Na ciência, trabalhamos com hipóteses, que serão de 0 a 100% corretas, com todas as tonalidades de cinza no meio desses números. O melhor texto que conheço sobre o assunto - e uma bela lição de vida - é o famoso "The median isn't the message", que conta como em 1982 o paleontólogo e divulgador da ciência Stephen Jay Gould foi diagnosticado com um mesotelioma abdominal (um câncer raro) e, ao invés de engolir a sentença de morte de 8 meses que a literatura médica lhe evidenciava, resolveu entender a estatística por trás do estudo. Nas palavras dele:
"When I learned about the eight-month median, my first intellectual reaction was: fine, half the people will live longer; now what are my chances of being in that half. I read for a furious and nervous hour and concluded, with relief: damned good. I possessed every one of the characteristics conferring a probability of longer life: I was young; my disease had been recognized in a relatively early stage; I would receive the nation's best medical treatment; I had the world to live for; I knew how to read the data properly and not despair."Gould viveu 20 anos mais depois do seu diagnóstico - ou seja, superou o valor da mediana em quase 30 vezes.
Veja bem, o fato dele ter vivido 30 vezes a mais que a literatura médica previa não significava que a tal literatura estava errada - porque a nossa percepção do mundo da ciência é probabilística, não certeira. Ele apenas soube interpretar tal dado da forma adequada, com as variáveis pertinentes, e enfrentar o problema sem querer se enganar com uma certeza inexistente. Ele preferiu apostar na incerteza estatística - e foi um vencedor.
Para continuar a ler, clique aqui.
sábado, setembro 29, 2007
quinta-feira, setembro 27, 2007
Theory of collective firing induced by noise or diversity in excitable media
C. J. Tessone, A. Scirè, R. Toral, and P. Colet -->
Institut Mediterrani d'Estudis Avançats, IMEDEA (CSIC-UIB), Campus Universitat Illes Balears, E-07122 Palma de Mallorca, Spain
(Received 20 June 2006; published 9 January 2007)
Large variety of physical, chemical, and biological systems show excitable behavior, characterized by a nonlinear response under external perturbations: only perturbations exceeding a threshold induce a full system response (firing). It has been reported that in coupled excitable identical systems noise may induce the simultaneous firing of a macroscopic fraction of units. However, a comprehensive understanding of the role of noise and that of natural diversity present in realistic systems is still lacking. Here we develop a theory for the emergence of collective firings in nonidentical excitable systems subject to noise. Three different dynamical regimes arise: subthreshold motion, where all elements remain confined near the fixed point; coherent pulsations, where a macroscopic fraction fire simultaneously; and incoherent pulsations, where units fire in a disordered fashion. We also show that the mechanism for collective firing is generic: it arises from degradation of entrainment originated either by noise or by diversity.
domingo, setembro 23, 2007
Authors: Yeon-Mu Choi, Hyun-Joo Kim
Physica A, Volume 382, Issue 2, p. 665-671 (2007).
Abstract: We study the Greek and Roman mythology using the network theory. We construct a directed network by using a dictionary of Greek and Roman mythology in which the nodes represent the entries listed in the dictionary and we make directional links from an entry to other entries that appear in its explanatory part. We find that this network is clearly not a random network but a directed scale-free network. Also measuring the various quantities which characterize the mythology network, we analyze the Greek and Roman mythology and compare it to other real networks.
Comments: 4 pages, 5 figures
Subjects: Physics and Society (physics.soc-ph); Statistical Mechanics (cond-mat.stat-mech)
Cite as: arXiv:physics/0506142v1
Resonant effects can be clearly seen in the radial distribution of the asteroids. Some orbital resonances are destabilizing, creating minima in the distribution, called "Kirkwood gaps" after Daniel Kirkwood, the astronomer who first recognized them. The main asteroid belt is bounded by the 4:1 and 2:1 orbital resonances with Jupiter. The stable 3:2 and 1:1 resonances account, respectively, for the Hilda family of asteroids and the Jupiter Trojans. The semimajor axis is one-half the long dimension of an object’s elliptical orbit. One astronomical unit is the Earth-Sun distance.
Graph by Barbara Aulicino. Distribution of asteroids courtesy of the Minor Planet Center.
A few years ago, Rory Barnes and Thomas Quinn at the University of Washington used computer simulations to examine the stability of extrasolar systems having two or more planets. They found that almost all systems with planets that are close enough to affect one another gravitationally lie near the edge of instability. The simulations showed that small alterations in the orbits of the planets in those systems would lead to catastrophic disruptions.
