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Results for “nonlinear analyses”

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PubMed2006

Orthostatic decrease in cardiac chaos during the head-up tilt test in patients with vasovagal syncope.

BACKGROUND: Autonomic dysfunction contributes to orthostatic intolerance in vasovagal syncope (VVS), but as it has not been identified by spectral analysis of heart rate variability (HRV) in previous studies, the present hypothesis was that nonlinear analysis of HRV would identify the orthostatic intolerance in VVS. METHODS AND RESULTS: Twenty-six patients with VVS and 14 matched controls were subjected to 80-degree head-up tilt test (positive: 13 patients; negative: 13 patients and 14 controls). There were no differences in the orthostatic changes in the indices of spectral analyses of HRV among the 3 groups. The Lyapunov exponent (LE) was calculated from 200 consecutive RR-intervals to investigate chaotic behavior, and cardiac chaos was defined as the incidence of the presence of a positive finite LE. Orthostatic decreases in cardiac chaos were observed in the VVS patients (both the positive and negative groups), although there was no orthostatic decrease in the control group (ANOVA: p = 0.008). The receiver-operator characteristic curve indicated that cardiac chaos during the tilt identified VVS regardless of the results of the tilt (p < 0.001, sensitivity: 85.7%, specificity: 96.2%). CONCLUSIONS: The decrease in cardiac chaos during the tilt test was specific to patients with VVS, even if their response to the test was negative.

Adult↗

PubMed2006

Photorefractivity of poly-N-vinylindole-based materials as compared with that of poly-N-vinylcarbazole-based blends.

The asymmetric two-beam coupling technique has been employed to measure the photorefractivity of thin films of polymer blends containing 2,5-dimethyl-4-(p-nitrophenylazo)anisole as the nonlinear optical component. Poly-(1-vinylindole) and poly-(2,3-dimethyl-1-vinylindole) were the photoconductive polymer counterparts. The values of the photorefractivity are reported. It appears that they are comparable with those of similar blends based on the well-known poly-(9-vinylcarbazole) (PVK), here considered as a reference standard. Careful differential scanning calorimetry analyses have been accomplished on the different blends taken into account to rationalize the significantly longer shelf lifetime of the indolyl-based films with respect to the PVK-based blends.

Journal Article↗

PubMed1999

Finite element analysis of vertebral body mechanics with a nonlinear microstructural model for the trabecular core.

In this study, a finite element model of a vertebral body was used to study the load-bearing role of the two components (shell and core) under compression. The model of the vertebral body has the characteristic kidney shape transverse cross section with concave lateral surfaces and flat superior and inferior surfaces. A nonlinear unit cell based foam model was used for the trabecular core, where nonlinearity was introduced as coupled elastoplastic beam behavior of individual trabeculae. The advantage of the foam model is that architecture and material properties are separated, thus facilitating studies of the effects of architecture on the apparent behavior. Age-related changes in the trabecular architecture were considered in order to address the effects of osteoporosis on the load-sharing behavior. Stiffness changes with age (architecture and porosity changes) for the trabecular bone model were shown to follow trends in published experimental results. Elastic analyses showed that the relative contribution of the shell to the load-bearing ability of the vertebra decreases with increasing age and lateral wall curvature. Elasto-plastic (non-linear) analyses showed that failure regions were concentrated in the upper posterior region of the vertebra in both the shell and core components. The ultimate load of the vertebral body model varied from 2800 N to 5600 N, depending on age (architecture and porosity of the trabecular core) and shell thickness. The model predictions lie within the range of experimental results. The results provide an understanding of the relative role of the core and shell in vertebral body mechanics and shed light on the yield and post-yield behavior of the vertebral body.

Aged↗

PubMed2006

Looking inside an invasion wave of cells using continuum models: proliferation is the key.

Recently, a suite of cell migration assays were conducted to investigate the migration of neural crest (NC) cells along the gut during the development of the enteric nervous system (ENS). The NC cells colonise the gastro-intestinal tract as a rostro-caudal wave. Local behaviour was shown to be controlled by position relative to the leading edge of the wavefront. The assays involved chick-quail grafting techniques allowing the total invading population to be considered as a two-species system. A two-species continuum model with logistic proliferation and a migration mechanism is developed here to simulate the chick-quail graft experiments and provide a means of looking at the processes occurring within the invasion wave. Five migration mechanisms are considered--linear diffusion, two cases of nonlinear diffusion, chemokinesis and chemotaxis. The model results agree with the experimental observations, regardless of the specific type of migration mechanism. The results show that NC cell invasion is driven by proliferation and cell motility at the leading edge of the wave. Furthermore, logistic proliferation exerts the dominant control on the system. This observation is confirmed by analysing some simplified invasion models. Once the basic experiments were mathematically replicated, the mathematical models were used in turn to make some predictions that were yet to be experimentally tested. This involved conducting a sensitivity analysis of the system by interrupting the proliferation and/or migration ability of the leading edge. Numerical results show that the system is stable against these changes. Of the three experiments suggested, one was carried out and the experimental results were concordant with the theoretical predictions. The outcome of two other suggested experiments are predicted and left for future experimental validation.

