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Results for “spatial synchrony”

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PubMed2004

Comparison of methods for detecting nondeterministic BOLD fluctuation in fMRI.

Functional MR imaging (fMRI) has been used in detecting neuronal activation and intrinsic blood flow fluctuations in the brain cortex. This article is aimed for comparing the methods for analyzing the nondeterministic flow fluctuations. Fast Fourier Transformation (FFT), cross correlation (CC), spatial principal component analysis (sPCA), and independent component analysis (sICA) were compared. 15 subjects were imaged at 1.5 T. Three quantitative measures were compared: (1) The number of subjects with identifiable fluctuation, (2) the volume, and (3) mean correlation coefficient (MCC) of the detected voxels. The focusing on cortical structures and the overall usability were qualitatively assessed. sICA was spatially most accurate but time consuming, robust, and detected voxels with high temporal synchrony. The CC and FFT were fast suiting primary screening. The CC detected highest temporal synchrony but the subjective detection for reference vector produced excess variance of the detected volumes. The FFT and sPCA were not spatially accurate and did not detect adequate temporal synchrony of the voxels.

Anesthesia↗

PubMed1999

Spatiotemporal analysis of local field potentials and unit discharges in cat cerebral cortex during natural wake and sleep states.

The electroencephalogram displays various oscillation patterns during wake and sleep states, but their spatiotemporal distribution is not completely known. Local field potentials (LFPs) and multiunits were recorded simultaneously in the cerebral cortex (areas 5-7) of naturally sleeping and awake cats. Slow-wave sleep (SWS) was characterized by oscillations in the slow (<1 Hz) and delta (1-4 Hz) frequency range. The high-amplitude slow-wave complexes consisted in a positivity of depth LFP, associated with neuronal silence, followed by a sharp LFP negativity, correlated with an increase of firing. This pattern was of remarkable spatiotemporal coherence, because silences and increased firing occurred simultaneously in units recorded within a 7 mm distance in the cortex. During wake and rapid-eye-movement (REM) sleep, single units fired tonically, whereas LFPs displayed low-amplitude fast activities with increased power in fast frequencies (15-75 Hz). In contrast with the widespread synchronization during SWS, fast oscillations during REM and wake periods were synchronized only within neighboring electrodes and small time windows (100-500 msec). This local synchrony occurred in an apparent irregular manner, both spatially and temporally. Brief periods (<1 sec) of fast oscillations were also present during SWS in between slow-wave complexes. During these brief periods, the spatial and temporal coherence, as well as the relation between units and LFPs, was identical to that of fast oscillations of wake or REM sleep. These results show that natural SWS in cats is characterized by slow-wave complexes, synchronized over large cortical territories, interleaved with brief periods of fast oscillations, characterized by local synchrony, and of characteristics similar to that of the sustained fast oscillations of activated states.

Animals↗

PubMed1984

The development of detour ability during infancy.

3 longitudinal studies were conducted to examine the generalization of detour ability across motor responses and barrier types and the relationship between the development of object permanence and detour ability. In Experiment 1, 12 8-month-olds were tested every 3 weeks for 4 months on 4 different detour problems and Stage 4 and 6 object permanence tasks. In the detour problems, infants had to reach or move around a transparent or opaque barrier to obtain an object. The results indicated that infants made reaching detours before corresponding locomotor ones and generally made detours around opaque barriers before transparent ones. Infants also solved the Stage 4 task before the detour problems but failed to solve the Stage 6 task before testing ended. The results of Experiments 2 and 3 suggested that the difference in reaching and locomotor detour performance was not an artifact of barrier length or the infant's position relative to the barrier. The overall results are discussed in relation to issues of developmental synchrony and Piaget's theory of infant spatial development.

Child Development↗

PubMed2005

A spectrum from pure post-spike effects to synchrony effects in spike-triggered averages of electromyographic activity during skilled finger movements.

