Efficiency and ambiguity in an adaptive neural code

We examine the dynamics of a neural code in the context of stimuli whose statistical properties are themselves evolving dynamically. Adaptation to these statistics occurs over a wide range of timescales-from tens of milliseconds to minutes. Rapid components of adaptation serve to optimize the information that action potentials carry about rapid stimulus variations within the local statistical ensemble, while changes in the rate and statistics of action-potential firing encode information about the ensemble itself, thus resolving potential ambiguities. The speed with which information is optimized and ambiguities are resolved approaches the physical limit imposed by statistical sampling and noise.     

Real and optimal neural images in early vision.

It has been suggested that the first steps in visual processing strive to compress as much information as possible about the outside world into the limited dynamic range of the visual channels. Here I compare measured neural images with theoretical calculations based on maximizing information, taking into account the statistical structure of natural images. Neural images were obtained by scanning an image while recording from a second-order neuron in the fly visual system. Over a 5.5-log-units-wide range of mean intensities, experiment and theory correspond well. At high mean intensities, redundancy in the image is reduced by spatial and temporal antagonism. At low mean intensities, spatial and temporal low-pass filtering combat noise and increase signal reliability.     

An ultra-sparse code underlies the generation of neural sequences in a songbird

Sequences of motor activity are encoded in many vertebrate brains by complex spatio-temporal patterns of neural activity; however, the neural circuit mechanisms underlying the generation of these pre-motor patterns are poorly understood. In songbirds, one prominent site of pre-motor activity is the forebrain robust nucleus of the archistriatum (RA), which generates stereotyped sequences of spike bursts during song and recapitulates these sequences during sleep. We show that the stereotyped sequences in RA are driven from nucleus HVC (high vocal centre), the principal pre-motor input to RA. Recordings of identified HVC neurons in sleeping and singing birds show that individual HVC neurons projecting onto RA neurons produce bursts sparsely, at a single, precise time during the RA sequence. These HVC neurons burst sequentially with respect to one another. We suggest that at each time in the RA sequence, the ensemble of active RA neurons is driven by a subpopulation of RA-projecting HVC neurons that is active only at that time. As a population, these HVC neurons may form an explicit representation of time in the sequence. Such a sparse representation, a temporal analogue of the 'grandmother cell' concept for object recognition, eliminates the problem of temporal interference during sequence generation and learning attributed to more distributed representations.     

提高固有频率测量精度的声频法及其工程应用

提出了敲击声的频率测量方法,事实上这种声频就是固有频率;对于钢、铝和铜等材料制作的自由梁,试验说明了它们的声频与横向振动的固有频率是一致的,这种声频法提高了固有频率测量的精度;文章还给出了用弹性梁的声频基频推算材料弹性模量的方法,该方法对于薄而短的试件是非常成功的,已应用于蠕变损伤的测量。     

基于WNN的双目摄像机标定方法

针对传统摄像机标定方法需要建立复杂的数学模型,且计算量大、实时性不好的问题,引入了人工神经网络来有效处理非线性映射问题,准确地建立起立体视觉中三维空间特征点与它在图像平面上像点之间的非线性关系;但现有的神经网络标定法仍存在实时性差、标定精度不够、泛化能力差的缺点,于是该文提出了一种基于小波神经网络（waveletneuralnetwork,WNN）的方法,同时用粒子群优化算法对学习算法进行改进,并对小波网络与BP网络的标定结果进行比较．实验结果表明,基于小波神经网络的双目视觉标定方法能够达到较高的实时性、标定精度和泛化能力的要求．     

伽玛暴微光变特征时标研究

伽玛暴光变曲线一般由若干个脉冲组成的“大轮廓”叠加着一些微光变组成的,大部分伽玛暴光变曲线没有呈现周期性,而这些微光变成分可能携带重要的辐射区性质.采用平滑滤波器把伽玛暴光变曲线中的“大轮廓”成分和微光变成分分离,然后采用“最大熵”功率谱密度估算法计算微光变信号的功率谱密度,并分析其周期特征时标.文中分析了Swift/...     

Reconciling the temperature dependence of respiration across timescales and ecosystem types

Ecosystem respiration is the biotic conversion of organic carbon to carbon dioxide by all of the organisms in an ecosystem, including both consumers and primary producers. Respiration exhibits an exponential temperature dependence at the subcellular and individual levels, but at the ecosystem level respiration can be modified by many variables including community abundance and biomass, which vary substantially among ecosystems. Despite its importance for predicting the responses of the biosphere to climate change, it is as yet unknown whether the temperature dependence of ecosystem respiration varies systematically between aquatic and terrestrial environments. Here we use the largest database of respiratory measurements yet compiled to show that the sensitivity of ecosystem respiration to seasonal changes in temperature is remarkably similar for diverse environments encompassing lakes, rivers, estuaries, the open ocean and forested and non-forested terrestrial ecosystems, with an average activation energy similar to that of the respiratory complex (approximately 0.65?electronvolts (eV)). By contrast, annual ecosystem respiration shows a substantially greater temperature dependence across aquatic (approximately 0.65?eV) versus terrestrial ecosystems (approximately 0.32?eV) that span broad geographic gradients in temperature. Using a model derived from metabolic theory, these findings can be reconciled by similarities in the biochemical kinetics of metabolism at the subcellular level, and fundamental differences in the importance of other variables besides temperature—such as primary productivity and allochthonous carbon inputs—on the structure of aquatic and terrestrial biota at the community level.     

