Pavlov (1927) first reported that following appetitive conditioning of multiple stimuli, extinction of one CS attenuated responding to others which had not undergone direct extinction. Four experiments with rat subjects investigated potential mechanisms of this secondary extinction effect. Experiment 1 assessed whether secondary extinction would be more likely to occur with target CSs that have themselves undergone some prior extinction. Two CSs were initially paired with shock. One CS was subsequently extinguished before the second CS was tested. The target CS was partially extinguished for half the rats and not extinguished CS for the other half. A robust secondary extinction effect was obtained with the non-extinguished target CS. Experiment 2 investigated whether secondary extinction occurs if the target CS is tested outside the context where the first CS is extinguished. Despite the context switch secondary extinction was observed. Extinction of one CS was also found to thwart renewal of suppression to a second CS when it was tested in a neutral context. Experiment 3 examined whether secondary extinction can be attributed to mediated generalization caused by association of the CSs with a common US during conditioning. Rats received conditioning with three CSs and then extinction with one of them. Secondary extinction was observed with a shock-associated CS when the extinguished CS had been associated with either food pellets or shock, suggesting that secondary extinction is not US-specific and is thus not explained by this mediated generalization mechanism. Experiment 4 examined whether intermixing trials with the two stimuli during conditioning is necessary for secondary extinction to occur. Rats were either conditioned with intermixed trials as in Experiments 1-3, or with blocked trials of each CS presented in conditioning sessions separated by a day. Secondary extinction was observed only in the former condition. The results are consistent with the hypothesis that CSs must be associated with a common temporal context for secondary extinction to occur.

Using the "revealed operant" technique, variability during acquisition and extinction was examined with measures of response rate and a detailed analysis of response topography. During acquisition, subjects learned to emit four response patterns. A continuous schedule of reinforcement (CRF) for 100 repetitions was used for each pattern and a 30 min extinction phase immediately followed. One group of subjects learned the response patterns via a "trial-and-error" method. This resulted in a wide range of variability during acquisition and extinction. Only one subject emitted a substantial amount of resurgent behavior. A second group of subjects was given instructions on what keys to press to earn reinforcers. This group had less variability in acquisition and extinction and resurgent responding was prevalent.

This study examined effects of two response acquisition procedures on topography of responding using the revealed operant technique and compared results to previous experiments on this topic. Subjects emitted 100 repetitions each of 4 response patterns on a continuous schedule of reinforcement. A 30-min extinction condition followed acquisition. One group of subjects learned the first response through a series of shaping steps designed to reduce acquisition variability. Another group of subjects was instructed in the correct response topography and was told there was no penalty for attempting other sequences. The first group of subjects produced high variability during extinction despite reduced variability in acquisition. The second group of subjects responded with moderate to high variability during extinction and little variability during acquisition. Most extinction responses for the first group were variations of the last pattern reinforced. Most extinction responses for the second group were repetitions of the last pattern reinforced.

High-mass stars form within star clusters from dense, molecular regions (DMRs), but is the process of cluster formation slow and hydrostatic or quick and dynamic? We link the physical properties of high-mass star-forming regions with their evolutionary stage in a systematic way, using Herschel and Spitzer data. In order to produce a robust estimate of the relative lifetimes of these regions, we compare the fraction of DMRs above a column density associated with high-mass star formation, N(H-2) > 0.4-2.5 x 10(22) cm(-2), in the "starless" (no signature of stars >= 10 M circle dot forming) and star-forming phases in a 2 degrees x 2(degrees) region of the Galactic Plane centered at l = 30 degrees. Of regions capable of forming high-mass stars on similar to 1 pc scales, the starless (or embedded beyond detection) phase occupies about 60%-70% of the DMR lifetime, and the star-forming phase occupies about 30%-40%. These relative lifetimes are robust over a wide range of thresholds. We outline a method by which relative lifetimes can be anchored to absolute lifetimes from large-scale surveys of methanol masers and UCHII regions. A simplistic application of this method estimates the absolute lifetime of the starless phase to be 0.2-1.7 Myr (about 0.6-4.1 fiducial cloud free-fall times) and the star-forming phase to be 0.1-0.7 Myr (about 0.4-2.4 free-fall times), but these are highly uncertain. This work uniquely investigates the star-forming nature of high column density gas pixel by pixel, and our results demonstrate that the majority of high column density gas is in a starless or embedded phase.