Animals↗

PubMed2005

Application of artificial neural network to fMRI regression analysis.

We used an artificial neural network (ANN) to detect correlations between event sequences and fMRI (functional magnetic resonance imaging) signals. The layered feed-forward neural network, given a series of events as inputs and the fMRI signal as a supervised signal, performed a non-linear regression analysis. This type of ANN is capable of approximating any continuous function, and thus this analysis method can detect any fMRI signals that correlated with corresponding events. Because of the flexible nature of ANNs, fitting to autocorrelation noise is a problem in fMRI analyses. We avoided this problem by using cross-validation and an early stopping procedure. The results showed that the ANN could detect various responses with different time courses. The simulation analysis also indicated an additional advantage of ANN over non-parametric methods in detecting parametrically modulated responses, i.e., it can detect various types of parametric modulations without a priori assumptions. The ANN regression analysis is therefore beneficial for exploratory fMRI analyses in detecting continuous changes in responses modulated by changes in input values.

Adult↗

PubMed1998

Non-linear dynamical analysis of the EEG in Alzheimer's disease with optimal embedding dimension.

We used non-linear analysis to investigate the dynamical properties underlying the EEG in patients with Alzheimer's disease. We calculated the correlation dimension D2 and the first positive Lyapunov exponent L1. We employed a new method, which was proposed by Kennel et al., to calculate the non-linear invariant measures. That method determined the proper minimum embedding dimension by looking at the behavior of nearest neighbors under a change in the embedding dimension d from d to d + 1. We demonstrated that for limited noisy data, our algorithm was strikingly faster and more accurate than previous ones. Also, we found that, in almost all channels, patients with Alzheimer's disease had significantly lower D2 and L1 values than those for age-approximated healthy controls. These results suggest that brains afflicted by Alzheimer's disease show behaviors which are less chaotic than those of normal healthy brains. In this paper, we show that non-linear analysis can provide a fruitful tool for detecting relative changes, which cannot be detected by conventional linear analysis, in the complexity of brain dynamics. We propose that non-linear dynamical analyses of the EEGs from patients with Alzheimer's disease will be a diagnostic modality in the appropriate clinical setting.

Aged↗

PubMed2003

Estimating DOC regime in a wastewater treatment plant by UV deconvolution.

A UV-deconvolution method was modified, and applied to estimation of dissolved organic carbon (DOC) along a municipal wastewater treatment plant (WWTP) in Catalonia, Spain. One grab sample was taken every 2h at four sampling points, for 1 week (336 samples), in order to characterise day/night and weekday/weekend DOC regimes along the plant. Samples were centrifuged before DOC determination or estimation. Four components were selected for describing wastewater composition and spectra. Reference spectra for these components were taken from the literature and proved to correctly explain the sample spectra. A two-step deconvolution method was developed, which avoided negative nitrate coefficients while keeping deconvolution error low. The calibration file for DOC estimation was determined by analysing DOC and acquiring UV spectra from 48 samples. DOC values were correlated to UV spectra by multiple linear regression. Determination coefficient and standard error were comparable to the values found in the literature. In raw or diluted samples with an absorbance between 2.0 and 2.5, DOC was probably underestimated by the method. This points to some nonlinearity for absorbances above 2.0, rather than the 2.5 limit suggested by the original method. DOC calculation through UV deconvolution allowed for the estimation of DOC regime along the WWTP. Time bands for higher and lower DOC concentrations were determined and characterised at each sampling point, for weekdays and the weekend. Except for the plant effluent, clear time bands were found. In effluent, DOC was always low, and very small oscillations were detected, due to DOC removal and intense mixing in the biological process. DOC profiles at each point are discussed in this paper. The modified deconvolution method has proved to be an accurate and efficient technique for estimating DOC of a large number of raw and diluted samples.

Calibration↗

PubMed1988

Selective responses of visual cortical cells do not depend on shunting inhibition.