自然崩落采矿法的颗粒流数值模拟

以自然崩落采矿法为研究对象,利用二维颗粒流数值模拟(PFC2D)的原理和方法,研究了自然崩落法矿体崩落规律. 以某镍铜矿的地质条件及矿岩物理力学性质为依据,采用数值模拟的方法分析了自然崩落法的崩落规律. 结果表明:PFC2D模型在分析自然崩落过程中力学机理的同时,能有效地模拟自然崩落法采矿过程,并且能直观地给出矿体在崩落过程中各种参数的具体形态变化. 利用PFC2D模型预测该镍铜矿自然崩落法初始崩落拉底半径为10 m,连续崩落的拉底半径为22 m,有效地指导该矿自然崩落法的放矿.     

用BP神经网络提高锂离子电池化成系统采样精度

针对自行设计的YX-20A型锂离子电池化成柜采样精度不高的问题,分别采用动量梯度下降法和L-M优化法以三层BP神经网络为预测模型对采样电流数据进行校正;并用校正后的采样数据通过TL494芯片调节设定基准和充放电电流实测值的偏差。研究结果表明:L-M算法能快速收敛,效果优于动量梯度下降法,当隐含层节点数为9时,L-M算法效果最佳;校正后的电流最大相对误差由原来的5%降到1.1%左右,平均误差小于0.5%;校正后基准电流和实测值间的相对误差波动较平缓,其最大相对误差比校正前有明显下降。     

旋转对超新星塌缩时标的影响

基于超新星前身型模型WS15M⊙,详细计算了在一维球对称未旋转条件和旋转条件下的流体动力学时标和电子俘获时标,进行了比较,发现在形成激波前很短时间内电子俘获时标将小于流体动力学时标,这种差异在旋转的星体下更为明显.计算结果支持了新近提出一种超新星爆发图像,可能对超新星的爆发机制研究有新的影响.       

夏季北太平洋副热带高压年际变化的特征时间尺度及其时空演变

 应用三维数据的时间小波变换方法分析研究了夏季北太平洋副热带高压年际变化的特征时间尺度及其时空演变.结论为:夏季北太平洋副热带高压年际变化的特征时间尺度分别约为2,5 a和更大的时间尺度;2 a和5 a特征时间尺度的最大振幅能量活动中心分别位于北太平洋西部的(27.5°N,135°E)附近和北部的(35°N,167.5°E)附近;其相应的最大振幅能量活动中心在纬向上表现出了明显的东西向波动的时间演变特征;在经向上则表现出了明显的阶段性和较小振幅的南北向波动的时间演变特征;从较长时间的长期演变来看,5a特征时间尺度的最大振幅能量活动中心的位置有一种向南移动的趋势.     

Small modulation of ongoing cortical dynamics by sensory input during natural vision

During vision, it is believed that neural activity in the primary visual cortex is predominantly driven by sensory input from the environment. However, visual cortical neurons respond to repeated presentations of the same stimulus with a high degree of variability. Although this variability has been considered to be noise owing to random spontaneous activity within the cortex, recent studies show that spontaneous activity has a highly coherent spatio-temporal structure. This raises the possibility that the pattern of this spontaneous activity may shape neural responses during natural viewing conditions to a larger extent than previously thought. Here, we examine the relationship between spontaneous activity and the response of primary visual cortical neurons to dynamic natural-scene and random-noise film images in awake, freely viewing ferrets from the time of eye opening to maturity. The correspondence between evoked neural activity and the structure of the input signal was weak in young animals, but systematically improved with age. This improvement was linked to a shift in the dynamics of spontaneous activity. At all ages including the mature animal, correlations in spontaneous neural firing were only slightly modified by visual stimulation, irrespective of the sensory input. These results suggest that in both the developing and mature visual cortex, sensory evoked neural activity represents the modulation and triggering of ongoing circuit dynamics by input signals, rather than directly reflecting the structure of the input signal itself.     

