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PubMed1997

[Stochastic relations between conscious and automatic memory processes: an expansion of the process-dissociation procedure].

This paper presents an extension of the process dissociation procedure with wordstem completion, allowing the measurement of the stochastic relation between conscious and automatic processes. The conditional probabilities of conscious remembering with and without automatic processes can be determined. The determination of these conditional probabilities succeeds due to the extension of the process dissociation procedure by an indirect wordstem completion test, without referring to the usual assumptions of the dissociation paradigm. For the evaluation of this extended process dissociation procedure a multinomial model is presented which allows the distinction between voluntary and involuntary conscious memory. The model is applied to a study by ourselves and to an experiment published by Toth, Reingold, and Jacoby (1994). The results show that the assumptions of stochastic independence or redundancy as well as exclusivity of conscious and automatic processes are often violated. Two conscious processes with different probabilities of occurrence are found; voluntary and involuntary conscious memory processes.

Awareness↗
PubMedDate not supplied

Modelling cost-effectiveness issues in the treatment of clinical depression.

The cost to the National Health Service of treatment for clinical depression for England and Wales has been estimated as being in the area of 416 million pounds (1990 price level), and the social burden in terms of increased morbidity and mortality due to depression is known to be considerable. Prescription of antidepressants is the most common treatment for people with clinical depression. The majority are diagnosed by general practitioners who issue 95% of all prescriptions for antidepressants. In 1992 the English National Health Service spent 81.1 million pounds on antidepressant drugs. However, the understanding of the disease process, the health, and economic impact of various treatment options are surrounded by much uncertainty. Few cost-effectiveness studies of antidepressive treatments can be found in the literature. They are often based on small sample sizes, a short time horizon, and a narrow focus on subjective measures of process or intermediate outcome and are therefore less than robust when generalized to a wider population of patients with clinical depression. We have developed a stochastic simulation model aimed at analysing cost-effectiveness aspects of treatment for depression and have used it to test the consequences of a range of treatment policies. Features of the model are discussed in this paper. The model is described with a flow chart that shows patients' pathways through the health-care system. Based on the incidence approach, the model simulates a cohort of patients, using decision and chance nodes which occur during treatment. A range of critical decision variables can be changed to assess the consequences for cost-effectiveness.

Antidepressive Agents↗
PubMed2001

Macroscopic dynamics in separable neural networks.

Parallel dynamics of neural networks with separable coupling is given starting from Coolen-Sherrington theory. Away from saturation, it is shown that this parallel retrieve dynamics is equivalent to the sequential dynamics for finite temperature. But the finite-size effects were found to be governed by a homogeneous Markov process, not by a time-dependent Ornstein-Uhlenbeck process in sequential dynamics.

Biophysics↗
PubMed2006

Fast and quiet MRI using a swept radiofrequency.

A novel fast and quiet method of magnetic resonance imaging (MRI) is introduced which creates new opportunities for imaging in medicine and materials science. The method is called SWIFT, sweep imaging with Fourier transformation. In SWIFT, time-domain signals are acquired in a time-shared manner during a swept radiofrequency excitation of the nuclear spins. With negligible time between excitation and signal acquisition, new possibilities exist for imaging objects consisting of spins with extremely fast transverse relaxation rates, such as macromolecules, semi-solids, and quadrupolar nuclei. The field gradient used for spatial-encoding is not pulsed on and off, but rather is stepped in orientation in an incremental manner, which results in low acoustic noise. This unique acquisition method is expected to be relatively insensitive to sample motion, which is important for imaging live objects. Additionally, the frequency-swept excitation distributes the signal energy in time and thus dynamic range requirements for proper signal digitization are reduced compared with conventional MRI. For demonstration, images of a plastic object and cortical bone are shown.

Algorithms↗
PubMed2003

Possibility of tissue separation caused by cell adhesion.

Adhesive molecules are suggested to play an important role when a single tissue is separated into two in developmental processes, illustrated by tissue-specific cadherins in the neural tube formation of amphibians. In this paper, we study the possibility for tissue separation to be carried out only by differential cell adhesion and random cell movement without any other morphogenetic mechanisms. We consider a two-dimensional regular triangular lattice filled with cells of three types (black, white, and gray). In the initial state, a cluster of black cells and a cluster of white cells are in contact and are surrounded by gray cells. Nearest-neighbor cells exchange their location at random, but the movement occurs faster if it increases the total adhesion. We considered separation to be successful if, in the final state, black cells and white cells kept their clusters but two clusters lost their direct contact with each other as gray cells are inserted between them. The maximum total adhesion (MTA) rule conjectures that the spatial pattern achieving maximum total adhesion might be that obtained in the final state. In the computer simulation, the runs for successful separation satisfied the condition predicted by the MTA rule. However, the condition for successful separation was more restricted than that predicted by the MTA rule. For some combinations of adhesions, it took an extremely long time to accomplish tissue separation. Finally, we discuss the role of homophilic adhesion molecules (such as cadherins) in the tissue separation processes, and show that the new expression of homophilic adhesion molecules cannot perform tissue separation without the change in other morphogenetic processes.

