A new perceptual illusion reveals mechanisms of sensory decoding

Perceptual illusions are usually thought to arise from the way sensory signals are encoded by the brain, and indeed are often used to infer the mechanisms of sensory encoding. But perceptual illusions might also result from the way the brain decodes sensory information, reflecting the strategies that optimize performance in particular tasks. In a fine discrimination task, the most accurate information comes from neurons tuned away from the discrimination boundary, and observers seem to use signals from these 'displaced' neurons to optimize their performance. We wondered whether using signals from these neurons might also bias perception. In a fine direction discrimination task using moving random-dot stimuli, we found that observers' perception of the direction of motion is indeed biased away from the boundary. This misperception can be accurately described by a decoding model that preferentially weights signals from neurons whose responses best discriminate those directions. In a coarse discrimination task, to which a different decoding rule applies, the same stimulus is not misperceived, suggesting that the illusion is a direct consequence of the decoding strategy that observers use to make fine perceptual judgments. The subjective experience of motion is therefore not mediated directly by the responses of sensory neurons, but is only developed after the responses of these neurons are decoded.     

Genetic tracing reveals a stereotyped sensory map in the olfactory cortex.

The olfactory system translates myriad chemical structures into diverse odour perceptions. To gain insight into how this is accomplished, we prepared mice that coexpressed a transneuronal tracer with only one of about 1,000 different odorant receptors. The tracer travelled from nasal neurons expressing that receptor to the olfactory bulb and then to the olfactory cortex, allowing visualization of cortical neurons that receive input from a particular odorant receptor. These studies revealed a stereotyped sensory map in the olfactory cortex in which signals from a particular receptor are targeted to specific clusters of neurons. Inputs from different receptors overlap spatially and could be combined in single neurons, potentially allowing for an integration of the components of an odorant's combinatorial receptor code. Signals from the same receptor are targeted to multiple olfactory cortical areas, permitting the parallel, and perhaps differential, processing of inputs from a single receptor before delivery to the neocortex and limbic system.     

A Jurassic mammal from South America

The Jurassic period is an important stage in early mammalian evolution, as it saw the first diversification of this group, leading to the stem lineages of monotremes and modern therian mammals. However, the fossil record of Jurassic mammals is extremely poor, particularly in the southern continents. Jurassic mammals from Gondwanaland are so far only known from Tanzania and Madagascar, and from trackway evidence from Argentina. Here we report a Jurassic mammal represented by a dentary, which is the first, to our knowledge, from South America. The tiny fossil from the Middle to Late Jurassic of Patagonia is a representative of the recently termed Australosphenida, a group of mammals from Gondwanaland that evolved tribosphenic molars convergently to the Northern Hemisphere Tribosphenida, and probably gave rise to the monotremes. Together with other mammalian evidence from the Southern Hemisphere, the discovery of this new mammal indicates that the Australosphenida had diversified and were widespread in Gondwanaland well before the end of the Jurassic, and that mammalian faunas from the Southern Hemisphere already showed a marked distinction from their northern counterparts by the Middle to Late Jurassic.     

Rapid degradation of a large fraction of newly synthesized proteins by proteasomes

MHC class I molecules function to present peptides eight to ten residues long to the immune system. These peptides originate primarily from a cytosolic pool of proteins through the actions of proteasomes, and are transported into the endoplasmic reticulum, where they assemble with nascent class I molecules. Most peptides are generated from proteins that are apparently metabolically stable. To explain this, we previously proposed that peptides arise from proteasomal degradation of defective ribosomal products (DRiPs). DRiPs are polypeptides that never attain native structure owing to errors in translation or post-translational processes necessary for proper protein folding. Here we show, first, that DRiPs constitute upwards of 30% of newly synthesized proteins as determined in a variety of cell types; second, that at least some DRiPs represent ubiquitinated proteins; and last, that ubiquitinated DRiPs are formed from human immunodeficiency virus Gag polyprotein, a long-lived viral protein that serves as a source of antigenic peptides.     

