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PubMed1994

[Evaluation of multislice dynamic MR imaging of the whole liver].

In 47 patients with liver cirrhosis, we performed dynamic MRI with a multisection FLASH technique that enabled us to obtain 13 T 1-weighted images of the entire liver within a single breath hold. Computed tomographic arterial portography (CTAP), US, CT, angiography (AOG) and MRI (spin echo [SE] and dynamic MRI) were performed in all 47 patients. Except for cyst, hemangioma and metastatic tumor, 104 focal nodules less than 3 cm in diameter were detected. These 104 focal lesions were divided into three groups according to the pattern of CTAP: 69 portal supply negative, 11 portal supply decreased, and 24 portal supply normal. In the portal supply negative group, 63 lesions (91%) were detected by dynamic MRI, which was superior to other modalities (US 77%, CT 41%, AOG 70%, MRI-SE 61%). The superiority of dynamic MRI resulted from its excellent ability to detect liver lesions less than 1 cm in diameter. We confirmed histologically that dynamic MRI had almost the same ability to detect hepatocellular carcinoma (HCC) as CTAP. Dynamic MRI should be clinically useful as a noninvasive examination for the detection of HCC.

Carcinoma, Hepatocellular↗

PubMed1993

A dynamical neural network model for motor cortical activity during movement: population coding of movement trajectories.

As a dynamical model for motor cortical activity during hand movement we consider an artificial neural network that consists of extensively interconnected neuron-like units and performs the neuronal population vector operations. Local geometrical parameters of a desired curve are introduced into the network as an external input. The output of the model is a time-dependent direction and length of the neuronal population vector which is calculated as a sum of the activity of directionally tuned neurons in the ensemble. The main feature of the model is that dynamical behavior of the neuronal population vector is the result of connections between directionally tuned neurons rather than being imposed externally. The dynamics is governed by a system of coupled nonlinear differential equations. Connections between neurons are assigned in the simplest and most common way so as to fulfill basic requirements stemming from experimental findings concerning the directional tuning of individual neurons and the stabilization of the neuronal population vector, as well as from previous theoretical studies. The dynamical behavior of the model reveals a close similarity with the experimentally observed dynamics of the neuronal population vector. Specifically, in the framework of the model it is possible to describe a geometrical curve in terms of the time series of the population vector. A correlation between the dynamical behavior of the direction and the length of the population vector entails a dependence of the "neural velocity" on the curvature of the tracing trajectory that corresponds well to the experimentally measured covariation between tangential velocity and curvature in drawing tasks.

Animals↗

PubMed1993

Dynamic visual acuity of normal subjects during vertical optotype and head motion.

PURPOSE: To characterize the effect of passive vertical head motion on dynamic visual acuity of young, normally sighted subjects wearing telescopic spectacles, and to relate this to the velocity of images on the retina. METHODS: Static visual acuity was measured without motion. Dynamic visual acuity was measured during vertical, sinusoidal motion of either optotypes or of a servo-driven rotating chair in which subjects were seated. Dynamic visual acuity for head motion was measured unaided, as well as with 1.9X, 4X, and 6X telescopic spectacles. Vertical eye movements were recorded using magnetic search coils. RESULTS: During optotype motion, acuity declined with increasing velocity to a minimum of approximately 20/200 at 100 degrees/sec. Pursuit gain (eye velocity/optotype velocity) for moving optotypes was low except for optotype velocities of 20 degrees/sec of less. Dynamic visual acuity without telescopic spectacles was not sensitive to head motion. Static visual acuity improved with increasing telescopic spectacle power, but dynamic visual acuity became progressively impaired by head motion as telescopic spectacle power was increased. Compared with static visual acuity, head motion with peak velocity of 40 degrees/sec reduced acuity two-fold for 1.9X telescopic spectacles, fourfold for 4X telescopic spectacles, and eightfold for 6X telescopic spectacles. Visual vestibulo-ocular reflex gain with telescopic spectacles increased to values markedly above 1.0, but was always less than telescopic spectacle magnification. There was visual tolerance of slip velocities of 2 degrees/sec or less, above which acuity declined in proportion to the 0.6 power of retinal slip velocity. Above 2 degrees/sec, retinal slip velocity accounted for 95% of the variance in dynamic visual acuity. CONCLUSIONS: These results confirm that acuity is sensitive to retinal image motion in the vertical direction, and extend this finding to indicate that sensitivity of acuity to vertical head motion during wearing of telescopic spectacles is attributable to retinal image slip velocity.