Animals↗
PubMed2006

Remedy for the fermion sign problem in the diffusion Monte Carlo method for few fermions with antisymmetric diffusion process.

We suggest an exact approach to help remedy the fermion sign problem in diffusion quantum Monte Carlo simulations. The approach is based on an explicit suppression of symmetric modes in the Schrödinger equation by means of a modified stochastic diffusion process (antisymmetric diffusion process). We introduce this algorithm and illustrate it on potential models in one dimension (1D) and show that there it solves the fermion sign problem exactly and converges to the lowest antisymmetric state of the system. Then, we discuss extensions of this approach to many-dimensional systems on examples of quantum oscillator in 2D-20D and a toy model of three and four fermions on harmonic strings in 2D and 3D. We show that in all these cases our method shows a performance comparable to that of a fixed-node approximation with an exact node.

Journal Article↗
PubMed1997

Partitioning of the Golgi apparatus during mitosis in living HeLa cells.

The Golgi apparatus of HeLa cells was fluorescently tagged with a green fluorescent protein (GFP), localized by attachment to the NH2-terminal retention signal of N-acetylglucosaminyltransferase I (NAGT I). The location was confirmed by immunogold and immunofluorescence microscopy using a variety of Golgi markers. The behavior of the fluorescent Golgi marker was observed in fixed and living mitotic cells using confocal microscopy. By metaphase, cells contained a constant number of Golgi fragments dispersed throughout the cytoplasm. Conventional and cryoimmunoelectron microscopy showed that the NAGT I-GFP chimera (NAGFP)-positive fragments were tubulo-vesicular mitotic Golgi clusters. Mitotic conversion of Golgi stacks into mitotic clusters had surprisingly little effect on the polarity of Golgi membrane markers at the level of fluorescence microscopy. In living cells, there was little self-directed movement of the clusters in the period from metaphase to early telophase. In late telophase, the Golgi ribbon began to be reformed by a dynamic process of congregation and tubulation of the newly inherited Golgi fragments. The accuracy of partitioning the NAGFP-tagged Golgi was found to exceed that expected for a stochastic partitioning process. The results provide direct evidence for mitotic clusters as the unit of partitioning and suggest that precise regulation of the number, position, and compartmentation of mitotic membranes is a critical feature for the ordered inheritance of the Golgi apparatus.

Cells, Cultured↗
PubMed2003

Detecting dynamical changes within a simulated neural ensemble using a measure of representational quality.

Technological advances allowing simultaneous recording of neuronal ensembles have led to many developments in our understanding of how the brain performs neural computations. One key technique for extracting information from neural populations has been population reconstruction. While reconstruction is a powerful tool, it only provides a value and gives no indication of the quality of the representation itself. In this paper, we present a mathematically and statistically justified measure for assessing the quality of a representation in a neuronal ensemble. Using a simulated neural network, we show that this measure can distinguish between system states and identify moments of dynamical change within the system. While the examples used in this paper all derive from a standard network model, the measure itself is very general. It requires only a representational space, measured tuning curves, and neural ensembles.

Animals↗
PubMed2004

The apparent immunity of high-frequency "transposed" stimuli to low-frequency binaural interference.

Discrimination of interaural temporal disparities (ITDs) was measured with either conventional or transposed "targets" centered at 4 kHz. The targets were presented either in the presence or absence of a simultaneously gated diotic noise centered at 500 Hz, the interferer. As expected, the presence of the low-frequency interferer resulted in substantially elevated threshold-ITDs for the conventional high-frequency stimuli. In contrast, these interference effects were absent for ITDs conveyed by the high-frequency transposed targets. The binaural interference effects observed with the conventional high-frequency stimuli were well accounted for, quantitatively, by the model described by Heller and Trahiotis [L. M. Heller and C. Trahiotis, J. Acoust. Soc. Am. 99, 3632-3637 (1996)]. The lack of binaural interference effects observed with the high-frequency transposed stimuli was not predicted by that model. It is suggested that transposed stimuli may be one of a class of stimuli that do not foster an obligatory combination of binaural information between low- and high-frequency regions. Under those conditions that do foster such an obligatory combination, one could still consider models of binaural interference, such as the one described in Heller and Trahiotis, to be valid descriptors of binaural processing.

Acoustic Stimulation↗
PubMed1989

Mean and quasideterministic equivalence for linear stochastic dynamics.

In linear, stochastic dynamics it is shown that the quasideterministic population size is equivalent to the mean population size. The quasideterministic dynamics are defined by the conditional infinitesimal mean of the process. The stochastic component of the dynamics includes both Gaussian and Poisson white noise, with amplitude coefficients proportional to the population size. Generalizations are given for nonautonomous coefficients and for distributed Poisson jump amplitudes. A counter example--an exactly integrable nonlinear jump model--shows that the equivalence result does not hold for nonlinear stochastic dynamics.

