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PubMedDate not supplied

Wavelet analysis of EEG for three-dimensional mapping of epileptic events.

This paper is aimed at understanding epileptic patient disorders through the analysis of surface electroencephalograms (EEG). It deals with the detection of spikes or spike-waves based on a nonorthogonal wavelet transform. A multilevel structure is described that locates the temporal segments where abnormal events occur. These events are then visually interpreted by means of a 3D mapping technique. This 3D display makes use of a ray tracing scheme and combines both the functional (the EEG but also its wavelet representation) and the morphological data (acquired from computed tomography [CT] or magnetic resonance imaging [MRI] devices). The results show that a significant reduction of the clinical workload is obtained while the most important episodes are better reviewed and analyzed.

Algorithms↗
PubMed2006

A multilevel modeling approach to assessing regional and local landscape features for lake classification and assessment of fish growth rates.

The ecoregion and watershed frameworks are landscape-based classifications that have been used to group waterbodies with respect to measures of community structure; however, they have yet to be evaluated for grouping lakes for demographic characteristics of fish populations. We used a multilevel modeling approach to determine if variability in mean fish length at age could be partitioned by ecoregions and watersheds. For the ecoregions analysis, we then examined if within-ecoregion variability could be explained by local water quality and lake morphometry characteristics. We used data from agency surveys conducted during 1974-1984 for age 2 and 3 fish of seven common warm and coolwater fish species. Variance in mean length at age between ecoregions for all species was not significant, and between-watershed variance estimates were only significant in 3 out of 14 analyses; however, the total amount of variation between watersheds was very small (ranging from 1.8% to 3.7% of the total variance), indicating that ecoregions and watersheds were ineffective in partitioning variability in mean length at age. Within ecoregions, water quality and lake morphometric characteristics accounted for 2%-23% of the variation in mean length at age. Measures of lake productivity were the most common significant covariates, with mean length at age increasing with increasing lake productivity. Much of the variability in mean length at age was not accounted for, suggesting that other local factors such as biotic interactions, fish density, and exploitation are important. The results indicate that the development of an effective regional framework for managing inland lakes will require a substantial effort to understand sources of demographic variability and that managers should not rely solely on ecoregions or watersheds for grouping lakes with similar growth rates.

Animals↗
PubMed2005

Area variation in mortality in Tasmania (Australia): the contributions of socioeconomic disadvantage, social capital and geographic remoteness.

This study investigated the association between socioeconomic disadvantage, social capital, geographic remoteness and mortality in the Australian state of Tasmania. The analysis is based on death rates among persons aged 25-74 years in 41 statistical local areas (SLA) for the period 1998-2000. Multilevel binomial regression indicated that death rates were significantly higher in disadvantaged areas. There was little support for an association between social capital and mortality, thereby contesting the often held notion that social capital is universally important for explaining variations in population health. Similarly, we found little evidence of a link between geographic remoteness and mortality, which contrasts with that found in other Australian states; this probably reflects the small size of Tasmania, and limited variation in the degree of remoteness amongst its SLA.

Adult↗
PubMed1993

Hyperhomocysteinemia in patients operated for lower extremity ischaemia below the age of 50--effect of smoking and extent of disease.

Moderate hyperhomocysteinemia may be a risk factor for atherosclerotic peripheral vascular disease (PVD). In order to develop PVD at an early age risk factors are more strongly expressed and hyperhomocysteinemia may be one such factor. Homocysteine is derived from methionine and is metabolised by cystathionine-synthase to cystathionine or remethylated to methionine. Cystathionine-synthase activity is dependent on vitamin B6 while the remethylation of homocysteine is dependent on vitamin B12 and folate. The present study analyses homocysteine in patients operated on for lower extremity ischaemia before the age of 50. Homocysteine before and after loading with methionine, vitamin B6, B12 and folate were measured at follow-up. The patients were compared to age- and sex-matched controls. Significantly more patients than controls had hyperhomocysteinemia, 16/58 vs. 4/65, defined as fasting total homocysteine above 18.6 mumol/l. Loading with methionine did not further discriminate between patients and controls. Smoking patients had higher levels of homocysteine than non-smoking patients or smoking and non-smoking controls. Smoking patients also had lower levels of vitamin B6. When comparing patients with suprainguinal, infrainguinal and multilevel disease the highest homocysteine levels were seen in the latter group. Also, in this group smoking patients had higher homocysteine levels. Multivariate analysis revealed that homocysteine was associated with low levels of vitamin B12, folate and smoking. Smoking therefore seems to be connected to increased homocysteine levels in patients with early development of atherosclerosis, partly explained by decreased levels of B6, B12 and folate.

