sexta-feira, fevereiro 29, 2008

Árvores fazem chover?

Há alguns anos atrás eu li em algum lugar que as arvores da floresta amazônica emitem partículas para a atmosfera de tal modo que o ar úmido que vai passando seja nucleado e chuvas caiam sobre ela. Ou seja, as plantas "manipulam" o clima, ou pelo menos a chuva. Será que alguém poderia confirmar essa informação, com referências?

Em todo caso, saiu nesta semana este artigo na Science que vai nessa direção.

Brent C. Christner, Cindy E. Morris, Christine M. Foreman, Rongman Cai, David C. Sands

Despite the integral role of ice nucleators (IN) in atmospheric processes leading to precipitation, their sources and distributions have not been well established. We examined IN in snowfall from mid- and high-latitude locations and found that the most active were biological in origin. Of the IN larger than 0.2 micrometer that were active at temperatures warmer than -7°C, 69 to 100% were biological, and a substantial fraction were bacteria. Our results indicate that the biosphere is a source of highly active IN and suggest that these biological particles may affect the precipitation cycle and/or their own precipitation during atmospheric transport.

quinta-feira, fevereiro 28, 2008

Cientistas devem se vestir melhor?

Faço minhas as palavras do Cosma Shalize: roupas impõe respeito de um modo que manifestações verbais não o fazem. Aprofundando o argumento: roupas fazem parte de códigos não verbais e comunicação subliminar cuja profundidade e sutileza não compreendemos de todo (em especial cientistas nerds). Assim, talvez seja mesmo melhor começar a usar calça comprida em vez de bermudas aqui no departamento, camisas em vez de camisetas, sapatos em vez de tênis etc. Ou melhor ainda, alguma roupa bem fashion. Ou seja, parece que vou ter que ficar mais conservador e renunciar à liberdade de vestimenta típica dos físicos. E aguentar o calor de Ribeirão, mas... quem sabe compense. Tudo por causa dos estudantes, sempre os estudantes...

Academics are supposed to impart knowledge and skills to their students, to critique their work, to direct their intellectual and to some extent their moral development; in all these tasks they are supposed to exercise authority over students. They may also be called upon to supervise student or other employees, which is another exercise of authority. They will do so more effectively if they display the recognized external markers of high status and of seriousness, which includes dressing in certain ways and adopting certain demeanors. In fact, if they do this, their authority is more likely to be accepted as legitimate, leading to fewer occasions on which it must be explicitly insisted upon and made into naked acts of domination. Furthermore, academics are often called upon to represent their schools and/or their scholarly communities to the outside world, and this, too, will be done more effectively if they dress in ways which their audiences take to convey seriousness.

Prá compensar...

Prá compensar a foto do Dyson, vai aqui uma foto de Angelina Jolie na conferência de Davos, junto com o link Wikipedia da moça.

Mudança de opinião

Freeman Dyson, no When facts change your mind, that's not always science. It may be history. I changed my mind about an important historical question: did the nuclear bombings of Hiroshima and Nagasaki bring World War Two to an end? Until this year I used to say, perhaps. Now, because of new facts, I say no. This question is important, because the myth of the nuclear bombs bringing the war to an end is widely believed. To demolish this myth may be a useful first step toward ridding the world of nuclear weapons.

Until the last few years, the best summary of evidence concerning this question was a book, "Japan's Decision to Surrender", by Robert Butow, published in 1954. Butow interviewed the surviving Japanese leaders who had been directly involved in the decision. He asked them whether Japan would have surrendered if the nuclear bombs had not been dropped. His conclusion, "The Japanese leaders themselves do not know the answer to that question, and if they cannot answer it, neither can I". Until recently, I believed what the Japanese leaders said to Butow, and I concluded that the answer to the question was unknowable.

Facts causing me to change my mind were brought to my attention by Ward Wilson. Wilson summarized the facts in an article, "The Winning Weapon? Rethinking Nuclear Weapons in the Light of Hiroshima", in the Spring 2007 issue of the magazine, "International Security". He gives references to primary source documents and to analyses published by other historians, in particular by Robert Pape and Tsuyoshi Hasegawa. The facts are as follows:

1. Members of the Supreme Council, which customarily met with the Emperor to take important decisions, learned of the nuclear bombing of Hiroshima on the morning of August 6, 1945. Although Foreign Minister Togo asked for a meeting, no meeting was held for three days.

2. A surviving diary records a conversation of Navy Minister Yonai, who was a member of the Supreme Council, with his deputy on August 8. The Hiroshima bombing is mentioned only incidentally. More attention is given to the fact that the rice ration in Tokyo is to be reduced by ten percent.

3. On the morning of August 9, Soviet troops invaded Manchuria. Six hours after hearing this news, the Supreme Council was in session. News of the Nagasaki bombing, which happened the same morning, only reached the Council after the session started.