This remarkable result might seem surprising. But the prevalence of such marginally stable systems makes sense, Barnes and Quinn concluded, if planets form within unstable systems that become more stable by ejecting massive bodies. The investigators remarked, "As unsettling as it may be, it seems that a large fraction of planetary systems, including our own, lie dangerously close to instability."
Barnes, now at the University of Arizona, and Sean N. Raymond, at the University of Colorado, went on to hypothesize that all planetary systems are packed as tightly as possible, as Laskar had suggested earlier. In some of the observed extrasolar systems, Barnes and Raymond identified apparently empty regions of stability around the central star. Those regions, they predict, contain planets small enough to have evaded detection.
For example, the star 55 Cancri has four known giant planets, three of them close in with short orbital periods and a more distant planet with a period of nearly 15 years. Between the inner three and the outermost planet lies a large area in which, Barnes and Raymond predict, one or more new planets will eventually be found. This region includes the "habitable zone," where a planet's surface temperature would allow liquid water to exist.
What we have here is a fascinating new hypothesis, which posits that our solar system and other mature planetary systems are filled nearly to capacity. The present configurations of such systems contain about as many planets as they can hold, spaced about as closely together as stability allows. Such is the expected outcome of the chaotic process that makes planets. A family of planetary embryos grows by feeding on a vast swarm of smaller objects in a debris disk until the system loses its brakes. Global instability then erupts, and the larger worlds consume or eject the more erratic ones until the system settles down into the mature state of marginal stability. The process is one of self-organization, increasing order within the system by exporting disorder to the external environment, in this case the Galaxy.
Module identification in bipartite and directed networks
Roger Guimerà, Marta Sales-Pardo, and Luís A. Nunes Amaral -->
Northwestern Institute on Complex Systems (NICO) and Department of Chemical and Biological Engineering, Northwestern University, Evanston, Illinois 60208, USA
(Received 7 June 2007; published 6 September 2007)
Modularity is one of the most prominent properties of real-world complex networks. Here, we address the issue of module identification in two important classes of networks: bipartite networks and directed unipartite networks. Nodes in bipartite networks are divided into two nonoverlapping sets, and the links must have one end node from each set. Directed unipartite networks only have one type of node, but links have an origin and an end. We show that directed unipartite networks can be conveniently represented as bipartite networks for module identification purposes. We report on an approach especially suited for module detection in bipartite networks, and we define a set of random networks that enable us to validate the approach.
©2007 The American Physical Society
PACS: 89.75.Hc, 89.65.-s, 05.50.+q
Full Text: PDF GZipped PS
Unveiling community structures in weighted networks
Nelson A. Alves -->
Departamento de Física e Matemática, FFCLRP Universidade de São Paulo, Avenida Bandeirantes 3900, CEP 14040-901, Ribeirão Preto, São Paulo, Brazil
(Received 28 February 2007; published 4 September 2007)
Random walks on simple graphs in connection with electrical resistor networks lead to the definition of Markov chains with transition probability matrix in terms of electrical conductances. We extend this definition to an effective transition matrix Pij to account for the probability of going from vertex i to any vertex j of the original connected graph G. Also, we present an algorithm based on the definition of this effective transition matrix among vertices in the network to extract a topological feature related to the manner by which graph G has been organized. This topological feature corresponds to the communities in the graph.
©2007 The American Physical Society
PACS: 89.75.Hc, 05.10.-a
Full Text: PDF GZipped PS
The protein folding network
MD simulations of aggregation
F. Rao and A. Caflisch, J. Mol. Biol. 342, 299-306 (2004) [pdf] [Supp. Mat.]
quinta-feira, setembro 20, 2007
It surprises most people to learn that this is even a question. Every other basic bodily function—like eating, drinking, breathing, urinating, or defecating—has a pretty clear purpose. Our bodies and brains need food, water, and oxygen to stay alive and to replenish themselves, and if they didn't rid themselves of the byproducts of this metabolism, we'd be awash in our own toxic waste. Likewise, sleep must be for something important. We all spend decades in this strange state, immobilized, unconscious, and vulnerable. But what exactly does sleep do for us?
