共查询到20条相似文献,搜索用时 15 毫秒
1.
M A García-Pérez 《Spatial Vision》1988,3(2):129-142
A multichannel model incorporating visual inhomogeneity is presented in this paper. The parameters that describe inhomogeneity have been experimentally obtained both at threshold and in several suprathreshold conditions. At threshold, probability summation is taken into account in order to determine the spatial extent of visual channels from experimental data showing an asymptotic increase in sensitivity with increasing grating area. At suprathreshold contrast, the region where luminance variations at several scales are visible has also been found. The results support a spatially limited multichannel model of early visual processing and set out a basis for studying perceptual phenomena from the viewpoint of linear space-variant visual processing. 相似文献
2.
Mathematical models have been used to study different aspects of the germinal centre reaction, in particular, affinity maturation of antibodies and the hypothesis of recycling. So far, interpretation of several theoretical and experimental results has pointed to the existence of recycling. However, theoretical models have seldom been compared with experimental data from specific immune responses and the potential relevance of recycling in the germinal centre is still an open problem. In this article, we propose a model without recycling that takes into account selection mechanisms that were previously uncovered experimentally. We apply the model to several experimental systems that use different Ag and compare the results with experimental data of affinity maturation whenever available. The results obtained for a primary immune response to the hapten (4-hydroxy-3-nitrophenyl)-acetyl show that recycling is not a necessary mechanism to achieve the level of affinity maturation observed in germinal centre reactions. Similar levels of affinity maturation are obtained for other responses, although for antibodies involving several affinity-enhancing mutations the affinity maturation obtained with the model is much lower. Interpretation of these results and consequences towards the concept of recycling are discussed. 相似文献
3.
酚类抗氧化剂清除自由基活性的理论表征与应用 总被引:34,自引:1,他引:34
筛选高效、低毒、价廉的抗氧化剂一直是自由基生物学的核心课题之一。由于筛选工作的复杂性与艰巨性,近来开始用理论方法加快这一过程。已发现许多理论参数与抗氧化剂清除自由基速率常数的对数有良好的相关性,其中尤以有征O-H解离能的△HOF值研究得最深入。借助这一理论指标可解释酚类抗氧化剂的构效关系。对复杂抗氧化体系中抗氧化剂的活性差异也能给予较好的预测,而且还可指导新型抗氧化剂的理论构建,至此这一理论的有效 相似文献
4.
P Sonneveld F W Schultz J A Mulder 《Analytical and quantitative cytology and histology / the International Academy of Cytology [and] American Society of Cytology》1988,10(4):261-268
A numerical method for deriving the fractions of cells in different phases of the cell cycle from a single observed DNA histogram is presented. The observed histogram is regarded as a polluted version (containing allocation errors) of the true histogram. A mathematical model is used to describe the pollution process. A theoretical histogram, representing the true histogram, is constructed so that G1 cells are put into one channel and G2M cells into another; the distribution of S cells in between is approximated with a set of harmonic functions. This theoretical histogram is subsequently disturbed with Gaussian dispersion functions to stimulate the pollution, yielding a predicted histogram. Using a maximum likelihood estimation technique, the model parameters are adjusted iteratively, matching the predicted histogram to the actually observed one. With the final parameter values substituted, the corresponding final theoretical histogram is regarded as a reliable reconstruction of the true histogram. From the latter, the required percentages can be read directly. The advantage of this approach over other mathematical analysis methods is that it allows a wide range of different, continuous distributions for relatively few model parameters (thus featuring flexibility and realism and a diminished risk of encountering computational problems). In addition, estimation errors providing a measure of accuracy can be obtained. To test the method, it was used to analyze various observed histograms from the literature that have been obtained by either simulation or actual flow cytometric measurements. The method appeared to perform well, as compared to the reported results of several other methods of analysis applied to the same data. 相似文献
5.
The structural parameters of fluid phase bilayers composed of phosphatidylcholines with fully saturated, mixed, and branched fatty acid chains, at several temperatures, have been determined by simultaneously analyzing small-angle neutron and X-ray scattering data. Bilayer parameters, such as area per lipid and overall bilayer thickness have been obtained in conjunction with intrabilayer structural parameters (e.g. hydrocarbon region thickness). The results have allowed us to assess the effect of temperature and hydrocarbon chain composition on bilayer structure. For example, we found that for all lipids there is, not surprisingly, an increase in fatty acid chain trans-gauche isomerization with increasing temperature. Moreover, this increase in trans-gauche isomerization scales with fatty acid chain length in mixed chain lipids. However, in the case of lipids with saturated fatty acid chains, trans-gauche isomerization is increasingly tempered by attractive chain-chain van der Waals interactions with increasing chain length. Finally, our results confirm a strong dependence of lipid chain dynamics as a function of double bond position along fatty acid chains. 相似文献
6.
