Neurophysiological investigation of the basis of the fMRI signal.

Functional magnetic resonance imaging (fMRI) is widely used to study the operational organization of the human brain, but the exact relationship between the measured fMRI signal and the underlying neural activity is unclear. Here we present simultaneous intracortical recordings of neural signals and fMRI responses. We compared local field potentials (LFPs), single- and multi-unit spiking activity with highly spatio-temporally resolved blood-oxygen-level-dependent (BOLD) fMRI responses from the visual cortex of monkeys. The largest magnitude changes were observed in LFPs, which at recording sites characterized by transient responses were the only signal that significantly correlated with the haemodynamic response. Linear systems analysis on a trial-by-trial basis showed that the impulse response of the neurovascular system is both animal- and site-specific, and that LFPs yield a better estimate of BOLD responses than the multi-unit responses. These findings suggest that the BOLD contrast mechanism reflects the input and intracortical processing of a given area rather than its spiking output.     

Sustained division of the attentional spotlight

By voluntarily directing attention to a specific region of a visual scene, we can improve our perception of stimuli at that location. This ability to focus attention upon specific zones of the visual field has been described metaphorically as a moveable spotlight or zoom lens that facilitates the processing of stimuli within its 'beam'. A long-standing controversy has centred on the question of whether the spotlight of spatial attention has a unitary beam or whether it can be divided flexibly to disparate locations. Evidence supporting the unitary spotlight view has come from numerous behavioural and electrophysiological studies. Recent experiments, however, indicate that the spotlight of spatial attention may be divided between non-contiguous zones of the visual field for very brief stimulus exposures (&<100 ms). Here we use an electrophysiological measure of attentional allocation (the steady-state visual evoked potential) to show that the spotlight may be divided between spatially separated locations (excluding interposed locations) over more extended time periods. This spotlight division appears to be accomplished at an early stage of visual-cortical processing.     

Hysteresis in the perception of motion direction as evidence for neural cooperativity

When elements of a parallel network, such as the human brain, are extensively interconnected, the network can exhibit 'cooperative behaviour'. Such behaviour, which is characterized by order-disorder transitions, multi-stable states, and a form of memory called 'hysteresis', has been observed in human stereopsis and has motivated models of stereopsis that incorporate cooperative networks. More recently, cooperative phenomena have also been observed in human visual motion perception. This report strongly supports a cooperative interpretation of motion perception by demonstrating hysteresis in the perception of motion direction. The results agree quantitatively with a mathematical model incorporating nonlinear excitatory and inhibitory interactions among direction-selective elements.     

Columns for visual features of objects in monkey inferotemporal cortex.

At early stages of the mammalian visual cortex, neurons with similar stimulus selectivities are vertically arrayed through the thickness of the cortical sheet and clustered in patches or bands across the surface. This organization, referred to as a 'column', has been found with respect to one-dimensional stimulus parameters such as orientation of stimulus contours, eye dominance of visual inputs, and direction of stimulus motion. It is unclear, however, whether information with extremely high dimensions, such as visual shape, is organized in a similar columnar fashion or in a different manner in the brain. Here we report that the anterior inferotemporal area of the monkey cortex, the final station of the visual cortical stream crucial for object recognition, consists of columns, each containing cells responsive to similar visual features of objects.     

Learning the parts of objects by non-negative matrix factorization.

Is perception of the whole based on perception of its parts? There is psychological and physiological evidence for parts-based representations in the brain, and certain computational theories of object recognition rely on such representations. But little is known about how brains or computers might learn the parts of objects. Here we demonstrate an algorithm for non-negative matrix factorization that is able to learn parts of faces and semantic features of text. This is in contrast to other methods, such as principal components analysis and vector quantization, that learn holistic, not parts-based, representations. Non-negative matrix factorization is distinguished from the other methods by its use of non-negativity constraints. These constraints lead to a parts-based representation because they allow only additive, not subtractive, combinations. When non-negative matrix factorization is implemented as a neural network, parts-based representations emerge by virtue of two properties: the firing rates of neurons are never negative and synaptic strengths do not change sign.     

Discovery of water vapour around IRC+10216 as evidence for comets orbiting another star.

Since 1995, planets with masses comparable to that of Jupiter have been discovered around approximately 60 stars. These planets have not been seen directly, but their presence has been inferred from the small reflex motions that they gravitationally induce on the star they orbit; these motions result in small periodic wavelength shifts in the stellar spectrum. The presence of analogues of the smaller bodies in our Solar System cannot, however, be determined using this technique, because the induced reflex motions are too small-so an alternative approach is needed. Here we report the observation of circumstellar water vapour around the ageing carbon star IRC+10216; water is not expected in measurable quantities around such a star. The only plausible explanation for this water is that the recent evolution of IRC+10216, which has been accompanied by a prodigious increase in its luminosity, is causing the vaporization of a collection of orbiting icy bodies-a process considered in an earlier theoretical study.     