Parents tell their children they need to sleep because they're tired and need to rest. But of course rest is not good enough. Lying still for eight hours is no substitute for sleep. My own mother had a different theory. She said I needed to sleep because I had too much "sleepy gas." It had been building up all day long, and so I needed to sleep to get rid of it. In fact, scientists observed a long time ago that if you keep a sheep awake continuously for several days and then inject some of its cerebrospinal fluid into another, well-rested sheep, that sheep will fall right asleep, presumably because some naturally-occurring sleep substance had reached a soporific level in the donor. But this line of research never quite solved the puzzle. Although a number of putative sleep substances have now been identified, we're not sure how they might work biochemically, or how sleep (as opposed to mere rest) might break them down
Other sleep-deprivation studies done in the early 1980's took a more brutal approach, keeping rats awake for weeks until they died from a lack of sleep, and then looking for the precise cause of death. Such studies (now outlawed) could not pinpoint any specific culprits, such as particular organ failures. One striking observation, however, was that the rats ate much more than normal and yet wasted away. Their metabolism seemed to be wrecked. So maybe sleep is for energy regulation, in some unspecified way. Other popular theories are that sleep is for tissue repair, or immune function, or for consolidating learning and memory.
The new development, and the cause for optimism, is an original approach to the problem that makes the first quantitative, testable predictions about the function of sleep. Two physicists, Van Savage (Harvard Medical School) and Geoff West (Santa Fe Institute), have analyzed how sleep varies across mammals of different species. Normally physiological time ticks slower for bigger animals. For example, elephants live much longer than mice and their hearts beat much slower. The interesting thing is that both animals' lifetimes and pulse times scale in the same way with their body mass—in direct proportion to their mass raised to the 1/4 power—with the curious implication that the hearts of mice and elephants will typically beat the same number of times in their lifetime.
quarta-feira, setembro 19, 2007
Navarro L, Martínez-vargas M, Murillo-rodríguez E, Landa A, Méndez-díaz M, Prospéro-garcía O. Neuroscience. 2003;120(3):855-9.
[Cannabinoid receptor 1 expression and pathological changes in rat hippocampus after deprivation of rapid eye movement sleep]
[Article in Chinese]
Jiang PF, Zhu T, Xia ZZ. Zhejiang Da Xue Xue Bao Yi Xue Ban. 2006 Sep;35(5):535-40.
Sleep modulates cannabinoid receptor 1 expression in the pons of rats.
Martínez-Vargas M, Murillo-Rodríguez E, González-Rivera R, Landa A, Méndez-Díaz M, Prospro-García O, Navarro L. Neuroscience. 2003;117(1):197-201.
Oleamide modulates memory in rats.
Murillo-Rodríguez E, Giordano M, Cabeza R, Henriksen SJ, Méndez Díaz M, Navarro L, Prospéro-García O. Neurosci Lett. 2001 Nov 2;313(1-2):61-4.
Grupo de Neurociencias, Departamento de Fisiología, Facultad de Medicina, Universidad Nacional Autónoma de México, A Postal 70-250 Mexico, D. F. 04510, Mexico.
Oleamide is a recently described lipid, obtained from the cerebrospinal fluid of sleep-deprived cats. It has been observed that oleamide possesses several biological effects, such as sleep induction, and immunological suppression as well as serotonin and gamma-aminobutyric acid receptors activation. In addition, oleamide also binds to the cannabinoid receptors. In this study, we have observed that oleamide facilitates memory extinction in a passive avoidance paradigm, reduces core temperature and pain perception, but does not affect significantly locomotion. These results suggest that oleamide modulates memory processes. However, we do not know if oleamide impairs the retrieval of the memory associated to the "not go" behavior, or facilitates the fast re-learning of the "go" behavior. In addition, since these effects are also induced by marijuana and anandamide, it is very likely that oleamide may be affecting the cerebral cannabinoid system to induce its effects.
Short-term memory is modulated by the spontaneous release of endocannabinoids: evidence from hippocampal population codes.
Deadwyler SA, Goonawardena AV, Hampson RE. Behav Pharmacol. 2007 Sep;18(5-6):571-80.
Department of Physiology and Pharmacology, Wake Forest University Health Sciences, Winston-Salem, North Carolina 27157-1083, USA. firstname.lastname@example.org
Population codes derived from ensembles of hippocampal neurons were assessed to determine whether endocannabinoids were active when rats performed a delayed-nonmatch-to-sample (DNMS) short-term memory task. Multivariate discriminant analyses of the firing patterns of ensembles of CA1 and CA3 hippocampal neurons extracted representations of information encoded at the time of the sample response (SmR codes) during individual DNMS trials. The 'strength' or distinctiveness of trial-specific SmR codes in normal sessions was compared with sessions in which either rimonabant, the well-characterized cannabinoid CB1 receptor antagonist, or WIN 55212-2 (WIN-2), a cannabinoid CB1 receptor agonist, were administered. Results show that performance on trials with delay intervals longer than 10 s was facilitated by rimonabant (2.0 mg/kg) owing to a significantly increased frequency of trials with stronger SmR codes. In contrast, WIN-2 (0.35 mg/kg) suppressed the strength of SmR codes necessary to perform trials with delays greater than 10 s. The positive influence of rimonabant on performance indicated that the action of endocannabinoids was to reduce SmR code strength, resulting in trials that were at risk for errors if the delay exceeded 10 s. Thus endocannabinoids, like exogenously administered cannabinoids, reduced hippocampal encoding necessary to perform long-delay trials. The findings therefore indicate a direct relationship between the actions of endocannabinoids on hippocampal processes and the ability to encode information into short-term memory.