4. The August 9 session of the Supreme Council resulted in the decision to surrender.

5. The Emperor, in his rescript to the military forces ordering their surrender, does not mention the nuclear bombs but emphasizes the historical analogy between the situation in 1945 and the situation at the end of the Sino-Japanese war in 1895. In 1895 Japan had defeated China, but accepted a humiliating peace when European powers led by Russia moved into Manchuria and the Russians occupied Port Arthur. By making peace, the emperor Meiji had kept the Russians out of Japan. Emperor Hirohito had this analogy in his mind when he ordered the surrender.

6. The Japanese leaders had two good reasons for lying when they spoke to Robert Butow. The first reason was explained afterwards by Lord Privy Seal Kido, another member of the Supreme Council: "If military leaders could convince themselves that they were defeated by the power of science but not by lack of spiritual power or strategic errors, they could save face to some extent". The second reason was that they were telling the Americans what the Americans wanted to hear, and the Americans did not want to hear that the Soviet invasion of Manchuria brought the war to an end.

In addition to the myth of two nuclear bombs bringing the war to an end, there are other myths that need to be demolished. There is the myth that, if Hitler had acquired nuclear weapons before we did, he could have used them to conquer the world. There is the myth that the invention of the hydrogen bomb changed the nature of nuclear warfare. There is the myth that international agreements to abolish weapons without perfect verification are worthless. All these myths are false. After they are demolished, dramatic moves toward a world without nuclear weapons may become possible.

quarta-feira, fevereiro 27, 2008


Via Entropicando Ciência:


Editorial Reviews

From Publishers WeeklyStarred Review. British author McDonald's outstanding SF novel channels the vitality of South America's largest country into an edgy, post-cyberpunk free-for-all. McDonald sets up three separate characters in different eras—a cynical contemporary reality-TV producer, a near-future bisexual entrepreneur and a tormented 18th-century Jesuit agent. He then slams them together with the revelation that their worlds are strands of an immense quantum multiverse, and each of them is threatened by the Order, a vast conspiracy devoted to maintaining the status quo until the end of time. As McDonald weaves together the separate narrative threads, each character must choose between isolation or cooperation, and also between accepting things as they are or taking desperate action to make changes possible. River of Gods (2004), set in near-future India, established McDonald as a leading writer of intelligent, multicultural SF, and here he captures Latin America's mingled despair and hope. Chaotic, heartbreaking and joyous, this must-read teeters on the edge of melodrama, but somehow keeps its precarious balance. (May) Copyright © Reed Business Information, a division of Reed Elsevier Inc. All rights reserved.

From Booklist*Starred Review* McDonald takes on frenetic, vast, fascinating Brazil in this epic interweaving three time strands: the contemporary world of TV producer Marcelina, whose proposal for a series based on a mock trial of an ex-soccer star who played in the most devastating championship game in Brazilian history gets her entangled with the strange truth about our world; the eighteenth century of a Jesuit whose "task most difficult" of returning a fellow Jesuit to the teachings of the church takes him to the Amazon, where the task becomes unexpectedly, unimaginably more difficult and bizarre; and the nearish future, in which Edson, risen from poverty and crime almost to his dream of wealth and a house by the sea, gets mired in the affairs of Fia, a quantumiera (she operates a quantum computer in an always-moving vehicle) who disables the quantum security chip his brother nearly died for stealing. The connections of these worlds through the various ways in which people can perceive all possible universes, and the implications of the universe's unavoidable quantum entanglements--ranging from the possibility of predicting the future to the existence of nigh-infinite doubles of everyone--prove startling. McDonald's Brasyl is a magnificent place, and the motivations and possible results of the battle over the multitude of quantum universes it posits are chilling and wonderful. Regina SchroederCopyright © American Library Association. All rights reserved

See all Editorial Reviews
Product Details
Hardcover: 357 pages
Publisher: Pyr (May 3, 2007)
Language: English
ISBN-10: 1591025435
ISBN-13: 978-1591025436
See all 14 customer reviews...

domingo, fevereiro 24, 2008

Cérebros culinários

OK, OK, o paper da culinária foi rejeitado, tentarei agora submetê-lo ao Physical Review E. Mas destaco abaixo esta notícia para os engraçadinhos que acham que culinária é um assunto de importância científica inferior (será isso um ranço sexista?)

Do blog Visões da Vida dr \reinaldo José Lopes, no G1:

Vida de macaco, pode acreditar, não é brincadeira. Ao pensar nos símios, a gente logo imagina os bichos descascando tranqüilamente uma banana e mastigando a guloseima. Acontece que, sem plantações gerenciadas por humanos por perto, os bichos são obrigados a deglutir coisas como esta ao lado.