Further molecular evidence for the Hordeum vulgare ssp. spontaneum in Tibet as ultimate progenitor of Chinese cultivated barley

106 aeeessions of Tibetan wild barley, including 50 accessions of the two-rowed wiht barley Hordeum vulgare ssp. sopntaneum （HS）, 27 accessions of the six-rowed bottle-shaped wild barley H. lagunculiforme （HL） and 29 accessions of the six-rowed wiht barley H. agriocrithon （HA） that separately represent different agrigeographical regions of Tibet, were used to study the genetic diversity and genetie differentiation using SSR markers selected from seven barley linkage groups. 229 allelic variants were identified with an average of 7.6 alleles/locus. The average of total number of alleles per locus in HA （6.4） is much higher than that in HS （3.9） and HL （3.4）. The genetie diversity and its standard deviation among the three subspecies were in the order of HS〉HL〉HA. Very significant genetic differentiation was observed among the three subspecies of wild barley. Comparisons of the results fiom this and previous studies showed a strong Oriental-Occidental differentiation of barley, and that Shannan region of Tibet might be the center of origin of the Tibetan two-rowed wild barley, thus supporting not only the hypothesis of a mono-phyletie origin of cultivated barley but also the proposition that the Tibetan two-rowed wild barley as ultimate progenitor of Chinese cultivated barley.     

面向移动对象范围的混合索引结构

许多时空应用(如火灾模拟等)需要高效地查询移动对象的变化范围,针对此需求提出了基于TPR-tree和GF索引方法的两种混合索引结构,以支持对移动对象当前和未来范围的预测时空查询.在代价模型分析的基础上,基于模拟数据集的实验结果表明,这种混合索引方法能够有效地支持对移动对象变化范围的预测查询.     

Stellar encounters as the origin of distant Solar System objects in highly eccentric orbits

The Kuiper belt extends from the orbit of Neptune at 30 au to an abrupt outer edge about 50 au from the Sun. Beyond the edge is a sparse population of objects with large orbital eccentricities. Neptune shapes the dynamics of most Kuiper belt objects, but the recently discovered planet 2003 VB12 (Sedna) has an eccentric orbit with a perihelion distance of 70 au, far beyond Neptune's gravitational influence. Although influences from passing stars could have created the Kuiper belt's outer edge and could have scattered objects into large, eccentric orbits, no model currently explains the properties of Sedna. Here we show that a passing star probably scattered Sedna from the Kuiper belt into its observed orbit. The likelihood that a planet at 60-80 au can be scattered into Sedna's orbit is about 50 per cent; this estimate depends critically on the geometry of the fly-by. Even more interesting is the approximately 10 per cent chance that Sedna was captured from the outer disk of the passing star. Most captures have very high inclination orbits; detection of such objects would confirm the presence of extrasolar planets in our own Solar System.     

D-S证据理论在小电流故障选线中的应用研究

介绍了在不同的情况下选取适当的故障选线算法来处理故障信息,并对故障信息进行D—S证据理论的信息融合,提高了故障选线的准确率．指出如何将选取的群体比幅比相算法和小波分析方法进行改进以适合证据理论的融合,如何设计算法计算D—S证据理论中的不确定性问题．     

用于非金属目标非接触位置检测的单片式电容传感器

利用印刷电路板加工工艺研制成了一种适于非金属目标位置检测的单片式电容传感器,工作原理是金属电极的近场效应。研究了传感器的阻抗频率特性以及传感器工作频率与检测系统灵敏度的关系,得到了使检测系统灵敏度最大的工作频率(1MHz)。用该传感器对36g/m2的胶片和10mm厚的玻璃体两种非金属目标位置进行检测。结果表明:在0.1mm量程内,检测系统的线性较好;在0.2mm量程内,非线性也不超过16.5%。     

视频编码中Lagrange乘子自适应调整算法

为改进视频编码算法的率失真性能,提出了一种应用于视频编码率失真优化算法中的Lagrange乘子自适应调整算法。该算法以信源数据的统计分布为理论基础,将视频编码序列的变换系数方差引入到Lagrange乘子的计算过程中来,使得Lagrange乘子的取值可以根据图像内容变化特性进行自适应的调整。实验结果证明,与传统的Lagrange乘子的取值算法相比,该算法可以取得更好的率失真性能,尤其应用在低码率的视频序列中效果更为明显,同等码率下可以将峰值信噪比提高0.1～0.2 dB。     

遗传算法-偏最小二乘法用于卵巢癌血清蛋白质组数据的特征挑选

统计学t检验结合引入的变量筛选方法——遗传算法-偏最小二乘法（GAPLS）对卵巢癌SELDI-TOF MS数据进行特征筛选,从15154个原始变量中筛选得到4个特征质荷比值,采用支持向量机（SVM）模型的留一法交叉验证结果为95.26%.结果表明这4个质荷比值具有重要的生物学意义,它们或许可以作为卵巢癌的生物标记物,同时GAPLS可以作为一种有效的蛋白质组数据的特征筛选方法.