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PubMed2002

Stochastic catastrophe analysis of switches in the perception of apparent motion.

Dynamical phenomena such as bistability and hysteresis have been found in a number of studies on perception of apparent motion. We show that new developments in stochastic catastrophe theory make it possible to test models of these phenomena empirically. Catastrophe theory explains discontinuous changes in responses caused by continuous changes in experimental parameters. We propose catastrophe models for two experimental paradigms on perception of apparent motion and present experiments that support these models. We test these models by using an algorithm for fitting stochastic catastrophe models. We derive from catastrophe theory the prediction that a dynamical phenomenon called divergence is necessary when hysteresis is found. This new prediction is supported by the data.

Humans↗
PubMed1997

Perceptual grouping in space and time: evidence from the Ternus display.

We report three experiments investigating the effect of perceptual grouping on the appearance of a bistable apparent-motion (Ternus) display. Subjects viewed a Ternus display embedded in an array of context elements that could potentially group with the Ternus elements. In contrast to several previous findings, we found that grouping influenced apparent motion perception. In Experiment 1, apparent motion perception was significantly affected via grouping by shape similarity, even when the visible persistence of the elements was controlled. In Experiment 2, elements perceived as moving without context were perceived as stationary without context were perceived as moving when grouped with moving elements perceived as stationary without context were perceived as moving when grouped with moving context elements. We argue that grouping in the spatial and temporal domains interact to yield perceptual experience of apparent-motion displays.

Adult↗
PubMed1997

Time course of perceptual grouping.

An investigation was made of the time course of perceptual grouping that is based on two qualitatively different spatial relationships: proximity and alignment. An index of grouping capacity was used to assess the processing time required before a backward pattern mask interfered with grouping. Stimuli consisted of bistable arrays of disjunct dots that were followed by a mask. Grouping cues, either proximity or alignment, were randomly assigned to either the horizontal or vertical orientation, and subjects indicated whether the dots appeared grouped as a series of horizontal or vertical lines. Spatial metrics of the cues were systematically altered until they no longer served as a cue for grouping thereby determining the grouping threshold. The stimulus onset asynchrony (SOA) of the mask, relative to the test stimulus, ranged from 33.3 to 150 msec. The SOA at which grouping thresholds first became elevated identified the point at which the mask first interfered with the grouping process, thereby identifying the processing time required for grouping by the specified cue. The processing time for grouping by proximity and alignment differed significantly, requiring means of 87.6 and 118.8 msec, respectively, for processing to be completed. These measurements serve to identify the processing time necessary for spatially integrating stimulus elements into unified forms, thereby delineating temporal constraints at this stage of visual processing.

Adult↗
PubMed1977

Combined ultrasound and needle aspiration cytology in the assessment and management of hypofunctioning thyroid nodule.

We evaluated the efficacy of combining B-scan bistable and gray-scale ultrasound with needle aspiration cytology in assessing and managing solitary hypofunctioning solid and cystic nodules. Of the 150 cases surveyed, 90 had surgical follow-up histology for comparison to preoperative aspiration cytology results. Overall neoplasm rate in these 90 cases was 66%, increasing to 93% if colloid nodules were included. Adequate material for cytology preoperatively was obtained in 83 (92%). In cases with adequate preoperative needle aspiration cytology, there were 66 solid and 17 cystic or mixed lesions. Overall accuracy for differentiating benign from malignant nodules for solid lesions was 63 of 66 (95%), compared to 15 of 17 (88%) for predominantly cystic or mixed lesions. Of the 17 cases of proven carcinoma, correct preoperative cytology diagnosis for malignancy was obtained in 12 (71%). Malignancy was most often correctly diagnosed for solid papillary and metastatic cancer lesions. No false-positives were noted and atypical adenomas and Hürthle-cell adenomas have been correctly diagnosed. The simplicity and safety of these diagnostic procedures justify their use for "selective" surgery and particularly for those cases that have been initially assigned to conservative, nonsurgical therapy.

Adenoma↗
PubMed1987

[Instabilities in tactile, visual and auditory sense--application of stochastic catastrophe model].

As a fundamental study on management of the working environment, instabilities in tactile, visual and auditory sense are discussed. The effects of localized vibration on the ability to discriminate the surface roughness are analyzed by means of the stochastic cusp catastrophe model. Also, bistable perception of the ambiguous man/girl figure is modelled in the same way. Furthermore, the instable phenomenon of a pure tone when heard under the masking noise is pointed out and analyzed by the catastrophe model.

Adult↗
PubMed2002

Neural correlates of structure-from-motion perception in macaque V1 and MT.