Sim, ISSO aí em cima é uma BANANA. Ou ao menos uma banana em sua versão ancestral, antes que a invenção da agricultura a transformasse no acompanhamento perfeito para a farinha láctea. As outras frutas que os primatas precisam encarar na natureza não são muito melhores. Eis aqui as palavras do bioantropólogo americano Richard Wrangham, que não me deixa mentir. “As frutas típicas da dieta dos chimpanzés são muito desagradáveis, muito fibrosas, bastante amargas. O efeito geral delas é que você não vai querer comer mais de duas ou três antes de sair correndo para tomar um copão d’água e dizer: ‘Esse não foi um experimento agradável. Espero não ficar doente’. Contêm pouco açúcar, e algumas fazem seu estômago revirar”, declarou ele à revista “Scientific American”. O malfadado teste levou Wrangham, que trabalha na Universidade Harvard, a uma conclusão inescapável: nossa espécie precisou inventar a culinária para chegar aonde está hoje.
Para ser mais exato, argumenta o bioantropólogo, só o uso do fogo para tornar os alimentos palatáveis teria sido capaz de proporcionar aos nossos ancestrais remotos a comida de alta qualidade que alimentaria seus cérebros sedentos de energia. Cozinhar, segundo essa perspectiva inovadora, estaria longe de ser a cereja do bolo (com o perdão do trocadilho) da nossa humanidade. Pelo contrário: primeiro teríamos virado chefs para só depois darmos um salto de capacidade mental. Wrangham ainda não conseguiu provar sua hipótese, e algumas peças do quebra-cabeça traçado por ele não se encaixam (mais sobre isso daqui a alguns parágrafos). Mas a idéia de uma humanidade eminentemente culinária tem diversos pontos intrigantes a seu favor, como veremos a seguir.
Miolos famintos
Como já tive a ocasião de mencionar algumas vezes nesta coluna, nossos cérebros são órgãos sequiosos de energia. Uma comparação entre massas iguais de tecido cerebral e tecido muscular revela um consumo 22 vezes maior de energia no cérebro. O mais lógico é que essa mesma densidade de energia requerida pelo cérebro esteja presente na densidade de energia da comida que o alimenta.E aí começam os problemas. Quando a nossa massa encefálica não era muito maior que a de um chimpanzé (situação que perdurou até cerca de 2 milhões de anos atrás), não era tão limitante assim usar uma dieta relativamente pouco nutritiva. Há 1,7 milhão de anos, contudo, surge o ancestral humano conhecido como Homo erectus – uma criatura cujo cérebro tinha quase o triplo do tamanho do cérebro de um chimpanzé, e cujos dentes sofreram uma redução considerável se comparados aos dos hominídeos mais antigos. Algum fator importante permitiu que o cérebro do Homo erectus crescesse e seus dentes encolhessem. A pergunta é: qual?
Existe uma correlação bem conhecida entre o tamanho proporcional do cérebro de primatas e as dimensões de seus intestinos. Como o organismo nunca dispõe de recursos ilimitados, parece haver uma “escolha” (obviamente inconsciente) entre investimentos de energia. Ora, intestinos volumosos são, em geral, uma necessidade quando é preciso digerir alimentos relativamente pobres em proteínas. Quanto a isso, os dados recolhidos por Wrangham não mentem: até os mais vegetarianos entre os caçadores-coletores de hoje têm uma dieta com, no máximo, 10% de fibra não-digerível. Já os chimpanzés têm de enfrentar 32% de fibra não-digerível no seu cardápio, o qual se compõe basicamente de frutos (60%), folhas e uma pequena quantidade de carne crua.
Se um Homo erectus tivesse de alimentar seu cérebro descomunal (ao menos para padrões chimpanzescos) com a mesma dieta, teria de obter três quilos de comida por pessoa ao dia, além de gastar SEIS horas diárias mastigando tudo isso antes de engolir. Não é um estilo de vida muito viável, para dizer o mínimo.
A coisa muda radicalmente de figura se o dito hominídeo aprender a cozinhar, no entanto. Uma série de alimentos se tornam incrivelmente mais fáceis de mastigar (basta comparar uma mandioca crua com outra cozida para saber do que eu estou falando). E, mais importante, muito mais fácil de digerir. Tomemos como exemplo a proteína animal, explica Wrangham: no formato cru, ela tende a se organizar de forma rígida, com uma estrutura ordenada que atrapalha a ação dos sucos digestivos. Ao ser cozida, seu estado natural de desorganização desaparece. É como abrir uma brecha nos muros de um castelo: fica menos difícil invadi-lo.
Resumindo a ópera, Wrangham aposta que o domínio do fogo como instrumento de cozimento disponibilizou, de forma quase instantânea, uma quantidade vasta de nutrientes para os ancestrais do Homo erectus. A fartura recém-adquirida tirou os hominídeos da encruzilhada entre cérebros e intestinos: agora, sobrava energia e não era mais preciso investir tantas tripas em obtê-la. Cérebros maiores, portanto, tornaram-se viáveis e, quando mutações ligadas a essa característica deram as caras, foram selecionadas e passadas adiante. Cadê o fogo que estava aqui?
Apesar da série impressionante de “encaixes” biológicos por trás da idéia, o que ainda não está claro é se o uso controlado do fogo é mesmo tão antigo quanto o Homo erectus. Os pesquisadores ainda batem cabeça em relação às evidências dessa tecnologia. Há quem veja indícios de fogueiras controladas no leste da África de 1,6 milhão de anos atrás – o que funcionaria relativamente bem para Wrangham.
Muita gente, no entanto, ainda contesta essa data. Por enquanto, a mais antiga data aceita para uma fogueira feita por mãos hominídeas vem do norte de Israel e corresponde a apenas 800 mil anos – uma época em que o Homo erectus talvez já estivesse desaparecendo da África e do Oriente Médio. Por mais saborosa que seja a hipótese de Wrangham, ainda falta o ingrediente final.
PS: Se a evidência mais antiga de fogueira é de 800.000 anos, dado que ela é fruto de uma amostragem sobre uma quantidade de eventos bastante rara (fogueiras fósseis potencialmente encontráveis pelos cientistas), então podemos concluir que certamente existem fogueiras mais antigas. quanto mais? Idéia: plote o número N(t) de fogueiras fósseis encontradas até hoje em função do tempo, considere a probabilidade p de uma fogueira fossilizar, faça uma extrapolação para o passado. Acho que não seria dificil dobrar a antiguidade das fogueiras...