We present a comprehensive study of interstellar X-ray extinction using the extensive Chandra supernova remnant (SNR) archive and use our results to refine the empirical relation between the hydrogen column density and optical extinction. In our analysis, we make use of the large, uniform data sample to assess various systematic uncertainties in the measurement of the interstellar X-ray absorption. Specifically, we address systematic uncertainties that originate from (i) the emission models used to fit SNR spectra; (ii) the spatial variations within individual remnants; (iii) the physical conditions of the remnant such as composition, temperature, and non-equilibrium regions; and (iv) the model used for the absorption of X-rays in the interstellar medium. Using a Bayesian framework to quantify these systematic uncertainties, and combining the resulting hydrogen column density measurements with the measurements of optical extinction toward the same remnants, we find the empirical relation N-H = (2.87 +/- 0.12) x 10(21) A(V) cm(-2), which is significantly higher than the previous measurements.

Ce travail est consacré à l'étude des interactions entre combustible, flamme et gouttes d'eau générées par des systèmes de type brouillard d'eau. Il est complété par le développement d'un modèle d'extinction dans le logiciel FDS (Fire Dynamics Simulator), code CFD largement utilisé dans la communauté scientifique et dans l'ingénierie. L'acquisition de connaissances sur les phénomènes physiques induits par l'application d'un spray sur un feu résulte de l'analyse d'une campagne expérimentale à échelle réelle. 84 essais de feux confinés et ventilés ont ainsi été réalisés faisant intervenir un brouillard d'eau et une instrumentation qui permet d'étudier l'influence sur l'extinction: du combustible, du temps de combustion avant déclenchement du brouillard d'eau, du diamètre du bac pour les feux de liquide inflammable, du nombre de buses et du placement du foyer. L'ajout d'un post-traitement au logiciel FDS sous la forme de bilans de masse et d'énergie dans le code permet de compléter les observations expérimentales et de quantifier l'importance des différents transferts thermiques avant et pendant l'aspersion d'eau. Deux modèles complémentaires permettent de déterminer l'extinction par refroidissement du combustible et par refroidissement de la flamme / inertage. Ils sont étudiés pour des cas appropriés sur les versions 5 et 6 de FDS. La capacité du code complet modifié à prédire l'extinction est évaluée / This work is devoted to the study of interactions between fuel, flame and water droplets generated by water mist systems. It is completed by an extinction model development in FDS (Fire Dynamics Simulator) software, which is a CFD code widely used by the scientific and engineering communities. Knowledge on physical phenomena induced by water application on fire is acquired thanks to an experimental campaign at real scale. 84 confined and ventilated fire tests have been carried out involving water mist and metrology allowing to study the influence on extinction of: fuel, combustion time before water mist application, pool diameter for liquid fuel fires, number of nozzles and fuel location. A post-processing in the form of mass and energy balances has been added to FDS, allowing to complete experimental observations and to quantify the different heat transfer modes before and during water application. Two complementary models allow extinction determination by fuel cooling and flame cooling / inerting effects. Their capability is analyzed for appropriate cases on the versions 5 and 6 of FDS. The capability of the complete modified code to predict extinction is finally evaluated

The term "resurgence" generally refers to the reappearance of certain behaviors during extinction. Different definitions describe these behaviors as previously reinforced, previously extinguished, or simply previously learned. At first glance, these definitions seem the same. And, researchers have not given much thought to the differences between them. However, these definitions could refer to different initial teaching procedures, and these differences may produce different results during extinction. The present study used the Portable Operant Research and Teaching Lab (PORTL) to examine how differences in the initial teaching procedure affected the behavior of college students during extinction. In the first condition, participants learned four behaviors. Each behavior was extinguished before the next behavior was taught. When all four behaviors were put on extinction, they resurged in the reverse order from how they were taught. A second condition followed the same procedure as the first with one difference. Each behavior was not extinguished before the next behavior was taught. When these four behaviors were put on extinction, they resurged in the order they were learned. These results indicate that the initial training procedure can influence the order in which behaviors appear during extinction.

A teoria da descendência com modificação de Darwin, que explica a origem de espécies através da seleção natural, é considerada um marco na história da ciência. A possibilidade de unificação de toda a biologia e a mudança que ela trouxe para nossos valores e para a nossa compreensão da posição da humanidade no universo ainda causam um grande impacto na sociedade e na relação entre ciência e filosofia. O objetivo do presente estudo é compreender alguns aspectos dos desenvolvimentos da ciência que antecederam essa teoria. Dois elementos foram estabelecidos como referências para essa análise: a teoria de Jean-Baptiste Lamarck, a mais importante a propor o conceito da transformação das espécies antes de Darwin e o problema científico da explicação da extinção. As várias teorias elaboradas para dar conta da diversidade de espécies na Terra, bem como para explicar o fenômeno da extinção são discutidas através das obras de Georges Cuvier, Geoffroy Saint-Hilaire e Richard Owen. Nessas teorias, as questões da adaptação e das noções teleológicas são destacadas devido à sua relação com o problema da extinção. A abordagem de Darwin para o problema da extinção é discutida em sua relação com o conceito de seleção natural e com o conceito de adaptação defendido pela teologia natural britânica. / Darwin\'s theory of descent with modification, which explains the origin of species by natural selection, is considered a milestone in the history of science. The possibility of unification of the entire field of biology and the changes that it brought to our values and to our understanding of the position of mankind in the universe are still causing great impact in society and in the relationship between science and philosophy. The aim of this study is to understand some aspects of the developments of science that preceded this theory. Two elements were established as references for this analysis: the theory of Jean-Baptiste Lamarck, the most important work proposing the concept of transformation of species before Darwin, and the scientific problem of the explanation of extinction. The several theories elaborated to account for the diversity of species on Earth as well as to explain the phenomenon of extinction are discussed through the works of Georges Cuvier, Geoffroy Saint-Hilaire and Richard Owen. Within these theories, the issues of adaptation and teleological notions are stressed because of their connection to the problem of extinction. Darwin\'s approach to the problem of extinction is discussed for its relation to the concept of natural selection and to the concept of adaptation defended by the british natural theology.