During individuated finger movements, a high proportion of synchrony effects was found in spike-triggered averages (SpikeTAs) of rectified electromyographic activity aligned on the spikes discharged by primary motor cortex (M1) neurons. Because synchrony effects can be produced even if the trigger neuron itself provides no direct synaptic connections to motoneurons, such nonoscillatory synchrony effects often are discounted when considering control of motoneuron pools. We therefore examined the distinctions between pure postspike effects and synchrony effects. The criteria usually applied to distinguish pure and synchrony effects-onset latency and peak width-failed to separate the present SpikeTA effects objectively into distinct subpopulations. Synchrony effects generally were larger than pure effects. Many M1 neurons produced pure effects in some muscles while producing synchrony effects in others. M1 neurons producing no effects, only pure effects, only synchrony effects, or both pure and synchrony effects did not fall into different groups based on discharge characteristics during finger movements. Nor were neurons producing different types of SpikeTA effects segregated spatially in M1. These observations suggest that neurons producing pure and synchrony SpikeTA effects come from similar M1 populations. We discuss potential mechanisms that might have produced a continuous spectrum of variation from pure to synchrony effects in the present monkeys. Although synchrony effects cannot be taken as evidence of mono- or disynaptic connections from the recorded neuron to the motoneuron pool, the functional linkages indicated by synchrony effects represent a substantial fraction of M1 input to motoneuron pools during skilled, individuated finger movements.

Action Potentials↗

PubMed2001

Large-scale synchrony in weakly interacting automata.

We study the behavior of two spatially distributed (sandpile) models which are weakly linked with one another. Using a Monte Carlo implementation of the renormalization-group and algebraic methods, we describe how large-scale correlations emerge between the two systems, leading to synchronized behavior.

Journal Article↗

PubMed2000

Synchronous oscillation in the cerebral cortex and object coherence: simulation of basic electrophysiological findings.

A lumped continuum model for electrocortical activity was used to simulate several established experimental findings of synchronous oscillation which have not all been previously embodied in a single explanatory model. Moving-bar visual stimuli of different extension, stimuli moving in different directions, the impact of non-specific cortical activation upon synchronous oscillation, and the frequency content of EEG associated with synchrony were considered. The magnitude of zero lag synchrony was primarily accounted for by the properties of the eigenmodes of the travelling local field potential superposition waves generated by inputs to the cortex, largely independent of the oscillation properties and associated spectral content. Approximation of the differences in cross-correlation observed with differently moving bar stimuli, and of the impact of cortical activation, required added assumptions on (a) spatial coherence of afferent volleys arising from parts of a single stimulus object and (b) the presence of low-amplitude diffuse field noise, with enhancement of cortical signal/noise ratio with respect to the spatially coherent inputs, at higher levels of cortical activation. Synchrony appears to be a ubiquitous property of cortex-like delay networks. Precision in the modelling of synchronous oscillation findings will require detailed description of input pathways, cortical connectivity, cortical stability, and aspects of cortical/subcortical interactions.

Artifacts↗

PubMed2001

Synchronous activity of inhibitory networks in neocortex requires electrical synapses containing connexin36.

Inhibitory interneurons often generate synchronous activity as an emergent property of their interconnections. To determine the role of electrical synapses in such activity, we constructed mice expressing histochemical reporters in place of the gap junction protein Cx36. Localization of the reporter with somatostatin and parvalbumin suggested that Cx36 was expressed largely by interneurons. Electrical synapses were common among cortical interneurons in controls but were nearly absent in knockouts. A metabotropic glutamate receptor agonist excited LTS interneurons, generating rhythmic inhibitory potentials in surrounding neurons of both wild-type and knockout animals. However, the synchrony of these rhythms was weaker and more spatially restricted in the knockout. We conclude that electrical synapses containing Cx36 are critical for the generation of widespread, synchronous inhibitory activity.

Animals↗

PubMed2000

Spatial synchronization of vole population dynamics by predatory birds.

Northern vole populations exhibit large-scale, spatially synchronous population dynamics. Such cases of population synchrony provide excellent opportunities for distinguishing between local intrinsic and regional extrinsic mechanisms of population regulation. Analyses of large-scale survey data and theoretical modelling have indicated several plausible synchronizing mechanisms. It is difficult, however, to determine the most important one without detailed data on local demographic processes. Here we combine results from two field studies in southeastern Norway--one identifies local demographic mechanisms and landscape-level annual synchrony among 28 enclosed experimental populations and the other examines region-level multi-annual synchrony in open natural populations. Despite fences eliminating predatory mammals and vole dispersal, the growth rates of the experimental populations were synchronized and moreover, perfectly linked with vole abundance in the region. The fates of 481 radio-marked voles showed that bird predation was the synchronizing mechanism. A higher frequency of risky dispersal movements in slowly growing populations appeared to accelerate predation rate. Thus, dispersal may induce a feedback-loop between predation and population growth that enhances synchrony.

Animals↗

PubMed2002

Investigation of the neuronal aggregate generating seizures in the rat tetanus toxin model of epilepsy.