quarta-feira, fevereiro 20, 2008

ANTI-STDP e homeostase sináptica

A teoria do unlearning de atratores profundos (pensamentos obsessivos, sejam ameaçadores sejam prazeirosos - afinal ansiedade (ter medo) e ansiar (desejar) estão relacionados, pelo menos na amidala) poderia ser enunciada numa frase: durante o sonho é promovida uma homeostase entre atratores. Acho que isso deveria ser verificado também nos sistemas motores (atratores fortes = tremores) e sistemas corticais (atratores fortes = músicas recorrentes - catchy songs)...

Mais uma referência para o paper com Juliana. Notar que anti-STDP equivale ao mecanismo anti-hebbiano de unlearning de Crick-Mitchison. Notar também que Rumsey e Abott discutem a possibilidade de que anti-STDP seja um processo desencadeado durante o sono. É óbvio que Abott têm em mente os trabalhos de unlearning da década de 80 e 90. Acho que o que é novo na minha proposta é o envolvimento do sistema endocanabinoide na implementação biofísica do processo anti-STDP. O tempo dirá... Que pena que o meu paper de 2002 não foi aceito pela BBS. Ah, sim, a versão do Arxiv têm aquele misprint "receptores DB1" em vez de "receptores CB1". Não tive tempo pra corrigir isso, o farei na nova versão...

91: 1941-1942, 2004; doi:10.1152/jn.01246.2003

0022-3077/04 $5.00

Editorial Focus

Rebuilding Dendritic Democracy. Focus on "Equalization of Synaptic Efficacy by Activity- and Timing-Dependent Synaptic Plasticity"

Hebbian learning has become a very influential paradigm, which states that learning in neural networks is achieved by modifying synaptic connections between neurons in proportion to their correlated activity (Gerstner and Kistler 2002Go). While this process is desirable to detect relevant inputs to be learned, it also represents a positive feedback mechanism that becomes discriminatory when different synapses do not have equal starting points to begin with. For example, activation of a "strong" synapse from neuron A onto neuron B will increase the activity of neuron B, resulting in an increase in the correlation between the two neurons and hence an increase in its own strength—and conversely, a further weakening of weak synapses. Starting points are unequal when applying Hebbian learning to synapses located on dendrites, as are most excitatory synapses in the CNS. Postsynaptic potentials are attenuated as they travel from the synapse toward the soma in a manner that depends on the cable structure and the membrane properties of dendrites. Thus distal synapses are weaker to start with, compared to more proximal synapses, in their ability to influence neuronal output via the axon. Facing such a situation, distal dendritic synapses might as well give up completely. Not only are they attenuated, but according to Hebbian learning their destiny is extinction.

Au contraire. In several cell types in which synaptic conductance has been measured directly as a function of the distance of the synapses from the soma, it was at least constant on average (Williams and Stuart 2002Go) or even increased with distance from the soma (Magee and Cook 2000Go; for review, see Häusser 2001Go). This distance-dependent scaling of the synaptic conductance appears to counterbalance the voltage attenuation, rendering excitatory postsynaptic potential (EPSP) amplitude at the soma insensitive to its dendritic origin, a situation that has been called "dendritic democracy" (Häusser 2001Go; but see London and Segev 2001Go). But how can a synapse possibly tell its distance from the soma to scale its conductance properly?