Theoretical analyses of the electrical behaviour of the highly branched processes of nerve cells has focused attention on the possibility that single cells perform complex logical operations rather than simply summing their synaptic inputs. In particular, it has been suggested that the orientation and direction selectivity of cells in the visual cortex results from the action of a nonlinear 'shunting' inhibition that emulates an AND-NOT logical operation. The characteristic biophysical feature of this proposed inhibitory mechanism is that it evokes a large and relatively sustained increase in the conductance of the neuronal membrane while leaving the membrane potential unaffected. This shunting mechanism contrasts with linear 'summative' inhibition in which conductance changes are less prominent, and inhibition is achieved by hyperpolarization of the membrane potential. In a direct experimental test of the hypothesis that the selectivity of visual cortical neurons depends on shunting inhibition we found no evidence for the large conductance changes predicted by the theory.

Action Potentials↗

PubMed1994

Computerized detection of masses in digital mammograms: automated alignment of breast images and its effect on bilateral-subtraction technique.

An automated technique for the alignment of right and left breast images has been developed for use in the computerized analysis of bilateral breast images. In this technique, the breast region is first identified in each digital mammogram by use of histogram analysis and morphological filtering operations. The anterior portions of the tracked breast border and computer-identified nipple positions are selected as landmarks for use in image registration. The paired right and left breast images, either from mediolateral oblique or craniocaudal views, are then registered relative to each other by use of a least-squares matching method. This automated alignment technique has been applied to our computerized detection scheme that employs a nonlinear bilateral-subtraction method for the initial identification of possible masses. The effectiveness of using bilateral subtraction in identifying asymmetries between corresponding right and left breast images is examined by comparing detection performances obtained with various computer-simulated misalignments of 40 pairs of clinical mammograms. Based on free-response receiver operating characteristic and regression analyses, the detection performance obtained with the automated alignment technique was found to be higher than that obtained with simulated misalignments. Detection performance decreased gradually as the amount of simulated misalignment increased. These results indicate that automatic alignment of breast images is possible and that mass-detection performance appears to improve with the inclusion of asymmetric anatomic information but is not sensitive to slight misalignment.

Breast Neoplasms↗

PubMed1985

Extraction and enhancement of spectral structure by the cochlea.

The intensity dependence of signal processing in the cat cochlea was studied in responses of single auditory-nerve fibers for harmonic complexes having various amplitude and phase spectra. Analyses were based on information present in temporal discharge cadences, and they consisted of assessing Fourier spectra of period histograms synchronized to the period of the waveform fundamental. At low intensities, response spectra resembled filtered versions of the stimulus spectrum, with the amounts of filtering being determined by fibers' tuning curves. At high intensities, response spectra exhibited nonlinear behavior and could differ dramatically from spectra obtained at low intensities. The high-intensity response typically emphasized one or more aspects of the stimulus spectrum. When the stimulus possessed equal component amplitudes and phases, the features that were emphasized at high intensities were the high- and low-frequency edges of the spectrum, and when the component at fiber CF was changed in phase or amplitude relative to the others, fibers primarily signaled the presence of the phase- or amplitude-shifted component. Many of the intensity-dependent changes in response spectra are accounted for by considering the effects of the compressive input-output nonlinearity operating at or peripheral to the hair-cell level on the temporal waveform.

Acoustic Stimulation↗

PubMed1991

Effects of intravitreal dexamethasone on concentration of intravitreal vancomycin in experimental methicillin-resistant Staphylococcus epidermidis endophthalmitis.

Intravitreal corticosteroids in the treatment of bacterial endophthalmitis remain controversial. We utilized an experimental rabbit model of methicillin-resistant Staphylococcus epidermidis endophthalmitis (i) to calculate the intravitreal vancomycin concentration in rabbit eyes receiving intravitreal vancomycin alone or in combination with intravitreal dexamethasone and (ii) to determine whether an intravitreal steroid has any effect on intravitreal vancomycin levels. All right eyes were infected and all left eyes were uninfected. The rabbits were divided into two treatment groups: (i) 32 eyes (group I) were injected with intravitreal vancomycin, 1.0 mg (0.1 ml); (ii) 32 additional eyes (group II) were injected with intravitreal dexamethasone, 400 micrograms (0.1 ml), in addition to vancomycin. Measurement of intravitreal vancomycin concentration was performed following sacrifice, utilizing a microbiologic agar diffusion assay. Analyses of intravitreal vancomycin concentrations were performed by using model-independent parameters, with area under the concentration-time curves derived by trapezoidal approximation. The intravitreal vancomycin concentration was significantly lower in both uninfected and infected group II eyes (P less than 0.002). Analysis of intravitreal vancomycin concentration-time relationships was performed by using a nonlinear least-squares regression program; data best fit a one-compartment model. In addition, no vancomycin-dexamethasone interaction could be demonstrated. The reduced level of intravitreal vancomycin in the presence of intravitreal dexamethasone may have important clinical implications.

