The brain processes underlying sensory comparison

自然界的刺激在许多感官属性上是不同的，比如亮度、色调、方向、纹理、运动的速度和方向，以及与观察者的距离。人类的视觉系统是如此毫不费力地、迅速地比较两个刺激物的其中一个属性，以至于很少有关于这种比较是如何在大脑中进行的正式讨论。我们认为有两种截然不同的机制潜藏在这种感知比较之下。当比较是局部的-例如，比较相邻物体的亮度-比较可能是由专门的，硬连线的神经细胞进行的，这些神经细胞从视网膜的相邻区域吸收拮抗输入（兴奋性和抑制性）。这种固有的比较很容易理解。生理学家已经发现，大脑皮层中的细胞会对物体的纹理、运动速度或与观察者的距离等方面的局部差异做出反应。但是，当观察者被要求比较落在视野中两个完全分开的点上的刺激时，该如何进行比较呢？这个问题很少被提出。是否存在一种硬连线的、提取差异的神经细胞来处理视野中每一对可能的位置以及视觉刺激的每一种特性（色调、方向、速度等）？这似乎不太可能，因为需要大量的神经细胞和连接。另一种说法是，大脑包含一个类似于人造互联网的通信系统，同样的纤维，每时每刻都携带着不同的信息包，每一个信息包都与发送者和接收者的地址相关联。根据这种说法，关于两种刺激的信息将以抽象形式编码，并通过大脑的互联网（“大脑总线”）发送到决策中心，可能位于前额皮质。我们计划的研究有实证和理论两个部分。我们建议通过感知实验来揭示——对于特定的条件和特定的视觉属性——观察者是在使用来自硬连线神经比较器的信号，还是在比较两个刺激的更抽象的表征。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下降，因为已知视觉系统中的横向连接随着距离而变得稀疏。同样，如果刺激在某些不相关的属性上也存在差异，那么比较的准确性可能会下降——比如，正在比较的是速度，但刺激的方向相反。对于专门的比较细胞来说，它们通常针对刺激的一个以上属性。然而，这两种实验操作都不会损害依赖于更抽象的刺激表征的比较。我们将特别关注的视觉属性是速度和运动方向。两组移动的圆点将在观察者视野的不同位置简短地呈现，他或她将被要求指出哪一组移动得更快。研究运动是有实际意义的，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。在初步研究中，我们发现人们在判断移动阵列的相对速度方面存在显著的个体差异：我们的工作可能会导致对这种判断至关重要的职业的选择测试。我们工作的理论部分旨在促进对信息在大脑白质束上传输的格式和控制传输的“协议”的讨论。在束的起源处是否有专门的组织学结构？发射器和接收器之间是否有“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?