Evidence that three structural genes code for human alkaline phosphatases.

The number of structural gene loci that code for the different molecular forms of human alkaline phosphatase is unknown. Physical properties of the enzymes, immunological data, chemical inhibition and genetic studies suggest that at least three structural genes are involved: one coding for alkaline phosphatase from placenta, another for the enzyme from intestine, and one or more for the enzymes from liver, kidney and bone. Badger and Sussman have shown that alkaline phosphatases from human liver and placenta are products of different structural genes, and Greene and Sussman have shown that alkaline phosphatase from a metastasised bronchogenic carcinoma was nearly identical to the enzyme from placenta. However, other tumour-associated alkaline phosphatases and the enzymes from normal tissue other than placenta and liver have not been identified by conclusive structural criteria, and thus it is not known whether these onco-alkaline phosphatases represent ectopic production or unusual post-translational modification of the enzymes found in normal tissues. We present here, using a sensitive peptide-mapping technique, structural evidence that the enzyme forms from liver, kidney and serum from a patient with Paget's disease of bone (osteitis deformans) are products of the same structural gene and can be easily distinguished from either the intestinal or placental isoenzymes. The technqiue seems to be useful for the classification of tumour-associated alkaline phosphatases on a structural basis.     

Monoclonal mice generated by nuclear transfer from mature B and T donor cells

Cloning from somatic cells is inefficient, with most clones dying during gestation. Cloning from embryonic stem (ES) cells is much more effective, suggesting that the nucleus of an embryonic cell is easier to reprogram. It is thus possible that most surviving clones are, in fact, derived from the nuclei of rare somatic stem cells present in adult tissues, rather than from the nuclei of differentiated cells, as has been assumed. Here we report the generation of monoclonal mice by nuclear transfer from mature lymphocytes. In a modified two-step cloning procedure, we established ES cells from cloned blastocysts and injected them into tetraploid blastocysts to generate mice. In this approach, the embryo is derived from the ES cells and the extra-embryonic tissues from the tetraploid host. Animals cloned from a B-cell nucleus were viable and carried fully rearranged immunoglobulin alleles in all tissues. Similarly, a mouse cloned from a T-cell nucleus carried rearranged T-cell-receptor genes in all tissues. This is an unequivocal demonstration that a terminally differentiated cell can be reprogrammed to produce an adult cloned animal.     

论中国古代小说“实-虚”观念的变迁与快节奏叙述的嬗变

中国古代小说“快节奏叙述”的地位有一个明显的由主到次的转变过程。该过程与小说家的创作意识由“崇实”到“纪虚”，修饰意识由“尚质”到“尚文”的嬗变过程相一致，它们是小说家的观念形态和实践创作从幼稚走向成熟的重要证据。     

试论圆锯的振动、噪音及减少噪音的途径

<正>引 言 圆锯作为一种主要的切割工具被广泛地应用于胶合板、刨花板、纤维板、制材、细木工、家具等各个木材加工企业。据估计,目前世界上使用各种类型圆锯的机床和设备,其总数达数百万。这些圆锯在运转中所发射的噪音,一般声压级都在90分贝(A)以上,有时甚至超过100分贝(A)。 噪音对于人体的健康有着不良影响。在90分贝(A)以上或略低于90分贝(A)的较强的噪音环境中长时间工作,人们都会感到刺耳难受,导致听觉逐渐迟钝,生理机能失调,甚至耳聋,以及人体神经系统、内分泌系统、心血管系统的某些疾病。     

基于输运模型的网络相变过程

 网络相变过程需重点关注网络上的总负荷数、网络上的消失负荷数、节点的排队负荷数等指标随单位时间进入网络的负荷数R 的变化规律。为此建立了无标度网络上的输运模型,用于定量计算这3 种负荷数的变化规律。仿真结果表明：最大介数节点最先产生拥堵,导致网络的进入和消失负荷数出现不平衡,进而导致网络进入拥堵状态;当R小于临界值R_c时,网络上的消失负荷数随R同步增长。当R超过临界值R_c时,消失负荷数与R的比值持续下降,表明随着R的增加,负荷到达目的地越来越困难。     