In this issue of the Journal of Neurophysiology (p. 2273–2280), Rumsey and Abbott provide an elegant demonstration of how synapses can solve this problem and rebuild dendritic democracy using an anti-Hebbian spike-timing-dependent plasticity (anti-STDP) rule. The key to their method relies on the important distinction between "synaptic strength" and "synaptic efficacy." The first reflects the biophysical parameters of the synapse that are modified by the learning rule (e.g., postsynaptic conductance, presynaptic release probability), whereas the second measures the effect of the synapse on neuronal output. When a presynaptic spike precedes the postsynaptic spike, the synaptic strength is reduced according to the anti-STDP rule and vice versa. This makes the algorithm sensitive to the implicit synaptic efficacy: a proximal dendritic synapse that initially tends to increase the probability of the postsynaptic neuron to fire will be weakened because its presynaptic spike will tend to occur before the postsynaptic spikes, whereas a distal synapse will become stronger. When applied to dendritic neurons, anti-STDP therefore results in distance-dependent synaptic scaling, leading to equalization of synaptic efficacies (rather than equalization of somatic EPSP amplitudes).

While this is a simple and efficient solution to a potentially serious problem, Rumsey and Abbott point out that many questions remain. For example, how is anti-STDP implemented as a biophysical mechanism? How can anti-STDP coexist peacefully with (Hebbian) STDP mechanisms that strengthen correlated synaptic inputs and would therefore be expected to undo the work of anti-STDP? Probably a range of Hebbian and homeostatic mechanisms (Turrigiano and Nelson 2000Go) is operating in parallel, at different time scales, and synapses between different types of neurons may be governed by different STDP mechanisms. This may apply even to synapses at different dendritic locations on the same postsynaptic neuron because the long-term depression (LTD) part of the anti-STDP timing rule depends on retrograde signaling via the backpropagating action potential (bAP), which informs the dendritic synapses that the axon has fired (Stuart et al., 1997Go). However, backpropagation is decremental in most cell types (Stuart et al. 1997Go; Vetter et al. 2001Go; Waters et al. 2003Go), and the decreasing amplitude of the bAP is likely to influence the biophysical mechanisms underlying the timing rule (Sourdet and Debanne 1999Go). In fact, the amplitude and waveform of the bAP might be an alternative source of information from which synapses can read out their dendritic position. It will not be easy to validate experimentally that anti-STDP is responsible for distance-dependent synaptic scaling because it is difficult to measure synaptic efficacy and because the synaptic changes are probably happening slowly.

In the neocortex, synaptic connections between pairs of neurons are typically made by several individual synaptic contacts (Markram et al., 1997aGo). Because they share the same presynaptic axon, their activity is highly correlated, and it is their joint efficacy that would be equalized to that of other presynaptic axons in the model of Rumsey and Abbott. This kind of equalization is not observed experimentally, however, as the unitary somatic EPSP amplitude of synaptic connections between the same types of neurons varies over more than an order of magnitude (Markram et al., 1997aGo). This could be due to (Hebbian) STDP operating in parallel (Markram et al., 1997bGo), and of course due to different patterns of correlations between different presynaptic axons. Intriguingly, different synaptic contacts within a given connection between a pair of neurons, which share similar histories of pre- and postsynaptic activity, do show a normalization of synaptic strength (Koester et al., 2003Go). The work of Rumsey and Abbott should provide a stimulus for further experimental and theoretical research into STDP timing rules and location dependence of synaptic plasticity.

Arnd Roth and Michael London

Wolfson Institute for Biomedical Research and Department of Physiology, University College London, London WC1E 6BT, United Kingdom

Sono e homeostase sináptica

Blogando de Recife. Mais uma referência para meu trabalho com Juliana Dias:

Nature Reviews Neuroscience 9, 166-167 (March 2008) | doi:10.1038/nrn2348

Synaptic plasticity: Rise and shine (we are all morning people!)

Monica Hoyos Flight

Synaptic plasticityRise and shine (we are all morning people!)

The effects of sleep on learning and memory are well documented — many studies have shown a decline in the performance of memory tasks following sleep deprivation — yet the effects of sleep on synaptic plasticity are not well understood. The latest findings from Giulio Tononi's group, published in Nature Neuroscience, suggest that sleep performs a homeostatic function by resetting synaptic potentiation.

First the authors investigated changes in the activity of signalling proteins involved in synaptic potentiation that occur during the early stages of sleep and wakefulness, using synaptoneurosomes (preparations containing enriched pinched-off, resealed presynaptic structures attached to resealed postsynaptic processes) from rat cortex and hippocampus. They found a correlation between the levels and activity of these long-term potentiation (LTP)/long-term depression (LTD)-associated molecules and the amount of sleep the animals had experienced in the preceding 6 hours. A 50% increase in the levels of GluR1, an alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-receptor subunit that can be used as a molecular fingerprint of LTP, was observed in the cortical preparations from animals that had been awake for at least 4 hours during the 6-hour period (awake group) compared with those that had slept for at least 4 hours (sleep group). The levels of phosphorylated GluR1, which is associated with membrane incorporation, and of one of the kinases responsible (CAMKII) were also increased, further supporting the idea that glutamatergic synapses might be stronger after periods of wakefulness. In addition, in hippocampal preparations, increased levels of the NMDA (N-methyl-D-aspartate)-receptor subunit NR2A and of phosphorylated glycogen synthase kinase 3beta — proteins that have been shown to increase and decrease in concentration during hippocampal LTP and LTD, respectively — were also found in the awake group.