Adult↗
PubMed2002

Predicting local cell deformations in engineered tissue constructs: a multilevel finite element approach.

A multilevel finite element approach is applied to predict local cell deformations in engineered tissue constructs. Cell deformations are predicted from detailed nonlinear FE analysis of the microstructure, consisting of an arrangement of cells embedded in matrix material. Effective macroscopic tissue behavior is derived by a computational homogenization procedure. To illustrate this approach, we simulated the compression of a skeletal muscle tissue construct and studied the influence of microstructural heterogeneity on local cell deformations. Results show that heterogeneity has a profound impact on local cell deformations, which highly exceed macroscopic deformations. Moreover, microstructural heterogeneity and the presence of neighboring cells leads to complex cell shapes and causes non-uniform deformations within a cell.

Cell Size↗
PubMed2002

Neighbourhood socioeconomic context and self reported health and smoking: a secondary analysis of data on seven cities.

OBJECTIVE: Many studies have shown that poor health status and harmful health behaviour occur more frequently in deprived neighbourhoods. Most studies show modest associations between area level socioeconomic factors, the neighbourhood context, and health outcomes. However, estimates for the contextual effects vary. It is unclear if this variation is attributable to differences in methodology. This study examines whether contextual neighbourhood differences in health outcomes really vary between cities or that differences in methodology may account for these differences. DESIGN: Secondary analysis of data from health interview surveys in seven large Dutch cities in the 1990s comprising 23 269 residents of 484 neighbourhoods, using multilevel logistic models. SETTING: General population aged 16 and over. MAIN OUTCOME MEASURES: Self reported health, smoking of cigarettes. MAIN RESULTS: The socioeconomic context of neighbourhoods is associated with health outcomes in all large Dutch cities. The strength of the association varies between cities, but variation is much smaller in the age group 25-64. Furthermore, neighbourhood differences vary in size between native and other residents. Contextual neighbourhood differences are about two times larger for self reported health than for the smoking of cigarettes, but for native Dutch people they are of similar size. CONCLUSIONS: A comparatively large improvement in health may be gained in deprived neighbourhoods, because of the poorer health status to which the context of these neighbourhoods also contributes. Health promoting interventions should be aimed at the residents and at the context of deprived neighbourhoods, taking differences between ethnic groups and age groups into account.

Adolescent↗
PubMed1994

Application of random-effects probit regression models.

A random-effects probit model is developed for the case in which the outcome of interest is a series of correlated binary responses. These responses can be obtained as the product of a longitudinal response process where an individual is repeatedly classified on a binary outcome variable (e.g., sick or well on occasion t), or in "multilevel" or "clustered" problems in which individuals within groups (e.g., firms, classes, families, or clinics) are considered to share characteristics that produce similar responses. Both examples produce potentially correlated binary responses and modeling these person- or cluster-specific effects is required. The general model permits analysis at both the level of the individual and cluster and at the level at which experimental manipulations are applied (e.g., treatment group). The model provides maximum likelihood estimates for time-varying and time-invariant covariates in the longitudinal case and covariates which vary at the level of the individual and at the cluster level for multilevel problems. A similar number of individuals within clusters or number of measurement occasions within individuals is not required. Empirical Bayesian estimates of person-specific trends or cluster-specific effects are provided. Models are illustrated with data from mental health research.

Cluster Analysis↗
PubMed1990

Proximal percutaneous balloon angioplasty and distal bypass for multilevel arterial occlusion. Veterans Administration Cooperative Study No. 199.

Multiple sites of atherosclerotic occlusion in high risk patients may be treated by angioplasty of the iliac obstruction and distal reconstruction. We report 18 male patients with symptomatic peripheral vascular disease in whom proximal iliac percutaneous transluminal angioplasty was combined with femoropopliteal bypass (11), femorotibial bypass (2), or femorofemoral bypass (5). There were no operative deaths. The pretreatment ankle brachial index of 0.40 +/- 0.04 was increased to 0.64 +/- 0.04 by discharge (p = 0.0001), and remained significantly increased through 27 months (0.65 +/- 0.07) (p = 0.0001). During the follow-up period of 2-57 (mean 27 months) one dilated iliac artery required repeated percutaneous transluminal angioplasty and revision of the femoropopliteal bypass at three months. Two late amputations of study limbs occurred at two years and three years due to progression of distal disease in the infrapopliteal segment. Four patients died during the follow-up period of ischemic heart disease (3) and lung carcinoma (1). Life table analysis shows a 76% success rate for the combined procedures at two years. In selected, high risk patients, proximal iliac dilatation and distal bypass is an acceptable alternative reconstruction for multilevel occlusion.