7.
A steady laminar flow of blood in a uniform tapered tube has been examined. Blood rheology is assumed to be described by a polar fluid. The analytical expressions for velocities (both axial and radial), total angular velocity, wall shear and pressure drop have been obtained. In literature, the parameters N (coupling number) and L (length ratio) have been chosen independently. But, in the present analysis, it is found that they are interrelated. Variation of the flow variables with suspension concentration and tapered angle have been investigated. Some of the theoretical models for the flow through tapered tubes have been critically examined. The pressure-flow relationship has been studied numerically over the flow rate range 0.01-0.1 cc/sec and compared with experimental results. It has been shown that the existing experimental results are for the tapered tubes of larger diameter which correspond to the flow under Newtonian conditions. Finally, some biological implications and future developments of this theory have been indicated. 相似文献
8.
R Aquilani G Poggi 《Bollettino della Società italiana di biologia sperimentale》1979,55(16):1545-1551
The effect of diaphragmatic respiration in broncopneumopathic patients has been investigated. By using a computerized method of analysis, VO2, VCO2, VE, VA and several other respiratory and metabolic parameters have been investigated. The results obtained in these patients, at rest, have shown that particularly in supine position the abdominal respiration improves the CO2 output by increasing VCO2 and reducing VO2. The reduction in the breathing rate, reduces VD and increases VA, in this way the efficiency of ventilation increases accordingly. The increase of CO2 output at the same metabolic rate is discussed in light of the fact that diaphragmatic respiration seems to improve the VA/Q ratio. 相似文献
9.
Modeling of R.A. Fisher's ideas about the evolution of male ornamentation using quantitative genetics began in the 1980s. Following an initial period of enthusiasm, interest in these models began to wane when theoretical studies seemed to show that the rapid evolution of ornaments would not occur if there were costs associated with female mate choice. Recent theoretical work has shown, however, that runaway evolution and other kinds of extensive diversification of ornaments and preferences can occur, even when female choice is costly. These new models highlight crucial parameters that profoundly influence evolutionary trajectories, but these parameters have been neglected in empirical studies. Here, we review quantitative genetic models of sexual selection with the aim of fostering communication and synergism between theoretical and empirical enterprises. We also point out several areas in which additional empirical work could distinguish between alternative models of evolution. 相似文献
10.
Theoretical treatment of resonance interaction of amide-I vibration has been done in a dipole–dipole approximation on the basis of perturbation theory. A single infinite antiparallel-chain pleated sheet as well as different kinds of its finite fragments have been considered. A good agreement has been obtained between calculated spectral parameters of amide I of the infinite sheet and observed ones in infrared and Raman spectra of synthetic polypeptides and fibrous proteins. A theoretical dependence of the resonance frequency shift of the main component and frequency splitting of two components active in the infrared spectra on the number of polypeptide chains in the finite sheet has been found. 相似文献
11.
Klevanik AV 《Membrane & cell biology》2000,13(4):485-501
The low-temperature absorption spectra of the Chlorobium tepidum FMO bacteriochlorophyll-protein complex at various pressures have been calculated within the framework of mini-exciton theory. The dependences of the Qy transition energies of the monomeric pigments on pressure have been found by means of functional minimization. This functional includes the parameters of both theoretical and experimental absorption spectra at low temperatures and various pressures. The dependences obtained are compared with those derived for the exciton transition energies, which have been obtained by deconvoluting absorption spectra with seven Gaussian components at each pressure. The pressure increase has been shown to result in the increased coupling energy between both the pigment molecules themselves and pigments and amino acid residues. The pigment molecules capable of binding histidines and water molecules have been shown to have the greatest and smallest responses to increased pressure, respectively. The couplings of Bchl molecules with the surrounding amino acid residues have been shown to change both the exciton delocalization index and the exciton distribution between the pigment molecules within the protein subunit; the increased pressure does not change these parameters significantly. 相似文献
12.