Adult↗

PubMed1993

[Transesophageal echocardiography study with dynamic spontaneous contrast of the left atrial appendage, its relationship with the anatomy and thrombus formation].

OBJECTIVE: Transesophageal two-dimensional echocardiographic study of anatomical characteristics of the left atrial appendage and its relation to spontaneous dynamic echocardiographic contrast. DESIGN: Outpatients undergoing a prospective two-dimensional transesophageal echocardiographic study. SETTING: Consecutive outpatients studied at the Echocardiographic Laboratory of Gregorio Marañon General Hospital, Madrid. MATERIAL AND METHODS: In each patient at the level of the left atrial appendage we calculated the following transesophageal echocardiographic parameters: end-systolic and end-diastolic maximal longitudinal and transversal diameters, total systolic and diastolic areas, percentage of systolic fractional shortening, presence of left atrial appendage thrombus and spontaneous dynamic echo-contrast. MAIN RESULTS: Left atrial appendage spontaneous dynamic contrast was observed in 48% of the total population. In the group of patients with left atrial spontaneous echo-contrast we observed larger longitudinal systolic (44 +/- 14 mm vs 28 +/- 13 mm, p = 0.01) and diastolic (52 +/- 16 mm vs 38 +/- 12 mm, p = 0.005) diameters, larger transversal systolic (25 +/- 10 mm vs 19 +/- 6 mm, p = 0.03) and diastolic (28 +/- 8 mm vs 25 +/- 9 mm, p = NS) diameters and also larger systolic (601 +/- 204 mm2 vs 337 +/- 110 mm2, p < 0.0001) and diastolic (715 +/- 230 mm2 vs 507 +/- 184 mm2, p = 0.001) areas, compared to the group without this dynamic echocardiographic phenomena. Left atrial appendage percentage of fractional shortening was considerably reduced in patients with spontaneous dynamic echo-contrast (15 +/- 14% vs 39 +/- 18%, p = 0.001) and related to local thrombus formation (13% vs 1%, p < 0.001). CONCLUSIONS: Left atrial spontaneous dynamic echo-contrast is more common in patients with enlarged left atrial appendage systo-diastolic diameters and areas. In this group of patients the presence of left atrial spontaneous echo-contrast is related to a significant reduction in left atrial appendage contractile function and thrombus formation. Parameter analysis of left atrial appendage anatomy by two-dimensional transesophageal echocardiography may have clinical relevance in the assessment of patients with high risk for left atria thromboembolic phenomena.

Diastole↗

PubMed1993

[Dynamic sequential study of hepatic nodular lesions with ultra-fast sequences and fast bolus of gadolinium-DTPA].

Eighty-five patients with single/multiple nodular hepatic lesions (10 focal nodular hyperplasias, 37 liver hemangiomas, 24 metastases and 16 hepatocellular carcinomas; 2 patients had associated lesions) were examined with dynamic single-slice sequences and fast i.v. bolus injection of Gd-DTPA. The dynamic single-slice technique was used to evaluate the peculiar features of the dynamic enhancement. The snapshot sequence proved best to provide the high temporal resolution of the dynamic parenchymal enhancement (scanning time < 1 second, one frame every third second). The following variables were investigated: nodular lesion intensity in the first basal snapshot image, enhancement appearance and its contemporaneity with arterial, venous or portal flows, enhancement gradient relative to surrounding liver parenchyma, morphologic features of the enhancement and its centrifugal/centripetal patterns. The enhancement curve of focal nodular hyperplasia increased very quickly during the first 20-25 seconds. This enhancement was quite similar to the arterial one and always occurred before portal and systemic venous times. Hemangiomas exhibited a typically slow growth-curve in the first 120 seconds, with a positive final gradient value relative to liver parenchyma. The appearance of peripheral contrast enhancement was a typical sign after 30 seconds. Metastases exhibited similar dynamic enhancement to hemangiomas, but peripheral enhancement was never observed and, after 120 seconds, gradient was null or negative relative to the adjacent liver parenchyma. Moreover, enhancement always followed portal times. Hepatocellular carcinomas showed an early growth curve, preceding portal times, but less marked than in hyperplasia. The study of our series provided the preliminary semiology of early dynamic enhancement patterns, which is quite specific to recognize and differentiate nodular hepatic lesions.