Structure-from-motion (SFM) is the perception of three-dimensional shape from motion cues. We used a bistable SFM stimulus, which can be perceived in one of two different ways, to study how neural activity in cortical areas V1 and MT is related to SFM perception. Monkeys performed a depth-order task, where they indicated in which direction the front surface of a rotating SFM cylinder display was moving. To prevent contamination of the neural data because of eye position effects, all experiments with significant effects of radius, vergence, and velocity were excluded. As expected, the activity of approximately 50% of neurons in V1 and approximately 80% of neurons in MT is affected by the stimulus. Furthermore, the activity of 20% of neurons in area V1 is modulated with the percept. This proportion is higher in MT, where the activity of >60% of neurons is modulated with the percept. In both areas, this perceptual modulation occurs only in neurons with activity that is also affected by the stimulus. The perceptual modulation is not correlated with neural tuning properties in area V1, but it is in area MT. Together, these results suggest that V1 is not directly involved in the generation of the SFM percept, whereas MT is. The perceptual modulation in V1 may be attributable to top-down feedback from MT.

Action Potentials↗
PubMed2000

A cooperative model for Ca(++) efflux windowing from cell membranes exposed to electromagnetic radiation.

We propose a simplified version of a cooperative lattice membrane model given by Grodsky to explain observed Ca(++) efflux windowing effects from cell membranes exposed to electromagnetic radiation. Assuming that field induced conformational interactions occur only between bistable receptor sites and glycoprotein Ca(++) sites on the surface of the membrane, the model is shown to be equivalent to an Ising model. This model is known to have a phase transition to an ordered state in which a macroscopic number of Ca(++) sites are either occupied or unoccupied. We identify such states with enhanced Ca(++) efflux from cell membranes. By further assuming an averaged signal, sinusoidally varying coupling between receptor and Ca(++) sites and a power-law dependence of the characteristic time constant on the induced power-density of the applied field, we show that the model is consistent with published experimental results on power density windowing effects for particular values of model parameters. For these parameter values, the model predicts further power densities where windowing effects may be observed under appropriate conditions.

Algorithms↗
PubMed2000

Effects of delay on the type and velocity of travelling pulses in neuronal networks with spatially decaying connectivity.

We study a one-dimensional model of integrate-and-fire neurons that are allowed to fire only one spike, and are coupled by excitatory synapses with delay. At small delay values, this model describes a disinhibited cortical slice. At large delay values, the model is a reduction of a model of thalamic networks composed of excitatory and inhibitory neurons, in which the excitatory neurons show the post-inhibitory rebound mechanism. The velocity and stability of propagating continuous pulses are calculated analytically. Two pulses with different velocities exist if the synaptic coupling is larger than a minimal value; the pulse with the lower velocity is always unstable. Above a certain critical value of the constant delay, continuous pulses lose stability via a Hopf bifurcation, and lurching pulses emerge. The parameter regime for which lurching occurs is strongly affected by the synaptic footprint (connectivity) shape. A bistable regime, in which both continuous and lurching pulses can propagate. may occur with square or Gaussian footprint shapes but not with an exponential footprint shape. A perturbation calculation is used in order to calculate the spatial lurching period and the velocity of lurching pulses at large delay values. For strong synaptic coupling, the velocity of the lurching pulse is governed by the tail of the synaptic footprint shape. Moreover, the velocities of continuous and lurching pulses have the same functional dependencies on the strength of the synaptic coupling strength gsyn: they increase logarithmically with gsyn for an exponential footprint shape, they scale like (In gsyn)1/2 for a Gaussian footprint shape, and they are bounded for a square footprint shape or any shape with a finite support. We find analytically how the axonal propagation velocity reduces the velocity of continuous pulses; it does not affect the critical delay. We conclude that the differences in velocity and shape between the front of thalamic spindle waves in vitro and cortical paroxysmal discharges stem from their different effective delays.

Algorithms↗
PubMed2001

Dynamics of low-threshold spike activation in relay neurons of the cat lateral geniculate nucleus.

