视觉注意与神经振荡

人脑每时每刻都要接收大量视觉信息，由于人脑加工信息的能力有限，所以在较大视野内将注意分配给相关信息，同时抑制引起注意分散的不相关信息，对执行目标导向的行为至关重要。这种对视觉信息的选择性和主动性加工以适应当前目标的过程被称作视觉注意（visual attention），且视觉注意可分为自上而下的注意与自下而上的注意两种不同功能。由于来自大脑电信号的神经振荡活动在认知加工中发挥重要作用，已有研究综述了视觉注意与神经振荡（neural oscillation）的密切关系，但并未涉及不同的注意功能与神经振荡的关系。本文系统性调查了不同注意功能与神经振荡的关系，发现额-顶区域的theta频带振荡活动反映了自上而下的认知控制，而后部脑区的theta振荡与自下而上的注意相关。顶-枕区域alpha振荡的偏侧化有助于注意分配，而alpha频带的大规模同步促成了注意对视皮层自上而下的影响。Beta振荡介导了自上而下的信息与自下而上的信息之间的互动，作为信息载体促进了视觉信息处理。Gamma振荡则可能与自上而下和自下而上的注意间整合相关。本文就视觉注意功能与神经振荡关系的研究现状展开综述，旨在揭示不同的神经振荡活动在特定的视觉注意功能中的作用。     

基于初级视觉皮层的同步振荡目标检测模型

提出一种基于初级视觉皮层的目标检测模型,该模型只采用方位选择性细胞和皮层内水平连接等Ｖ１基本单元,它以链码表示的目标轮廓作为知识,允许该知识以时间脉冲的形式控制Ｖ１区内神经细胞的动态活动,使与知识轮廓形状相符合的轮廓内的细胞进入同步振荡状态,实现对视野中特定目标轮廓的识别。计算机仿真结果表明,在较高级皮层的“知识”控制之下,初级视觉皮层结构上实现简单的目标检测是可行的。     

动态神经网络中的同步振荡

目前有一种假设认为同一视觉对象是由一群神经元的同步振荡活动来表征的。这一神经元发放活动的时间特性,是解决视觉信息处理中“结合问题（Ｂｉｎｄｉｎｇｐｒｏｂｌｅｍ）”的可能机制。本文用我们所提出的一种简化现实性神经网络模型［１］所构造的时滞非线性振子网络［２］,模拟生物神经网络的同步振荡活动。并考虑了振子各参数的设置与振荡活动的关系,以及网络振子间耦联对同步活动的影响．     

知觉相关的神经振荡-外界节律同步化现象

具有特定频率的节律性刺激能同步大脑内相应频率的神经振荡,使神经活动与外界刺激发生相位锁定,称之为神经振荡-外界节律同步化(neural entrainment).这种同步化的现象伴随着大脑内神经元集群兴奋水平的周期性波动,并与节律信息加工、知觉及注意等认知过程存在关联.得益于其非侵入、易操作以及能有效调控神经活动的特性,神经振荡-外界节律同步化成为了研究神经振荡与知觉和认知功能关系的有力手段,也为认知障碍诊断及干预提供了新的思路和方法.     

综述: 知觉相关的神经振荡-外界节律同步化现象

具有特定频率的节律性刺激能同步大脑内相应频率的神经振荡,使神经活动与外界刺激发生相位锁定,称之为神经振荡-外界节律同步化(neural entrainment).这种同步化的现象伴随着大脑内神经元集群兴奋水平的周期性波动,并与节律信息加工、知觉及注意等认知过程存在关联.得益于其非侵入、易操作以及能有效调控神经活动的特性,神经振荡-外界节律同步化成为了研究神经振荡与知觉和认知功能关系的有力手段,也为认知障碍诊断及干预提供了新的思路和方法.     

