocampus interacts with cortical regions during successful memory formation. Here, we used event-related functional magnetic resonance imaging (fMRI) to identify areas that exhibited differential functional connectivity with the hippocampus during processing of novel objects that were subsequently remembered or forgotten on a postscan test. Functional connectivity with the hippocampus was enhanced during successful, as compared with unsuccessful, memory formation, in a distributed network of limbic cortical areas,including perirhinal, orbitofrontal, and retrosplenial/posterior cingulate cortex,that are anatomically connected with the hippocampal formation. Increased connectivity was also observed in lateral temporal, medial parietal, and medial occipital cortex. These findings demonstrate that successful memory formation is associated with transient increases in cortico-hippocampal interaction. © 2005 Wiley-Liss, Inc. [source] Somatosensory working memory performance in humans depends on both engagement and disengagement of regions in a distributed networkHUMAN BRAIN MAPPING, Issue 1 2010Saskia Haegens Abstract Successful working memory (WM) requires the engagement of relevant brain areas but possibly also the disengagement of irrelevant areas. We used magnetoencephalography (MEG) to elucidate the temporal dynamics of areas involved in a somatosensory WM task. We found an increase in gamma band activity in the primary and secondary somatosensory areas during encoding and retention, respectively. This was accompanied by an increase of alpha band activity over task-irrelevant regions including posterior and ipsilateral somatosensory cortex. Importantly, the alpha band increase was strongest during successful WM performance. Furthermore, we found frontal gamma band activity that correlated both with behavioral performance and the alpha band increase. We suggest that somatosensory gamma band activity reflects maintenance and attention-related components of WM operations, whereas alpha band activity reflects frontally controlled disengagement of task-irrelevant regions. Our results demonstrate that resource allocation involving the engagement of task-relevant and disengagement of task-irrelevant regions is needed for optimal task execution. Hum Brain Mapp, 2010. © 2009 Wiley-Liss, Inc. [source] Neural correlates of consolidation in working memoryHUMAN BRAIN MAPPING, Issue 3 2007Nelly Mainy Abstract Many of our daily activities rely on a brain system called working memory, which implements our ability to encode information for short-term maintenance, possible manipulation, and retrieval. A recent intracranial study of patients performing a paradigmatic working memory task revealed that the maintenance of information involves a distributed network of oscillations in the gamma band (>40 Hz). Using a similar task, we focused on the encoding stage and targeted a process referred to as short-term consolidation, which corresponds to the encoding of novel items in working memory. The paradigm was designed to manipulate the subjects' intention to encode: series of 10 letters were presented, among which only five had to be remembered, as indicated by visual cues preceding or following each letter. During this task we recorded the intracerebral EEG of nine epileptic patients implanted in mesiotemporal structures, perisylvian regions, and prefrontal areas and used time,frequency analysis to search for neural activities simultaneous with the encoding of the letters into working memory. We found such activities in the form of increases of gamma band activity in a set of regions associated with the phonological loop, including the Broca area and the auditory cortex, and in the prefrontal cortex, the pre- and postcentral gyri, the hippocampus, and the fusiform gyrus. Hum Brain Mapp, 2007. © 2006 Wiley-Liss, Inc. [source] Tactile discrimination of grating orientation: fMRI activation patternsHUMAN BRAIN MAPPING, Issue 4 2005Minming Zhang Abstract Grating orientation discrimination is employed widely to test tactile spatial acuity. We used functional magnetic resonance imaging (fMRI) to investigate the neural circuitry underlying performance of this task. Two studies were carried out. In the first study, an extensive set of parietal and frontal cortical areas was activated during covert task performance, relative to a rest baseline. The active regions included the postcentral sulcus bilaterally and foci in the left parietal operculum, left anterior intraparietal sulcus, and bilateral premotor and prefrontal cortex. The second study examined selective recruitment of cortical areas during discrimination of grating orientation (a task with a macrospatial component) compared to discrimination of grating spacing (a purely microspatial task). The foci activated on this contrast were in the left anterior intraparietal sulcus, right postcentral sulcus and gyrus, left parieto-occipital cortex, bilateral frontal eye fields, and bilateral ventral premotor cortex. These findings not only confirm and extend previous studies of the neural processing underlying grating orientation discrimination, but also demonstrate that a distributed network of putatively multisensory areas is involved. Hum Brain Mapp, 2005. © 2005 Wiley-Liss, Inc. [source] Topographic controls on the chemistry of subsurface stormflowHYDROLOGICAL PROCESSES, Issue 10 2001Daniel L. Welsch Abstract Models are needed that describe how topography and other watershed characteristics affect the