Analysis of a potential trigger of an acute illness

Sometimes certain short-term risk exposures are postulated to act as a trigger for the onset of a specific acute illness. When the incidence of the illness is low it is desirable to investigate this possible association using only data on cases detected during a specific observation period. Here we propose an analysis for such a study based on a model expressed in terms of the probability that the exposure triggers the illness and a random delay from a triggered illness until its diagnosis. Both the natural hazard rate for the illness and the probability that the exposure triggers the illness are assumed to be small and possibly dependent on age and covariates such as sex and duration or severity of the exposure. The method of analysis is illustrated with a study of the association between long flights and hospitalization for venous thromboembolism.     

Spatiotemporal organization of simple-cell receptive fields in area 18 of cat''''s cortex

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Growth direction of nodal roots in rice: its variation and contribution to root system formation

The root system of a rice plant (Oryza sativa L.) consists of numerous nodal roots and their laterals. The growth direction of these nodal roots affects the spatial distribution of the root system in soil, which seems to relate to yield and lodging resistance. The growth angle of a nodal root varies with the type and timing of emergence of the nodal root. The body of a rice plant can be recognized as an integrated set of shoot units, each unit consisting of an internode with a leaf and several roots. Nodal roots formed at the apical part of a shoot unit often elongate horizontally, whereas those formed at the basal part of the shoot unit show various growth directions depending on both the growth stages of the plant and the environmental conditions. Moreover, nodal roots that emerge from the most basal shoot unit of a tiller are usually thick and grow downwards. External factors such as planting density and nitrogen application affect the growth direction of nodal roots, probably partly because of the changing tillering pattern of the shoot. In addition to the growth angle of nodal roots, size of nodal roots may be another important factor determining the spatial distribution of the root system in soil.     

Comparing measures of travel distances in primates: Methodological considerations and socioecological implications

Travel costs can influence numerous aspects of the lives of primates, including net energy balance (and therefore reproductive success of females) and maximum group size. Despite their potential impact, there has been no systematic comparison of different measures of travel distance. We compared three measures of travel distance in 30 min (actual distance of individuals, straight‐line distance of individuals, and straight‐line distance of groups) and their ratios in a small group and a large group of vervet monkeys (Cercopithecus aethiops) and between the large group of vervets and a group of patas monkeys (Erythrocebus patas) of roughly similar size. The large group of vervets traveled farther than the small group regardless of the measure used, but the ratios of the different measures were not significantly different between those groups. Patas monkeys traveled significantly farther than the large group of vervets regardless of the measure used. In both vervets and patas, straight‐line distances of individuals (ISLD) and groups (GSLD) underestimated actual distances traveled by individuals (IAD), but the degree to which they did so differed between species. IAD is more accurate than the other two measures and is preferred for studies of energetics and individual reproductive success, although ISLD or GSLD may be substituted when the ratios of IAD/ISLD or IAD/GSLD do not differ between groups or species. The ratio of IAD/ISLD was larger in vervets than in patas, suggesting that individual vervets meander more over short periods of time than patas. The ratio of ISLD/GSLD was larger in patas than in vervets, suggesting that patas move at angles or across the group's center‐of‐mass whereas vervets move more consistently along with others in their group. This has implications for the formation of spatial subgroups and alliances within groups. Am. J. Primatol. 48:87–98, 1999. © 1999 Wiley‐Liss, Inc.     

云南拉沙山黑白仰鼻猴群体规模对冬春季日移动距离和活动时间分配的影响

生态限制模型(Ecological constraints model)认为随种群规模增加,灵长类种群会增加日移动距离、移动时间和取食时间,减少休息时间.果食性灵长类为取食斑块分布的高质量食物资源(如果实)而存在群内分摊竞争(Within-group scramble competition),很好地验证了生态限制模型...     

