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PubMed1997

Probabilistic independence networks for hidden Markov probability models.

Graphical techniques for modeling the dependencies of random variables have been explored in a variety of different areas, including statistics, statistical physics, artificial intelligence, speech recognition, image processing, and genetics. Formalisms for manipulating these models have been developed relatively independently in these research communities. In this paper we explore hidden Markov models (HMMs) and related structures within the general framework of probabilistic independence networks (PINs). The paper presents a self-contained review of the basic principles of PINs. It is shown that the well-known forward-backward (F-B) and Viterbi algorithms for HMMs are special cases of more general inference algorithms for arbitrary PINs. Furthermore, the existence of inference and estimation algorithms for more general graphical models provides a set of analysis tools for HMM practitioners who wish to explore a richer class of HMM structures. Examples of relatively complex models to handle sensor fusion and coarticulation in speech recognition are introduced and treated within the graphical model framework to illustrate the advantages of the general approach.

Algorithms↗
PubMed2002

De Vries-Weber gain control and dark adaptation in human vision.

Thresholds for seeing light from a stimulus are determined by a mechanism that pairs subliminal excitations from both halves of a twin unit. Such excitations stem from a package of k > or = 1 receptor responses. A half-unit contains one red or one green cone and P rods. The receptor's "Weber machine" controls the receptor's gain. Each half of a twin unit contains a "de Vries machine," which controls the half's k number. In the dark the receptor's dark noise events reset its Weber machine and the receptor's relation to its de Vries machine. A pairing product for light perception also represents a direction event. The local time signs of the two subliminal excitations are crucial for the polarity, size, and pace of the direction event. In relation to the time when and the area in which the stimulus is presented, these signs have average latency periods that depend on intensity and average locations that depend on movement. Polarity depends on which of the two subliminal excitations happens to arrive first at the twin's pairing facility. The intra- and inter-twin pairings in a persepton for the perceptions of light, edge and movement and the probability summation of the pairing products of the mutually independent three sets of twins of the retrinet improve intensity discrimination. Cross-pairings of intra-receptor pairings in red and green cones of a trion for yellow improve visual discrimination further. Discrimination of stimuli that exploit the model's entire summation mechanisms and pairing facilities represents "what the perfect human eye sees best." For the model this threshold of modulation in quantum absorption is the ideal limit that is prescribed by statistical physics. The lateral and meta interaction in a twin unit enhance the contrast of an edge and of a temporal transient. The precision of the local time sign of a half's stimulation determines the spatiotemporal hyperfunctions for location and speed. The model's design for the perfect retinal mosaic consists of red twins situated along clockwise and counterclockwise spirals and green twins along circles that are concentric with the fovea. The model's descriptions of discrimination, adaptation, and hyperfunctions agree with experimental data.

Dark Adaptation↗
PubMed2004

Molecular dynamics simulations of duplex stretching reveal the importance of entropy in determining the biomechanical properties of DNA.

Advances in nanomanipulation techniques have made it possible to measure the response of an individual biomolecule to a force applied in the laboratory. Experiments that stretch a single molecule of duplex DNA have been difficult to interpret theoretically, particularly as the major changes in molecular structure caused by the force cannot be measured. In principle, computer simulation can calculate these conformational changes in atomic level detail, but to date such studies have failed to reproduce the experimental data due to the computational expense of the calculations. Here we show that a combination of molecular modeling and statistical physics can be used successfully to understand the stretching behavior of DNA. Our simulations provide new information about the dynamics of DNA denaturation under force in atomic level detail and also show the importance of entropy in determining biomechanical properties in general.

Biomechanical Phenomena↗
PubMedDate not supplied

[Maxillofacial trauma in pediatric age].

The different incidence of various kinds of maxillofacial fractures in children are examined and the causes of such differences are analyzed. The most significant are the variations in morphology of the maxillofacial skeleton during the children's growth and the physical-statistic problems strictly connected. Attention is drawn on the diagnostic difficulties of traumas due to fractures during childhood, sometimes unknown, especially with regard to the heavy functional and aesthetic exits which may occur.

Age Factors↗
PubMed1999

Associative memory properties of multiple cortical modules.