A extinção de grandes mamíferos terrestres no final do Pleistoceno (entre 50 e 11 mil anos atrás) é um dos temas mais debatidos em ecologia. A maioria dos estudos sobre as causas das extinções do Pleistoceno tem como foco o papel de fatores externos como mudanças climáticas e a chegada do homem. Entretanto, a forma como uma comunidade ecológica responde a perturbações depende de suas propriedades, como o número e composição de espécies e a forma como essas espécies interagem. O objetivo final dos estudos reunidos nessa tese foi entender como estavam organizadas as interações ecológicas entre os mamíferos do Pleistoceno e o possível papel dessas interações no episódio de extinção da megafauna. Em primeiro lugar adaptei modelos de teias tróficas para reproduzir redes formadas por diferentes tipos de interações entre consumidores e recursos. Em seguida, utilizei esses modelos para reconstruir redes de interação entre predadores e presas da megafauna do Pleistoceno e examinei as propriedades estruturais e dinâmicas dessas redes. Por fim, investiguei uma das possíveis consequências da extinção da megafauna: a perda de serviços de dispersão de sementes. Os resultados aqui apresentados mostram que (i) diferentes tipos de redes de interação entre consumidores e recursos compartilham características estruturais e podem ser reproduzidas por modelos de teias tróficas; (ii) redes de interação entre grandes mamíferos do Pleistoceno estavam, provavelmente, estruturadas de forma similar aos sistemas atuais na África. Entretanto, as comunidades do Pleistoceno seriam especialmente vulneráveis às mudanças estruturais e na dinâmica causadas pela chegada de um predador como o homem; (iii) entre as consequências da extinção do Pleistoceno está a reorganização de outros tipos de rede de interação como as redes de dispersão de sementes. Em conjunto os resultados apresentados aqui enfatizam a importância de considerarmos o possível papel das interações ecológicas em modular os efeitos de perturbações ao estudarmos eventos de extinção / The extinction of large terrestrial mammals during the late Pleistocene (between 50 and 11 kyrs ago) is one of the most debated topics in ecology. Most studies on the causes of Pleistocene extinctions focus on the role of external factors such as climate changes and the arrival of humans. Nevertheless, the way an ecological community responds to perturbations depends on its properties, such as its number of species, species composition and the way these species interact. This thesis encloses studies with the final objective of understanding how ecological interactions between Pleistocene large mammals were organized and the potential role of such interactions in the Pleistocene extinction episode. First, I adapted food-web models to reproduce networks depicting different types of ecological interactions between consumers and resources. Then, I used these models to reconstruct predator-prey interaction networks between Pleistocene large mammals and examined the structural and dynamic properties of these systems. Finally, as an overview of the ecological impacts of Pleistocene extinctions, I discuss one of the possible consequences of the demise of Pleistocene large mammals: the loss of seed-dispersal services. The results presented here show that (i) different types of interaction networks between consumers and resources share structural properties and can be reproduced by food-web models; (ii) interactions between Pleistocene large mammals were most likely structured in a similar way to modern large-mammals assemblages in Africa, but the former were especially vulnerable to the changes in structure and dynamics caused by a newly arriving predator such as humans; (iii) among the consequences of Pleistocene extinctions is the reconfiguration of other types of interaction networks such as seed-dispersal networks. Taken together these findings emphasize how important it is to consider the role of ecological interactions in modulating the effects of perturbations when studying extinctions events

Recent improvements in equipment used by firefighters has increased the value of manual suppression in buildings. However, because there is no evaluation method available, the effectiveness of manual suppression can not be incorporated into a fire safety analysis of a building. This thesis develops a method for evaluating manual suppression in buildings. he evaluation is done through an analysis of the paths through a building firefighters will use to attack a fire. The analysis considers the building, fire and fire department factors influencing progress towards teh fire. The fire attack path analysis yeilds a value relating the relative difficulty of a path.