Contrast Media↗

PubMed1995

Dynamic wedge factors for a comprehensive range of fields.

Assessing the methods available to model wedge factors as applied to dynamic wedges over a comprehensive range of field elongations is the primary intention of this paper. Wedge factors for dynamically produced wedges were investigated by a series of measurements for square and rectangular fields. A Varian 600C linear accelerator was used for the experimental work. Dynamic wedge angles of 15, 30, 45 and 60 degrees were studied. (The wedge factor was defined as the ratio of the central axis dose reading at 10 cm deep of the wedged field to the reading at 10 cm depth for the same sized open field). The possibility of improving dynamic wedge factors for elongated fields by modelling the rectangular field with the equivalent square (Worthley) and also by the square field of equal area (Arthur) methods was evaluated. Dynamic wedge factors were found to depend on the field size in the wedged direction dominantly. The influence of field elongation in the non-wedged axis was found to be negligible. The experiments indicate that the derivation of a dynamic wedge factor for use with rectangular fields is best based on a square field with a side equal to that in the wedged axis. The application of two square field models (Worthley and Arthur) for elongated fields was found to produce significantly worsened wedge factors for the majority of the fields considered.

Humans↗

PubMed1995

Dynamic patient data bases: the foundation of an integrated approach to outcome measures for the healthcare professionals.

In recent years there has been a tremendous need among healthcare professionals to assess the effectiveness, efficiency, and appropriateness of the patient care services being provided through criteria-based outcome and program evaluation. Although the need for a tool which could evaluate the effectiveness of patient care is widely recognized, such an undertaking has been severely limited due to the lack of any automated means to collect and analyze patient data on a routine, continuous basis within a clinical setting. We have developed and implemented at Mineral Springs Hospital, Banff, Alberta an integrated and automated hospital information system that not only continuously collects administrative, financial, and patient data, but also contains an intelligent component for automated outcome measure and program evaluation. The system collects various non-duplicated data elements from each routine work process within the facility on a continuous basis. Through the creation of a dynamic patient database, data is transformed into information--a powerful decision support tool. The system provides flexible user-defined reports in patient-specific resource utilization, direct and/or indirect specific financial costs, result reporting of each intervention, service provided and user-defined criteria-based outcome, and program evaluation. The system design incorporates expert rules, dynamic data entry forms, quantitative models, and user-defined access control. Using information derived from the dynamic common database, managers and front-line clinicians can easily evaluate and modify management decisions or careplans on a macro or micro level. An external review is planned to evaluate whether the system has helped the assessment of effectiveness, efficiency and appropriateness of healthcare services being provided at the hospital. The fundamental concept behind the system design is that the patient is the center of activity for data collection. The system provides the answers to the 5 W's (who, what, where, when, and why) together with intervention and service result reports. A dynamic common patient database is the center of the system and is accessible to all with proper authorization. Common data elements are collected from routine work flow without extra data entry and this information is subsequently shared. Data collection is a continuous process. We believe that every process is the outcome of another sub-process or event. The design of the dynamic patient database incorporates patient-specific costing and outcome evaluation, user-defined flexible data entry forms, user-defined access control, outcome evaluation rules and information semantic rules. Such a patient database would provide the flexibility needed to accommodate diverse methodologies to evaluate outcomes whether it they be medical, cost, access and/or other combination of measures. The system was developed on a PC-based Network technology, using FOXPRO (XBase) as the database development tool incorporating advanced technology such as distributed processing and fault tolerant computing. We chose PC-based technology because it is economical, having relatively low maintenance costs and requires no major dependency on vendors. The developed system produces patient-specific reports with many dimensions. The reports are user-defined. The system reports general data, CMG, RGN, LOS, Expected LOS, and other user-defined demographic data. Resource utilization, financial costs, and result reportings are produced together with rule-based outcome assessments of any type of measures, including, but not limited to, pre-set functional/health goals, user satisfaction, clinicianUs text or codified comments etc. It provides the framework for continually capturing data at a practical, work-flow level. The incorporation of a dynamic patient database as the driving forece of an integrated, rule-based administration, financial and patient data system will provdie the tools for healthcar