Endocannabinoid signaling and synaptic plasticity in the brain.
Zhu PJ. Crit Rev Neurobiol. 2006;18(1-2):113-24.
Laboratory for Integrative Neuroscience, National Institute on Alcohol Abuse and Alcoholism, National Institutes of Health and National Chemical Genomics Center, National Human Genome Research Institute, Bethesda, MD, USA. email@example.com
Repetitive firing neuron or activation of synaptic transmission plays an important role in the modulation of synaptic efficacy, such as long-term potentiation (LTP) and long-term depression (LTD). These activity-dependent changes in synaptic efficacy are thought to be critical to learning and memory; however, the underlying mechanisms remain to be defined. Endogenous cannabinoids (eCBs) are diffusible modulators that are released from depolarized postsynaptic neurons and act on presynaptic terminals. Persistent release of eCBs can lead to long-term modulation of synaptic plasticity in the brain. Given a broad distribution of eCB receptors in the brain, the eCB signaling system could contribute to use-dependent modification of brain functions.
Effects of intra-amygdala infusion of CB1 receptor agonists on the reconsolidation of fear-potentiated startle.
Lin HC, Mao SC, Gean PW. Learn Mem. 2006 May-Jun;13(3):316-21. Epub 2006 May 16.
Institute of Basic Medical Sciences and Department of Pharmacology, National Cheng-Kung University, Tainan, Taiwan. firstname.lastname@example.org
The cannabinoid CB1 receptor has been shown to be critically involved in the extinction of fear memory. Systemic injection of a CB1 receptor antagonist prior to extinction training blocked extinction. Conversely, administration of the cannabinoid uptake inhibitor AM404 facilitated extinction in a dose-dependent manner. Here we show that bilateral infusion of CB1 receptor agonists into the amygdala after memory reactivation blocked reconsolidation of fear memory measured with fear-potentiated startle. The effect was dose-dependent and could be blocked by AM251, a specific CB1 receptor antagonist. In contrast, the effect of CB1 agonists on reconsolidation was no longer seen if memory reactivation was omitted. Concomitant with block of reconsolidation, CB1 agonist-treated animals did not exhibit shock-induced reinstatement or spontaneous recovery of fear. The absence of recovery was not attributable to permanent damage to the amygdala in WIN-treated rats, nor did the effect result from alteration of baseline startle or shock reactivity. These results suggest that CB1 agonists could impair fear memory via blocking reconsolidation.
segunda-feira, setembro 17, 2007
O termo 'louco por você' agora tem um valor clínico. Segundo um estudo da Universidade da Basiléia, na Suíça, publicado pelo 'Journal of Adolescent Health', os efeitos da paixão nos adolescentes são os mesmos de uma síndrome chamada hipomania, considerada o primeiro estágio do distúrbio bipolar, ou variação de humor.
domingo, setembro 16, 2007
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sábado, setembro 15, 2007
Da Scientific American:
Mark Pagel, head of the evolutionary biology group at the University of Reading in England and editor of The Encyclopedia of Evolution, fills us in:
We humans are conspicuous among the 5,000 or so mammal species in that we are effectively naked. Just consider what your pet dog or cat (or, for that matter, a polar bear) would look like, and how it might feel, if its furry coat were shorn. Scientists have suggested three main explanations for why humans lack fur. All revolve around the idea that it may have been advantageous for our evolving lineage to have become less and less hairy during the six million years since we shared a common ancestor with our closest living relative, the chimpanzee.
The aquatic-ape hypothesis suggests that six million to eight million years ago apelike ancestors of modern humans had a semiaquatic lifestyle based on foraging for food in shallow waters. Fur is not an effective insulator in water, and so the theory asserts that we evolved to lose our fur, replacing it, as other aquatic mammals have, with relatively high levels of body fat. Imaginative as this explanation is—and helpful in providing us with an excuse for being overweight—paleontological evidence for an aquatic phase of human existence has proven elusive.
