胼胝体在初级视觉信息加工中的应用

在综述初级视区胼胝体功能研究的基础上,提出了视网膜双侧投射带的中枢拓扑联系模型,它不但符合一系列生理学和形态学实验的结果,而且解释了胼胝体存在的必要性：它传送的信号补偿了由于存在视网膜双侧投射带而在每侧皮层上所造成的信息损失。     

视觉刺激对猴初级视皮层神经元眼球位置效应的影响

用细胞外记录的方法,研究了视觉刺激对清醒猴初级视皮层神经元眼球位置效应的影响,当猴注视电视屏幕上２５个不同位置的小光点时,屏蔽上分别给予两种不同形式的视觉刺激：注视点周围的闪光圆环和感受野内的移动光条。这两种刺激都能增强初级视皮层神经元的眼球位置相关活动,并相应地使受眼球位置调制的神经元的比例明显增加。     

功能磁共振视网膜皮层映射成像在视觉系统研究中的应用

功能磁共振视网膜皮层映射成像能提供关于视觉皮层的丰富信息,是研究生理及病理状态下视觉皮层定位以及功能特性的强有力工具。本文就该成像技术的原理、应用及前景作一综述。     

综述: 初级视皮层的亮度信息加工

亮度(luminance)是最基本的视觉信息.与其他视觉特征相比,由于视神经元对亮度刺激的反应较弱,并且许多神经元对均匀亮度无反应,对亮度信息编码的神经机制知之甚少.初级视皮层部分神经元对亮度的反应要慢于对比度反应,被认为是由边界对比度诱导的亮度知觉(brightness)的神经基础.我们的研究表明,初级视皮层许多神经元的亮度反应要快于对比度反应,并且这些神经元偏好低的空间频率、高的时间频率和高的运动速度,提示皮层下具有低空间频率和高运动速度通路的信息输入对产生初级视皮层神经元的亮度反应有贡献.已经知道初级视皮层神经元对空间频率反应的时间过程是从低空间频率到高空间频率,我们发现的早期亮度反应是对极低空间频率的反应,与这一时间过程是一致的,是这一从粗到细的视觉信息加工过程的第一步,揭示了处理最早的粗的视觉信息的神经基础.另外,初级视皮层含有偏好亮度下降和高运动速度的神经元,这群神经元的活动有助于在光照差的环境中检测高速运动的低亮度物体.     

纹外皮层PMLS区反馈投射对初级视皮层神经元反应特性的影响

神经系统中存在大量下行投射,与上行输入一起形成复杂的前馈与反馈回路,调控神经信号的传导和处理,但目前对皮层内反馈投射的功能作用认识还比较薄弱.通过微量注射抑制性神经递质γ-氨基丁酸(γ-aminobutyric acid,GABA),使猫纹外皮层后内侧外上雪氏区(area posteromedial lateral suprasylvian,PMLS)局部可逆性失活,使用胞外记录方法,研究初级视皮层17区神经元反应特性的变化.实验结果显示,PMLS区失活后,17区细胞对运动刺激的反应总体减弱,反应的相对稳定性基本不变,最高发放率/自发之比有所下降.与此同时,细胞的方向选择性指数减小,朝向选择性无显著变化.除少数"双向"反应细胞外,绝大部分细胞的最优方向基本不变.进一步分析发现,细胞对各个方向刺激的反应普遍下降,最优方向上的下降程度最大,是导致方向选择性减弱的主要原因.这些结果表明,PMLS区反馈投射可增强初级视皮层的方向选择性,而对朝向选择性影响有限.这一作用特点体现了PMLS区在皮层中偏重处理运动视觉信息的功能.     