Next they examined electrophysiological correlates of LTP and LTD. Local field potentials (LFPs) have been shown to increase after LTP-inducing procedures in vivo, and the authors showed that after 1 hour of wakefulness the slope of LFPs evoked by electrical stimulation increased significantly, whereas it decreased after 2 hours of sleep. These effects were exacerbated after longer periods of wakefulness or sleep. Similarly, the amplitude of cortical evoked responses increased after wakefulness and decreased after sleep. These effects are not dependent on the time of day, as experiments with rats that had undergone enforced wakefulness yielded similar results.

Interestingly, it was easier to induce LTP in rats shortly after a period of sleep than after several hours of wakefulness, suggesting that prolonged wakefulness might saturate neuronal circuits and that sleep performs a restorative function by correcting the imbalance that occurs during wake time.

This study shows that both molecular and electrophysiological indicators of synaptic efficacy in the cortex and hippocampus change as a function of wake and sleep history. Whereas wakefulness favours synaptic potentiation, sleep appears to favour net synaptic depression. Although the effects await examination at the level of the synapse, these findings indicate that sleep has a homeostatic influence that is important for resetting neuronal excitability and facilitating plasticity in our waking hours.

sábado, fevereiro 16, 2008

Mudando de opinião

When thinking changes your mind, that's philosophy.

When God changes your mind, that's faith.

When facts change your mind, that's science.


Science is based on evidence. What happens when the data change? How have scientific findings or arguments changed your mind?"

165 contributors; 112,600 words

"The world's finest minds have responded with some of the most insightful, humbling, fascinating confessions and anecdotes, an intellectual treasure trove. ... Best three or four hours of intense, enlightening reading you can do for the new year. Read it now." — Mark Morford, San Francisco Chronicle
"As in the past, these world-class thinkers have responded to impossibly open-ended questions with erudition, imagination and clarity." — J. Peder Zane, The News & Observer
"A jolt of fresh thinking...The answers address a fabulous array of issues. This is the intellectual equivalent of a New Year's dip in the lake — bracing, possibly shriek-inducing, and bound to wake you up."— Margaret Wente, The Globe and Mail
"Answers ring like scientific odes to uncertainty, humility and doubt; passionate pleas for critical thought in a world threatened by blind convictions." — Sandro Contenta, The Toronto Star
"For an exceptionally high quotient of interesting ideas to words, this is hard to beat. ...What a feast of egg-head opinionating!" — John Derbyshire, National Review Online
"Even the world’s best brains have to admit to being wrong sometimes: here, leading scientists respond to a new year challenge." — Lewis Smith, The Times
"Provocative ideas put forward today by leading figures." — Roger Highfield, The Telegraph
"The splendidly enlightened Edge website ( has rounded off each year of inter-disciplinary debate by asking its heavy-hitting contributors to answer one question. I strongly recommend a visit."— Boyd Tonkin, The Independent
"A remarkable feast of the intellect... an amazing group of reflections on science, culture, and the evolution of ideas. Reading the Edge question is like being invited to dinner with some of the most interesting people on the planet." — Tim O'Reilly, O'Reilly Radar
"A great event in the Anglo-Saxon culture." — El Mundo
"As fascinating and weighty as one would imagine." — Comment (Leading Article), The Independent
"They are the intellectual elite, the brains the rest of us rely on to make sense of the universe and answer the big questions. But in a refreshing show of new year humility, the world's best thinkers have admitted that from time to time even they are forced to change their minds." — James Randerson, The Guardian

Últimos abismos

Como vocês sabem, eu tenho colocado aí na barra lateral todos os blogs de ciência em lingua portuguesa que tenho conhecimento, por ordem alfabética, sem classificar por constância, autoridade etc. Quem sabe, a partir daí, um futuro portal de blogs científicos brasileiros e/ou em lingua portuguesa possa ser organizado.
Pouco a pouco vou adicionando blogs que encontro aqui. Se você tem um blog de ciência, ou se seu link está quebrado, etc, coloque um comentário aqui com o novo link. Por enquanto, vai aqui uma auto-descrição do blog Últimos Abismos do jornalista científico Igor Zolnerkevik.

Do que trata este blog?