A key question in epilepsy is the organization and size of the neuronal networks necessary for generating seizures. Hypotheses include: a single focal neuronal network drives seizure discharges across the brain, which may or may not be identical with the circuits that generate interictal spikes; or multiple neuronal networks link together in re-entrant loops or other long-range networks. It remains unclear whether any of these hypotheses apply to spontaneous seizures in freely moving animals. We used the tetanus toxin chronic model of epilepsy to test the different predictions made by each hypothesis about the propagation and interaction of epileptic discharges during seizures. Seizures could start in either the injected or noninjected dorsal hippocampus, suggesting that seizures have multifocal onsets in the tetanus toxin model. During seizures, individual bursts propagated in either direction, both between the right and left dorsal hippocampi, and between CA3 and the dentate gyrus in the same hippocampus. These findings argue against one site "driving" seizures or seizures propagating around a limbic loop. Specifically, the side leading each burst switched a median of three times during the first 20 s of a seizure. Analysis of bursts during seizures suggested that the network at each recording site acted like a neuronal oscillator. Coupling of population spikes in right and left CA3 increased during the early part of seizures, but the cross-correlation of their whole-discharge waveforms changed little over the same period. Furthermore, the polarity of the phase difference between population spikes did not follow the phase difference for complete discharges. We concluded that the neuronal aggregate necessary for seizures in our animals comprises multiple spatially distributed neuronal networks and that the increased synchrony of the output (population spike firing) of these networks during the early part of seizures may contribute to seizure generation.

Action Potentials↗

PubMed1998

BacSim, a simulator for individual-based modelling of bacterial colony growth.

The generic, quantitative, spatially explicit, individual-based model BacSim was developed to simulate growth and behaviour of bacteria. The potential of this approach is in relating the properties of microscopic entities - cells - to the properties of macroscopic, complex systems such as biofilms. Here, the growth of a single Escherichia coli cell into a colony was studied. The object-oriented program BacSim is an extension of Gecko, an ecosystem dynamics model which uses the Swarm toolkit for multi-agent simulations. The model describes bacterial properties including substrate uptake, metabolism, maintenance, cell division and death at the individual cell level. With the aim of making the model easily applicable to various bacteria under different conditions, the model uses as few as eight readily obtainable parameters which can be randomly varied. For substrate diffusion, a two-dimensional diffusion lattice is used. For growth-rate-dependent cell size variation, a conceptual model of cell division proposed by Donachie was examined. A mechanistic version of the Donachie model led to unbalanced growth at higher growth rates, whereas including a minimum period between subsequent replication initiations ensured balanced growth only if this period was unphysiologically long. Only a descriptive version of the Donachie model predicted cell sizes correctly. For maintenance, the Herbert model (constant specific rate of biomass consumption) and for substrate uptake, the Michaelis-Menten or the Best equations were implemented. The simulator output faithfully reproduced all input parameters. Growth characteristics when maintenance and uptake rates were proportional to either cell mass or surface area are compared. The authors propose a new generic measure of growth synchrony to quantify the loss of synchrony due to random variation of cell parameters or spatial heterogeneity. Variation of the maximal uptake rate completely desynchronizes the simulated culture but variation of the volume-at-division does not. A new measure for spatial heterogeneity is introduced: the standard deviation of substrate concentrations as experienced by the cells. Spatial heterogeneity desynchronizes population growth by subdividing the population into parts synchronously growing at different rates. At a high enough spatial heterogeneity, the population appears to grow completely asynchronously.

Algorithms↗

PubMed1998

Saccade-vergence trajectories under free- and instrument-space environments.

PURPOSE: The purpose of this study was to examine in detail the binocular fixation top-view trajectories of saccade-vergence responses to asymmetrical targets, and to compare latency difference between saccade and vergence, under the free- and instrument-space viewing environments. METHODS: Binocular eye movements were recorded using the infrared reflection technique in five visually-normal subjects. Responses were obtained for various asymmetrical target positions under both free- and instrument-space environments. RESULTS: Four types of top-view trajectories that represented normal variations in saccade and vergence control were found: straight, overshoot, undershoot, and saccade-vergence. Also, it was found that under the instrument-space environment, there was a predominance of saccade-vergence trajectories and a scarcity of overshoot trajectories, whereas under the free-space environment, there was a predominance of overshoot trajectories, and a scarcity of saccade-vergence trajectories. Further, under the instrument-space environment, latency was significantly longer for saccade than vergence (35.9 +/- 15.7 msec; t = 5.1, degrees of freedom (df) = 4, P < 0.01), whereas under the free-space environment, there was no latency difference (-10.5 +/- 14.8 msec; = -1.6, df = 4, P > 0.05). CONCLUSIONS: The differences in response profiles under the two viewing environments could be accounted for by differences in timing of saccade and vergence onset. Moreover, in contrast to some recent investigations that suggest higher center control of individual trajectories, which was dependent on the naturalistic scene, these trajectories could be accounted for by known neural and oculomotor mechanisms, with the higher centers using a priori information about spatial location of the target, to assist in the synchrony of saccade and vergence onset under the free-space environment.