强直电刺激嗅内皮质-海马环路诱发CA1神经元的癫痫同步性膜电位振荡行为

目的和方法 :4 0 0～ 5 0 0 μm大鼠水平脑切片含有封闭的EC 海马环路。强直电刺激 (60Hz ,2s)海马Schaeffer侧支诱发癫痫放电 ,全细胞记录CA1胞体层单个神经元电活动 ,同步记录相应树突区细胞外场电位 ,探讨单个神经元膜电位振荡特性与细胞外癫痫电活动之间的关系。结果 :①强直电刺激诱发CA1神经元膜电位后放性振荡行为呈宽频特征 (3～ 10 0Hz)。以θ节律多见 ,跟随在刺激引起的膜电位去极化或超极化偏移 (paroxysmaldepolarizingorhyperpolaringshift,PDSorPHS)之后 ,振荡波的上升支和下降支分别由膜电位去极化 超极化或超极化 去极化成分构成 ;②逐渐增强的IPSP构成了膜电位振荡的起搏成分 ,继而反弹形成锋电位和阈下振荡 ,与细胞外癫痫样电活动同步 ,并促成癫痫放电由紧张性向阵挛性形式转变 ;③发现了电偶联电位 (spikelets)以及细胞之间的染料偶联现象。结论 :单个神经元作为振荡器可以启动群体神经元超同步化癫痫样电活动 ;缝隙连接可能参与了膜电位振荡的启动与场电位癫痫样电活动的同步作用。     

神经耦合振荡的量子孤波分析

为了分析神经耦合振荡产生频率同步的原因,在神经信息波动方程孤立子解的基础上讨论了神经耦合振荡方程组的稳定解。分别从同地和异地耦合孤波的同相和反相四种情况给出了孤波之间距离随时间的变化规律。阐述了大脑视觉皮层神经元集群之间的耦合振荡在神经孤波模式下呈现出的波粒二象性,体现了知觉具有量子化的特点。因而神经振荡必将引发集体突发,形成脑波频率同步。     

非快速眼动睡眠期间神经同步活动与记忆巩固关系研究进展

神经同步活动被认为是神经系统信息处理的关键。脑内存在多种不同频段的局部同步活动和区域间同步活动,这些神经同步活动与多种行为和认知功能相关。记忆作为脑的高级认知功能,其形成和巩固的过程与神经同步活动关系密切。本文主要从体内非快速眼动(non-rapid eye movement, NREM)睡眠期间多个脑区间的神经振荡活动以及体外培养神经网络的同步爆发活动两个层面综述了神经同步活动与记忆巩固关系的研究进展,分析了当前研究存在的问题,并对今后的相关研究作出展望。     

生物钟的转录后与翻译后水平调控进展

哺乳动物中的昼夜节律系统由位于下丘脑SCN核内的生物钟主钟和位于多数外周细胞中的子钟组成。在分子水平上,生物钟的节律振荡由生物钟基因及其编码蛋白的转录和翻译形成的自主的反馈环路组成,并接受外界因素的影响与环境周期保持同步。为此,就生物钟的调控机制而言,除了转录水平的基因表达调控外,生物钟转录产物和蛋白质的修饰也可以显著影响生物钟基因的表达时相。讨论了一些转录后与翻译后的修饰作用及其对生物钟的影响,并对其今后的研究方向作了展望。     

神经元群简化模型的动态特性的研究

神经发放率的振荡和同步现象是视觉神经生理学的最新实验发现,也是Ｍａｌｓｂｕｒｇ等人关于物体感知的时间相关性神经群理论的重要假设。本文在神经元群简化模型动力学方程的基础上加以改进,研究了噪声对振荡的相关性和独立性的影响,取得与生物实验相似的结果;发现在具有时间结构的输入刺激下单个神经元群发放的锁频现象,进一步讨论了两个神经元群之间的同步现象及其机理。     

Bulbocortical interplay in olfactory information processing via synchronous oscillations

Emergence of synchronous oscillatory activity is an inherent feature of the olfactory systems of insects, mollusks and mammals. A class of simple computational models of the mammalian olfactory system consisting of olfactory bulb and olfactory cortex is constructed to explore possible roles of the related neural circuitry in olfactory information processing via synchronous oscillations. In the models, the bulbar neural circuitry is represented by a chain of oscillators and that of cortex is analogous to an associative memory network with horizontal synaptic connections. The models incorporate the backprojection from cortical units to the bulbar oscillators in particular ways. They exhibit rapid and robust synchronous oscillations in the presence of odorant stimuli, while they show either nonoscillatory states or propagating waves in the absence of stimuli, depending on the values of model parameters. In both models, the backprojection is shown to enhance the establishment of large-scale synchrony. The results suggest that the modulation of neural activity through centrifugal inputs may play an important role at the early stage of cortical information processing.     