Databases, Factual↗

PubMed1996

[Comparison of dynamic and isometric loading in echocardiographic examinations in patients with ischemic heart disease].

The objective of the work was to compare functional indicators of the left ventricle during isometric (HG) and dynamic (Ergo) echocardiographic load examinations in patients with different grades of ischaemic heart disease. In group A they compared results in 34 patients with affection of three coronary arteries and in group B 15 patients with 1-2 coronary arteries affected. They evaluated the systolic, diastolic blood pressure, heart rate (HR), the index of the diastolic end volume EDV/m2, the index of the systolic end volume ESV/m2, Limacher's index for evaluation of left ventricular motility and the left ventricular ejection fraction (EF) at rest and after both types of loads. The EDV/m2 index increased in group A by 5.2 ml during an isometric load and by 10.4 ml during a dynamic load, in group B it increased by 1.6 and 7.0 ml resp. The values of the ESV/m2 index increased in group A by 8.5 ml during the isometric load and by 14.0 ml during the dynamic load, in group B by 4.1 and 10.8 ml resp. EF values at rest were similar in both groups: group A: 51.4%, B: 51.5%. During the isometric load the EF declined to 45.3% in group A and to 47,1% in group B. During the dynamic load the EF declined to 42% in group A and 42.5 in group B. The dynamic load produces, as compared with the isometric one, a statistically more significant drop of Limacher's index and greater volumetric changes of the left ventricular cavity in the group with 1-2 affected coronary arteries, as compared with the isometric load. In patients with three affected coronary arteries the volumetric changes during an isometric load are comparable with changes after a dynamic load.

Coronary Disease↗

PubMed1997

Dynamical energy systems and modern physics: fostering the science and spirit of complementary and alternative medicine.

When systems theory is carefully applied to the concept of energy, some novel and far-reaching implications for modern physics and complementary medicine emerge. The heart of systems theory is dynamic interactions: systems do not simply act on systems, they interact with them in complex ways. By definition, systems at any level (e.g., physical, biological, social, ecological) are open to information, energy, and matter to varying degrees, and therefore interact with other systems to varying degrees. We first show how resonance between two tuning forks, a classic demonstration in physics, can be seen to reflect synchronized dynamic interactions over time. We then derive how the dynamic interaction of systems in mutual recurrent feedback relationships naturally create dynamic "memories" for their interactions over time. The mystery of how a photon (or electron) "knows" ahead of time whether to function as a particle or wave in the single slit/double slit quantum physics paradigm is potentially solved when energetic interactions inherent in the experimental system are recognized. The observation that energy decreases with the square of distance is shown not to be immutable when viewed from a dynamical energy systems perspective. Implications for controversial claims in complementary and alternative medicine, such as memory for molecules retained in water (homeopathy), remote diagnosis, and prayer and healing, are considered. A dynamical energy systems framework can facilitate the development of what might be termed "relationship consciousness," which has the potential to nurture both the science and spirit of complementary medicine and might help to create integrated medicine.

Complementary Therapies↗

PubMedDate not supplied

Assessment of breast cancer with dynamic gadolinium-enhanced MR imaging combined with magnetization transfer contrast using a newly developed breast surface coil for the supine position.