Leaf physiognomic traits vary predictably along climatic and environmental gradients. The relationships between leaf physiognomy and climate have been investigated on different continents, but so far an investigation based on European vegetation has been missing. A grid data set (0.5 degrees x 0.5 degrees latitude/longitude) has been compiled in order to determine spatial patterns of leaf physiognomy across Europe. Based on distribution maps of native European hardwoods, synthetic chorologic flora lists were compiled for all grid cells. Every synthetic chorologic flora was characterised by 25 leaf physiognomic traits and correlated with 16 climatic parameters. Clear spatial patterns of leaf physiognomy have been observed, which are statistically significant related to certain, temperature-related climate parameters. Transfer functions for several climatic parameters have been established, based on the observed relationships. The study provides evidence that synthetically generated floras represent a powerful tool for analysing spatial patterns of leaf physiognomy and their relationships to climate. The transfer functions from the European data set indicate slightly different relationships of leaf physiognomy and environment compared with results obtained from other continents. 相似文献
13.
L. F. M. Fresco 《Plant Ecology》1982,48(2):175-185
The variation in species composition of plant communities has been described by means of the Gaussian distribution (relations between plant populations and environmental variables) and by linear equations (mutual interference between mixed populations). An important presupposition is the existence of a theoretical population, covering 100% of the soil surface. A numerical approximation of the parameters can be obtained by means of an iterative procedure. This technique has been applied to heath vegetation. A small sample has been used to demonstrate the algorithm. The cover ofCalluna vulgaris, Erica tetralix andMolinia caerulea could be explained to a large extent from phosphate and ground water table. A comparison of the results with those of experiments by other authors showed a reasonable similarity. The linear coefficients of interference as means to describe relations between mixed plant populations in a local area have been evaluated by means of dynamic simulation. 相似文献
14.
Abstract Ecological interpretation has been subject to several divisive controversies, involving, for example, the significance of density dependence and interspecific competition as ecological processes. Generally, resolution has been obtained through compromise and concensus or calls for yet more data. Essentially, both sides in the discussion are seen to have been correct to some extent. As a consequence the debates have been portrayed widely as having been sterile. We agree, but only because they have been conducted at a level so superficial that the relevance of the original criticisms to the theoretical structure of ecology has not been widely appreciated, nor resolved. Debate that deals with ecological generalizations must be conducted at a level appropriate to such aims. 相似文献
15.
O. V. Baum I. A. Chaikovsky L. A. Popov V. I. Voloshin L. S. Fainzilberg M. M. Budnyk 《Biophysics》2010,55(5):812-821
The peculiarities of the genesis of electrocardiosignal under myocardial ischemia in connection with an increase of its electrical
inhomogeneity, as well as the electrophysiological mechanisms of morphological changes of the T wave of the electrocardiogram,
including its “symmetrization” have been considered. A systemic approach to the problem has been used, which combines the
mathematical, computer, and physiological modeling of the cardiac electrical activity with studies of the electric field of
the human heart in terms of biophysical models. A database for the repolarization parameters of experimental electrocardiosignals
(“Norm” and “Ischemia” samples) has been formed. The parameters of the ST-T interval and T wave, which could characterize
the symmetry of the latter, and some additional properties of the repolarization process have been obtained. The methods of
mathematical modeling were used also. Computer experiments were carried out on a system for 3D modeling of the cardiac electrical
activity at different structural levels of the object. By the results of preliminary analysis, the β
T
index, which is calculated as the ratio of two maximum absolute values of the derivative of the cardiosignal at the left
and right of the T wave apex, has been chosen as one of the main diagnostic markers of ischemia. There is reason to believe
that the use of the β
T
index allows one to recognize those deviations from the norm that are usually hidden from a physician in traditional ECG
analysis. The ratios of repolarization intervals inside of the generalized QT interval have also appeared potentially informative.
With the purpose to test and correct the hypotheses for conducting further investigations, some preliminary experiments on
a low-resolution model for several alternatives of the degree, localization, and extensiveness of ischemia have been carried
out. The results obtained at the first stage of the team-work are essential to the understanding of mechanisms of the genesis
of an electrocardiographic image for myocardial ischemia and of interest for the biophysically sound development of new algorithms
for computer cardiodiagnostics. 相似文献
16.
