Memory encoding by theta phase precession in the hippocampal network

Recent experimental evidence on spike-timing-dependent plasticity and on phase precession (i.e., the theta rhythm dependent firing of rat hippocampal cells) associates the contribution of phase precession to episodic memory. This article aims at clarifying the role of phase precession in memory storage. Computer simulations show that the memory storage in the behavioral timescale varies in timescale of the temporal sequence from half a second to several seconds. In contrast, the memory storage caused by traditional rate coding is restricted to the temporal sequence within 40 ms. During phase precession, memory storage of a single trial experience is possible, even in the presence of noise. It is therefore concluded that encoding by phase precession is appropriate for memory storage of the temporal sequence in the behavioral timescale.     

θ相移在单次学习过程中促进神经网络对空间位置顺序记忆的研究

θ相移是在大鼠海马中发现的位置细胞放电的特殊模式．随着大鼠在某个位置场中行进,相应位置细胞发放脉冲的相位（相对于局部电位中的θ节律）会逐渐提前．一些学者认为,该现象可以将大鼠在运动中所经过的一系列位置场的顺序编码成时间上压缩,并且多次重复出现的脉冲模式,因此可以促进大鼠对其在运动中经过的空间位置的顺序的记忆．本文建立了一个模型,对该现象进行了研究．首先,本文建立了能够产生θ相移现象的单个海马神经元模型．这一模型建立在HarrisKD等及MageeJC的电生理实验研究的基础上,根据神经元真实的生理特性来建模．并且以整合与发放的脉冲神经元模型取代H—H模型,大大简化了计算量．而模拟结果又能较好的重现实验中真实神经元的表现．为了研究θ相移对空间位置顺序记忆的作用,在单神经元模型的基础上,又建立了一个基于STDP的学习型神经网络．通过对网络的研究发现,空间位置顺序的信息在模拟中只要输入一次,就可以使该网络对这一顺序形成一定程度的记忆,并且有一定的比率能达到很高的准确率．而如果在单神经元模型中去除θ相移功能,则在单次学习过程中,根本无法形成对空间位置顺序的记忆,代表各个空间位置的神经元几乎同时发放,基本上不能代表顺序信息．     

Kernel-based topographic map formation by local density modeling

We introduce a new learning algorithm for kernel-based topographic map formation. The algorithm generates a gaussian mixture density model by individually adapting the gaussian kernels' centers and radii to the assumed gaussian local input densities.     

Tailoring the formation of metastable Mg through interfacial engineering: A phase stability analysis

A classical thermodynamic approach has been used for describing the pseudomorphic growth in Mg/Nb multilayer films. The bi-phase diagram of these films has been predicted theoretically and the predictions were verified experimentally by growing multiple Mg/Nb thin films that were characterized through high resolution transmission electron micrograph (HRTEM) and X-ray diffraction (XRD) measurement. The good agreement between predictions and experiments shows that the stability of multilayer films can be explained through simple thermodynamic analysis on the competition between bulk and interfacial energies.     

Step sequence planning for a biped robot by means of a cylindrical shape model and a high-resolution 2.5D map

A novel step sequence planning (SSP) method for biped-walking robots is presented. The method adopts a free space representation custom-designed for efficient biped robot motion planning. The method rests upon the approximation of the robot shape by a set of 3D cylindrical solids. This feature allows efficient determination of feasible paths in a 2.5D map, comprising stepping over obstacles and stair climbing. A SSP algorithm based on A^∗-search is proposed which uses the advantages of the aforementioned environment representation. The efficiency of the proposed approach is evaluated by a series of simulations performed for eight walking scenarios.     

Topographic map formation by maximizing unconditional entropy: aplausible strategy for “online” unsupervised competitivelearning and nonparametric density estimation

An unsupervised competitive learning rule, called the vectorial boundary adaptation rule (VBAR), is introduced for topographic map formation. Since VBAR is aimed at producing an equiprobable quantization of the input space, it yields a nonparametric model of the input probability density function. Furthermore, since equiprobable quantization is equivalent to unconditional entropy maximization, we argue that this is a plausible strategy for maximizing mutual information (Shannon information rate) in the case of "online" learning. We use mutual information as a tool for comparing the performance of our rule with Kohonen's self-organizing (feature) map algorithm.     

Surface contraction and expansion waves correlated with differentiation in axolotl embryos--I. Prolegomenon and differentiation during invagination through the blastopore, as shown by the fate map

We have discovered a series of expansion and contraction, solitary waves that correlate with discrete steps of differentiation in the urodele amphibian axolotl embryo (Ambystoma mexicanum). Here we examine in detail the proposition that the blastopore is a set of differentiation waves. We superimposed the image of the axolotl fate map onto our digitized video images of normal gastrulation and matched the fate map to pigmentation irregularities on the embryo. We were then able to track the invagination of the fate map by tracking the variegated pigmentation on several embryos as gastrulation proceeded. We show a particular expansion and contraction wave sequence for every tissue in the blastula stage fate map and can now explain precisely why the fate map has the shape it does and its relationship to the embryo at subsequent stages. Each tissue can be assigned a differentiation code and placed on a hierarchical, binary differentiation tree.