Aged↗
PubMed2006

Mapping and measuring social disparities in premature mortality: the impact of census tract poverty within and across Boston neighborhoods, 1999-2001.

The identification and documentation of health disparities are important functions of public health surveillance. These disparities, typically falling along lines defined by gender, race/ethnicity, and social class, are often made visible in urban settings as geographic disparities in health between neighborhoods. Recognizing that premature mortality is a powerful indicator of disparities in both health status and access to health care that can readily be monitored using routinely available public health surveillance data, we undertook a systematic analysis of spatial variation in premature mortality in Boston (1999-2001) across neighborhoods and sub-neighborhoods in relation to census tract (CT) poverty. Using a multilevel model based framework, we estimated that the incidence of premature mortality was 1.39 times higher (95% credible interval 1.09-1.78) among persons living in the most economically deprived CTs (>/=20% below poverty) compared to those in the least impoverished tracts (<5% below poverty). We present maps of model-based standardized mortality ratios that show substantial within-neighborhood variation in premature mortality and a sizeable decrease in spatial variation after adjustment for CT poverty. Additionally, we present maps of model-based direct standardized rates that can more readily be compared to externally published rates and targets, as well as maps of the population attributable fraction that show that in some of Boston's poorest neighborhoods, the proportion of excess deaths associated with CT poverty reaches 25-30%. We recommend that these methods be incorporated into routine analyses of public health surveillance data to highlight continuing social disparities in premature mortality.

Adolescent↗
PubMed2003

Neighborhood differences in social capital: a compositional artifact or a contextual construct?

Assessment of social capital at the neighborhood level is often based on aggregating individual perceptions of trust and reciprocity. Individual perceptions, meanwhile, are influenced through a range of individual attributes. This paper examines the socioeconomic and demographic attributes that systematically correlate with individual perception of social capital and determines the extent to which such attributes account for neighborhood differences in social capital. Using improved multilevel modeling procedures, we ascertain the extent to which differences in social capital perception can be ascribed to true neighborhood-level variations. The analysis is based on the 1994-95 Community Survey of the Project on Human Development in Chicago Neighborhoods (PHDCN). The response measure is based on survey respondent's perceptions of whether people in their neighborhood can be trusted. The results suggest that even after accounting for individual demographic (age, sex, race, marital status) and socioeconomic characteristics (income, education), significant neighborhood differences remain in individual perceptions of trust, substantiating the notion of social capital as a true contextual construct.

Adult↗
PubMed2004

Biomechanical analysis of anterior poly-methyl-methacrylate reconstruction following total spondylectomy for metastatic disease.

STUDY DESIGN: Three reconstruction options were evaluated biomechanically following total spondylectomy using human cadaveric spine specimens. OBJECTIVES.: To evaluate and compare the stability of combined anterior and posterior fixation incorporating poly-methyl-methacrylate with alternative accepted reconstruction techniques. SUMMARY OF BACKGROUND DATA: Total spondylectomy represents the most radical option for decompression in metastatic spinal cord compression. Poly-methyl-methacrylate is considered a useful adjunct in spinal column stabilization and arthrodesis; however, there is little published biomechanical data to support its use in this setting. METHODS: Ten fresh-frozen human cadaveric spines (T9-L3) were used. After intact analysis, a total spondylectomy was performed at T12. Three potential reconstruction techniques were tested for their ability to restore stiffness to the specimen: 1) multilevel posterior pedicle screw instrumentation from T10-L2; 2) anterior instrumentation (ATL Z plate II) and rib graft at T11-L1 with multilevel posterior pedicle screw instrumentation from T10-L2; and 3) anterior cement (Simplex P) and pins construct (T12) with multilevel posterior pedicle screw instrumentation from T10-L2. Each of the three potential reconstruction techniques was tested on each specimen in random order using nondestructive testing under load control. RESULTS: Only combined stabilization techniques (e.g., anterior instrumentation and rib graft with multilevel posterior pedicle screw instrumentation and anterior cement-and-pins construct with multilevel posterior pedicle screw instrumentation) restored stiffness to a level equivalent to or higher than that of the intact spine in all loading modes (P < 0.05). Anterior cement-and-pins construct with multilevel posterior pedicle screw instrumentation provided more stability to the specimen than anterior instrumentation and rib graft with multilevel posterior pedicle screw instrumentation in compression and flexion testing (P < 0.05). Posterior instrumentation alone did not restore stiffness to the intact level in compression and flexion testing (P < 0.005). CONCLUSIONS: Combined anterior and posterior reconstruction using a cement construct provides equal to or more stability than the intact spine in all testing modes. Posterior stabilization alone is an inferior method of reconstruction following total spondylectomy. Poly-methyl-methacrylate has the advantage over traditional anterior reconstruction techniques in that it can be inserted using a posterior approach.