The second theory is that we lost our fur in order to control our body temperature when we adapted to life on the hot savannah. Our ape ancestors spent most of their time in cool forests, but a furry, upright hominid walking around in the sun would have overheated. The body-cooling idea seems sensible, but even though lacking fur might have made it easier for us to lose heat during the day, we also would have lost more heat at night, when we needed to retain it.
Recently, a colleague and I suggested that ancestors to modern humans became naked as a means to reduce the prevalence of external parasites that routinely infest fur. A furry coat provides an attractive and safe haven for insects such as ticks, lice, biting flies and other "ectoparasites." These creatures not only bring irritation and annoyance but carry viral, bacterial and protozoan-based diseases such as malaria, sleeping sickness, West Nile and Lyme disease, all of which can cause chronic medical problems and, in some cases, death. Humans, by virtue of being able to build fires, construct shelters and produce clothes, would have been able to lose their fur and thereby reduce the numbers of parasites they were carrying without suffering from the cold at night or in colder climates.
Human lice infections, which are confined to the hairy areas of our bodies, seem to support the parasite hypothesis. Naked mole rats, animals that can be described as resembling "overcooked sausages with buck teeth," also seem to support the theory: They live underground in large colonies, in which parasites would be readily transmitted. But the combined warmth of their bodies and the confined underground space probably negate the problem of losing heat to cold air for these animals, allowing them also to become naked.
Once hairlessness had evolved this way, it may have become subject to sexual selection—being a feature in one sex that appealed to another. Smooth, clear skin may have become a signal of health, like a peacock's tail, and could explain why women are naturally less hairy than men and why they put more effort into removing body hair. Despite exposing us to head lice, humans probably retained head hair for protection from the sun and to provide warmth when the air is cold. Pubic hair may have been retained for its role in enhancing pheromones or the airborne odors of sexual attraction.
Na verdade, acho que o Rantala publicou primeiro...
quinta-feira, setembro 13, 2007
Os neurônios do cérebro dos liberais e de conservadores reagem de forma diferente diante de decisões difíceis. A conclusão é de um estudo da Universidade de Nova York publicado pela revista britânica "Nature Neuroscience".
A pesquisa foi liderada pelo cientista político da Universidade de Nova York David Amodio, que teve o auxílio de diversos pesquisadores.
Desde que Aristóteles afirmou que o ser humano é um animal político por natureza, dezenas de estudos estabeleceram uma forte relação entre persuasão política e certos aspectos da personalidade dos indivíduos.
Os conservadores tendem a buscar a ordem e a estrutura em suas vidas e são mais coerentes na hora de tomar decisões. Os liberais, por sua vez, mostram maior tolerância para a ambigüidade e para a complexidade, e se adaptam mais facilmente a circunstâncias inesperadas, indicou o estudo divulgado no último domingo (9).
Um grupo de 43 pessoas se dispôs a realizar uma série de testes que avaliaram sua resposta diante de pautas criadas para romper uma rotina estabelecida.
"Costumamos voltar do trabalho para casa pelo mesmo caminho, um dia após o outro, até que se forme um hábito e não seja preciso pensar muito", explicou Amodio. "Mas, ocasionalmente, a rua está em obras, não se pode usar esse caminho e temos de romper a resposta habitual para que possamos processar a nova informação."
Usando eletroencefalogramas, que medem impulsos cerebrais, os cientistas examinaram a atividade de uma parte determinada do cérebro, o córtex cingulado anterior, que está fortemente vinculada ao processo de auto-regulação do controle de conflitos.
Aqueles que se autodenominavam liberais mostraram "uma atividade cerebral relacionada à gestão de conflitos significativamente maior" quando a situação hipotética necessitava de uma mudança na rotina.
Os conservadores, entretanto, foram menos flexíveis e se negavam a mudar velhos hábitos "apesar de assinalar que estes deviam mudar".
Se essa situação é má ou boa depende obviamente da perspectiva de cada um: uma linha de interpretação pode afirmar que os liberais possuem uma mente aberta e que os conservadores são rígidos e teimosos.
Outra corrente pode concluir que os liberais não têm personalidade nem defendem suas idéias, enquanto que os conservadores são leais.
O que vem primeiro: o modelo de atividade cerebral que cada um tem ou a orientação política? Amodio mostra-se relutante em dar uma resposta definitiva.
"Os mecanismos neurais para o controle de conflitos se formam muito cedo na infância" e provavelmente se originam na herança genética dos indivíduos, disse o pesquisador. "Mas, ainda que proporcionem um modelo para orientações mais liberais ou mais conservadoras, a atuação e a importância dos genes são influenciadas principalmente pelo ambiente que cerca o indivíduo ao longo do seu desenvolvimento."