Adult↗

PubMed2006

Task-modulation of functional synchrony between spontaneous low-frequency oscillations in the human brain detected by fMRI.

Recent developments in functional MRI (fMRI) technology with high spatial and temporal resolution have made it possible to noninvasively detect spontaneous low-frequency oscillations (SLOs) and quantify their functional synchrony in the human brain. In the present fMRI study the dynamic characteristics of the functional synchrony between SLOs were quantitatively determined by the phase shift index (PSI). With the use of an fMRI-guided voxel-selection method, the SLOs and their functional synchrony were found to be modulated by different memory tasks. The results demonstrate that SLOs in episodic memory-related circuitry have significantly higher synchrony during the performance of declarative memory-encoding activities compared to nondeclarative memory-encoding activities. It is suggested that the dynamic property of SLOs and the quantitative assessment of their functional synchrony could be utilized as a biomarker to noninvasively characterize localized pathophysiological functions in the human brain.

Adult↗

PubMed2005

Density-dependent dispersal and spatial population dynamics.

The synchronization of the dynamics of spatially subdivided populations is of both fundamental and applied interest in population biology. Based on theoretical studies, dispersal movements have been inferred to be one of the most general causes of population synchrony, yet no empirical study has mapped distance-dependent estimates of movement rates on the actual pattern of synchrony in species that are known to exhibit population synchrony. Northern vole and lemming species are particularly well-known for their spatially synchronized population dynamics. Here, we use results from an experimental study to demonstrate that tundra vole dispersal movements did not act to synchronize population dynamics in fragmented habitats. In contrast to the constant dispersal rate assumed in earlier theoretical studies, the tundra vole, and many other species, exhibit negative density-dependent dispersal. Simulations of a simple mathematical model, parametrized on the basis of our experimental data, verify the empirical results, namely that the observed negative density-dependent dispersal did not have a significant synchronizing effect.

Animals↗

PubMed2004

Calcium oscillations in a triplet of pancreatic acinar cells.

We use a mathematical model of calcium dynamics in pancreatic acinar cells to investigate calcium oscillations in a ring of three coupled cells. A connected group of cells is modeled in two different ways: 1), as coupled point oscillators, each oscillator being described by a spatially homogeneous model; and 2), as spatially distributed cells coupled along their common boundaries by gap-junctional diffusion of inositol trisphosphate and/or calcium. We show that, although the point-oscillator model gives a reasonably accurate general picture, the behavior of the spatially distributed cells cannot always be predicted from the simpler analysis; spatially distributed diffusion and cell geometry both play important roles in determining behavior. In particular, oscillations in which two cells are in synchrony, with the third phase-locked but not synchronous, appears to be more dominant in the spatially distributed model than in the point-oscillator model. In both types of model, intercellular coupling leads to a variety of synchronous, phase-locked, or asynchronous behaviors. For some parameter values there are multiple, simultaneous stable types of oscillation. We predict 1), that intercellular calcium diffusion is necessary and sufficient to coordinate the responses in neighboring cells; 2), that the function of intercellular inositol trisphosphate diffusion is to smooth out any concentration differences between the cells, thus making it easier for the diffusion of calcium to synchronize the oscillations; 3), that groups of coupled cells will tend to respond in a clumped manner, with groups of synchronized cells, rather than with regular phase-locked periodic intercellular waves; and 4), that enzyme secretion is maximized by the presence of a pacemaker cell in each cluster which drives the other cells at a frequency greater than their intrinsic frequency.

Animals↗

PubMed2000

Position and behavioral modulation of synchronization of hippocampal and accumbens neuronal discharges in freely moving rats.