To assess the value of dynamic gadolinium-enhanced MR imaging combined with magnetization transfer contrast (MTC) of breast cancer by SPGR sequence, 15 patients with breast cancer were imaged in the supine position with the newly developed breast coil in a 1.5 Tesla imager. Dynamic gadolinium-enhanced MR imaging combined with MTC (10 cases) and conventional dynamic gadolinium-enhanced MR imaging (5 cases) were performed after the administration of Gd-DTPA (0.2 ml/kg). Sagittal images were obtained every 22 seconds during the first 264 seconds. Thus, a total of 15 images were obtained in each lesion. The signal intensity ratio (SIR) of lesion to mammary glarid was calculated for each image as follows: (SI lesion/SI mammary gland)/(SI (pre)lesion/SI (pre)mammary gland). Dynamic gadolinium-enhanced MR imaging with MTC always allowed better SIR at 88 seconds than conventional dynamic gadolinium-enhanced MR imaging after bolus injection (p < 0.01). Dynamic gadolinium-enhanced MR imaging with MTC improved detection of the boundary of the lesion and mammary gland.

Adult↗

PubMed1997

Comparison of dynamic contrast-enhanced gradient-echo and spin-echo sequences in MR of head and neck neoplasms.

PURPOSE: To investigate the utility of dynamic contrast-enhanced gradient-echo MR imaging of head and neck lesions and to compare this technique with the commonly used spin-echo contrast-enhanced fat-saturation technique. METHODS: Twenty-two patients with a total of 23 head and neck neoplasms underwent dynamic gradient-echo and spin-echo MR imaging studies. The spin-echo and dynamic gradient-echo images were compared in each case by consensus of three observers for differences in tumor conspicuity and delineation of margins, particularly with regard to invasion of adjacent structures. When possible, pathologic and/or surgical confirmation of tumor extent was obtained. Relative contrast was also calculated to determine objectively the degree of tumor enhancement with respect to background mucosa. RESULTS: The dynamic gradient-echo images showed better or equal delineation of the tumor margins by subjective observation in all but two cases. Temporally different enhancement patterns were noted for lesions, background mucosa, and adjacent reaction and edema. The dynamic gradient-echo technique provided better relative contrast than the spin-echo technique in 17 (77%) of 22 lesions. CONCLUSION: Dynamic gradient-echo MR imaging is superior to conventional contrast-enhanced spin-echo imaging in delineating the margins and extent of tumor. This technique provided observers with added confidence in their interpretations and suffered from fewer technical limitations.

Adult↗

PubMed1997

[Nursing research on the dynamics of hope in the elderly--a review].

The dynamics of hope--the dialectics between hope and hopelessness--is a central dimension in the essence of human being, in health and illness, as well as in nursing. In this paper nursing research on hope and hopelessness in elderly people is described. CINAHL- and MEDLINE EXPRESS-database were used as sources of research material. The purposes, the target populations, the methods of data collection and the central results from 10 research articles were analyzed using the method of content analysis. In addition qualitative content analysis was used to illustrate the dynamics of hope. The research on the dynamics of hope in nursing science has mostly focused on young adults, and less on elderly people. Most of the studies on hope and hopelessness in elderly people were descriptive, cross-sectional research based on quantitative assumptions. The dynamics of hope was described as the dynamics of life and death in which elderly people's significant relationships, feelings and the ability to act were central elements. Qualitative, longitudinal designs are needed in nursing science in order to capture the essence of the dynamics of hope in elderly human beings.

Aged↗

PubMed1997

[Magnetic resonance angiography of the vessels of the neck: the optimization of a dynamic technic during the rapid infusion of a paramagnetic contrast medium].