A pulsating auroral X-ray hot spot on Jupiter

Jupiter's X-ray aurora has been thought to be excited by energetic sulphur and oxygen ions precipitating from the inner magnetosphere into the planet's polar regions. Here we report high-spatial-resolution observations that demonstrate that most of Jupiter's northern auroral X-rays come from a 'hot spot' located significantly poleward of the latitudes connected to the inner magnetosphere. The hot spot seems to be fixed in magnetic latitude and longitude and occurs in a region where anomalous infrared and ultraviolet emissions have also been observed. We infer from the data that the particles that excite the aurora originate in the outer magnetosphere. The hot spot X-rays pulsate with an approximately 45-min period, a period similar to that reported for high-latitude radio and energetic electron bursts observed by near-Jupiter spacecraft. These results invalidate the idea that jovian auroral X-ray emissions are mainly excited by steady precipitation of energetic heavy ions from the inner magnetosphere. Instead, the X-rays seem to result from currently unexplained processes in the outer magnetosphere that produce highly localized and highly variable emissions over an extremely wide range of wavelengths.     

Recovery rates reflect distance to a tipping point in a living system

Tipping points, at which complex systems can shift abruptly from one state to another, are notoriously difficult to predict. Theory proposes that early warning signals may be based on the phenomenon that recovery rates from small perturbations should tend to zero when approaching a tipping point; however, evidence that this happens in living systems is lacking. Here we test such 'critical slowing down' using a microcosm in which photo-inhibition drives a cyanobacterial population to a classical tipping point when a critical light level is exceeded. We show that over a large range of conditions, recovery from small perturbations becomes slower as the system comes closer to the critical point. In addition, autocorrelation in the subtle fluctuations of the system's state rose towards the tipping point, supporting the idea that this metric can be used as an indirect indicator of slowing down. Although stochasticity prohibits prediction of the timing of critical transitions, our results suggest that indicators of slowing down may be used to rank complex systems on a broad scale from resilient to fragile.     

Multiple evolutionary origins of prochlorophytes within the cyanobacterial radiation.

The taxonomic group Prochlorales (Lewin 1977) Burger-Wiersma, Stal and Mur 1989 was established to accommodate a set of prokaryotic oxygenic phototrophs which, like plant, green algal and euglenoid chloroplasts, contain chlorophyll b instead of phycobiliproteins. Prochlorophytes were originally proposed (with concomitant scepticism) to be a monophyletic group sharing a common ancestry with these 'green' chloroplasts. Results from molecular sequence phylogenies, however, have suggested that Prochlorothrix hollandica is not on a lineage that leads to plastids. Our results from 16S ribosomal RNA sequence comparisons, which include new sequences from the marine picoplankter Prochlorococcus marinus and the Lissoclinum patella symbiont Prochloron sp., indicate that prochlorophytes are polyphyletic within the cyanobacterial radiation, and suggest that none of the known species is specifically related to chloroplasts. This implies that the three prochlorophytes and the green chloroplast ancestor acquired chlorophyll b and its associated structural proteins in convergent evolutionary events. We report further that the 16S rRNA gene sequence from Prochlorococcus is very similar to those of open ocean Synechococcus strains (marine cluster A), and to a family of 16S rRNA genes shotgun-cloned from plankton in the north Atlantic and Pacific Oceans.     

Genetic history of an archaic hominin group from Denisova Cave in Siberia

Using DNA extracted from a finger bone found in Denisova Cave in southern Siberia, we have sequenced the genome of an archaic hominin to about 1.9-fold coverage. This individual is from a group that shares a common origin with Neanderthals. This population was not involved in the putative gene flow from Neanderthals into Eurasians; however, the data suggest that it contributed 4-6% of its genetic material to the genomes of present-day Melanesians. We designate this hominin population 'Denisovans' and suggest that it may have been widespread in Asia during the Late Pleistocene epoch. A tooth found in Denisova Cave carries a mitochondrial genome highly similar to that of the finger bone. This tooth shares no derived morphological features with Neanderthals or modern humans, further indicating that Denisovans have an evolutionary history distinct from Neanderthals and modern humans.     