家兔眼内压升高对ERG和VECP作用的对比研究

在脲酯麻醉的家兔,同时记录了闪光刺激引起的视网膜电图(ERG)和视皮层诱发电位(VECP)。直接测定的家兔正常眼内压为21.4±3.0mmHg(S.D.,n=7)。当用自制液压式加压装置使眼内压升至49mmHg 时,VECP 和 ERG 反应振幅随时间逐渐下降,在4min内均达最低值(约为正常值的40%),随后 VECP 逐渐回升至对照值的75%左右,但 ERG b波振幅恢复不如 VECP 明显。随着眼内压的进一步升高,VECP 和 ERG 均继续下降,但VECP 较 ERG 下降更明显。当眼内压超过一定水平(此水平因个体而异),VECP 反应骤然消失,ERG 反应虽变小却仍然存在,这种差别可能是由于眼内压的增高更易影响神经节细胞的活动所致。当撤去外加压力,眼内压恢复到正常水平后,VECP 和 ERG 均逐渐恢复正常。     

2 Hz与100 Hz刺激在诱导大鼠视皮层长持续长时程增强中的不同作用

在哺乳动物的视皮层,多种不同参数的刺激可诱导出长时程增强(long-term potentiation,LTP)现象。但关于刺激参数与持续时间长于3h的长持续LTP(long lasting LTP,L-LTP)之间关系的研究较少。本研究用3周龄的大鼠视皮层脑片标本,在Ⅳ层刺激而在Ⅱ／Ⅲ层记录场电位,待场电位稳定后施加强直刺激诱导LTP,探讨2Hz与100Hz的强直刺激在诱发持续时间长于3h的L-LTP中的作用。结果表明,多于300个脉冲不同频率的刺激可稳定地诱导出L-LTP;2Hz与100Hz的刺激诱发的L-LTP有明显不同的表达形式,100Hz刺激可诱导出较大的L-LTP;频率相同而脉冲数不同的强直刺激诱发的L-LTP有相同的表达形式。以上结果提示,不同频率的强直刺激诱发的L-LTP机制可能不同;相同频率的刺激(脉冲数不同)诱发的L-LTP可能有相同的机制。     

fMRI在视觉研究中的应用和进展

视觉研究对于揭示大脑的奥秘有着极其重要的意义.功能性磁共振成像（functional magnetic resonance imaging,fMRI）用于研究人脑的功能结构,主要是基于静脉毛细血管内血氧浓度的变化.fMRI可以无损伤地在几毫米级的空间分辨率和少于1 s的时间分辨率上观察清醒状态下人脑的活动,因此自90年代以来fMRI已经成为研究人脑的重要方法.fMRI在视觉研究中的应用已经使人们对视觉系统的功能性组织有了更好的理解,并取得了很多成果.今后的研究方向是进一步探讨人脑的意识、注意、记忆等高级功能的神经机制.     

金黄地鼠视皮层通过胼胝体顺行和逆行激活的神经元的电活动和分布

用细胞外记录技术研究了金黄地鼠一侧视皮层神经元对电刺激另一侧相应部位的反应及其对视觉刺激反应的特点。(1)在17—18a 交界区大多数(78%)细胞能被电刺激对侧相应点所驱动。在17区内部,被驱动细胞数随离开17—18a 交界距离增加而减少。(2)按对电刺激的反应形式,记录的细胞可分为四类:a.逆行驱动细胞,占17—18a 交界区记录细胞数的20%;b.顺行驱动细胞,占52%中;c.自发发放因电刺激而减少或完全停止的细胞,占6%,d.其余细胞(22%)对电刺激没有可察觉的反应。(3)在17—18a 交界,除了Ⅰ层以外的各层都可记录到顺行驱动细胞,而逆行驱动细胞则主要集中在Ⅲ及Ⅴ层。(4)17—18a 交界细胞感受野(RF)多位于垂直中线附近的对侧视野内,有些 RF 跨越中线,其边界延伸到同侧约10°处。(5)逆行驱动细胞包括刁和 Blakemore 以前在视皮层中记录到的所有细胞类型和各种程度的眼优势,但单眼细胞的比例增加,非对称式 RF 和中心对称式的给-撤型 RF 的比例有所减少。     

基于初级视觉皮层的图像匹配模型

提出一种基于初级视觉皮层的图像匹配模型。该模型只采用方位选择性细胞和皮层内有限范围水平连接等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,逐步进入并维持在兴奋状态,最终实现对视野中特定目标轮廓的提取     