The low-threshold spike (LTS), generated by the transient Ca(2+) current I(T), plays a pivotal role in thalamic relay cell responsiveness and thus in the nature of the thalamic relay. By injecting depolarizing current ramps at various rates to manipulate the slope of membrane depolarization (dV/dt), we found that an LTS occurred only if dV/dt exceeded a minimum value of approximately 5-12 mV/sec. We injected current ramps of variable dV/dt into relay cells that were sufficiently hyperpolarized to de-inactivate I(T) completely. Higher values of dV/dt activated an LTS. However, lower values of dV/dt eventually led to tonic firing without ever activating an LTS; apparently, the inactivation of I(T) proceeded before I(T) could be recruited. Because the maximum rate of rise of the LTS decreased with slower activating ramps of injected current, we conclude that slower ramps allow increasing inactivation of I(T) before the threshold for its activation gating is reached, and when the injected ramps have a sufficiently low dV/dt, the inactivation is severe enough to prevent activation of an LTS. In the presence of Cs(+), we found that even the lowest dV/dt that we applied led to LTS activation, apparently because Cs(+) reduced the K(+) "leak" conductance and increased neuronal input resistance. Nonetheless, under normal conditions, our data suggest that there is neither significant window current (related to the overlap of the inactivation and activation curves for I(T)), rhythmogenic properties, nor bistability properties for these neurons. Our theoretical results using a minimal model of LTS excitability in these neurons are consistent with the experimental observations and support our conclusions. We suggest that inputs activating very slow EPSPs (i.e., via metabotropic receptors) may be able to inactivate I(T) without generating sizable I(T) and a spurious burst of action potentials to cortex.

Action Potentials↗
PubMed2001

The role of activity-dependent network depression in the expression and self-regulation of spontaneous activity in the developing spinal cord.

Spontaneous episodic activity occurs throughout the developing nervous system because immature circuits are hyperexcitable. It is not fully understood how the temporal pattern of this activity is regulated. Here, we study the role of activity-dependent depression of network excitability in the generation and regulation of spontaneous activity in the embryonic chick spinal cord. We demonstrate that the duration of an episode of activity depends on the network excitability at the beginning of the episode. We found a positive correlation between episode duration and the preceding inter-episode interval, but not with the following interval, suggesting that episode onset is stochastic whereas episode termination occurs deterministically, when network excitability falls to a fixed level. This is true over a wide range of developmental stages and under blockade of glutamatergic or GABAergic/glycinergic synapses. We also demonstrate that during glutamatergic blockade the remaining part of the network becomes more excitable, compensating for the loss of glutamatergic synapses and allowing spontaneous activity to recover. This compensatory increase in the excitability of the remaining network reflects the progressive increase in synaptic efficacy that occurs in the absence of activity. Therefore, the mechanism responsible for the episodic nature of the activity automatically renders this activity robust to network disruptions. The results are presented using the framework of our computational model of spontaneous activity in the developing cord. Specifically, we show how they follow logically from a bistable network with a slow activity-dependent depression switching periodically between the active and inactive states.

Animals↗
PubMedDate not supplied

[New spatial-temporal dynamics in a reaction-electrodiffusion system].

The properties of a system reaction-electrodiffusion were studied using two-component model of interaction and diffusion of charged particles near membrane in solutions of low ionic strength to which traditional assumptions about local electroneutrality of medium are not applicable. It is shown that the effect of self-consistent electric field leads to bistability, the appearance of localized structures with contrast charge distribution and regimes aperiodical in time and in space.

Algorithms↗
PubMed2005

Parameterization of a nonlinear genotype to phenotype map using molecular networks.

Mathematical models of networks of molecular interactions controlling the expression of traits could theoretically be used as genotype to phenotype (GP) maps. Such maps are nonlinear functions of the environment and the genotype. It is possible to use nonlinear least square minimization methods to fit a model to a set of phenotypic data but the convergence of these methods is not automatic and may lead to a multiplicity of solutions. Both factors raise a number of questions with respect to using molecular networks as nonlinear maps. A method to fit a molecular network representing a bistable switch to various types of phenotypic data is introduced. This method relies on the identification of the model stable steady states and the estimation of the proportion of cells in each of them. By using environmental perturbations, it is possible to collect time-series of phenotypic data resulting in a smooth objective function leading to a good estimate of the parameters used to generate the simulated phenotypes.

Chromosome Mapping↗
PubMedDate not supplied

Geometry of liver system functioning: from a helix to a trajectory on curved space.

Autoisodiasostasis of the liver, i.e., its self-maintenance without change over the course of time, is characterized by a bistable equilibrium between two extreme states called homopoiesis (during which the liver system repairs its worn structures and replicates its cells) and homoerhesis (during which it satisfies the needs of the body). Retinol binding protein traffic within the hepatocyte, made visible by means of immunohistology, has been used as a prototype for studying the competition processes and dynamics of autoisodiasostasis. An account is given of the oscillation of autoisodiasostasis between homopoiesis and homeorhesis through intermediate phases. The phase cycle of autoisodiasostasis is illustrated in the form of bi-dimensional and tri-dimensional diagrams. The temporal behaviour of the systems is represented as a helix trajectory obtained by projecting homopoiesis-homeorhesis phase cycles onto a time series. A unified topological representation of the uncoupled but combined processes of competition and autoisodiasostasis is proposed.

Humans↗
PubMed1990

Graph-theoretic approach to metabolic pathways.