Visual deprivation modifies oscillatory activity in visual and auditory centers

Loss of vision may enhance the capabilities of auditory perception, but the mechanisms mediating these changes remain elusive. Here, visual deprivation in rats resulted in altered oscillatory activities, which appeared to be the result of a common mechanism underlying neuronal assembly formation in visual and auditory centers. The power of high-frequency β and γ oscillations in V1 (the primary visual cortex) and β oscillations in the LGN (lateral geniculate nucleus) was increased after one week of visual deprivation. Meanwhile, the power of β oscillations in A1 (the primary auditory cortex) and the power of β and γ oscillations in the MGB (medial geniculate body) were also enhanced in the absence of visual input. Furthermore, nerve tracing revealed a bidirectional nerve fiber connection between V1 and A1 cortices, which might be involved in transmitting auditory information to the visual cortex, contributing to enhanced auditory perception after visual deprivation. These results may facilitate the better understanding of multisensory cross-modal plasticity.     

Attractor analysis of asynchronous Boolean models of signal transduction networks

Prior work on the dynamics of Boolean networks, including analysis of the state space attractors and the basin of attraction of each attractor, has mainly focused on synchronous update of the nodes’ states. Although the simplicity of synchronous updating makes it very attractive, it fails to take into account the variety of time scales associated with different types of biological processes. Several different asynchronous update methods have been proposed to overcome this limitation, but there have not been any systematic comparisons of the dynamic behaviors displayed by the same system under different update methods. Here we fill this gap by combining theoretical analysis such as solution of scalar equations and Markov chain techniques, as well as numerical simulations to carry out a thorough comparative study on the dynamic behavior of a previously proposed Boolean model of a signal transduction network in plants. Prior evidence suggests that this network admits oscillations, but it is not known whether these oscillations are sustained. We perform an attractor analysis of this system using synchronous and three different asynchronous updating schemes both in the case of the unperturbed (wild-type) and perturbed (node-disrupted) systems. This analysis reveals that while the wild-type system possesses an update-independent fixed point, any oscillations eventually disappear unless strict constraints regarding the timing of certain processes and the initial state of the system are satisfied. Interestingly, in the case of disruption of a particular node all models lead to an extended attractor. Overall, our work provides a roadmap on how Boolean network modeling can be used as a predictive tool to uncover the dynamic patterns of a biological system under various internal and environmental perturbations.     

Synchronous and asynchronous systems of threshold elements

The role of synchronism in systems of threshold elements (such as neural networks) is examined. Some important differences between synchronous and asynchronous systems are outlined. In particular, important restrictions on limit cycles are found in asynchronous systems along with multi-frequency oscillations which do not appear in synchronous systems. The possible role of deterministic chaos in these systems is discussed.     

Cholinergic enhancement of visual attention and neural oscillations in the human brain

Cognitive processes such as visual perception and selective attention induce specific patterns of brain oscillations. The neurochemical bases of these spectral changes in neural activity are largely unknown, but neuromodulators are thought to regulate processing. The cholinergic system is linked to attentional function in vivo, whereas separate in vitro studies show that cholinergic agonists induce high-frequency oscillations in slice preparations. This has led to theoretical proposals that cholinergic enhancement of visual attention might operate via gamma oscillations in visual cortex, although low-frequency alpha/beta modulation may also play a key role. Here we used MEG to record cortical oscillations in the context of administration of a cholinergic agonist (physostigmine) during a spatial visual attention task in humans. This cholinergic agonist enhanced spatial attention effects on low-frequency alpha/beta oscillations in visual cortex, an effect correlating with a drug-induced speeding of performance. By contrast, the cholinergic agonist did not alter high-frequency gamma oscillations in visual cortex. Thus, our findings show that cholinergic neuromodulation enhances attentional selection via an impact on oscillatory synchrony in visual cortex, for low rather than high frequencies. We discuss this dissociation between high- and low-frequency oscillations in relation to proposals that lower-frequency oscillations are generated by feedback pathways within visual cortex.     