Aged↗
PubMed1997

The knowledge used in vision and where it comes from.

Knowledge is often thought to be something brought from outside to act upon the visual messages received from the eye in a 'top-down' fashion, but this is a misleadingly narrow view. First, the visual system is a multilevel heterarchy with connections acting in all directions so it has no 'top'; and second, knowledge is provided through innately determined structure and by analysis of the redundancy in sensory messages themselves, as well as from outside. This paper gives evidence about mechanisms analysing sensory redundancy in biological vision. Automatic gain controls for luminance and contrast depend upon feedback from the input, and there are strong indications that the autocorrelation function, and other associations between input variables, affect the contrast sensitivity function and our subjective experience of the world. The associative structure of sensory message can provide much knowledge about the world we live in, and neural mechanisms that discount established associative structure in the input messages by recoding them can improve survival by making new structure more easily detectable. These mechanisms may be responsible for illusions, such as those produced by a concave face-mask, that are classically attributed to top-down influences.

Cognition↗
PubMed2003

Depression in adult women: age changes and cohort effects.

OBJECTIVES: We sought to separate age and cohort associations with depression, assessed 3 times within a 10-year period in 701 women born between 1928 and 1958. METHODS: We used regression analysis to examine age differences in women with depression in 2 birth cohorts, pre-1945 and post-1944, who were assessed at comparable ages. Multilevel modeling was used to estimate changes with age in successive birth year cohorts. RESULTS: An age by cohort interaction indicated more depression among younger than older women in the post-1944 cohort but a flat age profile in the pre-1945 cohort. Longitudinal analyses indicated declines in depression with age in more recent cohorts but increases in earlier ones. CONCLUSIONS: Increases in depression in younger women in successive cohorts may be offset by decreases in middle age.

Adult↗
PubMed2005

Pressure tuning of the Fermi resonance in liquid methanol: implications for the analysis of high-pressure vibrational spectroscopy experiments.

It has been argued that pressure tuning allows for unambiguous assignment of the nonperturbed bands involved in the Fermi coupling of molecular systems in the condensed phase. Here we study the pressure evolution of the Fermi resonance occurring in liquid methanol between the symmetric methyl-stretch fundamental and the methyl-bending overtones. Our analysis is based on Raman experiments in both stretching and bending fundamental regions, which are used to evaluate the effect of pressure on accidental degeneracies occurring in the vibrational spectra of liquid methanol. We emphasize that the difference in frequency of the Fermi doublet constitutes the governing quantity to determine the condition at which the exact degeneracy of the unperturbed modes occurs. Analysis based on the intensity ratio of the Fermi doublet must be disregarded. We confirm the necessity of measuring the full vibrational spectrum under pressure in order to obtain the Fermi coupling parameters unambiguously and to give a correct assignment of the bands involved in the resonance phenomenon. We also analyze the possible occurrence of several simultaneous resonance effects using a multilevel perturbation model. This model provides an appropriate description of the frequencies observed in the experiments over the whole pressure range if we consider that the main resonance occurs between nu3 and 2nu10, in contrast to previous assignments. Our global analysis leads to some general rules concerning measurement and interpretation of high-pressure vibrational spectroscopy experiments.

Methanol↗
PubMedDate not supplied

Multilevel wavelet-transform encoding in MRI.