INTRODUCTION: A new noninvasive technique, dynamic Magnetic Resonance Angiography (MRA) during rapid infusion of paramagnetic contrast medium, has been recently developed for the study of neck vessels. This study was carried out to optimize technical parameters. MATERIALS AND METHODS: Twenty healthy volunteers (10 men and 10 women; age range: 25 to 50 years) were examined with a 1.5 T MR unit equipped with fast imaging software and a dedicated coil. Dynamic MRA was performed using fast spoiled gradient recalled (fast SPGR) sequences acquired on the coronal plane 13 s after contrast medium injection. Imaging parameters were: Echo Time (TE): 1 ms, Repetition Time (TR): 8 ms, flip angle: 60 degrees, matrix 256 x 128, number of excitations: 1, field of view: 18 x 13 cm, number of partitions per slab: 28, slice thickness: 1 mm, acquisition time: 32 s. A dose of .2 mmol/kg of paramagnetic contrast medium was administered with an MR compatible automatic injector, at a speed rate of 1.5 ml/s. Angiographic images were postprocessed with the maximum intensity projection (MIP) and targeted MIP algorithms. RESULTS: Using the above parameters and rapid contrast medium injection, the carotid arteries on the coronal plane (including the vessels from the proximal aspect of the common carotid arteries to the intracranial tract of the internal carotid arteries) in 18 cases. In the other two cases the tortuosity and the position of the carotid arteries on two different planes, prevented their complete visualization. The vertebral arteries were completely demonstrated in 50% of the investigated population. Partial or total overlapping of jugular veins did not affect the recognition of the carotid vessels in 10 cases, also with targeted MIP postprocessing. DISCUSSION: The vascular semiology of dynamic gadolinium enhanced MRA seems to be closer to that of conventional angiography than of conventional MRA, likely due to the same modality of vessel depiction, based on contrast medium administration. Flow artifacts, the major pitfall of conventional MRA causing signal void and overestimation of stenosis grade, are not frequent in dynamic MRA. CONCLUSIONS: Gadolinium enhanced dynamic MRA ensures panoramic and high resolution angiographic-like depiction of the neck vessels, providing rapid and excellent definition of vascular morphology. Thus dynamic MRA appears to be a substantial alternative to conventional MRA and conventional angiography.

Adult↗

PubMed1997

Complexity, dynamic cellular network, and tumorigenesis.

A holistic approach to tumorigenesis is proposed. The main element of the model is the existence of dynamic cellular network. This network comprises a molecular and an energetistic structure of a cell connected through the multidirectional flow of information. The interactions within dynamic cellular network are complex, stochastic, nonlinear, and also involve quantum effects. From this non-reductionist perspective, neither tumorigenesis can be limited to the genetic aspect, nor the initial event must be of molecular nature, nor mutations and epigenetic factors are mutually exclusive, nor a link between cause and effect can be established. Due to complexity, an unstable stationary state of dynamic cellular network rather than a group of unrelated genes determines the phenotype of normal and transformed cells. This implies relativity of tumor suppressor genes and oncogenes. A bifurcation point is defined as an unstable state of dynamic cellular network leading to the other phenotype-stationary state. In particular, the bifurcation point may be determined by a change of expression of a single gene. Then, the gene is called bifurcation point gene. The unstable stationary state facilitates the chaotic dynamics. This may result in a fractal dimension of both normal and tumor tissues. The co-existence of chaotic dynamics and complexity is the essence of cellular processes and shapes differentiation, morphogenesis, and tumorigenesis. In consequence, tumorigenesis is a complex, unpredictable process driven by the interplay between self-organisation and selection.

Animals↗

PubMed1998

[Dynamic obstruction to left ventricular outflow during dobutamine stress echocardiography: the probable mechanisms and clinical implications].

We observed the development of left ventricular outflow tract dynamic obstruction in some patients during dobutamine stress echocardiography. The purpose of this study was to identify the possible mechanisms and to consider the clinical implications. From 11/04/94 to 01/09/97 we studied 547 patients; 42 patients developed dynamic obstruction, defined as a late peak Doppler velocity profile that exceeded baseline outflow velocity by at least 1 m/s. The encountered mechanisms were: increased myocardial contractility; systolic anterior motion of the mitral valve; decreased venous return to the left ventricle, and peculiar characteristics of the left ventricular geometry. The results of this study show that the dynamic obstruction is mainly due to the first mechanism and secondarily to some characteristics of the left ventricular geometry. The hypotension observed in a few cases is not related to the dynamic obstruction but to beta 2 receptor hypersensibility to dobutamine. The symptoms, like dyspnea and chest pain, experienced by these patients are related to the dynamic obstruction rather than to the presence of coronary artery disease. In conclusion, we think that patients who develop dynamic obstruction, without wall motion abnormalities, during dobutamine stress echocardiography, may behave pathophysiologically as patients with obstructive hypertrophic cardiomyopathy, in whom diastolic dysfunction and outflow tract obstruction are responsible for symptoms. Therefore, these patients require a pharmacological treatment with beta blockers and/or non-dihydropyridine calcium channel blockers.