The existence of recurrent collateral connections between pyramidal cells within a cortical area and, in addition, reciprocal connections between connected cortical areas, is well established. In this work we analyse the properties of a tri-modular architecture of this type in which two input modules have convergent connections to a third module (which in the brain might be the next module in cortical processing or a bi-modal area receiving connections from two different processing pathways). Memory retrieval is analysed in this system which has Hebb-like synaptic modifiability in the connections and attractor states. Local activity features are stored in the intra-modular connections while the associations between corresponding features in different modules present during training are stored in the inter-modular connections. The response of the network when tested with corresponding and contradictory stimuli to the two input pathways is studied in detail. The model is solved quantitatively using techniques of statistical physics. In one type of test, a sequence of stimuli is applied, with a delay between them. It is found that if the coupling between the modules is low a regime exists in which they retain the capability to retrieve any of their stored features independently of the features being retrieved by the other modules. Although independent in this sense, the modules still influence each other in this regime through persistent modulatory currents which are strong enough to initiate recall in the whole network when only a single module is stimulated, and to raise the mean firing rates of the neurons in the attractors if the features in the different modules are corresponding. Some of these mechanisms might be useful for the description of many phenomena observed in single neuron activity recorded during short term memory tasks such as delayed match-to-sample. It is also shown that with contradictory stimulation of the two input modules the model accounts for many of the phenomena observed in the McGurk effect, in which contradictory auditory and visual inputs can lead to misperception.

Action Potentials↗
PubMed2003

Prevalence of lower extremity pain and its association with functionality and quality of life in elderly women in Australia.

OBJECTIVE: To determine the prevalence of self-reported lower extremity pain and the impact on functionality and quality of life in a population based study of elderly women in Western Australia. METHODS: One thousand four hundred eighty-six women, 6.2% of 24,800 women aged over 70 in Perth, were recruited. An index of relative socioeconomic disadvantage (SES) was derived from postcode. Self-reported lower extremity pain at the hip, knee, and foot was collected by questionnaire. The frequency of lower extremity pain was classified into 5 groups. Mobility was measured by the Timed Up and Go Test (TUG). Quality of life was measured using the Medical Outcome Study Short Form 36 (SF-36) summary statistics: physical and mental component scores (PCS and MCS). RESULTS: The prevalence of women reporting any hip, knee, and foot pain was 39%, 52%, and 34% respectively. Fourteen percent experienced pain at all sites whereas 28% had no pain. There was no age difference between the various pain groups. Women with more pain were heavier and had higher BMI scores. At all lower limb sites, women with more frequent pain had reduced mobility and lower quality of life as measured by TUG, PCS, and MCS. For the TUG test, significant determinants in stepwise regression were age, BMI, knee and hip pain. For the SF-36 PCS, significant predictors were age, SES, BMI, and foot, knee, and hip pain. For the SF-36 MCS, SES and foot pain were significant predictors. CONCLUSIONS: Our results confirm the high prevalence of lower extremity pain in elderly women in Australia. Lower extremity pain significantly reduced both physical and mental aspects of the quality of life as well as mobility. In view of the availability of effective interventions to reduce joint pain, more aggressive intervention in the most disabled is indicated.

Activities of Daily Living↗
PubMed2006

Kernel methods for melanoma recognition.

Skin cancer is a spreading disease in the western world. Early detection and treatment are crucial for improving the patient survival rate. In this paper we present two algorithms for computer assisted diagnosis of melanomas. The first is the support vector machines algorithm, a state-of-the-art large margin classifier, which has shown remarkable performances on object recognition and categorization problems. The second method, spin glass-Markov random fields, combines results of statistical physics of spin glasses with Markov random fields. We compared the two approaches using color histograms as features. We benchmarked our methods with another algorithm presented in the literature, which uses a sophisticated segmentation technique and a set of features especially designed for melanoma recognition. To our knowledge, this algorithm represents the state of the art on skin lesions classification. We show with extensive experiments that the support vector machines approach outperforms the existing method and, on two classes out of three, it achieves performances comparable to those obtained by expert clinicians.

Algorithms↗
PubMedDate not supplied

[The Ising model for the description of allosteric kinetics of polymeric enzymes].