Animals↗

PubMed2006

Triple dissociation in the medial temporal lobes: recollection, familiarity, and novelty.

Memory for past events may be based on retrieval accompanied by specific contextual details (recollection) or on the feeling that an item is old (familiarity) or new (novelty) in the absence of contextual details. There are indications that recollection, familiarity, and novelty involve different medial temporal lobe subregions, but available evidence is scarce and inconclusive. Using functional magnetic resonance imaging (MRI), we isolated retrieval-related activity associated with recollection, familiarity, and novelty by distinguishing between linear and nonlinear oldness functions derived from recognition confidence levels. Within the medial temporal lobes (MTLs), we found a triple dissociation among the posterior half of the hippocampus, which was associated with recollection, the posterior parahippocampal gyrus, which was associated with familiarity, and anterior half of the hippocampus and rhinal regions, which were associated with novelty. Furthermore, multiple regression analyses based on individual trial activity showed that all three memory signals, i.e., recollection, familiarity, and novelty, make significant and independent contributions to recognition memory performance. Finally, functional dissociations among recollection, familiarity, and novelty were also found in posterior midline, left parietal cortex, and prefrontal cortex regions. This is the first study to reveal a triple dissociation within the MTL associated with distinct retrieval processes. This finding has direct implications for current memory models.

Adult↗

PubMed2003

Steady-state analysis of structured population models.

Our systematic formulation of nonlinear population models is based on the notion of the environmental condition. The defining property of the environmental condition is that individuals are independent of one another (and hence equations are linear) when this condition is prescribed (in principle as an arbitrary function of time, but when focussing on steady states we shall restrict to constant functions). The steady-state problem has two components: (i). the environmental condition should be such that the existing populations do neither grow nor decline; (ii). a feedback consistency condition relating the environmental condition to the community/population size and composition should hold. In this paper we develop, justify and analyse basic formalism under the assumption that individuals can be born in only finitely many possible states and that the environmental condition is fully characterized by finitely many numbers. The theory is illustrated by many examples. In addition to various simple toy models introduced for explanation purposes, these include a detailed elaboration of a cannibalism model and a general treatment of how genetic and physiological structure should be combined in a single model.

Cannibalism↗

PubMed1988

Response surface analyses of the effects of dietary protein on feeding and growth patterns in mice from weaning to maturity.

Responses of food intake, protein intake, gross energy intake and body weight of mice from weaning to maturity to dietary protein level were studied. Thirteen groups of ddY male mice were fed purified diets covering the entire range (0-100%) of protein concentration on a gross energy basis for 68 d. The food intake and body weight data in each group were analyzed by nonlinear regression to obtain values of the six parameters in Parks' feeding and growth equations. These parameters, found as a function of the protein content of diet, were then used to construct the response surfaces of food intake and body weight over the dietary protein versus age space. At every protein level the daily energy intake rose rapidly with age to a plateau at about 7-14 d after weaning and was maintained throughout the experiment. Daily energy intake was independent of the dietary protein content except at very low dietary protein contents. Consequently, from 21 to 89 d of age daily protein intake was directly proportional to dietary protein level. These results imply that mice ate for energy rather than protein when fed diets varying in protein content in the range of 10-80%. Body weight as a function of time after weaning increased to a plateau at every dietary protein level, but the details of the pattern of growth were affected by the dietary protein level. Further, the response surface of body weight versus dietary protein level and age was analyzed to find the protein requirement of mice for maximal growth during a given interval from weaning.

Aging↗

PubMed2007

Nonlinear anisotropic stress analysis of anatomically realistic cerebral aneurysms.