Aged↗

PubMed1999

Finding the Missing Link between Landscape Structure and Population Dynamics: A Spatially Explicit Perspective.

We construct and explore a general modeling framework that allows for a systematic investigation of the impact of changes in landscape structure on population dynamics. The essential parts of the framework are a landscape generator with independent control over landscape composition and physiognomy, an individual-based spatially explicit population model that simulates population dynamics within heterogeneous landscapes, and scale-dependent landscape indices that depict the essential aspects of landscape that interact with dispersal and demographic processes. Landscape maps are represented by a grid of [Formula: see text] cells and consist of good-quality, poor-quality, or uninhabitable matrix habitat cells. The population model was shaped in accordance to the biology of European brown bears (Ursus arctos), and demographic parameters were adjusted to yield a source-sink configuration. Results obtained with the spatially explicit model do not confirm results of earlier nonspatial source-sink models where addition of sink habitat resulted in a decrease of total population size because of dilution of high-quality habitat. Our landscape indices, which describe scale-dependent correlation between and within habitat types, were able to explain variations in variables of population dynamics (mean number of females with sink home ranges, mean number of females with source home ranges, and mean dispersal distance) caused by different landscape structure. When landscape structure changed, changes in these variables generally followed the corresponding change of an appropriate landscape index in a linear way. Our general approach incorporates source-sink dynamics as well as metapopulation dynamics, and the population model can easily be modified for other species groups.

habitat connectivity↗

PubMed2002

Cyclic dynamics in simulated plant populations.

Despite the general interest in nonlinear dynamics in animal populations, plant populations are supposed to show a stable equilibrium that is attributed to fundamental differences compared with animals. Some studies find more complex dynamics, but empirical studies usually are too short and most modelling studies ignore important spatial aspects of local competition and establishment. Therefore, we used a spatially explicit individual-based model of a hypothetical, non-clonal perennial to explore which mechanisms might generate complex dynamics, i.e. cycles. The model is based on the field-of-neighbourhood approach that describes local competition and establishment in a phenomenological manner. We found cyclic population dynamics for a wide spectrum of model variants, provided that mortality is determined by local competition and recruitment is virtually completely suppressed within the zone of influence of established plants. This destabilizing effect of local processes within plant populations might have wide-ranging implications for the understanding of plant community dynamics and coexistence.

Ecosystem↗

PubMed2004

Modeling nonlinear effects in longitudinal survival data: implications for the physiological dynamics of biological systems.

Despite the wealth of longitudinal data on the health dynamics of human populations, information on covariates (risk factors) changes in those studies has not been systematically and fully exploited. In this work we use the 46-year follow-up of the Framingham Heart Study to analyze dynamics of these risk factors in survival models that go far beyond the standard linear dynamic formulation. We focus on improving the inferences about the physiology of human aging processes and its plasticity and on modeling state trajectories for individuals considering the effect of nonlinear interactions among covariates. We find that using standard statistical methods to construct models describing the age dependence of health status might give rise to surprising results with highly "diluted" dynamics, but with significantly improved statistical criteria. It is found that problems with the dynamics are a consequence of the intrinsic nonlinear nature of these models. We show that evolution of the risk factors measured in the Framingham study is more complicated for females than for males (i.e., female health status is more sensitive to nonlinear interactions among risk factors). We suggest that this is due to the rapid rate of decline of estrogen production after menopause.

Aging↗