Temporal hyperacuity in single neurons of electric fish

Behavioural studies have revealed that animals can resolve temporal disparities in the microsecond range. This resolution is far superior to that of individual receptors, and it must therefore be achieved through central neuronal mechanisms. It is unclear, however, whether such sensitivity ever emerges at the level of single neurons, or whether it is apparent only at the behavioural level through the collective action of many less-sensitive neurons. We have found that single neurons in the pre-pacemaker nucleus of a weakly electric fish are sensitive to temporal disparities as small as 1 microsecond, the highest temporal sensitivity ever observed at the single-neuron level. The remarkable temporal resolution of these pre-pacemaker neurons results from a high degree of spatial convergence of afferent inputs. These neurons represent the final elements of a sensory hierarchy and directly control the jamming avoidance response by which these fish regulate the frequency of their electric organ discharges.     

Predation enhances complexity in the evolution of electric fish signals.

Theories of sexual selection assume that predation is a restrictive, simplifying force in the evolution of animal display characters and many empirical studies have shown that predation opposes excessive elaboration of sexually selected traits. In an unexpected turnaround, I show here that predation pressure on neotropical, weakly electric fish (order Gymnotiformes) seems to have selected for greater signal complexity, by favouring characters that have enabled further signal elaboration by sexual selection. Most gymnotiform fish demonstrate adaptations that lower detectability of their electrolocation/communication signals by key predators. A second wave phase added to the ancestral monophasic signal shifts the emitted spectrum above the most sensitive frequencies of electroreceptive predators. By using playback trials with the predatory electric eel (Electrophorus electricus), I show that these biphasic signals are less detectable than the primitive monophasic signals. But sexually mature males of many species in the family Hypopomidae extend the duration of the second phase of their electric signal pulses and further amplify this sexual dimorphism nightly during the peak hours of reproduction. Thus a signal element that evolved for crypsis has itself been modified by sexual selection.     

Temporal evolution of the electric field accelerating electrons away from the auroral ionosphere.

The bright night-time aurorae that are visible to the unaided eye are caused by electrons accelerated towards Earth by an upward-pointing electric field. On adjacent geomagnetic field lines the reverse process occurs: a downward-pointing electric field accelerates electrons away from Earth. Such magnetic-field-aligned electric fields in the collisionless plasma above the auroral ionosphere have been predicted, but how they could be maintained is still a matter for debate. The spatial and temporal behaviour of the electric fields-a knowledge of which is crucial to an understanding of their nature-cannot be resolved uniquely by single satellite measurements. Here we report on the first observations by a formation of identically instrumented satellites crossing a beam of upward-accelerated electrons. The structure of the electric potential accelerating the beam grew in magnitude and width for about 200 s, accompanied by a widening of the downward-current sheet, with the total current remaining constant. The 200-s timescale suggests that the evacuation of the electrons from the ionosphere contributes to the formation of the downward-pointing magnetic-field-aligned electric fields. This evolution implies a growing load in the downward leg of the current circuit, which may affect the visible discrete aurorae.     

教学意义下的Fermat-Steiner问题

Fermat Steiner问题是初等几何研究领域中的经典名题.就Fermat Steiner问题进行研究并对解决问题过程进行分析,可以为研究解决初等几何问题提供一个范例.     

大学生心理一致感与社会支持的关系研究

采用《心理一致感量表》和《社会支持评定量表》,调查1110名大学生的心理一致感的特点及其与社会支持水平的关系。结果表明:在是否独生变量上,大学生心理一致感的意义感维度差异显著(t=-2.39,P<0.05);在年级变量上,可理解性(F=4.14,P<0.01)与意义感(F=2.86,P<0.05)两个维度差异显著;社会支持各维度与心理一致感显著相关,社会支持对心理一致感有预测作用;大学生中非独生高于独生,大四学生心理一致感最高,大二最低。     

极小代数意义下行列式的若干性质

给出极小代数意义下行列式的定义,研究了极小代数意义下行列式的性质,得出了若干结果,为简化极小代数意义下行列式的计算提供了方便.     

广义泰勒展式的推广

本文对Feller在[1]中利用差商和普阿松分布的有关性质,证明的广义泰勒展式进行了推广,放宽了定理中对函数有界的要求,扩大了定理的使用范围。     

时态信息与时态信息管理研究

时态信息和时态信息管理是数据库技术和信息管理技术的一种新的研究领域,对于信息的时态要求是信息应用需求多元化的必然.本文概括了时态信息管理中的经典模型,研究了时态信息管理的常用软件,并对其相关的时态支持进行了分析归纳.     

Generalized solutions to the Benjamin-Ono equations in sense of Colombeau

This paper discusses the existence and uniqueness of the generalized solution in the sense of Colombeau to the Benjamin-Ono (B-O) equation and the relationship between the new generalized solution and the classical solution.     

谈学生的责任教育

生活中屡见不鲜的大学生责任缺失现象,向学校的责任教育提出了挑战.学校应该改变教学方式、完善教学内容,形成一个责任教育体系,与家庭、社会共同努力,创造一个有利于培养学生责任意识的环境.     

例谈五种转化意识在解题中的作用

从五种角度说明转化意识在解数学题中的积极作用,是数学教学中应予以关注的方法．     

时态数据库的时态代数分析

以传统关系数据模型为基础建立了多种时态数据库模型。传统数据库具有一些集合运算：选择、投影、连接、笛卡尔积、差、并和交，这些关系代数只是一种瞬像代数，没有考虑时态关系。本文重点介绍了适合于时态数据库的时态关系代数。结果证明时态关系代数运算与传统关系代数运算是相容的。     

时间顺序原则和时间范围原则述评

本文对戴浩一提出的时间顺序原则和时间范围原则进行介绍和评论.认为这两个原则具有一定的合理性和可操作性,但还存在着一些不足.笔者立足于认知概念结构,引入方位系统中的原位、中位和终位概念,指出方位主义观能弥补这些不足,且其解释简洁.     

用函数观点理解数列

用函数的观点理解数列,将两个概念有机地结合起来,可以加深学生对这两个概念本质的理解,也可以利用其关系解决一些难题．     

论匀速率运动电荷的电磁场

论述了匀速率运动电荷在空间任意点产生的电磁场强度,以及它们之间的定量关系,为求解该类问题提供了一种新方法.     

匀强电场的电势参考点分析

讨论在匀强电场中未放入导体球前选择球心处的电势ψ0,并给出了确定ψ0的具体方法.