The brain processes underlying sensory comparison

自然界的刺激在许多感官属性上有所不同，如亮度、色调、方向、质地、运动速度和方向，以及与观察者的距离。人类的视觉系统可以毫不费力、迅速地将两种刺激与这些属性中的一种进行比较，以至于很少有人正式讨论这种比较是如何在大脑中进行的。我们认为，在这种知觉比较的基础上，有两种非常不同的机制。当比较是局部的--例如，相邻物体的亮度的比较--比较可能由专用的、硬连接的神经细胞执行，这些神经细胞从视网膜的相邻区域获得拮抗输入(兴奋和抑制)。这种硬性比较是相当容易理解的。生理学家已经在大脑皮层中发现了对纹理、运动速度或距离观察者的局部差异做出反应的细胞。但是，当观察者被要求比较落在视野内两个完全分开的点上的刺激时，如何进行比较？这个问题很少被提出来。对于视野中的每一对可能的位置和视觉刺激的每一种属性(色调、方向、速度等)，是否有一个硬连接的、提取差异的神经细胞？这似乎是不太可能的，因为需要大量的神经细胞和连接。另一种选择是，大脑包含一个类似于人造互联网的通信系统，因为相同的纤维每时每刻都承载着不同的信息包，每个信息包都与发送者和收件人的地址相关联。根据这种说法，关于这两种刺激的信息将被以抽象的形式编码，并通过大脑的互联网(大脑巴士)发送到决策中心，可能是在前额叶皮质。我们计划的研究包括经验和理论两个部分。我们提出了知觉实验，对于特定的条件和特定的视觉属性，应该揭示观察者是在使用来自硬连接的神经比较器的信号，还是在比较这两个刺激的更抽象的表示。因此，如果使用专用的比较器神经元，那么比较的精度预计会随着空间分离而恶化，因为众所周知，视觉系统中的横向连接随着距离的增加而变得更加稀疏。同样，如果刺激物在某些不相关的属性上也不同--比方说，如果正在比较速度，但刺激物朝着相反的方向移动，那么比较的精确度可能会下降。对于专用的比较器，细胞通常对刺激的一个以上属性是特异的。然而，这两种实验操作都不会损害依赖于更抽象的刺激表示的比较。我们将特别关注的视觉属性是运动的速度和方向。两组移动点将被短暂地呈现在观察者视野中的不同位置，他或她将被要求指出哪一组移动得更快。研究运动有实际的兴趣，因为在现实世界中，外围的相对运动指导着我们的平衡和运动。在试点研究中，我们发现人们判断移动阵列相对速度的能力存在显著的个体差异：我们的工作可能会导致对职业的选择测试，在这些测试中，这种判断是关键的。我们工作的理论部分旨在促使人们讨论信息在大脑白质束上传输的格式和控制传输的“协议”。在纤维束的起始处有专门的组织结构吗？发送器和接收器之间是否存在“握手”，以防止缓冲区溢出？

The stimuli of the natural world differ in many sensory attributes, such as lightness, hue, orientation, texture, the speed and direction of motion, and distance from the observer. So effortlessly and swiftly does the human visual system compare two stimuli with respect to one of these attributes, that there has been rather little formal discussion of how such comparisons are performed within the brain. We believe that there are two, very distinct, types of mechanism underlying such perceptual comparisons. When the comparison is local - for example, a comparison of the lightness of adjacent objects - the comparison may be performed by dedicated, hard-wired, nerve cells that draw antagonistic inputs (excitatory and inhibitory) from adjacent regions of the retina. This type of hard-wired comparison is rather well understood. Physiologists have found cells in the cortex of the brain that respond to local differences in texture, or in speed of motion or in distance from the observer. But how is the comparison performed when the observer is required to compare stimuli that fall at two well separated points within the visual field? This question has seldom been raised. Is there a hard-wired, difference-extracting nerve cell for every possible pair of positions in the visual field and for each of the properties of visual stimuli (hue, orientation, speed etc)? This seems a priori unlikely, owing to the bulk of the nerve cells and the connections that are needed. An alternative is that the brain contains a communication system that resembles the man-made Internet, in that the same fibres, from moment to moment, carry different packages of information, each associated with the addresses of the sender and of the addressee. By this account, the information about the two stimuli would be encoded in abstract form and sent over the brain's Internet (the 'cerebral bus') to a decision-making centre, perhaps in the prefrontal cortex.Our planned research has empirical and theoretical components. We propose perceptual experiments that should reveal - for particular conditions and particular visual attributes - whether the observer is using the signals from hard-wired neural comparators or is comparing more abstract representations of the two stimuli. Thus if dedicated comparator neurons are used, then the precision of comparison is expected to deteriorate with spatial separation, since lateral connections in the visual system are known to become sparser with distance. Similarly, the precision of comparison might be expected to deteriorate if the stimuli also differ in some irrelevant attribute - if, say, speed is being compared but the stimuli move inopposite directions. For dedicated comparator cells are often specific to more than one attribute of the stimulus. However, neither of these experimental manipulations may impair comparisons that depend on more abstract representations of the stimuli.The visual attributes we shall particularly concentrate on are speed and direction of motion. Two arrays of moving dots will be briefly presented at different positions in the observer's visual field and he or she will be asked to indicate which array is, say, moving faster. There is a practical interest in studying motion, since in the real world relative motion in the periphery guides our balance and our locomotion. In pilot studies, we found remarkable individual differences between people in their ability to judge the relative speed of moving arrays: our work may lead to selection tests for professions in which such judgements are critical.The theoretical part of our work is intended to prompt discussion of the format in which information is transmitted over the white-matter tracts of the brain and the 'protocol' that governs transmission. Are there specialized histological structures at the origins of the tracts? Is there 'handshaking' between transmitter and receiver, to prevent overflow of buffers?