This paper explores an alternative to standard Fourier MRI called wavelet-transform encoding. Spatially selective radiofrequency (RF) pulses are used to excite slice profiles in the shape of the wavelet-basis functions of a discrete wavelet transform. Our implementation on a standard commercial whole body MRI system resolves one spatial dimension through wavelet encoding and orthogonal directions through conventional frequency encoding and rectangular-slice selection. The wavelet transform is described as a method for multiresolution analysis, and we show how a high resolution MR image can be constructed from lower resolution images, representing the approximation and detail structures of the object. A multiple level architecture of the wavelet reconstruction is described, thereby allowing wavelet-encoded images to be reconstructed through several modes. Multilevel wavelet-encoded images of a gel phantom are presented that show comparable image quality to Fourier-encoded images of similar signal-to-noise ratio. However, a chemical-shift artifact is identified with this particular implementation.

Artifacts↗
PubMed2004

Multilevel cycle of anthropogenic copper.

A comprehensive contemporary cycle for stocks and flows of copper is characterized and presented, incorporating information on extraction, processing, fabrication and manufacturing, use, discard, recycling, final disposal, and dissipation. The analysis is performed on an annual basis, ca. 1994, at three discrete governmental unit levels--56 countries or country groups that together comprise essentially all global anthropogenic copper stocks and flows, nine world regions, and the planet as a whole. Cycles for all of these are presented and discussed, and a "best estimate" global copper cycle is constructed to resolve aggregation discrepancies. Among the most interesting results are (1) transformation rates and recycling rates in apparently similar national economies differ by factors of two or more (country level); (2) the discard flows that have the greatest potential for copper recycling are those with low magnitude flows but high copper concentrations--electronics, electrical equipment, and vehicles (regional level); (3) worldwide, about 53% of the copper that was discarded in various forms was recovered and reused or recycled (global level); (4) the highest rate of transfer of discarded copper to repositories is into landfills, but the annual amount of copper deposited in mine tailings is nearly as high (global level); and (5) nearly 30% of copper mining occurred merely to replace copper that was discarded. The results provide a framework for similar studies of other anthropogenic resource cycles as well as a basis for supplementary studies in resource stocks, industrial resource utilization, waste management, industrial economics, and environmental impacts.

Conservation of Natural Resources↗
PubMed2004

Subjective reactions to alcohol cue exposure: a qualitative analysis of patients' self-reports.

Ninety-one individuals receiving outpatient alcohol treatment received 1 session of behavioral cue exposure to their usual alcoholic beverage. Verbatim transcripts of the session were analyzed with a multilevel process that focused on the classification of participants' verbal reactions to alcohol cues. Participant responses were first classified into 1 of 2 categories: (a) approach toward alcohol or (b) avoidance of alcohol. Next, 7 approach categories (e.g., smells good) and 6 avoidance categories (e.g., not tempted/disinterested) were identified. Interrater agreement for 12 of the 13 categories was above .80. Post hoc analyses of several craving variables provided support for the qualitative data analyses. Overall, the results support M. J. Breiner, W. G. K. Stritzke, and A. R. Lang's (1999) motivational model of alcohol craving that incorporates both inclinations to approach alcohol as well as avoid alcohol.

Adult↗
PubMed1994

Iliac angioplasty as a prelude to distal arterial bypass.

BACKGROUND: Patients with limb ischemia and diffuse aortoiliac or combined aortoiliac and femoropopliteal disease present as a difficult problem for surgeons. This study was done to determine whether or not angioplasty can be successfully used as a prelude to distal bypass in patients with multilevel arterial occlusive disease. STUDY DESIGN: This is a retrospective review of an entire hospital experience with iliac angioplasty used as a prelude to distal arterial bypass. Kaplan-Meier life table analysis was performed and comparisons were made using log rank method. RESULTS: During a six year period, 37 patients underwent percutaneous iliac angioplasty before distal arterial reconstruction. Subsequent arterial reconstructions included femorofemoral bypass in nine patients, femoropopliteal bypass in 25 patients, femorotibial bypass in two patients, and common femoral endarterectomy in the remaining patient. The primary graft patency rate was 81 percent at five years. The overall success of iliac angioplasty was 76 percent at five years. Limb salvage was achieved in 78 percent of the patients with threatened extremities, and there was no perioperative mortality. CONCLUSIONS: Iliac angioplasty can successfully be used as a prelude to distal arterial bypass in patients with multilevel disease. The rate of angioplasty failure is low and grafts often remain patent in the face of early hemodynamic inflow failure, allowing salvage by operation or repeat dilatation. Close follow-up evaluation will allow early detection and appropriate correction of changes in inflow hemodynamics that occur before graft closure.

Aged↗