BACKGROUND: Static deformation analysis and estimation of wall stress distribution of patient-specific cerebral aneurysms can provide useful insights into the disease process and rupture. METHOD OF APPROACH: The three-dimensional geometry of saccular cerebral aneurysms from 27 patients (18 unruptured and nine ruptured) was reconstructed based on computer tomography angiography images. The aneurysm wall tissue was modeled using a nonlinear, anisotropic, hyperelastic material model (Fung-type) which was incorporated in a user subroutine in ABAQUS. Effective material fiber orientations were assumed to align with principal surface curvatures. Static deformation of the aneurysm models were simulated assuming uniform wall thickness and internal pressure load of 100 mm Hg. RESULTS: The numerical analysis technique was validated by quantitative comparisons to results in the literature. For the patient-specific models, in-plane stresses in the aneurysm wall along both the stiff and weak fiber directions showed significant regional variations with the former being higher. The spatial maximum of stress ranged from as low as 0.30 MPa in a small aneurysm to as high as 1.06 MPa in a giant aneurysm. The patterns of distribution of stress, strain, and surface curvature were found to be similar. Sensitivity analyses showed that the computed stress is mesh independent and not very sensitive to reasonable perturbations in model parameters, and the curvature-based criteria for fiber orientations tend to minimize the total elastic strain energy in the aneurysms wall. Within this small study population, there were no statistically significant differences in the spatial means and maximums of stress and strain values between the ruptured and unruptured groups. However, the ratios between the stress components in the stiff and weak fiber directions were significantly higher in the ruptured group than those in the unruptured group. CONCLUSIONS: A methodology for nonlinear, anisotropic static deformation analysis of geometrically realistic aneurysms was developed, which can be used for a more accurate estimation of the stresses and strains than previous methods and to facilitate prospective studies on the role of stress in aneurysm rupture.

Anisotropy↗

PubMed1976

Impaired plasma protein binding of phenytoin in uremia and displacement effect of salicylic acid.

The plasma protein binding of phenytoin (DPH) was studied by equilibrium dialysis at 37 degrees C in plasma from uremic patients and healthy subjects. Scatchard plot analyses demonstrated a decreased association constant Ka for the DPH-albumin interaction in the uremic plasma (mean 1.76 - 10(3) M-1 +/-SD 0.12 and a mean 4.10 - 10(3) M-1 +/- SD 0.24 in normal plasma). Studies on separated fractions of serum did not indicate any significant binding of DPH to proteins other than albumin. The nonlinear mathematical relationship between bound DPH and serum albumin could be linearized at low drug concentrations by plotting the ratio of bound/unbound DPH against albumin concentration. The displacement effect of salicylic acid at a concentration of 276 mug/ml (2mM) and DPH was considerable in plasma from normal subjects. In uremic plasma the effect was of much smaller magnitude.

Adult↗

PubMed1997

Time series analysis and its relationship with longitudinal analysis.

This paper discusses the relationship between longitudinal analysis of inter-individual (co-)variation and time series analysis of intra-individual (co-)variation. To set the stage for this discussion, first a tutorial overview of modern techniques of multivariate time series analysis is given which highlights the central role of rate-space modeling. Some increasingly general instances of the state-spuce model are presented, followed by a concise description of two recent applications involving nonlinear state-space modeling of oscillatory finger motion and nonlinear growth, respectively. We then consider the question under which conditions longitudinal factor analysis of inter-individual covariation will yield the same results as dynamic factor analysis of single-subject time series data. The conditions concerned can be derived from ergodicity theory and turn out to be very restrictive. This implies that the results obtained in analyses of inter-individual variation (like the construction and validation of measurement scales) cannot be generalized to the assessment and prediction of individual developmental processes (e.g., in single-subject counseling). A simple illustration with simulated data is given, to the best of our knowledge for the first time, in which a factor analysis of inter-individual covariation yields a satisfactorily fitting solution that has no relationship whatsoever to the factor structures characterizing the intra-individual covariation of each subject in the sample.

Epidemiologic Methods↗

PubMed1989

Analysis of the Om(1D) locus in Drosophila ananassae.

From the ca;px stock, which is the progenitor of Om mutants caused by insertions of the tom retrotransposon, 50 kb of genomic DNA including the Om(1D) locus was cloned by tom tagging and chromosome walking. Southern blot analyses of six Om(1D) mutants exposed one or two tom elements inserted at five nonrandom sites within an 18-kb distal segment of the restriction map; the phenotypic uniformity between these mutants was not affected by variations in the position, number or orientation of their inserts. Spontaneous revertants or more extreme derivatives of Om(1D) alleles were nonlinearly associated with losses or gains of tom inserts. Seven of eight radiation induced derivatives of Om(1D) mutants had one breakpoint of a chromosome rearrangement in polytene section 13A which includes the Om(1D) locus. Two Om(1D) derivatives, a spontaneous revertant and an induced extreme allele, were associated with overlapping deficiencies which define a region that is likely to contain the Om(1D) coding seguences proximal to the tom insertion sites. Incidental results confirm the previously indicated homology of the Om(1D) locus with the Bar locus of Drosophila melanogaster.

Animals↗