chemical composition of runoff waters, yet little spatially distributed data exist to develop such models. A topographically driven flushing mechanism for nitrate (NO3,) and dissolved organic carbon has been described in recent literature; however, this mechanism has not yet been thoroughly tested. A 24 ha catchment in the Catskill Mountains of New York was clearcut in the winter of 1996,97, resulting in elevated NO3, concentrations in soil water, groundwater and streamflow. We sampled shallow subsurface stormflow (SSSF) and streamflow six times during the spring and summer of 1998, 1 year after the harvest. We used a spatially distributed network of piezometers to investigate the relationship between topography and SSSF chemistry. Several indices of topography were computed, including the commonly employed topographic index of Beven and Kirkby (1979; Hydrological Sciences Bulletin24: 43,69). Topographic index was positively correlated with NO3, concentrations in SSSF. The strength of the NO3,,topography relationship was best explained by antecedent soil temperature and antecedent precipitation conditions. Results suggest a topographically driven flushing of high NO3, shallow soil at the site during storm events. Copyright © 2001 John Wiley & Sons, Ltd. [source] Parallel Algorithms for Dynamic Shortest Path ProblemsINTERNATIONAL TRANSACTIONS IN OPERATIONAL RESEARCH, Issue 3 2002Ismail Chabini The development of intelligent transportation systems (ITS) and the resulting need for the solution of a variety of dynamic traffic network models and management problems require faster-than-real-time computation of shortest path problems in dynamic networks. Recently, a sequential algorithm was developed to compute shortest paths in discrete time dynamic networks from all nodes and all departure times to one destination node. The algorithm is known as algorithm DOT and has an optimal worst-case running-time complexity. This implies that no algorithm with a better worst-case computational complexity can be discovered. Consequently, in order to derive algorithms to solve all-to-one shortest path problems in dynamic networks, one would need to explore avenues other than the design of sequential solution algorithms only. The use of commercially-available high-performance computing platforms to develop parallel implementations of sequential algorithms is an example of such avenue. This paper reports on the design, implementation, and computational testing of parallel dynamic shortest path algorithms. We develop two shared-memory and two message-passing dynamic shortest path algorithm implementations, which are derived from algorithm DOT using the following parallelization strategies: decomposition by destination and decomposition by transportation network topology. The algorithms are coded using two types of parallel computing environments: a message-passing environment based on the parallel virtual machine (PVM) library and a multi-threading environment based on the SUN Microsystems Multi-Threads (MT) library. We also develop a time-based parallel version of algorithm DOT for the case of minimum time paths in FIFO networks, and a theoretical parallelization of algorithm DOT on an ,ideal' theoretical parallel machine. Performances of the implementations are analyzed and evaluated using large transportation networks, and two types of parallel computing platforms: a distributed network of Unix workstations and a SUN shared-memory machine containing eight processors. Satisfactory speed-ups in the running time of sequential algorithms are achieved, in particular for shared-memory machines. Numerical results indicate that shared-memory computers constitute the most appropriate type of parallel computing platforms for the computation of dynamic shortest paths for real-time ITS applications. [source] Acoustic estimation of wildlife abundance: methodology for vocal mammals in forested habitatsAFRICAN JOURNAL OF ECOLOGY, Issue 3 2010Mya E. Thompson Abstract Habitat loss and hunting pressure threaten mammal populations worldwide, generating critical time constraints on trend assessment. This study introduces a new survey method that samples continuously and non-invasively over long time periods, obtaining estimates of abundance from vocalization rates. We present feasibility assessment methods for acoustic surveys and develop equations for estimating population size. As an illustration, we demonstrate the feasibility of acoustic surveys for African forest elephants (Loxodonta africana cyclotis). Visual surveys and vocalizations from a forest clearing in the Central African Republic were used to establish that low-frequency elephant calling rate is a useful index of elephant numbers (linear regression P < 0.001, radj.2 = 0.58). The effective sampling area was 3.22 km2 per acoustic sensor, a dramatic increase in coverage over dung survey transects. These results support the use of acoustic surveys for estimating elephant abundance over large remote areas and in diverse habitats, using a distributed network of acoustic sensors. The abundance estimation methods presented can be applied in surveys of any species for which an acoustic abundance index and detection function have been established. This acoustic survey technique provides an opportunity to improve management and conservation of many acoustically-active taxa whose populations are currently under-monitored. Résumé La perte d'habitat et la pression de la chasse menacent des populations de mammifères