Direction Selectivity and Spatiotemporal Separability in Simple Cortical Cells

Simple cells in mammalian visual cortex are quasi-linear mechanisms whose behavior departs from true linearity in a very consistent manner. Empirical research on direction selectivity (DS) clearly illustrates these characteristics. A linear DS cell will be DS for all stimuli, whereas a linear non-DS cell will not be DS for any stimuli. However, many simple cells have opposite preferred directions for stimuli of reversed polarity, and some cells are DS for some stimuli (e.g., moving bars) but not for others (e.g., drifting gratings). Also, linear non-DS cells must have separable spatiotemporal receptive fields (RFs), and linear DS cells must have inseparable RFs. Yet many actual DS cells have separable RFs. Here we present a nonlinear model of simple-cell behavior that reproduces all of these empirical behaviors. The model is a variant of the current linear model, amended to include an interleaved nonlinearity (half-wave rectification) that allows it to mimic the (im)balance of push-pull mechanisms. We present simulation results showing that balanced push-pull mechanisms result in linear behavior, while imbalanced push-pull arrangements produce all of the incongruent DS-related behaviors that have been reported for simple cells.     

A Continuous Attractor Network Model Without Recurrent Excitation: Maintenance and Integration in the Head Direction Cell System

Motivated by experimental observations of the head direction system, we study a three population network model that operates as a continuous attractor network. This network is able to store in a short-term memory an angular variable (the head direction) as a spatial profile of activity across neurons in the absence of selective external inputs, and to accurately update this variable on the basis of angular velocity inputs. The network is composed of one excitatory population and two inhibitory populations, with inter-connections between populations but no connections within the neurons of a same population. In particular, there are no excitatory-to-excitatory connections. Angular velocity signals are represented as inputs in one inhibitory population (clockwise turns) or the other (counterclockwise turns). The system is studied using a combination of analytical and numerical methods. Analysis of a simplified model composed of threshold-linear neurons gives the conditions on the connectivity for (i) the emergence of the spatially selective profile, (ii) reliable integration of angular velocity inputs, and (iii) the range of angular velocities that can be accurately integrated by the model. Numerical simulations allow us to study the proposed scenario in a large network of spiking neurons and compare their dynamics with that of head direction cells recorded in the rat limbic system. In particular, we show that the directional representation encoded by the attractor network can be rapidly updated by external cues, consistent with the very short update latencies observed experimentally by Zugaro et al. (2003) in thalamic head direction cells.     

Optimization-based region-of-interest reconstruction for X-ray computed tomography based on total variation and data derivative

Region-of-interest (ROI) and interior reconstructions for computed tomography (CT) have drawn much attention and can be of practical value for potential applications in reducing radiation dose and hardware cost. The conventional wisdom is that the exact reconstruction of an interior ROI is very difficult to be obtained by only using data associated with lines through the ROI. In this study, we propose and investigate optimization-based methods for ROI and interior reconstructions based on total variation (TV) and data derivative. Objective functions are built by the image TV term plus the data finite difference term. Different data terms in the forms of L1-norm, L2-norm, and Kullback–Leibler divergence are incorporated and investigated in the optimizations. Efficient algorithms are developed using the proximal alternating direction method of multipliers (ADMM) for each program. All sub-problems of ADMM are solved by using closed-form solutions with high efficiency. The customized optimizations and algorithms based on the TV and derivative-based data terms can serve as a powerful tool for interior reconstructions. Simulations and real-data experiments indicate that the proposed methods can be of practical value for CT imaging applications.     

Traveling or stopping of migrating birds in relation to wind: an illustration for the osprey

Although it is often assumed that birds strongly prefer tailwindsfor their migratory flights, we predict that a strategy of nowind selectivity (traveling independently of winds) may be morefavorable than wind selectivity (traveling on tailwind occasionsbut stopping to rest under headwind occasions) for birds withlow energy costs of travel relative to rest and for birds thatcannot use stopover time for efficient fuel deposition. We testthis prediction by analyzing the daily traveling or stoppingas recorded by satellite tracking of five ospreys Pandion haliaetus,a species often using energy-saving thermal soaring, duringtheir migration between northern Europe and Africa. Besideswind, precipitation is another weather factor included in theanalyses because thermal soaring migrants are expected to stopand rest in rainy weather. In logistic regression analyses,taking into account the effects of latitude, behavior on previousday, season, date, and individual for discriminating betweentraveling and stopping days, we found a lack of influence ofwinds, suggesting that the ospreys travel or stop without regardto wind. This lack of wind selectivity under light and moderatewinds is in agreement with our prediction. We expect a low degreeof wind selectivity and thus regular flights under headwindsalso among other types of birds that cannot use stopping timefor efficient foraging and fuel deposition. We also found anunexpected lack of influence of precipitation, possibly becauseof relatively few instances with rainfall in combination withpoor geographic precision for estimates of this weather variable.