The dynamics of two diffusively coupled predator-prey populations

I analyze the dynamics of predator and prey populations living in two patches. Within a patch the prey grow logistically and the predators have a Holling type II functional response. The two patches are coupled through predator migration. The system can be interpreted as a simple predator-prey metapopulation or as a spatially explicit predator-prey system. Asynchronous local dynamics are presumed by metapopulation theory. The main question I address is when synchronous and when asynchronous dynamics arise. Contrary to biological intuition, for very small migration rates the oscillations always synchronize. For intermediate migration rates the synchronous oscillations are unstable and I found periodic, quasi-periodic, and intermittently chaotic attractors with asynchronous dynamics. For large predator migration rates, attractors in the form of equilibria or limit cycles exist in which one of the patches contains no prey. The dynamical behavior of the system is described using bifurcation diagrams. The model shows that spatial predator-prey populations can be regulated through the interplay of local dynamics and migration.     

A cross correlation analysis of the electrical activity of different regions of the cerebral cortex during visual pattern recognition in children]

Characteristics of spatial-temporal organization of brain biopotentials were examined in one to 2.5 year old children during recognition of visual images. Crosscorrelation EEG analysis of frontal, motor, inferior parietal, temporal and occipital cortical zones has shown that recognition of familiar visual objects is accompanied by an increase in spatial synchronization of biopotentials, especially in the inferior parietal zones of both hemispheres and occipital centres of the left hemisphere. There is a considerable increase in the number of highly synchronous synphasic oscillations at the 4--5 per sec frequency with an intensified periodicity of processes. Recognition of unfamiliar objects does not produce a similar effect. Temporal organization of biopotentials of the associative (frontal and inferior parietal) and projection visual areas in the course of recognition of images depends on the existence of a notion of the whole object in the child's memory.     

Emergent properties of electrically coupled smooth muscle cells

Asynchronous and synchronous calcium oscillations occur in a variety of cells. A well-established pathway for intercellular communication is provided by gap junctions which connect adjacent cells and can mediate electrical and chemical coupling. Several experimental studies report that cells presenting only a transient increase when freshly dispersed may oscillate when they are coupled. Such observations suggest that the role of gap junctions is not only to coordinate calcium oscillations of adjacent cells. Gap junctions may also be important to generate oscillations. Here we illustrate the emergent properties of electrically coupled smooth muscle cells using a model that we recently proposed. A bifurcation analysis in the case of two cells reveals that synchronous and asynchronous calcium oscillations can be induced by electrical coupling. In a larger population of smooth muscle cells, electrical coupling may result in the creation of groups of cells presenting synchronous calcium oscillations. The elements of one group may be distant from each other. Moreover, our results highlight a general mechanism by which gap junctional electrical coupling can give rise to out of phase calcium oscillations in smooth muscle cells that are non-oscillating when uncoupled. All these observations remain true in the case of non-identical cells, except that the solution corresponding to synchronous calcium oscillations disappears and that the formation of groups is sensitive to the degree of heterogeneity. The first two authors contributed equally to this work.     

Oscillations of trypsinogen activation

Trypsin activity oscillations are shown by the autocatalytic activation of trypsinogen at 0 degrees C in aqueous solution. The oscillations were observed for 3-4 days and show only slight decrease in enzyme activity. The zymogen has been kept at ice water temperature and pH 8.2 in the presence of Mn2+ ion. The mean periods of around 1.5 hr are about half of those found previously at -10 degrees C in frozen aqueous solution, while the amplitudes related to the mean activity are about one-fourth of that in the frozen experiments. The phenomenon of oscillation is interpreted in terms of coupling between the inhomogeneities of protein and ion concentrations of the unstirred solution and a Mn3+/Mn2+ system, causing synchronous, periodic reduction-oxidation of some cystine bridges in the protein chain. These nonequilibrium conditions, together with synchronous transitions among several conformational states, may produce the observed activity oscillations.