To understand how hippocampal signals are processed by downstream neurons, we analyzed the relative timing between neuronal discharges in simultaneous recordings in the hippocampus and nucleus accumbens of rats performing in a plus maze. In all, 154 pairs of cells (composed of 65 hippocampal and 56 accumbens neurons) were examined during the 1 s period prior to reward delivery. Cross-correlation analyses over a +/- 300-ms window with 10-ms bins revealed that 108 pairs had at least one significant histogram bin (P < 0.01). The most frequently occurring peaks of hippocampal firing prior to accumbens discharges appeared at latencies from -30-0 ms, corresponding to published values of the latency of the hippocampal pathway to the nucleus accumbens. Other peaks appeared most often at latencies multiples of about 110 ms prior to and after this, corresponding to theta rhythmicity. Since firing synchronization can result from several types of connectivity patterns (such as common inputs), a group of 18 hippocampus-accumbens pairs was selected as those most likely to have monosynaptic connections. The criterion was the presence of at least one highly significant peak (P < 0.001) at latencies corresponding to field potentials evoked in the accumbens by hippocampal stimulation. A significant peak occurred on all four maze arms for only one of these cell pairs, indicating positional modulation for the others. In addition, behavior dependence of the synchrony between these nucleus accumbens and hippocampus neurons was examined by studying data in relation to three different synchronization points: reward box arrival, box departure, and arrival at the center of the maze. This indicates that the functional connectivity between hippocampal and accumbens neurons was stronger when the rat was near reward areas. Ten of the hippocampal neurons in these 18 cell pairs showed 9-Hz (theta) rhythmic activity in autocorrelation analyses. Of these 10 cells, cross-correlograms from eight hippocampal-accumbens pairs also showed theta rhythmicity. Overall, these results indicate that the synchrony between hippocampus and nucleus accumbens neurons is modulated by spatial position and behavior, and theta rhythm may play an important role for this synchronization.

Animals↗

PubMed2006

Multisensory integration of speech signals: the relationship between space and time.

Integrating audiovisual cues for simple events is affected when sources are separated in space and time. By contrast, audiovisual perception of speech appears resilient when either spatial or temporal disparities exist. We investigated whether speech perception is sensitive to the combination of spatial and temporal inconsistencies. Participants heard the bisyllable /aba/ while seeing a face produce the incongruent bisyllable /ava/. We tested the level of visual influence over auditory perception when the sound was asynchronous with respect to facial motion (from -360 to +360 ms) and emanated from five locations equidistant to the participant. Although an interaction was observed, it was not related to participants' perception of synchrony, nor did it indicate a linear relationship between the effect of spatial and temporal discrepancies. We conclude that either the complexity of the signal or the nature of the task reduces reliance on spatial and temporal contiguity for audiovisual speech perception.

Acoustic Stimulation↗

PubMed2000

Activity synchrony and social cohesion: a fission-fusion model.

A social group can only be spatially coherent if its members synchronize activities such as foraging and resting. However, activity synchronization is costly to individuals if it requires them to postpone an activity that would be personally more profitable in order to do what the rest of the group is doing. Such costs will be particularly high in groups whose members belong to different age, size or sex classes since the optimal allocation of time to various activities is likely to differ between such classes. Thus, differences in the costs of activity synchronization between and within classes could cause non-homogenous groups to be less stable than homogenous groups, with the result that homogenous groups predominate in the population: that is, they could cause 'social segregation' of animals of different sex, size or age. We develop a model that predicts the degree of social segregation attributable to differences in activity synchronization between homogenous and non-homogenous groups and use this model in determining whether activity synchronization can explain intersexual social segregation in red deer (Cervus elaphus). Differences in activity synchronization between mixed-sex and unisex groups of red deer explained 35% of the observed degree of intersexual social segregation, showing that activity synchronization is an important cause of social segregation in this species.

Animals↗

PubMed2006

Perceived self-motion in two visual contexts: dissociable mechanisms underlie perception.

We evaluated the influence of moving visual scenes and knowledge of spatial and physical context on visually induced self-motion perception in an immersive virtual environment. A sinusoidal, vertically oscillating visual stimulus induced perceptions of self-motion that matched changes in visual acceleration. Subjects reported peaks of perceived self-motion in synchrony with peaks of visual acceleration and opposite in direction to visual scene motion. Spatial context was manipulated by testing subjects in the environment that matched the room in the visual scene or by testing them in a separate chamber. Physical context was manipulated by testing the subject while seated in a stable, earth-fixed desk chair or in an apparatus capable of large linear motions, however, in both conditions no actual motion occurred. The compellingness of perceived self-motion was increased significantly when the spatial context matched the visual input and actual body displacement was possible, however, the latency and amplitude of perceived self-motion were unaffected by the spatial or physical context. We propose that two dissociable processes are involved in self-motion perception: one process, primarily driven by visual input, affects vection latency and path integration, the other process, receiving cognitive input, drives the compellingness of perceived self-motion.

Adolescent↗