Summary This article is concerned with the determination of kinetic parameters of the Calvin photosynthesis cycle which is described by seventeen nonlinear ordinary differential equations. It is shown that the task requires dynamic data for several sets of initial conditions. The numerical technique is based upon an algorithm for non-linear optimization and Gear's numerical integration scheme for stiff systems of differential equations. The sensitivity of the parameters to noise in the data is tested with a method adapted from Rosenbrook and Storey. A preliminary set of parameters has been obtained from a preliminary set of experimental data. The numerical methods are then tested with synthetic data derived from these parameters. The mathematical model and the results obtained in the simulation are used as an aid in designing new experiments. 相似文献
17.
A theoretical study on the quantification of surface thermal response of cancerous human skin using the frequency modulated thermal wave imaging (FMTWI) technique has been presented in this article. For the first time, the use of the FMTWI technique for the detection and the differentiation of skin cancer has been demonstrated in this article. A three dimensional multilayered skin has been considered with the counter-current blood vessels in individual skin layers along with different stages of cancerous lesions based on geometrical, thermal and physical parameters available in the literature. Transient surface thermal responses of melanoma during FMTWI of skin cancer have been obtained by integrating the heat transfer model for biological tissue along with the flow model for blood vessels. It has been observed from the numerical results that, flow of blood in the subsurface region leads to a substantial alteration on the surface thermal response of the human skin. The alteration due to blood flow further causes a reduction in the performance of the thermal imaging technique during the thermal evaluation of earliest melanoma stages (small volume) compared to relatively large volume. Based on theoretical study, it has been predicted that the method is suitable for detection and differentiation of melanoma with comparatively large volume than the earliest development stages (small volume). The study has also performed phase based image analysis of the raw thermograms to resolve the different stages of melanoma volume. The phase images have been found to be clearly individuate the different development stages of melanoma compared to raw thermograms. 相似文献
18.
Skin impedance measurements at various skin sites yield different impedance loci for normal skin and special skin sites. The results of skin impedance measurements taken at such sites with a two-electrode measurement system are presented herein. Some of these sites can be identified as acupuncture points. Data from 4 volunteers were acquired by means of a data acquisition board and a measuring system consisting of the measurement circuit, including several electrode types, and a power supply. The Cole model is a model for an equivalent electrical circuit of the skin-electrode system. The system was used to derive skin-typical parameters from the Bode plot of the whole system. These parameters are the fractional power a, the pseudo-capacity K, the parallel resistance Rp, and the serial resistance Rs of the equivalent electrical circuit. The results show that the measured parameters differ between normal skin and special skin sites. These effects have not previously been discovered by other authors, since there has been no systematic investigation of many acupuncture points to date, and there has been no apparent need for such an investigation. A number of necessary criteria for acupuncture point detection can be derived from the results obtained. 相似文献
19.
F. Garcia-Carmona F. Garcia-Cánovas J.L. Iborra J.A. Lozano 《Biochimica et Biophysica Acta (BBA)/General Subjects》1982,717(1):124-131
The first part of the melanization pathway from l-dopa to dopachrome has been studied as a system of various chemical reactions coupled by an enzymatic reaction. A theoretical and experimental kinetic approach is proposed for such a system. Rate constants for the implicated chemical steps at different pH and temperature values can be evaluated from measurement of the lag period arising from the accumulation of dopachrome that takes place when l-Dopa was oxidized at acid pH. The thermodynamic parameters of the chemical steps, the deprotonation of dopaquinone-H+ into dopaquinone and the internal cyclization of dopaquinone into leukodopachrome, have been obtained. From the results presented, an alternative series of chemical reactions to the Raper-Mason scheme are proposed and discussed. 相似文献
20.
Aydin A Ibrikçi T Akçali ID 《Computer methods in biomechanics and biomedical engineering》2012,15(7):753-759
In the field of orthopaedics, treatment of extremity deformities can be realised by means of external fixators. However, control of such biomedical system is very difficult. Some different mathematical models have been developed to improve quality of this service. Most of the parameters, which are used in these models, have been obtained from two orthogonal X-ray images: one from anteroposterior, AP, direction and the other from a lateral, L, direction. The quality of the results of this model is dependent on the accuracy of the input parameters. Measuring these parameters is a time-consuming issue, and the accuracy of the results is also low. To increase the quality of the measurement, the reference points should be chosen from the edges of the biomedical system, and it is important to find the edges without noise. To achieve this purpose, Sobel edge detector, binary large object analysis, thresholding and inverting are applied as image processing steps. The results are compared with manual measurement values which have been obtained earlier. The results show that semi-automatic measurement of the parameters is more accurate and faster than manual measurement. It shows that the efficiency of the fixator method has been improved. 相似文献