dans le monde entier, ce qui entraîne des contraintes de temps critiques pour l'évaluation des tendances. Cette étude présente une nouvelle méthode de recherche qui échantillonne sur de longues périodes de façon constante et non intrusive et qui permet d'obtenir des estimations d'abondance à partir des taux de vocalisation. Nous présentons les méthodes d'évaluation de la faisabilité d'études acoustiques et nous développons des équations pour estimer la taille des populations. Pour illustrer ceci, nous montrons la faisabilité d'études acoustiques chez les éléphants de forêts africains Loxodonta africana cyclotis. Nous avons utilisé des études visuelles et des vocalisations d'une clairière forestière de République Centrafricaine pour établir que le taux d'appel à basse fréquence des éléphants est un indice intéressant du nombre d'éléphants (régression linéaire P < 0,001, radj² = 0,58). La superficie d'échantillonnage effective était de 3,22 km² par senseur acoustique, ce qui est une augmentation spectaculaire par rapport à la couverture des transects d'études par comptage des crottes. Ces résultats appuient le recours aux études acoustiques pour estimer l'abondance des éléphants dans de vastes zones retirées et dans des habitats variés, en utilisant un réseau de senseurs acoustiques bien répartis. Les méthodes d'estimation de l'abondance présentées peuvent être appliquées à toute espèce pour laquelle on a établi un indice d'abondance acoustique et une fonction de détection. Cette technique d'étude acoustique donne la possibilité d'améliorer la gestion et la conservation de nombreux taxons actifs au point de vue acoustique et dont les populations sont actuellement trop peu suivies. [source] Revised terminology and concepts for organization of seizures and epilepsies: Report of the ILAE Commission on Classification and Terminology, 2005,2009EPILEPSIA, Issue 4 2010Anne T. Berg Summary The International League Against Epilepsy (ILAE) Commission on Classification and Terminology has revised concepts, terminology, and approaches for classifying seizures and forms of epilepsy. Generalized and focal are redefined for seizures as occurring in and rapidly engaging bilaterally distributed networks (generalized) and within networks limited to one hemisphere and either discretely localized or more widely distributed (focal). Classification of generalized seizures is simplified. No natural classification for focal seizures exists; focal seizures should be described according to their manifestations (e.g., dyscognitive, focal motor). The concepts of generalized and focal do not apply to electroclinical syndromes. Genetic, structural,metabolic, and unknown represent modified concepts to replace idiopathic, symptomatic, and cryptogenic. Not all epilepsies are recognized as electroclinical syndromes. Organization of forms of epilepsy is first by specificity: electroclinical syndromes, nonsyndromic epilepsies with structural,metabolic causes, and epilepsies of unknown cause. Further organization within these divisions can be accomplished in a flexible manner depending on purpose. Natural classes (e.g., specific underlying cause, age at onset, associated seizure type), or pragmatic groupings (e.g., epileptic encephalopathies, self-limited electroclinical syndromes) may serve as the basis for organizing knowledge about recognized forms of epilepsy and facilitate identification of new forms. [source] Intellectual abilities and white matter microstructure in development: A diffusion tensor imaging studyHUMAN BRAIN MAPPING, Issue 10 2010Christian K. Tamnes Abstract Higher-order cognitive functions are supported by distributed networks of multiple interconnected cortical and subcortical regions. Efficient cognitive processing depends on fast communication between these regions, so the integrity of the connections between them is of great importance. It is known that white matter (WM) development is a slow process, continuing into adulthood. While the significance of cortical maturation for intellectual development is described, less is known about the relationships between cognitive functions and maturation of WM connectivity. In this cross-sectional study, we investigated the associations between intellectual abilities and development of diffusion tensor imaging (DTI) derived measures of WM microstructure in 168 right-handed participants aged 8,30 years. Independently of age and sex, both verbal and performance abilities were positively related to fractional anisotropy (FA) and negatively related to mean diffusivity (MD) and radial diffusivity (RD), predominantly in the left hemisphere. Further, verbal, but not performance abilities, were associated with developmental differences in DTI indices in widespread regions in both hemispheres. Regional analyses showed relations with both FA and RD bilaterally in the anterior thalamic radiation and the cortico-spinal tract and in the right superior longitudinal fasciculus. In these regions, our results suggest that participants with high verbal abilities may show accelerated WM development in late childhood and a subsequent earlier developmental plateau, in contrast to a steadier and prolonged development in participants with average verbal abilities. Longitudinal data are needed to validate these interpretations. The results provide insight into the neurobiological underpinnings of intellectual development. Hum Brain Mapp, 2010. © 2010 Wiley-Liss, Inc. [source] Toward brain correlates of natural behavior: fMRI during violent video gamesHUMAN BRAIN MAPPING, Issue 12 2006Klaus Mathiak Abstract Modern video games represent highly advanced virtual reality simulations and often contain virtual violence. In a significant amount of young males, playing video games is a quotidian activity, making it an almost natural behavior. Recordings of brain activation with functional magnetic resonance imaging (fMRI) during gameplay may reflect neuronal correlates of real-life behavior. We recorded 13 experienced gamers (18,26 years; average 14 hrs/week playing) while playing a violent first-person shooter game (a violent computer game played in self-perspective) by means of distortion and dephasing reduced fMRI (3 T; single-shot triple-echo echo-planar imaging [EPI]). Content analysis of the video and sound with 100 ms time resolution achieved relevant behavioral variables. These variables explained significant signal variance across large distributed networks. Occurrence of violent scenes revealed significant neuronal correlates in an event-related design. Activation of dorsal and deactivation of rostral anterior cingulate and amygdala characterized the mid-frontal pattern related to virtual violence. Statistics and effect sizes can be considered large at these areas. Optimized imaging strategies allowed for single-subject and for single-trial analysis with good image quality at basal brain structures. We propose that virtual environments can be used to study neuronal processes involved in semi-naturalistic behavior as determined by content analysis. Importantly, the activation pattern reflects brain-environment interactions rather than stimulus responses as observed in classical experimental designs. We relate our findings to the general discussion on social effects of playing first-person shooter games. Hum Brain Mapp, 2006. © 2006 Wiley-Liss, Inc. [source] Formation of a geometric pattern with a mobile wireless sensor networkJOURNAL OF FIELD ROBOTICS (FORMERLY JOURNAL OF ROBOTIC SYSTEMS), Issue 10 2004Justin Lee Mobile wireless sensor networks (MWSNs) will enable information systems to gather detailed information about the environment on an unprecedented scale. These self-organizing, distributed networks of sensors, processors, and actuators that are capable of movement have a broad range of potential applications, including military reconnaissance, surveillance, planetary exploration, and geophysical mapping. In many of the foreseen applications, the MWSN will need to form a geometric pattern without assistance from the user. In military reconnaissance, for example, the nodes will be dropped onto the battlefield from a plane and land at random positions. The nodes will be expected to arrange themselves into a predetermined formation in order to perform a specific task. Thus, we present algorithms for forming a line, circle, and regular polygon from a given set of random positions. The algorithms are distributed and use no communication between the nodes to minimize energy consumption. Unlike past studies of geometric problems where algorithms are either tested in simulations where each node has global knowledge of all the other nodes or implemented on a small number of robots, the robustness of our algorithms has been studied with simulations that model the sensor system in detail. The simulations demonstrate that the algorithms are robust against random errors in the sensors and actuators. © 2004 Wiley Periodicals, Inc. [source] Resting interhemispheric functional magnetic resonance imaging connectivity predicts performance after strokeANNALS OF NEUROLOGY, Issue 3 2010Alex R. Carter MD Objective Focal brain lesions can have important remote effects on the function of distant brain regions. The resulting network dysfunction may contribute significantly to behavioral deficits observed after stroke. This study investigates the behavioral significance of changes in the coherence of spontaneous activity in distributed networks after stroke by measuring resting state functional connectivity (FC) using functional magnetic resonance imaging. Methods In acute stroke patients, we measured FC in a dorsal attention network and an arm somatomotor network, and determined the correlation of FC with performance obtained in a separate session on tests of attention and motor function. In particular, we compared the behavioral correlation with intrahemispheric FC to the behavioral correlation with interhemispheric FC. Results In the attention network, disruption of interhemispheric FC was significantly correlated with abnormal detection of visual stimuli (Pearson r with field effect = ,0.624, p = 0.002). In the somatomotor network, disruption of interhemispheric FC was significantly correlated with upper extremity impairment (Pearson r with contralesional Action Research Arm Test = 0.527, p = 0.036). In contrast, intrahemispheric FC within the normal or damaged hemispheres was not correlated with performance in either network. Quantitative lesion analysis demonstrated that our results could not be explained by structural damage alone. Interpretation These results suggest that lesions cause state changes in the spontaneous functional architecture of the brain, and constrain behavioral output. Clinically, these results validate using FC for assessing the health of brain networks, with implications for prognosis and recovery from stroke, and underscore the importance of interhemispheric interactions. ANN NEUROL 2010;67:365,375 [source] | ||||||||||||||||