Linear Models↗
PubMed2005

Spatially correlated disturbances in a locally dispersing population model.

The basic contact process in continuous time is studied, where instead of single occupied sites becoming empty independently, larger-scale disturbance events simultaneously remove the population from contiguous blocks of sites. Stochastic spatial simulations and pair approximations were used to investigate the model. Increasing the spatial scale of disturbance events increases spatial clustering of the population and variability in growth rates within localized regions, reduces the effective overall population density, and increases the critical reproductive rate necessary for the population to persist. Pair approximations yield a closed-form analytic expression for equilibrium population density and the critical value necessary for persistence.

Animals↗
PubMed1994

Quantification of evolution from order to randomness in practical time series analysis.

The principal focus of this chapter is the description of a recently developed, readily usable regularity statistic, ApEn, that quantifies the continuum from perfectly orderly to completely random in time series data. Several properties of ApEn facilitate its utility for practical time series analysis: (1) ApEn is nearly unaffected by noise of magnitude below a de facto specified filter level; (2) ApEn is robust to outliers; (3) ApEn can be applied to time series of 100 or more points, with good confidence (established by standard deviation calculations); (4) ApEn is finite for stochastic, noisy deterministic, and composite (mixed) processes, the last of which are likely models for complicated biological systems; (5) increasing ApEn corresponds to intuitively increasing process complexity in the settings of (4). This applicability to medium-sized data sets and general stochastic processes is in marked contrast to capabilities of "chaos" algorithms such as the correlation dimension, which are properly applied to low-dimensional iterated deterministic dynamical systems. The potential uses of ApEn to provide new insights in biological settings are thus myriad, from a perspective complementary to that given by classic statistical methods. The ApEn statistic is typically calculated by a computer program, with a FORTRAN listing for a "basic" code referenced above. It is imperative to view ApEn as a family of statistics, each of which is a relative measure of process regularity. For proper implementation, the two input parameters m (window length) and r (tolerance width, de facto filter) must remain fixed in all calculations, as must N, the data length, to ensure meaningful comparisons. Guidelines for m and r selection are indicated above. We have found normalized regularity to be especially useful; "r" is chosen as a fixed percentage (often 15 or 20%) of the SD of the subject rather than of a group SD. This version of ApEn has the property that it is decorrelated from process SD, in that it remains unchanged under uniform process magnification or reduction; thus we can entirely separate the questions of SD change and regularity change in data analysis. Because regularity questions are thematically orthogonal to the type of information that, for example, moment statistics ascertain, we highly recommend that ApEn be used in conjunction with other such statistics, rather than as a sole indicator of process typicality. Last and yet foremost, we recommend the following order of detail in Practical analysis. First, either visually or algorithmically, eliminate outliers.(ABSTRACT TRUNCATED AT 400 WORDS)

Activity Cycles↗
PubMed1999

Cytoplasm dynamics and cell motion: two-phase flow models.

The motion of amoeboid cells is characterized by cytoplasmic streaming and by membrane protrusions and retractions which occur even in the absence of interactions with a substratum. Cell translocation requires, in addition, a transmission mechanism wherein the power produced by the cytoplasmic engine is applied to the substratum in a highly controlled fashion through specific adhesion proteins. Here we present a simple mechano-chemical model that tries to capture the physical essence of these complex biomolecular processes. Our model is based on the continuum equations for a viscous and reactive two-phase fluid model with moving boundaries, and on force balance equations that average the stochastic interactions between actin polymers and membrane proteins. In this paper we present a new derivation and analysis of these equations based on minimization of a power functional. This derivation also leads to a clear formulation and classification of the kinds of boundary conditions that should be specified at free surfaces and at the sites of interaction of the cell and the substratum. Numerical simulations of a one-dimensional lamella reveal that even this extremely simplified model is capable of producing several typical features of cell motility. These include periodic 'ruffle' formation, protrusion-retraction cycles, centripetal flow and cell-substratum traction forces.

Actins↗
PubMed2003

Determination of multicanonical weight based on a stochastic model of sampling dynamics.

Based on the stochastic interpretation of the sampling process modeled by a Langevin equation, we present an effective iteration scheme to determine the weight in multicanonical molecular dynamics. Our method enables an automatic determination of the weight producing a uniform energy sampling via an iterative cancellation of the deterministic force in a Langevin equation. The deterministic force has been calculated from the energy trajectory by identifying the moments of the transition probability of a Fokker-Planck equation associated with a Langevin equation. The intimate relationship between the sampling process and the stochastic dynamics has been verified by applying the iteration scheme to a helix-coil transition of the 8-polyalanine system in a gas phase.

Journal Article↗