A host parasite interaction rescues Drosophila oogenesis defects

The cytoplasmically inherited bacterium Wolbachia pipientis is a widespread parasite of arthropods that manipulates the reproductive biology of its hosts, often to their detriment, in order to foster its own transmission through egg cytoplasm. Here we report that infection by Wolbachia restores fertility to Drosophila melanogaster mutant females prevented from making eggs by protein-coding lesions in Sex-lethal (Sxl), the master regulator of sex determination. Suppression of sterility by Wolbachia discriminates markedly among similar germline-specific Sxl alleles, and is not observed for mutations in other genes that produce similar 'tumorous ovary' phenotypes, including one that blocks Sxl germline expression. This allele and gene specificity indicates that suppression probably results from a specific interaction with Sxl protein, rather than from a bypass of the normal germline requirement for this developmental regulator or from an effect on Sxl expression. The Sxl-Wolbachia interaction provides a rare opportunity to explore host-parasite relationships at the molecular level in a model insect. Furthermore, demonstration that a parasite infection can counteract the deleterious effects of mutations in host genes illustrates how hosts might become dependent on parasites.     

The sources of sodium escaping from Io revealed by spectral high definition imaging

On Jupiter's moon Io, volcanic plumes and evaporating lava flows provide hot gases to form an atmosphere that is subsequently ionized. Some of Io's plasma is captured by the planet's strong magnetic field to form a co-rotating torus at Io's distance; the remaining ions and electrons form Io's ionosphere. The torus and ionosphere are also depleted by three time-variable processes that produce a banana-shaped cloud orbiting with Io, a giant nebula extending out to about 500 Jupiter radii, and a jet close to Io. No spatial constraints exist for the sources of the first two; they have been inferred only from modelling the patterns seen in the trace gas sodium observed far from Io. Here we report observations that reveal a spatially confined stream that ejects sodium only from the wake of the Io-torus interaction, together with a visually distinct, spherically symmetrical outflow region arising from atmospheric sputtering. The spatial extent of the ionospheric wake that feeds the stream is more than twice that observed by the Galileo spacecraft and modelled successfully. This implies considerable variability, and therefore the need for additional modelling of volcanically-driven, episodic states of the great jovian nebula.     

Demographic study of a wild house sparrow population by DNA fingerprinting

Over the past twenty years, several techniques from biochemical and molecular genetics, such as enzyme electrophoresis and isoelectric focusing, have been widely and successfully applied to the study of population differentiation and evolution. However, they have been less applicable to demographic problems such as assigning parentage to individuals within a population. This stems from a general weakness of data derived from enzyme loci: allele frequencies at polymorphic loci are sufficiently skewed that the majority of individuals are of one or two genotypes. Many enzyme systems can only be examined post mortem, so that the loci are of little use if the animals are to be studied in the wild. The search for new and more sensitive techniques for detecting genetic variation has continued, and recently a major discovery has come from molecular biology. Jeffreys et al. have reported the detection of a type of hypervariable 'minisatellite' DNA that is extraordinarily polymorphic in human populations. We have applied their technique to several bird species and particularly to a population of house sparrows (Passer domesticus) near Nottingham. We report here that one of the human minisatellite clones is a suitable probe for sparrow DNA and that it reveals variation as extensive as that found in man. These results suggest that analysis of minisatellite DNA will be a powerful tool in the study of demographic population genetics.     