A graph-theoretic approach is shown to be applicable within the framework of the metabolic control analysis. Kinetic differential equations linearized near a steady state are presented as kinetic graphs (schemes), their structure being correlated with kinetic properties of corresponding metabolic networks. The global properties may be expressed in terms of the local properties for steady states of metabolic systems. Instability, bistability, and concentrational oscillations are shown to be induced by specific graph fragments. The approach is illustrated by an example of systems showing the oscillatory kinetic behaviour.

Biotransformation↗
PubMed1985

Excitation dynamics: insights from simplified membrane models.

Excitable nerve membranes and models for their electrical activity exhibit a broad repertoire of dynamic behavior. To reveal these behaviors the theoretician seeks a model that is simple enough to analyze yet one that retains adequate biophysical realism. Here we strike such a balance by describing a two-variable simplification of the Hodgkin-Huxley (HH) model, which exhibits many membrane phenomena and reproduces, with good agreement, many HH responses. Comparisons and illustrations are presented for the single spike response, repetitive firing (and its cessation by a brief current pulse), and bistable behavior for increased extracellular K+ concentrations.

Animals↗
PubMedDate not supplied

Liver system. III. Prototype to study its unstable equilibrium on human biopsy samples.

Autoisodiasostasis of the liver, i.e., its self-maintenance without change over the course of time, is characterized by a bistable equilibrium between two extreme phases called homopoiesis, during which the liver system repairs its worn structures and replicates its cells, and homeorhesis, during which it satisfies the body's needs. Albumin traffic through the hepatocyte, rendered visible by means of an immunohistological method, has been used as a prototype model to study the dynamics of autoisodiasostasis. An account is given of the oscillation of autoisodiasostasis between homopoiesis and homeorhesis through two intermediate phases. The phase cycle of autoisodiasostasis is illustrated in the form of atemporal bidimensional and tridimensional diagrams. The temporal behavior of the system is represented as a helix trajectory obtained by the projections of homopoiesis-homeorhesis phase cycle on time series.

Adult↗
PubMed1987

Dynamic model of the pathogenesis of Mengo virus infection in mice.

A mathematical model of the pathogenesis of experimental Mengo virus infection in mice has been developed and fitted using kinetic data of both virus multiplication in different organs and mortality. The behaviour of the model proved to be bistable. In contrast to the widely accepted hypothesis that an acutely virus-infected host dies when virus replication has attained a critical level in the main target organ, the present results showed the following: the maximum virus titre in brain, the main target organ, has been reached already 24 hr post infection (p.i.) but the animals began to die since 60 hr. Hence, it was postulated and confirmed by a good model fit to the experimental data that the so-called AUC (area under the curve) of the virus multiplication kinetics may be a critical quantity. From this finding a hypothesis was deduced assuming that in the presence of high amounts of the virus the antiviral effect of IFN wanes with time. Since this process accounts for death, it may be a potential target of antiviral therapy.

Animals↗
PubMed1987

N-methyl-D-aspartate receptor-induced, inherent oscillatory activity in neurons active during fictive locomotion in the lamprey.

Bath application of N-methyl-aspartate induces fictive locomotor activity in the isolated spinal cord preparation of the lamprey, as well as TTX-resistant membrane potential oscillations in many individual neurons. This inherent oscillatory activity is shown to depend on a specific activation of N-methyl-D-aspartate (NMDA) receptors. This activation initiates voltage-dependent, magnesium-requiring membrane potential bistability, presumably due to a development of a region of negative slope conductance in the current-voltage relation of the neuron. When sodium ions were removed from the bathing solution, oscillations disappeared, and the membrane potential was maintained at a hyperpolarized level, suggesting that the depolarizing current during the oscillatory cycle is mainly carried by sodium ions. Replacing Ca2+ with Ba2+ also leads to a cessation of oscillatory activity, with the membrane potential remaining at the more depolarized level. This indicates an involvement of a Ca2+-dependent K+ current during the repolarization phase. These findings, together with the voltage dependence, can account for the main characteristics of the NMDA receptor-induced, TTX-resistant membrane potential oscillations. This oscillatory behavior has been demonstrated in motoneurons and in several interneurons including CC interneurons but has not been found in edge cells, dorsal cells, or lateral interneurons. The possibility that inherent oscillatory membrane properties may contribute to the activity pattern during fictive locomotion was investigated in experiments with intracellular current injection in the absence of TTX. The stimulation effects obtained required the presence of magnesium ions and were analogous to the stimulation effects seen during oscillations after TTX blockade. Together with similarities in, for instance, frequency and amplitude between the locomotor oscillatory activity and the TTX-resistant oscillations, the results are compatible with an involvement of inherent, oscillatory membrane properties during fictive locomotion in the lamprey spinal cord.

Animals↗