Tropical climate changes at millennial and orbital timescales on the Bolivian Altiplano

Tropical South America is one of the three main centres of the global, zonal overturning circulation of the equatorial atmosphere (generally termed the 'Walker' circulation). Although this area plays a key role in global climate cycles, little is known about South American climate history. Here we describe sediment cores and down-hole logging results of deep drilling in the Salar de Uyuni, on the Bolivian Altiplano, located in the tropical Andes. We demonstrate that during the past 50,000 years the Altiplano underwent important changes in effective moisture at both orbital (20,000-year) and millennial timescales. Long-duration wet periods, such as the Last Glacial Maximum--marked in the drill core by continuous deposition of lacustrine sediments--appear to have occurred in phase with summer insolation maxima produced by the Earth's precessional cycle. Short-duration, millennial events correlate well with North Atlantic cold events, including Heinrich events 1 and 2, as well as the Younger Dryas episode. At both millennial and orbital timescales, cold sea surface temperatures in the high-latitude North Atlantic were coeval with wet conditions in tropical South America, suggesting a common forcing.     

Temporal dynamics of a neural solution to the aperture problem in visual area MT of macaque brain

A critical step in the interpretation of the visual world is the integration of the various local motion signals generated by moving objects. This process is complicated by the fact that local velocity measurements can differ depending on contour orientation and spatial position. Specifically, any local motion detector can measure only the component of motion perpendicular to a contour that extends beyond its field of view. This "aperture problem" is particularly relevant to direction-selective neurons early in the visual pathways, where small receptive fields permit only a limited view of a moving object. Here we show that neurons in the middle temporal visual area (known as MT or V5) of the macaque brain reveal a dynamic solution to the aperture problem. MT neurons initially respond primarily to the component of motion perpendicular to a contour's orientation, but over a period of approximately 60 ms the responses gradually shift to encode the true stimulus direction, regardless of orientation. We also report a behavioural correlate of these neural responses: the initial velocity of pursuit eye movements deviates in a direction perpendicular to local contour orientation, suggesting that the earliest neural responses influence the oculomotor response.     

用BP神经网络构成三维视觉测量系统的新方法

提出一种基于人工神经网络的新的三维视觉测量方法，建立在三角测量原理上的传统的三维立体视觉测量技术会带入成像畸变非线性误差，这种新方法可以消除非线性因素的影响，采用正弦函数作为传递函数加快了网络的训练速度，提高了学习精度，对空间点位和尺寸测量的结果验证了方法的可行性，对影响系统测量精度的因素进行了讨论，并与传统立体视觉测量方法进行了比较。     

Synchronicity of Antarctic temperatures and local solar insolation on orbital timescales

The Milankovitch theory states that global climate variability on orbital timescales from tens to hundreds of thousands of years is dominated by the summer insolation at high northern latitudes. The supporting evidence includes reconstructed air temperatures in Antarctica that are nearly in phase with boreal summer insolation and out of phase with local summer insolation. Antarctic climate is therefore thought to be driven by northern summer insolation. A clear mechanism that links the two hemispheres on orbital timescales is, however, missing. We propose that key Antarctic temperature records derived from ice cores are biased towards austral winter because of a seasonal cycle in snow accumulation. Using present-day estimates of this bias in the 'recorder' system, here we show that the local insolation can explain the orbital component of the temperature record without having to invoke a link to the Northern Hemisphere. Therefore, the Antarctic ice-core-derived temperature record, one of the best-dated records of the late Pleistocene temperature evolution, cannot be used to support or contradict the Milankovitch hypothesis that global climate changes are driven by Northern Hemisphere summer insolation variations.     

图象编码系统中解码的唯一性和即时性探讨

一个图象编码系统由编码器、信道、解码器三个部分组成．通过编码,降低了数字图象对存储量的要求,缩短了图象传输所需的时间,但这是以牺牲压缩与解压的时间为代价的．解码的唯一性和即时性在图象处理过程中不容忽视．本文给出了解码的唯一性和即时性的概念,举例讨论了解码过程中遇到的唯一性和即时性问题以及它们对解码结果的影响．     

A hierarchy of timescales in protein dynamics is linked to enzyme catalysis

The synergy between structure and dynamics is essential to the function of biological macromolecules. Thermally driven dynamics on different timescales have been experimentally observed or simulated, and a direct link between micro- to milli-second domain motions and enzymatic function has been established. However, very little is understood about the connection of these functionally relevant, collective movements with local atomic fluctuations, which are much faster. Here we show that pico- to nano-second timescale atomic fluctuations in hinge regions of adenylate kinase facilitate the large-scale, slower lid motions that produce a catalytically competent state. The fast, local mobilities differ between a mesophilic and hyperthermophilic adenylate kinase, but are strikingly similar at temperatures at which enzymatic activity and free energy of folding are matched. The connection between different timescales and the corresponding amplitudes of motions in adenylate kinase and their linkage to catalytic function is likely to be a general characteristic of protein energy landscapes.