Os Últimos Abismos é meu sistema pessoal de organização de conteúdo, anotado e aumentado pelos comentários de vocês, leitores. É meu repositório para minhas reportagens publicadas aqui e ali; reportagens, resenhas, crônicas e ensaios escritos especialmente para o blog; idéias e observações súbitas sobre algum link interessante ou sobre algo no mundo lá fora.
Tá bom, mas este blog trata do que afinal?
Bem, minha especialidade são as ciências físicas (ciências que estudam as coisas sem vida, ou os organismos vivos de maneira indireta). Por isso, costumo acompanhar as novidades publicadas em periódicos científicos, tais como Nature, Science, Physical Review Letters, Astrophysical Journal Letters, Geophysical Review Letters, Icarus, Astrobiology, etc.
Meu outro interesse principal são os pesquisadores brasileiros. Nesse caso, escrevo sobre qualquer disciplina, qualquer pesquisa científica de qualidade feita no Brasil, ou por brasileiros no exterior, que eu tiver acesso em primeira mão.
Também sou um leitor compulsivo de tudo quanto é tipo de gênero. Além disso, trabalho para a Prefeitura de São Paulo como curador de uma futura biblioteca especialisada em divulgação científica. Vez ou outra, publicarei aqui uma resenha de livro.
Quem sou eu?
Formado em física pela
USP, com mestrado em relatividade geral pela UNESP, trabalhei como estagiário na Assessoria de Comunicação e Imprensa da UNESP, onde produzi um portal de notícias de física, o Universo Físico. Atualmente escrevo para jornais e revistas, e trabalho em um projeto de biblioteca pública de ciências.

quinta-feira, fevereiro 14, 2008



February 13, 2008, 9:25 pm
By Oliver Sacks

As a child, I was fascinated by patterns, starting with the patterns in our house — the square colored floor tiles we had on the porch, the tessellation of small pentagonal and hexagonal ones in the kitchen; the herringbone pattern on the curtains in my room, and the pattern on my father’s check suit. When I was taken to the synagogue for services, I was more interested in the mosaics of tiny tiles on the floor than in the religious liturgy. And I was fascinated by a pair of antique Chinese cabinets we had in our drawing room, for embossed on their lacquered surfaces were patterns of wonderful intricacy, patterns on different scales, patterns nested within patterns, all surrounded by clusters of tendrils and leaves.
These geometric and scrolling motifs seemed somehow familiar to me, though it did not dawn on my until years later that this was because I had seen them not only in my environment but in my own head, that these patterns resonated with my own inner experience of the intricate tilings and swirls of migraine.
Much later still, when I first saw photographs of the Alhambra, with its intricate geometric mosaics, I started to wonder whether what I had taken to be a personal experience and resonance might in fact be part of a larger whole, whether certain basic forms of geometric art, going back for tens of thousands of years, might also reflect the external expression of universal experiences. Migraine-like patterns, so to speak, are seen not only in Islamic art, but in classical and medieval motifs, in Zapotec architecture, in the bark paintings of Aboriginal artists in Australia, in Acoma pottery, in Swazi basketry — in virtually every culture. There seems to have been, throughout human history, a need to externalize, to make art from, these internal experiences, from the decorative motifs of prehistoric cave paintings to the psychedelic art of the 1960s. Do the arabesques in our own minds, built into our own brain organization, provide us with our first intimations of geometry, of formal beauty?
Whether or not this is the case, there is an increasing feeling among neuroscientists that self-organizing activity in vast populations of visual neurons is a prerequisite of visual perception — that this is how seeing begins. Spontaneous self-organization is not restricted to living systems — one may see it equally in the formation of snow crystals, in the roilings and eddies of turbulent water, in certain oscillating chemical reactions. Here, too, self-organization can produce geometries and patterns in space and time, very similar to what one may see in a migraine aura. In this sense, the geometrical hallucinations of migraine allow us to experience in ourselves not only a universal of neural functioning, but a universal of nature itself.
Leia o artigo aqui.

quarta-feira, fevereiro 13, 2008


Candido Portinari. Portrait of João Candido with Horse (1941)

Há algum tempo atrás, conheci por puro acaso Bill Hinchberger, jornalista responsável pelo site que contém informações e reportagens sobre o Brasil para estrangeiros. Batemos papo e tomamos uma bebidas no restaurante Viva México de Ribeirão Preto. Hoje ele me enviou um e-mail que imagino que ele gostaria que eu divulgasse. Aqui vai:

Foi um prazer conhecer você durante minha visita à Riberão Preto em dezembro. Queria reforçar meu convite para visitar o BrazilMax,, “the hip guide to Brazil” (o guia do Brasil para gringos espertos). Ganhador do prêmio “Best of the Web 2006” da revista Transitions Abroad e elogiado nos jornais The Guardian de Londres, SonntagsZeitung de Zurique, e O Dia do Rio de Janeiro, BrazilMax é um ponto de referência para estrangeiros que querem melhor entender o Brasil.

Temos também um programa de radio online:

Estou à disposição para conversar sobre possíveis parcerias. (Pretendo acrescentar conteúdo sobre Riberão Preto em breve.) Obrigado pela atenção.