A new class of models for the description of the complex allosteric kinetics of oligomeric enzymes has been offered. Its regulation can be realized at the expense of two types of the cooperative interactions. First, the cooperative interaction of the neighboring enzyme protomers is examined on the basis of one-dimensional Ising model; second, subunits that compose the protomer are described by analogy with the model of indirect cooperation of Monod--Wyman--Changeux. The methods of statistical physics open approach to the unification of the models of allosteric regulation in the modern biochemistry. A detailed analysis of the one-ligand model of the polymeric enzymes was performed and possible ways of its generalization were shown.

Allosteric Regulation↗
PubMed1984

Cell movement analysis in a necrotactic assay.

The methods of statistical physics have been applied to analysis of cell movement. Human leukocytes (granulocytes) were observed using time-lapse photography. The paths of the migrating cells were determined. The chemokinetic response at 35 degrees C is described by the diffusion constant (D = 233 micron2/min) and the track velocity (25 micron/min). A time-dependent chemotactic gradient is created by killing an erythrocyte by an intense laser flash. The chemotactic response at 35 degrees C is described by the degree of polar orientation (P1 = 0.85), the track velocity 24 micron/min, and the drift velocity towards the necrotactic source (v parallel = 20 micron/min). The track velocity as well the drift velocity show a broad distribution. The half-width of the velocity distribution. The half-width of the velocity distribution is approximately 5 micron/min. Cell movement can be described by elementary moving states. The characteristic time of the internal clock of the migrating cell is approximately 0.5 min. We found that the information transfer from the necrotactic gradient to the migrating cell is 1 bit per change in directed movement. A migrating cell cannot be stimulated within a period of approximately 10 s after the last decision to adapt a new moving direction.

Biophysical Phenomena↗
PubMed1984

Analysis of cell movement.

The methods of statistical physics have been applied to the analysis of cell movement. Human leukocytes (granulocytes) were observed using time-lapse photography. The center of gravity of a cell, variations of cell shape, and cell orientation were investigated. This analytical description leads to a better understanding of cell movement. Stationary motion of a cell is described by the anisotropy of the cell shape. The cell displacement can be characterized by three different types of movement: The persistent mode where the cell moves away from an arbitrary chosen origin with its track velocity. The diffusion mode where the cells become dispersed in space by a random walk process. The drift mode where the cell moves with a drift velocity, v parallel, in a concentration gradient of chemoattractant molecules. The chemokinetic response is described by the diffusion constant D (= 240 microns2/min) and the track velocity vc (= 30 microns/min). The chemotactic response is described by the degree of orientation P1 (= 0.8), which is identical with the McCutcheon index and the chemotropism index. Cell movement can be described by elementary moving states, and the life time of such a moving state is 0.5 min. The survival probability of the moving state is determined by an internal program. It is not described by a stochastic process. The angular change in moving direction is also programmed, as the square root of the mean square angular change is +/- 50 degrees. The plus and minus direction are equally probable in a chemokinetic response. However, in a chemotactic assay the plus and minus directions are not equally probably. We found that the information transfer from the chemotactic gradient to the migrating cell is 1 bit per change in moving direction. A disturbance in this information transfer leads to an order-disorder transition. Furthermore, we found that the migrating cell exhibits a directional memory of 75 s.

Biophysical Phenomena↗
PubMed1995

Evaluation of physical exertion by statistical analysis of worker's heart rate at log skidding.

Results of investigation into the physical exertion of the log skidding workers: tractor driver, winch operator and choker are presented. The investigation consisted of laboratory and field measurements and included measurements of the heart rate and assessment of the work effect, the work time structure, and the worker's physical exertion and energy consumption. According to the average rate during daily work, the physical exertion of the tractor driver and winch operator was classified as low exertion (75-95 min-1), whereas that of the choker was established as medium exertion (96-115 min-1). Energy consumption was calculated for the daily working time of 262 minutes, according to field measurements and for normal eight-hour work. According to field measurement values the tractor driver's and winch operator's work was categorized as light work (1.23-2.51 MJ) and that of the choker as heavy work (2.52-6.30 MJ). According to the values for eight-hour work, the tractor driver's and the winch operator's jobs were classified as heavy work (2.52-6.30 MJ/8 h) and the choker's job as the heaviest (6.31-10.47 MJ/8 h).