Developmental Upregulation of Ephrin-B1 Silences Sema3C/Neuropilin-1 Signaling during Post-crossing Navigation of Corpus Callosum Axons

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Shaping of Receptive Fields in the Visual Cortex During Retinal Maturation

We present a computational study of the formation of simple-cell receptive field patterns in the primary visual cortex. Based on the observation that the spatial frequency of the retinal filter increases postnatally, our results explain differences in the time course of the development of orientation selectivity in binocularly deprived and normally reared kittens. Development after eye-opening in normal animals is modelled by training with natural images, whereas in the case of binocular deprivation noise-like stimulation continues. Further, it is shown that different orientation selectivities are obtained for network models trained with natural images in contrast to random phase images of identical second order statistics. The latter finding suggests that higher-order statistics of the inputs influences development of primary visual cortex. Finally, we search for quantities that identify possible signatures of natural image statistics in order to specify the amount of constructiveness that visual experience has on the formation of receptive fields.     

Integration of Visual Information in Auditory Cortex Promotes Auditory Scene Analysis through Multisensory Binding

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A Method to Quantify Visual Information Processing in Children Using Eye Tracking

Visual problems that occur early in life can have major impact on a child''s development. Without verbal communication and only based on observational methods, it is difficult to make a quantitative assessment of a child''s visual problems. This limits accurate diagnostics in children under the age of 4 years and in children with intellectual disabilities. Here we describe a quantitative method that overcomes these problems. The method uses a remote eye tracker and a four choice preferential looking paradigm to measure eye movement responses to different visual stimuli. The child sits without head support in front of a monitor with integrated infrared cameras. In one of four monitor quadrants a visual stimulus is presented. Each stimulus has a specific visual modality with respect to the background, e.g., form, motion, contrast or color. From the reflexive eye movement responses to these specific visual modalities, output parameters such as reaction times, fixation accuracy and fixation duration are calculated to quantify a child''s viewing behavior. With this approach, the quality of visual information processing can be assessed without the use of communication. By comparing results with reference values obtained in typically developing children from 0-12 years, the method provides a characterization of visual information processing in visually impaired children. The quantitative information provided by this method can be advantageous for the field of clinical visual assessment and rehabilitation in multiple ways. The parameter values provide a good basis to: (i) characterize early visual capacities and consequently to enable early interventions; (ii) compare risk groups and follow visual development over time; and (iii), construct an individual visual profile for each child.     

Visual cortex encodes timing information in humans and mice

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Information in the Neuronal Representation of Individual Stimuli in the Primate Temporal Visual Cortex

To analyze the information provided about individual visual stimuliin the responses of single neurons in the primate temporal lobevisual cortex, neuronal responses to a set of 65 visual stimuli wererecorded in macaques performing a visual fixation task and analyzedusing information theoretical measures. The population of neuronsanalyzed responded primarily to faces. The stimuli included 23 facesand 42 nonface images of real-world scenes, so that the function ofthis brain region could be analyzed when it was processing relativelynatural scenes.It was found that for the majority of the neurons significantamounts of information were reflected about which of several of the23 faces had been seen. Thus the representation was not local, forin a local representation almost all the information available canbe obtained when the single stimulus to which the neuron respondsbest is shown. It is shown that the information available about anyone stimulus depended on how different (for example, how manystandard deviations) the response to that stimulus was from theaverage response to all stimuli. This was the case for responsesbelow the average response as well as above.It is shown that the fraction of information carried by the lowfiring rates of a cell was large—much larger than that carried bythe high firing rates. Part of the reason for this is that theprobability distribution of different firing rates is biased towardlow values (though with fewer very low values than would bepredicted by an exponential distribution). Another factor is thatthe variability of the response is large at intermediate and highfiring rates.Another finding is that at short sampling intervals (such as 20 ms)the neurons code information efficiently, by effectively acting asbinary variables and behaving less noisily than would be expectedof a Poisson process.     

Projection of Orthogonal Tiling from the Retina to the Visual Cortex

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