Bill Hinchberger

Abaixo coloco uma das reportagens do BrazilMax que une os temas ciência, arte, e minha região (Brodósqui, perto de Ribeirão Preto):

by Bill Hinchberger

Rio de Janeiro - Visiting the celebrated Brazilian artist Candido Portinari on the eve of his death in 1962, the Italian art critic Eugenio Luraghi noted a remarkable shift in the outlook of his distinguished friend. “Portinari told me how much he admired scientific progress and the most recent discoveries in astronomy, physics, robotics, computers,” wrote Luraghi. “He was delighted with the progress of (his son) João in his difficult studies at the Sorbonne and expressed his suspicion that science had supplanted art, given that the most intelligent people were being absorbed by research and scientific discovery. Thus the evolution of painting, where everything had already been done, was coming to an end, without a future.”
As if to confirm that deathbed prophecy, the scientific method permeates the spirit of the non-profit organization dedicated to preserving the Portinari legacy. Founded in 1979 by the painter’s son and former math professor João Candido Portinari, the Portinari Project sits in some deceptively antiquated digs in the basement of an old manor on the campus of the Catholic University of Rio de Janeiro (PUC-Rio). The project can count itself among the frontrunners in international efforts to apply the developments in information technology to the preservation and dissemination of humanity’s artistic and cultural heritage.
Who says so? IBM, for instance. The US multinational tabbed the Portinari Project, along with the Vatican, to showcase its new Digital Library Technology, designed to effectively catalogue and cross-reference images, text, audio and multimedia content. Plus the University of California, Berkeley, and Cambridge University – two among the dozens of institutions that have hosted talks by Prof. Portinari. The Portinari Project uses cutting edge Israeli fine art digital printing technology to produce “replicas” for traveling shows that seem worthy, at a distance, of Hans van Meegeren, the famed Vermeer forger. Speaking of forgeries, the Portinari Project has been co-sponsoring unprecedented research into the use of artificial intelligence to detect forgeries at the PUC-Rio campus in Rio de Janeiro’s southern district Gávea. (Para continuar a ler, clique aqui).

terça-feira, fevereiro 12, 2008

BlogPulse indica que Obama ganha de Clinton?

Talvez... Bom, na última eleição, BlogPulse tinha esse padrão na relação Lula-Alckmin.
No caso do Technorati temos o mesmo padrão (na verdade, Obama recebe atenção duas vezes maior...)

Posts that contain Barack Obama per day for the last 30 days.
Technorati Chart
Get your own chart!

Posts that contain Hillary Clinton per day for the last 30 days.
Technorati Chart
Get your own chart!

segunda-feira, fevereiro 11, 2008

Porque d = 3?

Há alguns posts atrás eu conjecturei sobre a possibilidade de que a não trivialidade de grande parte dos modelos de Física Estatística em d = 3 tivesse algo a ver com o fato de vivermos em um universo complexo também com d = 3. Este artigo abaixo vai na mesma direção.

Authors: Pierre-Henri Chavanis
(Submitted on 14 Aug 2007)
Abstract: We discuss the statistical mechanics of a system of self-gravitating fermions in a space of dimension $D$. We plot the caloric curves of the self-gravitating Fermi gas giving the temperature as a function of energy and investigate the nature of phase transitions as a function of the dimension of space. We consider stable states (global entropy maxima) as well as metastable states (local entropy maxima). We show that for $D\ge 4$, there exists a critical temperature (for sufficiently large systems) and a critical energy below which the system cannot be found in statistical equilibrium. Therefore, for $D\ge 4$, quantum mechanics cannot stabilize matter against gravitational collapse. This is similar to a result found by Ehrenfest (1917) at the atomic level for Coulombian forces. This makes the dimension D=3 of our universe very particular with possible implications regarding the anthropic principle. Our study enters in a long tradition of scientific and philosophical papers who studied how the dimension of space affects the laws of physics.
Statistical Mechanics (cond-mat.stat-mech)
Journal reference:
Phys. Rev. E, 69, 066126 (2004)
Cite as:
arXiv:0708.1888v1 [cond-mat.stat-mech]

Estatística sem matemática?

quinta-feira, fevereiro 07, 2008


Localizado mais um blog sobre neurociência computacional: Neurobot.

Welcome to Neurobot, a neuroscience and bioengineering blog Neurobot is a weblog system in the fields of neuroscience and bioengineering, held in the Laboratory of Animal Physiology, Aristotle University, Greece.
Neurobot's suggested reading is available through Neurobot's Corner

O SISNE (Laboratório de Sistemas Neurais) do DFM-FFCLRP-USP também tem o seu blog, que anda um pouco desatualizado. Será que não existe algum estudante que gostaria de mantê-lo em dia?

Andróides sonham com ovelhas elétricas?

Para referência, da Wikipédia: Freud suggested that bad dreams let the brain learn to gain control over emotions resulting from distressing experiences.[14]

Saiu a edição final de Blade Runner. Ainda não vi como ficou a sequência do sonho de Dekard.

Juliana Dias e eu estamos reescrevendo o paper sobre sonhos, a hipótese Crick-Mitchison e os endocannabinóides. Iremos submeter algo disso para o encontro
Universidade Federal do Maranhão
June 24-27, 2008 • São Luís • Brazil

Por enquanto, achei um índice daquela edição especial sobre sonhos da Revista Mente e Cérebro. Nela tem um artigo de divulgação sobre minha abordagem que propõe itinerância caótica entre atratores profundos como sendo o equivalente dinâmico da narrativa onírica. Você pode comprar a edição aqui.