Opening of dihydropyridine calcium channels in skeletal muscle membranes by inositol trisphosphate

In many non-muscle cells, D-inositol 1,4,5-trisphosphate (InsP3) has been shown to release Ca2+ from intracellular stores, presumably from the endoplasmic reticulum. It is thought to be a ubiquitous second messenger that is produced in, and released from, the plasma membrane in response to extracellular receptor stimulation. By analogy, InsP3 in muscle cells has been postulated to open calcium channels in the sarcoplasmic reticulum (SR) membrane, which is the intracellular Ca2+ store that releases Ca2+ during muscle contraction. We report here that InsP3 may have a second site of action. We show that InsP3 opens dihydropyridine-sensitive Ca2+ channels in a vesicular preparation of rabbit skeletal muscle transverse tubules. InsP3-activated channels and channels activated by a dihydropyridine agonist in the same preparation have similar slope conductance and extrapolated reversal potential and are blocked by a dihydropyridine antagonist. This suggests that in skeletal muscle, InsP3 can modulate Ca2+ channels of transverse tubules from plasma membrane, in contrast to the previous suggestion that the functional locus of InsP3 is exclusively in the sarcoplasmic reticulum membrane.     

混凝土双参数断裂模型和尺寸效应模型R阻力曲线比较

对混凝土的两类基本断裂模型(双参数断裂模型和尺寸效应模型)求解得到的第Ⅰ类几何形式下的R阻力曲线进行了详细的分析比较.首先对有效裂纹长度和裂纹尖端张口位移进行了修正,在此基础上由双参数断裂模型和尺寸效应模型得到了R阻力曲线的解析表达式.通过两类R阻力曲线计算得到的荷载-位移曲线与试验结果的对比,发现尺寸效应模型得到的临界有效裂纹长度稍大于双参数断裂模型的结果,而根据双参数断裂模型R阻力曲线得到的最大荷载大于尺寸效应模型的结果,且当相对初始裂纹长度较大时,双参数断裂模型R阻力曲线得到的最大荷载远大于试验值.双参数断裂模型的R阻力曲线适合于初始裂纹长度相对试件高度较小的情况.     

beta-Carboline-3-carboxylic acid ethyl ester antagonizes diazepam activity

Analogous to the progression of events in the opiate receptor-enkaphalin area, the first reports that benzodiazepines have selective and specific high-affinity binding sites in brain have stimulated a search for the endogenous 'ligand' or substance that might normally act at these sites. Braestrup and co-workers have extracted from human urine a gamma-fraction (ref. 10) which they have recently identified as beta-carboline-3-carboxylic acid ethyl ester (beta CEE). They reported that this substance is extremely potent in displacing 3H-diazepam from brain binding sites and proposed that a beta-carboline-3-carboxylic acid derivative might, in part, be the endogenous ligand for the brain benzodiazepine receptor. We have examined several synthetically derived beta-carboline-3-carboxylic acid analogues and now present data obtained from testing only the beta CEE described by Braestrup et al. In addition to confirming these workers' observation that this compound is a potent displacer of 3H-diazepam from brain tissue, our pharmacological data indicate that beta CEE has activity that is opposite to, rather than similar to, that of diazepam.     

Increased terrestrial methane cycling at the Palaeocene-Eocene thermal maximum

The Palaeocene-Eocene thermal maximum (PETM), a period of intense, global warming about 55 million years ago, has been attributed to a rapid rise in greenhouse gas levels, with dissociation of methane hydrates being the most commonly invoked explanation. It has been suggested previously that high-latitude methane emissions from terrestrial environments could have enhanced the warming effect, but direct evidence for an increased methane flux from wetlands is lacking. The Cobham Lignite, a recently characterized expanded lacustrine/mire deposit in England, spans the onset of the PETM and therefore provides an opportunity to examine the biogeochemical response of wetland-type ecosystems at that time. Here we report the occurrence of hopanoids, biomarkers derived from bacteria, in the mire sediments from Cobham. We measure a decrease in the carbon isotope values of the hopanoids at the onset of the PETM interval, which suggests an increase in the methanotroph population. We propose that this reflects an increase in methane production potentially driven by changes to a warmer and wetter climate. Our data suggest that the release of methane from the terrestrial biosphere increased and possibly acted as a positive feedback mechanism to global warming.