Energy Metabolism↗
PubMed2005

Mean particle diameters. From statistical definition to physical understanding.

Mean particle diameters may be used to describe and to model physical, chemical, or physiological properties of products or materials containing dispersed phases. There are different notation systems for these mean diameters, which may cause much confusion. This equally applies to their nomenclature. This article introduces the Moment-Ratio definition system and evaluates briefly the ISO definition system. The ISO system appears to have serious drawbacks. Mean particle diameters can be estimated from histograms of size distributions by Summation (M-R system) and by Integration (ISO system) over the histogram intervals. Summation tends to be more accurate than Integration and is less sensitive to low values of the lower limit of size distributions. The Summation method equations are straightforward and generally applicable. The mathematical formulas of the Integration method are difficult to apply in daily practice, and their complexity may easily hide the physical background of a mean particle diameter. A coherent nomenclature system for denoting mean particle diameters is recommended. This nomenclature system does not contain any ambiguities and clearly conveys the physical meanings of mean particle diameters. This article deals also with an empirical method to select the proper type of mean diameter to describe a physical, chemical, or physiological property of a product or material containing dispersed phases. After calculation of the mean diameters from experimental data, the relationships between the product property and these mean diameters are investigated statistically. The selection method has been illustrated by two examples. The dataset of each example consists of a set of particle size distributions and the corresponding physical product properties that are influenced by the particle sizes. Hypotheses are formulated to explain the types of selected mean diameters. Sharing results from all over the world of applications of the developed selection method will lead to a buildup of knowledge of physical meanings and application areas of the types of mean particle diameters, which will support decision making in product development.

Algorithms↗
PubMed2004

Calculating the statistical significance of physical clusters of co-regulated genes in the genome: the role of chromatin in domain-wide gene regulation.

Physical clusters of co-regulated, but apparently functionally unrelated, genes are present in many genomes. Despite the important implication that the genomic environment contributes appreciably to the regulation of gene expression, no simple statistical method has been described to identify physical clusters of co-regulated genes. Here we report the development of a model that allows the direct calculation of the significance of such clusters. We have implemented the derived statistical relation in a software program, Pyxis, and have analyzed a selection of Saccharomyces cerevisiae gene expression microarray data sets. We have identified many gene clusters where constituent genes exhibited a regulatory dependence on proteins previously implicated in chromatin structure. Specifically, we found that Tup1p-dependent gene domains were enriched close to telomeres, which suggested a new role for Tup1p in telomere silencing. In addition, we identified Sir2p-, Sir3p- and Sir4p-dependent clusters, which suggested the presence of Sir-mediated heterochromatin in previously unidentified regions of the yeast genome. We also showed the presence of Sir4p-dependent gene clusters bordering the HMRa heterothallic locus, which suggested leaky termination of the heterochromatin by the boundary elements. These results demonstrate the utility of Pyxis in identifying possible higher order genomic features that may contribute to gene regulation in extended domains.

Chromatin↗
PubMed1998

A statistical profile of physical therapists, 1980 and 1990.

BACKGROUND AND PURPOSE: To plan for future needs, human resource analysts require demographic data. In this research, US census data were used to develop a profile of physical therapists. SUBJECTS: Data were extracted from the Public Use Microdata Samples of the US censuses of population from 1980 and 1990. Samples of 3,112 physical therapists from 1990 and 1,530 therapists from 1980 were obtained. METHODS: A profile was generated by use of descriptive statistics to examine geographic distribution, social characteristics, employment characteristics, and income. Linear regression was used to determine factors that influence income. RESULTS: During the 1980s, physical therapy demonstrated remarkable growth, with trends in physical therapist location, gender, age, and place of employment. Even as the profession aged, it stayed an occupation composed predominantly of women, but one less concentrated in hospitals. Geographically, physical therapists remained clustered in the Northeast and along the Pacific Coast. Income generated by physical therapists was predicted by social and geographic characteristics. CONCLUSION AND DISCUSSION: This study presents a new data source to examine physical therapist characteristics. It provides information necessary for health care planners and analysts to better understand the nature of the profession and those who practice.

Adult↗